Prometheus expects a Content-Type header for POST queries ( application/x-www-form-urlencoded ). If the proxy manipulates this, Prometheus throws a 400.
4 minutes We have all been there. You have built a beautiful Grafana dashboard, or you are trying to fetch metrics via curl to debug a latency spike. You type the command, hit enter, and instead of a JSON payload of time series data, you are greeted with the digital equivalent of a slammed door: "400 Bad Request - There was an error returned querying the Prometheus API" It is vague. It is frustrating. And it usually happens at 2 AM during an incident. Prometheus expects a Content-Type header for POST queries
In this post, we will tear down what a 400 actually means for Prometheus, why you are seeing it, and how to fix it fast. A 400 Bad Request means the Prometheus server received your query but refused to process it because the request syntax was wrong or violated a server limit. The problem is your query, not the server. The Usual Suspects Prometheus is strict. Its query engine (PromQL) is powerful, but it expects perfect syntax and reasonable resource boundaries. Here are the top 5 reasons you are seeing this error. 1. The Unescaped Curly Brace (Grafana/Linux users) This is the most common culprit. When using curl in a Linux terminal or running queries via scripts, special characters like , , | , or $ often get interpreted by the shell before they reach Prometheus. You have built a beautiful Grafana dashboard, or
Troubleshooting the Ghost: Decoding the β400 Bad Requestβ from the Prometheus API And it usually happens at 2 AM during an incident
Increase your step interval ( step=300 for 5m steps) or reduce your time range. 4. Header Conflicts (The Reverse Proxy Issue) If you run Prometheus behind an NGINX or AWS ALB, the proxy might strip or modify required headers.