Context overflow 400 is retried because the error message contains "500000" (substring match of "500") #52464

Description

The session retry layer's RETRYABLE_MESSAGE_PATTERNS in packages/opencode/src/session/retry.ts matches status codes as raw substrings:

/429|500|502|503|504|524/i

Any error message that merely contains those digits inside a larger number is misclassified as retryable. For example, a deterministic 400 whose message includes a large token count:

• "The input (523659 tokens) is longer than the model's context length (500000 tokens)" → matches "500"
• "... 4290 requests ..." → matches "429"
• "... id 503123 not found ..." → matches "503"

When such an error reaches retryable() as a plain APIError (i.e. when it was not classified as ContextOverflowError upstream), the policy retries a request that can never succeed, up to 5 times with exponential backoff, before surfacing the error to the user.

This is a distinct failure mode from the context-overflow classification in packages/llm/src/provider-error.ts (isContextOverflow): the classifier handles the known phrasings, but any 4xx error carrying one of these numbers in its text — from a gateway, proxy, or a provider phrasing not in the pattern list — still falls through to the retry layer and hits the substring match.

Steps to reproduce

Unit-level (deterministic):

1. An APIError is constructed from a 4xx response whose message contains a large number, e.g. ... context length (500000 tokens)
2. SessionRetry.retryable(error) → matchesRetryableMessage(message) matches "500" inside "500000"
3. The retry policy treats the deterministic failure as retryable

Live: send a request that a gateway rejects with 4xx and an error body containing any of 429/500/502/503/504/524 as a substring of a larger number.

Expected behavior

Status codes embedded in larger numbers must not make an error retryable; a deterministic 4xx should surface immediately.

OpenCode version

v1.18.25 through current dev (packages/opencode/src/session/retry.ts)

Operating System / Terminal

macOS / any — logic is OS-independent

Additional context

Fix proposed in #52460: word-bounded status code pattern plus a non-retryable guard for common context-overflow phrasings at the retry layer.