Invalid field detected by input validation returns a jsonrpc error instead of a result with isError:true

[iwyrkore]

Describe the bug
When you call a tool with an argument field that's not in the input schema, it returns a jsonrpc "error" message instead of a result with isError:true so the client LLM can see it.

To Reproduce
Steps to reproduce the behavior:

1. Call any tool with an invalid argument.
2. Check the result error.

It looks something like this:

data: {"jsonrpc":"2.0","id":2,"error":{"code":0,"message":"unmarshaling: json: unknown field \"foobar\""}}

Expected behavior
I expected to see an error something like this:

 {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"error unmarshaling: json: unknown field \"foobar\")"}],"isError":true}}

[findleyr]

Hmm, I'm not sure this is right: the typescript SDK also treats parse and validation errors as JSONRPC errors:
https://github.com/modelcontextprotocol/typescript-sdk/blob/ebf39330f3a90fe9e74512788c11533a9be720e1/src/server/mcp.ts#L167

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#error-handling is a little ambiguous: what's the difference between "invalid arguments" and "invalid input data"?

[findleyr]

(Need to check the behavior of more SDKs).

[iwyrkore]

I read "invalid arguments" as an invalid jsonrpc message or content and "invalid input data" as a schema-mismatch for the arguments the tool expects. From what I understood (can't remember where I read it) the content of the Protocol Errors may not make it back to the LLM to be able to fix, although obviously an error like an unknown tool should.

[findleyr]

Yeah, that's what I thought as well. It makes sense to return this as a tool error so that the LLM can fix it.

I'll do more investigation. If you have evidence that other official SDKs behave differently, please let me know.

[iwyrkore]

Well I guess some LLMs get both types of error content because my MCP servers (with both typescript and go SDKs) work and the LLM in Cursor can see when it sends invalid args. But not in Claude.

I was using the typescript SDK and it returns an error like {"jsonrpc":"2.0","id":7,"error":{"code":-32602,"message":"MCP error -32602: Invalid arguments for tool xyz: [\n {\n \"code\": \"invalid_union\",\n \"unionErrors\": [\n ... for invalid args and Claude can't see them:

> when you got an error the first time, did you see details of what the error was?

No, I didn't see any specific error details. When the tool execution failed, I only received a generic "Tool execution failed" message without any additional information about what went wrong.