TypeScript SDK tool with outputSchema swallows thrown errors (MCP error -32602)

Question

When registering a tool with an outputSchema using the McpServer class in @modelcontextprotocol/typescript-sdk, errors thrown inside the tool call are swallowed and replaced with MCP error -32602: Tool ${toolName} has an output schema but no structured content was provided. The same happens when returning { content: ..., isError: true }. Which SDK version fixes this, and what is the workaround for older versions?

Originally asked on GitHub: https://github.com/modelcontextprotocol/typescript-sdk/issues/699

AgentArmorHQ team · Accepted Answer

Fixed in @modelcontextprotocol/typescript-sdk 1.15.0. The issue was closed as stale (NOT_PLANNED), but a commenter verified the fix on 1.15.0: the framework now automatically catches errors thrown inside a tool call and converts them to the standard message format with `{isError: true, ...}`, so your original error propagates instead of being replaced by the confusing `MCP error -32602: Tool ${toolName} has an output schema but no structured content was provided` message. Upgrade to 1.15.0 or later and re-test.

If you cannot upgrade and must use outputSchema with the McpServer class, the workaround shown in the thread is to always include a `structuredContent` object in the tool result that matches the outputSchema, in addition to the text content. For example, a tool with an outputSchema describing `{temperature, conditions, humidity}` should return:

```json
{
  "content": [
    {
      "type": "text",
      "text": "{\"temperature\": 22.5, \"conditions\": \"Partly cloudy\", \"humidity\": 65}"
    }
  ],
  "structuredContent": {
    "temperature": 22.5,
    "conditions": "Partly cloudy",
    "humidity": 65
  }
}
```

This requirement is visible in the source around `src/server/mcp.ts` lines 203-204 but is not documented, which is why the error message is misleading. As general advice: treat `structuredContent` as mandatory whenever a tool declares an outputSchema, since older SDK versions do not map thrown errors to `isError` on their own.

AgentArmorHQ team · Answer

**What a GitHub user tried on GitHub** (Didn't work)

Registered a tool with an outputSchema and threw an error inside the tool call. The thrown error was swallowed and replaced with `MCP error -32602: Tool ... has an output schema but no structured content was provided`; returning `{ content: ..., isError: true }` had the same result.

Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/699

AgentArmorHQ team · Answer

**What @sergii-kamenskyi tried on GitHub** (Worked)

Upgraded to @modelcontextprotocol/typescript-sdk 1.15.0 and let the tool throw errors. The framework now automatically catches the error and converts it to the message format with `{isError: true, ...}`.

Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/699#issuecomment-3035370884

AgentArmorHQ team · Answer

**What @z3media tried on GitHub** (Outcome unclear)

Suggested adding a `structuredContent` object that matches the outputSchema to every tool result alongside the text content, providing a full tool definition and JSON-RPC response example to demonstrate the expected shape.

Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/699#issuecomment-3570254234

TypeScript SDK tool with outputSchema swallows thrown errors (MCP error -32602)

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TypeScript SDK tool with outputSchema swallows thrown errors (MCP error -32602)

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