_find_tail_cut_by_tokens aligns cut_idx away from tool-call/result
boundaries (_align_boundary_backward), and both tail anchors re-align after
moving it. The final statement then raised the result to head_end + 1 so
compression always claims at least one message — without that floor the
caller's compress_start >= compress_end guard turns the pass into a no-op
that re-runs forever.
That raise discarded the alignment. When the floor landed inside a tool
group, the parent assistant(tool_calls) fell in the summarised region while
its tool results started the tail, and _sanitize_tool_pairs dropped those
orphans outright — so the tool output was neither summarised nor kept. It
vanished. That is exactly the silent loss _align_boundary_backward's own
docstring says the alignment exists to prevent.
Two back-to-back tool calls are enough to trigger it on default settings
(protect_first_n=3):
system, assistant(call_1), tool, tool, assistant(call_2), tool
aligned cut = 4 (keeps call_2's group together)
returned cut = 5 (floor overrode it)
summarised region = [assistant(call_2)]
tail = [tool(call_2)] -> orphan -> dropped
Sweeping every well-formed block layout up to length 6 (21840 transcripts),
5623 of them — 26% — split a call/result pair this way.
Re-align FORWARD after applying the floor. Forward, never backward: pulling
back would hand return the message the floor just claimed and reopen the
no-op loop. Sliding forward instead moves the cut past the end of the group,
so the whole call/result pair is summarised together and nothing is
orphaned. The same sweep reports 0 violations after the change, and the
progress guarantee is pinned by its own test.
Drexuxux
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