Upstash REST SUBSCRIBE and MONITOR use SSE: set the Accept header, avoid serverless

TL;DR: SUBSCRIBE, PUBLISH, and MONITOR over REST use Server-Sent Events. Without the right Accept header and method you get hangs or errors. Forgetting the Accept header: the request completes without streaming, or hangs waiting for a body that never comes. Verify Open the subscribe stream, then publish from another terminal.

The fix

1. Subscribe: POST /subscribe/chat with header Accept text/event-stream. The connection stays open and messages arrive as SSE events.
2. Publish: POST /publish/chat/hello. Plain request, immediate response.
3. Monitor: POST /monitor with Accept text/event-stream. Streams every command hitting the database.
4. Forgetting the Accept header: the request completes without streaming, or hangs waiting for a body that never comes.
5. Using these in serverless functions: an open SSE stream holds the function instance for the stream lifetime. That is one instance per subscriber, billed the whole time.
6. Trying MONITOR from a browser: it streams every command including other clients traffic. It is a debugging tool, not a feature.

When to use this

• This covers exactly what the title says: Upstash REST SUBSCRIBE and MONITOR use SSE.
• You are setting this up for the first time, or auditing an existing setup.
• You want the key gotchas in one place before you start.

When not to use this

• You are doing a different workflow with Upstash; these steps are specific to the title above.
• You need the full reference docs; this is the short path, not the manual.

Compatibility

• Not pinned to a specific version; follows current Upstash behavior.

Verified against Upstash REST API docs: SUBSCRIBE endpoint works via SSE, request with Accept text/event-stream; MONITOR likewise; PUBLISH is a plain POST to /publish/channel/message. Source: https://upstash.com/docs/redis/features/restapi