For a conventional API, create an HTTPS Cloud Function that validates each request, uses the Firebase Admin SDK to read or write Firestore, and returns a deliberate HTTP status and JSON response. Use a callable function instead when your caller is a Firebase app and you want Firebase’s client SDK to handle available authentication and App Check tokens. Use Firestore’s REST API when you need direct service-level access to Firestore rather than your own application endpoint.
The right choice depends on who calls the API and which authorization layer should govern access. The examples below show a protected HTTPS endpoint, explain the callable and direct-REST alternatives, and cover local testing and deployment.
Choose the Firebase API shape that fits the caller
Firebase offers several ways to expose or use an API; they are not interchangeable. Cloud Functions can respond to HTTPS requests as well as Firebase events. A function is a good fit when you need your own request and response contract, validation, business logic, or a server-side Firestore operation. Firebase’s official starter tutorial demonstrates the basic pattern with an addmessage endpoint that accepts text, writes a Firestore document, and responds to the caller.
| Approach | Caller and protocol | Authentication and authorization | Best fit |
|---|---|---|---|
| HTTPS Cloud Function | Ordinary HTTP clients, including non-Firebase clients | Your handler validates credentials and applies application authorization; server-side Admin SDK operations are privileged | A REST-style API with a contract you control |
| Callable Cloud Function | Firebase app using the Firebase client SDK’s callable protocol | When available, Firebase Authentication, FCM, and App Check tokens are included automatically; the callable trigger validates tokens and deserializes the request | A Firebase client that benefits from built-in token handling |
| Firestore REST API | HTTP requests to Firestore endpoints under https://firestore.googleapis.com/v1/ | Firebase ID-token requests are governed by Firestore Security Rules; service-account OAuth requests are governed by IAM | Direct Firestore access without a custom function endpoint |
For a user-facing client that should be constrained by database rules, use a Firebase ID token and Security Rules. For trusted server-to-server administration, use a service account and IAM, and keep its credentials on the server. A service-account identity is not the same as an authenticated end user.
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Set up a project and Functions workspace
You need a Firebase project with Firestore and Cloud Functions initialized. The Firebase CLI tutorial starts with these commands:
firebase login
firebase init firestore
firebase init functionsChoose JavaScript, TypeScript, or Python for Cloud Functions; those are the languages listed in Firebase’s Functions documentation. The example below uses JavaScript and the Functions SDK layout generated by the CLI. Run commands from the project directory initialized for Functions.
Build a protected HTTPS endpoint
This example accepts a JSON body containing text, verifies a Firebase ID token supplied as a bearer token, writes a document through the Admin SDK, then returns JSON. It rejects other HTTP methods, missing credentials, invalid tokens, and malformed input. The Admin SDK runs on the server; do not put privileged server code or service-account credentials in a browser or mobile client.
const functions = require("firebase-functions");
const admin = require("firebase-admin");
admin.initializeApp();
exports.addMessage = functions.https.onRequest(async (req, res) => {
if (req.method !== "POST") {
return res.status(405).json({ error: "Method not allowed" });
}
const authorization = req.get("authorization") || "";
const match = authorization.match(/^Bearers+(.+)$/i);
if (!match) {
return res.status(401).json({ error: "Bearer token required" });
}
let user;
try {
user = await admin.auth().verifyIdToken(match[1]);
} catch (error) {
return res.status(401).json({ error: "Invalid or expired token" });
}
const text = req.body && req.body.text;
if (typeof text !== "string" || text.trim().length === 0) {
return res.status(400).json({ error: "text must be a non-empty string" });
}
try {
const doc = await admin.firestore().collection("messages").add({
text: text.trim(),
uid: user.uid,
createdAt: admin.firestore.FieldValue.serverTimestamp()
});
return res.status(201).json({ id: doc.id, ok: true });
} catch (error) {
console.error("Could not create message", error);
return res.status(500).json({ error: "Could not create message" });
}
});The sample assumes the project’s Functions environment has the Firebase Functions and Admin SDK dependencies installed as part of CLI initialization. It accepts only a non-empty string, but production APIs should also impose an appropriate maximum length, validate every field and nested value, and authorize the requested operation for that specific user. Returning a generic server error avoids exposing internal exception details to callers; the server log retains diagnostic context.
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Call the endpoint from an HTTP client
Obtain a Firebase ID token through the Firebase Authentication client flow, then send it as a bearer token. Replace the endpoint URL with the deployed function URL and the token placeholder with a real, short-lived ID token. Do not place a service-account credential in this request.
curl -X POST "YOUR_DEPLOYED_FUNCTION_URL"
-H "Authorization: Bearer YOUR_FIREBASE_ID_TOKEN"
-H "Content-Type: application/json"
-d '{"text":"Hello from my app"}'A successful request returns HTTP 201 with a document ID and ok value. A missing or invalid token returns 401, a non-POST request returns 405, and invalid input returns 400. A Firestore failure returns 500; use the function logs to investigate rather than returning privileged details to the client.
When callable functions or direct REST are better
Callable functions for Firebase clients
Callables use the Firebase client SDK protocol, not an arbitrary REST endpoint contract. When available, Firebase Authentication, FCM, and App Check tokens are automatically included in callable requests. The callable trigger validates those tokens and deserializes the request body. This reduces protocol plumbing for a Firebase application, but a non-Firebase HTTP integration may be more naturally served by an HTTPS function.
Firestore REST for direct database operations
Use the Firestore REST API if a caller needs to access Firestore itself rather than invoke your own application logic. The REST guide places endpoints under https://firestore.googleapis.com/v1/. Authenticate a user-context request with a Firebase ID token so Firestore Security Rules apply. Authenticate server-level access with Google OAuth 2.0 using a service-account token, subject to IAM. Decide deliberately which identity and authorization model is appropriate; do not make privileged service access available to an untrusted client.
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Firebase Authentication also provides REST operations for creating users, signing in, and editing or deleting users. HTTPS is required for those operations. Authentication REST endpoints are distinct from a custom Cloud Function API and from Firestore’s data API.
Test locally before deploying
Use the Firebase Local Emulator Suite as an offline sandbox before touching production services. Firebase’s Functions tutorial uses it to test HTTP functions and Firestore-triggered functions. Exercise both normal requests and failure cases while connected to emulated services, and verify the authorization and data paths that matter to your application.
- Initialize Firestore and Functions with
firebase init firestoreandfirebase init functionsif you have not already done so. - Start the Emulator Suite with
firebase emulators:startfrom the initialized project directory. - Send the same HTTP request shape you plan to use in production to the local function endpoint shown by the emulator.
- Check success, malformed input, missing or invalid authentication, and Firestore failure behavior. Confirm that test writes go to the emulator rather than production.
Local testing is especially useful for authorization: verify that user requests can perform only the intended actions, and that server-side Admin SDK code is not being confused with requests governed by Firestore Security Rules.
Deploy and operate the API
Deploy Cloud Functions with the Firebase CLI. The official Firebase Functions tutorial states that deployment requires the Blaze pricing plan. That requirement is separate from the cost of your particular usage; check the current Firebase and Google Cloud billing details for your project before deploying.
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firebase deploy --only functionsAfter deployment, use the Google Cloud console to monitor logs and operational behavior. Cloud Functions manages instances and scales them with load, but that does not remove the need to observe errors, request volume, latency, or downstream Firestore behavior. Keep validation and authorization in the function, avoid unnecessary database operations, and make failure responses consistent so clients can distinguish bad requests from authentication and server failures.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshoot common API failures
| Symptom | Likely cause | What to check |
|---|---|---|
401 or UNAUTHENTICATED |
The request has no valid user token, or a token is expired or malformed | Send a Firebase ID token in the expected bearer format; do not substitute a service-account token for a user token |
PERMISSION_DENIED |
The identity lacks permission under Firestore Security Rules or IAM | Identify which authorization path the request uses. User-token requests use Security Rules; service-account OAuth requests use IAM |
400 or INVALID_ARGUMENT |
A required field is missing, malformed, or the wrong type | Check the JSON content type and body shape; validate input types and required fields before database work |
RESOURCE_EXHAUSTED |
A service limit or available resource has been exceeded | Inspect the error and project usage in the relevant Google Cloud or Firebase tooling; reduce request load or address the indicated quota/resource issue |
| Method not allowed | The caller used an unsupported HTTP method | Use POST for the sample and confirm the URL and client method |
| Function deploy fails | Functions deployment requirements are not met, including the project billing-plan requirement | Confirm the selected Firebase project and that it uses the Blaze plan required by the Firebase tutorial |
| Function returns 500 | The server-side operation failed | Inspect function logs for the logged error; check Firestore initialization and the operation’s data path |
Firestore REST documents the HTTP error classes PERMISSION_DENIED, UNAUTHENTICATED, INVALID_ARGUMENT, and RESOURCE_EXHAUSTED. A custom HTTPS function should return a similarly understandable, consistent error contract even though its own handler controls the response body.
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ScreenshotNeo is a separate website screenshot API and MCP server; it does not build, host, or authenticate a Firebase API. If your adjacent task is capturing a website rather than implementing Firebase endpoints, one GET request returns a screenshot or PDF. See the ScreenshotNeo API documentation.
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Frequently asked questions
Can an HTTPS function return something other than JSON?
Yes. An HTTPS handler controls its HTTP response, so the sample’s JSON response is an API design choice, not a requirement to return that format for every endpoint.
Does the sample create a Firestore collection in advance?
No manual collection setup appears in the example. It writes a document to the messages collection through the Admin SDK when a valid request succeeds.
Can I use Python instead of JavaScript for Cloud Functions?
Yes. Firebase’s current Functions documentation lists Python alongside JavaScript and TypeScript. The implementation shown here is JavaScript, so its code and dependency setup should not be copied unchanged into a Python function.
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Frequently Asked Questions
Can an HTTPS function return something other than JSON?
Yes. An HTTPS handler controls its HTTP response, so the sample’s JSON response is a design choice rather than a requirement.
Does the sample require creating the Firestore collection first?
No manual collection setup is shown; a successful write through the Admin SDK creates a document in the messages collection.
Can I use Python for Cloud Functions?
Yes. Firebase’s Functions documentation lists Python as supported, although the implementation here is JavaScript.
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