fix(mcp): drop protocol-reserved _meta keys from model-visible output

packages/agent-core-v2/src/agent/mcp/output.ts

@@ -7,19 +7,31 @@
  *     (dropping unsupported shapes).
  *  2. Wrap media-only outputs in `<mcp_tool_result name="…">` tags so the
  *     model can attribute binary output when several tools return media.
- *  3. Apply the 100K text/think character budget to the tool's own text.
+ *  3. Serialize `structuredContent` and server `_meta` into a trailing
+ *     `<mcp-structured-result>` text part — appended after the media wrap so
+ *     a media-only result keeps its attribution tags, and before the text
+ *     budget so oversized payloads stay bounded. Literal closing tags inside
+ *     the serialized payload are stripped so server data cannot fake an
+ *     early end of the block. `_meta` keys with a protocol-reserved prefix
+ *     (a dot-separated label list containing `modelcontextprotocol` or
+ *     `mcp`, per the spec's key-name rules) are dropped first: they carry
+ *     host/protocol plumbing rather than model-facing data, while unprefixed
+ *     and vendor-prefixed keys pass through because their semantics belong
+ *     to the server. Non-serialisable payloads drop the whole block rather
+ *     than failing the call.
+ *  4. Apply the 100K text/think character budget to the tool's own text.
  *     This runs BEFORE captions exist, so a chatty tool (page text + a
  *     screenshot) can never evict or slice the compression caption — that
  *     would silently reintroduce the very degradation the caption reports.
- *  4. Compress oversized inline images, announcing each compression with a
+ *  5. Compress oversized inline images, announcing each compression with a
  *     caption (original vs. sent size, readback path to the persisted
  *     original) so downsampling is never silent. The captions ride the
  *     result's `note` side channel — projected to the model at fold time, but
  *     kept out of `output` so UIs never render them.
- *  5. Apply the per-part 10 MB binary cap: oversized binary parts
+ *  6. Apply the per-part 10 MB binary cap: oversized binary parts
  *     (image/audio/video URLs) collapse to a notice, so a single
  *     screenshot cannot evict every text part.
- *  6. Collapse a single-text-part result to a plain string output; otherwise
+ *  7. Collapse a single-text-part result to a plain string output; otherwise
  *     emit the `ContentPart[]` as-is.
  *
  * `mcpResultToExecutableOutput` is the single entry point; the per-step
@@ -146,32 +158,23 @@ export async function mcpResultToExecutableOutput(
   }
 
   const wrapped = wrapMediaOnly(converted, qualifiedToolName);
-  // Structured payloads (structuredContent per MCP spec, plus server metadata
-  // in _meta) carry machine-readable contracts such as browser-handoff URLs.
-  // Appended AFTER the media wrap so a media-only result keeps its
-  // <mcp_tool_result> attribution, and BEFORE the text budget so oversized
-  // payloads stay bounded. Literal closing tags inside the serialized
-  // payload are stripped so server data cannot fake an early end of the
-  // block.
   const structuredExtras: Record<string, unknown> = {};
   if (result.structuredContent !== undefined) {
     structuredExtras['structuredContent'] = result.structuredContent;
   }
   if (result._meta !== undefined) {
-    structuredExtras['_meta'] = result._meta;
+    const meta = stripReservedMetaKeys(result._meta);
+    if (meta !== undefined) {
+      structuredExtras['_meta'] = meta;
+    }
   }
   if (Object.keys(structuredExtras).length > 0) {
-    try {
-      const serialized = JSON.stringify(structuredExtras).replaceAll(
-        '</mcp-structured-result>',
-        '',
-      );
+    const serialized = serializeStructuredExtras(structuredExtras);
+    if (serialized !== undefined) {
       wrapped.push({
         type: 'text',
         text: `\n<mcp-structured-result>\n${serialized}\n</mcp-structured-result>`,
       });
-    } catch {
-      // Non-serialisable payloads are dropped rather than failing the call.
     }
   }
 
@@ -202,6 +205,33 @@ export async function mcpResultToExecutableOutput(
   };
 }
 
+function serializeStructuredExtras(extras: Record<string, unknown>): string | undefined {
+  try {
+    return JSON.stringify(extras).replaceAll('</mcp-structured-result>', '');
+  } catch {
+    return undefined;
+  }
+}
+
+function stripReservedMetaKeys(
+  meta: Record<string, unknown>,
+): Record<string, unknown> | undefined {
+  const out: Record<string, unknown> = {};
+  for (const [key, value] of Object.entries(meta)) {
+    if (!isReservedMetaKey(key)) {
+      out[key] = value;
+    }
+  }
+  return Object.keys(out).length > 0 ? out : undefined;
+}
+
+function isReservedMetaKey(key: string): boolean {
+  const slash = key.indexOf('/');
+  if (slash <= 0) return false;
+  const labels = key.slice(0, slash).split('.');
+  return labels.some((label) => label === 'modelcontextprotocol' || label === 'mcp');
+}
+
 function wrapMediaOnly(parts: readonly ContentPart[], qualifiedToolName: string): ContentPart[] {
   const hasMedia = parts.some(
     (p) => p.type === 'image_url' || p.type === 'audio_url' || p.type === 'video_url',

packages/agent-core-v2/test/agent/mcp/output.test.ts

@@ -317,6 +317,40 @@ describe('mcpResultToExecutableOutput', () => {
     expect(joined.split('</mcp-structured-result>')).toHaveLength(2);
   });
 
+  test('drops protocol-reserved _meta keys and keeps vendor namespaces', async () => {
+    const out = await mcpResultToExecutableOutput(
+      {
+        content: [{ type: 'text', text: 'ok' }],
+        isError: false,
+        _meta: {
+          'modelcontextprotocol.io/progress': 1,
+          'tools.mcp.com/trace': 'x',
+          'example.com/custom': 2,
+          vendorKey: 3,
+        },
+      },
+      'mcp__s__t',
+    );
+    const parts = out.output as ContentPart[];
+    const joined = parts.map((p) => (p.type === 'text' ? p.text : '')).join('');
+    expect(joined).not.toContain('modelcontextprotocol.io/progress');
+    expect(joined).not.toContain('tools.mcp.com/trace');
+    expect(joined).toContain('"example.com/custom":2');
+    expect(joined).toContain('"vendorKey":3');
+  });
+
+  test('omits the structured block when every _meta key is protocol-reserved', async () => {
+    const out = await mcpResultToExecutableOutput(
+      {
+        content: [{ type: 'text', text: 'ok' }],
+        isError: false,
+        _meta: { 'mcp.dev/internal': true },
+      },
+      'mcp__s__t',
+    );
+    expect(out).toEqual({ output: 'ok', isError: false });
+  });
+
   test('returns an empty output array when the content array is empty', async () => {
     const out = await mcpResultToExecutableOutput(result([]), 'mcp__s__t');
     expect(out).toEqual({ output: [], isError: false });

packages/agent-core/src/mcp/output.ts

@@ -193,13 +193,17 @@ export async function mcpResultToExecutableOutput(
   // <mcp_tool_result> attribution, and BEFORE the text budget so oversized
   // payloads stay bounded. Literal closing tags inside the serialized
   // payload are stripped so server data cannot fake an early end of the
-  // block.
+  // block. Protocol-reserved _meta keys are dropped first: those carry
+  // host/protocol plumbing, not model-facing data.
   const structuredExtras: Record<string, unknown> = {};
   if (result.structuredContent !== undefined) {
     structuredExtras['structuredContent'] = result.structuredContent;
   }
   if (result._meta !== undefined) {
-    structuredExtras['_meta'] = result._meta;
+    const meta = stripReservedMetaKeys(result._meta);
+    if (meta !== undefined) {
+      structuredExtras['_meta'] = meta;
+    }
   }
   if (Object.keys(structuredExtras).length > 0) {
     try {
@@ -257,6 +261,41 @@ export async function mcpResultToExecutableOutput(
   };
 }
 
+/**
+ * Drop protocol-reserved `_meta` keys before the payload reaches the model.
+ *
+ * Per the MCP spec's `_meta` key-name rules, a key may carry a dot-separated
+ * label prefix terminated by `/`, and prefixes containing a
+ * `modelcontextprotocol` or `mcp` label are reserved for protocol use (e.g.
+ * `modelcontextprotocol.io/…`, `tools.mcp.com/…`). Reserved entries carry
+ * host/protocol plumbing — progress and task wiring, UI component payloads —
+ * that servers do not address to the model, so forwarding them would leak
+ * side-channel data into the conversation. Unprefixed and vendor-prefixed
+ * keys pass through untouched: their semantics belong to the server, and the
+ * host cannot know which of them the model is meant to see.
+ *
+ * Returns `undefined` when nothing survives, so callers can omit the `_meta`
+ * section entirely.
+ */
+function stripReservedMetaKeys(
+  meta: Record<string, unknown>,
+): Record<string, unknown> | undefined {
+  const out: Record<string, unknown> = {};
+  for (const [key, value] of Object.entries(meta)) {
+    if (!isReservedMetaKey(key)) {
+      out[key] = value;
+    }
+  }
+  return Object.keys(out).length > 0 ? out : undefined;
+}
+
+function isReservedMetaKey(key: string): boolean {
+  const slash = key.indexOf('/');
+  if (slash <= 0) return false;
+  const labels = key.slice(0, slash).split('.');
+  return labels.some((label) => label === 'modelcontextprotocol' || label === 'mcp');
+}
+
 /**
  * If `parts` contains media but no non-empty text, surround it with
  * `<mcp_tool_result name="…">` text tags so the model can attribute the

packages/agent-core/test/mcp/output.test.ts

@@ -316,6 +316,40 @@ describe('mcpResultToExecutableOutput', () => {
     expect(joined.split('</mcp-structured-result>')).toHaveLength(2);
   });
 
+  test('drops protocol-reserved _meta keys and keeps vendor namespaces', async () => {
+    const out = await mcpResultToExecutableOutput(
+      {
+        content: [{ type: 'text', text: 'ok' }],
+        isError: false,
+        _meta: {
+          'modelcontextprotocol.io/progress': 1,
+          'tools.mcp.com/trace': 'x',
+          'example.com/custom': 2,
+          vendorKey: 3,
+        },
+      },
+      'mcp__s__t',
+    );
+    const parts = out.output as ContentPart[];
+    const joined = parts.map((p) => (p.type === 'text' ? p.text : '')).join('');
+    expect(joined).not.toContain('modelcontextprotocol.io/progress');
+    expect(joined).not.toContain('tools.mcp.com/trace');
+    expect(joined).toContain('"example.com/custom":2');
+    expect(joined).toContain('"vendorKey":3');
+  });
+
+  test('omits the structured block when every _meta key is protocol-reserved', async () => {
+    const out = await mcpResultToExecutableOutput(
+      {
+        content: [{ type: 'text', text: 'ok' }],
+        isError: false,
+        _meta: { 'mcp.dev/internal': true },
+      },
+      'mcp__s__t',
+    );
+    expect(out).toEqual({ output: 'ok', isError: false });
+  });
+
   test('returns an empty string when the content array is empty', async () => {
     const out = await mcpResultToExecutableOutput(result([]), 'mcp__s__t');
     // No parts survive; collapseSingleText has nothing to collapse so the