Call schema on inputSchema validation failure

[clemmy]

Is your feature request related to a problem? Please describe.

Currently, it's pretty hard to tie in custom logic to input schema validation. If a tool is invoked with the wrong input schema, I would like to be able to add logging logic to track this. This would be helpful for observability of my tool-calls and to better tune the tool names / params / descriptions to help with LLM usage.

Describe the solution you'd like

It would be ideal if we can specify a callback to call on input schema validation failures.

Describe alternatives you've considered

One option is to not pass in inputSchema and just completely handle input schema validation in the handler. This seems a bit unideal when the SDK already gives a more official inputSchema arg.

Another option is by intercepting the response downstream and "detecting" it's an inputSchema validation error. This also seems a bit hacky to do and at the wrong layer.

[aptsalt]

I'd like to take this. Adding an \ callback to \ that fires before the validation error is returned to the client. PR incoming.

[aptsalt]

I would like to take this. Adding an onInputValidationError callback to McpServerOptions that fires before the validation error is returned to the client. PR incoming.

[jb4e]

+1 The observability aspect is what is most important for us. We hit an issue where Claude was incorrectly passing null for a parameter and the validation was failing, but we had no way of knowing. We want to log validation errors so we can change the schema or provide extra guidance to the LLM.