Great question—let’s break down exactly what’s happening in both scenarios to understand the core differences in how Postman and your API server interact.

Scenario 1: The API isn’t running at all #

When you send a request to an API that’s offline, this is a low-level network failure before your request even touches application logic:

• Postman tries to start a TCP connection to the server’s IP and port (like localhost:5000).
• Since there’s no program listening on that port, your operating system immediately sends a "connection refused" signal back to Postman.
• Postman picks up this failure within seconds and shows you an error (like "Could not send request" or "Connection refused").
• Your request never makes it to an actual API application—it dies at the network level.

Scenario 2: API is running, but you set a breakpoint on the inbound request #

This is a completely different situation because a valid network connection is already established:

• Postman successfully completes the TCP handshake with your running API server.
• It sends the full request (headers, body, etc.) to the server, which receives and acknowledges it.
• The server’s runtime (whether it’s ASP.NET Core, Node.js, or another framework) pauses execution at your breakpoint—so the code that would generate and send a response is frozen.
• From Postman’s perspective, it’s done its part: sent the request, and now it’s waiting for the server to send back a response (per HTTP’s request-response cycle).
• Postman will keep waiting indefinitely (unless you set a custom timeout in its settings) until either:
  • You resume execution in the debugger, letting the server finish processing and send the response, or
  • The underlying TCP connection times out (a much longer wait, controlled by your OS).
• The request absolutely reached the server—it’s just stuck in the application’s execution pipeline, not being finished.

The critical distinction here is: in the first case, the connection fails before any request data is delivered, while in the second case, the connection works and the request is sitting on the server waiting for processing to resume.

内容的提问来源于stack exchange，提问作者Loreno