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The API is versioned by dated fn-version header with account pinning — see the versioning contract in the API reference. Two promises govern this page:
  • Additive changes (new fields, new event phases, new endpoints, new members of open enums) ship to every version and are announced here.
  • Nothing is ever removed without appearing in the Deprecations section first, with at least 6 months’ notice; dated versions are supported for at least 12 months after a newer date exists.

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2026-08-27 — honesty correction

The initial entry below overstated what had shipped. This entry corrects the record; nothing about the API itself changed:
  • Shipped and live: everything in the “shipped” list of the 2026-08-26 entry as corrected below.
  • Specced but not yet served (marked x-fn-status: planned in the API reference; these routes answer 501 not_implemented with a surface field naming what is missing): the whole URL-first surface (POST /v1/discover, /v1/extractions single and batch, …/refresh), session …/refresh, DELETE redaction (sessions and extractions), /v1/monitors, GET /v1/receipts/{id}, webhook test deliveries (POST /v1/webhooks/{id}/test) and redelivery, and the MCP server at api.fiveninelabs.com/mcp. Each will get its own dated changelog entry when it ships.
  • The decline contract is documented as production behaves: a query we refuse to spend on returns 201 and ends as a $0 abstained run (pre-spend signal: estimate.would_run: false) — not the 422 an earlier draft described. See declined_to_spend.

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2026-08-26 — initial version

The first dated version of the Five Nine Agent API. Shipped:
  • Sessions: create (streaming or async), estimate (free, always callable), events, stream with Last-Event-ID resume, long-poll result, rows, receipts, turns with priced proposals, stop.
  • Platform plane: /v1/me, /v1/balance, /v1/usage, webhook endpoint CRUD with the attempts log, /v1/keys (list, rename, revoke).
  • Test mode: fn_test_ keys with recorded-session replays and the FN_TEST_* sentinels.
Specced but not shipped in this version (see the 2026-08-27 correction above): extractions, discover, monitors, refresh, redaction, GET /v1/receipts/{id}, webhook test/redeliver, MCP.

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Deprecations

Nothing is deprecated.