Upstash REST 400 Bad Request: syntax errors, unsupported commands, bad header combos

TL;DR: REST 400 Bad Request covers syntax errors, unsupported commands, and bad header combos like base64 plus resp2 format. Confirm Read the error field. Fix - Rebuild the path in Redis protocol order.

The error

{ "error": "ERR wrong number of arguments for 'get' command" }

The fix

1. Malformed command path: wrong argument order or count versus the Redis protocol order. REST expects /COMMAND/arg1/arg2 in protocol order.
2. Unsupported command: blocking pops (BLPOP, BRPOP, BZPOPMAX, BZPOPMIN), blocking XREAD/XREADGROUP, CLUSTER commands, WATCH/UNWATCH/DISCARD.
3. Bad header combination: Upstash-Encoding base64 together with Upstash-Response-Format resp2 is rejected. Pick one.
4. Unknown Upstash-Response-Format value: only json and resp2 are allowed.
5. Unsupported HTTP method: only HEAD, GET, POST, PUT are allowed. Anything else is 405, but proxies sometimes rewrite methods.
6. "ERR wrong number of arguments" points at your URL path.
7. "unknown command" or similar points at an unsupported command.
8. An empty or generic 400 with no error field points at headers.
9. Rebuild the path in Redis protocol order. Test the same command in redis-cli to confirm the argument order.
10. For unsupported commands, check the REST compatibility table in the docs and pick the supported alternative (non-blocking XREAD, /multi-exec instead of WATCH).
11. Drop one of the two conflicting response headers.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: Upstash REST 400 Bad Request.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Compatibility

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