package agent

import (
	"bytes"
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"hash"
	"log/slog"
	"math"
	"os"
	"path/filepath"
	"runtime"
	"slices"
	"sort"
	"strings"
	"sync"
	"time"
	"unicode/utf8"

	"reasonix/internal/filelock"
	"reasonix/internal/fileutil"
	fileencoding "reasonix/internal/fileutil/encoding"
	"reasonix/internal/provider"
	"reasonix/internal/store"
)

const (
	cleanupPendingExt             = ".cleanup-pending.json"
	maxRecoveryParentStemBytes    = 80
	sessionLockSidecarSuffix      = ".jsonl.lock"
	sessionLeaseLockSidecarSuffix = ".jsonl.lease.lock"
	sessionLeaseInfoSidecarSuffix = ".jsonl.lease.json"
	guardianSidecarSuffix         = ".guardian.jsonl"
	// nameMaxBytes is the single-component filename limit shared by the
	// filesystems Reasonix targets (APFS, ext4, NTFS all cap at 255).
	nameMaxBytes = 255
	// maxSessionBasenameBytes bounds transcript basenames that reconciliation
	// leaves in place. Sidecars append up to ~16 bytes to the transcript name
	// or its stem (".lease.lock", ".cleanup-pending.json", ".guardian.jsonl"),
	// so 224 keeps every sidecar comfortably under nameMaxBytes with headroom
	// for future suffixes. Names past this bound come from the pre-bounded
	// recovery cascade and get renamed by reconcileOverlongSessionFilenames.
	maxSessionBasenameBytes = 224
)

var (
	sessionSaveLocks sync.Map
	// sessionFileLockWait bounds cross-process save-lock acquisition. Session
	// leases normally prevent competing writers, but CLI/legacy writers and a
	// stalled process can still hold the compatibility .lock file. Navigation
	// and desktop shutdown snapshot synchronously; waiting forever here wedges
	// the UI and keeps the session lease (and WebView) alive indefinitely.
	// Package vars let focused tests shorten the wait without slowing the suite.
	sessionFileLockWait         = 5 * time.Second
	sessionFileLockPollInterval = 25 * time.Millisecond
	sessionMetaLockWait         = 5 * time.Second
	ErrSessionSnapshotConflict  = errors.New("session snapshot conflicts with newer transcript")
	// ErrSessionExternallyRemoved means a live Session still has a verified
	// baseline for path, but every authoritative transcript artifact disappeared.
	// Treating it as a first save would silently recreate a file the user or an
	// external cleanup tool deliberately removed.
	ErrSessionExternallyRemoved = errors.New("session was removed while still open")
	ErrSessionRecoveryNotNeeded = errors.New("session recovery not needed")
	// ErrSessionFileLockHeld reports that another process kept the
	// compatibility save lock for the full bounded acquisition window. Callers
	// that are about to terminate can use this sentinel to persist a recovery
	// branch without waiting on the same stalled file again.
	ErrSessionFileLockHeld = errors.New("session file lock held")
	// ErrSessionRecoveryDepthExceeded is retained for older callers. New
	// recovery writes update one stable branch and no longer return it.
	ErrSessionRecoveryDepthExceeded = errors.New("session recovery chain depth exceeded")
	sessionWriterID                 = newSessionWriterID()
)

// SessionRecoveryMaxDepth is the historical nested-fork cap. New writes stamp
// RecoveryDepth=1 and update one stable path instead of deepening a chain.
const SessionRecoveryMaxDepth = 3

type sessionPersistState struct {
	path     string
	digest   [sha256.Size]byte
	version  uint64
	revision int64
	// revisionKnown marks revision as a real ledger value. It is false when
	// the baseline was established while the meta sidecar was unreadable
	// (torn or corrupt): the session must still open, but revision 0 must not
	// pose as a baseline or every honest on-disk revision would read as a
	// stale-runtime conflict. CAS checks fall back to digest+version until a
	// successful save re-learns the revision.
	revisionKnown bool
	// saveVerified marks a baseline established by a completed save in this
	// process, whose write path verified transcript and ledger agree. A
	// baseline adopted at load time pairs the disk transcript with whatever
	// the meta sidecar said — which can lag the transcript after an
	// interrupted save — so only save-verified baselines may arm the
	// snapshot no-op fast path; the first save after a load must run in full
	// and heal a stale ledger.
	saveVerified bool
	ok           bool
}

type sessionSaveMode int

const (
	sessionSaveSnapshot sessionSaveMode = iota
	sessionSaveRewrite
	sessionSaveRewriteCompact
)

type snapshotWriteDecision struct {
	revision         int64
	reservedRevision int64
	upToDate         bool
	appendFrom       int
	appendOnly       bool
	// repairLog is set when the on-disk event log was damaged (torn tail with
	// a lost suffix, or nothing decodable): the safe write shape is a full
	// rewrite that also compacts the log back to a healthy single event.
	repairLog bool
	// ledgerStale is set when the on-disk transcript already matches the
	// snapshot but the meta ledger still describes older content — the
	// aftermath of a save whose bytes landed and whose revision record then
	// failed. The up-to-date path must heal the ledger instead of skipping it.
	ledgerStale bool
}

type SessionSnapshotConflictKind string

const (
	SessionSnapshotConflictStalePrefix SessionSnapshotConflictKind = "stale_prefix"
	SessionSnapshotConflictDiverged    SessionSnapshotConflictKind = "diverged"
)

type SessionSnapshotConflictError struct {
	Path             string
	Kind             SessionSnapshotConflictKind
	ExistingMessages int
	SnapshotMessages int
	BaseRevision     int64
	DiskRevision     int64
}

func (e *SessionSnapshotConflictError) Error() string {
	if e == nil {
		return ErrSessionSnapshotConflict.Error()
	}
	switch e.Kind {
	case SessionSnapshotConflictStalePrefix:
		return fmt.Sprintf("%s: %s has %d messages at revision %d; stale snapshot has %d messages from revision %d",
			ErrSessionSnapshotConflict, e.Path, e.ExistingMessages, e.DiskRevision, e.SnapshotMessages, e.BaseRevision)
	default:
		return fmt.Sprintf("%s: %s diverged on disk (%d messages, revision %d) from snapshot (%d messages, revision %d)",
			ErrSessionSnapshotConflict, e.Path, e.ExistingMessages, e.DiskRevision, e.SnapshotMessages, e.BaseRevision)
	}
}

func (e *SessionSnapshotConflictError) Unwrap() error {
	return ErrSessionSnapshotConflict
}

func SnapshotConflictKind(err error) (SessionSnapshotConflictKind, bool) {
	var conflict *SessionSnapshotConflictError
	if errors.As(err, &conflict) && conflict != nil {
		return conflict.Kind, true
	}
	return "", false
}

const RecoveryBranchDefaultName = "Recovered unsaved changes from stale runtime"

type RecoveryBranchOptions struct {
	OriginalPath string
	Name         string
	Reason       string
	BranchMeta   BranchMeta
}

type RecoveryBranchInfo struct {
	Path     string
	Digest   string
	Existing bool
	Meta     BranchMeta
	Preview  string
	Turns    int
}

// Save persists the session using the normal CAS-protected snapshot protocol.
// It is kept as the convenient default for callers that do not need to spell
// out rewrite intent; it must never provide a force-overwrite escape hatch.
// The .jsonl file remains as a compatibility checkpoint and discovery anchor;
// the append-only event log is authoritative once present, while the .jsonl
// checkpoint also serves as the random-read model for history paging.
func (s *Session) Save(path string) error {
	return s.saveObserved(path, sessionSaveSnapshot)
}

// SaveSnapshot writes a normal autosave/snapshot only when doing so cannot hide
// a newer transcript already on disk. Explicit history rewrites such as rewind,
// compaction, and cancel recovery should call SaveRewrite instead.
func (s *Session) SaveSnapshot(path string) error {
	return s.saveObserved(path, sessionSaveSnapshot)
}

// SaveRewrite writes an intentional non-append history rewrite only while this
// Session still owns the current on-disk transcript baseline. It prevents a
// stale controller from force-rewinding a newer transcript written elsewhere.
func (s *Session) SaveRewrite(path string) error {
	return s.saveObserved(path, sessionSaveRewrite)
}

// SaveRewriteCompact performs a CAS-protected rewrite and folds the event log
// to one replace record. It is for destructive maintenance such as redaction:
// retaining old WAL records would keep the removed bytes recoverable on disk.
func (s *Session) SaveRewriteCompact(path string) error {
	return s.saveObserved(path, sessionSaveRewriteCompact)
}

func (s *Session) save(path string, mode sessionSaveMode) error {
	if path == "" {
		return fmt.Errorf("empty session path")
	}
	return s.withSessionSaveLocks(path, func() error {
		return s.saveLocked(path, mode)
	})
}

func (s *Session) withSessionSaveLocks(path string, fn func() error) error {
	if strings.TrimSpace(path) == "" {
		return fmt.Errorf("empty session path")
	}
	unlock := lockSessionSavePath(path)
	defer unlock()
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return fmt.Errorf("create session dir: %w", err)
	}
	unlockFile, err := lockSessionFile(path)
	if err != nil {
		return fmt.Errorf("lock session file: %w", err)
	}
	defer unlockFile()
	return fn()
}

func sessionArtifactExists(path string) bool {
	if _, err := os.Lstat(path); err == nil {
		return true
	}
	for _, artifact := range store.SessionSidecarFiles(path) {
		// A legacy v0 event transcript can share the native
		// `<id>.events.jsonl` name with the destination being imported. It is
		// intentionally left in place, and must not make SaveIfAbsent believe
		// that the v1 `<id>.jsonl` destination already exists. Native event logs
		// remain owned artifacts and still protect the destination from a second
		// writer.
		if artifact == store.SessionEventLog(path) {
			probe, probeErr := probeSessionEventLog(path)
			if probeErr != nil {
				return true
			}
			if !probe.native {
				continue
			}
		}
		if _, err := os.Lstat(artifact); err == nil {
			return true
		}
	}
	return false
}

func (s *Session) saveLocked(path string, mode sessionSaveMode) error {
	baseRevision := int64(0)
	releaseAuth, err := s.requireWriteAuthorityForSave(path)
	if err != nil {
		return err
	}
	defer releaseAuth()
	observeUnleasedSessionWrite(path, mode)
	// Heal an empty/missing checkpoint from a valid WAL before classification
	// so a 0-byte .jsonl never forces a false diverged recovery.
	if err := healEmptyCheckpointFromWAL(path); err != nil {
		return err
	}
	if mode == sessionSaveSnapshot && s.snapshotUpToDate(path) {
		// Nothing changed since the last successful save to this exact path:
		// skip the rest of the save — including the full transcript serialize
		// + digest + disk probe the up-to-date decision below would still
		// pay. Desktop switch/close/prune paths snapshot defensively on every
		// navigation, and on large sessions that per-save cost is the
		// user-visible seconds of UI freeze in #6607. Version bookkeeping
		// makes this exact: any Add/Replace/preview update bumps version, any
		// rewrite bumps rewriteVersion, and load-time repairs or log damage
		// disarm the fast path until a real save persists them. The check
		// runs under the save locks, not before them, so a saver that waited
		// on a concurrent writer still re-evaluates against the state it must
		// persist when it finally enters the critical section.
		return nil
	}
	// Capture the snapshot only while holding the save locks. Concurrent
	// in-process savers (turn-end snapshot, periodic autosave, shutdown
	// snapshot) that captured before locking could land out of order: the
	// stalest capture written last would then read the newer transcript it
	// lost the race to as a bogus stale-prefix conflict.
	msgs, version, rewriteVersion := s.snapshotWithVersion()
	digest, contentBytes, err := digestAndSizeSessionMessages(msgs)
	if err != nil {
		return err
	}
	probe, err := probeSessionEventLog(path)
	if err != nil {
		return err
	}
	if probe.futureSchema {
		return fmt.Errorf("session event log for %s uses schema %d; this build supports up to %d", path, probe.schemaVersion, sessionEventSchemaVersion)
	}
	if probe.native && probe.size > 0 {
		// Drop any torn tail a crashed or disk-full append left behind before
		// it can be buried under new records where replay would stop forever.
		if err := repairSessionEventLogTail(path); err != nil {
			return fmt.Errorf("repair session event log: %w", err)
		}
	}
	repairLog := false
	ownedRewrite := mode == sessionSaveRewrite || mode == sessionSaveRewriteCompact
	decision, err := s.classifySnapshotWriteForCommit(path, msgs, digest, version, ownedRewrite, mode)
	if err != nil {
		return err
	}
	if decision.upToDate && mode != sessionSaveRewriteCompact {
		// Disk already holds exactly this transcript. Rewriting it would only
		// bump the revision, invalidating the persistence baseline of every
		// other runtime resumed on this file and turning their next
		// legitimate save into a stale-runtime conflict. Skip the write and
		// adopt the current on-disk revision as this session's baseline.
		if decision.ledgerStale {
			// ...unless the ledger never learned about this transcript: a
			// prior save landed its bytes and then failed to record the
			// revision. Same-content retries are exactly the "later save"
			// that failure deferred to, and skipping here would strand the
			// ledger on the old digest forever. Record now, reproducing
			// the state the interrupted save would have left.
			revision, err := recordSessionContentRevision(path, digest, decision.revision, decision.reservedRevision)
			if err != nil {
				return err
			}
			displayModelCurrent := true
			if err := writeSessionMessages(path, msgs); err != nil {
				displayModelCurrent = false
				slog.Warn("session: keeping save after display read-model repair failure", "path", path, "err", err)
			}
			if probe.native {
				if err := writeSessionEventIndex(path, msgs, digest, revision); err != nil {
					// See the append path below: index loss must not fail a
					// save whose transcript and revision already landed.
					slog.Warn("session: keeping save after event index write failure", "path", path, "err", err)
				}
			}
			if displayModelCurrent {
				if err := refreshSessionDisplayIndex(path, msgs, digest, revision, -1); err != nil {
					// The display index is a derived sidecar; transcript durability
					// must not depend on rebuilding it successfully.
					slog.Warn("session: keeping save after display index write failure", "path", path, "err", err)
				}
			}
			s.markPersistedWithListing(path, digest, version, revision, rewriteVersion, msgs)
			return nil
		}
		s.markPersistedWithListing(path, digest, version, decision.revision, rewriteVersion, msgs)
		return nil
	}
	if decision.appendOnly && probe.native && mode != sessionSaveRewriteCompact {
		logSize := sessionEventLogSize(path)
		displayModelCurrent := false
		switch {
		case logSize == 0:
			if err := appendSessionReplaceEvent(path, msgs, digest, decision.revision, "snapshot"); err != nil {
				return err
			}
			displayModelCurrent, err = appendSessionDisplayReadModel(path, msgs, decision.appendFrom, decision.revision)
		case sessionEventLogOversized(logSize, contentBytes):
			// Fold history into one replace event and refresh the random-read
			// model atomically. Normal appends keep it current below too.
			if err := compactSessionEventLog(path, msgs, digest, decision.revision, "compact"); err != nil {
				return err
			}
			if err := writeSessionMessages(path, msgs); err != nil {
				return err
			}
			displayModelCurrent = true
		default:
			if err := appendSessionAppendEvent(path, decision.appendFrom, msgs[decision.appendFrom:], digest, decision.revision); err != nil {
				return err
			}
			displayModelCurrent, err = appendSessionDisplayReadModel(path, msgs, decision.appendFrom, decision.revision)
		}
		if err != nil {
			slog.Warn("session: keeping save after display read-model append failure", "path", path, "err", err)
		}
		revision, err := recordSessionContentRevision(path, digest, decision.revision, decision.reservedRevision)
		if err != nil {
			return err
		}
		if err := writeSessionEventIndex(path, msgs, digest, revision); err != nil {
			// The event index is only a listing accelerator; the transcript
			// and its revision are already durable above. Failing the save
			// here would skip markPersisted and leave the in-memory baseline
			// behind the disk state it just wrote, misreading the next save
			// as a stale-runtime conflict.
			slog.Warn("session: keeping save after event index write failure", "path", path, "err", err)
		}
		if displayModelCurrent {
			if err := refreshSessionDisplayIndex(path, msgs, digest, revision, decision.appendFrom); err != nil {
				// The append boundary lets the refresh extend the previous index
				// instead of re-encoding the whole transcript.
				slog.Warn("session: keeping save after display index write failure", "path", path, "err", err)
			}
		}
		s.markPersistedWithListing(path, digest, version, revision, rewriteVersion, msgs)
		return nil
	}
	baseRevision = decision.revision
	repairLog = decision.repairLog
	// Full-rewrite path: new snapshots, intentional history rewrites, and
	// damage repairs. The event log mutates first so a crash between the two
	// writes leaves the newer transcript authoritative; the anchor rewrite
	// keeps the compatibility .jsonl fresh for direct readers.
	reason := "save"
	switch mode {
	case sessionSaveSnapshot:
		reason = "snapshot"
	case sessionSaveRewrite:
		reason = "rewrite"
	case sessionSaveRewriteCompact:
		reason = "rewrite-compact"
	}
	if repairLog {
		reason = "repair"
	}
	logSize := sessionEventLogSize(path)
	switch {
	case !probe.native:
		// A foreign file (legacy import leftover) squats the native log path.
		// Never write into or over it — the session stays checkpoint-only.
	case mode == sessionSaveRewriteCompact:
		// Maintenance rewrites compact even a short log so redacted or otherwise
		// removed bytes do not remain recoverable in historical WAL records.
		if err := compactSessionEventLog(path, msgs, digest, baseRevision, reason); err != nil {
			return err
		}
	case repairLog, sessionEventLogOversized(logSize, contentBytes):
		if err := compactSessionEventLog(path, msgs, digest, baseRevision, reason); err != nil {
			return err
		}
	default:
		if err := appendSessionReplaceEvent(path, msgs, digest, baseRevision, reason); err != nil {
			return err
		}
	}
	if err := writeSessionMessages(path, msgs); err != nil {
		return err
	}
	revision, err := recordSessionContentRevision(path, digest, baseRevision, decision.reservedRevision)
	if err != nil {
		return err
	}
	if probe.native {
		if err := writeSessionEventIndex(path, msgs, digest, revision); err != nil {
			// See the append path above: index loss must not fail a save whose
			// transcript and revision already landed.
			slog.Warn("session: keeping save after event index write failure", "path", path, "err", err)
		}
	}
	if err := refreshSessionDisplayIndex(path, msgs, digest, revision, -1); err != nil {
		// Warn-only like the event index above: the display index is a pure
		// derived sidecar and must never fail a save.
		slog.Warn("session: keeping save after display index write failure", "path", path, "err", err)
	}
	s.markPersistedWithListing(path, digest, version, revision, rewriteVersion, msgs)
	return nil
}

func writeSessionMessages(path string, msgs []provider.Message) error {
	// Write to a sibling tmp file then rename, so a crash mid-write can't
	// leave a partial JSONL that won't reload. The fsync guards the anchor
	// against power loss — it is the fallback when the event log is damaged.
	fileutil.Crash("session-checkpoint", path)
	tmp, err := os.CreateTemp(filepath.Dir(path), ".session.*.tmp")
	if err != nil {
		return fmt.Errorf("create session tmp: %w", err)
	}
	tmpPath := tmp.Name()
	enc := json.NewEncoder(tmp)
	for _, m := range msgs {
		if err := enc.Encode(m); err != nil {
			tmp.Close()
			os.Remove(tmpPath)
			return fmt.Errorf("encode message: %w", err)
		}
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		os.Remove(tmpPath)
		return err
	}
	if err := tmp.Close(); err != nil {
		os.Remove(tmpPath)
		return err
	}
	if err := fileutil.ReplaceFile(tmpPath, path); err != nil {
		os.Remove(tmpPath)
		return err
	}
	return nil
}

// checkSnapshotWrite decides whether this session may write msgs over path, and
// whether the safe write shape is a no-op, append-only suffix, or full rewrite.
func (s *Session) checkSnapshotWrite(path string, next []provider.Message, nextDigest [sha256.Size]byte, nextVersion uint64, allowOwnedRewrite bool) (snapshotWriteDecision, error) {
	baseState := s.persistState(path)
	current, err := loadSessionUnlocked(path)
	if err != nil {
		if os.IsNotExist(err) {
			if baseState.ok {
				return snapshotWriteDecision{}, ErrSessionExternallyRemoved
			}
			return snapshotWriteDecision{}, nil
		}
		return snapshotWriteDecision{}, err
	}
	currentRevision, currentLedgerDigest, err := sessionContentRevision(path)
	if err != nil {
		return snapshotWriteDecision{}, err
	}
	existing := current.Snapshot()
	existingDigest, err := digestSessionMessages(existing)
	if err != nil {
		return snapshotWriteDecision{}, err
	}
	// raw is the transcript as stored, before load-time normalization repaired
	// it; it equals existing when no repair ran. The prefix checks below must
	// be able to fall back to it: a mid-turn snapshot legitimately cuts an
	// assistant tool call from its still-running result, normalization then
	// fabricates a placeholder answer on load, and the live session's real
	// result collides with that placeholder — misreading a pure append as
	// divergence (and forking a bogus recovery branch).
	raw, rawDigest := existing, existingDigest
	rawDiffers := current.normalizedDirty && len(current.rawMessages) > 0
	if rawDiffers {
		raw = current.rawMessages
		if rawDigest, err = digestSessionMessages(raw); err != nil {
			return snapshotWriteDecision{}, err
		}
	}
	contentUnchanged := bytes.Equal(existingDigest[:], nextDigest[:])
	exactAppend := messagesHavePrefix(next, existing)
	appendShaped := contentUnchanged || exactAppend || messagesHavePrefixWithCompatibleSystem(next, existing)
	repairPending := current.normalizedDirty
	if !appendShaped && rawDiffers {
		rawUnchanged := bytes.Equal(rawDigest[:], nextDigest[:])
		rawAppend := messagesHavePrefix(next, raw)
		if rawUnchanged || rawAppend || messagesHavePrefixWithCompatibleSystem(next, raw) {
			existing = raw
			contentUnchanged = rawUnchanged
			exactAppend = rawAppend
			appendShaped = true
			// The snapshot supersedes the repaired view — appending it lands
			// the real tool results where the placeholders were fabricated —
			// so no load-time repair is left to force a rewrite.
			repairPending = false
		}
	}
	if !appendShaped && baseState.ok && baseState.revisionKnown &&
		baseState.revision == currentRevision && !contentUnchanged {
		// Revision equality alone is not ownership proof. Require digest
		// ancestry or a live generation-bound write authority covering path.
		if s.ownsWritableBaseline(path, existingDigest, rawDigest, rawDiffers, currentRevision, currentLedgerDigest, nextVersion) {
			appendShaped = true
		}
	}
	if appendShaped {
		// An unknown-revision baseline (meta sidecar unreadable at load) cannot
		// vouch for revision equality; the digest/prefix checks above already
		// vouch for the content, so only a known baseline arms the CAS check.
		// Under an append-shaped write (at most a compatible leading-system
		// swap) a stale revision is ledger drift — a reset sidecar, a
		// same-content heal, or another runtime recording messages this
		// snapshot already contains — unless the transcript was rewound.
		// Locating the persisted baseline among the snapshot's prefixes and
		// requiring the disk transcript to still reach it tells the two apart:
		// drift keeps the baseline reachable, while a rewind cut below it and
		// appending would resurrect the suffix another runtime removed.
		if baseState.ok && baseState.revisionKnown && currentRevision != baseState.revision && !contentUnchanged &&
			!appendCoversPersistedBaseline(next, existing, baseState.digest) {
			return snapshotWriteDecision{}, snapshotConflict(path, existing, next, baseState.revision, currentRevision)
		}
		// A normalized-dirty load means LoadSession repaired the history on the
		// way in: the digests match but the raw bytes on disk do not, so the
		// repair still needs a real write to persist. A damaged event log
		// likewise needs a real write (rewrite + compact) even when the
		// replayable prefix already matches this snapshot.
		decision := snapshotWriteDecision{
			revision:  currentRevision,
			upToDate:  contentUnchanged && !repairPending && !current.eventLogDamaged,
			repairLog: current.eventLogDamaged,
		}
		// A ledger digest that describes different content than the transcript
		// on disk is the aftermath of a save whose bytes landed and whose
		// revision record then failed (crash or fail-closed record between the
		// two writes). Only a non-empty mismatch counts: a missing sidecar or
		// a legacy one without a digest is a legitimate state, and stamping it
		// here would bump revisions other runtimes still hold as baselines.
		if decision.upToDate && currentLedgerDigest != "" && currentLedgerDigest != digestString(nextDigest) {
			decision.ledgerStale = true
		}
		// An append is only chain-safe when existing measures the transcript
		// the event log actually replays. Under a pending load-time repair the
		// normalized view differs from the raw log, so an append event indexed
		// against it breaks the replay chain and orphans the appended suffix;
		// fall through to the full rewrite, which also persists the repair.
		if exactAppend && !contentUnchanged && len(existing) < len(next) && !current.eventLogDamaged && !repairPending {
			decision.appendOnly = true
			decision.appendFrom = len(existing)
		}
		return decision, nil
	}
	if allowOwnedRewrite {
		if s.ownsWritableBaseline(path, existingDigest, rawDigest, rawDiffers, currentRevision, currentLedgerDigest, nextVersion) {
			return snapshotWriteDecision{revision: currentRevision, repairLog: current.eventLogDamaged}, nil
		}
	}
	// Bound controllers: missing/stale authority must not fork recovery.
	if err := s.authorityErrorForPath(path); err != nil {
		return snapshotWriteDecision{}, err
	}
	if messagesHavePrefix(existing, next) || messagesHavePrefixWithCompatibleSystem(existing, next) ||
		(rawDiffers && (messagesHavePrefix(raw, next) || messagesHavePrefixWithCompatibleSystem(raw, next))) {
		return snapshotWriteDecision{}, &SessionSnapshotConflictError{
			Path:             path,
			Kind:             SessionSnapshotConflictStalePrefix,
			ExistingMessages: len(existing),
			SnapshotMessages: len(next),
			BaseRevision:     baseState.revision,
			DiskRevision:     currentRevision,
		}
	}
	return snapshotWriteDecision{}, &SessionSnapshotConflictError{
		Path:             path,
		Kind:             SessionSnapshotConflictDiverged,
		ExistingMessages: len(existing),
		SnapshotMessages: len(next),
		BaseRevision:     baseState.revision,
		DiskRevision:     currentRevision,
	}
}

func snapshotConflict(path string, existing, next []provider.Message, baseRevision, diskRevision int64) error {
	kind := SessionSnapshotConflictDiverged
	if messagesHavePrefix(existing, next) || messagesHavePrefixWithCompatibleSystem(existing, next) {
		kind = SessionSnapshotConflictStalePrefix
	}
	return &SessionSnapshotConflictError{
		Path:             path,
		Kind:             kind,
		ExistingMessages: len(existing),
		SnapshotMessages: len(next),
		BaseRevision:     baseRevision,
		DiskRevision:     diskRevision,
	}
}

func (s *Session) SaveRecoveryBranch(opts RecoveryBranchOptions) (RecoveryBranchInfo, error) {
	return s.saveRecoveryBranch(opts, false)
}

// SaveShutdownRecoveryBranch persists the current transcript to a distinct
// recovery branch after the normal shutdown snapshot failed with
// ErrSessionFileLockHeld. It deliberately does not re-lock or inspect the
// original session file: doing so would repeat the same bounded timeout and
// let process teardown discard the only remaining in-memory copy.
//
// The recovery filename includes this live Session's isolated lane, so another
// controller in the same process cannot replace the emergency copy.
// The result still uses the normal session, event-log, and branch-meta formats
// and is therefore discoverable and resumable through existing flows.
func (s *Session) SaveShutdownRecoveryBranch(opts RecoveryBranchOptions) (RecoveryBranchInfo, error) {
	return s.saveRecoveryBranch(opts, true)
}

// SaveConflictRecoveryBranch writes the depth-cap isolated copy (one path per
// live Session). Subsequent conflicts from that Session rewrite it in place.
func (s *Session) SaveConflictRecoveryBranch(opts RecoveryBranchOptions) (RecoveryBranchInfo, error) {
	return s.saveRecoveryBranch(opts, true)
}

func (s *Session) saveRecoveryBranch(opts RecoveryBranchOptions, shutdown bool) (RecoveryBranchInfo, error) {
	originalPath := strings.TrimSpace(opts.OriginalPath)
	if originalPath == "" {
		return RecoveryBranchInfo{}, fmt.Errorf("empty original session path")
	}
	opts.OriginalPath = originalPath
	msgs, version, rewriteVersion := s.snapshotWithVersion()
	preview, turns := SessionPreviewFromMessages(msgs)
	if turns == 0 {
		return RecoveryBranchInfo{}, ErrSessionRecoveryNotNeeded
	}
	digest, err := digestSessionMessages(msgs)
	if err != nil {
		return RecoveryBranchInfo{}, err
	}
	digestText := digestString(digest)

	if !shutdown {
		unlockOriginal := lockSessionSavePath(originalPath)
		unlockOriginalFile, lockErr := lockSessionFile(originalPath)
		if lockErr != nil {
			unlockOriginal()
			return RecoveryBranchInfo{}, fmt.Errorf("lock original session file: %w", lockErr)
		}
		current, loadErr := loadSessionUnlocked(originalPath)
		unlockOriginalFile()
		unlockOriginal()
		if loadErr != nil && !os.IsNotExist(loadErr) {
			return RecoveryBranchInfo{}, loadErr
		}
		if loadErr == nil && current != nil {
			existing := current.Snapshot()
			existingDigest, digestErr := digestSessionMessages(existing)
			if digestErr != nil {
				return RecoveryBranchInfo{}, digestErr
			}
			covered := bytes.Equal(existingDigest[:], digest[:]) ||
				messagesHavePrefix(existing, msgs) ||
				messagesHavePrefixWithCompatibleSystem(existing, msgs)
			if !covered && current.normalizedDirty && len(current.rawMessages) > 0 {
				// Judge coverage against the pre-repair transcript too, for the
				// same reason as checkSnapshotWrite: load-time normalization can
				// reshape what is actually stored, and a recovery fork is only
				// warranted when the stored bytes themselves fail to cover this
				// snapshot.
				raw := current.rawMessages
				rawDigest, rawErr := digestSessionMessages(raw)
				if rawErr != nil {
					return RecoveryBranchInfo{}, rawErr
				}
				covered = bytes.Equal(rawDigest[:], digest[:]) ||
					messagesHavePrefix(raw, msgs) ||
					messagesHavePrefixWithCompatibleSystem(raw, msgs)
			}
			if covered {
				return RecoveryBranchInfo{}, ErrSessionRecoveryNotNeeded
			}
		}
	}

	// One stable recovery file per (root branch, writer generation). Nested
	// -recovery- names peel back to the root so conflicts update in place.
	for range 8 {
		recoveryPath, lane := s.isolatedRecoverySessionPath(originalPath)
		info, collision, err := s.writeRecoveryBranchAtPath(recoveryPath, opts, msgs, digest,
			version, rewriteVersion, preview, turns, digestText, 1, shutdown)
		if err != nil {
			return RecoveryBranchInfo{}, err
		}
		if !collision {
			return info, nil
		}
		s.rotateRecoveryLane(lane)
	}
	return RecoveryBranchInfo{}, fmt.Errorf("allocate isolated recovery lane: too many existing collisions")
}

func (s *Session) saveRecoveryBranchMeta(path string, opts RecoveryBranchOptions, preview string, turns int, digest string, depth int) (BranchMeta, error) {
	meta := opts.BranchMeta
	meta.ID = BranchID(path)
	if strings.TrimSpace(meta.Name) == "" {
		meta.Name = firstNonEmpty(strings.TrimSpace(opts.Name), RecoveryBranchDefaultName)
	}
	if strings.TrimSpace(meta.ParentID) == "" {
		meta.ParentID = recoveryRootID(opts.OriginalPath)
	}
	meta.ForkTurn = -1
	meta.ForkMessageIndex = len(s.Snapshot())
	meta.Preview = preview
	meta.Turns = turns
	meta.SchemaVersion = BranchMetaCountsVersion
	meta.Recovered = true
	meta.RecoveryReason = firstNonEmpty(strings.TrimSpace(opts.Reason), "session snapshot conflict")
	meta.RecoveryDigest = digest
	// Always stamped from the parent chain, never trusted from opts: callers
	// copy tab/session meta wholesale and would carry a stale depth.
	meta.RecoveryDepth = depth
	if meta.Revision == 0 {
		meta.Revision = 1
	}
	if strings.TrimSpace(meta.ContentDigest) == "" {
		meta.ContentDigest = digest
	}
	stampSessionListingProjection(&meta)
	if strings.TrimSpace(meta.WriterID) == "" {
		meta.WriterID = SessionWriterID()
	}
	// Keep any in-flight turn marker across isolated in-place rewrites.
	if err := saveBranchMetaKeepInFlightTurn(path, meta); err != nil {
		return BranchMeta{}, err
	}
	if stored, ok, err := LoadBranchMeta(path); err != nil {
		return BranchMeta{}, err
	} else if ok {
		return stored, nil
	}
	return meta, nil
}

func recoveryParentStem(parent string) string {
	parent = strings.TrimSpace(parent)
	if parent == "" {
		return "session"
	}
	sum := sha256.Sum256([]byte(parent))
	if before, _, ok := strings.Cut(parent, "-recovery-"); ok {
		base := strings.Trim(before, "-_. ")
		if base == "" {
			base = "session"
		}
		base = strings.Trim(truncateUTF8Bytes(base, maxRecoveryParentStemBytes), "-_. ")
		if base == "" {
			base = "session"
		}
		return fmt.Sprintf("%s-%x", base, sum[:6])
	}
	if len(parent) <= maxRecoveryParentStemBytes {
		return parent
	}
	prefix := strings.Trim(truncateUTF8Bytes(parent, maxRecoveryParentStemBytes), "-_. ")
	if prefix == "" {
		prefix = "session"
	}
	return fmt.Sprintf("%s-%x", prefix, sum[:6])
}

func truncateUTF8Bytes(s string, max int) string {
	if max <= 0 {
		return ""
	}
	if len(s) <= max {
		return s
	}
	used := 0
	for i, r := range s {
		size := utf8.RuneLen(r)
		if size < 0 {
			size = 1
		}
		if used+size > max {
			return s[:i]
		}
		used += size
	}
	return s
}

func firstNonEmpty(values ...string) string {
	for _, value := range values {
		if strings.TrimSpace(value) != "" {
			return value
		}
	}
	return ""
}

func (s *Session) ownsPersistedState(path string, existingDigest [sha256.Size]byte, existingRevision int64, existingLedgerDigest string, nextVersion uint64) bool {
	state := s.persistState(path)
	if !state.ok || state.version > nextVersion || !bytes.Equal(existingDigest[:], state.digest[:]) {
		return false
	}
	// An unknown-revision baseline still owns the transcript it loaded — the
	// digest+version match proves it. Requiring revision equality here would
	// make every rewrite from such a baseline a permanent conflict, because
	// the revision can only be re-learned by a successful save.
	// A disk ledger with no recorded revision is the mirror case: recorded
	// revisions start at 1, so revision 0 means the sidecar was deleted or
	// rebuilt by a listing-only writer after this session's save. An absent
	// claim cannot revoke the ownership the digest+version match proves.
	if !state.revisionKnown || existingRevision == 0 || state.revision == existingRevision {
		return true
	}
	// A foreign revision stamp whose recorded digest still describes these
	// exact bytes (a same-content heal or no-op record by another runtime)
	// vouches for no content of its own: the transcript is byte-for-byte what
	// this session last persisted, so rewriting it destroys nothing of
	// theirs — at worst the conflict moves to the stamper's next divergent
	// save, where its in-memory history forks a recovery branch as usual.
	// A stamp that disagrees with the on-disk transcript (or a legacy stamp
	// with no digest) keeps revoking ownership: that is the aftermath of a
	// save whose bytes and record split, the bytes cannot be attributed, and
	// only the conservative conflict path preserves both sides.
	return existingLedgerDigest == digestString(existingDigest)
}

// snapshotUpToDate reports whether a snapshot save to path is a provable
// no-op from in-memory bookkeeping alone: the last successful save went to
// this same path with a known ledger revision, the transcript version and
// rewrite version have not moved since, and no load-time repair or event-log
// damage is waiting to be persisted. Every one of these flags fails open —
// when any is unset or stale the caller falls through to the full save path,
// which re-derives the truth from disk.
func (s *Session) snapshotUpToDate(path string) bool {
	key := canonicalSessionSavePath(path)
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.persisted.ok &&
		s.persisted.saveVerified &&
		s.persisted.path == key &&
		s.persisted.version == s.version &&
		s.persisted.revisionKnown &&
		s.rewriteVersion == s.persistedRewriteVersion &&
		!s.normalizedDirty &&
		!s.eventLogDamaged
}

func (s *Session) persistState(path string) sessionPersistState {
	key := canonicalSessionSavePath(path)
	s.mu.RLock()
	defer s.mu.RUnlock()
	if s.persisted.ok && s.persisted.path == key {
		return s.persisted
	}
	return sessionPersistState{}
}

// PersistedState is a read-only view of the baseline the session last
// persisted to (or loaded from) its transcript path. History paging uses it to
// validate a display-index sidecar against the live session without touching
// disk: an index built at the same revision with the same content digest
// describes exactly the persisted prefix of the in-memory log.
type PersistedState struct {
	// Digest is the content digest of the persisted transcript.
	Digest [sha256.Size]byte
	// DigestHex is Digest in the hex form sidecars store.
	DigestHex string
	// Revision is the CAS ledger revision of the persisted transcript. It is
	// meaningful only when RevisionKnown is true.
	Revision      int64
	RevisionKnown bool
	// RewriteEpoch is the highest rewriteVersion that has reached disk. It
	// changes only when a content rewrite (compaction, rewind, …) is saved, so
	// it doubles as a stable epoch token for history entry IDs: append-only
	// saves keep it, rewrites bump it.
	RewriteEpoch int
	// AppendOnlyTail reports that no rewrite landed after the baseline, so the
	// persisted transcript is still a prefix of the in-memory log (the tail,
	// if any, is unsaved appends).
	AppendOnlyTail bool
	// UnchangedSincePersisted reports that the in-memory log is exactly the
	// persisted transcript (no appends, no rewrites since the baseline).
	UnchangedSincePersisted bool
}

// PersistedState returns the session's persistence baseline for path, or
// false when the session has never persisted to (or loaded from) that path.
func (s *Session) PersistedState(path string) (PersistedState, bool) {
	key := canonicalSessionSavePath(path)
	s.mu.RLock()
	defer s.mu.RUnlock()
	if !s.persisted.ok || s.persisted.path != key {
		return PersistedState{}, false
	}
	return PersistedState{
		Digest:                  s.persisted.digest,
		DigestHex:               digestString(s.persisted.digest),
		Revision:                s.persisted.revision,
		RevisionKnown:           s.persisted.revisionKnown,
		RewriteEpoch:            s.persistedRewriteVersion,
		AppendOnlyTail:          s.rewriteVersion == s.persistedRewriteVersion,
		UnchangedSincePersisted: s.persisted.version == s.version,
	}, true
}

// SessionContentIdentity returns the ledger identity of the authoritative
// persisted transcript without loading its message bodies. It is intended for
// derived sidecars such as the desktop display index: a matching checkpoint
// size alone cannot prove that offsets still describe the event-log-backed
// transcript. The bool is false for legacy sessions that have no digest in
// their branch metadata; callers must then validate against the transcript
// bytes directly.
func SessionContentIdentity(path string) (PersistedState, bool, error) {
	revision, digestHex, err := sessionContentRevision(path)
	if err != nil {
		return PersistedState{}, false, err
	}
	if digestHex == "" {
		return PersistedState{}, false, nil
	}
	var digest [sha256.Size]byte
	decoded, err := hex.DecodeString(digestHex)
	if err != nil || len(decoded) != len(digest) {
		return PersistedState{}, false, fmt.Errorf("invalid session content digest")
	}
	copy(digest[:], decoded)
	return PersistedState{
		Digest:        digest,
		DigestHex:     digestString(digest),
		Revision:      revision,
		RevisionKnown: true,
	}, true, nil
}

// sessionContentRevision reads the CAS ledger (revision + content digest) from
// the branch-meta sidecar. A missing sidecar is revision 0 — a session that
// has never recorded one. An unreadable sidecar is an error: reporting it as
// revision 0 would desync every runtime baseline from the ledger and turn the
// next honest save into a bogus conflict (and a recovery branch).
func sessionContentRevision(path string) (int64, string, error) {
	meta, ok, err := loadBranchMetaRetry(path)
	if err != nil {
		return 0, "", err
	}
	if !ok {
		return 0, "", nil
	}
	return meta.Revision, strings.TrimSpace(meta.ContentDigest), nil
}

func recordSessionContentRevision(path string, digest [sha256.Size]byte, baseRevision, reservedRevision int64) (int64, error) {
	// Revision allocation is a read-modify-write transaction. Holding only the
	// final SaveBranchMeta lock would still let another writer replace the
	// sidecar between our read and write, so keep the ledger lock across the
	// increment and read-back as well.
	unlock, err := LockSessionMetaPath(path)
	if err != nil {
		return 0, err
	}
	defer unlock()

	meta, ok, err := loadBranchMetaRetry(path)
	if err != nil {
		// Fail the save instead of rebuilding the ledger from a bad read: the
		// transcript bytes already landed, so a later save can record the
		// revision once the sidecar reads cleanly again — a content-bearing
		// save lands here again, and a same-content retry heals through the
		// up-to-date ledgerStale path in save().
		return 0, err
	}
	if !ok {
		meta = BranchMeta{ID: BranchID(path)}
	}
	if reservedRevision > 0 {
		if reservedRevision != baseRevision+1 || meta.Revision != reservedRevision || meta.SchemaVersion != 0 {
			return 0, fmt.Errorf("session revision reservation changed: base=%d reserved=%d current=%d schema=%d", baseRevision, reservedRevision, meta.Revision, meta.SchemaVersion)
		}
	} else {
		if meta.Revision < baseRevision {
			meta.Revision = baseRevision
		}
		if meta.Revision == math.MaxInt64 {
			return 0, fmt.Errorf("session revision exhausted")
		}
		meta.Revision++
	}
	meta.ContentDigest = digestString(digest)
	meta.WriterID = SessionWriterID()
	if err := saveBranchMeta(path, meta, false); err != nil {
		return 0, err
	}
	stored, ok, err := loadBranchMetaRetry(path)
	if err != nil {
		return 0, err
	}
	if ok && stored.Revision > 0 {
		return stored.Revision, nil
	}
	return meta.Revision, nil
}

func digestString(digest [sha256.Size]byte) string {
	return fmt.Sprintf("%x", digest[:])
}

func SessionWriterID() string {
	return sessionWriterID
}

func newSessionWriterID() string {
	host, _ := os.Hostname()
	host = strings.TrimSpace(host)
	if host == "" {
		host = "unknown-host"
	}
	host = strings.Map(func(r rune) rune {
		switch {
		case r >= 'a' && r <= 'z', r >= 'A' && r <= 'Z', r >= '0' && r <= '9', r == '-', r == '_', r == '.':
			return r
		default:
			return '-'
		}
	}, host)
	var nonce [8]byte
	if _, err := rand.Read(nonce[:]); err != nil {
		return fmt.Sprintf("%s-%d-%d", host, os.Getpid(), time.Now().UnixNano())
	}
	return fmt.Sprintf("%s-%d-%x", host, os.Getpid(), nonce[:])
}

func digestSessionMessages(msgs []provider.Message) ([sha256.Size]byte, error) {
	digest, _, err := digestAndSizeSessionMessages(msgs)
	return digest, err
}

func messageForSessionIdentity(m provider.Message) provider.Message {
	// CreatedAt is local display metadata. Keep it out of transcript identity
	// so older builds that ignore the optional field can share the same event-
	// log revision and append without false conflicts.
	m.CreatedAt = 0
	return m
}

// digestAndSizeSessionMessages also reports the encoded transcript size, which
// the save path uses to bound the event log relative to the live content.
func digestAndSizeSessionMessages(msgs []provider.Message) ([sha256.Size]byte, int64, error) {
	h := sha256.New()
	size := int64(0)
	for _, m := range msgs {
		m = messageForSessionIdentity(m)
		b, err := json.Marshal(m)
		if err != nil {
			return [sha256.Size]byte{}, 0, err
		}
		if _, err := h.Write(b); err != nil {
			return [sha256.Size]byte{}, 0, err
		}
		if _, err := h.Write([]byte{'\n'}); err != nil {
			return [sha256.Size]byte{}, 0, err
		}
		size += int64(len(b)) + 1
	}
	var out [sha256.Size]byte
	copy(out[:], h.Sum(nil))
	return out, size, nil
}

// sessionTranscriptHasher accumulates the transcript digest one message at a
// time, exactly like digestAndSizeSessionMessages, so load paths hash during
// decode instead of re-serializing. A nil receiver disables hashing.
type sessionTranscriptHasher struct {
	h   hash.Hash
	err error
}

func newSessionTranscriptHasher() *sessionTranscriptHasher {
	return &sessionTranscriptHasher{h: sha256.New()}
}

// rehash restarts accumulation over msgs (event-log replace records).
func (s *sessionTranscriptHasher) rehash(msgs []provider.Message) {
	if s == nil {
		return
	}
	s.h.Reset()
	s.err = nil
	s.addAll(msgs)
}

func (s *sessionTranscriptHasher) addAll(msgs []provider.Message) {
	for _, m := range msgs {
		s.add(m)
	}
}

// add hashes one message and returns it for append chaining.
func (s *sessionTranscriptHasher) add(m provider.Message) provider.Message {
	if s == nil || s.err != nil {
		return m
	}
	b, err := json.Marshal(messageForSessionIdentity(m))
	if err != nil {
		s.err = err
		return m
	}
	_, _ = s.h.Write(b)
	_, _ = s.h.Write([]byte{'\n'})
	return m
}

// sum reports the accumulated digest; ok is false when hashing was disabled
// or a message failed to encode, and callers fall back to digestSessionMessages.
func (s *sessionTranscriptHasher) sum() (digest [sha256.Size]byte, ok bool) {
	if s == nil || s.err != nil {
		return digest, false
	}
	copy(digest[:], s.h.Sum(nil))
	return digest, true
}

func messagesHavePrefix(full, prefix []provider.Message) bool {
	if len(prefix) > len(full) {
		return false
	}
	for i := range prefix {
		if !messagesEqualForStorage(full[i], prefix[i]) {
			return false
		}
	}
	return true
}

// messagesPrefixDigestDepth returns the number of leading messages of msgs
// whose storage digest equals target, or -1 when no prefix matches. The
// digest accumulates exactly like digestAndSizeSessionMessages, so a match at
// depth k means msgs[:k] has the same transcript identity as target.
func messagesPrefixDigestDepth(msgs []provider.Message, target [sha256.Size]byte) int {
	h := sha256.New()
	sum := make([]byte, 0, sha256.Size)
	for i, m := range msgs {
		m = messageForSessionIdentity(m)
		b, err := json.Marshal(m)
		if err != nil {
			return -1
		}
		h.Write(b)
		h.Write([]byte{'\n'})
		sum = h.Sum(sum[:0])
		if bytes.Equal(sum, target[:]) {
			return i + 1
		}
	}
	return -1
}

// appendCoversPersistedBaseline reports whether an append-shaped write (disk
// transcript a prefix of next, modulo a compatible leading-system swap) still
// covers everything this session ever persisted: the baseline digest must be
// reachable as a prefix of the pending snapshot, and the disk transcript must
// still extend at least to that depth. A shorter disk transcript means some
// other runtime deliberately rewound below the baseline — appending over it
// would resurrect the removed suffix, so the caller must conflict instead.
func appendCoversPersistedBaseline(next, existing []provider.Message, baseDigest [sha256.Size]byte) bool {
	depth := messagesPrefixDigestDepth(next, baseDigest)
	if depth < 0 && len(next) > 0 && len(existing) > 0 &&
		next[0].Role == provider.RoleSystem && existing[0].Role == provider.RoleSystem &&
		!messagesEqualForStorage(next[0], existing[0]) {
		// A resume that swapped the system prompt persisted its baseline with
		// the previous system message — the one still on disk. Re-anchor the
		// search on that message so the swap alone doesn't hide the baseline.
		variant := append([]provider.Message{existing[0]}, next[1:]...)
		depth = messagesPrefixDigestDepth(variant, baseDigest)
	}
	return depth >= 0 && len(existing) >= depth
}

func messagesHavePrefixWithCompatibleSystem(full, prefix []provider.Message) bool {
	full = messagesWithoutLeadingSystem(full)
	prefix = messagesWithoutLeadingSystem(prefix)
	return messagesHavePrefix(full, prefix)
}

func messagesWithoutLeadingSystem(msgs []provider.Message) []provider.Message {
	if len(msgs) > 0 && msgs[0].Role == provider.RoleSystem {
		return msgs[1:]
	}
	return msgs
}

func messagesEqualForStorage(a, b provider.Message) bool {
	a = messageForSessionIdentity(a)
	b = messageForSessionIdentity(b)
	ab, err := json.Marshal(a)
	if err != nil {
		return false
	}
	bb, err := json.Marshal(b)
	if err != nil {
		return false
	}
	return bytes.Equal(ab, bb)
}

func messagesEqualForStorageList(a, b []provider.Message) bool {
	if len(a) != len(b) {
		return false
	}
	for i := range a {
		if !messagesEqualForStorage(a[i], b[i]) {
			return false
		}
	}
	return true
}

func messagesCompatibleForStorageBaseline(a, b []provider.Message) bool {
	if messagesEqualForStorageList(a, b) {
		return true
	}
	return messagesEqualForStorageList(messagesWithoutLeadingSystem(a), messagesWithoutLeadingSystem(b))
}

func lockSessionSavePath(path string) func() {
	key := canonicalSessionSavePath(path)
	v, _ := sessionSaveLocks.LoadOrStore(key, &sync.Mutex{})
	mu := v.(*sync.Mutex)
	mu.Lock()
	return mu.Unlock
}

// lockSessionFile waits briefly for the cross-process compatibility save lock.
// A short overlap with a legitimate writer is allowed to settle, but an
// stalled or indefinitely held lock fails the save instead of freezing tab
// switching or application shutdown. The caller keeps its in-memory transcript
// and can retry through the existing autosave/recovery paths.
func lockSessionFile(path string) (func(), error) {
	wait := sessionFileLockWait
	poll := sessionFileLockPollInterval
	if poll <= 0 {
		poll = time.Millisecond
	}
	deadline := time.Now().Add(wait)
	for {
		unlock, err := tryLockSessionFile(path)
		if err == nil {
			return unlock, nil
		}
		if !errors.Is(err, ErrSessionFileLockHeld) {
			return nil, err
		}
		remaining := time.Until(deadline)
		if wait <= 0 || remaining <= 0 {
			return nil, ErrSessionFileLockHeld
		}
		if poll > remaining {
			poll = remaining
		}
		time.Sleep(poll)
	}
}

// LockSessionMetaPath serializes a complete branch-meta read-modify-write
// cycle with both goroutines in this process and other Reasonix processes.
// Callers must hold it from the first read through the final replace.
func LockSessionMetaPath(path string) (func(), error) {
	if strings.TrimSpace(path) == "" {
		return nil, fmt.Errorf("empty session path")
	}
	canonical := canonicalSessionSavePath(path)
	if err := os.MkdirAll(filepath.Dir(canonical), 0o755); err != nil {
		return nil, fmt.Errorf("create session metadata dir: %w", err)
	}
	ctx, cancel := context.WithTimeout(context.Background(), sessionMetaLockWait)
	releaseFile, err := filelock.Acquire(ctx, sessionMetaLockPath(canonical))
	cancel()
	if err != nil {
		return nil, fmt.Errorf("lock session metadata: %w", err)
	}
	return func() {
		releaseFile()
	}, nil
}

func sessionMetaLockPath(path string) string {
	canonical := canonicalSessionSavePath(path)
	digest := sha256.Sum256([]byte(canonical))
	return filepath.Join(filepath.Dir(canonical), "."+hex.EncodeToString(digest[:8])+".meta.lock")
}

// UpdateBranchMeta is the owner-level metadata API. The callback runs while
// the cross-process metadata lock is held, so callers cannot accidentally
// load a stale sidecar and overwrite fields written by another runtime.
func UpdateBranchMeta(path string, touchUpdated bool, update func(*BranchMeta) error) error {
	unlock, err := LockSessionMetaPath(path)
	if err != nil {
		return err
	}
	defer unlock()
	m, err := ensureBranchMetaUnlocked(path)
	if err != nil {
		return err
	}
	if update != nil {
		if err := update(&m); err != nil {
			return err
		}
	}
	return saveBranchMeta(path, m, touchUpdated)
}

func canonicalSessionSavePath(path string) string {
	key := filepath.Clean(strings.TrimSpace(path))
	if abs, err := filepath.Abs(key); err == nil {
		key = abs
	}
	// Resolve physical identity, not just spelling. Otherwise a symlink or
	// junction alias can acquire a second sidecar lock for the same transcript.
	key = resolvePathThroughExistingAncestor(key)
	if runtime.GOOS == "windows" {
		if strings.HasPrefix(strings.ToUpper(key), `\\?\UNC\`) {
			key = `\\` + key[len(`\\?\UNC\`):]
		} else {
			key = strings.TrimPrefix(key, `\\?\`)
		}
		key = strings.ToLower(key)
	}
	return key
}

// resolvePathThroughExistingAncestor resolves the deepest existing ancestor
// and appends every still-missing component. Fresh sessions can be nested under
// directories that have not been created yet; resolving only the immediate
// parent leaves aliases above that directory split into different lease keys.
func resolvePathThroughExistingAncestor(path string) string {
	current := filepath.Clean(path)
	missing := make([]string, 0, 4)
	for {
		if resolved, err := filepath.EvalSymlinks(current); err == nil {
			for _, v := range slices.Backward(missing) {
				resolved = filepath.Join(resolved, v)
			}
			return resolved
		}
		parent := filepath.Dir(current)
		if parent == current {
			return path
		}
		missing = append(missing, filepath.Base(current))
		current = parent
	}
}

// CanonicalSessionPath is the identity key of a session path: cleaned,
// absolute, and case-folded on Windows, matching the key form used by the
// lease registry and the save-path locks. Any runtime bookkeeping that
// compares or maps session paths (desktop tabs, detached runtimes) must use
// this exact form, or the same file splits into distinct keys — e.g.
// `C:\Users\...` vs the lease's lowercased `c:\users\...`. Empty input stays
// empty instead of resolving to the working directory.
func CanonicalSessionPath(path string) string {
	if strings.TrimSpace(path) == "" {
		return ""
	}
	return canonicalSessionSavePath(path)
}

// LoadSession reads a saved session into a fresh Session value. New sessions
// replay the append-only event log; legacy sessions without an event log fall
// back to the compatibility .jsonl checkpoint. A damaged log is replayed to its
// last clean record (or the checkpoint when nothing decodes) and flagged so the
// next save heals it with a rewrite-and-compact.
// In-process loads share the save path mutex so they cannot observe a local
// SaveSnapshot between appending an event-log record and refreshing the index.
// Missing files surface as os.IsNotExist so callers can fall through to a
// new session.
func LoadSession(path string) (*Session, error) {
	unlock := lockSessionSavePath(path)
	defer unlock()
	return loadSessionUnlocked(path)
}

func loadSessionUnlocked(path string) (*Session, error) {
	hasher := newSessionTranscriptHasher()
	msgs, _, damaged, err := loadSessionMessagesWithLimits(path, defaultSessionReplayLimits, hasher)
	if err != nil {
		return nil, err
	}
	s := &Session{Messages: msgs, eventLogDamaged: damaged}
	// Repair persisted-history-safe issues before anything reads the session.
	// Old sessions (pre adde2d3e) and interrupted turns can carry empty tool-call
	// names, dangling tool_calls, or half-streamed argument JSON that DeepSeek
	// rejects with a 400 on replay. Wire-only cleanup, such as dropping orphan
	// tool messages, stays in the provider send path so Save/LoadSession keeps
	// its round-trip contract. The fast path returns the input slice unchanged
	// for a well-formed history, so we detect an actual repair by comparing
	// slice headers: when NormalizeSession allocated a new backing array, the
	// session is marked dirty so the next Save persists the fix.
	normalized := NormalizeSession(s.Messages)
	normalized = migrateLegacyProviderContent(normalized)
	if len(normalized) != len(s.Messages) || (len(s.Messages) > 0 && &normalized[0] != &s.Messages[0]) {
		s.normalizedDirty = true
		// Keep the pre-repair transcript: checkSnapshotWrite must be able to
		// recognize a snapshot that extends the bytes actually on disk, which
		// the repaired view no longer represents (an interrupted tool turn
		// gets a placeholder result fabricated here that the live session
		// answered for real).
		s.rawMessages = msgs
	}
	s.Messages = normalized
	// Decode already hashed the transcript; when the repairs above returned it
	// unchanged (the common case) reuse that digest, else re-hash the repair.
	digest, digestOK := hasher.sum()
	if s.normalizedDirty || !digestOK {
		if d, derr := digestSessionMessages(s.Messages); derr == nil {
			digest, digestOK = d, true
		} else {
			digestOK = false
		}
	}
	if digestOK {
		// Pair the raw pre-repair transcript when normalization changed it.
		diskView := s.Messages
		if s.normalizedDirty {
			diskView = s.rawMessages
		}
		if meta, ok, metaErr := loadBranchMetaRetry(path); metaErr != nil {
			// The sidecar exists but is unreadable even after retries (torn or
			// corrupt). The session must still open, but revision 0 must not
			// pose as a real baseline: the next save would misread the honest
			// on-disk revision as another runtime's write and fork a recovery
			// branch. Anchor the baseline on digest+version only until a
			// successful save re-learns the revision.
			s.markPersistedRevisionUnknown(path, digest, s.version, s.rewriteVersion, diskView)
		} else {
			revision := int64(0)
			if ok {
				revision = meta.Revision
			}
			s.markPersistedFromLoad(path, digest, s.version, revision, s.rewriteVersion, diskView)
		}
	}
	return s, nil
}

// SessionInfo summarises a saved session for the --resume picker: where it is on
// disk, when it was created/last active, the first user message as a preview, and
// a rough turn count.
type SessionInfo struct {
	Path           string
	CreatedAt      time.Time
	LastActivityAt time.Time
	ModTime        time.Time // compatibility alias for LastActivityAt
	Preview        string
	Turns          int
	CountsKnown    bool
	Scope          string
	WorkspaceRoot  string
	TopicID        string
	TopicTitle     string
	CustomTitle    string
	Recovered      bool
	RecoveryReason string
	RecoveryDigest string
	ParentID       string
}

// CleanupPendingMeta records that a session was logically removed but still has
// artifacts waiting for a background job to unwind before physical cleanup.
type CleanupPendingMeta struct {
	Operation string `json:"operation"`
	CreatedAt int64  `json:"createdAt"`
}

// CleanupPendingInfo describes one durable delayed-cleanup marker and the
// session transcript it belongs to.
type CleanupPendingInfo struct {
	SessionPath string
	MarkerPath  string
	Meta        CleanupPendingMeta
}

// CleanupPendingPath returns the durable marker path for a session transcript.
func CleanupPendingPath(sessionPath string) string {
	return store.SessionCleanupPending(sessionPath)
}

// MarkCleanupPending hides a logically removed session from resume/list surfaces
// until delayed physical cleanup has finished.
func MarkCleanupPending(sessionPath, operation string) error {
	path := CleanupPendingPath(sessionPath)
	if path == "" {
		return nil
	}
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return err
	}
	meta := CleanupPendingMeta{Operation: strings.TrimSpace(operation), CreatedAt: time.Now().UnixMilli()}
	b, err := json.MarshalIndent(meta, "", "  ")
	if err != nil {
		return err
	}
	// The marker controls session visibility during delayed cleanup. Publish it
	// atomically so a crash cannot leave malformed JSON that hides the session
	// and blocks reconciliation on the next startup.
	return fileutil.AtomicWriteFile(path, b, 0o644)
}

// ClearCleanupPending removes a delayed-cleanup marker after physical cleanup.
func ClearCleanupPending(sessionPath string) error {
	path := CleanupPendingPath(sessionPath)
	if path == "" {
		return nil
	}
	if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
		return err
	}
	return nil
}

// IsCleanupPending reports whether a session is hidden pending delayed cleanup.
func IsCleanupPending(sessionPath string) bool {
	path := CleanupPendingPath(sessionPath)
	if path == "" {
		return false
	}
	_, err := os.Stat(path)
	return err == nil
}

// IsVisibleSession reports whether a persisted session should appear on normal
// user/agent-facing list, restore, and retrieval surfaces.
func IsVisibleSession(sessionPath string) bool {
	return strings.TrimSpace(sessionPath) != "" && !IsCleanupPending(sessionPath)
}

// ListCleanupPending returns delayed-cleanup markers left in dir. A missing
// directory is not an error.
func ListCleanupPending(dir string) ([]CleanupPendingInfo, error) {
	dir = strings.TrimSpace(dir)
	if dir == "" {
		return nil, nil
	}
	entries, err := os.ReadDir(dir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, err
	}
	var out []CleanupPendingInfo
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), cleanupPendingExt) {
			continue
		}
		markerPath := filepath.Join(dir, e.Name())
		var meta CleanupPendingMeta
		b, err := fileencoding.ReadFileUTF8(markerPath)
		if err != nil {
			if os.IsNotExist(err) {
				continue
			}
			return nil, err
		}
		if strings.TrimSpace(string(b)) != "" {
			if err := json.Unmarshal(b, &meta); err != nil {
				return nil, fmt.Errorf("read cleanup-pending marker %s: %w", markerPath, err)
			}
		}
		name := strings.TrimSuffix(e.Name(), cleanupPendingExt) + ".jsonl"
		out = append(out, CleanupPendingInfo{
			SessionPath: filepath.Join(dir, name),
			MarkerPath:  markerPath,
			Meta:        meta,
		})
	}
	sort.Slice(out, func(i, j int) bool {
		return out[i].SessionPath < out[j].SessionPath
	})
	return out, nil
}

// ReconcileCleanupPending retries physical cleanup for leftover delayed-cleanup
// markers and stale lock/lease sidecars. It keeps going after individual
// cleanup errors and returns them joined.
func ReconcileCleanupPending(dir string, cleanup func(CleanupPendingInfo) error) error {
	var errs []error
	if err := ReconcileSessionSidecars(dir); err != nil {
		errs = append(errs, err)
	}
	if err := reconcileRecoveryTrashStages(dir); err != nil {
		errs = append(errs, err)
	}
	pending, err := ListCleanupPending(dir)
	if err != nil {
		errs = append(errs, err)
		return errors.Join(errs...)
	}
	for _, item := range pending {
		handled, err := reconcileRecoveryTrashPending(item)
		if !handled {
			if cleanup == nil {
				continue
			}
			err = cleanup(item)
		}
		if err != nil {
			errs = append(errs, fmt.Errorf("%s: %w", item.SessionPath, err))
		}
	}
	return errors.Join(errs...)
}

// ReconcileSessionSidecars renames transcripts whose filenames outgrew their
// sidecars and removes stale lock and lease files left beside sessions by
// older runtimes. It never removes .jsonl transcripts; recovered conversations
// may contain useful user history even when their names are ugly.
func ReconcileSessionSidecars(dir string) error {
	dir = strings.TrimSpace(dir)
	if dir == "" {
		return nil
	}
	var errs []error
	if err := reconcileOverlongSessionFilenames(dir); err != nil {
		errs = append(errs, err)
	}
	// Re-list after the rename pass: it retires old names and their sidecars.
	entries, err := os.ReadDir(dir)
	if err != nil {
		if os.IsNotExist(err) {
			return errors.Join(errs...)
		}
		errs = append(errs, err)
		return errors.Join(errs...)
	}
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		name := e.Name()
		sidecarPath := filepath.Join(dir, name)
		switch {
		case strings.HasSuffix(name, sessionLeaseInfoSidecarSuffix):
			base := filepath.Join(dir, strings.TrimSuffix(name, ".lease.json"))
			if err := removeStaleSessionLeaseInfoSidecar(base, sidecarPath); err != nil {
				errs = append(errs, fmt.Errorf("%s: %w", sidecarPath, err))
			}
		case strings.HasSuffix(name, sessionLeaseLockSidecarSuffix):
			base := filepath.Join(dir, strings.TrimSuffix(name, ".lease.lock"))
			if err := removeStaleSessionLeaseLockSidecar(base, sidecarPath); err != nil {
				errs = append(errs, fmt.Errorf("%s: %w", sidecarPath, err))
			}
		case strings.HasSuffix(name, sessionLockSidecarSuffix):
			base := filepath.Join(dir, strings.TrimSuffix(name, ".lock"))
			if err := removeStaleSessionLockSidecar(base, sidecarPath); err != nil {
				errs = append(errs, fmt.Errorf("%s: %w", sidecarPath, err))
			}
		}
	}
	return errors.Join(errs...)
}

func removeStaleSessionLockSidecar(basePath, sidecarPath string) error {
	basePath = canonicalSessionSavePath(basePath)
	if sessionLeaseHeldLocally(basePath) || SessionLeaseHeldByOtherRuntime(basePath) {
		return nil
	}
	lock, err := tryTakeSessionLockFile(sidecarPath)
	if err != nil {
		if errors.Is(err, ErrSessionFileLockHeld) {
			return nil
		}
		return err
	}
	// The removal is atomic with the release (unlink-under-flock on Unix,
	// delete-disposition on the held handle on Windows), so a concurrent
	// saver can never acquire a lock file that is being deleted under it.
	return lock.RemoveAndUnlock()
}

// removeStaleSessionLeaseLockSidecar retires a leftover .lease.lock. The file
// is the lease lock itself, so taking it non-blocking proves no runtime holds
// the lease, and RemoveAndUnlock deletes it atomically with the release.
func removeStaleSessionLeaseLockSidecar(basePath, _ string) error {
	basePath = canonicalSessionSavePath(basePath)
	if sessionLeaseHeldLocally(basePath) {
		return nil
	}
	lock, err := tryTakeSessionLeaseLock(basePath)
	if err != nil {
		if errors.Is(err, ErrSessionLeaseHeld) {
			return nil
		}
		return err
	}
	return lock.RemoveAndUnlock()
}

// removeStaleSessionLeaseInfoSidecar retires a leftover .lease.json while
// holding the lease lock, so no runtime can adopt the info file mid-removal.
// The info file itself is never held open by anyone, so a plain remove under
// the lock is safe on every platform.
func removeStaleSessionLeaseInfoSidecar(basePath, sidecarPath string) error {
	basePath = canonicalSessionSavePath(basePath)
	if sessionLeaseHeldLocally(basePath) {
		return nil
	}
	lockPath := basePath + ".lease.lock"
	if _, err := os.Stat(lockPath); err == nil {
		unlock, err := tryLockSessionLeaseFile(basePath)
		if err != nil {
			if errors.Is(err, ErrSessionLeaseHeld) {
				return nil
			}
			return err
		}
		removeErr := os.Remove(sidecarPath)
		if unlock != nil {
			unlock()
		}
		if removeErr != nil && !os.IsNotExist(removeErr) {
			return removeErr
		}
		return nil
	} else if !os.IsNotExist(err) {
		return err
	}
	// No lease lock file: holders keep it present (and locked) for their whole
	// lifetime, so the leftover info sidecar has no owner to race with.
	if err := os.Remove(sidecarPath); err != nil && !os.IsNotExist(err) {
		return err
	}
	return nil
}

func sessionLeaseHeldLocally(path string) bool {
	_, ok := sessionLeaseOwners.Load(canonicalSessionSavePath(path))
	return ok
}

// sessionLockSidecarFits reports whether basePath's .lock sidecar name stays
// within the filesystem's per-component limit; past it, no process can hold
// (or ever have held) the file lock, because the lock file cannot be created.
func sessionLockSidecarFits(basePath string) bool {
	return len(filepath.Base(basePath))+len(".lock") <= nameMaxBytes
}

// sessionLeaseSidecarFits is the lease-file analogue of sessionLockSidecarFits.
func sessionLeaseSidecarFits(basePath string) bool {
	return len(filepath.Base(basePath))+len(".lease.lock") <= nameMaxBytes
}

// reconcileOverlongSessionFilenames renames transcripts whose basenames grew
// past maxSessionBasenameBytes — the leftover shape of the pre-bounded
// recovery cascade (#5923), where lock and lease sidecars could no longer be
// created and the session became unsaveable. The conversation bytes are kept
// verbatim under a bounded name derived the same way new recovery branches
// are named; branch meta moves along with its ID rewritten, and sessions
// pointing at the old ID are re-parented so lineage survives the rename.
func reconcileOverlongSessionFilenames(dir string) error {
	entries, err := os.ReadDir(dir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return err
	}
	var errs []error
	renamed := map[string]string{} // old branch ID -> new branch ID
	for _, e := range entries {
		name := e.Name()
		if e.IsDir() || !store.IsSessionTranscriptName(name) {
			continue
		}
		if len(name) <= maxSessionBasenameBytes {
			continue
		}
		oldPath := filepath.Join(dir, name)
		if IsCleanupPending(oldPath) {
			// Being deleted; renaming would orphan the cleanup marker.
			continue
		}
		newID, err := renameOverlongSession(oldPath)
		if err != nil {
			errs = append(errs, fmt.Errorf("%s: %w", oldPath, err))
		}
		// A non-empty newID means the transcript rename landed even if some
		// sidecar migration failed; record it so children still re-parent —
		// this run is the only one that knows the old-to-new mapping.
		if newID != "" {
			renamed[BranchID(oldPath)] = newID
		}
	}
	if len(renamed) > 0 {
		if err := reparentSessionBranches(dir, renamed); err != nil {
			errs = append(errs, err)
		}
	}
	return errors.Join(errs...)
}

// renameOverlongSession moves one overlong transcript to its bounded name and
// migrates the sidecars that carry user state. It returns the new branch ID,
// or "" when the session was skipped because a runtime may still own it.
func renameOverlongSession(oldPath string) (string, error) {
	oldID := BranchID(oldPath)
	newID := recoveryParentStem(oldID)
	if newID == oldID {
		return "", nil
	}
	newPath := filepath.Join(filepath.Dir(oldPath), newID+".jsonl")
	if _, err := os.Stat(newPath); err == nil {
		return "", fmt.Errorf("rename target %s already exists", filepath.Base(newPath))
	} else if !os.IsNotExist(err) {
		return "", err
	}
	unlockOld := lockSessionSavePath(oldPath)
	defer unlockOld()
	unlockNew := lockSessionSavePath(newPath)
	defer unlockNew()
	if sessionLeaseHeldLocally(oldPath) {
		return "", nil
	}
	// Names past the sidecar limit cannot have lease or lock holders in any
	// process — the holder files themselves are uncreatable — so probing them
	// would only manufacture ENAMETOOLONG errors and wrongly skip the exact
	// sessions this pass exists to repair.
	if sessionLeaseSidecarFits(oldPath) && SessionLeaseHeldByOtherRuntime(oldPath) {
		return "", nil
	}
	var lockFile *sessionLockFile
	if sessionLockSidecarFits(oldPath) {
		lock, err := tryTakeSessionLockFile(oldPath + ".lock")
		if err != nil {
			if errors.Is(err, ErrSessionFileLockHeld) {
				return "", nil
			}
			return "", err
		}
		lockFile = lock
	}
	if err := os.Rename(oldPath, newPath); err != nil {
		// Nothing moved: the old transcript is intact and the next
		// reconciliation can retry, so its lock file stays in place too.
		if lockFile != nil {
			lockFile.Unlock()
		}
		return "", err
	}
	// The transcript is committed under its new name from here on. Sidecar
	// migration and lock cleanup failures are reported, but the new ID is
	// still returned so the caller re-parents children: the old name is gone,
	// and a later run would have no way to reconstruct this mapping.
	var errs []error
	if err := migrateSessionSidecars(oldPath, newPath, newID); err != nil {
		errs = append(errs, err)
	}
	// Retire the old disposable lease sidecars: any holder was ruled out
	// above, and nothing keeps these files open, so a plain remove is safe.
	if sessionLeaseSidecarFits(oldPath) {
		for _, stale := range []string{oldPath + ".lease.lock", oldPath + ".lease.json"} {
			if err := os.Remove(stale); err != nil && !os.IsNotExist(err) {
				errs = append(errs, err)
			}
		}
	}
	// The old .lock goes atomically with the release of the lock we hold on it.
	if lockFile != nil {
		if err := lockFile.RemoveAndUnlock(); err != nil {
			errs = append(errs, err)
		}
	}
	return newID, errors.Join(errs...)
}

// migrateSessionSidecars moves the user-state sidecars of a renamed session:
// branch meta (with its ID rewritten to match the new filename), goal state,
// and the checkpoint/job directories. Lock and lease files are disposable and
// are removed by the caller instead.
func migrateSessionSidecars(oldPath, newPath, newID string) error {
	var errs []error
	if len(filepath.Base(oldPath))+len(".meta") <= nameMaxBytes {
		if meta, ok, err := LoadBranchMeta(oldPath); err != nil {
			errs = append(errs, err)
		} else if ok {
			meta.ID = newID
			if err := SaveBranchMetaPreserveUpdated(newPath, meta); err != nil {
				errs = append(errs, err)
			} else if err := os.Remove(BranchMetaPath(oldPath)); err != nil && !os.IsNotExist(err) {
				errs = append(errs, err)
			}
		}
	}
	for _, pair := range [][2]string{
		{store.SessionGoalState(oldPath), store.SessionGoalState(newPath)},
		{store.SessionEventLog(oldPath), store.SessionEventLog(newPath)},
		{store.SessionEventLogDamaged(oldPath), store.SessionEventLogDamaged(newPath)},
		{store.SessionTurnEventLog(oldPath), store.SessionTurnEventLog(newPath)},
		{store.SessionTurnEventLogDamaged(oldPath), store.SessionTurnEventLogDamaged(newPath)},
		{store.SessionEventIndex(oldPath), store.SessionEventIndex(newPath)},
		{store.SessionConflictLog(oldPath), store.SessionConflictLog(newPath)},
		{store.SessionRecoveryState(oldPath), store.SessionRecoveryState(newPath)},
		{store.SessionCheckpointDir(oldPath), store.SessionCheckpointDir(newPath)},
		{store.SessionJobsDir(oldPath), store.SessionJobsDir(newPath)},
		{store.SessionInboxDir(oldPath), store.SessionInboxDir(newPath)},
	} {
		// A source name past the filesystem limit cannot exist; renaming it
		// would just manufacture ENAMETOOLONG instead of a clean not-exist.
		if len(filepath.Base(pair[0])) > nameMaxBytes {
			continue
		}
		if err := os.Rename(pair[0], pair[1]); err != nil && !os.IsNotExist(err) {
			errs = append(errs, err)
		}
	}
	return errors.Join(errs...)
}

// reparentSessionBranches rewrites ParentID references from renamed branch IDs
// to their bounded replacements so the branch tree stays connected.
func reparentSessionBranches(dir string, renamed map[string]string) error {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return err
	}
	var errs []error
	for _, e := range entries {
		name := e.Name()
		if e.IsDir() || !store.IsSessionTranscriptName(name) {
			continue
		}
		if len(name)+len(".meta") > nameMaxBytes {
			continue
		}
		path := filepath.Join(dir, name)
		unlock := lockSessionSavePath(path)
		meta, ok, err := LoadBranchMeta(path)
		if err == nil && ok {
			if newParent, hit := renamed[meta.ParentID]; hit && newParent != meta.ParentID {
				meta.ParentID = newParent
				err = SaveBranchMetaPreserveUpdated(path, meta)
			}
		}
		unlock()
		if err != nil {
			errs = append(errs, fmt.Errorf("%s: %w", path, err))
		}
	}
	return errors.Join(errs...)
}

// ListSessionOrder returns every *.jsonl session under dir in the same
// most-recently-active order used by ListSessions, using only file metadata and
// branch sidecars. A missing directory is not an error.
func ListSessionOrder(dir string) ([]SessionOrderInfo, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, err
	}
	var out []SessionOrderInfo
	for _, e := range entries {
		if e.IsDir() || !store.IsSessionTranscriptName(e.Name()) {
			continue
		}
		info, err := e.Info()
		if err != nil {
			continue
		}
		full := filepath.Join(dir, e.Name())
		if !IsVisibleSession(full) {
			continue
		}
		contentMod := SessionContentModTime(full)
		if contentMod.IsZero() {
			contentMod = info.ModTime()
		}
		createdAt := info.ModTime()
		lastActivityAt := contentMod
		scope := "global"
		workspaceRoot := ""
		topicID := ""
		topicTitle := ""
		customTitle := ""
		recovered := false
		recoveryReason := ""
		recoveryDigest := ""
		parentID := ""
		recoveryPreferred := false
		turns := 0
		preview := ""
		schemaVersion := 0
		revision := int64(0)
		contentDigest := ""
		listingRevision := int64(0)
		listingContentDigest := ""
		if meta, ok, err := LoadBranchMeta(full); err == nil && ok {
			if !meta.CreatedAt.IsZero() {
				createdAt = meta.CreatedAt
			}
			if !meta.UpdatedAt.IsZero() {
				lastActivityAt = meta.UpdatedAt
			}
			scope = meta.DefaultScope()
			workspaceRoot = meta.WorkspaceRoot
			topicID = meta.TopicID
			topicTitle = meta.TopicTitle
			customTitle = meta.CustomTitle
			recovered = meta.Recovered
			recoveryReason = meta.RecoveryReason
			recoveryDigest = meta.RecoveryDigest
			parentID = meta.ParentID
			recoveryPreferred = RecoveryPreferenceCurrent(full, meta)
			turns = meta.Turns
			preview = meta.Preview
			schemaVersion = meta.SchemaVersion
			revision = meta.Revision
			contentDigest = meta.ContentDigest
			listingRevision = meta.ListingRevision
			listingContentDigest = meta.ListingContentDigest
		}
		// Old recovery files may lack Recovered meta; filename still proves
		// automatic recovery lineage for catalog folding.
		if !recovered && LooksLikeRecoveryFilename(full) {
			recovered = true
			if parentID == "" {
				if parent, ok := RecoveryFilenameParentID(full); ok {
					parentID = parent
				}
			}
		}
		out = append(out, SessionOrderInfo{
			Path:                 full,
			CreatedAt:            createdAt,
			LastActivityAt:       lastActivityAt,
			ModTime:              lastActivityAt,
			Scope:                scope,
			WorkspaceRoot:        workspaceRoot,
			TopicID:              topicID,
			TopicTitle:           topicTitle,
			CustomTitle:          customTitle,
			Recovered:            recovered,
			RecoveryReason:       recoveryReason,
			RecoveryDigest:       recoveryDigest,
			ParentID:             parentID,
			RecoveryPreferred:    recoveryPreferred,
			Turns:                turns,
			Preview:              preview,
			SchemaVersion:        schemaVersion,
			Revision:             revision,
			ContentDigest:        contentDigest,
			ListingRevision:      listingRevision,
			ListingContentDigest: listingContentDigest,
		})
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].LastActivityAt.Equal(out[j].LastActivityAt) {
			return out[i].Path < out[j].Path
		}
		return out[i].LastActivityAt.After(out[j].LastActivityAt)
	})
	return out, nil
}

// ListSessions returns every non-empty *.jsonl session under dir,
// most-recently-active first, each with a preview line so the picker can show
// something the user recognises. It never decodes a transcript: legacy counts
// remain explicitly unknown until the session catalog's single repair worker
// validates them. A missing directory is not an error.
func ListSessions(dir string) ([]SessionInfo, error) {
	ordered, err := ListSessionOrder(dir)
	if err != nil {
		return nil, err
	}
	var out []SessionInfo
	for _, session := range ordered {
		preview, turns := session.Preview, session.Turns
		if !sessionListingProjectionFresh(session.SchemaVersion, turns, session.Revision, session.ListingRevision, session.ContentDigest, session.ListingContentDigest) {
			if !sessionArtifactsHaveContent(session.Path) {
				continue
			}
			if strings.TrimSpace(preview) == "" {
				preview = "History is being indexed — " + filepath.Base(session.Path)
			}
			out = append(out, sessionInfoFromOrder(session, preview, turns, false))
			continue
		}
		if turns == 0 {
			// Never had user interaction — an empty conversation that should not
			// appear in the history panel or the resume picker.
			continue
		}
		out = append(out, sessionInfoFromOrder(session, preview, turns, true))
	}
	return out, nil
}

// SessionPreview returns the same preview and user-turn count used by
// ListSessions for one session file.
func SessionPreview(path string) (string, int) {
	return previewSession(path)
}

// SessionPreviewWithError returns the same preview and user-turn count as
// SessionPreview, but preserves read and decode failures for callers that must
// not persist a fallback derived from an unreadable transcript.
func SessionPreviewWithError(path string) (string, int, error) {
	return previewSessionWithError(path)
}

// SessionPreviewFromMessages computes the same preview line and user-turn count
// as previewSession, but from an in-memory message slice. Session.Save writes
// exactly these messages to the .jsonl, so this is byte-for-byte equivalent to
// decoding the file — letting the autosave path persist the counts into the
// sidecar without a disk read.
func SessionPreviewFromMessages(msgs []provider.Message) (string, int) {
	first := ""
	turns := 0
	for _, m := range msgs {
		if IsUserAuthoredTurnMessage(m) {
			turns++
			if first == "" {
				first = truncatePreview(previewProse(UserMessageText(m)))
			}
		}
	}
	return first, turns
}

// previewSession returns the first user message (truncated) and the number of
// user-role messages so the picker can show "5 turns · 'help me debug the…'".
// Errors are swallowed — a malformed file just shows up with an empty preview.
func previewSession(path string) (string, int) {
	preview, turns, _ := previewSessionWithError(path)
	return preview, turns
}

// previewProse drops the leading @file references a prompt opens with so the
// preview shows what was asked rather than a row of paths. A prompt that is
// nothing but references keeps them — there is nothing else to show.
func previewProse(s string) string {
	rest := strings.TrimLeft(s, " \t")
	for strings.HasPrefix(rest, "@") {
		end := strings.IndexAny(rest, " \t\r\n")
		if end < 0 {
			return s
		}
		next := strings.TrimLeft(rest[end:], " \t")
		if strings.TrimSpace(next) == "" {
			return s
		}
		rest = next
	}
	if rest == "" {
		return s
	}
	return rest
}

// truncatePreview clamps a preview line to 80 runes with an ellipsis, matching
// what the pickers render.
func truncatePreview(s string) string {
	if r := []rune(s); len(r) > 80 {
		return string(r[:77]) + "…"
	}
	return s
}

// ContinueSessionPath returns where a conversation carried into a rebuilt
// controller (model switch, config change) should keep auto-saving: its existing
// file when it has one, so the continued session stays a single file instead of
// the old one being orphaned as an identical duplicate (#2807). A session with no
// file yet gets a fresh path; "" when persistence is disabled.
func ContinueSessionPath(prevPath, dir, model string) string {
	if prevPath != "" {
		return prevPath
	}
	if dir == "" {
		return ""
	}
	return NewSessionPath(dir, model)
}

// NewSessionPath returns the path to use for a fresh session, namespaced by
// the model so the filename hints at what the conversation was with. dir is
// typically config.SessionDir().
func NewSessionPath(dir, model string) string {
	safe := strings.NewReplacer("/", "-", "\\", "-", ":", "-", "<", "-", ">", "-", "\"", "-", "|", "-", "?", "-", "*", "-").Replace(model)
	if safe == "" {
		safe = "session"
	}
	return filepath.Join(dir, fmt.Sprintf("%s-%s.jsonl", time.Now().UTC().Format("20060102-150405.000000000"), safe))
}