diff --git a/config/scripts/vitest-sqlite-runtime-files.mjs b/config/scripts/vitest-sqlite-runtime-files.mjs
index ee5701d1159f..8275fe853d9e 100644
--- a/config/scripts/vitest-sqlite-runtime-files.mjs
+++ b/config/scripts/vitest-sqlite-runtime-files.mjs
@@ -5,6 +5,7 @@ export const SQLITE_RUNTIME_INCLUDE = [
   ...SQLITE_DATABASE_RUNTIME_INCLUDE,
   'src/main/acp/acp-structured-sign-in.test.ts',
   'src/main/acp/acp-structured-auth.test.ts',
+  'src/main/acp/acp-structured-prompt-failure.test.ts',
   'src/main/acp/acp-timeline-turn-failures.test.ts',
   'src/main/acp/acp-structured-session-adapter.test.ts',
   'src/main/pi/rpc-turn-races.test.ts',
diff --git a/src/main/acp/acp-dialects/grok-dialect.ts b/src/main/acp/acp-dialects/grok-dialect.ts
index 81d07fa34bc5..609a9c313c5e 100644
--- a/src/main/acp/acp-dialects/grok-dialect.ts
+++ b/src/main/acp/acp-dialects/grok-dialect.ts
@@ -177,10 +177,7 @@ export const GROK_ACP_DIALECT: AcpDialect = {
   toolBackgroundTasks: grokToolBackgroundTasks,
   notification,
   contextWindow,
-  promptErrorDetail: (error) =>
-    typeof error.data === 'string'
-      ? error.data
-      : promptErrorDataSchema.safeParse(error.data).data?.message,
+  promptErrorDetail: (error) => promptErrorDataSchema.safeParse(error.data).data?.message,
   failedTurnText: (stopReason) =>
     stopReason === 'rate_limit'
       ? 'Grok usage limit reached.'
diff --git a/src/main/acp/acp-dialects/omp-dialect.ts b/src/main/acp/acp-dialects/omp-dialect.ts
index 0e38a7c2db56..d09c455a7292 100644
--- a/src/main/acp/acp-dialects/omp-dialect.ts
+++ b/src/main/acp/acp-dialects/omp-dialect.ts
@@ -96,7 +96,6 @@ function normalizeToolUpdate(update: ToolCallUpdate): ToolCallUpdate {
 
 export const OMP_ACP_DIALECT: AcpDialect = {
   normalizeToolUpdate,
-  promptErrorDetail: (error) => promptErrorDataSchema.safeParse(error.data).data?.details,
   authenticationRequired: (error) => {
     const details = promptErrorDataSchema.safeParse(error.data).data?.details
     return (
diff --git a/src/main/acp/acp-structured-prompt-failure.test.ts b/src/main/acp/acp-structured-prompt-failure.test.ts
new file mode 100644
index 000000000000..44c452504803
--- /dev/null
+++ b/src/main/acp/acp-structured-prompt-failure.test.ts
@@ -0,0 +1,109 @@
+import { afterEach, describe, expect, it } from 'vitest'
+import { z } from 'zod'
+import { closeProviderTimelineRigs } from '../native-chat/agent-session-timeline/provider-timeline-assembler-test-support'
+import { ACP_LAUNCH_SPECS, type AcpLaunchSpec } from './acp-launch-specs'
+import {
+  openAcpAdapterRig,
+  PROVIDER_SESSION,
+  sendHello
+} from './acp-structured-adapter.test-support'
+
+const promptMeta = z.object({ _meta: z.object({ promptId: z.string() }).optional() })
+afterEach(closeProviderTimelineRigs)
+
+function specFor(agent: string): AcpLaunchSpec {
+  const spec = ACP_LAUNCH_SPECS.find((entry) => entry.agent === agent)
+  if (!spec) {
+    throw new Error(`${agent} launch specification missing`)
+  }
+  return spec
+}
+
+/** Fails the first prompt with `code`, after the turn started unless `beforeTurn`. */
+async function failFirstPrompt(
+  spec: AcpLaunchSpec,
+  error: { code: number; message: string; data?: unknown },
+  beforeTurn = false
+) {
+  const rig = await openAcpAdapterRig({
+    spec,
+    script: (agent) =>
+      agent.on('session/prompt', (frame) => {
+        if (!beforeTurn) {
+          agent.notify('session/update', {
+            sessionId: PROVIDER_SESSION,
+            _meta: promptMeta.parse(frame.params)._meta,
+            update: {
+              sessionUpdate: 'agent_message_chunk',
+              content: { type: 'text', text: 'started' }
+            }
+          })
+        }
+        agent.fail(frame, error.code, error.message, error.data)
+      })
+  })
+  await rig.acquire()
+  await sendHello(rig, 'first')
+  await rig.settle()
+  const failures = (await rig.rig.rows()).flatMap(({ body }) =>
+    body.kind === 'status' && body.tone === 'error' ? [body] : []
+  )
+  return { rig, failures }
+}
+
+const WORDS = 'Model quota exceeded for this workspace; try again in 5 minutes.'
+// OpenCode 1.18.31's recorded answer when its model provider fails: data is only metadata.
+const OPENCODE_PROVIDER_ERROR = {
+  code: -32603,
+  message: 'Internal error: CAPTURE_PROVIDER_400',
+  data: { service: 'session', errorName: 'APIError' }
+}
+
+describe.each(['opencode', 'grok'])('%s prompt failure detail', (agent) => {
+  const spec = specFor(agent)
+
+  it.each([
+    ['data.details', { code: -32603, message: 'Internal error', data: { details: WORDS } }, WORDS],
+    ['string data', { code: -32603, message: 'Internal error', data: WORDS }, WORDS],
+    ['no data', { code: -32603, message: 'Internal error' }, 'Internal error'],
+    ['metadata-only data', OPENCODE_PROVIDER_ERROR, 'Internal error: CAPTURE_PROVIDER_400']
+  ])('shows the agent words from %s', async (_label, error, text) => {
+    const { failures } = await failFirstPrompt(spec, error)
+    expect(failures).toHaveLength(1)
+    expect(failures[0]?.text).toBe(text)
+    expect(failures[0]?.failure?.kind).not.toBe('notSignedIn')
+  })
+
+  it('quotes data.details after the sign-in sentence', async () => {
+    const { failures } = await failFirstPrompt(spec, {
+      code: -32000,
+      message: 'Authentication required',
+      data: { details: WORDS }
+    })
+    expect(failures).toHaveLength(1)
+    expect(failures[0]?.failure).toMatchObject({ kind: 'notSignedIn' })
+    expect(failures[0]?.text).toMatch(/^Sign in to .+\./)
+    expect(failures[0]?.text).toContain(WORDS)
+    expect(failures[0]?.text).not.toContain('Authentication required')
+  })
+})
+
+describe('prompt refused before its turn began', () => {
+  const spec = specFor('grok')
+
+  it.each([
+    [{ code: -32603, message: 'Internal error', data: { details: WORDS } }, WORDS],
+    [OPENCODE_PROVIDER_ERROR, 'Internal error: CAPTURE_PROVIDER_400']
+  ])('quotes the agent words: %j', async (error, text) => {
+    const { rig, failures } = await failFirstPrompt(spec, error, true)
+    expect(failures).toHaveLength(0)
+    const rejected = rig.settled.find((item) => 'state' in item && item.state === 'rejected')
+    expect(rejected).toMatchObject({
+      clientMessageId: 'first',
+      rejection: expect.objectContaining({
+        kind: 'providerRejected',
+        detail: { text, audience: 'person' }
+      })
+    })
+  })
+})
diff --git a/src/main/acp/acp-turn-failures.ts b/src/main/acp/acp-turn-failures.ts
index f5f5f5dd463a..ba87ec6a5050 100644
--- a/src/main/acp/acp-turn-failures.ts
+++ b/src/main/acp/acp-turn-failures.ts
@@ -17,9 +17,21 @@ function acpStopReasonFailed(stopReason: string): boolean {
   return FAILED_STOP_REASONS.includes(stopReason)
 }
 
-/** The provider's words in its error answer to `session/prompt`. */
+/** The provider's words in its error answer to `session/prompt`. Agents often answer a generic
+ *  message ("Internal error") and keep their own words in `data`. */
 export function acpPromptErrorDetail(dialect: AcpDialect, error: AcpAgentError): string {
-  return dialect.promptErrorDetail?.(error) ?? error.message
+  return dialect.promptErrorDetail?.(error) ?? acpErrorDataWords(error.data) ?? error.message
+}
+
+// Other structured data is metadata (service, error class names), not words for a person.
+function acpErrorDataWords(data: unknown): string | undefined {
+  const words =
+    typeof data === 'string'
+      ? data
+      : typeof data === 'object' && data !== null && 'details' in data
+        ? data.details
+        : undefined
+  return typeof words === 'string' && words.trim() ? words : undefined
 }
 
 export function acpAuthenticationRequired(dialect: AcpDialect, error: unknown): boolean {