Trident stage 1 dropped the assistant message that carried a file tool
use while keeping the tool result that answered it, so the compacted
session ended up with orphaned `tool_result` blocks. Providers reject
such requests with HTTP 400 ("tool_result blocks must be a response to
a preceding tool_use"), which breaks the auto-compact retry path that
runs Trident before resending a request.
Stage 1 also counted a tool result as an independent file operation,
which pushed `last_write` past the write the result belonged to and made
the newest write look obsolete.
Only tool uses count as file operations now, and a tool result is
dropped together with the tool use that produced it.
Bougie
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