walkeros-create-destination

---

name: walkeros-create-destination description: Use when creating a new walkerOS destination to send events to a vendor or API (GA4/gtag, Meta/Facebook Pixel, Mixpanel, Amplitude, a custom HTTP API, Measurement Protocol), web or server-side. Example-driven workflow: research the vendor SDK and define step examples before implementing the destination interface, env pattern, and mappings.

Create a New Destination

Prerequisites

Before starting, read these skills:

• understanding-flow - How destinations fit in architecture
• understanding-destinations - Destination interface
• understanding-transformers - Transformer chaining to destinations
• understanding-mapping - Event transformation
• testing-strategy - How to test with env pattern (Phase 3 + Phase 8)
• using-step-examples - Authoritative pattern for Flow.StepExample structure and Three Type Zones
• writing-documentation - Documentation standards (for Phase 9)

Choose Your Template

Complexity	Template	When to Use
Simple	plausible/	Single SDK call, minimal config
Complex	gtag/	Multiple services, sub-destinations
Server	gcp/	Server-side, batching, SDK init

Process Overview

1. Research     → Deeply understand vendor SDK, API, and event taxonomy
2. Classify     → Determine vendor taxonomy type and integration approach
3. Examples     → Define in/out pairs FIRST (start with the end result)
4. Mapping      → Define walkerOS → vendor transformation
5. Scaffold     → Copy template and configure
6. Convention   → Add walkerOS.json metadata and buildDev
7. Implement    → Build using examples as test fixtures
8. Test         → Verify against example variations
9. Document     → Write README

---

Phase 1: Research

Goal: Deeply understand the vendor SDK before writing any code. Research quality determines implementation quality.

1.1 Find and Install Official SDK

Always prefer the vendor's official SDK package over raw HTTP API calls. The SDK handles transport, batching, retries, and plugin ecosystems — don't reinvent these.

• Install the SDK — npm install @vendor/sdk and read the actual source
• Read TypeScript types — Import types from the SDK package directly. Never duplicate type definitions. This ensures IntelliSense completeness and consistency with SDK updates.
• Understand the full API surface — List every public method, not just track(). Identity methods, property operations, group management, specialized event types (revenue, etc.)

# Search npm for official packages
npm search [vendor-name]
npm search @[vendor]

# Install and inspect actual types
npm install @vendor/analytics-browser
ls node_modules/@vendor/analytics-browser/lib/esm/

1.2 Understand SDK Architecture

• Init options — What does init() accept? What can be configured?
• Call ordering — Can you call methods before init()? Is there internal queuing? What are the race condition implications?
• Plugin system — Does the SDK support plugins? How are they added?
• Identity management — How does the SDK handle user/device/session IDs? Does it manage cookies/storage? Can this be disabled (since walkerOS manages identity)?
• Consent — Does the SDK have a consent mode or just binary opt-out?

1.3 Identify All Event Methods

Go beyond just track(). Most SDKs have specialized methods:

Method Category	Examples	walkerOS Handling
Event tracking	track(), logEvent()	Default push()
User properties	identify(), setUserProperties()	mapping.settings
Revenue/purchase	revenue(), purchase()	mapping.settings
Groups/accounts	setGroup(), groupIdentify()	mapping.settings
Identity setters	setUserId(), setDeviceId()	settings.identify
Opt-out	setOptOut(), consent()	on('consent')
Cleanup	flush(), reset()	destroy()

1.4 Check Existing Patterns

Review similar destinations in the codebase:

ls packages/web/destinations/

Gate: Research Complete

• Official SDK installed and types inspected
• All public methods listed with their purpose
• Init options understood (what to disable, what to pass through)
• Call ordering / race conditions understood
• Identity, consent, and plugin patterns documented

Checkpoint: Research Review (Optional)

If working with human oversight, pause here to confirm:

• SDK integration approach correct?
• Event mapping makes sense for the use case?
• Any vendor quirks or limitations to handle?

---

Phase 2: Classify Vendor Taxonomy

Goal: Understand what the vendor expects, which determines destination complexity.

2.1 Taxonomy Types

Type	Description	Mapping Needed	Example Vendors
Free-form	Any event name accepted, no prescribed schema	Minimal — names pass through	Most modern analytics
Rigid	Prescribed event names unlock specific reports	Essential — must map to exact names	Some legacy analytics
Schema-based	Self-describing events with formal schemas	Structural — must build schema objects	Data warehouse tools

2.2 Determine Integration Approach

Approach	When to use	Pattern
Vendor SDK as host	SDK has plugins, batching, retries	Load SDK, disable what walkerOS replaces, use as transport
Script + command queue	SDK loaded via script tag	Load script, use global function queue
HTTP API	No SDK available, or server-side	Direct HTTP calls via sendWeb() or fetch

Prefer the vendor SDK — it handles transport, retries, and plugin orchestration. HTTP API is a fallback when no SDK exists.

When using the vendor SDK:

• Disable features walkerOS replaces (autocapture, identity storage) by default — but allow users to re-enable via settings
• Pass through all SDK init options for full configurability
• Import types from the SDK package directly

Gate: Classification Complete

• Taxonomy type identified (free-form / rigid / schema-based)
• Integration approach chosen (SDK / script / HTTP)
• Know which SDK features walkerOS replaces vs. uses

---

Phase 3: Create Examples (BEFORE Implementation)

Mandatory. Examples are the test fixtures for Phase 8. Define expected in / out / mapping triples FIRST — start with the end result in mind. Without examples, you cannot test. Even for free-form vendors where no mapping is strictly "required," step examples serve as the single source of truth for tests, simulations, and documentation.

Authoritative pattern: See using-step-examples for the Three Type Zones and lifecycle. This skill reuses that contract — do not diverge.

3.1 Scaffold Directory Structure

mkdir -p packages/web/destinations/[name]/src/examples
mkdir -p packages/web/destinations/[name]/src/{schemas,types}

3.2 Required Files (3 files total)

All seven reference web destinations (gtag, meta, snowplow, plausible, piwikpro, api, demo) use exactly three files in src/examples/. Match this structure — no events.ts, outputs.ts, or standalone mapping.ts.

File	Purpose
examples/env.ts	Mock environment for testing (no real network calls)
examples/step.ts	Flow.StepExample entries with in / out / mapping? triples
examples/index.ts	Barrel exports: env and step

The step.ts file embeds the input event, the mapping config, and the expected vendor output together in one Flow.StepExample — subsuming what older skills described as separate events.ts / outputs.ts / mapping.ts files.

3.3 Typing Rules (strict)

No any. Every example value must be explicitly typed.

• Inputs use WalkerOS.Event (via getEvent() from @walkeros/core) — never hand-roll event literals.
• Outputs use vendor SDK types imported from the official package whenever the vendor publishes them. Do not invent local output types for payloads the vendor already types (e.g. use Meta Pixel's fbq argument types, not a local FbqCall interface).
• Step entries are typed Flow.StepExample from @walkeros/core.
• Mock env is typed against the destination's local Env type from ../types. No as any, no untyped {}.
• Vendor SDK types come from the SDK you installed in Phase 1 — reuse them rather than duplicating shapes.

3.4 Code Template — examples/step.ts

import type { Flow, WalkerOS } from '@walkeros/core';
import { getEvent } from '@walkeros/core';

// One step example per supported feature / setting.
// `in`  is a WalkerOS.Event (use getEvent for deterministic fixtures).
// `out` is the vendor-specific call we expect the destination to produce —
//       typed against the vendor SDK's published types where available.
// `mapping` is the mapping rule under test (optional — omit for default push).

// Set `title` + `description` for public examples; mark test-only fixtures
// with `public: false`. See
// [walkeros-using-step-examples](../walkeros-using-step-examples/SKILL.md).

export const purchase: Flow.StepExample = {
  in: getEvent('order complete', { timestamp: 1700000100 }),
  mapping: {
    name: 'purchase',
    data: {
      map: {
        transaction_id: 'data.id',
        value: 'data.total',
        currency: { key: 'data.currency', value: 'EUR' },
      },
    },
  },
  out: [
    'event',
    'purchase',
    {
      transaction_id: '0rd3r1d',
      value: 555,
      currency: 'EUR',
    },
  ],
};

export const pageView: Flow.StepExample = {
  in: getEvent('page view', { timestamp: 1700000102 }),
  mapping: undefined,
  out: ['event', 'page_view', { send_to: 'G-XXXXXX-1' }],
};

// For destinations that handle consent updates, use the command field
// to route `in` through elb('walker consent', in) instead of an event push:
export const consentGranted: Flow.StepExample = {
  command: 'consent',
  in: { marketing: true, functional: true },
  out: ['consent', 'update', { ad_storage: 'granted' }],
};

Init step example

Every destination ships an examples.step.init entry — the init is a first-class step example, not a hidden side effect.

• in mirrors the real Destination.Config shape users copy-paste — typically { loadScript: true, settings: { /* vendor-specific */ } }. Whatever a user would configure in their flow goes here verbatim.
• out is the ordered list of vendor calls the init() lifecycle produces (script tags, SDK initializers, queue setup). Each effect tuple follows the standard [callable, ...args] shape.

Test pattern. Call destination.init({ id, config, env, logger, collector }) directly in the test — no capture helpers, no capture.ts, no allowlists. Assert the captured vendor calls equal examples.step.init.out:

const calls: unknown[][] = [];
const env = wrapEnv(examples.env.init, (call) => calls.push(call));
await destination.init({
  id: 'test',
  config: examples.step.init.in as Destination.Config,
  env,
  logger,
  collector,
});
expect(calls).toEqual(examples.step.init.out);

Event step tests bootstrap once with examples.step.init.in, then slice the shared capture buffer to isolate push effects from init effects:

const pushCalls = calls.slice(examples.step.init.out.length);
expect(pushCalls).toEqual(example.out);

There are no hand-maintained allowlists or isInitEffect filters — the init example's out.length is the single source of truth for how many effects belong to init.

Multi-tool packages (like gtag, which drives GA4, Google Ads, and GTM) ship per-tool init examples — examples.step.ga4Init, adsInit, gtmInit — instead of a single init. The docs render each on its own page via <StepExample example={data.examples.step.ga4Init} />, and tests pick the right init per sub-tool.

For destinations, the Three Type Zones collapse to:

• in = walkerOS event (WalkerOS.Event)
• out = vendor output (typed against vendor SDK)
• mapping = rule under test (optional)

3.5 examples/index.ts (barrel)

export * as env from './env';
export * as step from './step';

3.6 examples/env.ts

Mock the vendor SDK surface and any DOM touchpoints. Never reach real network, real cookies, or real globals. Type the exports against your local Env:

import type { Env } from '../types';

export const init: Env | undefined = {
  /* pre-init state (vendor SDK not yet loaded) */
};

export const push: Env = {
  /* post-init state used for push() tests */
};

3.7 Test Fixture Contract (hard rule)

The examples authored here are the Phase 8 test fixtures. No parallel fixtures allowed.

• src/index.test.ts (or src/__tests__/stepExamples.test.ts) MUST iterate examples via it.each(Object.entries(examples.step)).
• Tests must NOT contain hardcoded payloads, vendor configs, or expected outputs.
• If a test needs a value that is not in examples.step, add it to step.ts first, then consume it from the test. Never inline test data.
• The only per-test setup allowed is deriving destination settings from the example's mapping.settings (e.g. enabling the right sub-tool).

See packages/web/destinations/gtag/src/__tests__/stepExamples.test.ts for a canonical reference.

3.8 Export via dev.ts

export * as schemas from './schemas';
export * as examples from './examples';

Phase 3 Acceptance Checklist

• src/examples/env.ts — mock env, no real network, typed against local Env
• src/examples/step.ts — one Flow.StepExample per supported feature / setting, typed in / out / mapping?
• src/examples/index.ts — barrel exports env and step
• No standalone events.ts, outputs.ts, or mapping.ts files
• All vendor SDK types imported from the official package — no any, no reinvented local output types
• src/index.test.ts (or __tests__/stepExamples.test.ts) iterates examples.step via it.each(Object.entries(...))
• Tests contain zero hardcoded payloads / vendor configs / expected outputs — everything flows from examples.step
• npm run build passes — examples compile against published types
• Each example traces: in → apply mapping → matches out

---

Phase 4: Define Mapping

Goal: Document transformation from walkerOS events to vendor format.

Mapping rules live inside each Flow.StepExample entry in step.ts — no separate mapping.ts file. Each step example embeds the exact mapping rule under test alongside its in event and expected out output.

Verify Mapping Logic

For each entry in step.ts, trace:

Input: examples.step.purchase.in     (WalkerOS.Event)
  ↓ Apply examples.step.purchase.mapping
  ↓ name transforms, data.map applied
Output: Should match examples.step.purchase.out

Gate: Mapping Verified

• Step examples cover: page view + at least one conversion event
• One step example per supported setting / sub-tool
• Each mapping traces correctly from in to out

---

Phase 5: Scaffold

Template destination: packages/web/destinations/plausible/

cp -r packages/web/destinations/plausible packages/web/destinations/[name]
cd packages/web/destinations/[name]

# Update package.json: name, description, repository.directory

Directory structure:

packages/web/destinations/[name]/
├── src/
│   ├── index.ts           # Main destination (init + push)
│   ├── index.test.ts      # Tests against examples
│   ├── dev.ts             # Exports schemas and examples
│   ├── examples/
│   ├── schemas/
│   └── types/
├── package.json
├── tsconfig.json
├── tsup.config.ts
├── jest.config.mjs
└── README.md

Transformer Chain Integration

Destinations can wire to transformer chains via before in the init config:

destinations: {
  myDestination: {
    code: destinationMyDestination,
    config: { settings: { /* ... */ } },
    before: 'redact'  // Events go through redactor before this destination
  }
}

---

Phase 6: walkerOS.json Convention

Every walkerOS package ships a walkerOS.json file for CDN-based schema discovery.

Add walkerOS field to package.json

{
  "walkerOS": { "type": "destination", "platform": "web" },
  "keywords": ["walkerOS", "walkerOS-destination", ...]
}

Use buildDev() in tsup.config.ts

Replace buildModules({ entry: ['src/dev.ts'] }) with buildDev():

import { buildDev } from '@walkeros/config/tsup';
// In defineConfig array:
buildDev(),

This auto-generates dist/walkerOS.json from your Zod schemas at build time.

Hints (Optional)

If your destination has capabilities, behaviors, or troubleshooting patterns not obvious from schemas alone, add hints. See walkeros-writing-documentation skill for full guidelines.

Create src/hints.ts:

import type { Hint } from '@walkeros/core';

export const hints: Hint.Hints = {
  'auth-methods': {
    text: 'Supports X, Y, and Z auth methods. See settings schema for all options.',
    code: [{ lang: 'json', code: '{ "settings": { ... } }' }],
  },
};

Export from src/dev.ts:

export * as schemas from './schemas';
export * as examples from './examples';
export { hints } from './hints';

Guidelines:

• Expand awareness — describe capabilities ("supports X, Y, Z"), don't prescribe one path
• Reference schemas and examples, don't duplicate them
• Verify every claim against actual implementation before publishing
• Key naming: kebab-case, group with prefixes (auth-*, storage-*, troubleshoot-*)
• Most destinations don't need hints — schemas and examples cover the common case

Gate: Convention Met

• walkerOS field in package.json with type and platform
• buildDev() in tsup.config.ts
• Build generates dist/walkerOS.json
• Keywords include walkerOS and walkerOS-destination

Runtime-only npm dependencies

If your package wraps a third-party npm dep that cannot be ESM-bundled (uses __dirname, ships a .node binary, etc.), declare it under walkerOS.bundle.external in your package.json. See walkeros-using-cli → Bundle externals for the complete contract.

---

Phase 7: Implement

Now write code to produce the outputs defined in Phase 3.

Template Files

Use these templates as your starting point:

File	Purpose	Template
types/index.ts	Type definitions	types.ts
index.ts	Main destination	index.ts

Key Patterns

1. Init receives context: Destructure config, env, logger, id from context
2. Push receives context: Includes data, rule (renamed from mapping), ingest
3. Use getEnv<Env>(env): Never access window/document directly, and never cast them. Declare your SDK global (declare global { interface Window { vendorSdk?: ... } }) and narrow Env['window'] to the SDK shape in types.ts. The generic getEnv<Env>(env) then returns your narrowed env intersected with the real DOM globals, so window.vendorSdk and document.createElement are fully typed with zero casts. No as any, as unknown, as Window, as Document, @ts-ignore, or @ts-expect-error is allowed in production or test code, ever. If something will not type check, fix the types, do not cast.
4. Return config from init: Allows updating config during initialization
5. Optional destroy method: Implement if the destination holds resources (DB connections, SDK clients, timers) that need cleanup on shutdown. Call flush() or equivalent on the vendor SDK.
6. Mapping-based settings (recommended): Use standard walkerOS mapping values (map, loop, key, value, condition) in mapping.settings.* for vendor-specific operations. Resolve via getMappingValue() in push() and interpret the resolved object's keys as SDK method instructions. This keeps config agnostic and reuses the mapping engine.
7. Consent two-layer: config.consent gates walkerOS event delivery. on('consent') controls vendor SDK internals (opt-out, pause capture, etc.). Both needed for complete consent compliance.
   • For step-example tests, use command: 'consent' on Flow.StepExample to invoke the on('consent') handler. Do not push consent data as an event.

Reserved fields

config.setup is reserved for the setup lifecycle (see Adding setup (optional)). Do not use the key setup for unrelated package metadata or hint keys. The framework wires this field to SetupFn via resolveSetup, and walkeros setup destination.<name> reads it. Repurposing the name will collide with that wiring.

Gate: Implementation Compiles

• npm run build passes
• npm run verify:touched -- <destination-name> passes (L1: typecheck + lint + test)

---

Adding setup (optional)

A destination package can implement an optional setup() function to provision external resources idempotently: BigQuery datasets and tables, Pub/Sub topics, SQLite tables, warehouse schemas, S3 buckets, webhook registrations on downstream platforms. Setup runs only when an operator explicitly types walkeros setup destination.<name>. The runtime never auto-invokes it from init(), push(), or destroy().

The framework provides the slot, the CLI command, and a resolveSetup helper. The package owns: what setup means, idempotency, error handling, return value.

For background on how setup fits the destination lifecycle, see understanding-destinations.

Types

// types/index.ts
import type { CoreDestination } from '@walkeros/core';

export interface Settings {
  /* runtime push settings */
}
export interface InitSettings {
  /* one-time init settings */
}
export interface Mapping {
  /* per-event mapping config */
}
export interface Env {
  /* injected platform deps (SDK clients, etc.) */
}

// The package's own setup options interface.
// Becomes the U slot of Types; surfaces as `config.setup: boolean | Setup` for users.
export interface Setup {
  // package-specific provisioning options
  // e.g. for BigQuery: location, partitioning, clustering, schema
  location?: string;
  partitioning?: { field: string; type: 'DAY' | 'HOUR' };
}

// Service-account / key-pair credentials. Becomes the trailing C slot of Types;
// surfaces as the optional top-level `config.credentials` (sibling of `settings`),
// $env-resolvable and validated per package.
export type Credentials =
  CoreDestination.Credential<CoreDestination.ServiceAccount>;

export type Types = CoreDestination.Types<
  Settings,
  Mapping,
  Env,
  InitSettings,
  Setup,
  Credentials
>;

Where credentials belong. Put service-account / key-pair credentials (the kind paired with $env.NAME to inject a secret) on the strictly-typed top-level config.credentials, not inside settings. Read them via config.credentials ?? settings.credentials ?? ADC so existing flows keep working, and omit them to fall back to Application Default Credentials. The raw settings.<sdk> passthrough (e.g. settings.bigquery) stays as the lower-level escape hatch for SDK-specific auth options. The package-specific settings.credentials still works but is deprecated.

Implementation

// setup.ts
import type { CoreDestination, SetupFn } from '@walkeros/core';
import { resolveSetup } from '@walkeros/core';
import type { Setup, Types } from './types';

const DEFAULT_SETUP: Setup = {
  location: 'EU',
};

export const setup: SetupFn<
  CoreDestination.Config<Types>,
  CoreDestination.Env<Types>
> = async ({ config, env, logger }) => {
  const options = resolveSetup(config.setup, DEFAULT_SETUP);
  if (!options) return; // config.setup is false or unset

  // Package-specific provisioning, idempotent.
  // Returning a structured object (e.g. { datasetCreated: true })
  // makes that data available to operators via `walkeros setup ... | jq`.
};

Wire it in your default export:

// index.ts
import { setup } from './setup';

export default {
  type: 'my-destination',
  push: /* ... */,
  setup,
};

When to implement

Implement setup() when your destination needs first-time provisioning of external resources before events can flow: warehouse tables, Pub/Sub topics, S3 buckets, schema bindings, IAM roles, webhook registrations on downstream platforms. Skip it when your destination only consumes credentials or HTTP endpoints the user already provisioned.

Contract

• Triggered only by walkeros setup <kind>.<name>. Never by runtime push, init, or destroy.
• Idempotency is your responsibility. Re-running setup against a fully provisioned environment must be a safe no-op. Use try-create-catch-409 on REST APIs, IF NOT EXISTS on SQL, native idempotent operations where available. The framework does not retry, track state, or detect drift.
• Return structured data from setup() when useful for operator scripting. The CLI emits non-undefined return values as JSON to stdout.
• For packages where setup: true (boolean form) is meaningless because mandatory fields have no safe defaults (e.g., Kafka numPartitions, GitHub webhook webhookUrl), reject the boolean form with a clear runtime error listing required fields:

if (config.setup === true) {
  throw new Error(
    'kafka destination setup requires explicit options: ' +
      '{ topic, numPartitions, replicationFactor }. There is no safe default.',
  );
}

---

Phase 8: Test Against Examples

Tests verify implementation against the examples from Phase 3. If examples are incomplete, tests will be incomplete.

See testing-strategy for the shared env / dev-examples conventions this phase depends on.

Verify implementation produces expected outputs.

Test Template

Use the test template: index.test.ts. Reference canonical implementation: packages/web/destinations/gtag/src/__tests__/stepExamples.test.ts.

Key Test Patterns

1. it.each(Object.entries(examples.step)) is mandatory — one iteration per step example. Do not write per-feature tests with hand-rolled payloads.
2. Use createPushContext() helper — standardizes context creation.
3. Include id field — required in context.
4. Use rule instead of mapping — property renamed in PushContext.
5. Zero hardcoded payloads — every input, vendor config, and expected output comes from examples.step or examples.env. If you need something new, add it to examples first.
6. Clone env per test — clone(examples.env.push) so vendor mocks don't leak across iterations.

Gate: Tests Pass

• npm run verify:touched -- <destination-name> passes (L1)
• Tests iterate via it.each(Object.entries(examples.step))
• Tests contain no hardcoded payloads, vendor configs, or expected outputs
• Every assertion reads from examples.step[...].out

---

Phase 9: Document

Follow the writing-documentation skill for:

• README structure and templates
• Example validation against apps/quickstart/
• Quality checklist before publishing

Key requirements for destination documentation:

• Event mapping table (walkerOS → vendor format)
• Configuration options table (use PropertyTable if schema exists)
• Working code example with imports
• Installation instructions

---

Validation Checklist

Beyond understanding-development requirements (build, test, lint, no any):

• Uses getEnv<Env>(env) pattern (never direct window/document access, never as Window/as Document/as unknown casts in src or tests)
• dev.ts exports schemas and examples
• Examples match type signatures
• Tests use examples for assertions (not hardcoded values)
• walkerOS.json generated at build time
• walkerOS field in package.json

---

Reference Files

What	Where
Simple template	packages/web/destinations/plausible/
Complex example	packages/web/destinations/gtag/
Types	packages/web/core/src/types/destination.ts

Related Skills

• understanding-destinations - Destination interface and env pattern
• using-step-examples — Authoritative Flow.StepExample pattern and Three Type Zones
• testing-strategy — Testing with env mocking and dev-examples-as-fixtures conventions
• writing-documentation - Documentation standards