building-nango-functions-remotely - SKILL.md Agent Skill

name: building-nango-functions-remotely description: Builds Nango Functions without a checked-out Nango project by calling Nango remote compile, dryrun, and deploy endpoints, resolving NANGO_SERVER_URL from env or .env, and using shared action and sync patterns. Use when creating or updating Nango actions or syncs remotely.

Build Nango Functions Remotely

Build Nango actions and syncs without a checked-out Nango project by sending a single-file TypeScript function to Nango's remote compile, dryrun, and deploy APIs.

When to use

User wants to build or modify a Nango function
User wants to build an action in Nango
User wants to build a sync in Nango

Sync Strategy Gate (required before writing code)

If the task is a sync, read references/syncs.md before writing code and state one of these paths first:

Checkpoint plan:
- change source (updated_at, modified_since, changed-records endpoint, cursor, page token, offset/page, since_id, or webhook)
- checkpoint schema
- how the checkpoint changes the provider request or resume state
- whether the request still walks the full dataset or returns changed rows only
- delete strategy
Full refresh blocker:
- exact provider limitation from the docs or sample payloads
- why checkpoints cannot work here

Invalid sync implementations:

full refresh because it is simpler
saveCheckpoint() without getCheckpoint()
reading or saving a checkpoint without using it in request params or pagination state
using syncType: 'incremental' or nango.lastSyncDate in a new sync
using trackDeletesStart() / trackDeletesEnd() with a changed-only checkpoint (modified_after, updated_after, changed-records endpoint). Those requests omit unchanged rows, so trackDeletesEnd() will falsely delete them.
using trackDeletesStart() / trackDeletesEnd() in an incremental sync that already has explicit deleted-record events

Choose the Path

Action:

One-time request, user-triggered, built with createAction()
Read references/actions.md before writing code

Sync:

Scheduled or webhook-driven cache updates built with createSync()
Complete the Sync Strategy Gate first
Read references/syncs.md before writing code

Required Inputs (Ask User if Missing)

Always:

Integration ID (provider name)
Connection ID (for validation or dryrun)
Script/function name (kebab-case)
API reference URL or sample response

Action-specific:

Use case summary
Input parameters
Output fields
Metadata JSON if required
Test input JSON for validation/dryrun (required; use {} for no-input actions)

Sync-specific:

Model name (singular, PascalCase)
Frequency (every hour, every 5 minutes, etc.)
Checkpoint schema (timestamp, cursor, page token, offset/page, since_id, or composite)
How the checkpoint changes the provider request or resume state
Delete strategy (deleted-record endpoint/webhook, or why full refresh is required)
If proposing a full refresh, the exact provider limitation that blocks checkpoints from the docs/sample response
Metadata JSON if required (team_id, workspace_id)

If any required external values are missing, ask a targeted question after checking the repo and provider docs. For syncs, choose a checkpoint plus deletion strategy whenever the provider supports one. If you cannot find a viable checkpoint strategy, state exactly why before writing a full refresh.

Non-Negotiable Rules

Shared platform constraints

Nango functions use createAction() / createSync().
You cannot add arbitrary packages. Use relative imports only when the chosen workflow supports them; built-ins include zod, crypto/node:crypto, and url/node:url.
Use the Nango HTTP API for connection lookup, credentials, and proxy calls outside function code. Do not invent CLI token or connection commands.
Add an API doc link comment above each provider call.
Action outputs cannot exceed 2MB.
File uploads and downloads cannot be implemented as actions (sandboxed runtime: no fs, no axios, 2 MB output limit). Use a proxy script in {integration}/proxy/ with @nangohq/node instead — see references/actions.md.
HTTP retries default to 0; set retries deliberately. Treat 3 as the normal maximum; for sync provider calls, values above 3 are effectively forbidden unless docs prove they are safe and necessary. Avoid retries for non-idempotent writes unless the API supports idempotency.

Sync rules

Sync records need a stable string id.
New syncs should define a checkpoint schema, call nango.getCheckpoint() first, and nango.saveCheckpoint() after each page or batch.
A checkpoint is valid only if it changes the request or resume state (since, updated_after, cursor, page_token, offset, page, since_id, etc.). Saving one without using it is not incremental sync.
New syncs must not use syncType: 'incremental' or nango.lastSyncDate.
Default to nango.paginate(...) + nango.batchSave(...). Avoid manual while (true) loops when cursor, link, or offset pagination fits.
Prefer batchDelete() when the provider returns deletions, tombstones, or delete webhooks.
Use full refresh only if the provider cannot return changes, deletions, or resume state, or if the dataset is tiny.
For full refresh, cite the exact provider limitation from docs or payloads. "It is easier" is not enough.
deleteRecordsFromPreviousExecutions() is deprecated. For full refresh, call trackDeletesStart() before fetch/save and trackDeletesEnd() only after a successful full fetch/save.
Never combine trackDeletesStart() / trackDeletesEnd() with changed-only checkpoints (modified_after, updated_after, changed-records endpoints, etc.). They omit unchanged rows, so trackDeletesEnd() would delete them.
Checkpointed full refreshes are still full refreshes. Call trackDeletesEnd() only in the run that finishes the full window.
If a sync requires metadata (e.g. team_id, workspace_id, guild_id), set autoStart: false. The sync cannot run until the caller has set the metadata, so starting it automatically would fail.

Conventions

Match field casing to the external API. Passthrough fields keep provider casing; non-passthrough fields should use the majority casing of that API.
Prefer explicit field names.
Add .describe() examples for IDs, timestamps, enums, and URLs.
Avoid any; use inline mapping types.
Prefer static Nango endpoint paths (avoid :id / {id} in the exposed endpoint); pass IDs in input or params.
List actions should expose cursor plus a next-cursor field in the majority casing of that API (next_cursor, nextCursor, etc.).
Use nango.zodValidateInput() only when you need custom validation or logging; otherwise rely on schemas plus the chosen validation workflow.

Schema Semantics

Default non-required inputs to .optional().
Use .nullable() only when null has meaning, usually clear-on-update; add .optional() when callers may omit the field too.
Raw provider schemas should match the provider: .optional() for omitted fields, .nullable() for explicit null, .nullish() only when the provider truly does both.
Final action outputs and normalized sync models should prefer .optional() and normalize upstream null to omission unless null matters.
Default generated schemas to .optional() for non-required inputs and normalized outputs; widen only when the upstream contract justifies it.
Prefer .nullable() over z.union([z.null(), T]) or z.union([T, z.null()]).
Return null only when the output schema allows it.
z.object() strips unknown keys by default. For provider pass-through use z.object({}).passthrough(), z.record(z.unknown()), or z.unknown() with minimal refinements.

Field Naming and Casing Rules

Use explicit suffixes in the API's majority casing: IDs (user_id, userId), names (channel_name, channelName), emails (user_email, userEmail), URLs (callback_url, callbackUrl), and timestamps (created_at, createdAt).

Mapping example (API expects a different parameter name):

const InputSchema = z.object({
    userId: z.string()
});

const config: ProxyConfiguration = {
    endpoint: 'users.info',
    params: {
        user: input.userId
    },
    retries: 3
};

If the API is snake_case, use user_id instead. The goal is API consistency.

References

Action patterns, CRUD examples, metadata usage, and ActionError examples: references/actions.md
Sync patterns, concrete checkpoint examples, delete strategies, and full refresh fallback: references/syncs.md

Useful Nango docs (quick links)

Functions runtime SDK reference: https://nango.dev/docs/reference/functions
Implement an action: https://nango.dev/docs/implementation-guides/use-cases/actions/implement-an-action
Implement a sync: https://nango.dev/docs/implementation-guides/use-cases/syncs/implement-a-sync
Checkpoints: https://nango.dev/docs/implementation-guides/use-cases/syncs/checkpoints
Deletion detection (full vs incremental): https://nango.dev/docs/implementation-guides/use-cases/syncs/deletion-detection
Testing integrations (dryrun, --save, Vitest): https://nango.dev/docs/implementation-guides/platform/functions/testing
Nango HTTP API reference: https://nango.dev/docs/reference/api

When API Docs Do Not Render

If web fetching returns incomplete docs (JS-rendered):

Ask the user for a sample response
Use existing Nango actions or syncs in the workspace as a pattern when they exist
Use the skill-specific validation or dryrun workflow until it passes

Preconditions (Do Before Writing Code)

No checked-out Nango project is required.
Resolve NANGO_SERVER_URL in this order: environment variable, .env file, then fallback to https://api.nango.dev.
Resolve NANGO_SECRET_KEY before calling remote endpoints.
Use the environment bound to that secret key.
Keep the function self-contained in one TypeScript file unless you have direct evidence that the remote endpoint accepts multi-file payloads.
Do NOT create or modify any files in the current project/directory. If you need to create files, use a temp folder

Workflow (required)

Decide whether this is an action or a sync.
Read the matching reference file: references/actions.md or references/syncs.md.
For syncs, inspect provider docs or payloads for checkpoints and deletes, decide whether the endpoint returns full data or changed rows, and complete the Sync Strategy Gate.
Gather required inputs and external values, including the NANGO_SECRET_KEY for the target environment and any metadata needed for dryrun.
Resolve the host from NANGO_SERVER_URL in the environment, then .env, then https://api.nango.dev.
Write or update the function as one self-contained TypeScript file using createAction() or createSync().
Compile with POST {host}/remote-function/compile until compilation passes.
Dryrun with POST {host}/remote-function/dryrun using the target connection plus input, metadata, or checkpoint as needed.
If compile or dryrun cannot pass, stop and report the missing external state, inputs, or API contract mismatch.
Deploy with POST {host}/remote-function/deploy only when requested.

Remote API Workflow (required)

Read references/api.md before making remote calls.

Required sequence:

Compile first with /remote-function/compile.
Dryrun second with /remote-function/dryrun.
Deploy last with /remote-function/deploy.

Rules:

These endpoints are relative. Always resolve them against the chosen NANGO_SERVER_URL.
Send Authorization: Bearer <NANGO_SECRET_KEY> and Content-Type: application/json.
Do not send query params unless the API docs or an existing caller prove they are supported.
Use the server's validation errors to correct payloads. Do not invent undocumented fields when the API rejects a request.
For actions, dryrun should include input and metadata only when needed.
For syncs, dryrun should include metadata and checkpoint when needed to simulate a resumed run. Do not introduce last_sync_date for a new sync design.
Remote dryrun does not expose CLI --validate or --save; it compiles before running and returns the execution result, but it does not record local mocks.

Final Checklists

Action:

references/actions.md was used for the action pattern
Schemas and types are clear, and the function stays self-contained in one file
createAction() includes endpoint, input, output, and scopes when required
Fields use passthrough casing or the API's majority casing
Provider call includes an API doc link comment and intentional retries
nango.ActionError is used for expected failures
Host was resolved from NANGO_SERVER_URL, .env, or https://api.nango.dev
Compile succeeds with POST /remote-function/compile
Dryrun succeeds with POST /remote-function/dryrun and the expected action output
Deploy succeeds with POST /remote-function/deploy when requested

Sync:

references/syncs.md was used for the sync pattern
Models map is defined, ids are stable strings, and normalized models prefer .optional() unless null matters
Incremental was chosen first, with a checkpoint schema unless full refresh is explicitly justified from docs or payloads
nango.getCheckpoint() is read at the start and nango.saveCheckpoint() runs after each page or batch
Checkpoint data changes the provider request or resume state (since, updated_after, cursor, page_token, offset, page, since_id, etc.)
Changed-only checkpoint syncs (modified_after, updated_after, changed-records endpoint) do not use trackDeletesStart() / trackDeletesEnd()
If checkpoints were not used, the response explains exactly why no viable checkpoint strategy exists
Provider API calls use retries: 3; no sync retry value exceeds 3 without a documented exception
The function stays self-contained in one file unless the remote API proves multi-file support
Host was resolved from NANGO_SERVER_URL, .env, or https://api.nango.dev
Compile succeeds with POST /remote-function/compile
Dryrun succeeds with POST /remote-function/dryrun and returns the expected change set
Deploy succeeds with POST /remote-function/deploy when requested