Claude Sonnet 4.5 Listed in "/models" but return 400 error

[Meredyk48]

Describe the bug

"Claude Sonnet 4.5 is listed in /model but returns 400 error when used"

I was using the model all day, then it simply stopped working and started displaying the following message after each request.

Full error Message:
"✗ Execution failed: CAPIError: 400 The requested model is not supported. (Request ID:EAA9:15725E:2CB259:3196D2:69D6A4D0)"

Affected version

v1.0.21

Steps to reproduce the behavior

1. Access the Copilot CLI.
2. /models to list the available models.
3. Select the Claude Sonnet 4.5 (default).
4. Make a request using the Claude Sonnet 4.5 model.
5. See the error.

Expected behavior

1. Access the Copilot CLI.
2. /models to list the available models.
3. Select the Claude Sonnet 4.5 (default).
4. Make a request using the Claude Sonnet 4.5 model.
5. Receive the Answer.

Additional context

No response

[daniloceano]

I’m seeing the same problem.

Environment:

• OS: Linux
• Client: VS Code
• Surface: Copilot CLI / Autopilot Preview
• copilot-cli version: 1.115

What I’m seeing:

• Claude Sonnet 4 -> fails with CAPIError: 400 The requested model is not supported
• Claude Sonnet 4.5 -> fails with the same error
• Claude Haiku 4.5 -> works normally in the same session

What I already tried:

• Sign out / sign in
• Reinstall Copilot extension
• Start a new session

This makes it look like the CLI is working, but Sonnet models are failing on this route/session.