[BUG] MCP tool calls with dict/array parameters serialized as strings, fails mid-session after working correctly

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

MCP tool calls with an object or array-shaped parameter fail validation with the parameter arriving as a JSON string instead of a parsed object/dict. Simple scalar-only parameters (plain strings/ints) continue to work normally. This matches the pattern already described in #5504, #3084, #2747, and #6249, but those threads are closed/locked, so filing fresh per the bot's own instruction on #5504.

What Should Happen?

The tool call's object/array parameter should arrive at the MCP server as a parsed dict/array, not as a raw JSON string, so the server's schema validation succeeds.

Error Messages/Logs

1 validation error for call[<tool_name>]
body
  Input should be a valid dictionary or instance of <SomeQuery> [type=model_type, input_value='{"data": {...}}', input_type=str]

Steps to Reproduce

1. In a long-running Claude Code session, call an MCP tool whose schema expects an object-shaped parameter (e.g. a "body" field containing a nested dict), passing a well-formed JSON object.
2. Observe the call succeeds initially.
3. Later in the same session (no code/prompt changes on the client side), call the same tool again with an equivalently-shaped object.
4. Observe it now fails with the error above, as if the object were stringified before reaching the server.
5. Restarting the session does not resolve it.
6. Tried multiple payload shapes/field names to rule out a schema mismatch; all fail identically, pointing at client-side serialization rather than the tool's own schema.

Affected in this case: two independent MCP servers (a Klaviyo integration and a separate image-generation service). Related closed/locked threads: #5504, #3084, #2747, #6249.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

same session, no version change

Claude Code Version

2.1.62 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_