> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fiveninelabs.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Changelog

> Every dated fn-version with its behavioral diffs, every additive change, and anything with a retirement clock.

The API is versioned by dated `fn-version` header with account pinning — see the
versioning contract in the [API reference](/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.

## 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](/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](/errors/declined_to_spend).

## 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.

## Deprecations

Nothing is deprecated.