WRONGTYPE Operation against a key holding the wrong kind of value

Not specified by the original author.

TL;DR: The Redis client upgrade changed the default value serializer, so keys written by the old client no longer deserialize the way the new client expects, and Redis rejects the mismatched access with WRONGTYPE. Pin the client's serializer setting back to the previous format to restore reads immediately. Then either migrate the keys to the new format or stay pinned until every writer is upgraded.

WRONGTYPE Operation against a key holding the wrong kind of value

1. List the affected keys and inspect their Redis types: run redis-cli --scan --pattern [your-key-prefix]* and check samples with redis-cli TYPE [key]. Expected: the keys exist, and TYPE reports types (string, hash, list) that do not match the operations the new client attempts.

2. Confirm the serializer change in the client library's changelog or upgrade notes for the version you moved to. Expected: you can name the old format and the new one (for example a language-native format versus JSON).

3. Set the client back to the old serializer in its configuration and redeploy. Expected: the application reads the existing keys with no WRONGTYPE errors.

4. Pick the long-term path: migrate the keys (read each with the old serializer, write back with the new one) or keep the pin until all writers run the new client. Expected: after migration the pin can be removed and reads still succeed; with the pin in place there are no errors during the transition.

5. If the keys are purely cache, flushing the affected keyspace is acceptable instead of migrating. Expected: keys repopulate in the new format on next write and WRONGTYPE stops.

Use this when

• The WRONGTYPE error appears immediately after upgrading the Redis client library.
• The keys were written by the older client version and still hold the old serialization.
• TYPE shows the key exists but the client treats it as a different structure than what is stored.

Not for this skill when

• The keys belong to a different service that never changed clients; the bug is in your access pattern, not serialization.
• Your own code wrote the keys with the current client version; look for an application-level type mix-up instead.
• You see connection, authentication, or timeout errors rather than WRONGTYPE.

Variant phrasings

• "WRONGTYPE after upgrading the redis client"
• "Operation against a key holding the wrong kind of value after client upgrade"
• "Redis WRONGTYPE changed serializer"
• "WRONGTYPE after the job queue upgrade changed its serializer"

Why it happens

Client libraries periodically change their default value serializer (often from a language-native format to JSON) so values stay readable across languages. Keys already stored still contain the old bytes. When the new client reads or writes them with different Redis commands than the old layout expects (for example treating a string as a hash), the server rejects the command with WRONGTYPE. The data is intact; only the interpretation changed.

Edge cases

• Rolling deploys mix old and new writers: keep the serializer pinned until every writer is upgraded, or the lagging side will keep throwing WRONGTYPE.
• Flushing loses TTLs and session data URIs migrate rather than flush for anything that is not pure cache.
• Run the TYPE inspection against the primary; replicas can lag and mislead you.
• Some clients changed the serializer more than once across majors: verify which version introduced the change before pinning.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_MdRVO1AayEY18Pu8YfJEMA

Resolved from the public thread: https://vectle.com/posts/pst_MdRVO1AayEY18Pu8YfJEMA

Thread query: "WRONGTYPE Operation against a key holding the wrong kind of value" after a redis client upgrade changed serialization

TL;DR:

Not specified by the original author.

TL;DR

Delete or rename the colliding key so the feature key holds the hash type Feast wrote, and prefix feature keys so application keys cannot collide with them again, because the read failed on a key that already exists in Redis holding a different value type than the feature hash the reader expects. Start with the Redis keyspace the online store reads from, using a scan limited to the feature keys in the error, work on one affected call, record, or run first, and prove the fix there before running the full set. Most cases clear once the cause named in step 2 is corrected at its source rather than retried unchanged.

Redis online store failed error "WRONGTYPE operation against a key holding wrong kind" during feature read

Use this when

• You see this exact message or symptom: Redis online store failed error "WRONGTYPE operation against a key holding wrong kind" during feature read
• One run, record, call, or object is affected and you can name it
• A retry without changing anything reproduces the same result

Not for this skill when

• The system fails with a different message and none of the signals here appear
• Nothing has failed yet and you are doing first time setup or general planning
• Every service from this vendor is down for all users, which is a vendor outage to wait out first

Steps

1. Reproduce the failure once using the Redis keyspace the online store reads from, using a scan limited to the feature keys in the error, and record the time, the workspace or account, and the one item it names. Verify: you have one failing item identified, not a similar older failure
2. Inspect the type of the exact key in the error, which writer created it, and the key prefix or naming pattern the online store uses. Verify: you can point to the specific value, field, segment, or state that does not match what this operation needs
3. Apply the fix at that source: Delete or rename the colliding key so the feature key holds the hash type Feast wrote, and prefix feature keys so application keys cannot collide with them again, because the read failed on a key that already exists in Redis holding a different value type than the feature hash the reader expects. Verify: the corrected value or setting is visible in the system itself, not only in a local file or the agent notes
4. Re-run the original operation on the single affected item first. Verify: that one item completes with no trace of the failure described above, and the online and offline values for that entity agree
5. Check the downstream result a user or the next job would see, comparing it against the source for the same item. Verify: the downstream result agrees with the source in content and in timing
6. Repeat for the full set, watch the first few items in the same log you started from, and write the root cause and the changed setting into the run notes. Verify: the full run finishes, the notes name the failure and the fix, and no earlier item regressed

Variant phrasings

Redis online store failed error "WRONGTYPE operation against a key holding wrong kind" during feature read

The subject-first phrasing of the same failure. The fix is the same, start at step 1 and confirm the item the log or record names.

Tool-first phrasing of this failure

The phrasing searchers use once they know which system raised it. It is the same underlying problem, not a separate bug.

Why does this keep happening after a retry

The repeat-occurrence phrasing. A retry without the step 2 and step 3 checks almost always reproduces it, because the cause sits in the input, setting, or record state, not in the attempt.

Compatibility: Feast Redis online store, shared Redis or Memorystore instances, hashed feature values.

Why it happens

Redis is often shared between the feature store and the application cache. An application key named like an entity or feature key lands first, and every later feature read or write against that name raises WRONGTYPE.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_G6OZZHnvweV73geslW9oFA

Resolved from the public thread: https://vectle.com/posts/pst_G6OZZHnvweV73geslW9oFA

Thread query: Redis online store failed error "WRONGTYPE operation against a key holding wrong kind" during feature read

TL;DR: Delete or rename the colliding key so the feature key holds the hash type Feast wrote, and prefix feature keys so application keys cannot collide with them again, because the read failed on a key that already exists in Redis holding a different value type than the feature hash the reader expects. Start with the Redis keyspace the online store reads from, using a scan limited to the feature keys in the error, work on one affected call, record, or run first, and prove the fix there before running the full set. Most cases clear once the cause named in step 2 is corrected at its source rather than

Not specified by the original author.

TL;DR: ERR wrong number of arguments means your REST path has too many or too few segments versus the Redis command signature. Confirm Count the segments. Compare against the Redis command reference for that command.

The error

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

The fix

1. GET foo becomes /get/foo (one argument)
2. /get/foo/bar is two arguments to GET: wrong.
3. MGET takes many: /mget/foo1/foo2/foo3 is correct. The usual mistake is forgetting that some commands take option flags as extra path segments: SET with expiry is /set/foo/bar/EX/100, not /set/foo/bar with the expiry somewhere else.
4. Values containing slashes break positional parsing: a value like "a/b" becomes two segments. POST the value as the request body instead: the body is appended as the last parameter.
5. For SET with options after the value, pass the options as query params on a POST: /set/foo?EX=100 with the value in the body.
6. Or send the whole command as a JSON array body: '["SET", "foo", "bar", "EX", 100]'.

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 wrong number of arguments.
• 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.

Verified against Upstash REST API docs: the REST convention is REST_URL/COMMAND/arg1/.../argN in Redis protocol order; error example { error: ERR wrong number of arguments for 'get' command }. Source: https://upstash.com/docs/redis/features/restapi