diff --git a/packages/cachekit/src/backends/cachekitio-lockable.test.ts b/packages/cachekit/src/backends/cachekitio-lockable.test.ts
index ec79408..5bc810a 100644
--- a/packages/cachekit/src/backends/cachekitio-lockable.test.ts
+++ b/packages/cachekit/src/backends/cachekitio-lockable.test.ts
@@ -47,6 +47,18 @@ describe('LockableCachekitIO', () => {
     expect(await lockable.releaseLock('key', 'lock-id')).toBe(true);
   });
 
+  it('releaseLock sends lock_id in the X-CacheKit-Lock-Id header, not the URL (CWE-532)', async () => {
+    fetchSpy.mockResolvedValueOnce(mockResponse(200, { success: true }));
+    expect(await lockable.releaseLock('key', 'lock-secret')).toBe(true);
+    const [url, opts] = fetchSpy.mock.calls[0];
+    expect(opts.method).toBe('DELETE');
+    // Capability token rides the header under the exact wire name...
+    expect(opts.headers['X-CacheKit-Lock-Id']).toBe('lock-secret');
+    // ...and never appears in the URL (no query smuggling / log leak).
+    expect(url).not.toContain('lock_id');
+    expect(url).not.toContain('lock-secret');
+  });
+
   it('delegates get to inner backend', async () => {
     fetchSpy.mockResolvedValueOnce(new Response(null, { status: 404 }));
     expect(await lockable.get('missing')).toBeNull();
diff --git a/packages/cachekit/src/backends/cachekitio-lockable.ts b/packages/cachekit/src/backends/cachekitio-lockable.ts
index 5d74e09..c6aae1f 100644
--- a/packages/cachekit/src/backends/cachekitio-lockable.ts
+++ b/packages/cachekit/src/backends/cachekitio-lockable.ts
@@ -3,6 +3,14 @@ import { CachekitIOCore } from './cachekitio.js';
 import { BackendError, TimeoutError } from '../errors.js';
 import { classifyHttpError, classifyNetworkError } from './error-classifier.js';
 
+/**
+ * Lock capability token travels in this request header, never the query string
+ * (CWE-532): a `?lock_id=` query leaks the token into access/proxy logs and
+ * OpenTelemetry `http.url` spans. SaaS dual-reads header + legacy query during
+ * rollout, preferring the header. See protocol spec/saas-api.md.
+ */
+const LOCK_ID_HEADER = 'X-CacheKit-Lock-Id';
+
 export class LockableCachekitIO implements LockableBackend {
   constructor(private readonly inner: CachekitIOCore) {}
 
@@ -50,8 +58,10 @@ export class LockableCachekitIO implements LockableBackend {
 
   async releaseLock(key: string, lockId: string): Promise<boolean> {
     try {
-      const url = `${this.inner['apiUrl']}/v1/cache/${encodeURIComponent(key)}/lock?lock_id=${encodeURIComponent(lockId)}`;
-      const response = await this.inner.requestJson('DELETE', url);
+      const url = `${this.inner['apiUrl']}/v1/cache/${encodeURIComponent(key)}/lock`;
+      const response = await this.inner.requestJson('DELETE', url, undefined, {
+        [LOCK_ID_HEADER]: lockId,
+      });
       if (!response.ok)
         throw new BackendError(
           `Lock release failed (HTTP ${response.status})`,
diff --git a/packages/cachekit/src/backends/cachekitio.ts b/packages/cachekit/src/backends/cachekitio.ts
index 36f18aa..5a0e20f 100644
--- a/packages/cachekit/src/backends/cachekitio.ts
+++ b/packages/cachekit/src/backends/cachekitio.ts
@@ -157,10 +157,15 @@ export class CachekitIOCore implements Backend {
   }
 
   /** Package-internal: JSON request for lock/TTL decorators. Not part of public API. */
-  async requestJson(method: string, url: string, body?: unknown): Promise<Response> {
+  async requestJson(
+    method: string,
+    url: string,
+    body?: unknown,
+    headers?: Record<string, string>
+  ): Promise<Response> {
     return this.request(method, url, {
       body: body ? new TextEncoder().encode(JSON.stringify(body)) : undefined,
-      headers: body ? { 'Content-Type': 'application/json' } : {},
+      headers: { ...(body ? { 'Content-Type': 'application/json' } : {}), ...headers },
     });
   }