Compaction fails with 'Conversation too long' at 48% context usage (Opus 4.6)

Bug Description

With Claude Code using Opus 4.6, the system reports "context full" prematurely and /compact fails, leaving the user stuck. First noticed in v2.1.32, still present in v2.1.34.

Steps to Reproduce

1. Use Claude Code v2.1.34 with claude-opus-4-6 model
2. Work normally until message tokens reach ~78k (still only 48% of 200k context)
3. Claude reports context is full
4. Run /compact
5. Compaction fails with error

Error Message

/compact
⎿ Error: Error during compaction: Error: Conversation too long. Press esc twice to go up a few messages and try again.

Context State at Time of Error

/context output shows plenty of remaining space:

claude-opus-4-6 · 97k/200k tokens (48%)

Estimated usage by category
System prompt: 3.3k tokens (1.6%)
System tools: 16.5k tokens (8.2%)
Memory files: 3.4k tokens (1.7%)
Skills: 290 tokens (0.1%)
Messages: 77.7k tokens (38.8%)
Free space: 66k (32.9%)
Autocompact buffer: 33k tokens (16.5%)

No MCP servers are configured.

Expected Behavior

• Context should not report "full" at 48% usage
• /compact should successfully summarize the conversation
• After compaction, work should be able to continue

Actual Behavior

• Context reports full prematurely
• /compact fails with "Conversation too long"
• User is stuck and cannot continue working
• Only workaround is pressing Escape twice to drop messages or starting a fresh conversation

Likely Cause

The compaction process appears to send the conversation to a smaller model for summarization. When message tokens reach ~78k, this exceeds the summarization model's context window — even though the primary model (Opus 4.6, 200k) has plenty of room. The compaction pipeline may not be scaled for Opus 4.6's larger context window.

Environment

• Claude Code version: v2.1.34 (first noticed in v2.1.32)
• Model: claude-opus-4-6
• Platform: macOS (Darwin 24.6.0)