Session compaction produces orphaned tool_use blocks in long tool-heavy sessions (400 from Anthropic) #17065

Description
After a compaction event in a long tool-heavy session, OpenCode constructs a message history where a tool_use block exists without a corresponding tool_result block. Anthropic's API then rejects every subsequent message with a 400:

tool_use ids were found without tool_result blocks

The session becomes unrecoverable — all further messages fail until the session is abandoned.

Root cause (investigated): compaction drops tool_result blocks while retaining their paired tool_use blocks. Confirmed by isolating variables — hai-proxy (used as a local proxy) is a transparent passthrough that forwards Anthropic's 400 verbatim. The stage-aware-compaction plugin only calls output.context.push(string) and never touches message history.

Plugins
stage-aware-compaction (inject-only experimental.session.compacting hook — does not modify message history)

OpenCode version
1.2.22

Steps to reproduce

1. Start a session with a Claude model that makes many tool calls (e.g. reading multiple files, running multiple shell commands).
2. Let the session run until it triggers compaction (high message count, e.g. ~300 messages).
3. Send the next user message after compaction completes.
4. Observe 400 from Anthropic: tool_use ids were found without tool_result blocks.

Screenshot and/or share link
N/A

Operating System
macOS 26.2

Terminal
macOS Terminal