When working with Google Cloud Endpoints as a proxy for Cloud Functions, it's common to encounter the 403 Forbidden error with messages like:

Error: Forbidden
Your client does not have permission to get URL /function1GET from this server

The root cause typically stems from insufficient IAM permissions between services. Your ESP (Extensible Service Proxy) needs explicit invocation rights:

gcloud beta functions add-iam-policy-binding function1 \
  --member "serviceAccount:id-compute@developer.gserviceaccount.com" \
  --role "roles/cloudfunctions.invoker" \
  --project "project_id"

For frontend applications (like Firebase Hosting) to access your endpoint, proper CORS handling is crucial. Your OpenAPI spec should include:

x-google-endpoints:
- name: "HOST"
  allowCors: "true"

Many developers make these mistakes in their OpenAPI spec:

• Incorrect HTTP method mapping (GET/POST in backend address)
• Missing x-google-backend address protocol (https:// prefix required)
• Region/project ID mismatches in function URL

Here's a verified spec that works in production:

swagger: '2.0'
info:
  title: Cloud Endpoints + GCF
  version: 1.0.0
host: api.example.com
x-google-endpoints:
- name: "api.example.com"
  allowCors: "true"
paths:
  /v1/function1:
    get:
      x-google-backend:
        address: https://us-central1-project-id.cloudfunctions.net/function1
      responses:
        '200':
          description: Success

1. Verify ESP service account exists in Cloud Functions invokers
2. Check Cloud Function logs for detailed auth errors
3. Test direct function URL with authentication header
4. Validate IAM changes with:

   gcloud functions get-iam-policy function1

For enterprise implementations, consider adding JWT verification:

securityDefinitions:
  google_id_token:
    authorizationUrl: ""
    flow: "implicit"
    type: "oauth2"
    x-google-issuer: "https://accounts.google.com"
    x-google-jwks_uri: "https://www.googleapis.com/oauth2/v3/certs"

When deploying Cloud Functions behind Cloud Endpoints, a common hurdle is encountering the 403 Forbidden error when trying to access the endpoint URL. The error typically appears as:

Error: Forbidden
Your client does not have permission to get URL /function1GET from this server

This occurs despite following Google's official documentation for setting up the integration between these services.

The fundamental issue stems from IAM permission configurations. The Extensible Service Proxy (ESP) needs explicit permission to invoke your Cloud Function. The critical command is:

gcloud beta functions add-iam-policy-binding function1 \
  --member "serviceAccount:id-compute@developer.gserviceaccount.com" \
  --role "roles/cloudfunctions.invoker" \
  --project "project_id"

However, there are several nuances to consider:

A properly configured openapi-functions.yaml should include these essential elements:

swagger: '2.0'
info:
  title: Cloud Endpoints + GCF
  version: 1.0.0
host: service-name.a.run.app
x-google-endpoints:
- name: "service-name.a.run.app"
  allowCors: true
schemes:
  - https
paths:
  /function1:
    get:
      x-google-backend:
        address: https://REGION-project-id.cloudfunctions.net/function1
      responses:
        '200':
          description: Success response

For web applications accessing the endpoint, CORS configuration is crucial. The x-google-endpoints section should include:

x-google-endpoints:
- name: "your-service.a.run.app"
  allowCors: true

Additionally, your Cloud Function should handle CORS headers in its response:

def function1(request):
    headers = {
        'Access-Control-Allow-Origin': '*',
        'Access-Control-Allow-Methods': 'GET',
        'Access-Control-Allow-Headers': 'Content-Type',
        'Access-Control-Max-Age': '3600'
    }
    return ('Response content', 200, headers)

When troubleshooting, verify these aspects:

1. ESP service account has Cloud Functions Invoker role
2. The function name in the backend address matches exactly
3. CORS configuration is properly set at both endpoints and function level
4. The region in the backend address is correct

If permissions persist as an issue, consider using Cloud Run instead of Cloud Functions, which provides more granular control:

gcloud run services add-iam-policy-binding gcf-proxy \
  --member="allUsers" \
  --role="roles/run.invoker" \
  --platform managed

This approach often resolves stubborn permission issues while maintaining security.