How do I test SignalR Hub methods using Postman in a .NET 8 application?

Below is a summary of the situation and some recommendations.

How SignalR Handshake Works

SignalR expects a specific handshake sequence. The client (or test tool) must start by sending a handshake request message in JSON that ends with the record separator character (U+001e). For example, a proper handshake message looks like:

JSONCopied
{"protocol":"json","version":1}\u001e

The server should then respond with an empty message (again, terminated by U+001e) before you send actual hub invocation messages.

Challenges with Using Postman

While Postman supports WebSocket connections, it does not automatically handle the SignalR protocol quirks. Even if you insert the Unicode character manually, Postman’s WebSocket interface may not reliably mimic a full SignalR client. This is why you may see an error like "An unexpected error occurred during connection handshake." That error typically means that the server did not receive the handshake response in the exact format (including the trailing U+001e).

Recommendations

1. Use a Dedicated WebSocket Client: Tools such as WS Client or “wscat” (a command‑line WebSocket client) allow you to more precisely control the exact message sent (including appending U+001e).

2. Use the SignalR Client Library: Instead of testing via Postman, consider writing a short client using the SignalR .NET client (or JavaScript client) so that the handshake and ongoing protocol are handled automatically.

3. Manual Testing Workarounds:
   If you want to force Postman to work, ensure that each message you send ends with the record separator. For example, your hub method invocation could be:

   JSONCopied
   { "arguments": ["just a test"], "target": "HubMethod", "type": 1 }\u001e

   However, even with this, Postman may not manage the full lifecycle (handshake, keep-alive, etc.) as a dedicated SignalR client.

Conclusion

It can be challenging to simulate the SignalR protocol using Postman because of its custom handshake requirements. For reliable testing, using a dedicated WebSocket client or the official SignalR client is recommended. These approaches ensure that the protocol details (like appending U+001e) are handled correctly.