Bug: outputSchema defaults to empty object, breaking tool output validation

[viktormarinho]

Summary

When tools don't define an outputSchema, the runtime defaults to z.object({}).shape instead of undefined. This causes the MCP SDK to validate tool outputs against an empty schema, failing with:

MCP error -32602: Structured content does not match the tool's output schema: 
data must NOT have additional properties, data must NOT have additional properties

Affected Versions

• Broken: 1.2.0, 1.2.1, 1.2.2, 1.2.3, 1.2.4 (all tested)

Location

packages/runtime/src/tools.ts - line ~685 (in v1.2.4)

Current Code (Broken)

outputSchema: isStreamableTool(tool)
  ? z.object({ bytes: z.record(z.string(), z.number()) }).shape
  : tool.outputSchema &&
      typeof tool.outputSchema === "object" &&
      "shape" in tool.outputSchema
    ? (tool.outputSchema.shape as ZodRawShape)
    : z.object({}).shape,  // ← BUG: Should be undefined

Problem

The MCP SDK's validateToolOutput() function skips validation when outputSchema is undefined:

async validateToolOutput(tool, result, toolName) {
    if (!tool.outputSchema) {
        return;  // ← Skips validation when undefined
    }
    // ... validates against schema
}

But when defaulting to z.object({}).shape, validation runs against an empty schema and fails for any tool that returns data with properties.

Suggested Fix

Change line ~685 from:

: z.object({}).shape,

To:

: undefined,

Reproduction

Any tool without explicit outputSchema that returns an object with properties:

createTool({
  id: "my_tool",
  inputSchema: z.object({}),
  // No outputSchema defined
  execute: async () => {
    return { foo: "bar" };  // ← Fails validation against empty schema
  },
});

Workaround

Postinstall patch to change the default to undefined.

Related

Discovered alongside #2326 (transport.close SSE bug)