feat(compact): reactive compaction with direct input support - #726

PR Review

Summary: Completes five compaction features from #30: previous_usage token estimation fast-path, configurable summary_prefix, hiding compaction items from input_items, persisting compaction responses, and an explicit POST /v1/responses/compact endpoint.

Overall: Solid implementation with good integration tests covering both the rehydrated and direct-input paths. The summary prefix plumbing is clean and consistent across both translation paths. A few issues below around blocking in async context, stale docstrings, and missing unit test coverage for the new endpoint.

Findings without inline placement

[Large] Stale docstrings — The struct-level doc comment on CompactFilter (~line 96 of mod.rs) says compaction "only applies to multi-turn requests where openai_responses_rehydrate has loaded stored conversation history". The generated docs page (docs/filters/openai_responses_compact.md, Configuration Notes section) says the same. Both are now incorrect — this PR adds a direct-input compaction path that runs without rehydration. Update both to match the module-level doc comment (lines 14-20), which was correctly updated.

[Large] Missing unit tests for explicit compact endpoint — The new pure/mostly-pure functions parse_compact_request_body, extract_stored_messages, is_explicit_compact_request, and ensure_compactable_state have no unit tests. Per project convention each testable function should have coverage. Suggested cases:

• parse_compact_request_body: empty body, invalid JSON, missing response_id, valid request with optional fields
• extract_stored_messages: empty array, non-array messages, valid array
• is_explicit_compact_request: POST to correct path, GET to correct path, POST to wrong path
• ensure_compactable_state: no ResponsesState, rehydrated state, non-rehydrated with compaction config, non-rehydrated without compaction config

Re-review (Aug 18 commits)

Previous review had 6 findings. The new commits addressed 4 fully (async conversion of fetch_response/build_and_persist_compaction, _body rename, response_id emptiness validation, tiktoken overhead restoration). One partially addressed (unit tests added for parse_compact_request_body and extract_stored_messages, but is_explicit_compact_request and ensure_compactable_state still lack coverage). One unaddressed (stale docstrings).

One new finding below from the rewritten explicit compact endpoint.

Outside the PR diff, but

"ResponseStoreFilter::should_init_store_for_request() returns false for POST /v1/responses/compact because the path is not is_responses_create and the classifier does not set has_previous_response_id for sub-resource routes. The store Arc is therefore never registered into ResponseStoreRegistry before CompactFilter::resolve_store_and_tenant() runs. On a freshly built pipeline (or after reload), the first explicit compact call fails with "response store not available" even when the backend is healthy. Extend should_init_store_for_request (or equivalent) to initialize the store for POST /v1/responses/compact, mirroring how rehydrate paths are handled."

PR Review (re-review)

The Aug 20 commits fixed the response_object finding from the previous re-review (now includes model, created_at, and output).

One new finding from the direct-input compaction path.

Re-review (Aug 24 commits)

The two new commits fix the summarization scope (exclude current input from summarization text in the rehydrated path), add is_compactable unit tests, add an explicit compact endpoint integration test, fix the stale docstrings on CompactFilter and the docs page, and add store initialization for the compact endpoint.

Previous findings addressed:

• Stale docstrings: fixed (struct doc and docs page updated)
• Missing is_compactable / ensure_compactable_state tests: fixed (extracted is_compactable and added 5 tests)

One new finding below.

P1 — Explicit compact rejects conforming requests. parse_compact_request_body requires nonstandard response_id. The checked-in API contract requires model and supports input/previous_response_id. Standard {model,input} requests therefore return 400.

P1 — Response violates CompactResource. The response builder returns object: "response" without usage; the contract requires object: "response.compaction" and usage.

P2 — Fail-open becomes fail-closed. Explicit compaction converts the Ok(None) produced by on_failure: open into a hard-coded 502.

P2 — Reactive compaction creates unreachable rows. apply_compaction generates and persists a response ID, then discards it without exposing or linking it. Every compaction leaves an orphan record despite the normal response subsequently persisting the compacted history.