claude-code - 💡(How to fix) Fix [Bug] Rails 8 allow_browser middleware blocks link preview bots with 406 response [1 participants]

[{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n at TWH (/$bunfs/root/src/entrypoints/cli.js:2736:30496)\n at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T04:05:34.047Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n at TWH (/$bunfs/root/src/entrypoints/cli.js:2736:30496)\n at async L (/$bunfs/root/src/entrypoints/cli.js:6300:11825)\n at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T04:05:34.460Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n at TWH (/$bunfs/root/src/entrypoints/cli.js:2736:30496)\n at async R (/$bunfs/root/src/entrypoints/cli.js:6300:11019)\n at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T04:05:34.462Z"}]

TL;DR

Remove the allow_browser versions: :modern middleware from the ApplicationController to prevent it from returning a 406 error to unrecognized User-Agents.

Guidance

• Identify if the allow_browser middleware is enabled in the ApplicationController and remove it if necessary.
• Verify that the removal of the middleware resolves the issue by checking the response body for OG tags when accessing the URL with an unrecognized User-Agent.
• Check for any other potential middleware or configuration that may be interfering with the response.
• Consider adding custom logic to handle unrecognized User-Agents instead of removing the middleware entirely.

Example

No code example is provided as the fix is straightforward and only requires removing a line of code from the ApplicationController.

Notes

The provided error messages do not seem to be related to the issue described, as they mention missing environment variables and MCP server errors.

Recommendation

Apply the workaround by removing the allow_browser versions: :modern middleware, as it is the root cause of the issue and removing it has been confirmed to resolve the problem.