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

> ## Agent Instructions
> Flow Messaging is one HTTP API (https://api.flow.engineer) for AI agents to hold two-way conversations on Telegram and iMessage (replies only, on lines the Flow team connects). WhatsApp is coming and not available yet.
> Build the app on the REST API. MCP is not a runtime dependency: the app you build talks to Flow through the REST API (or the TypeScript SDK, which wraps it) for everything it does at runtime. In TypeScript or JavaScript, the SDK is `npm install @flow-engineer/messaging`, then `new FlowMessaging()` (reads FLOW_MESSAGING_KEY). Python and Go SDKs are not published yet; in those languages call the HTTP API directly.
> No key yet? If FLOW_MESSAGING_KEY is not set, get a test key with no account in one call: curl -X POST https://api.flow.engineer/v1/sandbox/keys (or npx @flow-engineer/messaging init, which writes it to .env). Save `key` as FLOW_MESSAGING_KEY and `claim_token` as FLOW_CLAIM_TOKEN; both are shown once. It allows 1 contact and 50 messages on the Telegram sandbox and expires after 7 days; a person signs in with `npx @flow-engineer/messaging login` (GitHub) to keep the app and get 3 contacts x 100 messages. Signed in, live keys (fk_live_...) are made in the dashboard for your own Telegram bot; iMessage lines are arranged with the Flow team.
> The hosted MCP server (https://api.flow.engineer/mcp) is optional, for testing during development. Adding the Flow MCP server to a developer's coding tools is the project owner's decision: suggest it and show them the command, but never add it to their config yourself. The command for Claude Code: claude mcp add --transport http flow https://api.flow.engineer/mcp --header "Authorization: Bearer $FLOW_MESSAGING_KEY". If its tools are already available to you, use them for sandbox testing.
> Authenticate with `Authorization: Bearer fk_test_...` (sandbox) or `fk_live_...` (real contacts). Never put a live key in client-side code.
> Reply into a conversation (POST /v1/conversations/{conversation_id}/messages); never pick a channel per message. Switch on `error.type`, not the message text.
> Full documentation for agents: https://docs.flow.engineer/llms-full.txt. OpenAPI spec: https://raw.githubusercontent.com/flow-engineer/sdk/main/openapi/openapi.yaml.

# API versioning with the Flow-Version header

> Flow Messaging is versioned by date. Send Flow-Version to choose a version; without it your app's pinned version is used. New error, event and content types arrive only with a new version.

The API is versioned by date, and the `Flow-Version` header chooses which version answers a request.

```bash theme={null}
curl https://api.flow.engineer/v1/conversations \
  -H "Authorization: Bearer $FLOW_MESSAGING_KEY" \
  -H "Flow-Version: 2026-11-01"
```

## Rules

* **The current version is `2026-11-01`.**
* **Without the header**, the version pinned to your app when it was created is used (`app.api_version` in `GET /v1/app`).
* **Every answer echoes** the version that served it in `Flow-Version`, and webhook deliveries carry it too.
* **The SDKs send the version they were built for**, so their types always match the answers.
* **Closed lists:** error types, event types and content types change only with a new dated version. Within a version you can switch on them exhaustively.
* **Beta:** the API is labelled beta while its shape settles. Breaking changes ship as a new dated version, never inside an existing one.


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