Add JSON validation to MCP tool responses

packages/server/src/server/agent/mcp-server.ts

@@ -2,6 +2,7 @@ import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { z } from "zod";
 import { homedir } from "node:os";
 import { resolve } from "node:path";
+import { ensureValidJson } from "../json-utils.js";
 import type {
   AgentPromptInput,
   AgentProvider,
@@ -294,14 +295,14 @@ export async function createAgentMcpServer(
 
       return {
         content: [],
-        structuredContent: {
+        structuredContent: ensureValidJson({
           agentId: snapshot.id,
           type: provider,
           status: snapshot.status,
           cwd: snapshot.cwd,
           currentModeId: snapshot.currentModeId,
           availableModes: snapshot.availableModes,
-        },
+        }),
       };
     }
   );
@@ -333,12 +334,12 @@ export async function createAgentMcpServer(
 
       return {
         content: [],
-        structuredContent: {
+        structuredContent: ensureValidJson({
           agentId,
           status: result.status,
           permission: result.permission,
           activity,
-        },
+        }),
       };
     }
   );
@@ -372,10 +373,10 @@ export async function createAgentMcpServer(
 
       return {
         content: [],
-        structuredContent: {
+        structuredContent: ensureValidJson({
           success: true,
           status: snapshot?.status ?? "idle",
-        },
+        }),
       };
     }
   );
@@ -400,15 +401,16 @@ export async function createAgentMcpServer(
         throw new Error(`Agent ${agentId} not found`);
       }
 
+      const structuredSnapshot = await serializeSnapshotWithMetadata(
+        agentRegistry,
+        snapshot
+      );
       return {
         content: [],
-        structuredContent: {
+        structuredContent: ensureValidJson({
           status: snapshot.status,
-          snapshot: await serializeSnapshotWithMetadata(
-            agentRegistry,
-            snapshot
-          ),
-        },
+          snapshot: structuredSnapshot,
+        }),
       };
     }
   );
@@ -432,7 +434,7 @@ export async function createAgentMcpServer(
       );
       return {
         content: [],
-        structuredContent: { agents },
+        structuredContent: ensureValidJson({ agents }),
       };
     }
   );
@@ -454,7 +456,7 @@ export async function createAgentMcpServer(
       const success = await agentManager.cancelAgentRun(agentId);
       return {
         content: [],
-        structuredContent: { success },
+        structuredContent: ensureValidJson({ success }),
       };
     }
   );
@@ -475,7 +477,7 @@ export async function createAgentMcpServer(
       await agentManager.closeAgent(agentId);
       return {
         content: [],
-        structuredContent: { success: true },
+        structuredContent: ensureValidJson({ success: true }),
       };
     }
   );
@@ -513,14 +515,14 @@ export async function createAgentMcpServer(
       if (format === "curated") {
         return {
           content: [],
-          structuredContent: {
+          structuredContent: ensureValidJson({
             agentId,
             format: "curated" as const,
             updateCount: timeline.length,
             currentModeId: snapshot?.currentModeId ?? null,
             content: curateAgentActivity(timeline),
             updates: null,
-          },
+          }),
         };
       }
 
@@ -529,14 +531,14 @@ export async function createAgentMcpServer(
         : timeline;
       return {
         content: [],
-        structuredContent: {
+        structuredContent: ensureValidJson({
           agentId,
           format: "raw" as const,
           updateCount: timeline.length,
           currentModeId: snapshot?.currentModeId ?? null,
           content: "",
           updates: entries,
-        },
+        }),
       };
     }
   );
@@ -560,7 +562,7 @@ export async function createAgentMcpServer(
       await agentManager.setAgentMode(agentId, modeId);
       return {
         content: [],
-        structuredContent: { success: true, newMode: modeId },
+        structuredContent: ensureValidJson({ success: true, newMode: modeId }),
       };
     }
   );
@@ -593,7 +595,7 @@ export async function createAgentMcpServer(
 
       return {
         content: [],
-        structuredContent: { permissions },
+        structuredContent: ensureValidJson({ permissions }),
       };
     }
   );
@@ -617,7 +619,7 @@ export async function createAgentMcpServer(
       await agentManager.respondToPermission(agentId, requestId, response);
       return {
         content: [],
-        structuredContent: { success: true },
+        structuredContent: ensureValidJson({ success: true }),
       };
     }
   );

packages/server/src/server/json-utils.ts

@@ -0,0 +1,60 @@
+export type JsonValue =
+  | string
+  | number
+  | boolean
+  | null
+  | JsonValue[]
+  | { [key: string]: JsonValue };
+
+/**
+ * Ensure the provided value only contains JSON-safe primitives.
+ * Throws when undefined, functions, symbols, BigInts, etc. are found.
+ */
+export function ensureValidJson<T>(value: T, rootLabel: string = "root"): T {
+  const seen = new Set<object>();
+
+  const validate = (current: unknown, path: string): void => {
+    if (current === undefined) {
+      throw new Error(`Invalid JSON value at ${path}: undefined`);
+    }
+
+    if (current === null) {
+      return;
+    }
+
+    const type = typeof current;
+
+    if (type === "string" || type === "number" || type === "boolean") {
+      return;
+    }
+
+    if (Array.isArray(current)) {
+      current.forEach((item, index) =>
+        validate(item, `${path}[${index}]`)
+      );
+      return;
+    }
+
+    if (type === "object") {
+      const asObject = current as Record<string, unknown>;
+      if (!seen.has(asObject)) {
+        seen.add(asObject);
+        for (const [key, val] of Object.entries(asObject)) {
+          const nextPath = path === rootLabel ? key : `${path}.${key}`;
+          validate(val, nextPath);
+        }
+        seen.delete(asObject);
+      }
+      return;
+    }
+
+    throw new Error(
+      `Invalid JSON value at ${path}: ${
+        type === "symbol" ? "symbol" : String(current)
+      }`
+    );
+  };
+
+  validate(value, rootLabel);
+  return value;
+}

packages/server/src/server/terminal-mcp/server.ts

@@ -1,5 +1,6 @@
 import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { z } from "zod";
+import { ensureValidJson } from "../json-utils.js";
 import { TerminalManager } from "./terminal-manager.js";
 
 export interface TerminalMcpServerOptions {
@@ -98,7 +99,7 @@ export async function createTerminalMcpServer(
       const output = { commands };
       return {
         content: [],
-        structuredContent: output,
+        structuredContent: ensureValidJson(output),
       };
     }
   );
@@ -131,7 +132,7 @@ export async function createTerminalMcpServer(
       const result = await terminalManager.captureCommand(commandId, lines);
       return {
         content: [],
-        structuredContent: result,
+        structuredContent: ensureValidJson(result),
       };
     }
   );
@@ -190,7 +191,7 @@ export async function createTerminalMcpServer(
       const result = { output: output || null };
       return {
         content: [],
-        structuredContent: result,
+        structuredContent: ensureValidJson(result),
       };
     }
   );
@@ -238,7 +239,7 @@ export async function createTerminalMcpServer(
       const result = { output: output || null };
       return {
         content: [],
-        structuredContent: result,
+        structuredContent: ensureValidJson(result),
       };
     }
   );
@@ -272,7 +273,7 @@ export async function createTerminalMcpServer(
       const output = { success: true, killedCount };
       return {
         content: [],
-        structuredContent: output,
+        structuredContent: ensureValidJson(output),
       };
     }
   );