diff --git a/CHANGELOG.md b/CHANGELOG.md
index 75535a0bbd..f40f20567c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -145,6 +145,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixes
 
+- `codegraph sync` no longer reports success when it could not take the index lock: it exits with status 1 and prints one line on stderr, also with `--quiet`. When another process (an MCP server or another CLI mid-index) holds the lock, the line names it and asks you to retry; any other lock problem keeps its `codegraph unlock` hint. A script of your own that checks the exit code can now tell a skipped sync from an up-to-date one.
 - Rust calls on `self` now stay with the enclosing type instead of linking to an unrelated type’s same-named method. Thanks @L4XB. (#1861)
 
 - Turning telemetry off now resets its identity and stops running processes from recording, sending, or restoring unsent data. (#1869)
diff --git a/__tests__/cli-sync-reports-reason.test.ts b/__tests__/cli-sync-reports-reason.test.ts
new file mode 100644
index 0000000000..c746b24ba9
--- /dev/null
+++ b/__tests__/cli-sync-reports-reason.test.ts
@@ -0,0 +1,106 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { spawnSync } from 'child_process';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { CodeGraph } from '../src';
+
+const BIN = path.resolve(__dirname, '../dist/bin/codegraph.js');
+let root: string;
+
+function run(...args: string[]) {
+  return spawnSync(process.execPath, [BIN, ...args], {
+    cwd: root,
+    encoding: 'utf8',
+    timeout: 30_000,
+    // NODE_NO_WARNINGS: skipping the relaunch also skips its
+    // --disable-warning=ExperimentalWarning, and Node 22 then prints the
+    // node:sqlite warning to stderr, which the quiet case counts line by line.
+    env: { ...process.env, CODEGRAPH_NO_DAEMON: '1', CODEGRAPH_WASM_RELAUNCHED: '1', NO_COLOR: '1', NODE_NO_WARNINGS: '1' },
+  });
+}
+
+const lockPath = () => path.join(root, '.codegraph', 'codegraph.lock');
+
+beforeEach(async () => {
+  root = fs.mkdtempSync(path.join(os.tmpdir(), 'codegraph-sync-locked-'));
+  fs.writeFileSync(path.join(root, 'original.ts'), 'export function original() { return 1; }\n');
+  const cg = CodeGraph.initSync(root);
+  try { await cg.indexAll(); } finally { cg.close(); }
+});
+
+afterEach(() => fs.rmSync(root, { recursive: true, force: true }));
+
+describe('codegraph sync while another process holds the index lock', () => {
+  it('sync() reports the lock instead of all-zero success counts', async () => {
+    // The test process itself is the live holder — the same thing FileLock.acquire writes.
+    fs.writeFileSync(lockPath(), String(process.pid));
+    const cg = CodeGraph.openSync(root);
+    try {
+      const result = await cg.sync();
+      expect(result.skippedReason).toBe('locked');
+      expect(result.lockHolderPid).toBe(process.pid);
+      expect(result.filesChecked).toBe(0);
+    } finally { cg.close(); }
+  });
+
+  it.each([false, true])('CLI exits 1 with the reason on stderr, quiet=%s', (quiet) => {
+    fs.writeFileSync(path.join(root, 'added.ts'), 'export const added = 2;\n');
+    fs.writeFileSync(lockPath(), String(process.pid));
+
+    const result = run('sync', ...(quiet ? ['--quiet'] : []));
+    expect(result.status).toBe(1);
+    expect(result.stderr).toContain('another process holds the index lock');
+    expect(result.stderr).toContain(`PID ${process.pid}`);
+    expect(result.stderr).toContain('codegraph sync');
+    expect(result.stdout + result.stderr).not.toContain('Already up to date');
+    if (quiet) {
+      expect(result.stdout).toBe('');
+      expect(result.stderr.trim().split('\n')).toHaveLength(1);
+    }
+
+    // Nothing was synced and the foreign lock was left alone.
+    expect(fs.readFileSync(lockPath(), 'utf8')).toBe(String(process.pid));
+    const cg = CodeGraph.openSync(root);
+    try { expect(cg.getStats().fileCount).toBe(1); } finally { cg.close(); }
+  });
+
+  it('does not call a lock nobody holds contention, and keeps its unlock hint', async () => {
+    // A directory where the lock file should be: acquire fails, but no process holds it.
+    fs.mkdirSync(lockPath());
+    const cg = CodeGraph.openSync(root);
+    try {
+      const result = await cg.sync();
+      expect(result.skippedReason).toBe('lock-failed');
+      expect(result.lockHolderPid).toBeUndefined();
+      expect(result.lockError).toContain('codegraph unlock');
+    } finally { cg.close(); }
+
+    const cli = run('sync', '--quiet');
+    expect(cli.status).toBe(1);
+    expect(cli.stderr).not.toContain('another process holds the index lock');
+    expect(cli.stderr).toContain('the index lock could not be taken');
+    expect(cli.stderr).toContain('codegraph unlock');
+  });
+
+  it('does not call a lock left by a dead process contention', async () => {
+    // A PID that cannot be alive: sync reclaims the stale lock and runs.
+    fs.writeFileSync(lockPath(), '2147483646');
+    fs.writeFileSync(path.join(root, 'added.ts'), 'export const added = 2;\n');
+    const cg = CodeGraph.openSync(root);
+    try {
+      const result = await cg.sync();
+      expect(result.skippedReason).toBeUndefined();
+      expect(result.filesAdded).toBe(1);
+    } finally { cg.close(); }
+  });
+
+  it.each([false, true])('syncs normally once the lock is released, quiet=%s', (quiet) => {
+    fs.writeFileSync(path.join(root, 'added.ts'), 'export const added = 2;\n');
+    const result = run('sync', ...(quiet ? ['--quiet'] : []));
+    expect(result.status).toBe(0);
+    expect(result.stderr).not.toContain('index lock');
+    const cg = CodeGraph.openSync(root);
+    try { expect(cg.getStats().fileCount).toBe(2); } finally { cg.close(); }
+  });
+});
diff --git a/src/bin/codegraph.ts b/src/bin/codegraph.ts
index a6fe54c98e..8aaa81b15a 100644
--- a/src/bin/codegraph.ts
+++ b/src/bin/codegraph.ts
@@ -61,6 +61,7 @@ import { BROWSER_ENV, DEFAULT_UI_PORT } from '../ui-server/constants';
 import type { UiServerHandle } from '../ui-server';
 import { lookupSymbolNodes, describeSymbolNode, groupDefinitions } from '../graph/symbol-lookup';
 import type { Node, Edge } from '../types';
+import type { SyncResult } from '../extraction';
 import { isTestPath } from '../search/query-utils';
 
 // Decided once, before `--color`/`--no-color` are stripped from argv below
@@ -955,9 +956,29 @@ program
       const { default: CodeGraph } = await loadCodeGraph();
       const cg = await CodeGraph.open(projectPath);
 
+      // sync() returns all-zero counts when it could not take the index lock.
+      // That is not "up to date": say why, and exit 1 even under --quiet,
+      // which suppresses progress but never a failure. Only a live holder is
+      // contention; any other lock failure keeps the lock's own message.
+      const lockedMessage = (result: SyncResult): string | null => {
+        if (result.skippedReason === 'locked') {
+          const holder = result.lockHolderPid != null ? ` (PID ${result.lockHolderPid})` : '';
+          return `Nothing synced: another process holds the index lock${holder}. Retry, or run "codegraph sync" when it exits.`;
+        }
+        if (result.skippedReason === 'lock-failed') {
+          return `Nothing synced: the index lock could not be taken. ${result.lockError ?? ''}`.trimEnd();
+        }
+        return null;
+      };
+
       if (options.quiet) {
-        await cg.sync();
+        const result = await cg.sync();
         cg.destroy();
+        const locked = lockedMessage(result);
+        if (locked) {
+          process.stderr.write(`codegraph sync: ${locked}\n`);
+          process.exit(1);
+        }
         return;
       }
 
@@ -973,6 +994,12 @@ program
 
       await progress.stop();
 
+      const locked = lockedMessage(result);
+      if (locked) {
+        cg.destroy();
+        throw new Error(locked);
+      }
+
       const totalChanges = result.filesAdded + result.filesModified + result.filesRemoved;
 
       if (totalChanges === 0) {
diff --git a/src/extraction/index.ts b/src/extraction/index.ts
index 743f5f42ed..23abe1d76c 100644
--- a/src/extraction/index.ts
+++ b/src/extraction/index.ts
@@ -125,6 +125,18 @@ export interface SyncResult {
   filesRemoved: number;
   nodesUpdated: number;
   durationMs: number;
+  /**
+   * Set when NO reconciliation ran because the index lock
+   * (`.codegraph/codegraph.lock`) could not be taken: `locked` when a live
+   * process holds it (an MCP server or another CLI mid-sync), `lock-failed`
+   * for anything else (an unreadable lock, a directory in its place). The
+   * counts above are then all zero and must not be read as "up to date".
+   */
+  skippedReason?: 'locked' | 'lock-failed';
+  /** PID of the live process holding the lock (`locked`). */
+  lockHolderPid?: number;
+  /** The lock's own error message, with its `codegraph unlock` hint (`lock-failed`). */
+  lockError?: string;
   changedFilePaths?: string[];
   /** Paths not absorbed because reading or extraction failed; retain for status/retry. */
   failedFilePaths?: string[];
diff --git a/src/index.ts b/src/index.ts
index 7b8ef2ca78..5f15415d5c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -785,8 +785,17 @@ export class CodeGraph {
     return this.indexMutex.withLock(async () => {
       try {
         this.fileLock.acquire();
-      } catch {
-        return { filesChecked: 0, filesAdded: 0, filesModified: 0, filesRemoved: 0, nodesUpdated: 0, durationMs: 0 };
+      } catch (err) {
+        // Contention is a live process named in the lock file. Anything else
+        // (an unreadable lock, a directory where the file should be) is not
+        // contention, and keeps the lock's own message and its unlock hint.
+        const lockHolderPid = this.fileLock.readLiveHolderPid();
+        return {
+          ...(lockHolderPid != null
+            ? { skippedReason: 'locked' as const, lockHolderPid }
+            : { skippedReason: 'lock-failed' as const, lockError: err instanceof Error ? err.message : String(err) }),
+          filesChecked: 0, filesAdded: 0, filesModified: 0, filesRemoved: 0, nodesUpdated: 0, durationMs: 0,
+        };
       }
       // Defer WAL auto-checkpointing for the whole incremental run, exactly
       // as indexAll does for the bulk path (#1231): sync's store loop and its
diff --git a/src/utils.ts b/src/utils.ts
index dc860a3f49..37627e573b 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -295,6 +295,19 @@ export class FileLock {
     }
   }
 
+  /**
+   * PID written in the lock file when that process is alive — real contention —
+   * or null when the file is absent, unreadable, or names a dead process.
+   */
+  readLiveHolderPid(): number | null {
+    try {
+      const pid = parseInt(fs.readFileSync(this.lockPath, 'utf-8').trim(), 10);
+      return !Number.isNaN(pid) && this.isProcessAlive(pid) ? pid : null;
+    } catch {
+      return null;
+    }
+  }
+
   /**
    * Release the lock
    */