fix(agent): preserve todos as tool state after compression

[0xLeathery]

Summary

• Preserve active todo continuity across context compression as canonical assistant tool-call + tool result history instead of appending a synthetic user message.
• Keep completed and cancelled todos out of the compressed continuation state.
• Add regression coverage for user-turn preservation, provider-valid tool pairing, hydration, and insertion ordering.

Why

Context compression used to append the preserved todo snapshot as a fresh user message. That blurred internal continuity state with user intent, and could make preserved todo text look like the latest user turn after compaction.

This keeps the two concepts separate: real user messages stay user messages, and active todo state is carried as paired tool history that the existing todo hydration path can restore.

Unlike #35349, this does not fold todo state back into a user turn; it carries active todos as paired assistant/tool history.

Closes #26979.

Test plan

• python3.11 manual compression smoke: verified latest real user turn remains the user tail, one paired todo tool result is inserted, only active todos are preserved, and completed todos are omitted.
• python3 scripts/check-windows-footguns.py agent/conversation_compression.py tools/todo_tool.py tests/agent/test_context_compression_todo_leak.py tests/tools/test_todo_tool.py
• Public diff hygiene scan: no secret-like assignments, local paths, or private process context in the changed diff.
• scripts/run_tests.sh tests/agent/test_context_compression_todo_leak.py tests/tools/test_todo_tool.py tests/run_agent/test_run_agent.py tests/tools/test_read_loop_detection.py tests/agent/test_context_compressor.py tests/agent/test_context_compressor_summary_continuity.py tests/agent/test_context_compressor_temporal_anchoring.py tests/agent/test_compression_concurrent_fork.py tests/agent/test_compression_rotation_state.py tests/agent/test_compression_progress.py tests/agent/test_compressed_summary_metadata.py — 645 passed.

Platforms tested

• macOS local development checkout.

[alt-glitch]

This was generated by AI during triage.

Competing fix cluster for #26979 (all open, different mechanisms): this PR carries active todos as paired assistant/tool history; #26981 removes the injection entirely; #18883 strips subjects from the injection. #35349 (closed) folded the snapshot back into a user turn, which this PR explicitly avoids. Not a duplicate of any -- flagging the cluster so a maintainer can pick the canonical approach.

[tonydwb]

LGTM. The _STALE_TODO_SHOULD_NOT_BE_A_USER_MESSAGE sentinel strings are intentional test fixtures, not stale TODOs — confirmed by context. Core logic (preserving todos as tool state after compression) looks correct. Well-tested fix with dedicated unit tests.

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Reviewed by Hermes Agent

[teknium1]

Thanks for preserving todo state as a paired tool result rather than a synthetic user instruction. The underlying issue remains on current main: agent/conversation_compression.py:810-813 still appends format_for_injection() as role="user".

Problems

• The current insertion order is stale against main. The PR returns unchanged history when it contains no user turn (agent/conversation_compression.py:126-134 in PR head 557918825581), but current main then restores a real user turn in _ensure_compressed_has_user_turn() (agent/conversation_compression.py:418-456, added by 6e176e4c). In that assistant/tool-only compression case, active todos are not preserved. Current main has an explicit regression for this shape at tests/agent/test_compression_concurrent_fork.py:206-241.

Suggested changes

• Preserve the current user-turn guard, run it before inserting the assistant/tool todo pair, and add a regression covering the assistant-only compressor result plus todo hydration.

Automated hermes-sweeper review.

[teknium1]

Current main adds _ensure_compressed_has_user_turn() after this hunk. If compression returns assistant/tool-only history, this helper returns unchanged at line 134 and the later guard restores a user turn without any todo pair. Run the user-turn guard before this insertion and cover that current-main compatibility path.

[teknium1]

Merged via #69860 (commit 2ca38e5). Issue #26979 is fixed via #69860 (merge-into-trailing-turn). Your preserve-as-tool-state direction is noted in the merged PR body as the possible longer-term shape — if you want to pursue it, it would now build on the merged behavior. Thanks!