diff --git a/src/lib/api.ts b/src/lib/api.ts
index e6de2ba..f4e77a5 100644
--- a/src/lib/api.ts
+++ b/src/lib/api.ts
@@ -33,8 +33,14 @@ export class ApiError extends Error {
     readonly method: string,
     readonly path: string,
     readonly detail: string,
+    // Per-request id the server stamps on every response; quote it to us so a
+    // failure maps to an exact log line. Absent only for pre-request failures.
+    readonly requestId?: string,
   ) {
-    super(`${method} ${path} failed: ${status} ${detail}`)
+    super(
+      `${method} ${path} failed: ${status} ${detail}` +
+        (requestId ? ` (request id: ${requestId})` : ''),
+    )
     this.name = 'ApiError'
   }
 }
@@ -67,7 +73,8 @@ export class ApiClient {
       body: body === undefined ? undefined : JSON.stringify(body),
     })
     if (!res.ok) {
-      throw new ApiError(res.status, method, path, await errorDetail(res))
+      const { detail, requestId } = await parseErrorResponse(res)
+      throw new ApiError(res.status, method, path, detail, requestId)
     }
     // Some endpoints (DELETEs, acks) return empty bodies; tolerate that.
     const text = await res.text()
@@ -211,15 +218,25 @@ export function buildQuery(query?: Record<string, unknown>): string {
   return qs ? `?${qs}` : ''
 }
 
-// Surface the server's `{"detail": ...}` message when present; fall back to the
-// status text. Keeps `agent` error output actionable instead of bare codes.
-export async function errorDetail(res: Response): Promise<string> {
+// Pull the server's `{"detail": ...}` message and request id off a non-2xx
+// response. The detail keeps `agent` error output actionable instead of bare
+// codes; the id comes from the `X-Request-ID` header (set on every API response)
+// and falls back to the `request_id` field the server's 500 handler includes in
+// its body, so an error carries something we can grep our logs for.
+export async function parseErrorResponse(
+  res: Response,
+): Promise<{ detail: string; requestId?: string }> {
+  const headerRequestId = res.headers.get('x-request-id') ?? undefined
   try {
-    const body = (await res.json()) as { detail?: unknown }
-    if (typeof body.detail === 'string') return body.detail
-    if (body.detail) return JSON.stringify(body.detail)
+    const body = (await res.json()) as { detail?: unknown; request_id?: unknown }
+    const requestId =
+      headerRequestId ??
+      (typeof body.request_id === 'string' ? body.request_id : undefined)
+    if (typeof body.detail === 'string') return { detail: body.detail, requestId }
+    if (body.detail) return { detail: JSON.stringify(body.detail), requestId }
+    return { detail: res.statusText, requestId }
   } catch {
     // not JSON
+    return { detail: res.statusText, requestId: headerRequestId }
   }
-  return res.statusText
 }
diff --git a/test/api.test.ts b/test/api.test.ts
index bac4f27..76d9b12 100644
--- a/test/api.test.ts
+++ b/test/api.test.ts
@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
-import { ApiClient, ApiError, buildQuery, errorDetail } from '../src/lib/api'
+import { ApiClient, ApiError, buildQuery, parseErrorResponse } from '../src/lib/api'
 
 describe('buildQuery', () => {
   it('returns empty string for no/empty query', () => {
@@ -22,24 +22,55 @@ describe('buildQuery', () => {
   })
 })
 
-describe('errorDetail', () => {
+describe('parseErrorResponse', () => {
   it('extracts a string detail', async () => {
     const res = new Response(JSON.stringify({ detail: 'Invalid credentials' }), {
       status: 401,
     })
-    expect(await errorDetail(res)).toBe('Invalid credentials')
+    expect((await parseErrorResponse(res)).detail).toBe('Invalid credentials')
   })
 
   it('stringifies a structured detail (FastAPI 422)', async () => {
     const res = new Response(JSON.stringify({ detail: [{ loc: ['query', 'limit'] }] }), {
       status: 422,
     })
-    expect(await errorDetail(res)).toContain('limit')
+    expect((await parseErrorResponse(res)).detail).toContain('limit')
   })
 
   it('falls back to status text for non-JSON bodies', async () => {
     const res = new Response('<html>oops</html>', { status: 502, statusText: 'Bad Gateway' })
-    expect(await errorDetail(res)).toBe('Bad Gateway')
+    expect((await parseErrorResponse(res)).detail).toBe('Bad Gateway')
+  })
+
+  it('reads the request id from the X-Request-ID header', async () => {
+    const res = new Response(JSON.stringify({ detail: 'boom' }), {
+      status: 500,
+      headers: { 'x-request-id': 'request_abc123' },
+    })
+    expect(await parseErrorResponse(res)).toEqual({
+      detail: 'boom',
+      requestId: 'request_abc123',
+    })
+  })
+
+  it('falls back to the body request_id when the header is absent', async () => {
+    const res = new Response(
+      JSON.stringify({ detail: 'Internal Server Error', request_id: 'request_xyz' }),
+      { status: 500 },
+    )
+    expect((await parseErrorResponse(res)).requestId).toBe('request_xyz')
+  })
+
+  it('still parses a non-JSON body that carries the header', async () => {
+    const res = new Response('<html>oops</html>', {
+      status: 502,
+      statusText: 'Bad Gateway',
+      headers: { 'x-request-id': 'request_gw' },
+    })
+    expect(await parseErrorResponse(res)).toEqual({
+      detail: 'Bad Gateway',
+      requestId: 'request_gw',
+    })
   })
 })
 
@@ -80,6 +111,26 @@ describe('ApiClient.request', () => {
     await expect(api.whoami()).rejects.toThrow(/403 nope/)
   })
 
+  it('surfaces the request id from the response on non-2xx', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(
+        async () =>
+          new Response(JSON.stringify({ detail: 'Internal Server Error' }), {
+            status: 500,
+            headers: { 'x-request-id': 'request_deadbeef' },
+          }),
+      ),
+    )
+    const api = new ApiClient('http://api.test', 't')
+    await expect(api.whoami()).rejects.toMatchObject({
+      name: 'ApiError',
+      status: 500,
+      requestId: 'request_deadbeef',
+    })
+    await expect(api.whoami()).rejects.toThrow(/request id: request_deadbeef/)
+  })
+
   it('tolerates empty response bodies', async () => {
     vi.stubGlobal('fetch', vi.fn(async () => new Response(null, { status: 204 })))
     const api = new ApiClient('http://api.test', 't')