Neon Functions

FIRST: Use the parent neon skill for a Neon overview, getting started with Neon, Neon development best practices, and more.

If the neon skill is not installed, fetch it from https://neon.com/docs/ai/skills/neon/SKILL.md or install it with:

neon skills -s neon -y

Neon Functions

This is a public beta feature, currently available in us-east-2 and eu-central-1.

Neon Functions are long-running Node.js HTTP handlers deployed onto a Neon branch. Each function gets a public HTTPS URL, runs in the same region as your database, and — if the branch has Postgres — gets DATABASE_URL injected automatically. You deploy and manage them through the same Neon CLI, neon.ts, and API you already use.

Use this skill to help the user define, run locally, deploy, and manage functions next to their database. Deliver a deployed function with its invocation URL, a working local neon dev loop, or a precise answer from the official Neon docs.

When to Use

Reach for Neon Functions when the workload is a request/response handler that benefits from staying alive and staying close to the data:

• Long-running request/response flows that outlast lambda-style limits. Agents that make several LLM calls and tool invocations per request, or image/video generation, routinely blow past the ~10–60s execution caps and short streaming windows of traditional serverless functions. Neon Functions are long-running: the handler just needs to start responding within 15 minutes, and an open stream stays alive as long as bytes keep flowing. That's enough headroom for real agent workloads.
• Stateful streaming without bolting on Redis. Because a function stays alive across a request, it can host an SSE endpoint or a WebSocket server and hold the connection open in-process — no external state store (Redis, etc.) needed just to keep a stream coherent. Module-scope state (a pg pool, an in-memory counter) persists across requests on the same isolate.
• Compute that must sit next to Postgres. The function runs in the same region as the branch's database, so there are no cross-region round trips on every query. DATABASE_URL is injected for you.
• A backend that branches with your data. Each branch runs its own version of the function at its own URL, against its own isolated database (and storage, and gateway) state. Preview deployments, CI, and dev environments each get a self-contained backend — deploying to a child never affects the parent.
• Query Postgres from the Function (or an existing framework handler). Prefer that over the Data API. Use the Data API when the application already uses PostgREST or Supabase-js database calls, or is migrating that client.
• Webhooks, bots, and post-response work. Webhook handlers that fan out into multiple DB writes, Discord/WebSocket bots, and fire-and-forget follow-ups via waitUntil (analytics, audit logs) all fit.
• Recurring HTTP work. A Function Trigger POSTs to the function on a cron (type: "schedule"). Same fetch handler, same 15-minute time-to-first-byte limit. See Function Triggers.

If the workload is a pure static site, or something that must run outside the supported regions (us-east-2, eu-central-1) today, this isn't the right tool yet (see Timeouts and Runtime Limits and Availability).

What It Does

• Long-running & serverless — Built for WebSocket servers (see WebSocket Servers), SSE endpoints (see Server-Sent Events (SSE)), long agent HTTP streams, and APIs. Still scales to zero when idle.
• Web-standard handler — A function is any default export with a fetch(request) method returning a Response (Workers/WinterTC-compatible). A Hono app exports exactly that shape, so export default app just works. Runs on Node.js 24, so all Node APIs are available.
• Close to your database — Runs in the branch's region; DATABASE_URL injected automatically when the branch has Postgres.
• Branchable — Each branch runs its own function version at its own URL against its own isolated state.
• Same CLI/API — Deploy and manage via neon, neon.ts, or the Neon API.
• Function Triggers — Neon POSTs to the function on a cron. See Function Triggers.

Availability

Check this precondition before setting anything up: Neon Functions is a public beta feature currently available in us-east-2 and eu-central-1. Confirm the user's Neon project is in one of these regions. Functions usage isn't billed during the public beta.

Architecture: Where Functions Fit

Neon (Functions included) is backend primitives, not full-stack app hosting. Host your app on Vercel (or Netlify, or another frontend/app host); Functions are the long-running, stateful slice of your backend that lives next to your data. They compose with that platform in two ways:

• Add a Function to a full-stack app. Your Next.js / TanStack Start app on Vercel (or Netlify) owns UI, auth (e.g. Neon Auth), and talks directly to Lakebase Postgres and Object Storage. When one workload outgrows the host's short serverless limits — a WebSocket or SSE server, or a long-running agent that would time out — move just that piece onto a Neon Function. (See Functions as an Agent Backend for the client-direct pattern.)
• Run the whole backend control plane on Functions. Especially when the frontend is client-only — TanStack Router, React Router in client mode, and similar SPAs hosted on Vercel or Netlify — the client calls Functions directly. Build REST APIs and request/response agents, host MCP servers, and run anything stateful or that belongs close to Postgres and Object Storage.

Either way, secure a Function like any standalone REST API: verify a JWT or API key at the top of the handler (see the WARNING under Functions as an Agent Backend). Because a Function is just your backend, you can move pieces between your host and Neon — relocate an agent or a stateful WebSocket server onto a Function when it needs more runtime, and back if needed.

Prefer a Function, or an existing framework handler, that queries Postgres. Use Data API when the application already uses PostgREST/Supabase-js database calls or is migrating that client.

Setup

Functions are declared in neon.ts (see the neon skill for the branch-first workflow and neon.ts basics). Add @neon/config and declare functions under functions, keyed by slug:

// neon.ts
import { defineConfig } from "@neon/config/v1";

export default defineConfig({
  functions: {
    todos: {
      // slug: ^[a-z0-9]{1,20}$ — lowercase letters/digits, no hyphens
      name: "todo api", // display label only
      source: "src/index.ts", // entry file, relative to neon.ts
    },
  },
});

The slug is the function's permanent identity (it appears in the invocation URL and CLI commands) and can't be changed after the first deploy. Use name for a human-readable label.

A minimal function — a Hono app that queries the branch's Postgres via the injected DATABASE_URL:

// src/index.ts
import { Hono } from "hono";
import { drizzle } from "drizzle-orm/node-postgres";
import { Pool } from "pg";
import { parseEnv } from "@neon/env";
import { attachDatabasePool } from "@neon/functions";
import config from "../neon";
import { todos } from "./db/schema";

const env = parseEnv(config);
const pool = new Pool({ connectionString: env.postgres.databaseUrl, max: 5 });
attachDatabasePool(pool);
const db = drizzle(pool);

const app = new Hono();
app.get("/", (c) => c.text("Neon + Hono + Drizzle"));
app.post("/todos", async (c) => {
  const { text } = await c.req.json<{ text: string }>();
  const [row] = await db.insert(todos).values({ text }).returning();
  return c.json(row, 201);
});
app.get("/todos", async (c) => c.json(await db.select().from(todos)));

export default app;

Create the pg pool at module scope (reused across requests on the same isolate) and keep max small (e.g. 5), since each isolate keeps its own pool. Call attachDatabasePool(pool) so an idle disconnect is not an uncaughtException — see Connecting to Postgres.

parseEnv(config) requires every variable the config implies. A function that only talks to Postgres over the pooled URL can scope it to just that key — parseEnv then validates and returns only what you asked for (the keys autocomplete from your neon.ts):

const { postgres } = parseEnv(config, ["DATABASE_URL"]); // not the unpooled URL, auth, etc.
const pool = new Pool({ connectionString: postgres.databaseUrl, max: 5 });
attachDatabasePool(pool);

Develop Locally and Deploy

neon dev      # serves every function in neon.ts with hot reload; injects DATABASE_URL & friends
neon deploy --env <file>   # preferred full deploy from neon.ts; --env is the file Function env is read from

Keep .env or .env.local up to date with every key under functions.*.env. neon env pull writes Neon-managed vars only; add Function secrets to that file, then pass it as --env. neon deploy --env <file> loads that file into process.env each time, then uploads those values. A missing value is undefined and defineConfig throws. Omit the key from neon.ts if you do not want to write it. Never coerce a missing process.env value to an empty string (that uploads "" and deletes the live key). An empty assignment (KEY=) is also "". Use process.env.X! when TypeScript needs an assertion.

To deploy a single function without applying neon.ts: neon functions deploy <slug> --src src/index.ts (--src takes either the entry file or a directory containing index.ts, index.mjs, or index.js). That command's --env is KEY=VALUE (repeatable), not a file path. Use it for a targeted env update. Retrieve the public URL with neon functions get <slug> (the invocation_url field, of the form https://<branch_id>-<slug>.compute.<cell>.us-east-2.aws.neon.tech). Manage with neon functions list|get|delete.

When neon checkout creates a new branch and a neon.ts is present, it applies the policy automatically. Pass --env <file> on that create so Function env that reads process.env resolves (neon checkout feat --create --env .env.local). Existing process env wins over the file. Checking out an existing branch never reconciles it — apply config changes with neon deploy --env <file> (add --update-existing only after reviewing those changes).

Neon Infrastructure as Code (neon.ts)

The functions block from Setup is part of neon.ts, Neon's infrastructure-as-code file — one TypeScript file declares every function (its source, display name, and env) alongside any other branch services, in version control (see the neon skill for the full reference). Treat it like Terraform for your branch:

neon config status   # print the branch's live config (deployed functions)
neon config plan     # dry-run diff of what apply would change
neon config apply --env <file>  # bundle + deploy the declared functions  (neon deploy is an alias; pass --env when Function env reads process.env)

Functions are branch-scoped: each branch runs its own deployment at its own URL. When a neon.ts is present, neon checkout applies the policy as it creates a branch. Pass --env <file> on that create when Function env reads process.env. Checking out an existing branch doesn't redeploy — run neon deploy --env <file> to apply changes.

Per-branch deploy tuning (e.g. runtime) lives in the branch closure, keyed by slug, so it can vary by branch without changing which functions exist:

export default defineConfig({
  functions: { todos: { name: "todo api", source: "src/index.ts" } },
  branch: (branch) => ({
    functions: { todos: { runtime: "nodejs24" } },
  }),
});

Environment Variables

Neon injects branch-scoped connection strings and service URLs at runtime — you don't declare these or pass them at deploy time:

Object storage (AWS_*) and AI Gateway (NEON_AI_GATEWAY_*) vars are also injected when those services are declared — see the neon-object-storage and neon-ai-gateway skills.

neon env pull / neon-env run / neon dev emit NEON_BRANCH (and the connection strings) into your local dev environment too, so local runs mirror the deployed runtime.

Your own secrets are per-deployment. Preferred path: declare them in neon.ts and run neon deploy --env <file>. <file> is the gitignored file env pull already writes (.env if that file exists, otherwise .env.local). Env pull writes Neon-managed vars only; add Function secrets to that file. All declared Function env keys must be present. Omit a key from neon.ts if you do not want to write it. undefined means you asked to write the key and the value is missing (defineConfig throws). Never coerce a missing process.env value to an empty string: that uploads "" and deletes the live key. An empty assignment in the file (KEY=) is also "". If TypeScript needs an assertion, use process.env.X! and make sure the file has the value:

functions: {
  todos: {
    name: "todo api",
    source: "src/index.ts",
    env: { RESEND_API_KEY: process.env.RESEND_API_KEY! },
  },
}

neon functions deploy --env KEY=VALUE is the manual path (repeatable; --env KEY= deletes a key; unmentioned keys carry over). Use it for a targeted env update, not a full neon.ts apply.

Load Function secrets into the same file env pull wrote, then neon deploy --env <file>. Pull the branch's Neon-managed vars onto disk for local dev with neon env pull (link/checkout do this automatically; pass --no-env-pull to skip and use neon-env run -- <cmd> for runtime injection). Limits: ≤1,000 vars, ≤64 KiB total, and the NEON_ prefix is reserved.

Connecting to Postgres

When the branch has Postgres, Neon injects the connection strings at runtime — you don't declare them, pass them at deploy time, or hardcode anything. The two you'll use:

• DATABASE_URL — pooled connection string (routed through Neon's connection pooler). Use it for normal request/response query traffic. Kept un-prefixed because every Postgres ORM (Drizzle, Prisma, Knex, …) reads DATABASE_URL by default.
• DATABASE_URL_UNPOOLED — direct connection string to the same database. Use it for migrations, LISTEN/NOTIFY, and long multi-statement transactions.

Use Drizzle (or another ORM) on top of node-postgres (pg) for queries and schema management — not Neon's serverless driver. Functions are long-running and reuse an isolate across many requests, so a persistent pg pool is the right fit; the serverless driver's HTTP transport is meant for fully isolated, lambda-style runtimes.

Create the connection pool once at module scope and reuse it across requests — don't open a connection per request:

import { attachDatabasePool } from "@neon/functions";
import { drizzle } from "drizzle-orm/node-postgres";
import { Pool } from "pg";

const pool = new Pool({ connectionString: process.env.DATABASE_URL, max: 5 });
attachDatabasePool(pool);
const db = drizzle(pool);

node-postgres emits idle-client failures as error on the pool. With no listener that is an uncaughtException and Node exits the isolate. Call attachDatabasePool(pool) once after new Pool. Requires @neon/functions ≥ 0.8.0. Expected idle disconnects (ECONNRESET, EPIPE, ETIMEDOUT, Postgres 57P01, node-postgres's Connection terminated unexpectedly) are silent. Anything else is console.error, or onUnexpectedError if you pass it on the first call. The first call wins; a later call that passes onUnexpectedError is ignored and warns. This does not close the pool.

Pooling is recommended because an isolate is reused across many requests (and several requests can be in flight on the same isolate at once — see Timeouts and Runtime Limits). A module-scope pool is opened once on cold start and then shared by every subsequent request that isolate serves, so you amortize connection setup instead of paying it on every request and you avoid exhausting Postgres connections under load.

Keep max small (e.g. 5): each isolate keeps its own pool, so total connections to Postgres scale with the number of live isolates. You don't need to close the pool on shutdown — when the runtime evicts an isolate it sends SIGINT/SIGTERM, and Neon's pooler reclaims those connections for you, so an explicit drain handler is redundant.

Reading process.env.DATABASE_URL directly works everywhere. The function in Setup instead uses @neon/env's parseEnv(config) to read the same value in a typed, validated way — either is fine.

Timeouts and Runtime Limits

Functions are long-running but still serverless — they are a request/response runtime, not a background job runner. The hard limits:

• Time to first byte: 15 minutes. Your handler must begin returning a response within 15 minutes of receiving a request. Most handlers finish in seconds; the 15-minute ceiling exists so agent workloads like image/video generation have room.
• Heartbeat: 15 minutes. Open WebSocket/SSE connections stay alive as long as data flows. The timeout only fires when a connection goes silent — send at least one byte every 15 minutes to keep a quiet stream alive.
• waitUntil: 15 minutes. Work registered with waitUntil (from @neon/functions) keeps the invocation alive after the response is sent, up to 15 minutes — for cleanup like analytics writes and audit logs, not a background job runner. Off the Neon runtime (local neon dev, tests) it's a no-op: the promise still runs but isn't tracked.
• Idle eviction. With no active connections Neon shuts the function down; it may also evict/restart for operational reasons — e.g. maintenance, or moving the function to a different compute node (active functions can run for hours first). Treat eviction like a process restart — WebSocket/SSE clients must reconnect. Neon sends SIGINT before evicting, so a process.on("SIGINT", ...) handler lets you detect that the function is about to be evicted and run any last-minute cleanup. You don't need one just to close Postgres connections — Neon's pooler reclaims those on its own.
• Runtime: Node.js 24, memory fixed at 2048 MiB during the preview. Slugs must match ^[a-z0-9]{1,20}$. An isolate is reused across many requests — multiple requests can be in flight on the same isolate at once (interleaved on Node's single-threaded event loop), and under load the runtime runs several isolates in parallel, each with its own copy of module state. State held in module scope is therefore per-isolate (shared by every request that isolate handles) and in-memory only — persist anything that must survive eviction in Postgres. This reuse is exactly why you create a connection pool once at module scope rather than per request (see Connecting to Postgres).

Functions as an Agent Backend (Next.js and Similar Frameworks)

A Neon Function is a great home for an AI agent precisely because it doesn't time out the way lambda-style serverless does (15-minute budget, see Timeouts and Runtime Limits). But that advantage disappears the moment you proxy the agent stream through your web app's backend — a Next.js route handler, Remix/SvelteKit/Nuxt action, etc. hosted on Vercel, Netlify, Cloudflare, and the like. Those platforms cap serverless/edge execution at short windows (often ~10–60s, sometimes up to ~300s), so a long agent or image/video generation stream gets cut off mid-response even though the Neon Function would happily keep going.

Building the agent itself. The Vercel AI SDK and Mastra are the recommended ways to build the agent — point either at the Neon AI Gateway (see the neon-ai-gateway skill) for one credential across every model, with no extra provider keys. For a complete AI SDK agent running as a Function (streaming toUIMessageStreamResponse, multi-step tool calling next to Postgres, and persisting generated images to Object Storage), see references/ai-sdk.md; for the Mastra equivalent with built-in tracing, see references/mastra-studio.md.

The fix: call the function directly from the client. Don't route the long request through your app server.

Browser ──(Authorization: Bearer <JWT>)──▶  Neon Function (agent)   ✅ no host timeout
Browser ──▶ your app backend ──▶ Neon Function                       ❌ host cuts the stream

• Mint a short-lived JWT on your app backend (e.g. better-auth's jwt plugin, NextAuth, or your own signer) — that call is fast and well within host limits.
• Hand the token to the client and have it call the Neon Function directly (cross-origin), e.g. with the Vercel AI SDK: new DefaultChatTransport({ api: NEON_FUNCTION_URL, fetch }) where fetch attaches Authorization: Bearer <token>. Your app server is never in the path of the long stream.
• Add CORS so the browser can reach it (handle OPTIONS, set Access-Control-Allow-Origin/-Headers).

[!WARNING] A Neon Function has a public HTTPS URL — it is reachable by anyone. A direct client→function call means there is no app backend in front of it to gate access, so you must authenticate the function yourself. Verify a JWT (e.g. against your app's JWKS), check a shared secret / API key, or validate a session token at the top of the handler and reject anything else. Never deploy an unauthenticated agent.

// src/index.ts — verify the caller before doing any work
import { createRemoteJWKSet, jwtVerify } from "jose";

const jwks = createRemoteJWKSet(
  new URL(`${process.env.AUTH_BASE_URL}/api/auth/jwks`),
);

export default {
  async fetch(request: Request) {
    if (request.method === "OPTIONS")
      return new Response(null, { status: 204, headers: cors(request) });

    const auth = request.headers.get("authorization");
    if (!auth?.toLowerCase().startsWith("bearer ")) {
      return new Response("Unauthorized", {
        status: 401,
        headers: cors(request),
      });
    }
    try {
      const { payload } = await jwtVerify(auth.slice(7), jwks, {
        issuer: process.env.AUTH_BASE_URL,
        audience: process.env.AUTH_BASE_URL,
      });
      const userId = payload.sub; // scope the agent to this user
      // ... run the agent, return result.toUIMessageStreamResponse({ headers: cors(request) })
    } catch {
      return new Response("Unauthorized", {
        status: 401,
        headers: cors(request),
      });
    }
  },
};

Pass the JWKS/issuer URL to the function via its env (see Environment Variables). Persist anything you need to keep (generated images, history) in Postgres — module state doesn't survive eviction.

WebSocket Servers

A WebSocket server is the canonical Functions workload: a long-running handler holds connections open in-process, with no external state store needed to keep a stream coherent. The connection stays alive as long as bytes flow (15-minute heartbeat, see Timeouts).

Upgrade from inside fetch. Call upgradeWebSocket(request) from @neon/functions and return the response it gives you. Hono apps use the same primitive via @neon/functions/hono (see Hono below). There is one entrypoint and no WebSocket dependency to install:

import { upgradeWebSocket } from "@neon/functions";

export default {
  async fetch(req: Request): Promise<Response> {
    if (req.headers.get("upgrade")?.toLowerCase() !== "websocket") {
      return new Response("expected a websocket upgrade", { status: 426 });
    }

    const { socket, response } = upgradeWebSocket(req);
    socket.addEventListener("message", (event) => socket.send(event.data));
    return response;
  },
};

socket is a standard WebSocket, so addEventListener and the onopen/onmessage/onclose/onerror properties both work. It is still CONNECTING when you get it — the runtime writes the 101 only once your handler returns response, and the socket opens then.

Three rules that matter:

• Return response unchanged. A 101 can't be built as a plain Response (the fetch spec caps constructed responses at 200–599), so the runtime hands back an object carrying the pending upgrade. clone(), or rebuilding it with new Response(res.body, res) as response-rewriting middleware does, discards the upgrade and fails the request.
• Refuse a handshake by returning an ordinary Response. Return a 401, 403, or 404 from fetch, before you upgrade, to gate a socket. A browser client can't read why a handshake was refused; it sees only a generic connection failure, not your status or body. Refuse to keep clients out, but send any detail the client needs over a separate authenticated request.
• binaryType defaults to "arraybuffer", not the browser's "blob". event.data is a string for text frames and an ArrayBuffer for binary ones, so branch on typeof.

With auth. Browsers can't set headers on a WebSocket, so authenticate with a ?token= query param (verify it the same way as the agent backend: jwtVerify against your JWKS) and refuse before upgrading:

// src/index.ts
import { upgradeWebSocket } from "@neon/functions";

const clients = new Set<WebSocket>();

export default {
  async fetch(request: Request): Promise<Response> {
    if (request.headers.get("upgrade")?.toLowerCase() !== "websocket") {
      return new Response("WebSocket endpoint — connect with ?token=<jwt>");
    }

    const url = new URL(request.url);
    const identity = await verifyToken(url.searchParams.get("token"));
    if (!identity) return new Response("unauthorized", { status: 401 });

    const { socket, response } = upgradeWebSocket(request);
    clients.add(socket);
    socket.addEventListener("close", () => clients.delete(socket));
    socket.addEventListener("message", (event) => {
      if (typeof event.data !== "string") return;
      persist(identity.id, event.data); // fan out to every isolate — see below
    });
    return response;
  },
};

Subprotocols. Pass { protocol } to select one the client offered; it is echoed in Sec-WebSocket-Protocol and exposed as socket.protocol. Selecting one the client did not offer throws a TypeError. Omit it and no protocol is negotiated. No extensions are negotiated either — socket.extensions is always "" and permessage-deflate is not available.

Hono. Use upgradeWebSocket from @neon/functions/hono — the same primitive as Hono's own WebSocket helper, with no ws dependency and not the deprecated @hono/node-ws. Auth is ordinary middleware; gate upgrade requests before next():

// src/index.ts
import { Hono } from "hono";
import { upgradeWebSocket } from "@neon/functions/hono";

const clients = new Set<WebSocket>();

const app = new Hono<{ Variables: { userId: string } }>();

app.use("/ws", async (c, next) => {
  const identity = await verifyToken(c.req.query("token"));
  if (!identity) return c.text("Unauthorized", 401);
  c.set("userId", identity.id);
  await next();
});

app.get(
  "/ws",
  upgradeWebSocket((c) => ({
    onOpen(_event, ws) {
      clients.add(ws.raw);
      ws.send("welcome");
    },
    onClose(_event, ws) {
      clients.delete(ws.raw);
    },
    onMessage(event, ws) {
      ws.send(`echo: ${event.data}`);
    },
  })),
);

export default app;

Connect from the browser with the function's wss:// URL (from neon functions get <slug>), for example new WebSocket("wss://<branch>-<slug>.compute.<region>.aws.neon.tech/ws?token=<jwt>"). Reconnect on close — isolates are evictable and idle connections may be terminated after 15 minutes.

Do not put cors() on the upgrade route, and do not read c.res before await next() or call c.header() after it — both rebuild the 101 and break the upgrade. See @neon/functions README for the full middleware table.

Heartbeat (keep the socket alive)

A connection stays open only while bytes flow: Neon evicts a silent stream after 15 minutes (Timeouts and Runtime Limits), and intermediary proxies / load balancers are usually far stricter (often tens of seconds). Don't rely on the app being chatty enough — send a periodic keepalive from the server so the socket never goes quiet.

The standard WebSocket interface has no ping(), so send an application-level message the client filters out:

const HEARTBEAT_MS = 25_000; // comfortably under proxy idle timeouts

const beat = setInterval(() => {
  for (const socket of clients) {
    if (socket.readyState === socket.OPEN) socket.send('{"type":"ping"}');
  }
}, HEARTBEAT_MS);
beat.unref?.();

The client skips these when handling messages. There is no protocol-level shortcut here: the standard WebSocket from upgradeWebSocket has no ping(), and a browser can't send ping frames from JavaScript, so an application-level message is the only keepalive a browser client can use. (A Node ws client can send ping frames, and the server auto-replies with a pong, but a browser can't.)

Keeping clients in sync across isolates (do not skip this)

Under load the runtime runs several isolates in parallel, each with its own copy of module state — so each isolate has its own clients set. Broadcasting only to that local set means a client on isolate A never sees an event produced on isolate B, and the feed silently fractures. It's easy to miss: neon dev runs a single process (one isolate), so in-process broadcast always looks fine locally but breaks in production, where concurrent connections spread across many isolates.

Module state doesn't survive eviction anyway, so Postgres is the shared source of truth. Pick a fan-out strategy. In every snippet below, pool is a pooled pg client and clients is this isolate's Set of live connections.

1. Poll Postgres — the default, and the only option that keeps Scale to Zero. Each isolate re-reads the shared state (or rows past a cursor) on a short interval and pushes changes to its own clients. One query per isolate per tick (not per client), and none when the isolate has no clients — so an idle compute still suspends.

let lastId = "0"; // bigint id, so a string
let polling = false;

async function poll() {
  if (polling || clients.size === 0) return; // guard overlap; no clients → no query → compute can scale to zero
  polling = true;
  try {
    const { rows } = await pool.query(
      "SELECT id, payload FROM events WHERE id > $1 ORDER BY id",
      [lastId],
    );
    for (const { id, payload } of rows) {
      lastId = id;
      for (const socket of clients) {
        if (socket.readyState === socket.OPEN) socket.send(payload);
      }
    }
  } catch (err) {
    console.error("[poll]", err);
  } finally {
    polling = false;
  }
}

// Seed from the latest id so a fresh isolate sends only new rows, not the whole table, then poll.
pool
  .query("SELECT coalesce(max(id), 0)::text AS id FROM events")
  .then((seed) => {
    lastId = seed.rows[0].id;
  })
  .catch((err) => console.error("[seed]", err))
  .finally(() => setInterval(poll, 1000).unref?.());

• Latency: up to the interval (~1s) — fine for counters, chat, and dashboards.
• Scaling: database load grows with the number of live isolates, not clients. Keep the cursor on an indexed serial/bigserial PK and the interval sane.
• Scale to Zero: ✅ preserved — polling stops when no clients are connected, so the compute suspends on its normal timer.
• Ordering: WHERE id > cursor can skip a row that commits out of sequence: a transaction that took a lower id but commits after a higher one is already behind the cursor, so the poll never returns it. For a broadcast feed occasional loss is usually fine; when you need every row, use LISTEN/NOTIFY or poll by created_at with a small overlap window and dedupe by id.

2. LISTEN/NOTIFY — lowest latency, but requires disabling Scale to Zero. Each isolate LISTENs on a channel over a dedicated unpooled connection; broadcasting is NOTIFY, so every isolate (including the sender's) re-pushes to its sockets. Near-instant — but the listener holds an idle connection that does not count as active, so Scale to Zero suspends the compute and drops it, silently killing the feed. Only use it on an always-on compute (Scale to Zero disabled — a paid-plan setting).

import { attachDatabasePool } from "@neon/functions";
import { Pool, Client } from "pg";

const pool = new Pool({ connectionString: process.env.DATABASE_URL, max: 5 });
attachDatabasePool(pool);
const CHANNEL = "chat_events";

// One dedicated DIRECT connection per isolate, just to receive events.
// Use DATABASE_URL_UNPOOLED — LISTEN needs a real session, not a pooled one.
// Don't call attachDatabasePool here: it would silence the idle drop that killed the feed.
// The error listener keeps the process alive; reconnect the client on error in production (omitted here).
const listener = new Client({
  connectionString: process.env.DATABASE_URL_UNPOOLED,
});
listener.on("error", (err) => {
  console.error(err);
});
listener.connect().then(() => listener.query(`LISTEN ${CHANNEL}`));
listener.on("notification", (msg) => {
  if (!msg.payload) return;
  for (const socket of clients) {
    if (socket.readyState === socket.OPEN) socket.send(msg.payload);
  }
});

// Broadcast by NOTIFYing through the pool — every isolate's listener fires.
function broadcast(event: unknown) {
  return pool.query("SELECT pg_notify($1, $2)", [
    CHANNEL,
    JSON.stringify(event),
  ]);
}

3. External pub/sub (e.g. Upstash Redis) — best at scale. For high fan-out, sub-second latency at large connection counts, or multi-region, publish/subscribe through a dedicated broker. Highest throughput, and it doesn't touch Postgres or block Scale to Zero — at the cost of another service to run.

Rule of thumb: start with polling (works with Scale to Zero, no extra infra); switch to LISTEN/NOTIFY only on always-on compute that needs sub-second latency; move to Redis when fan-out outgrows Postgres.

Client must reconnect

Idle functions are evicted (and isolates restart for operational reasons), so a client's socket will drop — treat reconnection as normal, not exceptional. Reconnect with exponential backoff, capped, and re-mint a fresh token on every attempt (tokens are short-lived, so a stale one fails the upgrade auth check):

let closed = false,
  retry = 0,
  timer: ReturnType<typeof setTimeout>;

async function connect() {
  if (closed) return;
  const token = await getToken(); // re-mint each attempt; short-lived
  const ws = new WebSocket(`${WS_URL}?token=${encodeURIComponent(token)}`);
  ws.onopen = () => {
    retry = 0; // reset backoff on success
  };
  ws.onmessage = (e) => {
    /* apply the event */
  };
  ws.onclose = () => {
    if (!closed)
      timer = setTimeout(connect, Math.min(1000 * 2 ** retry++, 15000));
  };
  ws.onerror = () => ws.close(); // let onclose drive the retry
}
connect();

Together — upgradeWebSocket inside fetch, JWT auth over ?token=, cross-isolate fan-out, and client backoff — these compose into a complete realtime chat backend on a single function.

Server-Sent Events (SSE)

When you only need server → client streaming (live counters, notifications, progress, token streams), SSE is simpler than a WebSocket and needs no upgrade at all: a plain fetch handler returns a Response whose body is a ReadableStream with Content-Type: text/event-stream, and the runtime holds it open as long as bytes flow. The browser consumes it with EventSource, which reconnects on its own — so there's no client backoff to write.

// src/index.ts — minimal SSE endpoint
const encoder = new TextEncoder();
export default {
  fetch: () => {
    let t: ReturnType<typeof setInterval>;
    return new Response(
      new ReadableStream<Uint8Array>({
        start(controller) {
          controller.enqueue(encoder.encode("data: hello\n\n"));
          t = setInterval(
            () => controller.enqueue(encoder.encode(": ping\n\n")),
            25_000,
          );
        },
        cancel() {
          clearInterval(t); // fires when the client disconnects
        },
      }),
      {
        headers: {
          "Content-Type": "text/event-stream",
          "Cache-Control": "no-cache, no-transform",
        },
      },
    );
  },
};

The same rules as WebSockets apply. Heartbeat: a stream stays open only while bytes flow — Neon's window is 15 minutes (Timeouts and Runtime Limits) but proxies are usually far stricter, so emit a : ping\n\n comment every ~25–30s (shown above) to keep idle streams from being dropped. Keep state in Postgres, and fan out across isolates using one of the sync strategies (hold a Set of stream controllers and enqueue to each). EventSource is GET-only and can't set headers, so authenticate with a ?token= query param or cookie, exactly like the WebSocket case. references/sse.md has the full pattern — Hono variant, cross-isolate fan-out, wire format, client, and caveats.

Function Triggers

A Function Trigger POSTs JSON to your function on a cron. Declare it in neon.ts, apply with neon deploy, and authenticate the delivery with parseTrigger (Hono) or parseTriggerInvocation (a fetch handler). The only trigger type today is schedule. Prefer neon.ts; CLI and the Neon MCP trigger tools (list_triggers, create_trigger, …) are the backup.

Full field list, CLI, MCP, payload, inheritance, and both handler shapes: references/function-triggers.md.

MCP Servers

An MCP server is a natural Functions workload: a long-running HTTP handler that exposes tools to AI clients (Cursor, Claude

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