Problem

The Codex engine's remote GitHub MCP server configuration was using an outdated format that doesn't align with the new streamable HTTP requirements specified in OpenAI Codex documentation (@openai/codex/files/docs/config.md).

Solution

Updated the remote GitHub MCP configuration to use the new streamable HTTP format with the following changes:

Configuration Format Changes

Old format:

[mcp_servers.github]
type = "http"
url = "https://api.githubcopilot.com/mcp/"
headers = { "Authorization" = "Bearer ${{ secrets.GITHUB_MCP_TOKEN }}" }

New format:

# Streamable HTTP requires the experimental rmcp client
experimental_use_rmcp_client = true

[mcp_servers.github]
url = "https://api.githubcopilot.com/mcp/"
bearer_token_env_var = "GITHUB_MCP_TOKEN"

New format (read-only mode):

# Streamable HTTP requires the experimental rmcp client
experimental_use_rmcp_client = true

[mcp_servers.github]
url = "https://api.githubcopilot.com/mcp-readonly/"
bearer_token_env_var = "GITHUB_MCP_TOKEN"

Key Changes

1. Top-level experimental flag: Added experimental_use_rmcp_client = true at the root configuration level when GitHub tool is in remote mode
2. Removed deprecated field: Removed type = "http" which is no longer needed with the rmcp client
3. Authentication method: Changed from inline headers with Bearer token to bearer_token_env_var referencing an environment variable
4. Read-only mode: Uses dedicated readonly endpoint https://api.githubcopilot.com/mcp-readonly/ instead of X-MCP-Readonly header
5. Environment variable: Added GITHUB_MCP_TOKEN to the workflow execution step environment variables
6. dev.md workflow: Updated .github/workflows/dev.md to use the new remote GitHub MCP configuration by adding tools.github.mode: remote, changed trigger to workflow_dispatch, and updated task to write poetry

Backward Compatibility

• ✅ Local mode (Docker-based) configurations remain unchanged
• ✅ Only remote mode configurations are affected
• ✅ Claude and Copilot engines maintain their existing configuration formats
• ✅ All existing tests pass with updated expectations

Testing

• Updated github_remote_mode_test.go to verify the new configuration format
• Added checks to ensure the experimental flag is only present in remote mode
• Verified both standard and read-only remote mode configurations
• Confirmed local mode does not include the experimental flag
• Verified read-only mode uses /mcp-readonly/ endpoint instead of headers
• Verified dev.md workflow compiles successfully with the new remote configuration

Fixes requirements specified in issue referencing @openai/codex/files/docs/config.md.

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