№ 9: The OAuth 400 That Said Only "Error"

On March 16, 2026, Anthropic OAuth users started getting 400 errors with a response body of just {"message": "Error"}. We diagnosed it through proxy forensics, documented it for Context Hub, and built an MCP server to serve the fix.

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The full article covers:

  • Twenty-four hours of 500 errors across all OAuth users
  • Proxy forensics: comparing dibbur headers against claude-code
  • The two-line fix that resolved the 400s
  • Secondary 400s from retry-corrupted conversation state
  • How we documented it for Context Hub and built an MCP server

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