fix: Prevent JSON serialization errors and accept keyword keys in tool properties

[BuddhiLW]

Summary

This PR fixes two issues encountered when building an MCP server with the SDK.

Context

While developing emacs-mcp, an MCP server that integrates Claude Code with Emacs, we encountered a crash on server startup:

Execution error (JsonGenerationException) at cheshire.generate/generate (generate.clj:155).
Cannot JSON encode object of class: class emacs_mcp.tools$handle_eval_elisp

Investigating the stack trace led us to validate-spec!, which was trying to log the full server-spec (including handler functions) when validation failed. This revealed a second issue: the spec validation itself was failing because our tool definitions used keyword keys in inputSchema.properties (e.g., :function_name), but the spec only accepted string keys.

Changes

server.clj

• Modified validate-spec! to strip :handler keys from tools, prompts, and resources before logging
• Added docstring explaining the fix

specs.clj

• Changed :tool/properties spec from (s/map-of string? any?) to (s/map-of (s/or :str string? :kw keyword?) any?) to accept both string and keyword keys

Test plan

• MCP server starts without crashing when tools use keyword keys in inputSchema
• Spec validation errors are properly logged without JSON serialization failures

🤖 Generated with Claude Code

Co-Authored-By: Pedro Branquinho [email protected]
Co-Authored-By: Claude Opus 4.5 [email protected]

[vedang]

Please submit only the change of loggable-spec and the or check for str or keyword. Undo everything else -- the indentation changes and the change to your fork.

Happy to accept the PR post that.

Thank you for your contribution and for using this SDK!

[BuddhiLW]

@vedang Done! I've cleaned up the PR to include only the essential changes:

Changes retained:

1. loggable-spec in validate-spec! - strips :handler functions before logging
2. s/or :str string? :kw keyword? in :tool/properties spec

Removed:

• All indentation changes
• deps.edn fork reference

The diff is now minimal (10 additions, 3 deletions across 2 files).

Happy to use your project; it's very useful, and enables me to write very interesting tools. Happy to collaborate!

[BuddhiLW]

Side note:

In clojure-lsp/jsonrpc4clj#2 I have also submitted an improvement (to one of your project's dependencies) which was capping one of my emacs-mcp add-ons. Where I spawn a swarm of claudes controlled by a main-thread claude (which is talking to me and giving updates - keeping me on the loop - on a high-level).

That improvement enables my agents to do multiple mcp calls asynchronously.

If the changes go through, I will open another PR to bump the versions here. I intend to not necessarily use a custom fork in the future, in order to run my project :). Maybe it's also useful to other people, right?

Regards,
Pedro

[vedang]

Thank you, merged.

Yes, I'll be happy to accept PR with version bump once your changes are merged into jsonrpc4clj!

[BuddhiLW]

By the way, the coffee you are sipping in your photo seems very tasty 😃 . Love the photo, brother.