tai42-channel-whatsapp

A Meta WhatsApp API channel plugin for the TAI ecosystem. It delivers an ask_user question to a human on WhatsApp through the Cloud (Graph) API and bridges the human's reply back into the interactions store — so an agent can reach a person out-of-band instead of only showing the question in the Studio inbox. It implements the tai42_contract.channels.Channel protocol and registers under the name "whatsapp". It is for numbers hosted directly on Meta's Cloud API (no BSP/Twilio in front); the Twilio-hosted path is the sibling tai42-channel-twilio.

The TAI ecosystem

TAI is an open-source runtime for MCP tools, agents, and workflows. A Channel is "how a question reaches a human" — a pluggable deliverer the runtime resolves by name when ask_user is called with channel=.... This package is one such deliverer (WhatsApp API); siblings back the same contract with Twilio, Telegram, or Slack. The ecosystem is open-ended: any package can back the same contract, so this repo is this plugin's own full doc home, and the documentation site covers the platform-level story:

• Interactions concept: https://tai42.ai/concepts/interactions
• Build a channel plugin (author guide): https://tai42.ai/guides/authors/channel
• Ecosystem catalog: https://tai42.ai/reference/catalog

Its only tai-* dependencies are tai42-contract (the Channel protocol, ChannelDelivery, ChannelDeliveryError, and the tai42_app handle) and tai42-kit[redis] (HttpxClient, RedisClient, TaiBaseSettings, and the settings cache). Beyond those it depends on httpx, starlette, and pydantic / pydantic-settings. There is no Meta SDK: the send is one Bearer-auth JSON POST over httpx, and webhook signature validation is a few lines of stdlib hmac/hashlib.

Install

Requires Python 3.13+. Install from PyPI into the environment that runs the server:

uv add tai42-channel-whatsapp

Or from source — clone this repo and add it as an editable dependency; the tai42-* dependencies resolve in-tree from the workspace.

git clone https://github.com/tai42ai/tai42   # next to your app checkout
cd /path/to/your/app
uv add --editable ../tai42/plugins/channel-whatsapp

Discovery

The runtime discovers this plugin through the manifest's channel_modules key:

channel_modules: ["tai42_channel_whatsapp"]

At app load the runtime imports every module under the package, and register.py fires the registrations as its import side-effect: the "whatsapp" channel on tai42_app.channels, and — via the inbound import — the unauthenticated webhook route on tai42_app.http. A bare import tai42_channel_whatsapp registers nothing — the package is library-safe; only the register module carries the side-effect.

Configuration

Settings are read from the CHANNEL_WHATSAPP_ environment group (see WhatsAppSettings / WhatsAppRedisSettings):

One credential (ACCESS_TOKEN + APP_SECRET) serves many phone_number_ids: a bridge reply is sent from the exact phone_number_id that received the inbound message, while an ask_user delivery is sent from CHANNEL_WHATSAPP_DEFAULT_PHONE_NUMBER_ID. Recipient policy is split by what Meta itself fences. Freeform sends (questions, replies, media) reach any requested wa_id — Meta's own 24-hour customer-service window is the fence, so a send to a number outside it is rejected by Meta (error 131047) and raises. Template sends are the one send Meta delivers cold, so they keep an operator fence: the recipient must be on CHANNEL_WHATSAPP_ALLOWED_RECIPIENTS or be a known contact — a (phone_number_id, wa_id) pair the inbound webhook has seen within CHANNEL_WHATSAPP_TEMPLATE_CONTACT_WINDOW_DAYS days; a cold, unlisted number raises. A recipient is always caller-supplied — there is no default recipient, so a recipientless request raises. Secrets live only in the environment.

Two steps happen out-of-band (the plugin never mutates Meta app configuration at startup):

1. In the Meta App dashboard, point the WhatsApp webhook callback URL at {public base URL}/api/channels/whatsapp/inbound and set the verify token to CHANNEL_WHATSAPP_VERIFY_TOKEN. Meta issues a GET handshake the route answers by echoing hub.challenge.
2. Subscribe the app to the messages webhook field so message and delivery-status events reach the same POST endpoint.

How a human answers

A text question arrives as a normal WhatsApp message; the human just replies — no code to quote, no prefix. A select question renders natively: a few short options become tappable reply buttons, more options become an interactive list, and past those platform caps it falls back to a numbered plaintext list. A tap answers by a question-bound id that maps back to the exact option text; the human may always type an option instead. A form question renders as an in-chat WhatsApp Flow — one screen of typed fields the human fills and submits: a string becomes a text field, a string with an enum a dropdown, a boolean an opt-in toggle, and an integer/number a numeric text field. The Flow is created and published once per distinct answer schema (cached by a schema hash under CHANNEL_WHATSAPP_WABA_ID) and reused; the completed form returns as an nfm_reply, its values coerced to the schema's types before the answer object is forwarded. When the callback door rejects a forwarded form answer (400 — a schema rule the Flow could not enforce, e.g. a minimum or pattern), a completed Flow has no re-reply surface, so the channel re-sends a fresh Flow for the same interaction: same flow token, the cached flow id, and a body that repeats the question with the door's error line (which names the failing field). This is bounded — after a fixed number of rejections the channel stops re-sending, tells the participant once the form could not be processed, and lets the ask time out on its own deadline. The platform validates the answer schema against this subset when the question is asked — before the question is stored — so an out-of-subset schema (nested objects, arrays, unions, unknown types) never reaches this send; the Flow mapping rejects one defensively too. Correlation is fully out-of-band: any reply (typed, tapped, or a submitted form) from the recipient resolves the (phone_number_id, wa_id) pair's pending question, so one question can be pending per pair at a time; a second concurrent one is rejected loudly.

An agent notification (notify_user) advertises the full capability set — supports_media_notifications, supports_location_notifications, supports_template_notifications, supports_interactive_notifications, and supports_form_notifications — so a notification may carry the full outbound vocabulary:

• Media — each file item sends as its own native message: an image, document (with caption/filename), video (with caption), or audio (voice/clip; the Cloud API audio object carries no caption/filename); a link item is appended to the body text as a label: url line.
• Options (list[Option], a typed union, never bare strings) — a ReplyOption renders as a native reply button (or a list row past the button caps / for a described option), sending its authored id on the wire when set (echoed back on tap) else a minted index; a tap enters the conversation as a visitor message. A lone LinkOption renders as a cta_url interactive (one URL button); mixed reply+link or multiple links append the link(s) to the body as label: url lines (WhatsApp has no multi-URL button).
• Sections (list[OptionSection]) — a multi-section interactive list, each row carrying an optional description secondary line.
• Header + footer — a media header (image/video/document) rides the interactive header (an audio header is sent as its own message ahead of it); a text footer rides the interactive footer. Both require options/sections.
• Location (LocationElement) — a native location message (a pin with an optional name/address).
• Template — for a send outside the 24-hour window, a pre-approved ChannelTemplate by name: its header_media (image/video/document), body_parameters (positional body-text fills), and buttons (per-button quick-reply payload / url suffix) map onto the template-message components.
• Ask-less form (schema) — an in-chat WhatsApp Flow (see below).

template, a choice surface, and schema are mutually exclusive on one notification; options and sections are mutually exclusive with each other; media and location may combine with a choice surface.

An ask-less form notification renders exactly like a form ask — any media first, then a WhatsApp Flow whose body is the message — but stores no correlation: the flow token is minted in the tai42-nf: namespace (tai42-nf:{schema hash}:{random}), and the inbound webhook routes an nfm_reply by that prefix before any pending-question lookup, so a submission enters the conversation as a structured participant message (rendered label: value text plus the structured copy) and can never answer — or disturb — a question pending on the same pair. The answer schema is cached durably beside the published-flow id under the schema hash; a reply carries only the hash, so a lost cache entry cannot be repopulated from it — the reply then degrades to its raw (uncoerced) values, and is still accepted. Like every freeform send, a form notification delivers only inside Meta's 24-hour customer-service window — out of window the send fails loudly (error 131047), synchronously or as a failed status; it is never silently downgraded to a template.

A confirm or external question arrives as a tappable link and is answered in the browser via the callback door — no WhatsApp reply is expected or matched, and it never consumes the pair.

Inbound entry parameters

When an inbound message enters the conversation bridge as a fresh turn (a message with no pending question to answer), the channel forwards a set of opaque entry parameters alongside the turn text. They ride verbatim to a tool target's payload under its own params key (payload["params"]) — the platform attaches no meaning and no trust; a channel-agnostic consumer opts into whichever keys it understands. This is the channel's public inbound contract:

Participant media and location and reactions and contacts bridge as turns. The caption of a media message becomes the turn text (a faithful [image] / [document: file] / [voice message] / … placeholder when caption-less); an inbound location lands as a typed LocationElement on the turn's location (a machine-consumable field, not a param), with the place name/coordinates as the text.

Inbound media design note (design gap). WhatsApp inbound media arrives as a Graph media id; resolving it to bytes is a two-step Graph call that returns a short-lived, Bearer-authenticated lookaside URL — not a durable public https URL and not a valid MediaItem source. Minting a durable typed attachments entry needs a served-media ingestion seam (fetch → persist → mint a {MEDIA_ROUTE_PREFIX}{id} served reference); the platform's served-media store is not reachable from a channel plugin and handles only outbound data:image substitution, so no such seam exists today. Rather than invent infrastructure or fabricate an unfetchable URL, inbound media therefore bridges without a typed attachments entry — its identity rides the media_* params above and a consumer re-fetches via media_id with operator credentials. The durable fix is a platform served-media ingestion seam on the app handle.

Params ride only on the bridge path. A tap or quick-reply that answers a pending question does not surface them: the answer path forwards {"answer": …} to the callback door — a seam that carries no params — and a tap's id is already consumed there to select the option. Values are transport-bounded (per the platform's entry-param limits — count, key charset, value length, total size); an individual value over the per-value cap is dropped (never truncated), and in the rare case the aggregate still overflows a bound the whole set is dropped and the turn bridges without it — a participant message is never lost to a params bound.

Meta inbound error notices (e.g. an unsupported message type the participant sent, carried as an errors[] array) are logged at warning with the detail and are never bridged as participant turns.

Delivery statuses

WhatsApp reports delivery asynchronously: a send returns 2xx with a wamid, and a later statuses webhook carries sent/delivered/read/failed. The same POST endpoint records those receipts through the interactions facet — failed (e.g. a message outside the 24-hour session window, error 131047) marks the answer failed loudly; sent/delivered confirm it; read is informational and ignored.

A wamid the conversation bridge does not own is not a dead end: it may be a notify_user send (which runs no conversation record). The webhook then resolves the wamid through the send-outcome receipt index and, on a hit, posts a delivery_receipt event onto the send's originating monitoring trace, nested under its send span — ERROR for a failed receipt (carrying the provider errors), informational otherwise. This is observability only: it annotates the originating trace, never re-sending or failing the run. Only a genuine miss — neither the bridge nor any such send owns the wamid — keeps the untracked-message log; either way the status is acknowledged, never retried.

Security

• Inbound requests authenticate via X-Hub-Signature-256: sha256= + hex(HMAC-SHA256(app_secret, raw body)), validated fail-closed with a constant-time compare before the body is parsed. A missing/empty app secret is an operator error that raises loudly (logged 500) — never a soft 401 that reads like a bad signature.
• The GET verification handshake echoes hub.challenge only when hub.verify_token matches the configured token under a constant-time compare.
• Meta's signature scheme carries no timestamp, so a captured request validates forever; the wamid dedupe window (48h default) plus HTTPS are the replay guards.
• The access token, app secret, and verify token are SecretStr — never in a repr, log line, or traceback; the plaintext is read only at the Bearer-auth and HMAC seams.
• The unauthenticated route bounds its body read (1 MiB → 413) before any signature work.

Limits

Development

uv venv --python 3.13
uv pip install --no-sources --group dev --editable .
uv run --no-sync pytest --cov --cov-report=term-missing
uv run --no-sync ruff check .
uv run --no-sync ruff format --check .
uv run --no-sync pyright

The live integration suite (pytest -m integration) sends real messages via the WhatsApp API and runs only when the CHANNEL_WHATSAPP_* credentials are present in the environment; it skips cleanly otherwise.

License

Apache-2.0. See LICENSE and NOTICE.

Correlation surface (2.0)

Since 2.0 inbound answers resolve through the platform's shared inbound-answer ladder: the plugin exposes its correlation store over the contract's CorrelationStore port (reserve / peek / release) plus a transport ack, and the skeleton owns the forward / retry-in-place / bridge ladder. The plugin-local pop/restore correlation helpers from 1.x are gone.