fix(db): surface SQLite backend in status + actionable WASM-fallback banner (#148)

Closes the visibility gap behind issues #138 (WASM-on-macOS) and #139
(MCP "database is locked"). `better-sqlite3` is in optionalDependencies,
so when the native build fails npm install still succeeds and the
runtime silently falls back to node-sqlite3-wasm — 5-10x slower and
without WAL, so writers block readers (which is what makes the MCP
server appear to "lock the DB" in #139). The only existing signal was
a one-line `console.warn` to stderr that MCP transports typically
swallow.

This patch does NOT change install behavior — better-sqlite3 stays in
optionalDependencies so cross-platform installs keep working. It just
makes the substitution observable + recoverable.

## Visibility (4 surfaces)

- CLI `codegraph status`: new `Backend:` line under Index Statistics.
  `native` rendered green; `wasm` rendered yellow with an inline
  `npm rebuild better-sqlite3` nudge. Also exposed in `--json` as
  `backend: 'native' | 'wasm'`.
- MCP `codegraph_status`: new `**Backend:**` line. Native form reads
  `native (better-sqlite3)`; wasm form prepends a warning glyph and
  includes the full fix recipe.
- Stderr banner on fallback (`buildWasmFallbackBanner`): replaces the
  bare one-line `console.warn` with a multi-line bordered banner
  covering macOS + Linux fix steps and optionally appending the
  native load error.
- README troubleshooting: new "Indexing is slow / MCP database is
  locked / WASM fallback active" entry that walks users to the
  `Backend:` line and the fix.

## Per-instance backend tracking

`createDatabase` previously set a module-level `activeBackend` global.
MCP can open multiple project DBs in one process via the
`getCodeGraph()` cache, so the global would race / overwrite. Refactor:
`createDatabase` now returns `{db, backend}`, `DatabaseConnection`
carries `private backend` and exposes `getBackend()`, and
`CodeGraph.getBackend()` is the public surface. The CLI and MCP both
call `cg.getBackend()`.

## What this does NOT fix

The root cause of users landing on WASM is environment-specific (Mac
without Xcode CLT, Node version mismatch, etc.) and not fixable in
code without changing the optionalDependencies design. The README
entry tells users what to run; `Backend: native` after rebuild is the
confirmation signal.

## Tests

New `__tests__/sqlite-backend.test.ts` (6 tests) pins the banner
recipe content (so future edits can't strip the recovery commands),
the `WASM_FALLBACK_FIX_RECIPE` constant, and per-instance
`DatabaseConnection.getBackend()` / `CodeGraph.getBackend()` reporting.
Suite: 503 → 509, all passing.

Credit to @andreinknv whose analysis on #138 (and patches on his fork
at 6d0e7a2 + 69f7001) framed the visibility approach.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -439,6 +439,30 @@ The `.codegraph/config.json` file controls indexing:
 
 **Indexing is slow** — Check that `node_modules` and other large directories are excluded. Use `--quiet` to reduce output overhead.
 
+**Indexing is slow / MCP `database is locked` / WASM fallback active** — `codegraph` ships with a WASM SQLite fallback for environments where `better-sqlite3` (a native module, declared as `optionalDependencies`) can't install. The fallback is 5-10x slower than the native backend and uses a journal mode that lets writers block readers, so MCP queries can also hit `database is locked` while indexing runs. Run `codegraph status` and look at the `Backend:` line:
+
+- `Backend: native` — you're on the fast path, nothing to do.
+- `Backend: wasm` — you're on the slow fallback. Common causes: missing C build tools, prebuilt binary unavailable for your Node version, or your Node version changed after install. Fix:
+
+  ```bash
+  # macOS
+  xcode-select --install                                  # installs the C compiler
+
+  # Linux (Debian / Ubuntu)
+  sudo apt install build-essential python3 make
+
+  # Linux (RHEL / Fedora)
+  sudo yum groupinstall "Development Tools"
+
+  # Then rebuild on any platform:
+  npm rebuild better-sqlite3
+
+  # Or force-include as a hard dep:
+  npm install better-sqlite3 --save
+  ```
+
+  After the fix, `codegraph status` should show `Backend: native`.
+
 **MCP server not connecting** — Ensure the project is initialized/indexed, verify the path in your MCP config, and check that `codegraph serve --mcp` works from the command line.
 
 **Missing symbols** — The MCP server auto-syncs on save (wait a couple seconds). Run `codegraph sync` manually if needed. Check that the file's language is supported and isn't excluded by config patterns.

__tests__/sqlite-backend.test.ts

@@ -0,0 +1,86 @@
+/**
+ * SQLite backend visibility tests
+ *
+ * Pins the WASM-fallback banner content + the per-instance backend
+ * tracking. Closes the visibility gap behind issue #138.
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import * as fs from 'fs';
+import * as path from 'path';
+import * as os from 'os';
+import {
+  buildWasmFallbackBanner,
+  WASM_FALLBACK_FIX_RECIPE,
+} from '../src/db/sqlite-adapter';
+import { DatabaseConnection } from '../src/db';
+import { CodeGraph } from '../src';
+
+describe('buildWasmFallbackBanner — fix-recipe content', () => {
+  it('includes the macOS / Linux / cross-platform fix commands', () => {
+    const banner = buildWasmFallbackBanner();
+    expect(banner).toContain('WASM SQLite fallback active');
+    expect(banner).toContain('5-10x slower');
+    expect(banner).toContain('xcode-select --install');
+    expect(banner).toContain('apt install build-essential');
+    expect(banner).toContain('npm rebuild better-sqlite3');
+    expect(banner).toContain('npm install better-sqlite3 --save');
+    expect(banner).toContain('codegraph status');
+  });
+
+  it('appends the native load error when one is provided', () => {
+    const banner = buildWasmFallbackBanner(
+      "Cannot find module 'better-sqlite3'"
+    );
+    expect(banner).toContain(
+      "Native load error: Cannot find module 'better-sqlite3'"
+    );
+  });
+
+  it('omits the load-error block when no error is supplied', () => {
+    const banner = buildWasmFallbackBanner();
+    expect(banner).not.toContain('Native load error:');
+  });
+});
+
+describe('WASM_FALLBACK_FIX_RECIPE — single source of truth', () => {
+  it('mentions the three recovery commands', () => {
+    expect(WASM_FALLBACK_FIX_RECIPE).toContain('xcode-select --install');
+    expect(WASM_FALLBACK_FIX_RECIPE).toContain('npm rebuild better-sqlite3');
+    expect(WASM_FALLBACK_FIX_RECIPE).toContain(
+      'npm install better-sqlite3 --save'
+    );
+  });
+});
+
+describe('DatabaseConnection — per-instance backend reporting', () => {
+  let dir: string;
+
+  beforeEach(() => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'codegraph-backend-'));
+  });
+
+  afterEach(() => {
+    if (fs.existsSync(dir)) {
+      fs.rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
+  it('reports a concrete backend (native or wasm) for an initialized DB', () => {
+    const dbPath = path.join(dir, 'test.db');
+    const conn = DatabaseConnection.initialize(dbPath);
+    const backend = conn.getBackend();
+    expect(['native', 'wasm']).toContain(backend);
+    conn.close();
+  });
+
+  it('CodeGraph.getBackend() delegates to the underlying DatabaseConnection', async () => {
+    fs.writeFileSync(path.join(dir, 'x.ts'), `export function x(): void {}\n`);
+    const cg = await CodeGraph.init(dir, { index: true });
+    try {
+      expect(['native', 'wasm']).toContain(cg.getBackend());
+    } finally {
+      cg.destroy();
+    }
+  });
+});

src/bin/codegraph.ts

@@ -643,6 +643,7 @@ program
       const cg = await CodeGraph.open(projectPath);
       const stats = cg.getStats();
       const changes = cg.getChangedFiles();
+      const backend = cg.getBackend();
 
       // JSON output mode
       if (options.json) {
@@ -653,6 +654,7 @@ program
           nodeCount: stats.nodeCount,
           edgeCount: stats.edgeCount,
           dbSizeBytes: stats.dbSizeBytes,
+          backend,
           nodesByKind: stats.nodesByKind,
           languages: Object.entries(stats.filesByLanguage).filter(([, count]) => count > 0).map(([lang]) => lang),
           pendingChanges: {
@@ -677,6 +679,14 @@ program
       console.log(`  Nodes:     ${formatNumber(stats.nodeCount)}`);
       console.log(`  Edges:     ${formatNumber(stats.edgeCount)}`);
       console.log(`  DB Size:   ${(stats.dbSizeBytes / 1024 / 1024).toFixed(2)} MB`);
+      // Surface the active SQLite backend so users can spot the silent
+      // WASM fallback (5-10x slower). better-sqlite3 is in
+      // `optionalDependencies`, so `npm install` succeeds without it
+      // when the native build fails.
+      const backendLabel = backend === 'native'
+        ? chalk.green('native')
+        : chalk.yellow('wasm — slower fallback; run `npm rebuild better-sqlite3`');
+      console.log(`  Backend:   ${backendLabel}`);
       console.log();
 
       // Node breakdown

src/db/index.ts

@@ -4,13 +4,13 @@
  * Handles SQLite database initialization and connection management.
  */
 
-import { SqliteDatabase, createDatabase } from './sqlite-adapter';
+import { SqliteDatabase, SqliteBackend, createDatabase } from './sqlite-adapter';
 import * as fs from 'fs';
 import * as path from 'path';
 import { SchemaVersion } from '../types';
 import { runMigrations, getCurrentVersion, CURRENT_SCHEMA_VERSION } from './migrations';
 
-export { SqliteDatabase, getActiveBackend } from './sqlite-adapter';
+export { SqliteDatabase, SqliteBackend, WASM_FALLBACK_FIX_RECIPE } from './sqlite-adapter';
 
 /**
  * Database connection wrapper with lifecycle management
@@ -18,10 +18,12 @@ export { SqliteDatabase, getActiveBackend } from './sqlite-adapter';
 export class DatabaseConnection {
   private db: SqliteDatabase;
   private dbPath: string;
+  private backend: SqliteBackend;
 
-  private constructor(db: SqliteDatabase, dbPath: string) {
+  private constructor(db: SqliteDatabase, dbPath: string, backend: SqliteBackend) {
     this.db = db;
     this.dbPath = dbPath;
+    this.backend = backend;
   }
 
   /**
@@ -35,7 +37,7 @@ export class DatabaseConnection {
     }
 
     // Create and configure database
-    const db = createDatabase(dbPath);
+    const { db, backend } = createDatabase(dbPath);
 
     // Enable foreign keys and WAL mode for better performance
     db.pragma('foreign_keys = ON');
@@ -62,7 +64,7 @@ export class DatabaseConnection {
       ).run(CURRENT_SCHEMA_VERSION, Date.now(), 'Initial schema includes all migrations');
     }
 
-    return new DatabaseConnection(db, dbPath);
+    return new DatabaseConnection(db, dbPath, backend);
   }
 
   /**
@@ -73,7 +75,7 @@ export class DatabaseConnection {
       throw new Error(`Database not found: ${dbPath}`);
     }
 
-    const db = createDatabase(dbPath);
+    const { db, backend } = createDatabase(dbPath);
 
     // Enable foreign keys and WAL mode
     db.pragma('foreign_keys = ON');
@@ -88,7 +90,7 @@ export class DatabaseConnection {
     db.pragma('mmap_size = 268435456');
 
     // Check and run migrations if needed
-    const conn = new DatabaseConnection(db, dbPath);
+    const conn = new DatabaseConnection(db, dbPath, backend);
     const currentVersion = getCurrentVersion(db);
 
     if (currentVersion < CURRENT_SCHEMA_VERSION) {
@@ -105,6 +107,15 @@ export class DatabaseConnection {
     return this.db;
   }
 
+  /**
+   * Get the SQLite backend serving this connection. Per-instance so
+   * MCP cross-project queries report the right backend even when
+   * multiple project DBs are open in the same process.
+   */
+  getBackend(): SqliteBackend {
+    return this.backend;
+  }
+
   /**
    * Get database file path
    */

src/db/sqlite-adapter.ts

@@ -22,13 +22,51 @@ export interface SqliteDatabase {
 
 export type SqliteBackend = 'native' | 'wasm';
 
-let activeBackend: SqliteBackend | null = null;
+/**
+ * One-line summary of the recovery steps shown when WASM fallback is
+ * active. Single source of truth so the recipe can't drift between the
+ * stderr banner and the MCP status formatter.
+ */
+export const WASM_FALLBACK_FIX_RECIPE =
+  '`xcode-select --install` (macOS) or `apt install build-essential` (Debian/Ubuntu), ' +
+  'then `npm rebuild better-sqlite3`, or `npm install better-sqlite3 --save` to force-include it.';
 
 /**
- * Get the currently active SQLite backend.
+ * Multi-line banner shown to stderr when `createDatabase` falls back to
+ * WASM. Replaces a one-line `console.warn` that MCP transports (which
+ * take stdout for the protocol) typically swallow, leaving users on a
+ * 5-10x slower backend with no signal.
+ *
+ * Exported for unit testing — pinning the recipe content prevents
+ * future edits from silently stripping the recovery commands.
  */
-export function getActiveBackend(): SqliteBackend | null {
-  return activeBackend;
+export function buildWasmFallbackBanner(nativeError?: string): string {
+  const sep = '─'.repeat(72);
+  const lines = [
+    sep,
+    '[CodeGraph] WASM SQLite fallback active (better-sqlite3 unavailable)',
+    sep,
+    'Indexing and sync will be 5-10x slower than the native backend.',
+    '',
+    'Fix on macOS:',
+    '  xcode-select --install        # install C build tools',
+    '  npm rebuild better-sqlite3    # rebuild native binding for current Node',
+    '',
+    'Fix on Linux:',
+    '  sudo apt install build-essential python3 make    # Debian/Ubuntu',
+    '  # or: sudo yum groupinstall "Development Tools"  # RHEL/Fedora',
+    '  npm rebuild better-sqlite3',
+    '',
+    'Or force-include as a hard dependency on any platform:',
+    '  npm install better-sqlite3 --save',
+    '',
+    'Verify after fix: `codegraph status` should show `Backend: native`.',
+  ];
+  if (nativeError) {
+    lines.push('', `Native load error: ${nativeError}`);
+  }
+  lines.push(sep);
+  return lines.join('\n');
 }
 
 /**
@@ -192,9 +230,13 @@ class WasmDatabaseAdapter implements SqliteDatabase {
 
 /**
  * Create a database connection. Tries native better-sqlite3 first,
- * falls back to node-sqlite3-wasm.
+ * falls back to node-sqlite3-wasm. Returns the active backend
+ * alongside the db so each `DatabaseConnection` can report its own
+ * backend per-instance — MCP can open multiple project DBs in one
+ * process (`tools.ts` getCodeGraph cache), so a process-global would
+ * race / overwrite.
  */
-export function createDatabase(dbPath: string): SqliteDatabase {
+export function createDatabase(dbPath: string): { db: SqliteDatabase; backend: SqliteBackend } {
   let nativeError: string | undefined;
   let wasmError: string | undefined;
 
@@ -203,8 +245,7 @@ export function createDatabase(dbPath: string): SqliteDatabase {
     // eslint-disable-next-line @typescript-eslint/no-require-imports
     const Database = require('better-sqlite3');
     const db = new Database(dbPath);
-    activeBackend = 'native';
-    return db as SqliteDatabase;
+    return { db: db as SqliteDatabase, backend: 'native' };
   } catch (error) {
     nativeError = error instanceof Error ? error.message : String(error);
   }
@@ -212,9 +253,8 @@ export function createDatabase(dbPath: string): SqliteDatabase {
   // Fall back to WASM
   try {
     const db = new WasmDatabaseAdapter(dbPath);
-    activeBackend = 'wasm';
-    console.warn('[CodeGraph] Using WASM SQLite backend (native better-sqlite3 unavailable)');
-    return db;
+    console.warn(buildWasmFallbackBanner(nativeError));
+    return { db, backend: 'wasm' };
   } catch (error) {
     wasmError = error instanceof Error ? error.message : String(error);
   }

src/index.ts

@@ -612,6 +612,16 @@ export class CodeGraph {
     return stats;
   }
 
+  /**
+   * Active SQLite backend for this project's connection. `wasm` means
+   * the native better-sqlite3 install failed and the WASM fallback is
+   * serving requests at 5-10x the latency. Surfaced via `codegraph
+   * status` and the `codegraph_status` MCP tool.
+   */
+  getBackend(): import('./db').SqliteBackend {
+    return this.db.getBackend();
+  }
+
   // ===========================================================================
   // Node Operations
   // ===========================================================================

src/mcp/tools.ts

@@ -11,6 +11,7 @@ import { writeFileSync, readFileSync, existsSync } from 'fs';
 import { clamp, validatePathWithinRoot } from '../utils';
 import { tmpdir } from 'os';
 import { join } from 'path';
+import { WASM_FALLBACK_FIX_RECIPE } from '../db';
 
 /** Maximum output length to prevent context bloat (characters) */
 const MAX_OUTPUT_LENGTH = 15000;
@@ -973,10 +974,23 @@ export class ToolHandler {
       `**Total nodes:** ${stats.nodeCount}`,
       `**Total edges:** ${stats.edgeCount}`,
       `**Database size:** ${(stats.dbSizeBytes / 1024 / 1024).toFixed(2)} MB`,
-      '',
-      '### Nodes by Kind:',
     ];
 
+    // Surface the active SQLite backend. Without this, users on the
+    // silent WASM fallback (better-sqlite3 install failed) see "slow"
+    // indexing and DB-lock errors with no signal of why.
+    const backend = cg.getBackend();
+    if (backend === 'native') {
+      lines.push(`**Backend:** native (better-sqlite3)`);
+    } else {
+      lines.push(
+        `**Backend:** ⚠ wasm (better-sqlite3 unavailable) — ` +
+        `5-10x slower than native. Fix: ${WASM_FALLBACK_FIX_RECIPE}`
+      );
+    }
+
+    lines.push('', '### Nodes by Kind:');
+
     for (const [kind, count] of Object.entries(stats.nodesByKind)) {
       if ((count as number) > 0) {
         lines.push(`- ${kind}: ${count}`);