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Can't call tools #110
Description
Describe the bug
A clear and concise description of what the bug is.
Using the example tool in the readme and running that script to open a stdio server, you cannot call the tool with json_rpc. It returns an error.
To Reproduce
Steps to reproduce the behavior:
- Copy the example tool script from the readme into a ruby file and run it:
ruby <file.rb> - use json rpc in the same console to request the tool:
{"jsonrpc":"2.0","id":"2","method":"tools/call", "params":{"name":"example_tool"}}
{"jsonrpc":"2.0","id":"2","error":{"code":-32603,"message":"Internal error","data":"Internal error handling tools/call request"}}
Expected behavior
It should run the tool and give the proper output.
Activity
atesgoral commentedon Aug 20, 2025
@WriterZephos Are you using
paraminstead ofparamsor was that a typo in the issue description?WriterZephos commentedon Aug 25, 2025
@atesgoral That was a mistake in the issue description. I replaced it with a copy directly from my terminal.
koic commentedon Aug 29, 2025
As shown in the
tools/listresponse with"required":["message"], themessagefield is mandatory.{"jsonrpc":"2.0","id":"2","method":"tools/list"} {"jsonrpc":"2.0","id":"2","result":{"tools":[{"name":"example_tool","description":"A simple example tool that echoes back its arguments","inputSchema":{"type":"object","properties":{"message":{"type":"string"}},"required":["message"]}}]}}Therefore, the
tools/callrequest must include this required argument"message", for example:{"jsonrpc":"2.0","id":"3","method":"tools/call","params":{"name":"example_tool","arguments":{"message":"Hello"}}}I’ve opened #120 to add a
tools/callexample to the README.In the next release of the mcp gem, the error message will be updated as shown below, which should make it easier to identify the cause compared to the current mcp 0.2.0.
Before:
{"jsonrpc":"2.0","id":"2","method":"tools/call", "params":{"name":"example_tool"}} {"jsonrpc":"2.0","id":"2","error":{"code":-32603,"message":"Internal error","data":"Internal error handling tools/call request"}}After:
{"jsonrpc":"2.0","id":"2","method":"tools/call", "params":{"name":"example_tool"}} {"jsonrpc":"2.0","id":"2","error":{"code":-32603,"message":"Internal error","data":"Missing required arguments: message"}}[Docs] Add a `tools/call` example in README