The #83714 fix hardened only the tool-call args renderer; five other
renderers (plus three same-idiom siblings) still composed the bare
truncation marker, which the model imitated from replayed context into
new durable writes (#83435).
This routes all model-visible elision in agent/ through shared helpers
in agent/compression_marker.py:
- elide(text, limit): head + marker cap with accurate omitted/total counts
- elide_middle(text, head, tail): kept head/tail with the middle elided
The elision marker's first sentence is byte-identical to the args marker's,
so the existing _COMPRESSION_MARKER_RE (and the dispatch-boundary guard in
tool_dispatch_helpers) rejects a copied marker regardless of which renderer
leaked it; only the tool-call-specific second sentence is dropped so the
marker still fits small caps (the clarify summary cap is 199 chars).
Routed sites:
- context_compressor.py: static-fallback turn renderer, _sum_clarify(),
summarizer prompt builder (middle elision), active-task snapshot,
lean user-message quotes (2 sites)
- skill_preprocessing.py: inline-shell output embedded into skill bodies
- lsp/reporter.py: truncate() for <diagnostics> tool output
- verification_stop.py: verify-on-stop nudge output summary
Regression test asserts the imitable idiom appears nowhere in agent/
strings (comments excluded), so a new open-coded renderer cannot land
silently.
Fixes#121548
(cherry picked from commit 69c63d50b4ababedf4fb4ff88c08f5d1e144a79f)
77b9ea0831