feat(capture): v1 wire serialization transforms (capture v1, 2/6)

[eli-r-ph]

💡 Motivation and Context

Second PR in the stacked Capture V1 series (stacked on #701). Adds the pure, no-I/O serialization layer that turns a legacy-shaped queued message into a /i/v1/analytics/events wire event. Still inert — nothing calls these functions yet (transport + wiring come in later PRs).

New module posthog/capture_v1.py:

• to_v1_event(msg) — dict-to-dict transform proven against the server contract (rust/capture/src/v1/analytics/types.rs) and posthog-go's capture_v1.go:
  • Lifts the sentinel properties into the typed options object, including the $ignore_sent_at -> disable_skew_correction rename.
  • Promotes $session_id/$window_id to top-level string fields.
  • Relocates top-level $set/$set_once into properties — v1 has no top-level form, and the legacy set()/set_once() builders emit them at the top level, so without this person-property updates would silently vanish.
  • Strips $lib/$lib_version (the server injects them from the required PostHog-Sdk-Info header).
  • Coerces options to native JSON types or omits them. v1 deserializes options strictly; a wrong type (e.g. cookieless_mode: "true") 400s the entire batch, so a bad value is dropped rather than forwarded.
  • Pure: the input message is not mutated (safe for retries/callbacks).
• build_v1_batch_body(events, historical_migration) — the api_key/sent_at-free envelope with a tz-aware RFC3339 created_at (historical_migration omitted when false).
• Shared constants: endpoint path, required header names, result codes, retryable/terminal status sets (used by the transport PR).

💚 How did you test it?

New posthog/test/test_capture_v1.py (47 cases, parameterized): bool/string coercion across all variants; per-sentinel lift + rename + coercion; bad-coercion omitted-but-removed; top-level string sentinels; options == {} when empty; $lib stripping; non-wire keys (e.g. type) not leaked; input-not-mutated; top-level $set/$set_once relocation incl. merge-precedence with an existing properties.$set; $groups left untouched; tz-aware/passthrough/default timestamps; envelope shape + RFC3339 created_at + historical_migration toggling.

ruff format/check clean; mypy clean on the new module; regenerated references/public_api_snapshot.txt.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change (additive; transforms are not yet wired in).

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Authored with Cursor (Claude Opus 4.8) per the agreed plan. Architecture note: posthog-go builds v1 events from typed message structs, but posthog-python's queue holds already-assembled legacy dicts, so this is a dict-to-dict transform applied at send time — the wire shape and sentinel table are identical to go's. The strict-type/coerce-or-omit and $set relocation behaviors were verified directly against the Rust Event/Options structs and their serde tests.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(capture): add v1 wire serialization..." | Re-trigger Greptile}

[github-actions]

posthog-python Compliance Report

Date: 2026-06-28 00:55:07 UTC
Duration: 530108ms

✅ All Tests Passed!

45/45 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	517ms
Format Validation.Event Has Uuid	✅	10007ms
Format Validation.Event Has Lib Properties	✅	10007ms
Format Validation.Distinct Id Is String	✅	10006ms
Format Validation.Token Is Present	✅	10007ms
Format Validation.Custom Properties Preserved	✅	10007ms
Format Validation.Event Has Timestamp	✅	10006ms
Retry Behavior.Retries On 503	✅	18019ms
Retry Behavior.Does Not Retry On 400	✅	12004ms
Retry Behavior.Does Not Retry On 401	✅	10006ms
Retry Behavior.Respects Retry After Header	✅	16013ms
Retry Behavior.Implements Backoff	✅	30018ms
Retry Behavior.Retries On 500	✅	13009ms
Retry Behavior.Retries On 502	✅	16010ms
Retry Behavior.Retries On 504	✅	16010ms
Retry Behavior.Max Retries Respected	✅	30017ms
Deduplication.Generates Unique Uuids	✅	7002ms
Deduplication.Preserves Uuid On Retry	✅	16015ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	23018ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	16004ms
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	✅	10006ms
Batch Format.Flush With No Events Sends Nothing	✅	5005ms
Batch Format.Multiple Events Batched Together	✅	10005ms
Error Handling.Does Not Retry On 403	✅	12008ms
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	✅	9500ms
Request Payload.Flags Request Uses V2 Query Param	✅	10007ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	10006ms
Request Payload.Flags Request Omits Authorization Header	✅	10007ms
Request Payload.Token In Flags Body Matches Init	✅	10007ms
Request Payload.Groups Round Trip	✅	10006ms
Request Payload.Groups Default To Empty Object	✅	10006ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	10007ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	10006ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	10006ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	10007ms
Request Lifecycle.No Flags Request On Init Alone	✅	5003ms
Request Lifecycle.No Flags Request On Normal Capture	✅	10507ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	9510ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	10002ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	10510ms