fix: drop alias() calls with a missing identity (sdk-specs alias)

[posthog]

💡 Motivation and Context

The sdk-specs alias contract requires both identities to be present before an alias event is emitted:

• openspec/specs/alias/spec.md — Behavior → Server-side flow, step 1: "Validate inputs. Both the existing id and the alias target id must be present."
• Same file, scenario "Alias is dropped when required identities are missing (@both)" → "WHEN alias is called without a previous distinct id / THEN no event should be enqueued / AND the SDK should record a validation warning" (mirrored in acceptance/public/alias.feature)

Client.alias() did neither for previous_id — it was passed straight through with no check:

posthog.alias(None, "user-123")
# -> enqueued: {"event": "$create_alias", "distinct_id": null,
#               "properties": {"distinct_id": null, "alias": "user-123"}}
# -> returned a uuid, logged nothing

An alias event with a null source id cannot link anything, so this is a silently unusable event rather than a caught mistake. The same happened for previous_id="".

The other half of the validation — a missing alias target — was already dropped (get_identity_state reports personless when neither an explicit nor a context distinct id is available), but it returned None with no diagnostic at all, so the caller had no way to tell a dropped alias from a delivered one.

Change

• previous_id is stringified and checked; empty/missing drops the call, logs a warning, and returns None.
• The pre-existing missing-target drop now logs a warning too, so both spec-required "identity missing" paths record one.
• previous_id is stringified once and reused, so distinct_id and properties.distinct_id agree for non-string ids (alias(0, ...) previously produced "0" at the top level and 0 in properties — _enqueue only stringifies the top-level field).

Signatures and the public API snapshot are unchanged; previous_id stays annotated str since passing None was never a supported call shape.

⚠️ Behavior change

Callers who pass a missing or empty previous_id stop getting a $create_alias event, and alias() returns None instead of a UUID for those calls. The events this removes were already inert — a $create_alias with a null distinct_id links no identities — so no working alias is affected. Callers checking the return value for success will now correctly see the failure, and will see a warning in the logs explaining it. Valid calls, including non-string ids such as alias(0, "user-123"), are unchanged.

💚 How did you test it?

Added four tests in posthog/test/test_client.py: previous_id=None and previous_id="" are dropped with a warning and nothing posted, previous_id=0 still sends and is stringified consistently, and a missing distinct_id is dropped with a warning. Confirmed the pre-change behavior first by running the SDK locally against a mocked batch_post.

Ran pytest posthog/test/test_client.py posthog/test/test_module.py posthog/test/test_contexts.py (190 passed), ruff format --check ., ruff check ., mypy ... | mypy-baseline filter (clean), check_public_api.py (snapshot up to date), and python -W error -c "import posthog". No manual or integration testing beyond that.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed. — alias() docstring documents the drop behavior.
• No breaking change or entry added to the changelog. — behavior change, see above; changeset added.

If releasing new changes

• Ran sampo add to generate a changeset file — hand-written at .sampo/changesets/alias-validates-identities.md (the sampo CLI isn't available in this environment); please sanity-check the format.

🤖 Agent context

Autonomy: Fully autonomous

Opened by the SDK Spec Compliance Enforcer loop, which audits posthog-python against the contracts in PostHog/sdk-specs. This run re-checked the spec capabilities whose Applicability is both or server, using parallel read-only sub-agents (Claude Code general-purpose agents with Read/Grep/Bash) and confirming each candidate by exercising the SDK locally.

This finding was chosen because the spec scenario is explicit and tagged @both, the current behavior produces an event that provably cannot do its job, and the fix is contained to one method. Runners-up left for humans, all from the same audit: group_identify() has the identical missing-validation gap against its own @both "Group identify requires type and key" scenario (a natural follow-up, kept out of this PR to keep it focused); capture() does not reject an empty event name, which the capture spec's Behavior/Error-handling prose requires but no scenario covers; and the capture spec asks for UUIDv7 event ids where the SDK generates UUIDv4, which is a larger change needing a product call. Note the previously closed #794 (wontfix) is unrelated to this change.

Agent-authored — requires human review; not self-merged.

---

Created with PostHog Code

[github-actions]

posthog-python Compliance Report

Date: 2026-08-05 07:57:08 UTC
Duration: 338849ms

✅ All Tests Passed!

111/111 tests passed

---

Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	516ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	1011ms
Required Headers.Has Authorization Bearer Header	✅	1009ms
Required Headers.Has Content Type Json	✅	1008ms
Required Headers.Has Posthog Sdk Info Format	✅	1009ms
Required Headers.Has Posthog Attempt Header	✅	1009ms
Required Headers.Has Posthog Request Id	✅	1009ms
Required Headers.Has Posthog Request Timestamp	✅	1009ms
Required Headers.Has User Agent	✅	1010ms
Body Format.Body Has Created At And Batch	✅	1009ms
Body Format.No Api Key In Body	✅	1010ms
Body Format.No Sent At In Body	✅	1009ms
Event Format.Event Has Required Root Fields	✅	1008ms
Event Format.Event Uuid Is Valid	✅	1009ms
Event Format.Event Timestamp Is Rfc3339	✅	1010ms
Event Format.Distinct Id Is String	✅	1011ms
Event Format.Distinct Id At Root Not Properties	✅	1007ms
Event Format.Custom Properties Preserved	✅	1011ms
Event Format.Set Properties Preserved	✅	1009ms
Event Format.Set Once Properties Preserved	✅	1010ms
Event Format.Groups Properties Preserved	✅	1010ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1009ms
Event Format.Event Has Required Root Fields Batch	✅	1013ms
Event Format.Event Uuid Is Valid Batch	✅	1013ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	1013ms
Event Format.Distinct Id Is String Batch	✅	1013ms
Event Format.Distinct Id At Root Not Properties Batch	✅	1012ms
Event Format.Custom Properties Preserved Batch	✅	1012ms
Event Format.Set Properties Preserved Batch	✅	1013ms
Event Format.Set Once Properties Preserved Batch	✅	1014ms
Event Format.Groups Properties Preserved Batch	✅	1012ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1013ms
Batch Behavior.Multiple Events In Single Batch	✅	1507ms
Batch Behavior.Batch Envelope Smoke	✅	1015ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1005ms
Batch Behavior.Flush At Triggers Batch	✅	1508ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1012ms
Deduplication.Generates Unique Uuids	✅	1508ms
Deduplication.Different Events Same Content Different Uuids	✅	1507ms
Deduplication.Preserves Uuid On Retry	✅	7511ms
Deduplication.Preserves Timestamp On Retry	✅	7516ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7512ms
Deduplication.No Duplicate Events In Batch	✅	1503ms
Header Behavior On Retry.Attempt Header Starts At One	✅	1010ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	14515ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7514ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3518ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7513ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1007ms
Response Format Validation.Success Response Has Ok For Each Event	✅	1507ms
Response Format Validation.Success No Retry After When All Ok	✅	1507ms
Response Format Validation.Success Retry After Present When Retry Events	✅	2512ms
Response Format Validation.Success No Retry After When Drop Only	✅	1506ms
Response Format Validation.Response Echoes Request Id	✅	1009ms
Retry Behavior.Retries On 408	✅	7519ms
Retry Behavior.Retries On 500	✅	7510ms
Retry Behavior.Retries On 503	✅	9517ms
Retry Behavior.Retries On 504	✅	7511ms
Retry Behavior.Retryable Errors Have Retry After	✅	4507ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12519ms
Retry Behavior.Does Not Retry On 400	✅	3503ms
Retry Behavior.Does Not Retry On 401	✅	3508ms
Retry Behavior.Does Not Retry On 402	✅	3508ms
Retry Behavior.Does Not Retry On 413	✅	3507ms
Retry Behavior.Does Not Retry On 415	✅	3508ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3506ms
Retry Behavior.Implements Backoff	✅	23531ms
Retry Behavior.Max Retries Respected	✅	23521ms
Partial Batch Handling.Handles 200 Full Success	✅	3002ms
Partial Batch Handling.Handles 200 With All Ok	✅	4508ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	4506ms
Partial Batch Handling.Does Not Retry Limited Events	✅	4507ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	7515ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	7510ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7512ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7508ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7512ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7515ms
Partial Batch Handling.Respects Retry After On Partial	✅	9510ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4505ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4511ms
Compression.Sends Gzip Content Encoding	✅	1008ms
Compression.No Content Encoding When Disabled	✅	1009ms
Compression.Compressed Body Is Decompressible	✅	1009ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3508ms
Event Options.Cookieless Mode Override	✅	1008ms
Event Options.Disable Skew Correction Override	✅	1009ms
Event Options.Process Person Profile Override	✅	1011ms
Event Options.Product Tour Id Override	✅	1009ms
Event Options.Unset Options Omitted	✅	1009ms
Event Options.Options Override In Batch	✅	1014ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	1009ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	1010ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	1009ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	1006ms
Request Payload.Flags Request Uses V2 Query Param	✅	1007ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	1007ms
Request Payload.Flags Request Omits Authorization Header	✅	1006ms
Request Payload.Token In Flags Body Matches Init	✅	1006ms
Request Payload.Groups Round Trip	✅	1007ms
Request Payload.Groups Default To Empty Object	✅	1007ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	1006ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	1006ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	1006ms
Request Lifecycle.No Flags Request On Init Alone	✅	503ms
Request Lifecycle.No Flags Request On Normal Capture	✅	1507ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1010ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1003ms
Retry Behavior.Retries Flags On 502	✅	1006ms
Retry Behavior.Retries Flags On 504	✅	1007ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1509ms