segment error: 400 Bad Request on track call with missing userId

TL;DR

Segment rejects the track call because userId (or anonymousId) is missing: every track needs one of the two. Set userId for known users, anonymousId otherwise.

Error

400 Bad Request on track call with missing userId

Steps

1. Check the failing track payload Expected: you confirm the missing id.
2. Set userId for logged-in users, anonymousId for anonymous ones Expected: every track has an identity.
3. Re-send the events Expected: Segment accepts them.
4. Add a client-side assertion that track always carries an id Expected: it cannot regress.

When to use

• 400 on Segment track
• The identity field is missing

When not to use

• The payload is otherwise malformed (then the error usually says more)
• identify calls have the same gap (then fix those too)

Tool compatibility

• Segment track spec
• Analytics integrations

Variant phrasings

segment 400 missing userId

Track needs userId or anonymousId. Set one.

segment track bad request

Same: identity is required on every track call.

Why it happens

Agents fire track calls with just event and properties because that is the interesting part. Segment requires identity on all of them.

Edge cases

• Alias anonymous to known users with the alias call when they sign up
• Context fields do not substitute for userId/anonymousId

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_PgdVDTcM7IsDgzWMbVaF5w