400 'thinking blocks cannot be modified' when a run_in_background tool completes (interleaved thinking + Opus 1M)

Summary

With interleaved (extended) thinking enabled on Opus 4.8 (1M context), the session hard-fails with a 400 the instant an asynchronous tool result is injected back into the conversation — specifically when a run_in_background Bash task completes, and also during the harness's "Invalid tool parameters" error-recovery path. The error:

API Error: 400 messages.1.content.6: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.

This stops the session entirely; the only recovery is rolling back to before the offending turn.

Why this is a harness bug (not user/repo/compaction)

• Reproduces in a fresh session at ~64k of a 1M window — compaction/summarization is not involved (nothing to summarize).
• No hooks configured at user, project, or local scope (ruled out content-mutating hooks).
• The error coordinates messages.1.content.6 indicate the first assistant turn (messages[1]) produced an interleaved layout (thinking -> tool_use -> ... -> thinking at block 6), and the signed thinking block was modified during re-serialization rather than passed back byte-for-byte.
• The failure fires exactly when a run_in_background command completes and its result is injected, forcing a re-send of the latest assistant turn. The harness's invalid-tool-params recovery (which rebuilds the assistant message) is the same class of trigger.

Repro

1. Opus 4.8 with 1M context + interleaved thinking (default).
2. In a fresh session, issue a prompt that makes the model think and then fire a tool with run_in_background: true (long autonomous skills like /doc-audit, /backlog-sweep trigger this reliably because they spawn parallel/background tool calls).
3. When the background task completes, the next API request 400s with the message above.

Observed sequence (from the live session)

Bash(claude update)            -> ok (2.1.154 -> 2.1.156)
Bash(sleep 5)                  -> ok
Bash(claude update; claude --version)  -> ok
Bash(claude --version)  [run_in_background]
  -> Invalid tool parameters   (x2)
Bash(kill %1 ...)              -> done
Background command "Check installed Claude Code version" completed (exit code 0)
  -> API Error: 400 messages.1.content.6: `thinking` ... cannot be modified

The 400 appears on the background-completion injection, not during normal turn-taking.

Impact

• Affects any workflow that combines interleaved thinking with parallel or backgrounded tool calls — i.e. most long-running slash-command skills. Not specific to any one skill.
• Hard stop; lost turn; requires manual rollback.

Environment

• Claude Code: bug reproduced on 2.1.156 (also seen on 2.1.154). Updating to latest did not fix it.
• Model: Opus 4.8, 1M context beta.
• OS: Windows 11 Pro (10.0.26200), shell: Git Bash / PowerShell 7.
• Interleaved thinking: enabled (default). DISABLE_INTERLEAVED_THINKING unset.

Likely fix area

The async tool-result injection path (background-task completion + invalid-tool-params recovery) must preserve thinking/redacted_thinking blocks in the latest assistant message verbatim when rebuilding the request, instead of re-encoding/reordering them.

Workaround

Setting DISABLE_INTERLEAVED_THINKING=1 removes the signed interleaved blocks and is expected to avoid the failure mode.