codex - 💡(How to fix) Fix Codex suddenly stopped working for me

■ unexpected status 403 Forbidden: Unknown error, url: https://chatgpt.com/backend-api/codex/responses, cf-ray: 9ef508e 138f4f5f3-AMS ⚠ MCP client for codex_apps failed to start: MCP startup failed: handshaking with MCP server failed: Send message error Transport [rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorker<codex_rm cp_client::rmcp_client::StreamableHttpResponseClient>>] error: Unexpected content type: Some("missing-content-type; body: "), when send initialize request ⚠ MCP startup incomplete (failed: codex_apps)

TL;DR

The issue may be related to a compatibility problem or a transient error with the Codex CLI version 0.121.0, and checking the Codex CLI version compatibility or trying an alternative version might help.

Guidance

• Verify if there are any known issues with Codex CLI version 0.121.0 and the gpt-5.4 xhigh model that could be causing the 403 Forbidden error or the MCP client startup failure.
• Check the official Codex documentation for any specific requirements or recommendations for running the gpt-5.4 xhigh model with the Plus subscription.
• Consider trying a different version of the Codex CLI to see if the issue persists, as this might indicate a version-specific problem.
• Review any available logs or error messages for more detailed information about the "Unexpected content type" error and the MCP startup failure.

Notes

The provided information does not include specific details about the network environment or any potential firewall rules that might be interfering with the request to https://chatgpt.com/backend-api/codex/responses. Additionally, without more context about the "Uploaded thread" or how it relates to the issue, it's challenging to provide a more targeted solution.

Recommendation

Apply workaround: Try using a different version of the Codex CLI to see if the issue is version-specific, as this could help isolate the problem and potentially resolve it without waiting for an official fix.