How do I use plain JSON Schema instead of Zod for MCP tool schemas in typescript-sdk?

4 answers

• Best answerAgentArmor teamTeam2d ago

  Status: fixed. The issue is closed as completed. A contributor said that in SDK v2 (2.0.0, PR #1689), tool schemas can come from any Standard Schema library that can produce JSON Schema, and that fromJsonSchema() wraps a plain JSON Schema you already have. v1 still uses Zod for registerTool().

  For v1, a commenter said you can skip the helpers and use the low-level server. Register handlers on server.server for ListToolsRequestSchema (from @modelcontextprotocol/sdk/types.js) that return tools with a plain JSON Schema inputSchema, and a handler for CallToolRequestSchema that reads request.params.arguments. This was offered as a possibility; the thread does not confirm anyone tested it, and the example shown there does not validate input.

  General advice: if you use this route, validate request.params.arguments against your schema yourself (for example with Ajv) before using it, since the SDK does not do that for you in this path.

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• AgentArmor teamTeam2d ago

  What @Rutledge tried on GitHub (Didn't work)

  Passed plain JSON Schema generated by Stainless (from OpenAPI) to server.tool(). The SDK ran zodToJsonSchema on it, so parameters did not appear in MCP Inspector and tools received undefined for all parameters.

  Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/283#issuecomment-3016034013 (opens external site) ↗

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• AgentArmor teamTeam2d ago

  What @finom tried on GitHub (Didn't work)

  Had only a JSON Schema, using @vercel/mcp-adapter, and tried to convert it to Zod for the SDK. The conversion attempts were unsuccessful.

  Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/283#issuecomment-3015383861 (opens external site) ↗

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• AgentArmor teamTeam2d ago

  What @dan-kwiat tried on GitHub (Outcome unclear)

  Suggested bypassing the helpers by registering ListToolsRequestSchema and CallToolRequestSchema handlers on server.server and returning tools with a plain JSON Schema inputSchema. No one in the thread confirmed this was tested.

  Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/283#issuecomment-3368985570 (opens external site) ↗

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