Label Studio storage sync failed error "S3 AccessDenied when listing objects"

TL;DR

Fix this in Label Studio by acting on the exact message first: check the bucket policy and the IAM role used by storage sync, granting list and get on the exact bucket prefix, not the whole bucket. Do not rerun the whole job unchanged, because the same inputs produce the same failure. Prove the fix on one item, then rerun the full scope and reconcile the counts.

S3 AccessDenied when listing objects

Use this when

• The operation in Label Studio fails with this exact message: S3 AccessDenied when listing objects.
• The same failure repeats on retry with unchanged inputs.
• A review, export or check is blocked until this item completes.

Not for this skill when

• The message is a permission or outage notice from the platform. Resolve that first, then return here if this error remains.
• The operation completes and you only disagree with its output. That is a quality review, not this failure.

Steps

1. Reproduce the failure with the smallest scope that still shows it in Label Studio, and copy the full message plus the item, user or metric involved into the run notes. Verify: the error is reproduced on demand with a named item, not only seen in a log.
2. Apply the specific fix for this error: check the bucket policy and the IAM role used by storage sync, granting list and get on the exact bucket prefix, not the whole bucket. Verify: run a head and list check from the server with the same role and confirm both succeed.
3. Check the neighbouring items that share the same config, ontology, baseline or credential, because this class of failure rarely affects only one item. Verify: the neighbouring items are listed and each is marked affected or clear.
4. Compare the result counts, decisions or alerts against the source in Label Studio so a silent partial success is caught before sign off. Verify: counts match the source or every difference is named and explained.
5. Rerun the original operation in Label Studio on the previously failing scope, watching for the same message rather than assuming success from a quiet log. Verify: the operation completes and the original message does not appear.
6. Record the fix, the triggering input and the verification result on the item so the next occurrence is recognised immediately. Verify: the record names the error, the fix applied and the evidence of success.

Why it happens

Storage sync lists objects before it reads them; a role with get but no list on the prefix gets AccessDenied at the first step.

Variant phrasings

S3 AccessDenied when listing objects

The verbatim form searchers paste from the terminal or console. Treat it identically: same fix, same single item proof, then the full rerun.

How to fix: Label Studio storage sync failed error "S3 AccessDenied when listing o

The how-to phrasing for the same failure in Label Studio. The steps above apply unchanged; start at the reproduction step rather than skipping to the rerun.

Label Studio troubleshooting for this failure

The symptom-first phrasing used when the console hides the full message. Surface the full message first, confirm it matches the code block above, then follow the steps.

Compatibility: Label Studio at current versions, plus the platform API or console the query names; no extra tooling beyond a text editor and the source system access needed to verify counts.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstxfdJIZKI-QSk-4ze87c5g