diff --git a/internal/docrevision/ids.go b/internal/docrevision/ids.go
new file mode 100644
index 00000000..32c72b82
--- /dev/null
+++ b/internal/docrevision/ids.go
@@ -0,0 +1,103 @@
+package docrevision
+
+// advanceID 把进程内分配计数推进到不少于已落库 ID，避免下一临时 ID 撞上真实对象。
+func advanceID(counter *int64, id int64) {
+	if counter == nil || id <= 0 {
+		return
+	}
+	if id > *counter {
+		*counter = id
+	}
+}
+
+// nextFreeID 分配一个当前 map 中未被占用的临时 ID。
+func nextFreeID(counter *int64, occupied func(int64) bool) int64 {
+	if counter == nil {
+		return 0
+	}
+	for {
+		*counter++
+		id := *counter
+		if occupied == nil || !occupied(id) {
+			return id
+		}
+	}
+}
+
+func (s *Service) adoptSnapshotID(snap *Snapshot, dbID int64) {
+	if s == nil || snap == nil || dbID <= 0 {
+		return
+	}
+	oldID := snap.ID
+	if oldID != 0 && oldID != dbID {
+		if existing := s.snapshots[oldID]; existing == snap {
+			delete(s.snapshots, oldID)
+		}
+	}
+	snap.ID = dbID
+	s.snapshots[dbID] = snap
+	advanceID(&s.nextSnapID, dbID)
+}
+
+func (s *Service) adoptVersionID(ver *Version, dbID int64) {
+	if s == nil || ver == nil || dbID <= 0 {
+		return
+	}
+	oldID := ver.ID
+	if oldID != 0 && oldID != dbID {
+		if existing := s.versions[oldID]; existing == ver {
+			delete(s.versions, oldID)
+		}
+	}
+	ver.ID = dbID
+	s.versions[dbID] = ver
+	advanceID(&s.nextVerID, dbID)
+}
+
+func (s *Service) adoptEventID(ev *ChangeEvent, dbID int64) {
+	if s == nil || ev == nil || dbID <= 0 {
+		return
+	}
+	oldID := ev.ID
+	if oldID != 0 && oldID != dbID {
+		if existing := s.events[oldID]; existing == ev {
+			delete(s.events, oldID)
+		}
+	}
+	ev.ID = dbID
+	s.events[dbID] = ev
+	if ev.IdempotencyKey != "" {
+		s.idemp[ev.IdempotencyKey] = dbID
+	}
+	advanceID(&s.nextEvtID, dbID)
+}
+
+func (s *Service) adoptReportID(rep *ChangeReport, dbID int64) {
+	if s == nil || rep == nil || dbID <= 0 {
+		return
+	}
+	oldID := rep.ID
+	if oldID != 0 && oldID != dbID {
+		if existing := s.reports[oldID]; existing == rep {
+			delete(s.reports, oldID)
+		}
+	}
+	rep.ID = dbID
+	s.reports[dbID] = rep
+	advanceID(&s.nextRepID, dbID)
+}
+
+func (s *Service) adoptCandidateID(c *AgentCandidate, dbID int64) {
+	if s == nil || c == nil || dbID <= 0 {
+		return
+	}
+	oldID := c.ID
+	if oldID != 0 && oldID != dbID {
+		if existing := s.candidates[oldID]; existing == c {
+			delete(s.candidates, oldID)
+		}
+	}
+	c.ID = dbID
+	s.candidates[dbID] = c
+	advanceID(&s.nextCandID, dbID)
+}
diff --git a/internal/docrevision/ids_test.go b/internal/docrevision/ids_test.go
new file mode 100644
index 00000000..f979290a
--- /dev/null
+++ b/internal/docrevision/ids_test.go
@@ -0,0 +1,70 @@
+package docrevision
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNextFreeID_SkipsOccupied(t *testing.T) {
+	var counter int64 = 248
+	occupied := map[int64]bool{249: true}
+	got := nextFreeID(&counter, func(id int64) bool { return occupied[id] })
+	if got != 250 {
+		t.Fatalf("next free id=%d want 250", got)
+	}
+}
+
+func TestAdoptSnapshotID_DoesNotDeleteOtherObject(t *testing.T) {
+	s := NewService(nil)
+	baseline := &Snapshot{ID: 249, DocID: "doc_a", ContentSHA256: "aa", CreatedAt: time.Now().UTC()}
+	temp := &Snapshot{ID: 249, DocID: "doc_b", ContentSHA256: "bb", CreatedAt: time.Now().UTC()}
+	s.snapshots[249] = baseline
+	s.nextSnapID = 248
+
+	s.adoptSnapshotID(temp, 250)
+	if s.snapshots[249] != baseline {
+		t.Fatal("adopting a different object must not delete the real baseline")
+	}
+	if s.snapshots[250] != temp || temp.ID != 250 {
+		t.Fatalf("temp must live at 250, id=%d", temp.ID)
+	}
+	if s.nextSnapID != 250 {
+		t.Fatalf("counter=%d want 250", s.nextSnapID)
+	}
+}
+
+func TestAdoptSnapshotID_RemovesOnlySameObject(t *testing.T) {
+	s := NewService(nil)
+	snap := &Snapshot{ID: 1, DocID: "doc_a", ContentSHA256: "aa"}
+	s.snapshots[1] = snap
+	s.adoptSnapshotID(snap, 249)
+	if _, ok := s.snapshots[1]; ok {
+		t.Fatal("same-object remap must drop the old temp key")
+	}
+	if s.snapshots[249] != snap || snap.ID != 249 {
+		t.Fatal("same object must occupy the database id")
+	}
+	if s.nextSnapID < 249 {
+		t.Fatalf("counter must advance to db id, got %d", s.nextSnapID)
+	}
+}
+
+func TestPutSnapshotLocked_DoesNotClobberExistingBaseline(t *testing.T) {
+	s := NewService(nil)
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	baseline := &Snapshot{ID: 249, DocID: "doc_seed", ContentSHA256: "base"}
+	s.snapshots[249] = baseline
+	s.nextSnapID = 248
+	s.blobs["out"] = &ContentBlob{ContentSHA256: "out", ContentMarkdown: "# out\n"}
+	got := s.putSnapshotLocked("doc_new", "# out\n", "out", nil, SourceAgent, nil, "run", ActorSnapshot{}, "run", nil)
+	if got.ID == 249 {
+		t.Fatal("temporary snapshot must not reuse the occupied baseline id")
+	}
+	if s.snapshots[249] != baseline {
+		t.Fatal("baseline snapshot must remain")
+	}
+	if s.snapshots[got.ID] != got {
+		t.Fatal("new snapshot must be stored under its own id")
+	}
+}
diff --git a/internal/docrevision/pg_id_remap_test.go b/internal/docrevision/pg_id_remap_test.go
new file mode 100644
index 00000000..802264fa
--- /dev/null
+++ b/internal/docrevision/pg_id_remap_test.go
@@ -0,0 +1,198 @@
+package docrevision_test
+
+import (
+	"context"
+	"fmt"
+	"testing"
+	"time"
+
+	"chat-agent-system/internal/db"
+	"chat-agent-system/internal/docrevision"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func TestMaterializeAgentOutput_HighSequenceFirstGenKeepsBaseline_PG(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping PostgreSQL id-remap integration in -short")
+	}
+	dsn := db.SkipIfNoTestDatabaseURL(t)
+	ctx, cancel := context.WithTimeout(context.Background(), 45*time.Second)
+	defer cancel()
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		t.Skipf("postgres unavailable: %v", err)
+	}
+	defer pool.Close()
+	if err := pool.Ping(ctx); err != nil {
+		t.Skipf("postgres ping failed: %v", err)
+	}
+	if err := db.RunMigrationsOnceForTest(ctx, pool, dsn); err != nil {
+		t.Fatalf("migration failed: %v", err)
+	}
+	var n int
+	if err := pool.QueryRow(ctx, `SELECT COUNT(*) FROM information_schema.tables WHERE table_name='document_snapshots'`).Scan(&n); err != nil || n == 0 {
+		t.Skip("document_snapshots table missing")
+	}
+
+	suffix := time.Now().UnixNano()
+	seedDoc := fmt.Sprintf("doc_idremap_seed_%d", suffix)
+	genDoc := fmt.Sprintf("doc_idremap_gen_%d", suffix)
+	t.Cleanup(func() {
+		cleanupRevisionDocs(context.Background(), pool, seedDoc, genDoc)
+	})
+	cleanupRevisionDocs(ctx, pool, seedDoc, genDoc)
+
+	seedSnap := nextExplicitID(t, ctx, pool, "document_snapshots")
+	seedVer := nextExplicitID(t, ctx, pool, "document_versions")
+	seedEvt := nextExplicitID(t, ctx, pool, "document_change_events")
+	seedHighIDs(t, ctx, pool, seedDoc, seedSnap, seedVer, seedEvt)
+
+	svc := docrevision.NewService(pool)
+	seedState, err := svc.GetRevisionState(ctx, seedDoc)
+	if err != nil {
+		t.Fatalf("hydrate seed: %v", err)
+	}
+	if seedState.BaselineSnapshotID != seedSnap {
+		t.Fatalf("seed baseline=%d want %d", seedState.BaselineSnapshotID, seedSnap)
+	}
+
+	outMD := "# 首轮交付\n\n## 范围\n\nPG 序列碰撞回归正文\n"
+	res, err := svc.MaterializeAgentOutput(ctx, docrevision.AgentMaterializeRequest{
+		DocID: genDoc, RunID: fmt.Sprintf("run-idremap-%d", suffix), OutputMarkdown: outMD,
+		ProjectID: 1, WorkDir: "ws/idremap",
+		ActorKind:        docrevision.ActorAgentSquad,
+		ActorSnapshot:    docrevision.ActorSnapshot{SquadName: "bizdev-squad", Model: "test"},
+		AutoCommitUserID: 42,
+	})
+	if err != nil {
+		t.Fatalf("materialize first gen: %v", err)
+	}
+	if res.State == nil || res.State.CurrentVersionNo != 2 {
+		t.Fatalf("want V2 after auto-commit, got %+v", res.State)
+	}
+	if res.Candidate == nil || res.Candidate.Status != docrevision.CandidateCommitted {
+		t.Fatalf("candidate must be committed, got %+v", res.Candidate)
+	}
+	if res.State.BaselineSnapshotID == 0 {
+		t.Fatal("baseline snapshot id must remain")
+	}
+	versions, err := svc.ListVersions(ctx, genDoc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var v1, v2 *docrevision.Version
+	for i := range versions {
+		switch versions[i].VersionNo {
+		case 1:
+			v1 = &versions[i]
+		case 2:
+			v2 = &versions[i]
+		}
+	}
+	if v1 == nil || v2 == nil {
+		t.Fatalf("want V1+V2, got %+v", versions)
+	}
+	if v1.BaselineSnapshotID != seedSnap+1 {
+		t.Fatalf("first-gen template snapshot want %d (next after seed %d), got %d", seedSnap+1, seedSnap, v1.BaselineSnapshotID)
+	}
+	if v2.BaselineSnapshotID == v1.BaselineSnapshotID || v2.HeadSnapshotID == v1.BaselineSnapshotID {
+		t.Fatalf("V2 must not reuse/drop V1 baseline: v1=%+v v2=%+v", v1, v2)
+	}
+	if res.Candidate.InputSnapshotID != v1.BaselineSnapshotID {
+		t.Fatalf("candidate input=%d want V1 baseline %d", res.Candidate.InputSnapshotID, v1.BaselineSnapshotID)
+	}
+
+	seedAfter, err := svc.GetRevisionState(ctx, seedDoc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if seedAfter.BaselineSnapshotID != seedSnap || seedAfter.HeadSnapshotID != seedSnap {
+		t.Fatalf("seed doc must keep snapshot %d, got %+v", seedSnap, seedAfter)
+	}
+
+	restart := docrevision.NewService(pool)
+	st2, err := restart.GetRevisionState(ctx, genDoc)
+	if err != nil {
+		t.Fatalf("hydrate gen after new Service: %v", err)
+	}
+	if st2.CurrentVersionNo != 2 || st2.HeadSnapshotID == 0 {
+		t.Fatalf("restart hydrate want V2/head, got %+v", st2)
+	}
+	cand, err := restart.GetCandidate(ctx, genDoc, res.Candidate.ID)
+	if err != nil || cand == nil || cand.Status != docrevision.CandidateCommitted {
+		t.Fatalf("restart candidate: %+v err=%v", cand, err)
+	}
+	if st2.BaselineSnapshotID == 0 {
+		t.Fatal("restart must keep baseline")
+	}
+
+	seedRestart, err := restart.GetRevisionState(ctx, seedDoc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if seedRestart.BaselineSnapshotID != seedSnap {
+		t.Fatalf("restart seed baseline=%d", seedRestart.BaselineSnapshotID)
+	}
+}
+
+func nextExplicitID(t *testing.T, ctx context.Context, pool *pgxpool.Pool, table string) int64 {
+	t.Helper()
+	var maxID int64
+	q := fmt.Sprintf(`SELECT COALESCE(MAX(id), 0) FROM %s`, table)
+	if err := pool.QueryRow(ctx, q).Scan(&maxID); err != nil {
+		t.Fatalf("max id %s: %v", table, err)
+	}
+	return maxID + 50
+}
+
+func seedHighIDs(t *testing.T, ctx context.Context, pool *pgxpool.Pool, docID string, snapID, verID, evtID int64) {
+	t.Helper()
+	sha := "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+	if _, err := pool.Exec(ctx, `
+		INSERT INTO document_content_blobs (content_sha256, content_markdown, byte_size)
+		VALUES ($1, $2, 8)
+		ON CONFLICT (content_sha256) DO NOTHING
+	`, sha, "# seed\n"); err != nil {
+		t.Fatalf("seed blob: %v", err)
+	}
+	if _, err := pool.Exec(ctx, `
+		INSERT INTO document_snapshots (id, doc_id, content_sha256, source_kind)
+		VALUES ($1, $2, $3, 'template')
+	`, snapID, docID, sha); err != nil {
+		t.Fatalf("seed snapshot: %v", err)
+	}
+	if _, err := pool.Exec(ctx, `
+		INSERT INTO document_versions (id, doc_id, version_no, status, origin_kind, baseline_snapshot_id, head_snapshot_id)
+		VALUES ($1, $2, 1, 'open', 'template', $3, $3)
+	`, verID, docID, snapID); err != nil {
+		t.Fatalf("seed version: %v", err)
+	}
+	if _, err := pool.Exec(ctx, `
+		INSERT INTO document_change_events (
+			id, doc_id, version_id, event_no, to_snapshot_id, change_kind, actor_kind, idempotency_key
+		) VALUES ($1, $2, $3, 1, $4, 'import', 'template', $5)
+	`, evtID, docID, verID, snapID, "seed:"+docID); err != nil {
+		t.Fatalf("seed event: %v", err)
+	}
+	if _, err := pool.Exec(ctx, `SELECT setval('document_snapshots_id_seq', $1, true)`, snapID); err != nil {
+		t.Fatalf("setval snapshots: %v", err)
+	}
+	if _, err := pool.Exec(ctx, `SELECT setval('document_versions_id_seq', $1, true)`, verID); err != nil {
+		t.Fatalf("setval versions: %v", err)
+	}
+	if _, err := pool.Exec(ctx, `SELECT setval('document_change_events_id_seq', $1, true)`, evtID); err != nil {
+		t.Fatalf("setval events: %v", err)
+	}
+}
+
+func cleanupRevisionDocs(ctx context.Context, pool *pgxpool.Pool, docIDs ...string) {
+	for _, docID := range docIDs {
+		_, _ = pool.Exec(ctx, `DELETE FROM document_agent_candidates WHERE doc_id=$1`, docID)
+		_, _ = pool.Exec(ctx, `UPDATE document_versions SET change_report_id=NULL WHERE doc_id=$1`, docID)
+		_, _ = pool.Exec(ctx, `DELETE FROM document_change_reports WHERE doc_id=$1`, docID)
+		_, _ = pool.Exec(ctx, `DELETE FROM document_change_events WHERE doc_id=$1`, docID)
+		_, _ = pool.Exec(ctx, `DELETE FROM document_versions WHERE doc_id=$1`, docID)
+		_, _ = pool.Exec(ctx, `DELETE FROM document_snapshots WHERE doc_id=$1`, docID)
+	}
+}
diff --git a/internal/docrevision/pg_mutate.go b/internal/docrevision/pg_mutate.go
index 24be7374..28d49f50 100644
--- a/internal/docrevision/pg_mutate.go
+++ b/internal/docrevision/pg_mutate.go
@@ -26,8 +26,7 @@ func (s *Service) openTemplatePGLocked(ctx context.Context, docID, canon, sha st
 		if err != nil {
 			return err
 		}
-		sn.ID = id
-		s.snapshots[id] = sn
+		s.adoptSnapshotID(sn, id)
 
 		ver := &Version{
 			DocID: docID, VersionNo: 1, Status: VersionOpen, OriginKind: OriginTemplate,
@@ -37,8 +36,7 @@ func (s *Service) openTemplatePGLocked(ctx context.Context, docID, canon, sha st
 		if err != nil {
 			return err
 		}
-		ver.ID = vid
-		s.versions[vid] = ver
+		s.adoptVersionID(ver, vid)
 
 		ev := &ChangeEvent{
 			DocID: docID, VersionID: vid, EventNo: 1, ToSnapshotID: id,
@@ -50,9 +48,7 @@ func (s *Service) openTemplatePGLocked(ctx context.Context, docID, canon, sha st
 		if err != nil {
 			return err
 		}
-		ev.ID = eid
-		s.events[eid] = ev
-		s.idemp[ev.IdempotencyKey] = eid
+		s.adoptEventID(ev, eid)
 
 		root := &DocRootPointers{
 			DocID: docID, Kind: "controlled", ProjectID: projectID, WorkDir: workDir,
@@ -84,30 +80,20 @@ func (s *Service) persistHumanRevisionLocked(ctx context.Context, root *DocRootP
 		if err := s.ensureBlobTX(ctx, tx, blob); err != nil {
 			return err
 		}
-		oldSnapID := snap.ID
 		id, err := s.insertSnapshotTX(ctx, tx, snap)
 		if err != nil {
 			return err
 		}
-		if oldSnapID != 0 && oldSnapID != id {
-			delete(s.snapshots, oldSnapID)
-		}
-		snap.ID = id
-		s.snapshots[id] = snap
+		s.adoptSnapshotID(snap, id)
 
 		if isBaseline {
 			ver.BaselineSnapshotID = id
 			ver.HeadSnapshotID = id
-			oldVerID := ver.ID
 			vid, err := s.insertVersionTX(ctx, tx, ver)
 			if err != nil {
 				return err
 			}
-			if oldVerID != 0 && oldVerID != vid {
-				delete(s.versions, oldVerID)
-			}
-			ver.ID = vid
-			s.versions[vid] = ver
+			s.adoptVersionID(ver, vid)
 			root.LedgerVersionID = &vid
 			root.HeadSnapshotID = &id
 			ev.VersionID = vid
@@ -122,19 +108,11 @@ func (s *Service) persistHumanRevisionLocked(ctx context.Context, root *DocRootP
 			ev.VersionID = ver.ID
 		}
 
-		oldEv := ev.ID
 		eid, err := s.insertEventTX(ctx, tx, ev)
 		if err != nil {
 			return err
 		}
-		if oldEv != 0 && oldEv != eid {
-			delete(s.events, oldEv)
-		}
-		ev.ID = eid
-		s.events[eid] = ev
-		if ev.IdempotencyKey != "" {
-			s.idemp[ev.IdempotencyKey] = eid
-		}
+		s.adoptEventID(ev, eid)
 		return s.projectRootPointersTX(ctx, tx, root)
 	})
 }
@@ -151,41 +129,26 @@ func (s *Service) persistAgentCandidateLocked(ctx context.Context, outSnap *Snap
 		if err := s.ensureBlobTX(ctx, tx, blob); err != nil {
 			return err
 		}
-		oldSnap := outSnap.ID
 		sid, err := s.insertSnapshotTX(ctx, tx, outSnap)
 		if err != nil {
 			return err
 		}
-		if oldSnap != 0 && oldSnap != sid {
-			delete(s.snapshots, oldSnap)
-		}
-		outSnap.ID = sid
-		s.snapshots[sid] = outSnap
-		c.OutputSnapshotID = &sid
+		s.adoptSnapshotID(outSnap, sid)
+		c.OutputSnapshotID = &outSnap.ID
 		rep.ToSnapshotID = sid
 
-		oldRep := rep.ID
 		rid, err := s.insertReportTX(ctx, tx, rep)
 		if err != nil {
 			return err
 		}
-		if oldRep != 0 && oldRep != rid {
-			delete(s.reports, oldRep)
-		}
-		rep.ID = rid
-		s.reports[rid] = rep
-		c.ChangeReportID = &rid
+		s.adoptReportID(rep, rid)
+		c.ChangeReportID = &rep.ID
 
-		oldC := c.ID
 		cid, err := s.insertCandidateTX(ctx, tx, c)
 		if err != nil {
 			return err
 		}
-		if oldC != 0 && oldC != cid {
-			delete(s.candidates, oldC)
-		}
-		c.ID = cid
-		s.candidates[cid] = c
+		s.adoptCandidateID(c, cid)
 		return nil
 	})
 }
@@ -201,32 +164,22 @@ func (s *Service) persistAgentCommitLocked(ctx context.Context, root *DocRootPoi
 		if err := s.updateVersionTX(ctx, tx, oldVer); err != nil {
 			return err
 		}
-		oldVerID := newVer.ID
 		vid, err := s.insertVersionTX(ctx, tx, newVer)
 		if err != nil {
 			return err
 		}
-		if oldVerID != 0 && oldVerID != vid {
-			delete(s.versions, oldVerID)
-		}
-		newVer.ID = vid
-		s.versions[vid] = newVer
-		root.LedgerVersionID = &vid
+		s.adoptVersionID(newVer, vid)
+		root.LedgerVersionID = &newVer.ID
 		hid := outSnap.ID
 		root.HeadSnapshotID = &hid
 
 		ev.VersionID = vid
 		ev.ToSnapshotID = outSnap.ID
-		oldEv := ev.ID
 		eid, err := s.insertEventTX(ctx, tx, ev)
 		if err != nil {
 			return err
 		}
-		if oldEv != 0 {
-			delete(s.events, oldEv)
-		}
-		ev.ID = eid
-		s.events[eid] = ev
+		s.adoptEventID(ev, eid)
 
 		if err := s.updateCandidateTX(ctx, tx, c); err != nil {
 			return err
diff --git a/internal/docrevision/pg_store.go b/internal/docrevision/pg_store.go
index b0157f97..5541dbdc 100644
--- a/internal/docrevision/pg_store.go
+++ b/internal/docrevision/pg_store.go
@@ -457,15 +457,3 @@ func (s *Service) withTx(ctx context.Context, fn func(pgx.Tx) error) error {
 
 // Pool exposes the underlying pool for integration tests.
 func (s *Service) Pool() *pgxpool.Pool { return s.pool }
-
-// remapSnapshotID updates parent pointers after a snapshot receives its real PG id.
-func (s *Service) remapSnapshotID(oldID, newID int64) {
-	if oldID == 0 || oldID == newID {
-		return
-	}
-	if sn := s.snapshots[oldID]; sn != nil {
-		delete(s.snapshots, oldID)
-		sn.ID = newID
-		s.snapshots[newID] = sn
-	}
-}
diff --git a/internal/docrevision/service.go b/internal/docrevision/service.go
index e0c6597a..ce5c46d4 100644
--- a/internal/docrevision/service.go
+++ b/internal/docrevision/service.go
@@ -499,17 +499,15 @@ func (s *Service) CreateAgentCandidate(ctx context.Context, req AgentCandidateRe
 	if *root.HeadSnapshotID != req.InputSnapshotID {
 		status = CandidateStale
 	}
-	s.nextRepID++
-	rep.ID = s.nextRepID
+	rep.ID = nextFreeID(&s.nextRepID, func(id int64) bool { return s.reports[id] != nil })
 	rep.DocID = req.DocID
 	rep.FromSnapshotID = req.InputSnapshotID
 	rep.ToSnapshotID = outSnap.ID
 	rep.CreatedAt = time.Now().UTC()
 	s.reports[rep.ID] = rep
 
-	s.nextCandID++
 	c := &AgentCandidate{
-		ID: s.nextCandID, DocID: req.DocID, InputSnapshotID: req.InputSnapshotID,
+		ID: nextFreeID(&s.nextCandID, func(id int64) bool { return s.candidates[id] != nil }), DocID: req.DocID, InputSnapshotID: req.InputSnapshotID,
 		OutputSnapshotID: &outSnap.ID, RunID: req.RunID, ProposalID: req.ProposalID,
 		Status: status, AuthorizedScope: req.AuthorizedScope, ChangeReportID: &rep.ID,
 		ActorKind: req.ActorKind, ActorSnapshot: req.ActorSnapshot, CreatedAt: time.Now().UTC(),
@@ -621,9 +619,11 @@ func (s *Service) CommitAgentVersion(ctx context.Context, docID string, candidat
 		c.DecidedAt = &now
 		if s.usingPG() {
 			if err := s.withTx(ctx, func(tx pgx.Tx) error {
-				if _, err := s.insertEventTX(ctx, tx, ev); err != nil {
+				eid, err := s.insertEventTX(ctx, tx, ev)
+				if err != nil {
 					return err
 				}
+				s.adoptEventID(ev, eid)
 				return s.updateCandidateTX(ctx, tx, c)
 			}); err != nil {
 				s.forgetDocLocked(docID)
@@ -860,8 +860,7 @@ func (s *Service) ImportFreeformLegacy(ctx context.Context, docID string, ownerI
 		ID: 0, DocID: docID, VersionNo: startNo, Status: VersionOpen, OriginKind: OriginMigration,
 		BaselineSnapshotID: snap.ID, HeadSnapshotID: snap.ID, OpenedAt: time.Now().UTC(),
 	}
-	s.nextVerID++
-	ver.ID = s.nextVerID
+	ver.ID = nextFreeID(&s.nextVerID, func(id int64) bool { return s.versions[id] != nil })
 	s.versions[ver.ID] = ver
 	root := &DocRootPointers{
 		DocID: docID, Kind: "freeform", ProjectID: projectID,
@@ -939,9 +938,8 @@ func (s *Service) putSnapshotLocked(docID, canon, sha string, parent *int64, sou
 	} else {
 		blocks = BuildBlockMap(canon, clientBlocks)
 	}
-	s.nextSnapID++
 	snap := &Snapshot{
-		ID: s.nextSnapID, DocID: docID, ContentSHA256: sha, ParentSnapshotID: parent,
+		ID: nextFreeID(&s.nextSnapID, func(id int64) bool { return s.snapshots[id] != nil }), DocID: docID, ContentSHA256: sha, ParentSnapshotID: parent,
 		BlockMap: blocks, SourceKind: source, ActorUserID: userID, ActorRef: actorRef,
 		ActorSnapshot: actor, SourceRunID: runID, CreatedAt: time.Now().UTC(),
 	}
@@ -961,9 +959,8 @@ func (s *Service) openVersionLocked(docID, origin string, baselineID, headID int
 			v.SealedAt = &now
 		}
 	}
-	s.nextVerID++
 	ver := &Version{
-		ID: s.nextVerID, DocID: docID, VersionNo: maxNo + 1, Status: VersionOpen,
+		ID: nextFreeID(&s.nextVerID, func(id int64) bool { return s.versions[id] != nil }), DocID: docID, VersionNo: maxNo + 1, Status: VersionOpen,
 		OriginKind: origin, BaselineSnapshotID: baselineID, HeadSnapshotID: headID,
 		SourceRunID: runID, OpenedAt: time.Now().UTC(),
 	}
@@ -972,9 +969,8 @@ func (s *Service) openVersionLocked(docID, origin string, baselineID, headID int
 }
 
 func (s *Service) putEventLocked(docID string, versionID int64, eventNo int, from *int64, to int64, kind, actorKind string, userID *int64, actorRef string, actor ActorSnapshot, hunks []DiffHunk, editSession, requestID, idemp string) *ChangeEvent {
-	s.nextEvtID++
 	ev := &ChangeEvent{
-		ID: s.nextEvtID, DocID: docID, VersionID: versionID, EventNo: eventNo,
+		ID: nextFreeID(&s.nextEvtID, func(id int64) bool { return s.events[id] != nil }), DocID: docID, VersionID: versionID, EventNo: eventNo,
 		FromSnapshotID: from, ToSnapshotID: to, ChangeKind: kind, ActorKind: actorKind,
 		ActorUserID: userID, ActorRef: actorRef, ActorSnapshot: actor, ChangeSet: hunks,
 		EditSessionID: editSession, RequestID: requestID, IdempotencyKey: idemp, CreatedAt: time.Now().UTC(),