Bug: Claude/Anthropic 500/502/503/504 server errors are not retried

[rich-jojo]

What happened?

When Anthropic's API returns a transient server error (HTTP 500, 502, 503, or 504), OpenCode immediately fails with no retry:

{"type":"api_error","message":"Claude API returned a server error. This is usually temporary — try again in a moment."}

The same infrastructure already retries 429 and 529 — but 500/502/503/504 are silently dropped.

Root cause

parseAPICallError() in packages/opencode/src/provider/error.ts trusts input.error.isRetryable from the Vercel AI SDK for non-OpenAI providers. The @ai-sdk/anthropic package only sets isRetryable: true for 429 and 529. HTTP 500/502/503/504 get isRetryable: false, so SessionRetry.retryable() returns undefined and the retry loop exits.

OpenAI already has a isOpenAiErrorRetryable() override — the same pattern needs to apply universally.

Steps to reproduce

1. Configure Anthropic as provider
2. Trigger or wait for a 500/502/503/504 response (during outages or high load)
3. Observe: immediate error with no retry attempt

Expected behavior

OpenCode should retry 500/502/503/504 with exponential backoff, the same as 429/529.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Claude/Anthropic 500/502/503/504 server errors are not retried #21420: Identical report — same title, same root cause analysis, same fix suggestion (appears to be a double-submission)
• Anthropic 5xx errors incorrectly marked non-retryable, stopping agent loop #19394: Anthropic 5xx errors incorrectly marked non-retryable, stopping agent loop — same root cause and same proposed fix in error.ts
• fix: 5xx errors from non-standard providers are not retried #19203: 5xx errors from non-standard providers are not retried — same underlying problem of non-retryable 5xx classification
• [FEATURE]: retry automatically after internet server error #19100: Feature request for automatic retry after server errors — same user-facing behavior request

Please check if any of these already cover your report.