Autocompact not triggering with Custom model (mimo-v2.5-pro) despite exceeding context limit

Bug Report

Description

Autocompact feature does not trigger when using a Custom model, even when context usage exceeds the configured limit significantly.

Environment

• Claude Code Version: Latest (June 2026)
• Model: mimo-v2.5-pro (Custom model via OpenAI-compatible API)
• Platform: Windows 11
• Shell: Git Bash

Configuration

• Model context window: 1M (native support)
• Claude Code configured context: 200k (user preference)
• Autocompact threshold: 80% (default)

Steps to Reproduce

1. Configure Claude Code to use a Custom model (e.g., mimo-v2.5-pro) with 200k context limit
2. Have a long conversation with multiple Agent tool calls
3. Monitor context usage with /context
4. Observe that autocompact does NOT trigger even when context exceeds 200k (152%)

Expected Behavior

Autocompact should trigger when context usage reaches 80% of the configured 200k limit (160k tokens).

Actual Behavior

• Context reached 303k/200k (152%) without autocompact triggering
• Session eventually "ran out of context" and had to be manually resumed
• Manual /compact works correctly, reducing context to 18%

Evidence

Before manual compact:
Context Usage: 303k/200k tokens (152%)

After manual compact:
Context Usage: 36.2k/200k tokens (18%)

Root Cause Analysis

The issue appears to be that Claude Code calculates autocompact threshold based on the model's native context window (1M) rather than the configured context limit (200k):

• 303k ÷ 1M = 30% (below 80% threshold → no trigger)
• 303k ÷ 200k = 152% (should have triggered)

This is similar to the Opus 4.7 issue mentioned in changelog:

Fixed Opus 4.7 sessions showing inflated /context percentages and autocompacting too early — Claude Code was computing against a 200K context window instead of Opus 4.7's native 1M

Additional Context

• Multiple parallel Agent calls were used during the session
• The /context command correctly shows 200k as the limit
• The issue is specific to Custom models where native context > configured context

Suggestion

Allow Custom model configuration to explicitly set contextWindow parameter, or ensure autocompact uses the configured limit (200k) rather than the model's native context window (1M).