Claude models (and Gemini Argon) fail with HTTP 400 “request body is not valid JSON”; other models work #5139

Describe the bug

I consistently encounter errors when using Claude Sonnet 5.5 and Claude Opus 5.5 in GitHub Copilot CLI. Other models appear to work correctly in the same environment. EDIT: also found similar problems with Gemini Argon model

The issue affects me, but colleagues at the same company do not experience it. I have already performed a clean installation of Copilot CLI, and the problem persists. I have also ruled out MCP servers as the cause.

Environment

• GitHub Copilot CLI: 1.0.97
• OS: macOS, ARM64
• Affected models: Claude Sonnet 5.5 and Claude Opus 5.5
• Provider: GitHub Copilot

Steps to reproduce

1. Start a Copilot CLI session.
2. Select Claude Sonnet 5.5 or Claude Opus 5.5.
3. Send a prompt. Even a simple greeting such as “hola” failed with Opus.
4. The request fails before any assistant response or tool execution.
5. Switch to a non-Claude model; the session works.

Expected behavior

Claude models should respond normally, as they do for my colleagues.

Actual behavior

Requests to  /v1/messages  fail with HTTP 400 and JSON parsing errors.

For Claude Sonnet 5.5, the logs first show two HTTP 408 request-body upload timeouts for a 161,769-byte request. The CLI retries with a fresh connection, then reports:

Execution failed: 400 The request body is not valid JSON:
request body is not valid JSON: decoding JSON body:
invalid character 'p' after object key

For Claude Opus 5.5, the request fails almost immediately:

Execution failed: 400 The request body is not valid JSON:
request body is not valid JSON: decoding JSON body:
invalid character ':' after object key:value pair

Request IDs — October 14, 2026

• Sonnet, 11:04:06 UTC:  75B5:2DFF7A:51E12:5E166:6ACF61A5 
• Opus, 11:04:31 UTC:  6C22:3A95C8:560F6:62F7D:6ACF61BF 

Since this persists after a clean installation, affects both Claude models, and does not affect other models or my colleagues, could you investigate whether there is an account-specific or request-routing issue in the Claude integration? The available logs do not establish whether the invalid JSON originates in the CLI or in the service handling the request.

Affected version

GitHub Copilot CLI 1.0.97

Steps to reproduce the behavior

No response

Expected behavior

Claude models work as the others do

Additional context

No response