Invalid JSON format in tool call arguments" with Claude Haiku 4.5 in VS Code

[rojassartorio]

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Bug

Copilot Feature Area

VS Code

Body

Description

I'm getting repeated errors in VS Code with Copilot Chat while using the Claude Haiku 4.5 agent:

Sorry, your request failed. Please try again.
Copilot Request id: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
GH Request Id: xxxxx:xxxxx:xxxxx:xxxxx:xxxxxxxxxxxx
Reason: Request Failed: 400 {"error":{"message":"Invalid JSON format in tool call arguments","code":"invalid_tool_call_format"}}

This happens even with very simple prompts and after multiple retries.

What I've already tried

• Restarted VS Code and reloaded the window.
• Cleared chat history and restarted the chat agent from the command palette.
• Signed out / signed in to Copilot.
• Deleted all Copilot-related cache folders:
  • %APPDATA%\Code\User\globalStorage\github.copilot-chat
  • %APPDATA%\Code\User\globalStorage\github.copilot
  • %USERPROFILE%\.vscode\extensions\github.copilot-*
• Updated and reinstalled Copilot and Copilot Chat extensions.
• Tried selecting another agent (Grok, GPT-4, Claude Sonnet).
  But after selecting "Grok code fast", the system still keeps responding with Claude Haiku and the same error persists.
• Confirmed the error is not related to my prompts (even empty prompts trigger it).
• Confirmed it happens on multiple workspaces and after full clean reinstall.

Screenshots

Environment

• VS Code version: 1.85.x
• Copilot Chat extension: latest
• System: Windows 10/11 (please specify your OS if different)
• Agent: Claude Haiku 4.5 (problem), Grok (still responds with Haiku)

What I expect

• To switch agents properly (agent selection shows Grok, but keeps using Haiku).
• Not to receive tool call format errors for simple prompts.
• Normal usage of Copilot Chat.

Additional info

I've seen similar reports, but no official solution.
Please advise how to fix this or escalate to the product team.

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Spanish summary (Resumen en español):

Ya probé todas las soluciones recomendadas. El error persiste y no cambia de modelo aunque seleccione otro agente.

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[Maxlyu254]

I have encountered the same issue, and it seems like similar bugs appeared 2 months ago. Glad to see this is posted.

[soncco]

how is this answer a solution to the problem?