If you’re getting 401 and 400 intermittently from the upstream service, that strongly suggests authentication-related issues, often tied to token forwarding, expiration, or format mismatch.

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🔁 Quick Summary of Key Differences

Status	Meaning	Common Cause
401	Unauthorized	Missing/invalid/expired credentials
400	Bad Request	Malformed or incomplete request (e.g. OIDC token request)

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🧠 Intermittent 401 + 400: Common Root Causes

🔸 1. Expired or Reused Tokens

• Kong gets a token once, caches it, and keeps using it—but upstream expects a fresh one.
• Especially common with client credentials or authorization code flows.

Solution:

• Set token caching to short duration or disable it in the OIDC plugin: config: cache_ttl: 0 # Or a very short TTL like 5

🔸 2. Multiple Consumers with Invalid Secrets

• One client (consumer) is configured correctly, others are not.
• You see 401/400 when the bad client makes a request.

Solution:

• Enable verbose logging in Kong: export KONG_LOG_LEVEL=debug kong reload Then correlate consumer_id with the error.

🔸 3. Kong Not Forwarding Tokens Correctly

• Kong authenticates but doesn’t forward Authorization header to the upstream.
• Some plugins strip headers by default.

Solution:

• Add request-transformer plugin to pass the token: curl -X POST http://localhost:8001/services/YOUR_SERVICE/plugins \ --data "name=request-transformer" \ --data "config.add.headers=Authorization:Bearer $(jwt_token)"

🔸 4. OIDC Plugin Misconfiguration

If you’re using the OpenID Connect plugin:

• grant_type, client_id, or redirect_uri may be wrong or missing intermittently.
• Kong might request a new token but fail to pass a correct one.

Check:

• Kong OIDC plugin config
• Errors like: error=invalid_request error_description="Unsupported client authentication method"

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🧪 How to Debug Effectively

1. Set KONG_LOG_LEVEL=debug, reload Kong, and tail the logs: tail -f /usr/local/kong/logs/error.log
2. Inspect Upstream Request:
   • Look for what headers/body Kong is sending.
   • Especially Authorization, Content-Type, and request body if OIDC is involved.
3. Track Errors to a Specific Consumer:
   • Use consumer_id in the access log to trace.
   • Maybe only some consumers are misconfigured.
4. Try Curling the Upstream Directly with the exact payload Kong sends (use Postman or curl): curl -X POST https://upstream/token \ -H "Authorization: Bearer <your_token>" \ -H "Content-Type: application/x-www-form-urlencoded" \ -d "grant_type=client_credentials&client_id=...&client_secret=..."

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to check

• Kong plugin configs (especially OIDC/JWT)
• A few lines from Kong’s debug logs showing the upstream request/response
• Whether you’re using Ping Identity or a custom upstream