[BUG] Bedrock Opus 4.6 incorrectly gets 200k context window instead of 1M

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When using Claude Opus 4.6 via AWS Bedrock (us.anthropic.claude-opus-4-6-v1), Claude Code reports a 200k context window, auto-compacts at the 200k threshold, and hits the "Context limit reached" blocking limit prematurely. AWS Bedrock natively supports 1M context for this model without requiring any beta headers.

The root cause is in the internal contextWindow calculation. The 1M path is gated on two conditions, both of which fail for Bedrock:

1. Regex check (/\[1m\]/i) — tests if the model string contains the literal text [1m]. No real model ID contains this.
2. Beta flag check — requires sdkBetas to include "context-1m-2025-08-07", but sdkBetas is never populated for Bedrock providers since Bedrock doesn't use Anthropic SDK beta headers.

The model identification function correctly recognizes opus-4-6 in the Bedrock model ID, but it's only evaluated in conjunction with the beta flag:

// Simplified from minified source (v2.1.87)
function getContextWindow(model, betas) {
  // Path 1: regex /\[1m\]/i — doesn't match any real model ID
  if (regexCheck(model)) return 1_000_000;

  // Path 2: registry lookup — falls through if model not in registry
  let info = registryLookup(model);
  if (info?.max_input_tokens >= 100_000) {
    if (info.max_input_tokens > 200_000 && disable1MContext()) return 200_000;
    return info.max_input_tokens;
  }

  // Path 3: requires BOTH beta flag AND model match — betas is undefined for Bedrock
  if (betas?.includes("context-1m-2025-08-07") && isOpusOrSonnet4(model))
    return 1_000_000;

  return 200_000; // default fallback — this is what Bedrock Opus hits
}

This cascades into two additional problems:

• CLAUDE_CODE_AUTO_COMPACT_WINDOW cannot override upward: The env var is applied via Math.min(contextWindow, override), so setting it to 1M results in Math.min(200000, 1000000) = 200000. It can only make the window smaller.
• Blocking limit is too low: The "Context limit reached" blocking limit is calculated as contextWindow - reserved - 3000 ≈ 177k, preventing use of the full 1M context.

What Should Happen?

Bedrock Opus 4.6 (us.anthropic.claude-opus-4-6-v1) should be recognized as having a 1M context window. The isOpusOrSonnet4(model) check (which already correctly identifies Bedrock model IDs containing opus-4-6) should be sufficient to return 1M without requiring the beta flag, since Bedrock supports 1M natively.

• /context should show 1M for Bedrock Opus 4.6
• Auto-compact should trigger at the 1M boundary, not 200k
• CLAUDE_CODE_AUTO_COMPACT_WINDOW should use Math.max (or replace) instead of Math.min so users can override the detected window upward

Error Messages/Logs

/context output showing 200k:

⛁ ⛁ ⛀ ⛀ ⛀ ⛀ ⛶ ⛶ ⛶ ⛶   us.anthropic.claude-opus-4-6-v1 · 7k/200k tokens (4%)

Status line shows "0% until auto-compact" and eventually:

Context limit reached · /compact or /clear to continue · /model opus

This occurs well before reaching the actual 1M Bedrock limit.

Steps to Reproduce

1. Configure Claude Code to use AWS Bedrock with Opus 4.6:

``json
{
"env": {
"CLAUDE_CODE_USE_BEDROCK": "1",
"AWS_REGION": "us-west-2",
"ANTHROPIC_DEFAULT_OPUS_MODEL": "us.anthropic.claude-opus-4-6-v1"
},
"model": "opus"
}
``

1. Start a conversation
2. Run /context — observe it shows 200k instead of 1M
3. Use the conversation until ~177k tokens — observe "Context limit reached" blocking message appears

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.87

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

Other

Additional Information

• Claude Code version: 2.1.87
• OS: macOS (Darwin 25.3.0)
• Bedrock model ID: us.anthropic.claude-opus-4-6-v1 (cross-region inference profile)
• Workarounds:
• CLAUDE_CODE_BLOCKING_LIMIT_OVERRIDE=950000 — prevents the "Context limit reached" block until ~950k tokens (this env var uses direct replacement instead of Math.min)
• CLAUDE_CODE_AUTO_COMPACT_WINDOW=1000000 — does NOT help due to Math.min, but doesn't hurt
• /context display will still show 200k incorrectly regardless of workarounds
• The same issue likely affects any Bedrock model that supports >200k context, including Sonnet 4.6 if it has extended context on Bedrock, not fully tested yet.