Skip to content
Cade's Dotfiles

Codex MCP OAuth fails: "Authorization server response missing required issuer"

Page source

Source: Troubleshooting reference

Symptom. codex mcp login <server> (or first use of an OAuth MCP server in Codex) opens the browser, auth succeeds there, then the CLI dies with failed to handle OAuth callback … Authorization server response missing required issuer: expected <server url>. The same server connects fine from Claude Code.

Root cause. A Codex regression, not a server or config problem. Codex 0.143.0+ looks for an iss field in the token endpoint’s JSON response body — where RFC 6749 doesn’t put one — instead of using the RFC 9207 iss callback parameter it already validated. Spec-compliant authorization servers (Cloudflare’s among them) fail the check. Tracked in openai/codex#31573; introduced via a modelcontextprotocol/rust-sdk change.

Confirm. codex --version ≥ 0.143.0, the issue above still open, and the server works from another harness. For Cloudflare specifically, prove the server itself is healthy with a direct handshake:

Terminal window
curl -s -X POST https://mcp.cloudflare.com/mcp \
  -H "Authorization: Bearer $CLOUDFLARE_API_TOKEN" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"probe","version":"0.0.0"}}}'
# → {"result":{…"serverInfo":{"name":"cloudflare-api"…

Fix. Sidestep OAuth in Codex with a static bearer: the --codex-bearer <ENV_VAR> annotation in packages/mcp-servers.txt makes install/codex.sh emit bearer_token_env_var = "<ENV_VAR>" for that server while every other harness keeps OAuth. Cloudflare rides --codex-bearer CLOUDFLARE_API_TOKEN (from ~/.cloudflare.env, bash install/auth.sh cloudflare); mcp.cloudflare.com accepts API-token bearers directly, verified with the handshake above. Caveat: the token’s scopes bound what the tools can do — OAuth carried your full user grant, so mint a broader token if a tool call 403s. Remove the annotation once the upstream fix ships.