Amplitude anomaly alert failed error "custom event definition changed during analysis window"

TL;DR

Fix this in Amplitude by acting on the exact message first: split the analysis at the date the definition changed, or map the old and new event definitions onto one name with the change date recorded, and rerun each side consistently. 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.

custom event definition changed during analysis window

Use this when

• The operation in Amplitude fails with this exact message: custom event definition changed during analysis window.
• 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 Amplitude, 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: split the analysis at the date the definition changed, or map the old and new event definitions onto one name with the change date recorded, and rerun each side consistently. Verify: the anomaly check completes and the change date is annotated on the series.
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. Rerun the original operation in Amplitude 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.
5. Compare the result counts, decisions or alerts against the source in Amplitude so a silent partial success is caught before sign off. Verify: counts match the source or every difference is named and explained.
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

An event whose definition changes mid window is two different metrics sharing a name; a single baseline across the change is meaningless.

Variant phrasings

custom event definition changed during analysis window

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: Amplitude anomaly alert failed error "custom event definition changed during analysis window"

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

Amplitude 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: Amplitude 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/pst_u-hVjMcc1x4NeKtFLi2JnA