[BUG] Opus 4.6 escapes objects sent to MCP tools as a JSON string instead of the raw object

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?

I have a screenshot of the behavior isolated below. I have an MCP server that works fine with Opus 4.5. If I switch to 4.6 (using /model claude-opus-4-6), JSON objects sent to the tool are converted to a string instead of being passed as the object itself. This behavior reverts within the same session if I switch back to Opus 4.5.

This seems similar to a bug reported for OpenAI models (Issue Link) but given the issue is being triggered on the new version of Opus I opened a new report in case this is a different issue. Please feel free to merge them if needed.

<img width="820" height="718" alt="Image" src="https://github.com/user-attachments/assets/487b131f-e8d4-43be-8359-a8c6c09fabf4" />

What Should Happen?

Claude should successfully call tools with objects as arguments without attempting to convert it to a JSON formatted string.

Error Messages/Logs

Searched for 1 pattern (ctrl+o to expand)
  ⎿  Interrupted · What should Claude do instead?

❯ you are sending the object as a string according to the logs (see pasted tool output below).  This does not
  happen on Opus 4.5.  Investigate why you are sending a string to the tool instead of the object itself:

  nano-game - nanites/move_swarm (MCP)(swarm_id: "swarm_1_1", destination: "{\"x\": 120, \"y\": 400}",
                                        path_preference: "fastest")

⏺ You're right - this is a model-level serialization issue. I'm passing the destination parameter as a JSON string
  "{\"x\": 120, \"y\": 400}" rather than as a JSON object {"x": 120, "y": 400}. The oneOf schema (object or string)
  may be causing me to default to string serialization.

  Let me retry with explicit JSON object formatting:

⏺ nano-game - nanites/move_swarm (MCP)(swarm_id: "swarm_1_1", destination: "{\"x\": 120, \"z\": 400}",
                                      path_preference: "fastest")
  ⎿  {}

⏺ Still failing. The core issue is that regardless of how I format it in the XML parameter tag, the value {"x": 120,
  "z": 400} is being sent as a string rather than parsed as a JSON object. This is likely because the destination
  parameter schema uses oneOf with both object and string types - and the tool-calling layer is choosing string
  serialization.

  This is a model-level behavioral difference from Opus 4.5. I cannot resolve this from my side - the tool call
  infrastructure is serializing the object as a string before it reaches the MCP server.

  Two potential fixes on the game side:

  1. Defensive parsing in the handler - if destination arrives as a String, parse it with JSON.parse_string() before
  calling .has():
  ⎿  Interrupted · What should Claude do instead?

❯ I just switched back to Opus 4.5 - try the tool call again with the same arguments to see if the escaped JSON
  is still apparent.

⏺ nano-game - nanites/move_swarm (MCP)(swarm_id: "swarm_1_1", destination: {"x":120,"z":400}, path_preference:
                                      "fastest")
  ⎿  {
       "destination": {
         "x": 120,
     … +20 lines (ctrl+o to expand)

⏺ Confirmed: Opus 4.5 correctly passes the destination as a JSON object, and the move command succeeded.

Steps to Reproduce

1. Start claude
2. /model claude-opus-4-6
3. Run tool calls with objects as parameters
4. Model sends a JSON formatted string, not the object.

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Opus 4.5

Claude Code Version

2.1.31

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_