[BUG] Claude Code is submitting MCP requests as strings when using an SSE server

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?

When connecting claude code the my MCP server via SSE, it submits the requests for endpoint that has a nested structure as stringified JSON.

  2025-11-10T15:41:34.505916Z  WARN rmcp::service: response error, id: 10, error: ErrorData { code: ErrorCode(-32602), message: "failed to deserialize parameters: invalid type: string \"{\\\"schedulePatternType\\\": {\\\"weekly\\\": {\\\"mon\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"tues\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"weds\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"thurs\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"fri\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}}}}\", expected struct SchedulePattern", data: None }

I notice this for other types as well... where sometimes a field is optional and it will pass in the string "null" (instead of the null value).

This issue does NOT happen on Claude Desktop for the same MCP server.

The issue does happen for stdio server as well, and for a workaround, we have a bridge that is intercepting the requests and converting them into proper JSON objects.

What Should Happen?

JSON RPC requests should be submitted as proper JSON objects

Error Messages/Logs

2025-11-10T15:41:34.505916Z  WARN rmcp::service: response error, id: 10, error: ErrorData { code: ErrorCode(-32602), message: "failed to deserialize parameters: invalid type: string \"{\\\"schedulePatternType\\\": {\\\"weekly\\\": {\\\"mon\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"tues\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"weds\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"thurs\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}, \\\"fri\\\": {\\\"shift\\\": {\\\"shiftType\\\": {\\\"regular\\\": {\\\"isRegular\\\": true}}, \\\"shiftTemplate\\\": {\\\"startTime\\\": {\\\"time\\\": \\\"09:00:00\\\"}, \\\"duration\\\": {\\\"seconds\\\": 28800}, \\\"breakTemplates\\\": []}}, \\\"venueId\\\": \\\"019a6e6c-db70-7308-bb4d-eaaad7e049c4\\\", \\\"roleId\\\": \\\"019a6e6c-dbd3-7c61-b32e-df0bc823d243\\\"}}}}\", expected struct SchedulePattern", data: None }

Steps to Reproduce

Have an MCP endpoint that has a nested endpoint and ask claude code to use the specified tool

Claude Model

Not sure / Multiple models

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.0.36

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_