After context compaction, the preserved last assistant message retained
its structured tool_calls field, but the corresponding tool_result was
summarized away. This created orphaned tool_use blocks that violated
the Anthropic API constraint: 'Each tool_use block must have a
corresponding tool_result block in the next message', causing 400 errors.
Primary fix: clear tool_calls from the preserved assistant message in
extract_preserved_messages(). The tool call was already executed and
its result is captured in the summary.
Defense-in-depth: added strip_orphaned_tool_use() post-processing in
Anthropic convert_messages() to detect and strip any orphaned tool_use
blocks before they reach the API.
Added 7 tests: 3 unit tests for compaction stripping, 3 unit tests for
Anthropic orphan detection, 1 integration test reproducing the exact
bug scenario from the h3 session.
d61be719c2