How to Build E-Signatures Into Mobile Apps You Make With a0.dev

You built a mobile app with a0.dev and now you need real signatures inside it. A client has to sign a service agreement before onboarding, a parent has to approve a consent form, a gig worker has to accept a contract before their first shift. You don't want to bounce people out to a separate signing portal and you definitely don't want to wait weeks wiring up an enterprise e-signature platform. The good news is that adding legally binding e-signatures to an a0.dev app is something you can ship in an afternoon, and it costs about 3 cents per signature instead of the per-seat pricing most providers lock you into.

This guide walks through how Firma.dev fits into the React Native and Expo apps that a0.dev generates, what it costs, and how a0.dev's own AI chat can wire most of it up for you. The full implementation, with code, lives in the official a0.dev integration guide. This piece is the why and the what before you open the editor.

Building e-signatures into an a0.dev mobile app

a0.dev generates Expo and React Native apps, so the signing experience needs to feel native rather than like a tacked-on web page. Mobile products tend to need signatures in very specific moments: a customer accepting terms during signup, a contractor signing a statement of work, a tenant confirming a rental agreement, a patient giving consent before a telehealth visit. These are short, high-intent moments where dropping someone into an email thread or a third-party website kills the flow and loses you the signature.

Firma.dev solves this by letting the document get signed inside your app, in a screen that looks and behaves like the rest of your product. The signer never leaves. They tap, they sign, they're done, and your backend gets told the moment it happens. For a founder, the thing that matters is the outcome: you keep people inside the experience you built, and the legal weight of the signature is the same as any compliant e-signature solution.

The main path: Firma.dev inside the app you are building

Here is the shape of it without any code. Firma.dev is an API. Your app's backend calls that API to create a signing request from a template you set up once, like a consulting agreement or a consent form. Firma.dev returns a signing link. Your app opens that link in a native WebView, which is just a signing screen that lives inside your app rather than in a separate browser. The signer completes the document there, and your backend can react to the result.

The reason the call goes through your backend and not straight from the phone is security. Your API key has to stay on a server you control, never inside the app code that ships to a user's device. If you don't already have a backend, a small Express server or a Supabase Edge Function is enough, and the integration guide shows exactly how to set one up. There's also a lighter deep-link option for very simple flows that opens the signing page in the system browser, but for a real product the in-app WebView is the one you want.

What you need to have ready is short: a Firma.dev account and API key, an a0.dev project, the react-native-webview package added to it, and at least one template with your signing fields configured. That's the whole list.

The alternative: let a0.dev's AI wire it for you

a0.dev builds apps through an AI chat, and that same chat can do most of the integration work if you point it at accurate documentation. Firma.dev publishes a Docs MCP server, which is a way for AI tools to read the real API reference instead of guessing at endpoints and parameters. Connect it, and you can describe what you want in plain language, something like asking the chat to call your backend when a user taps a Send Contract button and then show the Firma.dev signing screen, and it will assemble the integration against the correct API details.

This is the secondary route, not the main event. It's genuinely useful for getting a first version stood up fast, but you still own the architecture decisions, especially keeping your API key on the backend. Treat the AI as a fast pair of hands, not a reason to skip understanding where your secrets live.

Why this is cheaper than the alternative

Firma.dev is pay-as-you-go at €0.029 per envelope, roughly 3 cents USD per signature. There are no upfront costs, no monthly minimums, and no annual contract to negotiate. You pay for the signatures you actually send and nothing else.

That pricing model matters most when you're early. Traditional e-signature platforms sell seats and tiers, which means you commit to a monthly bill before you've sent a single document, and the per-signature cost balloons once you go past your plan's envelope cap. A mobile app that sends a few hundred signatures one month and a few thousand the next fits pay-as-you-go far better than a fixed plan. You can model your cost as a clean line item: number of signatures times 3 cents. For a lot of teams that works out many times cheaper than the seat-based alternatives, which is the difference between e-signing being a rounding error and being a budget conversation.

Built for products with many customers

If your a0.dev app serves multiple business customers, Firma.dev's Customer Workspaces give each of them a private, partitioned space. Templates and signature usage stay isolated per customer, so one client's documents and volume never bleed into another's. You get clean separation without having to build any of that partitioning logic yourself, which is the kind of thing that quietly eats weeks if you try to roll it on your own.

Compliance, briefly

Firma.dev is designed to support the major electronic signature frameworks. In the US that means the ESIGN Act and UETA. In the EU and UK it aligns with eIDAS for simple and advanced electronic signatures, and it's built with GDPR in mind. For most mobile app use cases, a service agreement, a consent form, a contractor agreement, this is the level of assurance you need, and a Firma.dev signature carries the same legal standing as one collected through any other compliant provider. If you have a specific regulatory requirement, check it against your own legal guidance, but the common cases are well covered.

Get started

Adding e-signatures to an a0.dev app comes down to four things you already have or can set up in minutes: an account, a template, a small backend endpoint, and a WebView screen. The a0.dev integration guide has the full code for both the backend and the app side, and a0.dev's AI chat can handle a good chunk of the wiring once you connect the Docs MCP.

Get started with Firma.dev for free, no credit card required.