Auto-compact fails with Opus 4.5 extended thinking blocks

Bug Report: Auto-compact fails with Opus 4.5 extended thinking blocks

Description

When using Claude Code with the claude-opus-4-5-20251101 model, the automatic conversation compaction feature fails repeatedly with a 400 error. This causes the session to become unusable and all context is lost.

Environment

• Claude Code version: Latest (npm global install)
• Model: claude-opus-4-5-20251101
• OS: Windows 10/11
• Date: 2025-11-24

Error Message

400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.11.content.11: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response."},"request_id":"req_011CVTZiv4zz4a4AmyidNwVx"}

Steps to Reproduce

1. Start a Claude Code session with Opus 4.5 model
2. Have a long conversation that triggers auto-compaction
3. When Claude Code attempts PreCompact:auto, the API returns error 400
4. The error repeats on every subsequent message
5. Session becomes unusable

Debug Log Evidence

From the debug log (~/.claude/debug/*.txt):

2025-11-24T23:15:23.678Z [DEBUG] Skipping PreCompact:auto hook execution - workspace trust not accepted
2025-11-24T23:15:24.181Z [ERROR] Error streaming, falling back to non-streaming mode: 400 {"type":"error"...
2025-11-24T23:15:24.604Z [ERROR] Error in non-streaming fallback: 400 {"type":"error"...

Total failed compact attempts in session: 77

Root Cause Analysis

The Opus 4.5 model uses "extended thinking" which generates <thinking> blocks in responses. When Claude Code attempts to compact/summarize the conversation:

1. It modifies or reorganizes messages containing thinking blocks
2. The API rejects this because thinking blocks "cannot be modified" and "must remain as they were in the original response"
3. Both streaming and non-streaming fallback fail with the same error
4. The compaction never succeeds, making the session unrecoverable

Impact

• Severity: High
• Data Loss: Complete loss of conversation context
• User Experience: Session becomes completely unusable
• Workaround: Switch to Sonnet model (no extended thinking)

Expected Behavior

Claude Code should either:

1. Preserve thinking blocks exactly as-is during compaction
2. Handle the error gracefully and continue the session
3. Warn users before using Opus 4.5 about this limitation

Suggested Fix

When compacting conversations with Opus 4.5:

• Extract and preserve thinking/redacted_thinking blocks unchanged
• Only summarize the non-thinking content
• Or skip compaction for messages containing thinking blocks

Additional Context

The user was working on AutoCAD LISP development when this occurred. The session had grown large enough to trigger auto-compaction, at which point it entered an error loop and became unusable.

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Reported via Claude Code session analysis