diff --git a/packages/core/src/session.ts b/packages/core/src/session.ts
index fb29abb545..a120758a10 100644
--- a/packages/core/src/session.ts
+++ b/packages/core/src/session.ts
@@ -930,6 +930,8 @@ export interface TurnStateMessage {
   /** Diagnostic source for user/renderer-triggered aborts, e.g. renderer.stop_button. */
   abortSource?: string;
   errorClass?: string;
+  /** Bounded provider response summary for a failed turn. */
+  failureMessage?: string;
   partialOutputRetained: boolean;
 }
 
@@ -1056,6 +1058,8 @@ export interface TurnRecord {
   abortedAt?: number;
   abortSource?: string;
   errorClass?: string;
+  /** Bounded provider response summary for a failed turn. */
+  failureMessage?: string;
   partialOutputRetained: boolean;
 }
 
@@ -1163,6 +1167,7 @@ const TURN_STATE_MESSAGE_SHAPE = defineObjectShape<TurnStateMessage>()(
     'abortedAt',
     'abortSource',
     'errorClass',
+    'failureMessage',
   ],
 );
 const WORKHUB_DELEGATION_ASSIGNED_MESSAGE_SHAPE =
@@ -1409,7 +1414,8 @@ function decodeMessage(
         isOptionalString(message.parentSessionId) &&
         (message.abortedAt === undefined || isFiniteNumber(message.abortedAt)) &&
         isOptionalString(message.abortSource) &&
-        isOptionalString(message.errorClass)
+        isOptionalString(message.errorClass) &&
+        isOptionalString(message.failureMessage)
       )
         return message as unknown as TurnStateMessage;
       break;
@@ -1656,6 +1662,7 @@ export function deriveTurnRecords(messages: readonly StoredMessage[]): TurnRecor
         ...(latestState.abortedAt !== undefined ? { abortedAt: latestState.abortedAt } : {}),
         ...(latestState.abortSource ? { abortSource: latestState.abortSource } : {}),
         ...(latestState.errorClass ? { errorClass: latestState.errorClass } : {}),
+        ...(latestState.failureMessage ? { failureMessage: latestState.failureMessage } : {}),
         partialOutputRetained: latestState.partialOutputRetained || partialOutputRetained,
       };
     }
diff --git a/packages/runtime-host/src/__tests__/canonical-session-projection.test.ts b/packages/runtime-host/src/__tests__/canonical-session-projection.test.ts
index 06eff1cae3..9882440630 100644
--- a/packages/runtime-host/src/__tests__/canonical-session-projection.test.ts
+++ b/packages/runtime-host/src/__tests__/canonical-session-projection.test.ts
@@ -380,6 +380,9 @@ test('projects a failed Turn message from the canonical terminal event', async (
         recoverable: false,
         code: 'provider_error',
         message: 'canonical provider failure api_key=sk-test-secret-value',
+        details: {
+          providerSummary: 'provider rejected model (code=provider_error, requestId=req-123)',
+        },
       },
       context,
       memory,
@@ -417,7 +420,7 @@ test('projects a failed Turn message from the canonical terminal event', async (
     if (canonical?.rootTurn?.status === 'failed') {
       assert.equal(
         canonical.rootTurn.failureMessage,
-        'canonical provider failure api_key=[redacted]',
+        'provider rejected model (code=provider_error, requestId=req-123)',
       );
     }
   });
diff --git a/packages/runtime-host/src/__tests__/session-turns.test.ts b/packages/runtime-host/src/__tests__/session-turns.test.ts
index 945e36e780..a09b30f2e7 100644
--- a/packages/runtime-host/src/__tests__/session-turns.test.ts
+++ b/packages/runtime-host/src/__tests__/session-turns.test.ts
@@ -26,6 +26,7 @@ import {
   projectSessionTurnContribution,
   projectSessionTurnContributionForWire,
   SESSION_TURN_DIAGNOSTIC_MAX_BYTES,
+  SESSION_TURN_FAILURE_MESSAGE_MAX_BYTES,
   SESSION_TURN_LANDMARK_RESULT_MAX_BYTES,
 } from '../protocol/session-turns.js';
 
@@ -110,6 +111,47 @@ test('bounds turn diagnostics before publishing a contribution', () => {
   );
 });
 
+test('persists and bounds the provider failure summary in the Turn projection', () => {
+  const contribution = projectSessionTurnContributionForWire({
+    turnId: 'turn-1',
+    firstSequence: 0,
+    latestState: {
+      sequence: 0,
+      message: {
+        type: 'turn_state',
+        id: 'state-1',
+        turnId: 'turn-1',
+        ts: 1,
+        status: 'failed',
+        partialOutputRetained: false,
+        errorClass: 'rate_limit',
+        failureMessage: `provider says ${'x'.repeat(10_000)}`,
+      },
+    },
+    userPromptPreview: null,
+    hasAssistantMessage: false,
+    hasAssistantOutput: false,
+    hasToolResult: false,
+    hasFailedToolResult: false,
+    hasAbortNote: false,
+  });
+
+  assert.ok(
+    Buffer.byteLength(contribution.latestState!.message.failureMessage!, 'utf8') <=
+      SESSION_TURN_FAILURE_MESSAGE_MAX_BYTES,
+  );
+  const decoded = decodeSessionTurnsQueryResult({
+    sessionId: 'session-1',
+    throughSequence: 0,
+    contributions: [contribution],
+    nextPosition: null,
+  });
+  assert.equal(
+    decoded.contributions[0]!.latestState!.message.failureMessage,
+    contribution.latestState!.message.failureMessage,
+  );
+});
+
 test('rejects invalid turn-state references before publishing a contribution', () => {
   assert.throws(() =>
     projectSessionTurnContributionForWire({
diff --git a/packages/runtime-host/src/adapter/session-projector.ts b/packages/runtime-host/src/adapter/session-projector.ts
index 7924fa38ad..2fe8e3ba9e 100644
--- a/packages/runtime-host/src/adapter/session-projector.ts
+++ b/packages/runtime-host/src/adapter/session-projector.ts
@@ -290,7 +290,10 @@ export class RuntimeHostSessionProjector {
         ts: terminal.ts,
         recoverable: false,
         reason,
-        message: `Turn failed: ${reason}`,
+        message: terminal.failureMessage ?? `Turn failed: ${reason}`,
+        ...(terminal.failureMessage
+          ? { details: { providerSummary: terminal.failureMessage } }
+          : {}),
       });
     } else {
       events.push({
@@ -329,7 +332,8 @@ export class RuntimeHostSessionProjector {
           ts,
           recoverable: false,
           reason,
-          message: `Turn failed: ${reason}`,
+          message: turn.failureMessage ?? `Turn failed: ${reason}`,
+          ...(turn.failureMessage ? { details: { providerSummary: turn.failureMessage } } : {}),
         },
       ];
     }
@@ -483,6 +487,7 @@ export class RuntimeHostSessionProjector {
         recoverable: false,
         reason: root.failureClass,
         message: root.failureMessage ?? `Turn failed: ${root.failureClass}`,
+        ...(root.failureMessage ? { details: { providerSummary: root.failureMessage } } : {}),
       });
     } else {
       events.push({
diff --git a/packages/runtime-host/src/protocol/index.ts b/packages/runtime-host/src/protocol/index.ts
index abac95994a..cb0c4738be 100644
--- a/packages/runtime-host/src/protocol/index.ts
+++ b/packages/runtime-host/src/protocol/index.ts
@@ -100,7 +100,10 @@ export const RUNTIME_HOST_REGISTRATION_SCHEMA_VERSION = 1 as const;
 export const RUNTIME_HOST_PROTOCOL_VERSION = 0 as const;
 // Increment when the same protocol version no longer guarantees safe Client-Host
 // interoperability. Mismatches are rejected before domain commands are admitted.
-export const RUNTIME_HOST_COMPATIBILITY_EPOCH = 101 as const;
+export const RUNTIME_HOST_COMPATIBILITY_EPOCH = 102 as const;
+// 102: Session Turn projections carry bounded provider failure summaries for
+// live and reloaded failed-turn diagnostics. Older peers cannot preserve or
+// render this additional failure context safely.
 // 101: Session Turn requests can carry regeneration intents and Guests can
 // atomically withdraw pending requests. Older peers do not share this command
 // vocabulary or the expanded Guest operation grant.
diff --git a/packages/runtime-host/src/protocol/session-turns.ts b/packages/runtime-host/src/protocol/session-turns.ts
index c615b7152a..b3c0df009d 100644
--- a/packages/runtime-host/src/protocol/session-turns.ts
+++ b/packages/runtime-host/src/protocol/session-turns.ts
@@ -32,6 +32,7 @@ import { defineOperation } from './operation-spec.js';
 export const SESSION_TURN_QUERY_MAX_CONTRIBUTIONS = 128;
 export const SESSION_TURN_QUERY_RESULT_MAX_BYTES = 192 * 1024;
 export const SESSION_TURN_DIAGNOSTIC_MAX_BYTES = 128;
+export const SESSION_TURN_FAILURE_MESSAGE_MAX_BYTES = 256;
 export const SESSION_TURN_PROMPT_PREVIEW_MAX_BYTES = 256;
 export const SESSION_TURN_LANDMARK_MAX_ITEMS = 64;
 export const SESSION_TURN_LANDMARK_LABEL_MAX_BYTES = 96;
@@ -179,6 +180,14 @@ function projectTurnStateMessageForWire(message: TurnStateMessage): TurnStateMes
     ...(message.errorClass
       ? { errorClass: truncateUtf8(message.errorClass, SESSION_TURN_DIAGNOSTIC_MAX_BYTES) }
       : {}),
+    ...(message.failureMessage
+      ? {
+          failureMessage: truncateUtf8(
+            message.failureMessage,
+            SESSION_TURN_FAILURE_MESSAGE_MAX_BYTES,
+          ),
+        }
+      : {}),
     partialOutputRetained: message.partialOutputRetained,
   };
 }
@@ -205,6 +214,7 @@ export function projectSessionTurnContribution(contribution: SessionTurnContribu
       ...(state.abortedAt !== undefined ? { abortedAt: state.abortedAt } : {}),
       ...(state.abortSource ? { abortSource: state.abortSource } : {}),
       ...(state.errorClass ? { errorClass: state.errorClass } : {}),
+      ...(state.failureMessage ? { failureMessage: state.failureMessage } : {}),
       partialOutputRetained: state.partialOutputRetained || partialOutputRetained,
     };
   }
@@ -424,6 +434,13 @@ function decodeSessionTurnContribution(value: unknown): SessionTurnContribution
         SESSION_TURN_DIAGNOSTIC_MAX_BYTES,
       );
     }
+    if (message.failureMessage !== undefined) {
+      requireUtf8String(
+        message.failureMessage,
+        'Session turn failure message',
+        SESSION_TURN_FAILURE_MESSAGE_MAX_BYTES,
+      );
+    }
     latestState = {
       sequence: requireCount(state.sequence, 'Session turn state sequence'),
       message,
diff --git a/packages/runtime-host/src/server/canonical-turn-snapshot.ts b/packages/runtime-host/src/server/canonical-turn-snapshot.ts
index bdf7a3ae17..684c21a9ce 100644
--- a/packages/runtime-host/src/server/canonical-turn-snapshot.ts
+++ b/packages/runtime-host/src/server/canonical-turn-snapshot.ts
@@ -73,7 +73,10 @@ export async function readCanonicalTurnSnapshot(
       const failureMessage =
         fact.terminalEvent.content?.kind === 'error'
           ? truncateUtf8(
-              redactSecrets(fact.terminalEvent.content.message),
+              redactSecrets(
+                providerFailureSummaryFromRuntimeEvent(fact.terminalEvent) ??
+                  fact.terminalEvent.content.message,
+              ),
               TURN_FAILURE_MESSAGE_MAX_BYTES,
               '…',
             )
@@ -110,6 +113,14 @@ export async function readCanonicalTurnSnapshot(
   return { sessionId, turnId, runId, status: run.status };
 }
 
+function providerFailureSummaryFromRuntimeEvent(
+  event: import('@maka/core/runtime-event').RuntimeEvent,
+): string | undefined {
+  const details = event.content?.kind === 'error' ? event.content.details : undefined;
+  if (!details || Array.isArray(details)) return undefined;
+  const summary = details.providerSummary;
+  return typeof summary === 'string' && summary.length > 0 ? summary : undefined;
+}
 function readContextCompactionOutcome(value: unknown): ContextCompactionOutcome | undefined {
   if (!value || typeof value !== 'object') return undefined;
   const outcome = value as Record<string, unknown>;
diff --git a/packages/runtime-host/src/server/shared-session-transcript.ts b/packages/runtime-host/src/server/shared-session-transcript.ts
index 5c59dd8b0e..b113d376fc 100644
--- a/packages/runtime-host/src/server/shared-session-transcript.ts
+++ b/packages/runtime-host/src/server/shared-session-transcript.ts
@@ -136,6 +136,7 @@ export function projectSharedSessionTranscriptMessage(
         ...(message.abortedAt === undefined ? {} : { abortedAt: message.abortedAt }),
         ...(message.abortSource === undefined ? {} : { abortSource: message.abortSource }),
         ...(message.errorClass === undefined ? {} : { errorClass: message.errorClass }),
+        ...(message.failureMessage === undefined ? {} : { failureMessage: message.failureMessage }),
         partialOutputRetained: message.partialOutputRetained,
       };
     case 'token_usage':
diff --git a/packages/runtime/src/__tests__/model-adapter-onerror.test.ts b/packages/runtime/src/__tests__/model-adapter-onerror.test.ts
index c789c6548a..aa03e448da 100644
--- a/packages/runtime/src/__tests__/model-adapter-onerror.test.ts
+++ b/packages/runtime/src/__tests__/model-adapter-onerror.test.ts
@@ -154,6 +154,7 @@ describe('ModelAdapter.startStream onError', () => {
         message: 'Rate limit exceeded',
         retryable: true,
         retryAfterMs: 2500,
+        diagnosticSummary: 'rate limited (status=429)',
       },
     ]);
     const outcome = await requireAlreadySettled(result.outcome);
@@ -204,6 +205,7 @@ describe('ModelAdapter.startStream onError', () => {
         kind: 'unknown',
         message: 'Plaintext Responses reasoning item is missing final summary metadata',
         retryable: false,
+        diagnosticSummary: 'Plaintext Responses reasoning item is missing final summary metadata',
       },
     ]);
     const outcome = await requireAlreadySettled(result.outcome);
@@ -249,6 +251,7 @@ describe('ModelAdapter.startStream onError', () => {
       retryable: false,
       message: 'Rate limit exceeded',
       code: 'rate_limit_exceeded',
+      diagnosticSummary: 'Provider stopped the stream with an error (code=rate_limit_exceeded)',
     });
     assert.equal(outcome.usage?.rawFinishReason, 'rate_limit_exceeded');
   });
@@ -288,6 +291,7 @@ describe('ModelAdapter.startStream onError', () => {
         message: 'Rate limit exceeded',
         retryable: true,
         retryAfterMs: 2500,
+        diagnosticSummary: 'rate limited (status=429)',
       },
     ]);
     assert.deepEqual(await result.outcome, {
@@ -485,6 +489,8 @@ describe('ModelAdapter.startStream onError', () => {
           kind: 'network',
           message: 'Network error',
           retryable: true,
+          diagnosticSummary:
+            'Client network socket disconnected before secure TLS connection was established',
         },
       ]);
     } finally {
diff --git a/packages/runtime/src/__tests__/model-adapter.test.ts b/packages/runtime/src/__tests__/model-adapter.test.ts
index 3fa8751fe9..80ca297c7d 100644
--- a/packages/runtime/src/__tests__/model-adapter.test.ts
+++ b/packages/runtime/src/__tests__/model-adapter.test.ts
@@ -378,6 +378,7 @@ describe('ModelAdapter stream and error normalization', () => {
       code: '429',
       message: 'Rate limit exceeded',
       retryable: false,
+      diagnosticSummary: '429 rate limit (code=429)',
     });
     // The backend consumes the typed failure without recovering the raw
     // provider error shape.
@@ -385,6 +386,7 @@ describe('ModelAdapter stream and error normalization', () => {
     assert.equal(shaped.reason, 'rate_limit');
     assert.equal(shaped.code, '429');
     assert.equal(shaped.message, 'Rate limit exceeded');
+    assert.deepEqual(shaped.details, { providerSummary: '429 rate limit (code=429)' });
   });
 
   test('normalizes a status-less provider server_error into a retryable outage', () => {
@@ -408,6 +410,8 @@ describe('ModelAdapter stream and error normalization', () => {
         code: 'server_error',
         message: 'Provider returned an error',
         retryable: true,
+        diagnosticSummary:
+          'Streaming response failed: [502] Upstream error from Nvidia: Service temporarily overloaded (code=server_error)',
       },
     });
   });
@@ -842,6 +846,24 @@ describe('ModelAdapter stream and error normalization', () => {
     assert.equal(event.message.includes('sk-live-secret-token-value'), false);
   });
 
+  test('re-scrubs a pre-normalized provider diagnostic at the event boundary', () => {
+    const event = newAdapter().makeErrorEvent('turn-1', {
+      type: 'model_failure',
+      kind: 'rate_limit',
+      message: 'Rate limit exceeded',
+      retryable: false,
+      diagnosticSummary: `provider says api_key=sk-live-secret-token-value ${'x'.repeat(4_000)}`,
+    });
+
+    const providerSummary =
+      event.details && !Array.isArray(event.details) ? event.details.providerSummary : undefined;
+    assert.equal(
+      typeof providerSummary === 'string' && providerSummary.includes('sk-live-secret-token-value'),
+      false,
+    );
+    assert.equal(Buffer.byteLength(String(providerSummary ?? ''), 'utf8') <= 2 * 1024, true);
+  });
+
   test('normalizes cache and reasoning usage variants in the adapter module', () => {
     assert.deepEqual(
       normalizeAiSdkUsage({
diff --git a/packages/runtime/src/__tests__/openai-responses-model-adapter.test.ts b/packages/runtime/src/__tests__/openai-responses-model-adapter.test.ts
index 36052d5dd5..f760447cca 100644
--- a/packages/runtime/src/__tests__/openai-responses-model-adapter.test.ts
+++ b/packages/runtime/src/__tests__/openai-responses-model-adapter.test.ts
@@ -136,6 +136,8 @@ describe('OpenAI Responses ModelAdapter continuation', () => {
         retryable: true,
         code: 'OPENAI_RESPONSES_WEBSOCKET_TRANSPORT_ERROR',
         message: 'Network error',
+        diagnosticSummary:
+          'closed before completion (code=OPENAI_RESPONSES_WEBSOCKET_TRANSPORT_ERROR)',
       },
     ]);
   });
diff --git a/packages/runtime/src/__tests__/runtime-event-read-model.test.ts b/packages/runtime/src/__tests__/runtime-event-read-model.test.ts
index 73eb22437a..cbb36dfde6 100644
--- a/packages/runtime/src/__tests__/runtime-event-read-model.test.ts
+++ b/packages/runtime/src/__tests__/runtime-event-read-model.test.ts
@@ -1603,6 +1603,36 @@ describe('projectRuntimeEventsToStoredMessages', () => {
     assert.deepStrictEqual(out.diagnostics, []);
   });
 
+  test('carries the bounded provider summary into the durable failed turn', () => {
+    const out = projectRuntimeEventsToStoredMessages(
+      [
+        ev({
+          id: 'evt-provider-failed',
+          ts: ts + 9,
+          status: 'failed',
+          content: {
+            kind: 'error',
+            reason: 'rate_limit',
+            message: 'Rate limit exceeded',
+            details: {
+              providerSummary: `provider says api_key=sk-live-secret ${'x'.repeat(1_000)}`,
+            },
+          },
+          actions: { endInvocation: true },
+        }),
+      ],
+      {
+        runHeaders: [{ ...header, status: 'failed', failureClass: 'rate_limit' }],
+      },
+    );
+
+    const state = out.messages.find((message) => message.type === 'turn_state');
+    assert.equal(state?.type, 'turn_state');
+    assert.equal(state?.errorClass, 'rate_limit');
+    assert.equal(state?.failureMessage?.includes('sk-live-secret'), false);
+    assert.ok(Buffer.byteLength(state?.failureMessage ?? '', 'utf8') <= 256);
+  });
+
   test('tool step cap terminal fact projects a persistent system notice', () => {
     const out = projectRuntimeEventsToStoredMessages(
       [
diff --git a/packages/runtime/src/agent-run.ts b/packages/runtime/src/agent-run.ts
index cff8aaed16..da8030c59a 100644
--- a/packages/runtime/src/agent-run.ts
+++ b/packages/runtime/src/agent-run.ts
@@ -76,6 +76,7 @@ import {
   buildStatusPatch,
   isTerminalRunStatus,
   normalizeStopSessionSource,
+  providerFailureMessageFromEvent,
   statusFromEvent,
   turnStatusFromEvent,
 } from './session-projection-helpers.js';
@@ -119,7 +120,12 @@ export interface AgentRunHooks {
     turnId: string,
     status: TurnRecord['status'],
     lineage?: AgentRunLineage,
-    options?: { ts?: number; errorClass?: string; abortSource?: string },
+    options?: {
+      ts?: number;
+      errorClass?: string;
+      failureMessage?: string;
+      abortSource?: string;
+    },
   ): Promise<void>;
 }
 
@@ -905,6 +911,7 @@ export class AgentRun {
         {
           ts: ev.ts,
           errorClass: turnStatus.errorClass,
+          failureMessage: turnStatus.failureMessage,
           ...(turnStatus.status === 'aborted' && this.abortSource
             ? { abortSource: this.abortSource }
             : {}),
@@ -929,10 +936,15 @@ export class AgentRun {
           .appendTurnState(this.sessionId, this.turnId, 'failed', this.lineage, {
             ts: ev.ts,
             errorClass: ev.reason ?? ev.code ?? 'unknown',
+            failureMessage: ev.type === 'error' ? providerFailureMessageFromEvent(ev) : undefined,
           })
           .catch((error) => this.enqueueTraceWriteFailure(error, 'terminal session projection'));
 
-        this.markRunFailed(ev.reason ?? ev.code ?? 'unknown', ev.message, ev.ts);
+        this.markRunFailed(
+          ev.reason ?? ev.code ?? 'unknown',
+          (ev.type === 'error' ? providerFailureMessageFromEvent(ev) : undefined) ?? ev.message,
+          ev.ts,
+        );
       }
     }
   }
diff --git a/packages/runtime/src/model-adapter.ts b/packages/runtime/src/model-adapter.ts
index b5444bb529..b1e7a1a5d9 100644
--- a/packages/runtime/src/model-adapter.ts
+++ b/packages/runtime/src/model-adapter.ts
@@ -25,7 +25,8 @@ import {
   type RuntimeExecutionConnection,
 } from '@maka/core/llm-connections';
 import { lookupModelMetadata } from '@maka/core/model-metadata';
-import { generalizedErrorMessage } from '@maka/core/redaction';
+import { generalizedErrorMessage, redactSecrets } from '@maka/core/redaction';
+import { truncateUtf8 } from '@maka/core/diagnostic-log';
 import type { CacheMissInputSource } from '@maka/core/usage-stats/types';
 import { rawFinishReasonString } from './model-protocol.js';
 import type {
@@ -63,6 +64,7 @@ import {
 import {
   classifyError,
   errorPresentationFromClass,
+  PROVIDER_FAILURE_SUMMARY_MAX_BYTES,
   providerFailureSummary,
   providerRetryMetadata,
 } from './provider-error-classification.js';
@@ -540,6 +542,14 @@ export class ModelAdapter {
 
   makeErrorEvent(turnId: string, err: unknown, reasonOverride?: string): ErrorEvent {
     const failure = normalizeModelFailure(err);
+    const diagnosticSummary =
+      isModelFailure(err) && typeof err.diagnosticSummary === 'string'
+        ? truncateUtf8(
+            redactSecrets(err.diagnosticSummary),
+            PROVIDER_FAILURE_SUMMARY_MAX_BYTES,
+            '…',
+          )
+        : undefined;
     return {
       type: 'error',
       id: this.input.newId(),
@@ -553,6 +563,7 @@ export class ModelAdapter {
           ? { reason: failure.kind }
           : {}),
       message: failure.message,
+      ...(diagnosticSummary ? { details: { providerSummary: diagnosticSummary } } : {}),
     };
   }
 
@@ -1269,6 +1280,7 @@ function normalizeProviderFailure(error: unknown): ModelFailure {
   return {
     ...failure,
     ...(summary?.code !== undefined ? { code: summary.code } : {}),
+    ...(summary?.message !== undefined ? { diagnosticSummary: summary.message } : {}),
     ...(failure.kind === 'unknown' && summary !== undefined ? { message: summary.message } : {}),
   };
 }
diff --git a/packages/runtime/src/model-protocol.ts b/packages/runtime/src/model-protocol.ts
index 8e771cc3ff..6b902725a1 100644
--- a/packages/runtime/src/model-protocol.ts
+++ b/packages/runtime/src/model-protocol.ts
@@ -349,6 +349,8 @@ export interface ModelFailure {
   /** Provider-requested delay for the next physical attempt, in milliseconds. */
   retryAfterMs?: number;
   code?: string;
+  /** Bounded, redacted provider response retained for user diagnostics. */
+  diagnosticSummary?: string;
 }
 
 /**
diff --git a/packages/runtime/src/provider-error-classification.ts b/packages/runtime/src/provider-error-classification.ts
index aaa73bbe9b..91c54369a9 100644
--- a/packages/runtime/src/provider-error-classification.ts
+++ b/packages/runtime/src/provider-error-classification.ts
@@ -139,7 +139,7 @@ interface ProviderFailureSummary {
   code?: string;
 }
 
-const PROVIDER_FAILURE_SUMMARY_MAX_BYTES = 2 * 1024;
+export const PROVIDER_FAILURE_SUMMARY_MAX_BYTES = 2 * 1024;
 const PROVIDER_FAILURE_FIELD_MAX_BYTES = 256;
 
 const MAX_SAFE_TIMER_DELAY_MS = 2_147_483_647;
diff --git a/packages/runtime/src/runtime-event-backfill.ts b/packages/runtime/src/runtime-event-backfill.ts
index ac02be04bb..fafca4e7c1 100644
--- a/packages/runtime/src/runtime-event-backfill.ts
+++ b/packages/runtime/src/runtime-event-backfill.ts
@@ -470,6 +470,16 @@ function terminalRuntimeEvent(input: {
           ...(abortSource !== undefined ? { abortSource } : {}),
         },
       },
+      ...(status === 'failed' && turnState?.failureMessage
+        ? {
+            content: {
+              kind: 'error' as const,
+              message: turnState.failureMessage,
+              ...(turnState.errorClass ? { reason: turnState.errorClass } : {}),
+              details: { providerSummary: turnState.failureMessage },
+            },
+          }
+        : {}),
       ...(turnState ? { refs: { storedMessageId: turnState.id } } : {}),
     },
   };
diff --git a/packages/runtime/src/runtime-event-read-model.ts b/packages/runtime/src/runtime-event-read-model.ts
index 62ab789737..387874bf0d 100644
--- a/packages/runtime/src/runtime-event-read-model.ts
+++ b/packages/runtime/src/runtime-event-read-model.ts
@@ -22,6 +22,8 @@ import type { AssistantStepContentKind, StoredMessage, TurnStatus } from '@maka/
 import type { RuntimeEvent, RuntimeEventStatus } from '@maka/core/runtime-event';
 import type { ToolActivityKind, ToolResultContent } from '@maka/core/events';
 import { markPersisted } from '@maka/core/persisted-value';
+import { truncateUtf8 } from '@maka/core/diagnostic-log';
+import { redactSecrets } from '@maka/core/redaction';
 import {
   SANDBOX_BOUNDARY_REQUEST_STATUSES,
   validateSandboxBoundaryExpansion,
@@ -46,6 +48,7 @@ const SETTLED_SANDBOX_BOUNDARY_STATUSES: readonly SettledSandboxBoundaryStatus[]
   SANDBOX_BOUNDARY_REQUEST_STATUSES.filter(
     (status): status is SettledSandboxBoundaryStatus => status !== 'pending',
   );
+const TURN_FAILURE_MESSAGE_MAX_BYTES = 256;
 import type { CanonicalPermissionOutcomeRecord } from './interaction-authority.js';
 import { isArchivedToolResultPlaceholder } from './tool-result-archive.js';
 
@@ -1150,6 +1153,7 @@ function projectTerminalTurnState(
   const abortSource = status === 'aborted' ? abortSourceFromRuntime(event, header) : undefined;
   const failureClass =
     status === 'failed' ? failureClassFromRuntimeEvent(event, header) : undefined;
+  const failureMessage = status === 'failed' ? failureMessageFromRuntimeEvent(event) : undefined;
   const partialOutputRetained = messages.some(
     (message) =>
       message.turnId === event.turnId &&
@@ -1172,6 +1176,7 @@ function projectTerminalTurnState(
     ...(status === 'aborted' ? { abortedAt: event.ts } : {}),
     ...(abortSource ? { abortSource } : {}),
     ...(status === 'failed' ? { errorClass: failureClass ?? 'unknown' } : {}),
+    ...(failureMessage ? { failureMessage } : {}),
     partialOutputRetained,
   });
   if (failureClass === 'tool_step_cap_reached') {
@@ -1202,6 +1207,17 @@ function projectTerminalTurnState(
   return true;
 }
 
+function failureMessageFromRuntimeEvent(event: RuntimeEvent): string | undefined {
+  const content = event.content;
+  if (content?.kind !== 'error' || !content.details || Array.isArray(content.details)) {
+    return undefined;
+  }
+  const summary = content.details.providerSummary;
+  return typeof summary === 'string' && summary.length > 0
+    ? truncateUtf8(redactSecrets(summary), TURN_FAILURE_MESSAGE_MAX_BYTES, '…')
+    : undefined;
+}
+
 function attachPendingThinking(
   event: RuntimeEvent,
   state: ProjectionState,
@@ -1591,6 +1607,7 @@ function semanticMessage(message: StoredMessage): unknown {
         abortedAt: message.abortedAt,
         abortSource: message.abortSource,
         errorClass: message.errorClass,
+        failureMessage: message.failureMessage,
         partialOutputRetained: message.partialOutputRetained,
       };
     case 'system_note':
diff --git a/packages/runtime/src/runtime-kernel.ts b/packages/runtime/src/runtime-kernel.ts
index 1da029c273..4241770141 100644
--- a/packages/runtime/src/runtime-kernel.ts
+++ b/packages/runtime/src/runtime-kernel.ts
@@ -2731,7 +2731,13 @@ export class RuntimeKernel implements RuntimeKernelLike {
     turnId: string,
     status: TurnRecord['status'],
     lineage: AgentRunLineage = {},
-    options: { id?: string; ts?: number; errorClass?: string; abortSource?: string } = {},
+    options: {
+      id?: string;
+      ts?: number;
+      errorClass?: string;
+      failureMessage?: string;
+      abortSource?: string;
+    } = {},
   ): Promise<void> {
     const ts = options.ts ?? this.deps.now();
     await this.deps.store.appendMessage(
@@ -2744,6 +2750,7 @@ export class RuntimeKernel implements RuntimeKernelLike {
         lineage,
         ...(options.abortSource ? { abortSource: options.abortSource } : {}),
         ...(options.errorClass !== undefined ? { errorClass: options.errorClass } : {}),
+        ...(options.failureMessage !== undefined ? { failureMessage: options.failureMessage } : {}),
         partialOutputRetained: await this.turnHasRetainedOutput(sessionId, turnId),
       }),
     );
diff --git a/packages/runtime/src/session-manager.ts b/packages/runtime/src/session-manager.ts
index fe98e096f7..f100c10f08 100644
--- a/packages/runtime/src/session-manager.ts
+++ b/packages/runtime/src/session-manager.ts
@@ -4268,7 +4268,12 @@ export class SessionManager {
     turnId: string,
     status: TurnRecord['status'],
     lineage: AgentRunLineage = {},
-    options: { ts?: number; errorClass?: string; abortSource?: string } = {},
+    options: {
+      ts?: number;
+      errorClass?: string;
+      failureMessage?: string;
+      abortSource?: string;
+    } = {},
   ): Promise<void> {
     const ts = options.ts ?? this.deps.now();
     await this.deps.store.appendMessage(
@@ -4281,6 +4286,7 @@ export class SessionManager {
         lineage,
         ...(options.abortSource ? { abortSource: options.abortSource } : {}),
         ...(options.errorClass !== undefined ? { errorClass: options.errorClass } : {}),
+        ...(options.failureMessage !== undefined ? { failureMessage: options.failureMessage } : {}),
         partialOutputRetained: await this.turnHasRetainedOutput(sessionId, turnId),
       }),
     );
diff --git a/packages/runtime/src/session-projection-helpers.ts b/packages/runtime/src/session-projection-helpers.ts
index 468005fc24..ba6343aa19 100644
--- a/packages/runtime/src/session-projection-helpers.ts
+++ b/packages/runtime/src/session-projection-helpers.ts
@@ -18,6 +18,8 @@
  */
 
 import type { AgentRunHeader } from '@maka/core/agent-run';
+import { truncateUtf8 } from '@maka/core/diagnostic-log';
+import { redactSecrets } from '@maka/core/redaction';
 import { failureClassFromCompleteStopReason, type SessionEvent } from '@maka/core/events';
 import type {
   SessionBlockedReason,
@@ -46,6 +48,7 @@ export interface BuildTurnStateMessageInput {
   status: TurnRecord['status'];
   lineage?: TurnStateLineage;
   errorClass?: string;
+  failureMessage?: string;
   abortSource?: string;
   partialOutputRetained: boolean;
 }
@@ -80,6 +83,9 @@ export function buildTurnStateMessage(input: BuildTurnStateMessageInput): TurnSt
     ...(input.status === 'aborted' ? { abortedAt: input.ts } : {}),
     ...(input.status === 'aborted' && input.abortSource ? { abortSource: input.abortSource } : {}),
     ...(input.status === 'failed' ? { errorClass: input.errorClass ?? 'unknown' } : {}),
+    ...(input.status === 'failed' && input.failureMessage
+      ? { failureMessage: input.failureMessage }
+      : {}),
     partialOutputRetained: input.partialOutputRetained,
   };
 }
@@ -141,12 +147,18 @@ export function statusFromEvent(
 
 export function turnStatusFromEvent(
   event: SessionEvent,
-): { status: TurnRecord['status']; errorClass?: string } | undefined {
+): { status: TurnRecord['status']; errorClass?: string; failureMessage?: string } | undefined {
   switch (event.type) {
     case 'abort':
       return { status: 'aborted' };
     case 'error':
-      return { status: 'failed', errorClass: event.reason ?? event.code ?? 'unknown' };
+      return {
+        status: 'failed',
+        errorClass: event.reason ?? event.code ?? 'unknown',
+        ...(providerFailureMessageFromEvent(event)
+          ? { failureMessage: providerFailureMessageFromEvent(event) }
+          : {}),
+      };
     case 'complete': {
       if (event.stopReason === 'user_stop') return { status: 'aborted' };
       const errorClass = failureClassFromCompleteStopReason(event.stopReason);
@@ -158,6 +170,17 @@ export function turnStatusFromEvent(
   }
 }
 
+/** Extract the adapter's bounded provider summary without exposing arbitrary details. */
+export function providerFailureMessageFromEvent(
+  event: Extract<SessionEvent, { type: 'error' }>,
+): string | undefined {
+  if (!event.details || Array.isArray(event.details)) return undefined;
+  const summary = event.details.providerSummary;
+  return typeof summary === 'string' && summary.length > 0
+    ? truncateUtf8(redactSecrets(summary), 256, '…')
+    : undefined;
+}
+
 function blockedReasonFromErrorReason(reason: string | undefined): SessionBlockedReason {
   if (!reason) return 'unknown';
   if (reason === 'permission_required') return 'permission_required';
diff --git a/packages/ui/src/__tests__/chat-turn-steering-order.test.ts b/packages/ui/src/__tests__/chat-turn-steering-order.test.ts
index 2583e6c095..b871fd625e 100644
--- a/packages/ui/src/__tests__/chat-turn-steering-order.test.ts
+++ b/packages/ui/src/__tests__/chat-turn-steering-order.test.ts
@@ -71,3 +71,31 @@ test('renders steering where it arrived in the assistant timeline', () => {
     assert.equal(visibleText.split(text).length - 1, 1, `${text} should render exactly once`);
   }
 });
+
+test('renders provider failure detail collapsed beneath the classified banner', () => {
+  const turn: TurnViewModel = {
+    turnId: 'turn-provider-failure',
+    status: 'failed',
+    partialOutputRetained: false,
+    user: { id: 'user', role: 'user', text: 'request', ts: 1 },
+    tools: [],
+    notes: [],
+    startedAt: 1,
+    timeline: [{ kind: 'text', text: 'partial output', messageId: 'assistant', ts: 2 }],
+  };
+  const markup = renderToStaticMarkup(
+    createElement(LocaleProvider, {
+      locale: 'en',
+      children: createElement(TurnView, {
+        turn,
+        failedReasonLabel: 'Rate limit exceeded',
+        failedDiagnostic: '429 rate limit (code=rate_limit_exceeded, requestId=req-123)',
+      }),
+    }),
+  );
+  assert.match(markup, /<details[^>]*maka-turn-failed-diagnostic/);
+  assert.match(markup, /<summary>Provider response details<\/summary>/);
+  assert.match(markup, /429 rate limit/);
+  assert.doesNotMatch(markup, /<details[^>]*open/);
+  assert.ok(markup.indexOf('Rate limit exceeded') < markup.indexOf('429 rate limit'));
+});
diff --git a/packages/ui/src/__tests__/materialize.test.ts b/packages/ui/src/__tests__/materialize.test.ts
index 179be9ea18..ed83bc818a 100644
--- a/packages/ui/src/__tests__/materialize.test.ts
+++ b/packages/ui/src/__tests__/materialize.test.ts
@@ -485,6 +485,24 @@ describe("unfinished tools take their status from the turn", () => {
     ]);
     assert.equal(turn?.tools[0]?.status, "interrupted");
   });
+
+  test("restores provider failure detail from a reloaded turn state", () => {
+    const [turn] = materializeTurns([
+      userMsg("t1", 1, "run it"),
+      {
+        type: "turn_state",
+        id: "s1",
+        turnId: "t1",
+        ts: 2,
+        status: "failed",
+        errorClass: "rate_limit",
+        failureMessage: "provider says retry after 30 seconds",
+        partialOutputRetained: false,
+      },
+    ]);
+    assert.equal(turn?.errorClass, "rate_limit");
+    assert.equal(turn?.failureMessage, "provider says retry after 30 seconds");
+  });
 });
 
 describe("live tool status over persisted", () => {
diff --git a/packages/ui/src/chat-turn.tsx b/packages/ui/src/chat-turn.tsx
index 1fedef9546..4c38b26b53 100644
--- a/packages/ui/src/chat-turn.tsx
+++ b/packages/ui/src/chat-turn.tsx
@@ -396,6 +396,8 @@ export const TurnView = memo(function TurnView(props: {
    * is the outcome, this is the execution state, and both can be true.
    */
   failedExecutionStateLabel?: string;
+  /** Bounded provider response detail shown collapsed beneath the category. */
+  failedDiagnostic?: string;
   safeResumeAction?: {
     pending: boolean;
     detail?: string;
@@ -682,35 +684,43 @@ export const TurnView = memo(function TurnView(props: {
                   is one — it explains why the button did nothing, which
                   outranks execution state on the one turn that can have both. */}
               {ownsTurnChrome && turn.status === 'failed' && props.failedReasonLabel && (
-                <Banner
-                  status={props.failedSeverity ?? 'error'}
-                  container="section"
-                  className="maka-turn-failed-banner"
-                  title={props.failedReasonLabel}
-                  {...(props.safeResumeAction?.detail ?? props.failedExecutionStateLabel
-                    ? {
-                        description:
-                          props.safeResumeAction?.detail ?? props.failedExecutionStateLabel,
-                      }
-                    : {})}
-                  {...(props.safeResumeAction
-                    ? {
-                        endContent: (
-                          <UiButton
-                            variant="ghost"
-                            size="sm"
-                            isDisabled={props.safeResumeAction.pending}
-                            onClick={props.safeResumeAction.onResume}
-                            label={
-                              props.safeResumeAction.pending
-                                ? copy.safeResumePending
-                                : copy.safeResume
-                            }
-                          />
-                        ),
-                      }
-                    : {})}
-                />
+                <>
+                  <Banner
+                    status={props.failedSeverity ?? 'error'}
+                    container="section"
+                    className="maka-turn-failed-banner"
+                    title={props.failedReasonLabel}
+                    {...(props.safeResumeAction?.detail ?? props.failedExecutionStateLabel
+                      ? {
+                          description:
+                            props.safeResumeAction?.detail ?? props.failedExecutionStateLabel,
+                        }
+                      : {})}
+                    {...(props.safeResumeAction
+                      ? {
+                          endContent: (
+                            <UiButton
+                              variant="ghost"
+                              size="sm"
+                              isDisabled={props.safeResumeAction.pending}
+                              onClick={props.safeResumeAction.onResume}
+                              label={
+                                props.safeResumeAction.pending
+                                  ? copy.safeResumePending
+                                  : copy.safeResume
+                              }
+                            />
+                          ),
+                        }
+                      : {})}
+                  />
+                  {props.failedDiagnostic && (
+                    <details className="maka-turn-failed-diagnostic">
+                      <summary>{copy.failedDiagnostic}</summary>
+                      <pre>{props.failedDiagnostic}</pre>
+                    </details>
+                  )}
+                </>
               )}
               {ownsTurnChrome && props.liveStreaming && (
                 <>
diff --git a/packages/ui/src/chat-view.tsx b/packages/ui/src/chat-view.tsx
index ea812fe1be..2b220dcc3b 100644
--- a/packages/ui/src/chat-view.tsx
+++ b/packages/ui/src/chat-view.tsx
@@ -788,6 +788,9 @@ export function ChatView(props: {
                       failedExecutionStateLabel={
                         turnPresentation?.failedExecutionStateLabels[turn.turnId]
                       }
+                      failedDiagnostic={turnPresentation?.failedReasonLabels[turn.turnId]
+                        ? turn.failureMessage
+                        : undefined}
                       safeResumeAction={turnPresentation?.resumeCandidateTurnId === turn.turnId
                         ? props.safeResumeAction
                         : undefined}
diff --git a/packages/ui/src/conversation-copy.ts b/packages/ui/src/conversation-copy.ts
index 98ae0115c0..6f77e41096 100644
--- a/packages/ui/src/conversation-copy.ts
+++ b/packages/ui/src/conversation-copy.ts
@@ -280,6 +280,7 @@ export interface ConversationCopy {
     providerRetryReason: Record<ProviderRetryReason, string>;
     safeResumePending: string;
     safeResume: string;
+    failedDiagnostic: string;
     thinking: string;
     truncated: string;
     copied: string;
@@ -525,6 +526,7 @@ const CONVERSATION_COPY = {
     },
     messages: {
       you: '你', assistant: 'Maka', processing: '正在处理…', continuing: '继续中…', workingPhrases: ['正在琢磨…', '正在推敲…', '正在盘算…', '正在钻研…', '正在忙活…', '正在梳理…', '正在打磨…', '正在鼓捣…', '正在酝酿…', '正在攻坚…', '正在权衡…', '正在拾掇…'], providerRetryScheduled: (seconds, attempt, maxAttempts) => `${formatRetryDelay(seconds, 'zh')}后重试（${attempt}/${maxAttempts}）`, providerRetryStarted: (attempt, maxAttempts) => `正在重试（${attempt}/${maxAttempts}）`, providerRetryWaiting: (attempt, maxAttempts) => `等待重试（${attempt}/${maxAttempts}）`, providerRetryReason: { network: '网络中断', provider_capacity: '模型服务暂时满载', provider_unavailable: '模型服务暂时不可用', rate_limit: '触发模型速率限制', timeout: '请求超时', unknown: '模型请求失败' }, safeResumePending: '正在检查…', safeResume: '继续这一轮', thinking: '深度思考', truncated: '已截断', copied: '已复制', copying: '复制中', copyFailed: '复制失败', copy: '复制', editMessage: '编辑并重发', editMessageDisabledRunning: '当前回答仍在进行中，结束后再编辑', editMessageDisabledAttachments: '包含附件的历史消息暂不支持编辑并重发', editMessageDisabledQuotes: '包含引用的历史消息暂不支持编辑并重发', editMessageDisabledTransformedText: '包含已展开上下文的历史消息暂不支持编辑并重发',
+      failedDiagnostic: 'Provider 响应详情',
       editMessageDisabledDirectoryReferences: '包含文件夹引用的历史消息暂不支持编辑并重发',
       userAriaLabel: '你发送的消息', systemAriaLabel: '系统消息', assistantAriaLabel: 'Maka 的回答', answerActionsAriaLabel: (context) => `回答操作${context ? `：${context}` : ''}`, answerActionAriaLabel: (action, context) => `${action}回答${context ? `：${context}` : ''}`, messageActionAriaLabel: (action, context) => `${action}消息${context ? `：${context}` : ''}`, sourceAriaLabel: '本轮回答的来源', derivativesAriaLabel: '本轮回答的衍生', scheduledTaskTriggered: '定时任务触发', scheduledTaskTitle: (id) => `由定时任务触发 · ${id}`, legacyAutomationTriggered: '旧版自动化（仅历史）', legacyAutomationTitle: (id) => `由旧版自动化触发 · ${id} · 仅保留历史，不会再次执行`, goalContinued: 'Goal 自动继续', goalTitle: (id) => `由 Goal 继续执行 · ${id}`, agentGraphTriggered: 'Agent Graph 自动继续', agentGraphTitle: (graphId) => `由 Agent Graph 调度器触发 · ${graphId}`,
       thinkingTruncatedTitle: '部分 reasoning 已截断；显示的是最近的内容', outputTruncatedTitle: '助手输出已超过单次回合上限，超出部分未渲染。如需完整内容请重新生成或查看持久化的任务日志。', removeAttachmentAriaLabel: (name) => `移除 ${name}`, quoteLabel: '引用', quoteExpandAriaLabel: '展开引用全文', quoteCollapseAriaLabel: '收起引用', removeQuoteAriaLabel: '移除引用', aborted: '已中断', abortedByStop: '已中断 · 由停止按钮触发',
@@ -683,6 +685,7 @@ const CONVERSATION_COPY = {
     },
     messages: {
       you: 'You', assistant: 'Maka', processing: 'Working…', continuing: 'Continuing…', workingPhrases: ['Pondering…', 'Tinkering…', 'Untangling…', 'Digging in…', 'Mulling…', 'Chewing on it…', 'Wrangling…', 'Piecing it together…'], providerRetryScheduled: (seconds, attempt, maxAttempts) => `Retrying in ${formatRetryDelay(seconds, 'en')} (${attempt}/${maxAttempts})`, providerRetryStarted: (attempt, maxAttempts) => `Retrying (${attempt}/${maxAttempts})`, providerRetryWaiting: (attempt, maxAttempts) => `Waiting to retry (${attempt}/${maxAttempts})`, providerRetryReason: { network: 'Network interrupted', provider_capacity: 'The model service is temporarily at capacity', provider_unavailable: 'Model service temporarily unavailable', rate_limit: 'Model rate limit reached', timeout: 'Request timed out', unknown: 'Model request failed' }, safeResumePending: 'Checking…', safeResume: 'Continue this turn', thinking: 'Thinking', truncated: 'Truncated', copied: 'Copied', copying: 'Copying', copyFailed: 'Copy failed', copy: 'Copy', editMessage: 'Edit & resend', editMessageDisabledRunning: 'Wait for this answer to finish before editing', editMessageDisabledAttachments: 'Edit & resend does not yet support messages with attachments', editMessageDisabledQuotes: 'Edit & resend does not yet support messages with quotes', editMessageDisabledTransformedText: 'Edit & resend does not yet support messages with expanded context',
+      failedDiagnostic: 'Provider response details',
       editMessageDisabledDirectoryReferences: 'Edit & resend does not yet support messages with folder references',
       userAriaLabel: 'Your message', systemAriaLabel: 'System message', assistantAriaLabel: "Maka's response", answerActionsAriaLabel: (context) => `Response actions${context ? `: ${context}` : ''}`, answerActionAriaLabel: (action, context) => `${action} response${context ? `: ${context}` : ''}`, messageActionAriaLabel: (action, context) => `${action} message${context ? `: ${context}` : ''}`, sourceAriaLabel: 'Source of this response', derivativesAriaLabel: 'Responses derived from this one', scheduledTaskTriggered: 'Triggered by scheduled task', scheduledTaskTitle: (id) => `Triggered by scheduled task · ${id}`, legacyAutomationTriggered: 'Legacy Automation (history only)', legacyAutomationTitle: (id) => `Triggered by legacy Automation · ${id} · Historical only; it will not run again`, goalContinued: 'Continued by Goal', goalTitle: (id) => `Continued by Goal · ${id}`, agentGraphTriggered: 'Continued by Agent Graph', agentGraphTitle: (graphId) => `Triggered by the Agent Graph scheduler · ${graphId}`,
       thinkingTruncatedTitle: 'Some reasoning was truncated; showing the most recent content', outputTruncatedTitle: 'The assistant output exceeded the per-turn limit. Regenerate it or inspect the persisted task log for the complete content.', removeAttachmentAriaLabel: (name) => `Remove ${name}`, quoteLabel: 'Quote', quoteExpandAriaLabel: 'Show the full quoted excerpt', quoteCollapseAriaLabel: 'Collapse the quoted excerpt', removeQuoteAriaLabel: 'Remove quote', aborted: 'Interrupted', abortedByStop: 'Interrupted · Stop button',
diff --git a/packages/ui/src/materialize.ts b/packages/ui/src/materialize.ts
index 02dc73fde9..4ab0b82570 100644
--- a/packages/ui/src/materialize.ts
+++ b/packages/ui/src/materialize.ts
@@ -376,6 +376,7 @@ export interface TurnViewModel {
   abortedAt?: number;
   abortSource?: string;
   errorClass?: string;
+  failureMessage?: string;
   partialOutputRetained: boolean;
   user?: ChatItem;
   tools: ToolActivityItem[];
@@ -703,6 +704,7 @@ export function materializeTurns(
           : {}),
         ...(record?.abortSource ? { abortSource: record.abortSource } : {}),
         ...(record?.errorClass ? { errorClass: record.errorClass } : {}),
+        ...(record?.failureMessage ? { failureMessage: record.failureMessage } : {}),
         partialOutputRetained: record?.partialOutputRetained ?? false,
         tools: [],
         notes: [],
diff --git a/packages/ui/src/styles.css b/packages/ui/src/styles.css
index 659d9415f7..3f341f9fa9 100644
--- a/packages/ui/src/styles.css
+++ b/packages/ui/src/styles.css
@@ -790,6 +790,19 @@
 /* Astryx `Banner` paints the failed-turn surface; this only sets the block
    rhythm between it and the timeline entry above it. */
 .maka-turn-failed-banner { margin-block: var(--space-1) var(--space-0-5); }
+.maka-turn-failed-diagnostic {
+  margin-block: var(--space-0-5) var(--space-1);
+  padding-inline: var(--space-1);
+  color: var(--muted-foreground);
+  font-size: 0.8rem;
+}
+.maka-turn-failed-diagnostic summary { cursor: pointer; }
+.maka-turn-failed-diagnostic pre {
+  margin-block: var(--space-0-5) 0;
+  white-space: pre-wrap;
+  overflow-wrap: anywhere;
+  font: inherit;
+}
 
 .maka-turn-lineage-row,
 .maka-turn-footer {