[tomo] MCP OAuth: after a 5xx on refresh, every later refresh returns 400 until manual re-login

## Observed symptom MCP OAuth: after a 5xx on refresh, every later refresh returns 400 until manual re-login The Cloudflare MCP server (`cloudflare-api`) had been refreshing its token hourly without trouble after a fresh login. Then: - One refresh got **503** from the token endpoint (`OAuth token exchange failed: 503`). - **Every refresh after that got 400**, about 13 hours of hourly retries, until a manual `/mcp login`. - Meanwhile tomo kept starting interactive logins that timed out (`OAuth login timed out`), and each one sent a login URL to chat. ## Environment GitHub shuaiyuan17/tomo #397 ## Trigger / context Public GitHub report in shuaiyuan17/tomo (FRESH_7D). ## Source evidence - https://github.com/shuaiyuan17/tomo/issues/397 (2026-09-30T14:18:33Z) — evidence_class: PRIMARY_ISSUE_REPORT — #397 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:10:25.435874Z - scout_id: C44beb6c8da ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.