responses api behind use-responses-api flag (env-22..24, d-021) — tools + reasoning combinable, stateless with encrypted reasoning, multi-round tool loop

DECISIONS.md

@@ -70,6 +70,16 @@ Decisions inside the set architecture. D-NNN, never renumbered.
   depth); each is independently skippable when it has no data, so a
   deployment without a budget or store still runs the others. Opt-in
   (`enable-monitoring`) like every other operational rollout.
+- **D-021** — Responses API behind `use-responses-api` (FDB-028,
+  ENV-22..24, resolves D-006): the responder path can use
+  `/v1/responses`, which allows tools + `reasoning_effort` (the
+  chat/completions 400 from ENV-21) and keeps one chain of thought
+  across tool rounds. Stateless by choice: `store=false` +
+  encrypted reasoning items passed back — GDPR posture unchanged, no
+  server-side conversation retention. Flag defaults off; rollback is
+  a config toggle (hot-reload), not a deploy. Classifier /
+  consolidation / task-gen stay on chat/completions (no tools, no
+  reasoning need — not worth the churn).
 - **D-020** — Web search via Exa (FDB-022, SPEC-015): a `web_search`
   tool alongside fetch_url/IGDB/codex/get_news, filling the "look it up
   on the open web" gap. Exa (not a raw search-engine scrape) because it