Metadata-Version: 2.4
Name: turnstack
Version: 0.1.3
Summary: WhatsApp bot engine with node-based flows and interactive replies
Author-email: IdrisFallout <dev@waithakasam.com>
License: MIT
Project-URL: Homepage, https://github.com/IdrisFallout/turnstack
Project-URL: Repository, https://github.com/IdrisFallout/turnstack
Project-URL: Bug Tracker, https://github.com/IdrisFallout/turnstack/issues
Keywords: whatsapp,bot,chatbot,flow,engine
Classifier: Programming Language :: Python :: 3
Classifier: Programming Language :: Python :: 3.10
Classifier: Programming Language :: Python :: 3.11
Classifier: License :: OSI Approved :: MIT License
Classifier: Operating System :: OS Independent
Requires-Python: >=3.10
Description-Content-Type: text/markdown

# TurnStack — Developer Documentation

> **The WhatsApp conversation engine that gets out of your way.**
> You define the flow. TurnStack drives it.

---

## Table of Contents

1. [What TurnStack Is (and Isn't)](#1-what-turnstack-is-and-isnt)
2. [Core Concepts](#2-core-concepts)
3. [Quick Start](#3-quick-start)
4. [The Flow Tree](#4-the-flow-tree)
5. [Building Blocks — Node Reference](#5-building-blocks--node-reference)
   - [Menu](#51-menu)
   - [Input](#52-input)
   - [Confirm](#53-confirm)
   - [Action](#54-action)
   - [Router](#55-router)
   - [ListNode](#56-listnode)
   - [MediaReply](#57-mediareply)
   - [CtaUrl](#58-ctaurl)
   - [Carousel](#59-carousel)
   - [ContactReply](#510-contactreply)
6. [Field Types (inside Input)](#6-field-types-inside-input)
   - [Field / TextField](#61-field--textfield)
   - [MenuField](#62-menufield)
   - [ButtonsField](#63-buttonsfield)
   - [ImageField](#64-imagefield)
   - [DocumentField](#65-documentfield)
   - [LocationField](#66-locationfield)
   - [BranchField](#67-branchfield)
7. [Node → WhatsApp Widget Mapping](#7-node--whatsapp-widget-mapping)
8. [The Engine](#8-the-engine)
   - [Instantiation](#81-instantiation)
   - [process()](#82-process)
   - [IncomingMessage](#83-incomingmessage)
   - [Reply](#84-reply)
9. [Session & State](#9-session--state)
   - [Session object](#91-session-object)
   - [session.collected](#92-sessioncollected)
   - [session.context](#93-sessioncontext)
   - [session.pagination](#94-sessionpagination)
10. [Session Stores](#10-session-stores)
    - [InMemorySessionStore](#101-inmemorysessionstore)
    - [Custom stores](#102-custom-stores)
11. [Navigation — Built-in Commands](#11-navigation--built-in-commands)
12. [Sending Replies — Adapter Pattern](#12-sending-replies--adapter-pattern)
    - [The boilerplate send helper](#121-the-boilerplate-send-helper)
    - [Sending via pywa / any library](#122-sending-via-pywa--any-library)
13. [Wiring to a Webhook](#13-wiring-to-a-webhook)
14. [Validation & Transformation](#14-validation--transformation)
15. [Dynamic Content](#15-dynamic-content)
16. [Conditional Fields — BranchField](#16-conditional-fields--branchfield)
17. [Pagination — Automatic Behaviour](#17-pagination--automatic-behaviour)
18. [Custom Node Handlers](#18-custom-node-handlers)
19. [Error Handling](#19-error-handling)
20. [Debug Utilities](#20-debug-utilities)
21. [Complete Example — Customer Support Bot](#21-complete-example--customer-support-bot)

---

## 1. What TurnStack Is (and Isn't)

**TurnStack is a conversation-flow engine.** You give it a tree of nodes. It receives normalised WhatsApp messages, drives the user through the tree, manages all session state, and hands you back structured `Reply` objects ready to send.

**What TurnStack handles for you:**

- Session lifecycle (create, persist, expire, reset)
- Navigation state machine (current node, history stack, back/home/exit)
- Multi-step form collection with per-field validation and transformation
- Menu and list pagination (automatic, configurable)
- Interactive vs plain-text rendering hints via `reply.node_type`
- Unsupported message types (stickers, audio, reactions) — polite reply, no state change
- Media file delivery followed by the next node — both sent automatically
- Global navigation commands (`back`, `home`, `exit`) intercepted before dispatch

**What TurnStack does NOT do:**

- Send messages — that's your adapter (REST, pywa, Twilio, or anything else)
- Store sessions to a database — plug in your own `SessionStore`
- Parse raw WhatsApp webhook payloads — your webhook handler does that (it's a one-time ~50-line setup, and we give you the exact boilerplate below)
- Lock you into any web framework — FastAPI, Flask, Django, Lambda, raw asyncio — all fine

---

## 2. Core Concepts

```
Raw WA payload
      │
      ▼
  Your webhook  ──► builds IncomingMessage
      │
      ▼
  engine.process(incoming)
      │
      ▼
  List[Reply]   ──► your send adapter dispatches each reply to WA API
```

**FlowTree** — a dictionary of named nodes you build once at startup.

**Node** — a single step in the conversation. Each node has a type (menu, input, action, etc.) and a `next` key pointing to the next node.

**Session** — per-user state the engine manages. Contains `current_node`, collected form data, navigation history, and arbitrary context your code can read/write.

**IncomingMessage** — a normalised message object you build from the raw WA payload and pass to the engine.

**Reply** — a structured response object the engine returns. You read `reply.node_type` and `reply.options` to decide how to send it (interactive list, buttons, plain text, document, CTA, carousel, etc.).

---

## 3. Quick Start

```bash
pip install fastapi uvicorn httpx python-dotenv turnstack
```

```python
from turnstack import BotEngine, FlowTree, IncomingMessage
from turnstack.nodes import Menu, Input, Action, Option, Field

# 1. Build the tree
tree = FlowTree(entry="welcome")

tree.add("welcome", Menu(
    text="👋 Welcome! What would you like to do?",
    options=[
        Option("📝 Book appointment", next="book_form"),
        Option("ℹ️ About us",         next="about"),
    ],
))

tree.add("book_form", Input(
    title="Booking",
    fields=[
        Field("name", "What is your full name?"),
        Field("date", "What date works for you? (YYYY-MM-DD)"),
    ],
    next="confirm_booking",
))

tree.add("confirm_booking", Action(
    fn=lambda session, collected: f"✅ Booking confirmed for {collected['name']} on {collected['date']}!",
    next="welcome",
))

tree.add("about", Action(
    fn=lambda s, c: "We are an example company. Reply anything to go back.",
    next="welcome",
))

# 2. Create the engine
engine = BotEngine(tree=tree)

# 3. In your webhook, normalise the payload and call process()
async def handle_message(user_id: str, text: str):
    incoming = IncomingMessage(user_id=user_id, type="text", text=text)
    replies  = await engine.process(incoming)
    for reply in replies:
        print(reply.body)   # send this via your WhatsApp adapter
```

---

## 4. The Flow Tree

```python
from turnstack import FlowTree

tree = FlowTree(entry="welcome")
tree.add("welcome",  Menu(...))
tree.add("register", Input(...))
tree.add("done",     Action(...))
```

`FlowTree(entry="<node_key>")` — the `entry` key is where all new sessions start.

`tree.add(key, node)` — register a node. The key is a plain string; any node type is valid.

`tree.validate()` — called automatically when `BotEngine` starts. Raises if any `next` reference points to a missing node, or if no entry node is defined.

**Special destination key: `"__end__"`**

Use `next="__end__"` on any node to cleanly terminate the session. The engine sends the final message and the session is marked closed. The next message from the user starts a fresh session from the entry node.

```python
tree.add("goodbye", Action(
    fn=lambda s, c: "👋 Thanks for using our service. Goodbye!",
    next="__end__",
))
```

---

## 5. Building Blocks — Node Reference

### 5.1 Menu

Presents the user with a list of options. Renders as a WhatsApp interactive list message (with automatic pagination when options exceed the display limit).

```python
from turnstack.nodes import Menu, Option

tree.add("main_menu", Menu(
    text="What would you like to do?",
    options=[
        Option("🛒 Place order",   next="order_flow",  description="Create a new order"),
        Option("📦 Track order",   next="track_flow",  description="Check delivery status"),
        Option("🆘 Support",       next="support_flow"),
        Option("❌ Cancel order",  next="cancel_flow"),
    ],
    button_label="Main Menu",     # label on the interactive list button
    header="MyCo Services",       # optional header
    footer="Reply 00 for home",   # optional footer
    allow_numeric=True,           # also accept "1", "2", "3"…
))
```
**`Option` fields:**

| Field | Type | Description |
|--------|------|-------------|
| `label` | `str` | Displayed text (keep under 24 chars for WA interactive rows) |
| `next` | `str` | Node key to navigate to when selected |
| `value` | `str` | ID sent back when selected. Defaults to `next` if not set. |
| `description` | `str` | Optional subtitle in list-style menus (max 72 chars) |

When the user selects an option, the engine navigates to the `next` node. No code required.

---

### 5.2 Input

A multi-step form. Walks through a list of fields one at a time, validating each response before moving on. After all fields are collected, advances to `next`.

```python
from turnstack.nodes import Input, Field, MenuField, ButtonsField

tree.add("support_ticket", Input(
    title="Support Ticket",    # shown as "Support Ticket — Step 1 of 3"
    fields=[
        Field("summary",    "Briefly describe your issue:"),
        MenuField("priority", "How urgent is this?", options=[
            Option("🔴 Critical", value="critical"),
            Option("🟡 Medium",   value="medium"),
            Option("🟢 Low",      value="low"),
        ]),
        Field("contact_email", "What email should we reach you at?"),
    ],
    next="ticket_confirm",
))
```
| Argument | Type | Description |
|----------|------|-------------|
| `fields` | `List[Field \| ...]` | Ordered list of field objects (any mix of types) |
| `next` | `str` | Node to go to after all fields are collected |
| `title` | `str` | Optional flow title shown on each step |
The user can send `back` at any point to re-answer the previous field, or `0` to step back field by field within the same Input node.

---

### 5.3 Confirm

Presents a summary and asks the user to confirm before you commit a side effect. Renders as WhatsApp interactive reply buttons (max 3 options).

```python
from turnstack.nodes import Confirm, Option

tree.add("ticket_confirm", Confirm(
    text=lambda collected: (
        f"Please confirm your ticket:\n\n"
        f"Issue: {collected['summary']}\n"
        f"Priority: {collected['priority']}\n"
        f"Email: {collected['contact_email']}"
    ),
    options=[
        Option("✅ Submit",   next="ticket_action"),
        Option("✏️ Edit",     next="support_ticket"),
        Option("❌ Cancel",   next="main_menu"),
    ],
))
```

`text` can be a plain string or a callable `(collected: dict) -> str`. The callable receives `session.collected` so you can summarise what the user entered.

---

### 5.4 Action

Runs your Python function, sends the return value as a text message, then navigates to `next`.

```python
from turnstack.nodes import Action

def save_ticket(session, collected):
    ticket_id = db.create_ticket(
        user_id  = session.user_id,
        summary  = collected["summary"],
        priority = collected["priority"],
        email    = collected["contact_email"],
    )
    return f"✅ Ticket #{ticket_id} created. We'll reply to {collected['contact_email']}."

tree.add("ticket_action", Action(
    fn=save_ticket,
    next="main_menu",
))
```

**`fn` signature:** `(session: Session, collected: dict) -> str`

The string you return becomes the message body. Return `None` or `""` to send no text (useful when you only want a side effect before navigating to the next node).

`fn` can also be an `async` coroutine:

```python
async def async_action(session, collected):
    result = await external_api.call(collected["query"])
    return f"Result: {result}"
```

---

### 5.5 Router

Silently branches to a different node based on session state — no user input, no visible message. Use it as the entry point or at any junction where you need conditional routing.

```python
from turnstack.nodes import Router, Route

tree = FlowTree(entry="entry_router")

tree.add("entry_router", Router(
    before=load_user_profile,       # optional hook run before route conditions
    routes=[
        Route(when=lambda s: not s.context.get("user"),               next="onboarding"),
        Route(when=lambda s: s.context["user"]["role"] == "admin",    next="admin_menu"),
    ],
    default="main_menu",            # fallback when no route matches
))

def load_user_profile(session):
    """before hook — populate session.context before route conditions run."""
    row = db.get_user(session.user_id)
    if row:
        session.context["user"] = dict(row)
```

`before` is called once before any `when` condition is evaluated. Use it to load data from your database into `session.context` so route conditions stay clean and declarative.

`Route.when` receives the full `session` object and must return `bool`. Routes are evaluated in order; the first `True` wins.

---

### 5.6 ListNode

Renders a dynamic list fetched at runtime with built-in pagination and optional interactive selection.

```python
from turnstack.nodes import ListNode, Option

tree.add("product_list", ListNode(
    fetch            = fetch_products,
    item_label       = lambda p: f"{p['name']} — Ksh {p['price']:,}",
    item_description = lambda p: p.get("category", ""),
    on_select        = "product_detail",
    title            = "🛒 Our Products",
    empty_text       = "No products available right now.",
    interactive      = True,
    button_label     = "Browse",
    page_size        = 8,
    extra_options    = [
        Option("🔙 Back to menu", next="main_menu"),
    ],
))

def fetch_products(session):
    """Simple fetch — returns a flat list."""
    return db.get_all_products()
```

**Paginated fetch** (when you have thousands of records):

```python
def fetch_products(session, page: int, page_size: int):
    """Paginated fetch — return (items_on_this_page, total_count)."""
    rows  = db.get_products(offset=page * page_size, limit=page_size)
    total = db.count_products()
    return rows, total
```

The engine detects which signature you use (3 params = paginated) and calls accordingly. Prev/Next navigation is added automatically.

When the user selects an item, the selected item is stored in `session.context["selected_item"]` and the engine navigates to `on_select`.

| Argument | Type | Default | Description |
|----------|------|---------|-------------|
| `fetch` | `Callable` | required | Simple or paginated fetch function |
| `item_label` | `Callable[[item], str]` | required | Display label for each item |
| `on_select` | `str` | required | Node to go to on selection |
| `title` | `str` | `"Select an option"` | Heading above the list |
| `empty_text` | `str` | `"No items available."` | Shown when fetch returns empty |
| `item_description` | `Callable[[item], str]` | `None` | Optional subtitle per item |
| `extra_options` | `List[Option]` | `[]` | Static options appended on last page |
| `interactive` | `bool` | `False` | Render as interactive list |
| `button_label` | `str` | `"Options"` | Interactive list button label |
| `page_size` | `int` | `8` | Items per page (1–10) |

---

### 5.7 MediaReply

Generates a file (PDF, Excel, image, etc.), uploads it to WhatsApp, and sends it to the user. The engine then automatically navigates to `next` and sends the following node's reply. Your adapter receives two `Reply` objects — the file and the follow-up — just loop and send both.

```python
from turnstack.nodes import MediaReply
import io, openpyxl

def build_report(session, collected) -> bytes:
    wb = openpyxl.Workbook()
    ws = wb.active
    ws.append(["Name", "Town", "Units"])
    for row in db.get_properties(session.user_id):
        ws.append([row["name"], row["town"], row["units"]])
    buf = io.BytesIO()
    wb.save(buf)
    return buf.getvalue()

tree.add("export_report", MediaReply(
    generate  = build_report,
    filename  = lambda s, c: f"report_{s.user_id}.xlsx",
    mime_type = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
    caption   = "📊 Here is your property report.",
    next      = "main_menu",
))
```

`generate` can be sync or `async`. `filename` and `caption` can be plain strings or callables `(session, collected) -> str`.

The send adapter handles media in two steps: upload to `/{PHONE_ID}/media`, then send using the returned `media_id`. The boilerplate below handles this for you.

---

### 5.8 CtaUrl

Sends a WhatsApp interactive CTA URL card — a tappable button that opens a URL in the user's browser. No message is sent back to your webhook when the user taps; the engine automatically advances to `next` after sending the card.

```python
from turnstack.nodes import CtaUrl

# Static CTA
tree.add("view_pricing", CtaUrl(
    body   = "Check out our latest pricing plans.",
    url    = "https://example.com/pricing",
    button = "View Pricing",
    header = "Our Plans",               # plain text header
    footer = "Opens in your browser",
    next   = "main_menu",
))
```

**Dynamic CTA** (body, url, and button can be callables `(session, collected) -> str`):

```python
tree.add("my_portal", CtaUrl(
    body   = lambda s, c: f"Hi {s.context['user']['first_name']}! Your portal is ready.",
    url    = lambda s, c: f"https://example.com/portal/{s.user_id}",
    button = lambda s, c: "Open My Portal",
    header = "Your Dashboard",
    footer = "Secure · Personalised",
    next   = "main_menu",
))
```

**Rich headers** (image, video, or document — pass a dict):

```python
CtaUrl(
    body   = "Step 1: Review our pricing.",
    url    = "https://example.com/pricing",
    button = "View Pricing",
    header = {"type": "image",    "url": "https://example.com/banner.jpg"},
    # or:  {"type": "video",    "url": "https://example.com/intro.mp4"}
    # or:  {"type": "document", "url": "https://example.com/guide.pdf", "filename": "guide.pdf"}
    next   = "next_step",
)
```

**Chaining multiple CTA cards** (set `next` to another `CtaUrl` node):

```python
tree.add("step_1", CtaUrl(body="Step 1 ...", url="...", button="Open", next="step_2"))
tree.add("step_2", CtaUrl(body="Step 2 ...", url="...", button="Open", next="done"))
tree.add("done",   Action(fn=lambda s, c: "All done!", next="main_menu"))
```
| Argument | Type | Description |
|----------|------|-------------|
| `body` | `str | Callable` | Message body text (required) |
| `url` | `str | Callable` | URL the button opens |
| `button` | `str | Callable` | Button label (max 20 chars) |
| `header` | `str | dict | None` | Plain string for text header; dict for image/video/document |
| `footer` | `str | None` | Optional footer text (max 60 chars) |
| `next` | `str` | Node to advance to after sending |
---

### 5.9 Carousel

Sends a WhatsApp carousel message — a horizontally scrollable set of cards, each with a header image, body text, and quick-reply buttons. When the user taps a card button, the engine advances to that button's `next` node and stores the tapped button's ID in `session.collected[store_as]`.

```python
from turnstack.nodes import Carousel, Card, CarouselButton

tree.add("product_showcase", Carousel(
    body  = "🛒 Browse our top picks — tap a card to select:",
    cards = [
        Card(
            header_type = "image",
            header_url  = "https://example.com/product-a.jpg",
            body        = "Product A — Ksh 1,200",
            buttons     = [CarouselButton(id="prod_a", title="🛍️ Select", next="product_detail")],
        ),
        Card(
            header_type = "image",
            header_url  = "https://example.com/product-b.jpg",
            body        = "Product B — Ksh 850",
            buttons     = [CarouselButton(id="prod_b", title="🛍️ Select", next="product_detail")],
        ),
        Card(
            header_type = "image",
            header_url  = "https://example.com/product-c.jpg",
            body        = "Product C — Ksh 2,000",
            buttons     = [CarouselButton(id="prod_c", title="🛍️ Select", next="product_detail")],
        ),
    ],
    store_as = "selected_product",   # key in session.collected
))

tree.add("product_detail", Action(
    fn   = lambda s, c: f"You selected: {c.get('selected_product')}. We'll process your order!",
    next = "main_menu",
))
```
| Argument | Type | Description |
|----------|------|-------------|
| `body` | `str` | Intro text above the carousel |
| `cards` | `List[Card]` | 2–10 card objects |
| `store_as` | `str` | Key in `session.collected` where the tapped button ID is stored |

**`Card` fields:**

| Field | Type | Description |
|--------|------|-------------|
| `header_type` | `"image"` | Currently only image headers are supported by WA carousels |
| `header_url` | `str` | Publicly accessible image URL |
| `body` | `str \| None` | Card body text (max 1024 chars) |
| `buttons` | `List[CarouselButton]` | 1–2 quick-reply buttons per card |

**`CarouselButton` fields:**

| Field | Type | Description |
|--------|------|-------------|
| `id` | `str` | Value stored in `session.collected[store_as]` when tapped |
| `title` | `str` | Button label (max 20 chars) |
| `next` | `str` | Node to navigate to when tapped |
---

### 5.10 ContactReply

Sends a WhatsApp contact card (or multiple contacts) to the user, optionally with a caption text sent first. Use this to share support numbers, agent details, or any contact information.

```python
from turnstack.nodes import ContactReply

tree.add("share_support_contact", ContactReply(
    contacts = [
        {
            "name":   {"formatted_name": "Acme Support", "first_name": "Acme"},
            "phones": [{"phone": "+254 700 000000", "type": "MOBILE", "wa_id": "254700000000"}],
        }
    ],
    caption = "📞 Here's our support number. Save it and reach out anytime!",
    next    = "main_menu",
))
```

The caption is sent as a separate text message before the contact card (WhatsApp contact messages do not support a body field natively). The `contacts` list must use the WhatsApp Cloud API contact shape. To share a contact a user sent you, the engine stores it in parsed form — the send adapter reconstructs the required WA shape automatically.

| Argument | Type | Description |
|----------|------|-------------|
| `contacts` | `List[dict]` | One or more contacts in WA API shape |
| `caption` | `str \| None` | Text sent before the card(s) |
| `next` | `str` | Node to navigate to after sending |
---

## 6. Field Types (inside Input)

### 6.1 Field / TextField

Plain text input. Accepts any text message from the user.

```python
Field("full_name", "What is your full name?")
TextField("full_name", "What is your full name?")  # identical alias
```

With validation and transformation:

```python
Field(
    "age",
    "How old are you?",
    validate  = lambda v: "Must be a number." if not v.isdigit() else None,
    transform = int,
)
```

---

### 6.2 MenuField

Interactive list selection inside a form. The user picks one option; the value is stored in `session.collected`.

```python
from turnstack.nodes import MenuField, Option

MenuField(
    "town",
    "Which town are you in?",
    options = [
        Option("Nairobi",  value="nairobi"),
        Option("Mombasa",  value="mombasa"),
        Option("Kisumu",   value="kisumu"),
    ],
    button_label    = "Select Town",
    rejection_text  = "Please select a town from the list.",
)
```

`options` can be a static list or a callable `(session) -> List[Option]` for dynamic menus.

---

### 6.3 ButtonsField

Interactive reply buttons inside a form. Max 3 options (WhatsApp limit). The user taps a button; the value is stored in `session.collected`.

```python
from turnstack.nodes import ButtonsField, Option

ButtonsField(
    "tier",
    "Which plan are you on?",
    options = [
        Option("Standard", value="standard"),
        Option("Premium",  value="premium"),
    ],
)
```

---

### 6.4 ImageField

Prompts the user to send an image. The collected value is a dict:

```python
{
    "media_id":  "...",    # WhatsApp media ID (use to download via Media API)
    "mime_type": "image/jpeg",
}
```

```python
from turnstack.nodes import ImageField

ImageField(
    "photo",
    "Please send a photo of your property 📷",
    rejection_text = "Please send an image (JPG or PNG).",
)
```

---

### 6.5 DocumentField

Prompts the user to send a document. The collected value is a dict:

```python
{
    "media_id":  "...",
    "mime_type": "application/pdf",
    "filename":  "document.pdf",
}
```

```python
from turnstack.nodes import DocumentField

DocumentField(
    "id_doc",
    "Please send a copy of your ID (PDF or image).",
    rejection_text = "Please send a document or image file.",
)
```

---

### 6.6 LocationField

Sends a WhatsApp location request (a native UI button prompting the user to share their location). The collected value is a dict:

```python
{
    "latitude":  -1.286389,
    "longitude": 36.817223,
    "name":      "Nairobi CBD",    # may be None
    "address":   "Kenyatta Ave",   # may be None
}
```

```python
from turnstack.nodes import LocationField

LocationField(
    "pickup_location",
    "Please share your pickup location 📍",
    rejection_text = "⚠️ Please use the 📍 button to share your location.",
)
```

---

### 6.7 BranchField

Conditionally injects a group of fields into the form based on earlier answers. The step counter updates dynamically — the user only sees steps relevant to their path.

```python
Input(
    title="Loan Application",
    fields=[
        ButtonsField("employment_type", "Are you employed or self-employed?", options=[
            Option("Employed",      value="employed"),
            Option("Self-employed", value="self_employed"),
        ]),

        # Only shown for employed applicants
        BranchField(
            when=lambda s: s.collected.get("employment_type") == "employed",
            fields=[
                Field("employer_name",  "Who is your employer?"),
                Field("monthly_salary", "What is your monthly salary (KES)?",
                      validate=lambda v: None if v.isdigit() else "Enter a number."),
            ],
        ),

        # Only shown for self-employed applicants
        BranchField(
            when=lambda s: s.collected.get("employment_type") == "self_employed",
            fields=[
                Field("business_name",    "What is your business name?"),
                Field("monthly_revenue",  "What is your average monthly revenue (KES)?"),
            ],
        ),

        Field("loan_amount", "How much would you like to borrow (KES)?"),
    ],
    next="loan_confirm",
)
```

`BranchField` is not itself a field — it has no `name`. It's a conditional wrapper that flattens transparently at runtime. Branches can be nested.

A field's `skip_if` argument is an alternative for single-field conditional skipping:

```python
Field(
    "company_name",
    "What is your company name?",
    skip_if=lambda s: s.collected.get("employment_type") == "self_employed",
)
```

---

## 7. Node → WhatsApp Widget Mapping

Use this table to understand exactly what WhatsApp message type each TurnStack node and `reply.node_type` value maps to, so you can wire your send adapter correctly.

| TurnStack Node / `reply.node_type` | WhatsApp Message Type | Notes |
|------------------------------------|-----------------------|-------|
| `Menu` / `"menu"` | Interactive **list** message | Rows built from `reply.options`; button label from `reply.meta["button_label"]` |
| `ListNode` / `"menu"` with `reply.meta["sections"]` | Interactive **list** with sections | Pre-built sections in `reply.meta["sections"]`; use these directly |
| `Confirm` / `"confirm"` | Interactive **reply buttons** | Max 3 buttons; built from `reply.options` |
| `Input` + `TextField` / `"input"` | Plain **text** message | Simple question prompt |
| `Input` + `MenuField` / `"input_menu"` | Interactive **list** message | Same as Menu; built from `reply.options` |
| `Input` + `ButtonsField` / `"input_buttons"` | Interactive **reply buttons** | Max 3 buttons; built from `reply.options` |
| `Input` + `ImageField` / `"input_image"` | Plain **text** message | WA has no native image-request widget; send a plain prompt |
| `Input` + `DocumentField` / `"input_document"` | Plain **text** message | WA has no native document-request widget; send a plain prompt |
| `Input` + `LocationField` / `"input_location"` | Interactive **location request** | `type: "location_request_message"` with `action.name = "send_location"` |
| `MediaReply` / `reply.type == "media"` | **Document** or **Image** send | Upload file first via Media API; send using returned `media_id` |
| `CtaUrl` / `"cta_url"` | Interactive **CTA URL** button | `interactive.type = "cta_url"`; header can be text, image, video, or document |
| `Carousel` / `"carousel"` | Interactive **carousel** | Cards with image headers and quick-reply buttons |
| `ContactReply` / `reply.type == "contact"` | **Contacts** message | Caption sent first as plain text, then `type: "contacts"` |
| `Action` / `"text"` | Plain **text** message | Return value of `fn` |
| `Router` | *(no message)* | Silent branching — no WA message sent |
| Error / `reply.type == "error"` | Plain **text** message | Log the error; optionally send `reply.body` to the user |
| Session end / `reply.type == "end"` | Plain **text** message | Goodbye message before session closes |

---

## 8. The Engine

### 8.1 Instantiation

```python
from turnstack import BotEngine, FlowTree
from turnstack.stores.memory import InMemorySessionStore

engine = BotEngine(
    tree             = tree,
    session_store    = InMemorySessionStore(),  # default
    session_timeout  = 3600,                     # seconds of inactivity before expiry
    back_keywords    = {"0", "back", "go back"},
    home_keywords    = {"00", "home", "menu", "start over"},
    exit_keywords    = {"000", "exit", "quit", "reset", "goodbye", "bye"},
    unsupported_text = "⚠️ Sorry, I can't process that message. Please try again.",
)
```

All parameters except `tree` are optional. The engine validates the tree on startup and raises immediately if any node reference is broken.

---

### 8.2 process()

```python
replies: List[Reply] = await engine.process(incoming)
```

The single public method you call for every inbound message. Always returns a `List[Reply]`.

In the common case the list contains one item. When a `MediaReply` or `ContactReply` node fires, the list may contain two items — the file/contact reply and the follow-up node — sent in order. Just loop:

```python
for reply in replies:
    await send_whatsapp(user_id, phone, reply)
```

The engine handles everything internally: session load/create/expire, global command interception, node dispatch, state transition, and session save. You never touch the session store or call internal engine methods directly.

---

### 8.3 IncomingMessage

Build this from the raw WhatsApp webhook payload and pass it to `process()`.

```python
from turnstack import IncomingMessage

# Text message
IncomingMessage(user_id="2547XXXXXXXX", type="text", text="Hello", raw=raw_payload)

# Interactive selection (button or list reply)
IncomingMessage(user_id="2547XXXXXXXX", type="interactive", interactive_id="option_value")

# Image
IncomingMessage(user_id="2547XXXXXXXX", type="image",
                media_id=msg["image"]["id"], media_mime=msg["image"].get("mime_type"))

# Document
IncomingMessage(user_id="2547XXXXXXXX", type="document",
                media_id=msg["document"]["id"],
                media_mime=msg["document"].get("mime_type"),
                media_name=msg["document"].get("filename"))

# Location
IncomingMessage(user_id="2547XXXXXXXX", type="location",
                location={"latitude": loc["latitude"], "longitude": loc["longitude"],
                          "name": loc.get("name"), "address": loc.get("address")})

# Contacts (shared by user)
IncomingMessage(user_id="2547XXXXXXXX", type="contacts", contacts=parsed_contacts)

# Unsupported type (sticker, audio, reaction…) — engine replies politely, holds state
IncomingMessage(user_id="2547XXXXXXXX", type="sticker")
```

| Field | Type | Description |
|--------|------|-------------|
| `user_id` | `str` | Unique user identifier (WA phone number or user ID) |
| `type` | `str` | `"text"`, `"interactive"`, `"image"`, `"document"`, `"location"`, `"contacts"`, or any other |
| `text` | `str \| None` | Text body (`type="text"`) |
| `interactive_id` | `str \| None` | Selected option ID (`type="interactive"`) |
| `media_id` | `str \| None` | WhatsApp media ID (`type="image"` or `type="document"`) |
| `media_mime` | `str \| None` | MIME type of the media |
| `media_name` | `str \| None` | Original filename (documents) |
| `location` | `dict \| None` | Location dict with latitude/longitude/name/address |
| `contacts` | `list \| None` | List of parsed contact dicts (`type="contacts"`) |
| `raw` | `Any` | Original raw payload — stored for your reference; engine ignores it |

---

### 8.4 Reply

The object returned by `process()`. Read its fields to decide how to send the message.

```python
@dataclass
class Reply:
    type:              Literal["text", "media", "contact", "end", "error"]
    body:              str                 # message text / caption for media
    phone:             str                 # recipient (same as user_id by default)

    # media
    file_bytes:        Optional[bytes]
    filename:          Optional[str]
    mime_type:         Optional[str]

    # interactive hints
    options:           List[ReplyOption]   # populated for menu / confirm / buttons nodes
    node_type:         Optional[str]       # see table in §7 for all values
    suggested_replies: List[str]           # option labels for quick-reply chips

    # meta — extra hints specific to the node type
    meta:              Dict[str, Any]

    # navigation
    current_node:      Optional[str]
    session_state:     Optional[str]       # "new" | "active" | "expired"
```

**`ReplyOption`:**

```python
@dataclass
class ReplyOption:
    label:       str    # display text
    value:       str    # the id to send back when selected
    description: str    # optional subtitle (list menus)
```

**Notable `meta` keys by node type:**

| `node_type` | `meta` keys available |
|-------------|-----------------------|
| `"menu"` / `"input_menu"` | `button_label`, `sections` (ListNode pre-built sections) |
| `"cta_url"` | `url`, `button_label`, `header`, `footer` |
| `"carousel"` | `cards` (list of card dicts ready for WA API) |
| `"contact"` | `contacts` (list of WA-shaped contact dicts) |

---

## 9. Session & State

### 9.1 Session object

The engine manages this for you. You interact with it inside `fn`, `when`, `before`, `fetch`, `validate`, `transform`, and dynamic text callables.

```python
session.user_id          # str  — the user's identifier
session.current_node     # str  — which node the user is currently on
session.collected        # dict — all form values collected so far
session.context          # dict — your arbitrary data (not cleared between nodes)
session.nav_stack        # list — navigation history (for back/go home)
session.lifecycle_state  # "new" | "active" | "expired"
```

---

### 9.2 session.collected

Form data collected by `Input` nodes. Keys are the `name` values of your fields.

```python
def confirm_order(session, collected):
    return (
        f"Order summary:\n"
        f"Item:     {collected['item_name']}\n"
        f"Quantity: {collected['quantity']}\n"
        f"Address:  {collected['delivery_address']['address']}"
    )
```

`collected` is cleared when an `Input` node is entered fresh (not on back-navigation within it). Data from previous Input nodes persists until explicitly cleared or the session expires.

---

### 9.3 session.context

A free-form dict for your own data. The engine does not read or write it except:
- `ListNode` writes `context["selected_item"]` on item selection.
- `Carousel` writes `collected[store_as]` (not context) when a card button is tapped.

Persists for the lifetime of the session.

```python
# In a Router before hook
def load_user(session):
    session.context["user"] = db.get_user(session.user_id)

# In a Menu text callable
Menu(text=lambda s: f"Hello {s.context['user']['first_name']}! What can I do?", ...)

# In an Action
def process_order(session, collected):
    user = session.context["user"]
    ...
```

---

### 9.4 session.pagination

Stores page indices for menu and list pagination. Managed entirely by the engine — do not write to this directly. Readable for debugging.

---

## 10. Session Stores

### 10.1 InMemorySessionStore

The default. Fast, zero-config, but sessions are lost on server restart. Good for development.

```python
from turnstack.stores.memory import InMemorySessionStore

engine = BotEngine(tree=tree, session_store=InMemorySessionStore(session_timeout=3600))
```

---

### 10.2 Custom Stores

Implement the `SessionStore` interface to persist sessions to Redis, a database, or anywhere:

```python
from turnstack.session import SessionStore, Session
import json

class RedisSessionStore(SessionStore):

    def __init__(self, redis_client, timeout: int = 3600):
        self.redis   = redis_client
        self.timeout = timeout

    async def get(self, user_id: str) -> Session | None:
        data = await self.redis.get(f"session:{user_id}")
        if not data:
            return None
        return Session.from_dict(json.loads(data))

    async def save(self, session: Session) -> None:
        await self.redis.setex(
            f"session:{session.user_id}",
            self.timeout,
            json.dumps(session.to_dict()),
        )

    async def delete(self, user_id: str) -> None:
        await self.redis.delete(f"session:{user_id}")

engine = BotEngine(tree=tree, session_store=RedisSessionStore(redis, timeout=3600))
```

---

## 11. Navigation — Built-in Commands

The engine intercepts these plain-text messages before dispatching to any node handler. They work anywhere in the flow without any node configuration.

| Keyword(s) | Action |
|------------|--------|
| `0`, `back`, `go back` | Step back — previous field inside an Input, or previous node |
| `00`, `home`, `menu`, `start over` | Jump to the entry node, clearing the navigation stack |
| `000`, `exit`, `quit`, `reset`, `goodbye`, `bye` | End the session; next message starts a fresh one |

All keyword sets are configurable on `BotEngine`:

```python
engine = BotEngine(
    tree          = tree,
    back_keywords = {"b", "back"},
    home_keywords = {"h", "home"},
    exit_keywords = {"x", "exit"},
)
```

**Back within an Input node** is field-aware: pressing back steps to the previous field (clearing its collected value) rather than leaving the Input node entirely. Once at field 0, pressing back leaves the node and goes to the previous node in the stack.

---

## 12. Sending Replies — Adapter Pattern

TurnStack is send-agnostic. You read `reply.node_type` (and `reply.type` for media/contact/error) to decide how to format the outgoing WA message, then send it however you like.

The pattern is always:

```python
replies = await engine.process(incoming)
for reply in replies:
    await send_whatsapp(user_id, phone, reply)
```

### 12.1 The boilerplate send helper

Copy this into your app and adjust as needed. It covers every node type TurnStack can produce, using the WhatsApp Cloud API directly via `httpx`.

```python
import os
import httpx

WA_TOKEN    = os.getenv("WA_TOKEN", "")
WA_PHONE_ID = os.getenv("WA_PHONE_ID", "")

async def send_whatsapp(user_id: str, phone: str, reply) -> None:
    headers = {"Authorization": f"Bearer {WA_TOKEN}", "Content-Type": "application/json"}
    url     = f"https://graph.facebook.com/v25.0/{WA_PHONE_ID}/messages"

    # ── error / end ───────────────────────────────────────────────────
    if reply.type in ("error", "end"):
        body = reply.body or ("⚠️ Something went wrong." if reply.type == "error" else "👋 Goodbye!")
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "text", "text": {"body": body},
        }
        async with httpx.AsyncClient() as client:
            await client.post(url, json=payload, headers=headers)
        return

    # ── media file (MediaReply) ───────────────────────────────────────
    if reply.type == "media" and reply.file_bytes:
        upload_url = f"https://graph.facebook.com/v25.0/{WA_PHONE_ID}/media"
        async with httpx.AsyncClient() as client:
            upload_resp = await client.post(
                upload_url,
                headers={"Authorization": f"Bearer {WA_TOKEN}"},
                files={"file": (reply.filename, reply.file_bytes, reply.mime_type)},
                data={"messaging_product": "whatsapp"},
            )
        if upload_resp.status_code != 200:
            print(f"❌ Media upload failed: {upload_resp.text}")
            return
        media_id = upload_resp.json().get("id")
        mime = (reply.mime_type or "").lower()
        if mime.startswith("image/"):
            wa_type = "image"
            media_body: dict = {"id": media_id}
            if reply.body:
                media_body["caption"] = reply.body[:1024]
        else:
            wa_type = "document"
            media_body = {"id": media_id, "filename": reply.filename}
            if reply.body:
                media_body["caption"] = reply.body[:1024]
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": wa_type, wa_type: media_body,
        }
        async with httpx.AsyncClient() as client:
            await client.post(url, json=payload, headers=headers)
        return

    # ── CTA URL button (CtaUrl) ───────────────────────────────────────
    if reply.node_type == "cta_url":
        meta         = reply.meta or {}
        button_label = meta.get("button_label", "Open")[:20]
        link_url     = meta.get("url", "")
        footer_text  = meta.get("footer", "")
        interactive: dict = {
            "type": "cta_url",
            "body": {"text": reply.body[:1024] or " "},
            "action": {
                "name": "cta_url",
                "parameters": {"display_text": button_label, "url": link_url},
            },
        }
        header = meta.get("header", "")
        if isinstance(header, dict):
            h_type = header.get("type", "image")
            if h_type == "text" and header.get("text"):
                interactive["header"] = {"type": "text", "text": str(header["text"])[:60]}
            elif h_type in ("image", "video"):
                interactive["header"] = {"type": h_type, h_type: {"link": header.get("url", "")}}
            elif h_type == "document":
                doc: dict = {"link": header.get("url", "")}
                if header.get("filename"):
                    doc["filename"] = header["filename"]
                interactive["header"] = {"type": "document", "document": doc}
        elif isinstance(header, str) and header:
            interactive["header"] = {"type": "text", "text": header[:60]}
        if footer_text:
            interactive["footer"] = {"text": footer_text[:60]}
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "interactive", "interactive": interactive,
        }

    # ── carousel (Carousel) ───────────────────────────────────────────
    elif reply.node_type == "carousel":
        cards_data = (reply.meta or {}).get("cards", [])
        wa_cards = []
        for idx, card in enumerate(cards_data):
            card_obj: dict = {
                "card_index": idx,
                "type": "cta_url",
                "header": {
                    "type": card["header_type"],
                    card["header_type"]: {"link": card["header_url"]},
                },
                "action": {
                    "buttons": [
                        {"type": "quick_reply", "quick_reply": {"id": btn["id"], "title": btn["title"][:20]}}
                        for btn in card["buttons"]
                    ]
                },
            }
            if card.get("body"):
                card_obj["body"] = {"text": card["body"][:1024]}
            wa_cards.append(card_obj)
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "interactive",
            "interactive": {
                "type": "carousel",
                "body": {"text": reply.body[:1024]},
                "action": {"cards": wa_cards},
            },
        }

    # ── interactive list with named sections (ListNode / MenuField) ───
    elif reply.node_type in ("menu", "input_menu") and reply.meta and "sections" in reply.meta:
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "interactive",
            "interactive": {
                "type": "list",
                "body": {"text": reply.body[:1024] or "Select an option"},
                "action": {
                    "button": reply.meta.get("button_label", "Options"),
                    "sections": reply.meta["sections"],
                },
            },
        }

    # ── interactive list — flat (Menu / MenuField without sections) ───
    elif reply.node_type in ("menu", "input_menu") and reply.options and len(reply.options) >= 2:
        rows = [
            {"id": opt.value[:200], "title": opt.label[:24], "description": (opt.description or "")[:72]}
            for opt in reply.options
        ]
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "interactive",
            "interactive": {
                "type": "list",
                "body": {"text": reply.body[:1024] or "Choose an option"},
                "action": {
                    "button": (reply.meta.get("button_label", "Options") if reply.meta else "Options"),
                    "sections": [{"title": " ", "rows": rows}],
                },
            },
        }

    # ── interactive reply buttons (Confirm / ButtonsField) ───────────
    elif reply.node_type in ("confirm", "input_buttons") and reply.options:
        buttons = [
            {"type": "reply", "reply": {"id": opt.value[:256], "title": opt.label[:20]}}
            for opt in reply.options[:3]
        ]
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "interactive",
            "interactive": {
                "type": "button",
                "body": {"text": reply.body[:1024] or "Choose:"},
                "action": {"buttons": buttons},
            },
        }

    # ── location request (LocationField) ─────────────────────────────
    elif reply.node_type == "input_location":
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "interactive",
            "interactive": {
                "type": "location_request_message",
                "body": {"text": reply.body[:1024] or "Please share your location."},
                "action": {"name": "send_location"},
            },
        }

    # ── contact card (ContactReply) ───────────────────────────────────
    elif reply.type == "contact":
        wa_contacts = (reply.meta or {}).get("contacts", [])

        def _to_wa_contact(c: dict) -> dict:
            if "name" in c:
                return c  # already in WA API shape
            wa: dict = {"name": {"formatted_name": c.get("formatted_name", "Contact")}}
            for k in ("first_name", "last_name", "middle_name"):
                if c.get(k):
                    wa["name"][k] = c[k]
            if c.get("phones"):
                wa["phones"] = c["phones"]
            for extra in ("emails", "org", "addresses", "urls"):
                if c.get(extra):
                    wa[extra] = c[extra]
            return wa

        # Send caption text first if present
        if reply.body:
            caption_payload = {
                "messaging_product": "whatsapp", "recipient_type": "individual",
                "to": phone, "type": "text", "text": {"body": reply.body},
            }
            async with httpx.AsyncClient() as client:
                await client.post(url, json=caption_payload, headers=headers)

        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "contacts",
            "contacts": [_to_wa_contact(c) for c in wa_contacts],
        }

    # ── plain text (Action / TextField / error fallback) ─────────────
    else:
        payload = {
            "messaging_product": "whatsapp", "recipient_type": "individual",
            "to": phone, "type": "text", "text": {"body": reply.body},
        }

    async with httpx.AsyncClient() as client:
        resp   = await client.post(url, json=payload, headers=headers)
        status = "✅" if resp.status_code == 200 else "❌"
        print(f"{status} WA → {user_id} [{reply.node_type}] {resp.status_code}")
```

You own this function. Customise payloads, swap `httpx` for another HTTP client, add retry logic, or route to a different send library — TurnStack has no opinion about what happens after `Reply` is returned.

---

### 12.2 Sending via pywa / any library

If you use [pywa](https://github.com/david-lev/pywa) or another WhatsApp SDK, adapt the same `reply.node_type` switch to your library's API:

```python
from pywa import WhatsApp
from pywa.types import Button, SectionList, Section, SectionRow

wa = WhatsApp(phone_id=WA_PHONE_ID, token=WA_TOKEN)

async def send(reply):
    if reply.node_type in ("menu", "input_menu"):
        rows = [SectionRow(id=o.value, title=o.label) for o in reply.options]
        await wa.send_message(
            to=reply.phone,
            text=reply.body,
            buttons=SectionList(
                button_title=reply.meta.get("button_label", "Options"),
                sections=[Section(title="Options", rows=rows)],
            ),
        )
    elif reply.node_type in ("confirm", "input_buttons"):
        btns = [Button(id=o.value, title=o.label) for o in reply.options]
        await wa.send_message(to=reply.phone, text=reply.body, buttons=btns)
    else:
        await wa.send_message(to=reply.phone, text=reply.body)
```

The engine's output is always the same structured `Reply` — the send layer is fully swappable.

---

## 13. Wiring to a Webhook

Copy this boilerplate into your app. It handles all message types WhatsApp can send — text, interactive, image, document, location, contacts, and quick-reply buttons — and feeds each into the engine.

```python
import os
import traceback
import httpx
from fastapi import FastAPI, Request, Response, HTTPException
from dotenv import load_dotenv
from turnstack import BotEngine, IncomingMessage

load_dotenv()
WA_VERIFY_TOKEN = os.getenv("WA_VERIFY_TOKEN", "")
WA_TOKEN        = os.getenv("WA_TOKEN", "")
WA_PHONE_ID     = os.getenv("WA_PHONE_ID", "")

app = FastAPI()

# ── webhook verification ──────────────────────────────────────────────────────

@app.get("/api/v1/webhooks/whatsapp")
async def verify(request: Request):
    p = request.query_params
    if p.get("hub.mode") == "subscribe" and p.get("hub.verify_token") == WA_VERIFY_TOKEN:
        return Response(content=p.get("hub.challenge"), media_type="text/plain")
    raise HTTPException(403)


# ── webhook handler ───────────────────────────────────────────────────────────

@app.post("/api/v1/webhooks/whatsapp")
async def webhook(request: Request):
    raw = await request.json()

    # ── Step 1: parse the WA envelope ─────────────────────────────────
    try:
        value = raw["entry"][0]["changes"][0]["value"]
        if "messages" not in value:
            return {"status": "no_messages"}
        msg      = value["messages"][0]
        phone    = msg.get("from", "")
        user_id  = msg.get("from_user_id", phone)
        msg_type = msg.get("type", "")
    except Exception:
        traceback.print_exc()
        return {"status": "parse_error"}

    # ── Step 2: build IncomingMessage ──────────────────────────────────
    try:
        if msg_type == "text":
            incoming = IncomingMessage(
                user_id=user_id, type="text",
                text=msg["text"]["body"], raw=raw,
            )
        elif msg_type == "interactive":
            itype = msg["interactive"]["type"]
            iid   = (msg["interactive"]["button_reply"]["id"]
                     if itype == "button_reply"
                     else msg["interactive"]["list_reply"]["id"])
            incoming = IncomingMessage(
                user_id=user_id, type="interactive", interactive_id=iid, raw=raw,
            )
        elif msg_type == "button":
            # Quick-reply button payload (sent back from Carousel / template buttons)
            incoming = IncomingMessage(
                user_id=user_id, type="interactive",
                interactive_id=msg["button"]["payload"], raw=raw,
            )
        elif msg_type == "image":
            incoming = IncomingMessage(
                user_id=user_id, type="image",
                media_id=msg["image"]["id"],
                media_mime=msg["image"].get("mime_type"), raw=raw,
            )
        elif msg_type == "document":
            incoming = IncomingMessage(
                user_id=user_id, type="document",
                media_id=msg["document"]["id"],
                media_mime=msg["document"].get("mime_type"),
                media_name=msg["document"].get("filename"), raw=raw,
            )
        elif msg_type == "location":
            loc = msg["location"]
            incoming = IncomingMessage(
                user_id=user_id, type="location",
                location={
                    "latitude":  loc.get("latitude"),
                    "longitude": loc.get("longitude"),
                    "name":      loc.get("name"),
                    "address":   loc.get("address"),
                }, raw=raw,
            )
        elif msg_type == "contacts":
            raw_contacts = msg.get("contacts", [])
            parsed = []
            for c in raw_contacts:
                name_block = c.get("name", {})
                parsed.append({
                    "formatted_name": name_block.get("formatted_name", ""),
                    "first_name":     name_block.get("first_name", ""),
                    "last_name":      name_block.get("last_name", ""),
                    "phones":         c.get("phones", []),
                    "_raw":           c,
                })
            incoming = IncomingMessage(
                user_id=user_id, type="contacts", contacts=parsed, raw=raw,
            )
        else:
            # Sticker, audio, reaction, etc. — engine replies politely, holds state
            incoming = IncomingMessage(user_id=user_id, type=msg_type, raw=raw)
    except Exception:
        traceback.print_exc()
        return {"status": "message_parse_error"}

    # ── Step 3: process + send ─────────────────────────────────────────
    try:
        replies = await engine.process(incoming)
    except Exception:
        traceback.print_exc()
        # Engine crashed — send a plain fallback so the user isn't stuck
        try:
            async with httpx.AsyncClient() as client:
                await client.post(
                    f"https://graph.facebook.com/v25.0/{WA_PHONE_ID}/messages",
                    headers={"Authorization": f"Bearer {WA_TOKEN}"},
                    json={
                        "messaging_product": "whatsapp", "recipient_type": "individual",
                        "to": phone, "type": "text",
                        "text": {"body": "⚠️ Something went wrong. Please try again."},
                    },
                )
        except Exception:
            traceback.print_exc()
        return {"status": "engine_error"}

    for reply in replies:
        try:
            await send_whatsapp(user_id, phone, reply)
        except Exception:
            traceback.print_exc()
            # Log the failure but continue — one bad reply shouldn't block the rest

    return {"status": "ok"}
```

> **Note on Graph API version.** The boilerplate uses `v25.0`. Meta recommends using the latest stable version your app has been tested against. Update the version string as needed.

---

## 14. Validation & Transformation

Every field type (`Field`, `MenuField`, `ButtonsField`, `ImageField`, `DocumentField`, `LocationField`) supports two optional hooks:

**`validate(value) -> str | None`** — return an error message to reject the input; return `None` to accept.

```python
import re

def validate_email(v: str):
    if not re.match(r"^[^@]+@[^@]+\.[^@]+$", v):
        return "⚠️ That doesn't look like a valid email address."
    return None

def validate_positive_integer(v: str):
    if not v.isdigit() or int(v) <= 0:
        return "⚠️ Please enter a positive whole number."
    return None

Field("email",    "Your email address?",   validate=validate_email)
Field("quantity", "How many units?",       validate=validate_positive_integer)
```

When validation fails the engine re-asks the same question with the error message prepended. No state change occurs.

**`transform(value) -> Any`** — applied after validation passes, before storing in `session.collected`.

```python
Field("units",         "How many units?",
      validate=lambda v: None if v.isdigit() else "Enter a number.",
      transform=int)   # stored as int, not string

Field("full_name",     "Your full name?",
      transform=str.strip)

Field("date_of_birth", "Date of birth (YYYY-MM-DD)?",
      validate=lambda v: None if re.match(r"\d{4}-\d{2}-\d{2}", v) else "Format: YYYY-MM-DD",
      transform=lambda v: datetime.strptime(v, "%Y-%m-%d").date())
```

---

## 15. Dynamic Content

Most text-bearing arguments accept a callable so you can personalise the UI at runtime.

**Menu text** — callable receives `(session)`:

```python
Menu(
    text=lambda s: f"Hi {s.context.get('user', {}).get('name', 'there')}! What can I do for you?",
    options=[...],
)
```

**Confirm text** — callable receives `(collected)`:

```python
Confirm(
    text=lambda c: f"Confirm order for {c['item_name']} × {c['quantity']}?",
    options=[...],
)
```

**Dynamic options from a database:**

```python
MenuField(
    "branch",
    "Select your nearest branch:",
    options=lambda session: [
        Option(b["name"], value=str(b["id"]), description=b["address"])
        for b in db.get_branches(session.context.get("city"))
    ],
)
```

**Dynamic CtaUrl body, url, and button:**

```python
CtaUrl(
    body   = lambda s, c: f"Hi {s.context['user']['first_name']}! Your portal is ready.",
    url    = lambda s, c: f"https://example.com/portal/{s.user_id}",
    button = lambda s, c: "Open My Portal",
    next   = "main_menu",
)
```

**Dynamic filename and caption on MediaReply:**

```python
MediaReply(
    generate  = build_statement,
    filename  = lambda s, c: f"statement_{s.context['user']['account_no']}.pdf",
    caption   = lambda s, c: f"📄 Statement for {c['period']}",
    mime_type = "application/pdf",
    next      = "main_menu",
)
```

---

## 16. Conditional Fields — BranchField

See [Section 6.7](#67-branchfield) for the full reference. Quick pattern:

```python
Input(
    fields=[
        ButtonsField("type", "What are you reporting?", options=[
            Option("Bug",     value="bug"),
            Option("Feature", value="feature"),
        ]),
        BranchField(
            when=lambda s: s.collected.get("type") == "bug",
            fields=[
                Field("steps_to_reproduce", "How do you reproduce it?"),
                Field("expected_behaviour", "What did you expect to happen?"),
            ],
        ),
        BranchField(
            when=lambda s: s.collected.get("type") == "feature",
            fields=[
                Field("feature_description", "Describe the feature you'd like:"),
                Field("business_value",       "Why would this be valuable?"),
            ],
        ),
        Field("contact_email", "Your email for follow-up?"),
    ],
    next="submit_ticket",
)
```

The step counter shown to the user (`Step N of M`) reflects only the active fields for their path.

---

## 17. Pagination — Automatic Behaviour

**Menu pagination** kicks in automatically when a `Menu` or `MenuField` has more options than WhatsApp can show in a single interactive list. The engine:

1. Splits options into pages (max 8 real options per page, with Prev/Next controls)
2. Tracks the current page in `session.pagination`
3. Sends the correct page on each interaction

You do nothing — just define as many options as you need.

**ListNode pagination** works the same way. For large datasets use the paginated fetch signature `(session, page, page_size) -> (items, total)` to avoid loading all records into memory.

**Page size** on `ListNode` is configurable (1–10, default 8):

```python
ListNode(fetch=..., ..., page_size=5)
```

---

## 18. Custom Node Handlers

If you need a node type that doesn't exist in TurnStack, register a custom handler:

```python
from turnstack.handlers.base import NodeHandler
from turnstack.reply import Reply
from turnstack.session import Session
from turnstack.message import IncomingMessage
from turnstack.tree import FlowTree

class PaymentPromptHandler(NodeHandler):
    async def handle(
        self,
        node:    dict,
        session: Session,
        message: IncomingMessage,
        tree:    FlowTree,
    ) -> Reply:
        ref = payment_gateway.create_link(session.user_id, node["amount"])
        session.context["payment_ref"] = ref
        self._transition_to(session, node.get("next", "main_menu"))
        return Reply(
            type         = "text",
            body         = f"Please complete payment here: {ref['url']}",
            phone        = session.user_id,
            node_type    = "text",
            current_node = session.current_node,
        )

# Register with the engine
engine.register_handler("payment_prompt", PaymentPromptHandler())

# Use in the tree
tree.add("pay_now", {"type": "payment_prompt", "amount": 500, "next": "payment_confirm"})
```

---

## 19. Error Handling

The engine never raises exceptions to the caller. All internal errors produce a `Reply(type="error", ...)` with a descriptive `body`.

```python
for reply in replies:
    if reply.type == "error":
        logger.error(f"Engine error | node={reply.current_node} | {reply.body}")
        await send_plain_text(phone, "⚠️ Something went wrong. Please try again.")
        continue
    await send_whatsapp(user_id, phone, reply)
```

**Common error causes:**

- A `next` key references a node that doesn't exist (caught at startup by `tree.validate()`)
- A `generate` function in `MediaReply` raises an exception
- A `fetch` function in `ListNode` raises
- No handler registered for a node type (only with custom types)

**Exceptions in `Action.fn`** are caught and surfaced as error replies. Catch expected exceptions yourself for user-friendly messages:

```python
def save_order(session, collected):
    try:
        order_id = db.create_order(session.user_id, collected)
        return f"✅ Order #{order_id} placed!"
    except db.OutOfStockError:
        return "⚠️ Sorry, that item is out of stock. Please choose another."
    except Exception:
        logger.exception("Unexpected error saving order")
        return "⚠️ Something went wrong. Please try again later."
```

---

## 20. Debug Utilities

**Inspect all active sessions:**

```python
for user_id, session in engine.session_store.all().items():
    print(user_id, session.current_node, session.collected)
```

**Reset a single session:**

```python
await engine.session_store.delete("2547XXXXXXXX")
```

**Add debug endpoints to your API:**

```python
@app.get("/debug/sessions")
async def debug_sessions():
    return {
        uid: {
            "node":      s.current_node,
            "state":     s.lifecycle_state,
            "collected": s.collected,
            "context":   s.context,
        }
        for uid, s in engine.session_store.all().items()
    }

@app.delete("/debug/sessions/{user_id}")
async def reset_session(user_id: str):
    await engine.session_store.delete(user_id)
    return {"reset": user_id}

@app.get("/debug/db")
async def debug_db():
    """Show all database records (add your own queries)."""
    ...
```

**Log reply metadata** in your send function:

```python
print(f"[{reply.session_state}] node={reply.current_node} type={reply.node_type} → {reply.body[:60]}")
```

---

## 21. Complete Example — Customer Support Bot

A complete, runnable example showing the majority of TurnStack features together.

```python
"""
support_bot.py
==============
Customer support bot using TurnStack.
"""

import io
import asyncio
import traceback
import httpx
from fastapi import FastAPI, Request, Response, HTTPException
from turnstack import BotEngine, FlowTree, IncomingMessage
from turnstack.nodes import (
    Menu, Input, Confirm, Action, Router, ListNode, MediaReply, CtaUrl,
    Option, Field, MenuField, ButtonsField, ImageField, BranchField, Route,
)

# ── database (stub — replace with your real DB) ───────────────────────────────

users   = {}   # user_id -> {name, tier}
tickets = []   # list of ticket dicts

def get_user(user_id):   return users.get(user_id)
def save_user(user_id, name, tier):
    users[user_id] = {"name": name, "tier": tier}
def create_ticket(user_id, data):
    tid = len(tickets) + 1
    tickets.append({"id": tid, "user_id": user_id, **data})
    return tid
def get_tickets(user_id):
    return [t for t in tickets if t["user_id"] == user_id]


# ── router hooks ──────────────────────────────────────────────────────────────

def load_profile(session):
    user = get_user(session.user_id)
    if user:
        session.context["user"] = user


# ── action functions ──────────────────────────────────────────────────────────

def do_register(session, collected):
    save_user(session.user_id, collected["name"], collected["tier"])
    session.context["user"] = {"name": collected["name"], "tier": collected["tier"]}
    return f"✅ Welcome, {collected['name']}! Your account is set up."

def do_submit_ticket(session, collected):
    tid = create_ticket(session.user_id, {
        "type":     collected["ticket_type"],
        "summary":  collected["summary"],
        "detail":   collected.get("detail"),
        "image_id": collected.get("screenshot", {}).get("media_id"),
    })
    return f"✅ Ticket #{tid} submitted. Our team will respond within 24 hours."

def do_learn_more(session, collected):
    tier = session.context.get("user", {}).get("tier", "standard")
    if tier == "premium":
        return "⭐ As a Premium member you get 24/7 live support and dedicated SLAs."
    return "📋 Standard support includes email responses within 24 hours."

def build_report(session, collected) -> bytes:
    import openpyxl
    wb = openpyxl.Workbook()
    ws = wb.active
    ws.append(["#", "Type", "Summary"])
    for t in get_tickets(session.user_id):
        ws.append([t["id"], t["type"], t.get("summary", "")])
    buf = io.BytesIO()
    wb.save(buf)
    return buf.getvalue()


# ── flow tree ─────────────────────────────────────────────────────────────────

tree = FlowTree(entry="entry")

tree.add("entry", Router(
    before  = load_profile,
    routes  = [Route(when=lambda s: s.context.get("user") is None, next="welcome_new")],
    default = "main_menu",
))

tree.add("welcome_new", Menu(
    text    = "👋 Welcome to SupportBot! Looks like you're new here.",
    options = [
        Option("Get started", next="register"),
        Option("Learn more",  next="about_action"),
    ],
))

tree.add("about_action", Action(
    fn   = lambda s, c: "SupportBot lets you raise and track tickets, download reports, and manage your account — all on WhatsApp.",
    next = "welcome_new",
))

tree.add("register", Input(
    title  = "Registration",
    fields = [
        Field("name", "What is your name?",
              validate=lambda v: "At least 2 characters." if len(v.strip()) < 2 else None,
              transform=str.strip),
        ButtonsField("tier", "Which plan are you on?", options=[
            Option("Standard", value="standard"),
            Option("Premium",  value="premium"),
        ]),
    ],
    next = "register_action",
))

tree.add("register_action", Action(fn=do_register, next="main_menu"))

tree.add("main_menu", Menu(
    text    = lambda s: f"Hi {s.context.get('user', {}).get('name', 'there')} 👋 How can I help?",
    options = [
        Option("🎫 New ticket",      next="new_ticket",     description="Report an issue or request a feature"),
        Option("📋 My tickets",      next="my_tickets",     description="View your open tickets"),
        Option("📊 Download report", next="report_media",   description="Get an Excel report"),
        Option("ℹ️ My plan",          next="plan_action",   description="View plan details"),
        Option("🔗 API docs",        next="docs_cta",       description="Read the developer docs"),
    ],
    button_label = "Main Menu",
))

# New ticket with conditional fields
tree.add("new_ticket", Input(
    title  = "New Ticket",
    fields = [
        ButtonsField("ticket_type", "What type of issue is this?", options=[
            Option("🐛 Bug",      value="bug"),
            Option("💡 Feature",  value="feature"),
            Option("❓ Question", value="question"),
        ]),
        Field("summary", "Describe your issue in one sentence:"),
        BranchField(
            when=lambda s: s.collected.get("ticket_type") == "bug",
            fields=[
                Field("detail", "What steps reproduce the bug?"),
                ImageField("screenshot", "Attach a screenshot (optional — send any text to skip):"),
            ],
        ),
        BranchField(
            when=lambda s: s.collected.get("ticket_type") == "feature",
            fields=[Field("detail", "Describe the feature you'd like in more detail:")],
        ),
    ],
    next = "confirm_ticket",
))

tree.add("confirm_ticket", Confirm(
    text    = lambda c: (
        f"📋 Ticket summary:\n\n"
        f"Type: {c['ticket_type']}\nIssue: {c['summary']}\n"
        f"Details: {c.get('detail', '—')}\n\nSubmit this ticket?"
    ),
    options = [
        Option("✅ Submit",  next="submit_ticket_action"),
        Option("✏️ Edit",    next="new_ticket"),
        Option("❌ Cancel",  next="main_menu"),
    ],
))

tree.add("submit_ticket_action", Action(fn=do_submit_ticket, next="main_menu"))

# My tickets — dynamic list
tree.add("my_tickets", ListNode(
    fetch            = lambda session: get_tickets(session.user_id),
    item_label       = lambda t: f"#{t['id']} — {t['type']}",
    item_description = lambda t: t.get("summary", "")[:60],
    on_select        = "main_menu",
    title            = "📋 Your Tickets",
    empty_text       = "You haven't raised any tickets yet.",
    interactive      = True,
    button_label     = "My Tickets",
    extra_options    = [Option("🔙 Back", next="main_menu")],
))

# Report download
tree.add("report_media", MediaReply(
    generate  = build_report,
    filename  = lambda s, c: f"tickets_{s.user_id}.xlsx",
    mime_type = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
    caption   = "📊 Here is your ticket report.",
    next      = "main_menu",
))

# Plan info
tree.add("plan_action", Action(fn=do_learn_more, next="main_menu"))

# CTA — link to API docs
tree.add("docs_cta", CtaUrl(
    body   = "Read the full TurnStack developer documentation.",
    url    = "https://github.com/your-org/turnstack",
    button = "Open Docs",
    header = "TurnStack Docs",
    footer = "Opens in your browser",
    next   = "main_menu",
))

# ── engine ────────────────────────────────────────────────────────────────────

engine = BotEngine(tree=tree, session_timeout=3600)

# ── send helper + webhook — copy from §12 and §13 ────────────────────────────
# (paste send_whatsapp() and the FastAPI webhook handler here)
```

---

*TurnStack — build the conversation, not the plumbing.*
