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Zapier OAuth 2.0: Fix 404 on Redirect URI Mismatch

Fix Zapier OAuth 2.0 404 errors: ensure redirect_uri matches exactly, perform the code-to-token exchange, and set the token request in Zapier for private apps.

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OAuth 2.0 Integration with Zapier: Fixing 404 Error on Redirect URI for Private App

I’m setting up a private app in Zapier with OAuth 2.0 authorization to connect to my application. Here’s the authorization URL I’m constructing:

const zapierUrl =
    `https://xxxxxxxxx.ngrok-free.app/zapier-oauth/authorize` +
    `?token=x` +
    `&client_id=CLIENT_ID` +
    `&state=x` +
    `&redirect_uri=${encodeURIComponent(
      'https://zapier.com/dashboard/auth/oauth/return/APP_ID'
    )}` +
    `&response_type=code` +
    `&scope=${encodeURIComponent('read write')}`;

When I click to connect via Zapier, it redirects correctly to the redirect_uri with the authorization code, but Zapier shows a 404 error and the process stops. Is this issue caused by my app being private? How can I troubleshoot and resolve the 404 error in Zapier OAuth 2.0 integration?

The 404 error during Zapier OAuth 2.0 integration typically hits when the redirect_uri doesn’t perfectly match the authorized redirect URIs configured in your private app settings, causing Zapier to fail loading its callback page like https://zapier.com/dashboard/auth/oauth/return/APP_ID after receiving the authorization code. Private apps aren’t the direct culprit—it’s usually a precise URI mismatch, unhandled state parameter, or incomplete token exchange setup in Zapier’s interface. Fix it by double-checking your app’s OAuth configuration, testing the full flow with ngrok-exposed endpoints, and ensuring your access token request block points to your backend callback for code swapping.

Contents

Understanding Zapier OAuth 2.0 Flow for Private Apps

Zapier private apps let you connect custom services via OAuth 2.0 without public platform approval, but the flow demands precision. You build an authorization URL pointing users to your app’s /authorize endpoint—yours uses ngrok, which is smart for local testing. After user approval, it redirects back with a code and state to Zapier’s fixed callback: https://zapier.com/dashboard/auth/oauth/return/APP_ID.

Zapier then expects to swap that code for an access token using your backend endpoint, defined in the app’s “Access Token Request” block. If that swap fails or the initial redirect URI mismatches, you get the 404 because Zapier’s page can’t load properly. Developers often overlook that Zapier handles its own dashboard redirect, not your custom one during connection.

Why the 404 Error Occurs on Redirect URI

That 404 on https://zapier.com/dashboard/auth/oauth/return/APP_ID?code=...&state=... screams “page not found” right after code delivery. From real Zapier community cases, it happens when your redirect_uri in the auth URL doesn’t exactly match what’s whitelisted in your private app’s OAuth settings—no trailing slashes, precise APP_ID, HTTPS only.

Your code snippet looks solid, but subtle issues like unencoded scopes or mismatched client_id trigger it. Private apps amplify this since Zapier auto-appends its dashboard path, and if your app treats the code as a direct token (common OAuth pitfall), the exchange bombs. One forum thread nails it: Zapier grabs the code from URL params but needs your backend to POST it for a token, or it 404s on reload.

Mismatches also spark “invalid redirect uri” variants, but yours manifests as 404 because Zapier tries rendering its success page.

Step 1: Verify Authorized Redirect URIs

Head to your Zapier private app editor. Under OAuth 2.0 settings, scan the “Authorized redirect URIs” field. Paste in exactly:

https://zapier.com/dashboard/auth/oauth/return/APP_ID/

Replace APP_ID with your actual ID (find it in the app URL). No www, no query params—just the raw path. Save and test.

This fixes most cases, per troubleshooting guides. If your ngrok URL varies (e.g., xxxxxxxxx.ngrok-free.app), whitelist it too for dev, but production needs stable domains. Google OAuth pros stress exact matches to dodge redirect_uri_mismatch, which cascades to 404 in Zapier.

Step 2: Debug Your Authorization URL Construction

Log the full zapierUrl before redirecting. Hit it in a browser—does your /authorize endpoint approve and bounce back with ?code=...&state=...? Tweak like this for robustness:

javascript
const redirectUri = encodeURIComponent('https://zapier.com/dashboard/auth/oauth/return/YOUR_APP_ID');
const scope = encodeURIComponent('read write'); // Match your app's scopes exactly

const zapierUrl = `https://your-ngrok.ngrok-free.app/zapier-oauth/authorize?` +
  `client_id=${CLIENT_ID}&` +
  `state=${state}&` +  // Generate unique state!
  `redirect_uri=${redirectUri}&` +
  `response_type=code&` +
  `scope=${scope}`;

Ditch the token=x param—OAuth 2.0 doesn’t use it here; it confuses flows. Use tools like OAuth Debugger to validate.

Step 3: Handle Code Exchange and Access Token Requests

In Zapier’s app editor, set the “Access Token Request” to POST your backend /token endpoint. Example payload:

Method: POST
URL: https://yourdomain.com/zapier-oauth/token
Headers: Authorization: Basic ${base64(client_id:client_secret)}
Body: grant_type=authorization_code&code={code}&redirect_uri={your_redirect_uri}

Your server swaps code → token, returns JSON {access_token: "..."}. Zapier community threads confirm: direct code hunting skips this, causing 404 on callback.

Test with curl:

bash
curl -X POST https://your-ngrok.ngrok-free.app/zapier-oauth/token \
  -H "Authorization: Basic $(echo -n 'CLIENT_ID:SECRET' | base64)" \
  -d "grant_type=authorization_code&code=YOUR_TEST_CODE&redirect_uri=https://zapier.com/dashboard/auth/oauth/return/APP_ID"

Advanced Troubleshooting for Private Apps

Private apps skip review, so iterate fast. Check Zapier task history for logs—search “OAuth” errors. Ngrok inspect traffic: does the code POST hit your /token? 404 often masks auth creds fails.

Issue Symptom Quick Fix
URI Mismatch 404 immediate post-redirect Exact whitelist match
No Token Endpoint Code visible, no swap Define Access Token Request
State Loss “Invalid state” + 404 Unique state gen/validate
Staging 404 Webhook-like fails Public ngrok, no IP blocks

Whitelist Zapier’s IPs if firewalled. Staging sites 404 if private—expose fully.

Testing Your Zapier OAuth Integration

  1. Create test Zap: Trigger → your private app action.
  2. “Connect Account” → watch network tab.
  3. Success? Token persists in connections.
  4. Fail? Reproduce in incognito, note exact URL.

Use Postman collections for OAuth mock. Once live, monitor with Zapier audits.

Best Practices to Avoid Future 404 Errors

Stick to HTTPS URIs, dynamic state (uuid), scope subsets. Version your endpoints. For prod, swap ngrok for Vercel/Netlify. Audit monthly—OAuth drifts break Zaps silently.

Sources

Conclusion

Nail the redirect_uri match and token exchange to squash that Zapier OAuth 2.0 404 error—your private app will connect seamlessly. Follow the steps, test iteratively, and you’ll automate like a pro without URI headaches.

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Zapier OAuth 2.0: Fix 404 on Redirect URI Mismatch