diff --git a/src/alliance/tools/guided-query-tool.test.ts b/src/alliance/tools/guided-query-tool.test.ts
index bcc5b68..dcc5514 100644
--- a/src/alliance/tools/guided-query-tool.test.ts
+++ b/src/alliance/tools/guided-query-tool.test.ts
@@ -107,6 +107,36 @@ describe('guided_query tool handler', () => {
     expect(trace.rerank_status).toBe('skipped_no_model');
   });
 
+  it('guided_query: redacts API keys from returned MCP content', async () => {
+    const apiKey = 'pcsk_guided_secret_1234567890abcdef';
+    mockedGetClient.mockReturnValue({
+      query: vi.fn().mockResolvedValue(
+        makeHybridQueryResult({
+          degraded: true,
+          degradation_reason: `rerank_failed: ${apiKey}`,
+          results: [makeSearchResult({ reranked: false })],
+        })
+      ),
+      count: vi.fn().mockResolvedValue({ count: 7, truncated: false }),
+    } as never);
+
+    const server = createMockServer();
+    registerGuidedQueryTool(server as never);
+
+    const raw = await server.getHandler('guided_query')!({
+      user_query: 'What does the paper say about contracts?',
+      namespace: 'papers',
+      top_k: 8,
+      preferred_tool: 'auto',
+      enrich_urls: false,
+    });
+    const body = parseToolJson(raw);
+    const result = body.result as Record<string, unknown>;
+
+    expect(JSON.stringify(raw)).not.toContain(apiKey);
+    expect(result.degradation_reason).toBe('rerank_failed: ***');
+  });
+
   it('runs query_detailed path on auto when user asks for content', async () => {
     const server = createMockServer();
     registerGuidedQueryTool(server as never);
diff --git a/src/core/server/tool-response-redaction.test.ts b/src/core/server/tool-response-redaction.test.ts
new file mode 100644
index 0000000..e522e79
--- /dev/null
+++ b/src/core/server/tool-response-redaction.test.ts
@@ -0,0 +1,61 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { redactApiKey, setLogLevel } from '../../logger.js';
+import { classifyToolCatchError } from './tool-error.js';
+import { jsonErrorResponse, jsonResponse } from './tool-response.js';
+import { assertToolError, parseToolJson } from './tools/test-helpers.js';
+
+describe('MCP response redaction', () => {
+  const apiKey = 'pcsk_response_secret_1234567890abcdef';
+
+  afterEach(() => {
+    setLogLevel('INFO');
+  });
+
+  it('redacts pcsk_ Pinecone API key strings directly', () => {
+    expect(redactApiKey(`connection failed for ${apiKey}`)).toBe('connection failed for ***');
+  });
+
+  it('redacts tool error message and suggestion fields before returning MCP content', () => {
+    const raw = jsonErrorResponse({
+      code: 'PINECONE_ERROR',
+      message: `Pinecone SDK error for ${apiKey}`,
+      recoverable: false,
+      suggestion: `Rotate ${apiKey}`,
+    });
+    const err = assertToolError(raw);
+
+    expect(JSON.stringify(raw)).not.toContain(apiKey);
+    expect(err.message).toBe('Pinecone SDK error for ***');
+    expect(err.suggestion).toBe('Rotate ***');
+  });
+
+  it('redacts catch-all DEBUG error messages before returning MCP content', () => {
+    setLogLevel('DEBUG');
+    const raw = jsonErrorResponse(
+      classifyToolCatchError(new Error(`Pinecone connection string ${apiKey}`), 'fallback')
+    );
+    const err = assertToolError(raw);
+
+    expect(JSON.stringify(raw)).not.toContain(apiKey);
+    expect(err.message).toBe('Pinecone connection string ***');
+  });
+
+  it('redacts nested success payload metadata before returning MCP content arrays', () => {
+    const body = parseToolJson(
+      jsonResponse({
+        status: 'success',
+        degradation_reason: `rerank_failed: ${apiKey}`,
+        decision_trace: {
+          note: `Authorization: Bearer ${apiKey}`,
+        },
+        diagnostic_metadata: {
+          nested: [`api_key=${apiKey}`],
+        },
+      })
+    );
+
+    expect(JSON.stringify(body)).not.toContain(apiKey);
+    expect(body.degradation_reason).toBe('rerank_failed: ***');
+    expect(JSON.stringify(body)).toContain('***');
+  });
+});
diff --git a/src/core/server/tool-response.ts b/src/core/server/tool-response.ts
index 0c247a6..c0d2e7c 100644
--- a/src/core/server/tool-response.ts
+++ b/src/core/server/tool-response.ts
@@ -1,5 +1,6 @@
 import type { ToolError } from './tool-error.js';
 import { toolErrorSchema } from './tool-error.js';
+import { redactValue } from '../../logger.js';
 
 export type TextPayload = {
   content: Array<{ type: 'text'; text: string }>;
@@ -12,7 +13,7 @@ export function jsonResponse(payload: unknown): TextPayload {
     content: [
       {
         type: 'text',
-        text: JSON.stringify(payload, null, 2),
+        text: JSON.stringify(redactValue(payload), null, 2),
       },
     ],
   };
@@ -20,7 +21,7 @@ export function jsonResponse(payload: unknown): TextPayload {
 
 /** Build an MCP tool error payload with JSON-stringified {@link ToolError} and isError: true. */
 export function jsonErrorResponse(err: ToolError): TextPayload {
-  const validated = toolErrorSchema.parse(err);
+  const validated = toolErrorSchema.parse(redactValue(err));
   return {
     isError: true,
     content: [
diff --git a/src/core/server/tools/query-tool.test.ts b/src/core/server/tools/query-tool.test.ts
index ec18544..c913d6a 100644
--- a/src/core/server/tools/query-tool.test.ts
+++ b/src/core/server/tools/query-tool.test.ts
@@ -245,6 +245,33 @@ describe('query tool handler (preset-driven)', () => {
     expect(body.degradation_reason).toMatch(/rerank_skipped_no_model/);
   });
 
+  it('query: redacts API keys from degradation reasons before returning content', async () => {
+    const apiKey = 'pcsk_query_secret_1234567890abcdef';
+    mockedGetClient.mockReturnValue({
+      query: vi.fn().mockResolvedValue(
+        makeHybridQueryResult({
+          degraded: true,
+          degradation_reason: `rerank_failed: ${apiKey}`,
+        })
+      ),
+      count: vi.fn(),
+    } as never);
+
+    const server = createMockServer();
+    registerQueryTool(server as never);
+
+    const raw = await server.getHandler('query')!({
+      query_text: 'hello',
+      namespace: 'wg21',
+      top_k: 3,
+      preset: 'detailed',
+    });
+    const body = parseToolJson(raw);
+
+    expect(JSON.stringify(raw)).not.toContain(apiKey);
+    expect(body.degradation_reason).toBe('rerank_failed: ***');
+  });
+
   it('query: surfaces unreranked hits when client returns reranked:false (rerank fallback shape)', async () => {
     mockedGetClient.mockReturnValue({
       query: vi.fn().mockResolvedValue(
diff --git a/src/logger.ts b/src/logger.ts
index 1baf5d0..fbcacd3 100644
--- a/src/logger.ts
+++ b/src/logger.ts
@@ -59,13 +59,14 @@ export function redactApiKey(s: string): string {
     /[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}/g,
     '***'
   );
+  out = out.replace(/\bpcsk_[A-Za-z0-9_-]+\b/g, '***');
   out = out.replace(/(api[_-]?key["':\s=]+)([^\s"',}]+)/gi, '$1***');
   out = out.replace(/(Authorization:\s*Bearer\s+)([^\s"',}]+)/gi, '$1***');
   return out;
 }
 
 /** Recursively redact API keys from a serializable value. Returns a deep copy. */
-function redactValue(value: unknown, seen: WeakSet<object> = new WeakSet()): unknown {
+export function redactValue(value: unknown, seen: WeakSet<object> = new WeakSet()): unknown {
   if (typeof value === 'string') return redactApiKey(value);
   if (value === null || value === undefined) return value;
   if (typeof value === 'object') {