diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 38b0d8e..4a7b9f0 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -5,6 +5,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.8.5] - 2026-05-23
+
+Bugfix release. Removes the last concurrent-writer path that could still reach
+`SQLITE_BUSY` after 0.8.4 (RC5 in the SQLite busy incident postmortem).
+
+### Fixed
+
+- **Foreground `index_documents` could write concurrently with the file-watcher** — `server.callIndexDocuments` called `Engine.IndexDocuments` directly, bypassing the `indexWithLock` guard (`re.indexing`) that only the startup and file-watcher paths used. So a manual `index_documents` and a background watcher index could write to `vectors.db` at the same time. 0.8.4 made them queue politely (busy_timeout up to 5s), but a heavy index holding a write transaction longer than that could still hit `SQLITE_BUSY`. `IndexDocuments` now holds a dedicated `Engine.indexMu` for its whole duration, serialising every indexing run regardless of caller. The watcher's `indexing` flag is unchanged (it still coalesces debounced ticks); foreground callers now wait on the mutex and index for real instead of being skipped. Regression test `TestIndexDocumentsSerialisesWriters` asserts a second call blocks while the lock is held and completes once released. See `06-planning/2026-05-05-sqlite-busy-incident.md` §7 RC5.
+
 ## [0.8.4] - 2026-05-23
 
 Bugfix release. Closes the SQLITE_BUSY recurrence on `index_documents` that the
diff --git a/internal/rag/rag.go b/internal/rag/rag.go
index e736bde..b676384 100644
--- a/internal/rag/rag.go
+++ b/internal/rag/rag.go
@@ -95,6 +95,14 @@ type Engine struct {
 	vecService     *vectorService
 	mu             sync.Mutex
 	indexing       bool
+	// indexMu serialises the whole IndexDocuments write path so two indexing
+	// runs never write to vectors.db concurrently. The file-watcher path
+	// (indexWithLock) coalesces debounced ticks via the `indexing` flag, but
+	// the foreground index_documents MCP tool calls IndexDocuments directly —
+	// without this mutex the two could race and hit SQLITE_BUSY (RC5 in
+	// 06-planning/2026-05-05-sqlite-busy-incident.md). Foreground callers wait
+	// here and then index for real, rather than being skipped.
+	indexMu        sync.Mutex
 	lastIndexCheck time.Time
 	stopWatcher    chan struct{}
 	stopOnce       sync.Once
@@ -405,6 +413,11 @@ func (re *Engine) IndexDocuments(ctx context.Context) error {
 		return fmt.Errorf("RAG engine not available")
 	}
 
+	// Serialise all indexing runs (foreground tool + background watcher) so
+	// they never write to vectors.db concurrently. See Engine.indexMu.
+	re.indexMu.Lock()
+	defer re.indexMu.Unlock()
+
 	startTime := time.Now().UTC()
 	// chunkerVersion bumps trigger a full rebuild on next IndexDocuments.
 	//   char-v1     — naive char-budget splitter (initial)
diff --git a/internal/rag/rag_test.go b/internal/rag/rag_test.go
index 7bc90f5..0237a0c 100644
--- a/internal/rag/rag_test.go
+++ b/internal/rag/rag_test.go
@@ -182,6 +182,84 @@ func TestIndexDocumentsRecoversAfterDirtyCommitFailure(t *testing.T) {
 	}
 }
 
+// TestIndexDocumentsSerialisesWriters guards RC5: IndexDocuments must hold the
+// engine's write lock for its whole duration so the foreground index_documents
+// tool and the background file-watcher never write to vectors.db concurrently.
+// We hold indexMu (simulating an in-progress run) and assert a second call
+// blocks until it is released, then completes for real.
+func TestIndexDocumentsSerialisesWriters(t *testing.T) {
+	repoRoot := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(repoRoot, "docs"), 0o755); err != nil {
+		t.Fatalf("mkdir docs: %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(repoRoot, "docs", "runbook.md"), []byte("# Runbook\nrollback ingress safely"), 0o644); err != nil {
+		t.Fatalf("write runbook: %v", err)
+	}
+
+	embeddingServer := newTestEmbeddingServer(t)
+	defer embeddingServer.Close()
+
+	emb, err := embedder.New(embedder.Config{
+		OllamaBaseURL: embeddingServer.URL,
+		Dimension:     2,
+		Mode:          "local-only",
+		MaxRetries:    0,
+		Timeout:       time.Second,
+	}, zap.NewNop())
+	if err != nil {
+		t.Fatalf("embedder.New: %v", err)
+	}
+
+	store, err := vectorstore.NewSQLiteStore(filepath.Join(t.TempDir(), "vectors.db"), 2, zap.NewNop())
+	if err != nil {
+		t.Fatalf("NewSQLiteStore: %v", err)
+	}
+	t.Cleanup(func() { _ = store.Close() })
+
+	engine := &Engine{
+		config:   config.Config{RootPath: repoRoot, RAGMaxResults: 10},
+		repoRoot: repoRoot,
+		logger:   zap.NewNop(),
+		docService: newDocumentService(docServiceConfig{
+			RepoRoot:     repoRoot,
+			IndexDirs:    []string{"docs"},
+			ChunkSize:    2000,
+			ChunkOverlap: 200,
+		}, zap.NewNop()),
+		vecService: &vectorService{
+			config: vecServiceConfig{Embedder: emb, MaxResults: 10},
+			logger: zap.NewNop(),
+			store:  store,
+		},
+		stopWatcher: make(chan struct{}),
+	}
+
+	// Simulate an indexing run already holding the writer lock.
+	engine.indexMu.Lock()
+
+	done := make(chan error, 1)
+	go func() { done <- engine.IndexDocuments(context.Background()) }()
+
+	select {
+	case <-done:
+		engine.indexMu.Unlock()
+		t.Fatal("IndexDocuments ran while indexMu was held — writers are not serialised")
+	case <-time.After(150 * time.Millisecond):
+		// Expected: the second run is blocked on indexMu.
+	}
+
+	engine.indexMu.Unlock()
+
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatalf("IndexDocuments after lock release: %v", err)
+		}
+	case <-time.After(3 * time.Second):
+		t.Fatal("IndexDocuments did not complete after indexMu was released")
+	}
+}
+
 func TestCalculateIndexChanges_NewFiles(t *testing.T) {
 	e := &Engine{}