fix: Validate custom event UUIDs

[marandaneto]

💡 Motivation and Context

Fixes #153.

Invalid custom event UUIDs were previously passed through to ingestion, which could surface as a generic backend 400 response. This validates user-provided event UUIDs client-side so invalid values fail before upload with a descriptive error/log message, while valid UUID strings and uuid.UUID objects remain supported.

💚 How did you test it?

• uv run --extra test pytest -q posthog/test/test_client.py
• uv run --extra dev ruff check posthog/client.py posthog/args.py posthog/test/test_client.py
• uv run pytest -q posthog/test/test_client.py

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

A coding agent implemented the focused fix requested for issue #153 in an isolated worktree. The change is intentionally scoped to client-side UUID validation before events are queued or uploaded; it preserves generated UUID behavior when no UUID is supplied and accepts both valid UUID strings and uuid.UUID values.

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
posthog/client.py:120-124
The error message in the falsy-string branch doesn't include the actual value, while the second branch does. When `uuid=""` is passed, the caller sees a generic message with no indication of what was supplied, making it harder to debug than the `"not-a-uuid"` case.

```suggestion
    stringified = stringify_id(value)
    if not stringified:
        raise ValueError(
            f"Invalid event uuid {value!r}. Expected a valid UUID string or uuid.UUID instance."
        )
```

### Issue 2 of 2
posthog/test/test_client.py:234-263
Per the team's review standards, parameterised tests are always preferred. `test_capture_with_invalid_uuid_logs_and_does_not_send` and `test_capture_with_invalid_uuid_raises_in_debug` each exercise only a single invalid input (`"not-a-uuid"`). A `@parameterized.expand` over a range of bad values (e.g. `""`, `"not-a-uuid"`, `"1234"`, `123`) would give more coverage and follows the existing pattern already used elsewhere in this file.

_{Reviews (1): Last reviewed commit: "fix: validate custom event UUIDs" | Re-trigger Greptile}

[github-actions]

posthog-python Compliance Report

Date: 2026-06-19 12:53:44 UTC
Duration: 540109ms

✅ All Tests Passed!

45/45 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	516ms
Format Validation.Event Has Uuid	✅	10007ms
Format Validation.Event Has Lib Properties	✅	10007ms
Format Validation.Distinct Id Is String	✅	10007ms
Format Validation.Token Is Present	✅	10008ms
Format Validation.Custom Properties Preserved	✅	10007ms
Format Validation.Event Has Timestamp	✅	10008ms
Retry Behavior.Retries On 503	✅	18024ms
Retry Behavior.Does Not Retry On 400	✅	12004ms
Retry Behavior.Does Not Retry On 401	✅	10008ms
Retry Behavior.Respects Retry After Header	✅	16013ms
Retry Behavior.Implements Backoff	✅	32016ms
Retry Behavior.Retries On 500	✅	16014ms
Retry Behavior.Retries On 502	✅	16011ms
Retry Behavior.Retries On 504	✅	16010ms
Retry Behavior.Max Retries Respected	✅	32024ms
Deduplication.Generates Unique Uuids	✅	9996ms
Deduplication.Preserves Uuid On Retry	✅	16013ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	23021ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	16005ms
Deduplication.No Duplicate Events In Batch	✅	10002ms
Deduplication.Different Events Have Different Uuids	✅	10007ms
Compression.Sends Gzip When Enabled	✅	10007ms
Batch Format.Uses Proper Batch Structure	✅	10007ms
Batch Format.Flush With No Events Sends Nothing	✅	5005ms
Batch Format.Multiple Events Batched Together	✅	10005ms
Error Handling.Does Not Retry On 403	✅	12010ms
Error Handling.Does Not Retry On 413	✅	10007ms
Error Handling.Retries On 408	✅	14013ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	9502ms
Request Payload.Flags Request Uses V2 Query Param	✅	10007ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	10007ms
Request Payload.Flags Request Omits Authorization Header	✅	10008ms
Request Payload.Token In Flags Body Matches Init	✅	10007ms
Request Payload.Groups Round Trip	✅	10006ms
Request Payload.Groups Default To Empty Object	✅	10008ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	10007ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	10007ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	10007ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	10006ms
Request Lifecycle.No Flags Request On Init Alone	✅	5004ms
Request Lifecycle.No Flags Request On Normal Capture	✅	10508ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	9511ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	10003ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	10509ms

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
posthog/args.py:65-67
The phrase "ignored" implies silent behavior, but invalid UUIDs are actually logged at ERROR level (`self.log.error(...)`) before the fallback UUID is generated. ERROR-level logs can trigger production alerting. This same wording appears in `__init__.py` (three times) and `client.py` (five docstrings). Replacing "ignored" with "trigger an error log and" more accurately describes what callers can expect.

```suggestion
            UUID is returned, so you can correlate it with actions in your app. If provided,
            it must be a valid UUID string or uuid.UUID instance; invalid values trigger an
            error log and are replaced with a newly generated UUID.
```

### Issue 2 of 2
posthog/client.py:1117-1124
The `group_identify` signature was updated to `Optional[Union[str, UUID]]` in this PR, but the `alias` method's `uuid` parameter is still untyped. Since the docstring for `alias.uuid` was updated to document the same UUID validation behavior, the type annotation should match for consistency.

```suggestion
    def alias(
        self,
        previous_id: str,
        distinct_id: Optional[str],
        timestamp=None,
        uuid: Optional[Union[str, UUID]] = None,
        disable_geoip=None,
    ):
```

_{Reviews (2): Last reviewed commit: "docs: document custom UUID fallback beha..." | Re-trigger Greptile}