stdlib json was FastAPI's default. Every response body, every SSE frame,
and every add_log/state/payload write paid the stdlib encode cost.
- pyproject.toml: add orjson>=3.10 as a core dep.
- decnet/web/api.py: default_response_class=ORJSONResponse on the
FastAPI app, so every endpoint return goes through orjson without
touching call sites. Explicit JSONResponse sites in the validation
exception handlers migrated to ORJSONResponse for consistency.
- health endpoint's explicit JSONResponse → ORJSONResponse.
- SSE stream (api_stream_events.py): 6 json.dumps call sites →
orjson.dumps(...).decode() — the per-event frames that fire on every
sse tick.
- sqlmodel_repo.py: encode sites on the log-insert path switched to
orjson (fields, payload, state value). Parser sites (json.loads)
left as-is for now — not on the measured hot path.
anti
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