Beta header context-1m-2025-08-07 not sent for Claude Opus 4.6, causing 200k hard limit

Problem

Claude Opus 4.6 supports a 1M token context window, but the Anthropic API requires the beta header context-1m-2025-08-07 to unlock it. Without this header, the API enforces a hard 200k input token limit.

OpenCode currently sends this beta header for claude-sonnet-* models but does not include claude-opus-4-6. This causes the API to reject requests exceeding 200k tokens:

prompt is too long: 201965 tokens > 200000 maximum

Root Cause

The beta header logic only matches Sonnet model IDs. Opus 4.6 is the first Opus-class model with 1M context support and needs to be added to the header inclusion check.

Impact

Config overrides setting limit.context: 1000000 have no effect — the limit is enforced server-side by the Anthropic API
The status bar shows usage against 1M (e.g., 11% at 109k tokens), but the API rejects at 200k
Auto compaction doesn't trigger in time because OpenCode believes the model has 1M of headroom

Expected Behavior

OpenCode should send anthropic-beta: context-1m-2025-08-07 header for claude-opus-4-6 requests, matching the behavior already in place for Sonnet models.

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

fix(zen): 1M context window beta not sent for Opus 4.6 and Sonnet 4.5 #12452: fix(zen): 1M context window beta not sent for Opus 4.6 and Sonnet 4.5
Claude Opus 4.6 context window limits still 200k #12438: Claude Opus 4.6 context window limits still 200k

Feel free to ignore if none of these address your specific case.

Here's a workaround that worked for me: #12338 (comment)

For Anthropic direct API users, I made a plugin that injects the context-1m-2025-08-07 beta header via chat.headers hook — it appends to existing SDK beta flags (claude-code, interleaved-thinking, etc.) instead of replacing them.

Add to your opencode.json:

{
  "plugin": ["opencode-anthropic-context-1m"],
  "provider": {
    "anthropic": {
      "models": {
        "claude-opus-4-6":          { "limit": { "context": 1000000, "output": 128000 } },
        "claude-sonnet-4-6":        { "limit": { "context": 1000000, "output": 64000 } },
        "claude-sonnet-4-5":        { "limit": { "context": 1000000, "output": 64000 } },
        "claude-sonnet-4-20250514": { "limit": { "context": 1000000, "output": 64000 } }
      }
    }
  }
}

The plugin entry injects the beta header. The provider.anthropic.models entries override the default 200K limit so auto-compaction triggers at ~968K instead of ~168K.

Repo: https://github.com/DusKing1/opencode-anthropic-context-1m

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Beta header `context-1m-2025-08-07` not sent for Claude Opus 4.6, causing 200k hard limit #12507

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Beta header context-1m-2025-08-07 not sent for Claude Opus 4.6, causing 200k hard limit #12507

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github-actions commented on Feb 6, 2026

ItsWendell commented on Feb 16, 2026

DusKing1 commented on Mar 7, 2026

github-actions commented on Jul 3, 2026

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Beta header `context-1m-2025-08-07` not sent for Claude Opus 4.6, causing 200k hard limit #12507