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OAuth 400 Bad Request — redirect_uri Mismatch Between Client and Provider

Symptom

• OAuth authorization flow fails with 400 or 401
• Error: redirect_uri_mismatch, invalid_redirect_uri, or The redirect URI in the request did not match a registered redirect URI
• Happens on: Google OAuth, GitHub OAuth, any OAuth 2.0 provider
• Works in one environment (local) but fails in another (production)
• Error appears at the OAuth provider’s authorization page, not in your logs

Root Cause

OAuth providers require the redirect_uri in the authorization request to match exactly one of the registered redirect URIs — including protocol, domain, port, and path. Common mismatches:

• http://localhost:3000 vs http://localhost:3000/ (trailing slash)
• http:// vs https://
• localhost vs 127.0.0.1
• Port present in request but not in registered URI
• Different path suffix (/callback vs /auth/callback)

Fix

Step 1: Check the exact URI being sent in the request

from urllib.parse import urlencode, urlparse

# Log the exact URI being used in the OAuth request
redirect_uri = "https://your-app.com/auth/callback"
print(f"Using redirect_uri: {redirect_uri}")

# Build auth URL
params = {
    "client_id": CLIENT_ID,
    "redirect_uri": redirect_uri,
    "response_type": "code",
    "scope": "openid email profile",
}
auth_url = f"https://accounts.google.com/o/oauth2/auth?{urlencode(params)}"
print(f"Auth URL: {auth_url}")

Step 2: Match the exact URI in the provider dashboard

For Google OAuth:

• Go to Google Cloud Console → Credentials → OAuth Client
• “Authorized redirect URIs” must match exactly
• Add both with and without trailing slash if needed:
  • https://your-app.com/auth/callback
  • http://localhost:3000/auth/callback (for local dev)

For GitHub OAuth:

• GitHub App settings → Authorization callback URL
• GitHub allows ONE redirect URI — use the production URI, handle dev separately

For Generic OAuth provider:

# Find the registered URIs vs what you're sending
curl "https://provider.com/.well-known/openid-configuration" | jq '.authorization_endpoint'
# Compare with what your code sends

Step 3: Environment-aware redirect URI

import os

def get_redirect_uri():
    env = os.environ.get("ENVIRONMENT", "development")
    base_url = {
        "production": "https://your-app.com",
        "staging": "https://staging.your-app.com",
        "development": "http://localhost:3000",
    }[env]
    return f"{base_url}/auth/callback"

# Use this everywhere — consistent URI construction
REDIRECT_URI = get_redirect_uri()

Step 4: Register all environment URIs in the provider

For Google OAuth, add all variants at once:

https://your-app.com/auth/callback
https://staging.your-app.com/auth/callback
http://localhost:3000/auth/callback
http://127.0.0.1:3000/auth/callback

Common Mismatch Fix Table

Error	Registered	Fix
http vs https	https://app.com/callback	Update redirect_uri to use https
Trailing slash	https://app.com/callback	Remove trailing slash from request
Port difference	https://app.com/callback	Remove :443 from request URI
Path difference	https://app.com/callback	Check for /oauth/callback vs /callback

Expected Token Savings

Debugging OAuth redirect URI mismatch: ~5,000 tokens This fix: ~5 minutes of dashboard configuration

Environment

• Google OAuth, GitHub OAuth, any OAuth 2.0 provider
• Most common when deploying to new environment (staging → production)
• Source: direct experience, OAuth 2.0 RFC 6749 §10.6

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