Function Dev Agent Skill

Use when developing, deploying, or debugging Butterbase serverless functions, or when the user needs to add backend logic like webhooks, scheduled jobs, or custom API endpoints

3k tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

532

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/butterbase-ai/butterbase-skills --skill function-dev

The instruction itself

30 sections, as written by the author

Serverless Function Development on Butterbase

Guide for developing and deploying serverless functions on Butterbase's Deno runtime. Covers handler signatures, trigger types, database access, environment variables, and testing.

1. Handler Signature

Every function exports a single handler function with this signature:

export async function handler(
  request: Request,
  context: {
    db: PostgresClient,                                  // RLS-aware DB client
    env: Record<string, string>,                         // env vars set on the function
    user: { id: string } | null,                         // present for HTTP+auth:required; null for cron
    waitUntil: (p: Promise<unknown>) => void,            // background work after Response (≤30s)
    idempotency: {
      claim: (key: string, opts?: { scope?: string; ttlSeconds?: number }) => Promise<boolean>
    }                                                    // atomic dedup for webhook retries
  }
): Promise<Response>

CRITICAL: The handler MUST return new Response() (Web API standard). Do NOT return plain objects.

Correct:

return new Response(JSON.stringify({ message: "ok" }), {
  status: 200,
  headers: { "Content-Type": "application/json" }
});

Wrong (will fail):

return { status: 200, body: "ok" };  // NOT a Response object!

2. Trigger Types

HTTP Trigger

Invoke the function via an HTTP request.

{
  "trigger": {
    "type": "http",
    "config": { "method": "POST", "path": "/my-endpoint", "auth": "required" }
  }
}

Auth options:

"required" — request must include a valid JWT; ctx.user is always set
"optional" — JWT is parsed if present; ctx.user may be null
"none" — public endpoint; no auth needed; ctx.user is always null

Cron Trigger

Execute the function on a schedule.

{
  "trigger": {
    "type": "cron",
    "config": { "schedule": "0 9 * * *", "timezone": "UTC" }
  }
}

Uses standard 5-field cron expressions:

"*/5 * * * *" — every 5 minutes
"0 0 * * 0" — weekly, Sunday at midnight
"0 3 * * *" — daily at 3am UTC
"0 9 * * 1-5" — weekdays at 9am

Cron functions run as butterbase_service (RLS bypassed). ctx.user is always null.

WebSocket Trigger

Fire when a connected client sends a matching event over the realtime WebSocket.

{
  "trigger": {
    "type": "websocket",
    "config": { "event": "chat-message" }
  }
}

Fires when client sends matching event via realtime WebSocket connection. The request body contains the event payload sent by the client.

S3 Upload Trigger _(placeholder — not yet implemented)_

{
  "trigger": {
    "type": "s3_upload",
    "config": { "prefix": "uploads/", "contentTypes": ["image/*"] }
  }
}

3. Database Access

Use ctx.db.query(sql, params) for all database queries. Always use parameterized queries to prevent SQL injection — NEVER use string interpolation.

// Always use $1, $2 placeholders — never string interpolation
const { rows } = await ctx.db.query(
  'SELECT * FROM posts WHERE author_id = $1',
  [ctx.user.id]  // params array
);

SELECT

const { rows } = await ctx.db.query(
  'SELECT * FROM posts WHERE author_id = $1 AND published = true',
  [ctx.user.id]
);

INSERT

await ctx.db.query(
  'INSERT INTO logs (event, user_id) VALUES ($1, $2)',
  ['page_view', ctx.user.id]
);

UPDATE

await ctx.db.query(
  'UPDATE posts SET title = $1, updated_at = now() WHERE id = $2 AND author_id = $3',
  [newTitle, postId, ctx.user.id]
);

RLS Behavior by Invocation Type

| Invocation | Role | RLS |

|------------|------|-----|

| End-user JWT | butterbase_user | Enforced — ctx.db queries filtered by policies |

| API key (bb_sk_) | butterbase_service | Bypassed — sees all data |

| Cron trigger | butterbase_service | Bypassed — sees all data |

4. Environment Variables

Set at deploy time: pass envVars parameter to deploy_function
Update without redeploying: use update_function_env
Access in handler: ctx.env.VARIABLE_NAME
Encrypted at rest: values are never exposed in logs or API responses

Common uses: API keys, webhook secrets, external service URLs.

const apiKey = ctx.env.OPENAI_API_KEY;
const webhookSecret = ctx.env.WEBHOOK_SECRET;
const serviceUrl = ctx.env.EXTERNAL_SERVICE_URL;

5. Complete Working Examples

Example 1 — Protected API Endpoint (auth: required)

Returns the authenticated user's posts.

export async function handler(req, ctx) {
  const { rows } = await ctx.db.query(
    'SELECT id, title, created_at FROM posts WHERE author_id = $1 ORDER BY created_at DESC',
    [ctx.user.id]
  );
  return new Response(JSON.stringify(rows), {
    headers: { "Content-Type": "application/json" }
  });
}

Deploy:

deploy_function(
  app_id,
  name: "my-posts",
  code: ...,
  trigger: {
    type: "http",
    config: { method: "GET", path: "/my-posts", auth: "required" }
  }
)

Example 2 — Webhook Receiver (auth: none)

Accepts an incoming webhook, validates the signature, and stores the event.

export async function handler(req, ctx) {
  const body = await req.json();
  const signature = req.headers.get("x-webhook-signature");
  // Validate signature against ctx.env.WEBHOOK_SECRET
  await ctx.db.query(
    'INSERT INTO webhook_events (event_type, payload) VALUES ($1, $2)',
    [body.type, JSON.stringify(body)]
  );
  return new Response("ok", { status: 200 });
}

Deploy with: trigger: { type: "http", config: { method: "POST", path: "/webhook", auth: "none" } }

Example 3 — Cron Cleanup Job

Deletes expired sessions on a nightly schedule.

export async function handler(req, ctx) {
  const result = await ctx.db.query(
    "DELETE FROM sessions WHERE expires_at < now() RETURNING id"
  );
  return new Response(JSON.stringify({ deleted: result.rowCount }), {
    headers: { "Content-Type": "application/json" }
  });
}

Deploy with: trigger: { type: "cron", config: { schedule: "0 3 * * *" } }

Example 4 — External API Call

Proxies a request to an external AI service using a stored API key.

export async function handler(req, ctx) {
  const { prompt } = await req.json();
  const response = await fetch("https://api.openai.com/v1/chat/completions", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "Authorization": `Bearer ${ctx.env.OPENAI_API_KEY}`
    },
    body: JSON.stringify({
      model: "gpt-4",
      messages: [{ role: "user", content: prompt }]
    })
  });
  const data = await response.json();
  return new Response(JSON.stringify(data), {
    headers: { "Content-Type": "application/json" }
  });
}

Deploy with envVars: { OPENAI_API_KEY: "sk-..." } and trigger: { type: "http", config: { method: "POST", auth: "required" } }

Example 5 — Error Handling Pattern

Always wrap handler logic in try/catch and return a proper error Response.

export async function handler(req, ctx) {
  try {
    const { id } = await req.json();
    const { rows } = await ctx.db.query(
      'SELECT * FROM items WHERE id = $1',
      [id]
    );
    if (rows.length === 0) {
      return new Response(JSON.stringify({ error: "Not found" }), {
        status: 404,
        headers: { "Content-Type": "application/json" }
      });
    }
    return new Response(JSON.stringify(rows[0]), {
      status: 200,
      headers: { "Content-Type": "application/json" }
    });
  } catch (err) {
    return new Response(JSON.stringify({ error: "Internal server error" }), {
      status: 500,
      headers: { "Content-Type": "application/json" }
    });
  }
}

6. Testing & Debugging

The standalone tools deploy_function and invoke_function are unchanged. Everything else (logs, env updates, listing, deletion) is handled by manage_function with an action parameter.

Invoke a Function

invoke_function(
  app_id: "app_abc123",
  function_name: "my-function",
  method: "POST",
  body: { key: "value" }
)

Returns the full HTTP response (status, headers, body, duration_ms). Use this immediately after deploying to verify behavior.

View Error Logs

manage_function(
  app_id: "app_abc123",
  action: "get_logs",
  function_name: "my-function",
  level: "error"
)

Returns recent invocations with errors, stack traces, and captured console.log/warn/error output.

View All Logs

manage_function(
  app_id: "app_abc123",
  action: "get_logs",
  function_name: "my-function",
  limit: 100,
  since: "2026-01-15T00:00:00Z"
)

Filters: limit (default 100), since (ISO timestamp), level ("error" or "all").

List Functions & Metrics

manage_function(app_id: "app_abc123", action: "list")

Returns each function's name, trigger, URL, status, and metrics (invocationCount, errorRate, avgDuration, lastInvoked).

7. Common Mistakes

| Mistake | Fix |

|---------|-----|

| Returning plain object instead of Response | Always use new Response(JSON.stringify(data), { headers: {...} }) |

| SQL injection via string interpolation | Use parameterized queries: $1, $2 placeholders |

| Not wrapping in try/catch | Always catch errors and return a Response with error status |

| Forgetting async on handler | Handler must be async function handler(...) |

| Exceeding timeout (30s default) | Increase timeoutMs in deploy_function or optimize the function |

| Not setting Content-Type header | Always include "Content-Type": "application/json" for JSON responses |

8. Quick Reference

Deploy a Function

deploy_function(
  app_id: "app_abc123",
  name: "my-function",
  code: "export async function handler(req, ctx) { ... }",
  trigger: { type: "http", config: { method: "POST", auth: "required" } },
  envVars: { MY_SECRET: "value" },
  timeoutMs: 30000,       // default: 30s, max: 300s
  memoryLimitMb: 128      // default: 128MB
)

Update Env Vars (without redeploying)

manage_function(
  app_id: "app_abc123",
  action: "update_env",
  function_name: "my-function",
  env: { MY_SECRET: "new-value", DELETE_ME: null }   // null deletes the key
)

Delete a Function

manage_function(
  app_id: "app_abc123",
  action: "delete",
  function_name: "my-function"
)

Invocation URL Pattern

https://api.butterbase.ai/v1/{app_id}/fn/{function-name}

For HTTP triggers, this is the URL clients call directly.

If a docs/butterbase/00-state.md exists in the working directory, prefer invoking via /butterbase-skills:journey-functions so the journey orchestrator stays in sync.

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How to use it

Copy the folder

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