Summary

Salvages #31242 onto current main: when context compression's summary LLM fails (e.g. Codex 429 usage_limit_reached), the dropped-window placeholder is now a deterministic locally-extracted handoff instead of a content-free N messages were removed marker.

Cherry-picked 3 commits from @Hinotoi-agent's fix/deterministic-compression-fallback onto current main, authorship preserved.

What it does

Adds _build_static_fallback_summary() to agent/context_compressor.py. Only fires on the existing failure path (abort_on_summary_failure=False AND aux LLM returned empty/errored). From the compacted window it extracts:

• recent user asks
• assistant tool calls (name + args, dict and object shapes)
• file paths (path-mention regex + path/workdir/file_path/output_path keys in tool args)
• error / blocker snippets (error|failed|exception|traceback|timeout)
• bounded last-dropped-turn excerpts

Output is capped (8KB total, 700 chars/turn), deduped, and run through redact_sensitive_text + an extra gh-token-prefix scrub before insertion. Normal LLM-summarized compression is unchanged.

Why this matters

Reference incident: Discord user @damien hit Codex Plus usage_limit_reached mid-compression on a 145k-token session. The aux compressor 429'd → 142 messages dropped → the content-free placeholder gave the next turn nothing to work with → continuity collapsed. With this PR the same failure still drops the window, but the model gets back user asks, file paths, tool activity, and error snippets so it can keep going.

compression.abort_on_summary_failure=true remains the alternative — freeze the session instead of dropping. This PR only improves the drop path that runs when that flag is false (default).

Scope

• agent/context_compressor.py — new _build_static_fallback_summary(), helper extractors, fallback-branch wiring
• tests/agent/test_context_compressor.py — coverage for dict/object tool calls, path extraction (POSIX + Windows), gh-token redaction, bounded output, protected-tail exclusion, dropped-turn snippet behaviour

No provider, gateway, loop, CLI, or storage changes.

Validation

Notes on the ACP test failure on the original PR

Upstream PR #31242 had test (3) red on tests/acp/test_server.py::test_model_switch_uses_requested_provider with 'custom' == 'anthropic'. That was an unrelated xdist-worker flake caused by another test in the same worker mutating _KNOWN_PROVIDER_NAMES / _PROVIDER_ALIASES. Already pinned on current main by commits 3127a41 and 2b76853 (monkeypatch parse_model_input, drop the brittle tail-position assertion). Re-running 78/78 green here confirms.

Infographic

Credits

• @Hinotoi-agent — original implementation, all 3 commits cherry-picked with authorship preserved
• Closes fix: preserve context when summary generation fails #31242