---
title: "Test sign-up emails in apps built with Qoder · Proxy Mail QA"
description: "Check the real sign-up, login and reset emails after Qoder's agent changes your auth code, or let it check them itself through the Proxy Mail QA MCP server."
url: https://proxymail.qa/built-with/qoder
---

1. [Home](https://proxymail.qa/)
2. [AI app builders](https://proxymail.qa/built-with)

# Testing auth emails in apps you build with Qoder

Qoder's agent can plan and build sign-up, login and password reset across your codebase. Whichever auth service it uses, the emails only prove themselves when a real one lands in a real inbox, so check them after every change that touches auth.

Updated October 17, 2026

## Why the email path needs a real check

Qoder’s agent is good at the code around authentication: forms, routes, token handling and email templates. But a change can pass every test and still send an email that links to `localhost`, never arrives because the sender isn’t set up, or contains a code that expires before anyone could type it. Those show up only when a real email reaches a real inbox.

## Which login will it build?

Whichever you ask for, or the one your codebase already uses. Supabase Auth, Clerk, Firebase Authentication, Auth.js and Better Auth each send different emails: Clerk sends a code by default, Firebase a link, and Auth.js only magic links unless you build the rest. Name the methods you want in your request, then test each email the agent’s change sends. [Which emails each login service sends](https://proxymail.qa/built-with/login-methods#login-services-coding-agents-use).

## Check it after every auth change

1. **Run the app** locally, or open the preview deployment for your branch.
2. **Create an address** in Proxy Mail QA, one per scenario: new user, invited user, password reset.
3. **Go through the flow** the change touched, using the code or link from the address’s row.
4. **Read the whole email once** in the inbox view: sender, subject, wording and every link.
5. **Note what you found** on the address, so it stays with the test account.

Links to `localhost` on a preview or staging deployment are flagged on the row. [Here’s how to fix them](https://proxymail.qa/solve/localhost-links) in Supabase, Firebase, Auth.js and other stacks.

## Let Qoder’s agent check the email itself

Add the Proxy Mail QA MCP server: open **Qoder IDE Settings**, choose **MCP**, then **\+ Add** on the **My Servers** tab, and add it under `mcpServers`:

Qoder MCP settings

```json
{
  "mcpServers": {
    "proxymail-qa": {
      "type": "http",
      "url": "https://api.proxymail.qa/mcp"
    }
  }
}

```

Save it, and sign in to Proxy Mail QA in your browser when asked, choosing the project the agent may use. Set Qoder’s permission level to **Ask every time**, not Auto approve, and approve each Proxy Mail QA step when Qoder asks: under Auto approve, the agent may not find the tools and suggest the marketplace instead. Then ask in plain words: “Sign up on localhost:3000 with a new Proxy Mail QA address and confirm the email.” The agent creates the address, fills in your form, waits for the code or link and finishes the sign-up. Other ways to connect, including an API key, are in [MCP server setup](https://proxymail.qa/docs/mcp).

## Copy-paste prompts for Qoder

Prompt

```text
List every email this app sends: sign-up confirmation, magic link, password reset, invite and email change. For each, show where it's triggered in the code, which service sends it, how its link URL is built, and how long its code or link is valid. Flag hard-coded localhost URLs.

```

Prompt

```text
Build every link in auth emails from an environment variable for the app's public URL, with localhost only as the development default. Add the variable to .env.example and the deployment settings.

```

Prompt

```text
The email says the code is valid for 10 minutes. Make the server enforce the same expiry, and make requesting a new code invalidate the old one.

```

## A rule that prevents the usual bugs

Give the agent this rule, in your project’s rules or at the start of an auth task, so it follows it whenever it touches auth:

Prompt

```text
When changing sign-up, login, password reset, invites or any email-sending code:
- Build links from the app's public URL setting, never a hard-coded localhost.
- Keep the expiry stated in an email equal to the one the server enforces.
- Make single-use links and codes stop working once used or replaced.
- Tell me which emails to test by hand after the change.

```

## Try it on your next sign-up test

Free plan, no credit card. Create an address and watch the code or link arrive.

[Create a free account](https://app.proxymail.qa/?mode=signup&from=qoder)

## Questions

### Which auth services does this work with?

Any that send email: Supabase Auth, Firebase Authentication, Clerk, Auth0, Auth.js with an email provider, or your own code sending through Resend, Postmark or SendGrid. Proxy Mail QA receives the email like any inbox, as long as the provider can send to any address: one in sandbox or test mode, such as Amazon SES before production access or Resend without a verified domain, only sends to addresses you've verified.

### Can Qoder's agent read the code from the email itself?

Yes. Add the Proxy Mail QA MCP server in Qoder's MCP settings and set the permission level to Ask every time, and the agent can create an address, sign up and wait for the code or link. It gets only the code or link, never the email's text. Setup is at proxymail.qa/docs/mcp.

### Do I need to change my app to test with Proxy Mail QA?

No. Sign up with a Proxy Mail QA address like any user would.

## Related guides

- [LovableWhich Lovable Cloud logins send email, the Site URL and sender settings, and a check before you launch.](https://proxymail.qa/built-with/lovable)
- [ReplitReplit Auth, Clerk Auth or your own: which send email, and how to test each on the published app.](https://proxymail.qa/built-with/replit)
- [BoltTurn on email confirmation, fix the localhost Site URL and test every auth email before you publish.](https://proxymail.qa/built-with/bolt)
- [Base44Test the sign-up code, password reset and invites, and every user role, with a test account per role.](https://proxymail.qa/built-with/base44)
- [v0Set Supabase’s Site URL and redirects for Vercel deployments, or move Clerk to production, then test every login email.](https://proxymail.qa/built-with/v0)
- [Claude CodeCheck the auth emails in apps you build with Anthropic’s coding agent, from localhost to staging.](https://proxymail.qa/built-with/claude-code)
- [OpenAI CodexRun the real email flow once before you merge a Codex change that touches sign-up or login.](https://proxymail.qa/built-with/codex)
- [CursorCheck the real sign-up, magic-link and reset emails after the agent changes your auth code.](https://proxymail.qa/built-with/cursor)
- [Google AntigravityKeep unverified Firebase users out, check authorized domains and the sender, and let the agent run the check.](https://proxymail.qa/built-with/antigravity)
- [Login methods explainedEmail, codes, magic links, Google and Apple buttons, phone and SSO: what each sends, what breaks and how to test it.](https://proxymail.qa/built-with/login-methods)

[See every email testing problem we solve](https://proxymail.qa/solve)