diff --git a/packages/@n8n/instance-ai/evaluations/harness/scenario-execution.ts b/packages/@n8n/instance-ai/evaluations/harness/scenario-execution.ts
index 8aaba0834cf..d2574f95d5a 100644
--- a/packages/@n8n/instance-ai/evaluations/harness/scenario-execution.ts
+++ b/packages/@n8n/instance-ai/evaluations/harness/scenario-execution.ts
@@ -549,16 +549,20 @@ function buildScenarioContextBlock(
 			);
 		}
 		for (const req of nr.interceptedRequests) {
-			if (
-				typeof req.mockResponse === 'object' &&
-				req.mockResponse !== null &&
-				'_evalMockError' in (req.mockResponse as Record<string, unknown>)
-			) {
-				const msg = (req.mockResponse as Record<string, unknown>).message;
+			if (typeof req.mockResponse !== 'object' || req.mockResponse === null) continue;
+			const mockResponse = req.mockResponse as Record<string, unknown>;
+			// `_evalMockError` = HTTP-mock generation failure; `evalMockGenerationError`
+			// = LLM wire-server generation/translation failure. Flag both.
+			if ('_evalMockError' in mockResponse) {
+				const msg = mockResponse.message;
 				const msgStr = typeof msg === 'string' ? msg : 'unknown';
 				preAnalysis.push(
 					`⚠ MOCK ISSUE: "${nodeName}" ${req.method} ${req.url} → mock generation failed: ${msgStr}`,
 				);
+			} else if (typeof mockResponse.evalMockGenerationError === 'string') {
+				preAnalysis.push(
+					`⚠ MOCK ISSUE: "${nodeName}" ${req.method} ${req.url} → mock generation failed: ${mockResponse.evalMockGenerationError}`,
+				);
 			}
 		}
 	}
diff --git a/packages/cli/src/modules/instance-ai/eval/__tests__/llm-wire-server.test.ts b/packages/cli/src/modules/instance-ai/eval/__tests__/llm-wire-server.test.ts
index abe71346b0f..20e30bbb8e9 100644
--- a/packages/cli/src/modules/instance-ai/eval/__tests__/llm-wire-server.test.ts
+++ b/packages/cli/src/modules/instance-ai/eval/__tests__/llm-wire-server.test.ts
@@ -208,7 +208,43 @@ describe('LlmWireServer', () => {
 			expect(intercepts[0].rootName).toBe('LLM Chain');
 			expect(intercepts[0].method).toBe('POST');
 			expect(intercepts[0].nodeType).toBe(subNode.type);
-			expect(intercepts[0].mockResponse).toEqual({ content: 'reply' });
+			// The ledger records the wire envelope the SDK received, not the mock
+			// handler's `{ content }` shorthand — judges read this as the wire body.
+			expect(intercepts[0].mockResponse).toMatchObject({
+				object: 'chat.completion',
+				choices: [{ message: { role: 'assistant', content: 'reply' } }],
+			});
+		});
+
+		it('keeps the _evalMockError sentinel at the ledger top level so mock_issue stays attributable', async () => {
+			const intercepts: InterceptedTurn[] = [];
+			const mockHandler = vi.fn().mockResolvedValue({
+				body: { _evalMockError: true, message: 'Mock generation failed: upstream 429' },
+				headers: {},
+				statusCode: 200,
+			}) as unknown as EvalLlmMockHandler;
+
+			server = new LlmWireServer({
+				logger: mockLogger,
+				mockHandler,
+				rootToSubNode: new Map([['Agent', subNode]]),
+				onIntercept: (t) => intercepts.push(t),
+			});
+			const url = await server.start();
+
+			await postChatCompletion(url, '/eval/Agent/v1/chat/completions', {
+				model: 'gpt-4o',
+				messages: [{ role: 'user', content: 'ping' }],
+			});
+
+			// Consumers test `'_evalMockError' in mockResponse`; a translated chat
+			// envelope would bury the sentinel in choices[0].message.content and the
+			// failure would be misattributed to the builder.
+			expect(intercepts).toHaveLength(1);
+			expect(intercepts[0].mockResponse).toEqual({
+				_evalMockError: true,
+				message: 'Mock generation failed: upstream 429',
+			});
 		});
 
 		it('still returns 200 with a valid envelope when onIntercept throws (ledger failure is isolated)', async () => {
@@ -567,6 +603,11 @@ describe('LlmWireServer', () => {
 
 			expect(intercepts).toHaveLength(1);
 			expect(intercepts[0].rootName).toBe('Agent');
+			// Streamed turns record the equivalent non-streamed wire envelope.
+			expect(intercepts[0].mockResponse).toMatchObject({
+				object: 'chat.completion',
+				choices: [{ message: { content: 'streamed' } }],
+			});
 		});
 
 		it('uses the no-handler stub for streaming when no mock handler is attached', async () => {
@@ -945,6 +986,12 @@ describe('LlmWireServer', () => {
 
 			expect(intercepts).toHaveLength(1);
 			expect(intercepts[0].rootName).toBe('My Agent');
+			// The ledger records the canonical Responses envelope the SDK received —
+			// output[].content[].text — not the handler's internal shorthand.
+			expect(intercepts[0].mockResponse).toMatchObject({
+				object: 'response',
+				output: [{ type: 'message', content: [{ type: 'output_text', text: 'ok' }] }],
+			});
 			// Reverse translator uses the canonical OpenAI URL so mock-handler's
 			// service/endpoint extraction derives `/v1/responses` correctly.
 			expect(intercepts[0].url).toBe('https://api.openai.com/v1/responses');
diff --git a/packages/cli/src/modules/instance-ai/eval/llm-wire-server.ts b/packages/cli/src/modules/instance-ai/eval/llm-wire-server.ts
index 6835b896f4f..b9960dce017 100644
--- a/packages/cli/src/modules/instance-ai/eval/llm-wire-server.ts
+++ b/packages/cli/src/modules/instance-ai/eval/llm-wire-server.ts
@@ -4,6 +4,7 @@ import type { EvalLlmMockHandler, EvalMockHttpResponse } from 'n8n-core';
 import type { IHttpRequestOptions, INode } from 'n8n-workflow';
 import { type Server } from 'node:http';
 
+import { isMockErrorSentinel } from './mock-handler';
 import {
 	buildOpenAiErrorEnvelope,
 	extractRequestModel,
@@ -29,6 +30,17 @@ export interface InterceptedTurn {
 	method: string;
 	nodeType: string;
 	requestBody: unknown;
+	/**
+	 * Wire-level response body as served to the vendor SDK (post protocol
+	 * translation) — NOT the mock handler's internal shorthand. Judges read this
+	 * field as "what the node received"; recording the pre-translation shorthand
+	 * made them misattribute node-side failures to a malformed mock envelope.
+	 * For streamed turns this is the equivalent non-streamed envelope.
+	 *
+	 * Exception: generation/translation failures keep their sentinel shorthand
+	 * (`_evalMockError` / `evalMockGenerationError`) so consumers can still
+	 * attribute mock_issue — a wire envelope would hide it in message content.
+	 */
 	mockResponse: unknown;
 }
 
@@ -232,6 +244,41 @@ export class LlmWireServer {
 			`[EvalMock] wire turn root="${rootName}" stream=${String(stream)} responseHead=${JSON.stringify(mockResponse?.body ?? null).slice(0, 300)}`,
 		);
 
+		// Translate to the wire envelope BEFORE the ledger write so the ledger
+		// records exactly what the SDK receives (a translator throw records an
+		// error turn instead of leaking the handler's internal shorthand).
+		let wireBody: Record<string, unknown>;
+		try {
+			wireBody = adapter.forwardObject(mockResponse, model);
+		} catch (error) {
+			const message = error instanceof Error ? error.message : String(error);
+			this.options.logger.error(
+				`[EvalMock] Wire-server envelope translation failed for root "${rootName}": ${message}`,
+			);
+			try {
+				this.options.onIntercept?.({
+					rootName,
+					url: synthetic.url,
+					method: synthetic.method ?? 'POST',
+					nodeType: subNode.type,
+					requestBody: req.body,
+					mockResponse: { evalMockGenerationError: `envelope translation failed: ${message}` },
+				});
+			} catch {
+				// Ledger write must never block the error response.
+			}
+			this.respondWithError(adapter, res, message);
+			return;
+		}
+
+		// A handler-returned generation failure carries the `_evalMockError` sentinel
+		// that ledger consumers key on to attribute mock_issue. Translating it into a
+		// vendor envelope buries the sentinel in assistant message content, so record
+		// the shorthand verbatim for this branch — a failed generation has no
+		// meaningful "what the node received" beyond the failure itself.
+		const ledgerBody =
+			mockResponse && isMockErrorSentinel(mockResponse) ? mockResponse.body : wireBody;
+
 		// Ledger write BEFORE the response so consumers see the entry deterministically
 		// after `await fetch(...)`. `requestBody` is stored by reference (express.json
 		// never re-touches it); callers must not mutate. A thrown `onIntercept` never
@@ -243,7 +290,7 @@ export class LlmWireServer {
 				method: synthetic.method ?? 'POST',
 				nodeType: subNode.type,
 				requestBody: req.body,
-				mockResponse: mockResponse?.body,
+				mockResponse: ledgerBody,
 			});
 		} catch (error) {
 			const message = error instanceof Error ? error.message : String(error);
@@ -254,7 +301,7 @@ export class LlmWireServer {
 			if (stream) {
 				this.writeSseResponse(adapter, req, res, mockResponse, model);
 			} else {
-				res.status(200).json(adapter.forwardObject(mockResponse, model));
+				res.status(200).json(wireBody);
 			}
 		} catch (error) {
 			const message = error instanceof Error ? error.message : String(error);
diff --git a/packages/cli/src/modules/instance-ai/eval/mock-handler.ts b/packages/cli/src/modules/instance-ai/eval/mock-handler.ts
index edd719649f4..99a20e03015 100644
--- a/packages/cli/src/modules/instance-ai/eval/mock-handler.ts
+++ b/packages/cli/src/modules/instance-ai/eval/mock-handler.ts
@@ -271,7 +271,7 @@ function buildMockCacheKey(
 }
 
 /** True for the fallback body `generateMockResponse` returns when all attempts failed. */
-function isMockErrorSentinel(response: EvalMockHttpResponse): boolean {
+export function isMockErrorSentinel(response: EvalMockHttpResponse): boolean {
 	const body = response.body;
 	return (
 		typeof body === 'object' && body !== null && !Array.isArray(body) && '_evalMockError' in body