fix(provider): treat Anthropic billing_error as context overflow for auto-compaction

[codeg-old]

Problem

Anthropic returns HTTP 403 with billing_error type and the message:

"Extra usage is required for long context requests."

This occurs when a session context exceeds ~200K tokens for accounts not in Anthropic usage tier 4.

Root Cause

The OVERFLOW_PATTERNS array in provider/error.ts did not include this error message. As a result:

1. parseAPICallError() classifies it as a generic api_error (not context_overflow)
2. retry.ts treats it as retryable (depending on HTTP status interpretation)
3. No automatic compaction is triggered — the session keeps retrying with the same oversized context

Observed in production: 57 consecutive failures over 2 hours on a session with 452K tokens.

Fix

Add /extra usage is required for long context/i to OVERFLOW_PATTERNS.

This causes opencode to:

• Classify the error as ContextOverflowError instead of APIError
• Trigger automatic compaction to reduce the session context
• Retry with a smaller context that stays under Anthropic's threshold

Testing

Manually verified on a session with 452K+ tokens hitting this error. After fix, compaction is triggered automatically instead of infinite retry.

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Results

No duplicate PRs found.

However, I found several related PRs that address context overflow and compaction, though they address different aspects:

• PR fix(core): consider code: context_length_exceeded as context overflow in API call errors #17748: Similar error classification fix for context_length_exceeded codes
• PR fix(opencode): recover from ContextOverflowError via auto-compaction #14826: Recover from ContextOverflowError via auto-compaction
• PR fix: auto-resume agent loop after context compaction #12970: Auto-resume agent loop after context compaction

These PRs are in the same feature area but target different error scenarios or approaches, so they're not duplicates of PR #17766.

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