chore(capture): document capture_mode + release changeset (capture v1, 6/6)

.sampo/changesets/capture-v1-mode.md

@@ -0,0 +1,9 @@
+---
+pypi/posthog: minor
+---
+
+Add an opt-in `capture_mode` for the Capture V1 ingestion protocol (`POST /i/v1/analytics/events`). Set `capture_mode="v1"` on the client (or the `POSTHOG_CAPTURE_MODE=v1` environment variable) to use Bearer auth, per-event results, and partial retry. Defaults to `"v0"` (the legacy `/batch/` endpoint), so existing setups are unaffected.
+
+When using `capture_mode="v1"`, request bodies can be compressed via `capture_compression` (or `POSTHOG_CAPTURE_COMPRESSION`): `"gzip"`, `"deflate"`, `"zstd"` (requires the optional `posthog[zstd]` extra), or `"none"` (default). The legacy `gzip=True` flag is honored as a fallback.
+
+Per-event server verdicts are surfaced through the existing `on_error` handler: events the backend explicitly drops, or fails to accept after retries, raise a `CaptureV1Error` carrying the affected event UUIDs — so a rejection is never silently lost, even when the HTTP request itself succeeded.

AGENTS.md

@@ -9,6 +9,27 @@ Guidance for coding agents working in `posthog-python`.
 - The project uses `uv` for local development. See `CONTRIBUTING.md` for setup.
 - Keep edits targeted and follow existing patterns. Prefer adding or updating tests near the behavior you change.
 
+## Capture protocol (`capture_mode`)
+
+The client supports two ingestion wire protocols, selected by `capture_mode` (precedence: explicit `Client(capture_mode=...)` kwarg > `POSTHOG_CAPTURE_MODE` env var > default).
+
+- `"v0"` (default) — legacy `POST /batch/`. Upgrades stay transparent; existing callers are unaffected.
+- `"v1"` — `POST /i/v1/analytics/events`: Bearer auth, a typed event `options` object, per-event results, and partial retry.
+
+v1 request bodies can additionally be compressed via `capture_compression` (precedence: explicit `Client(capture_compression=...)` kwarg > `POSTHOG_CAPTURE_COMPRESSION` env var > the legacy `gzip` flag > none). Supported values are `"none"`, `"gzip"`, `"deflate"` (zlib-wrapped, RFC 1950, to match the server's decoder and the Go/Rust SDKs), and `"zstd"` (requires the optional `posthog[zstd]` extra; explicit zstd without the package raises, env-var zstd warns and falls back). v0 keeps using its own `gzip` flag; `capture_compression` is v1-only.
+
+Where the pieces live:
+
+- `posthog/capture_mode.py` — the `CaptureMode` enum and `_resolve_capture_mode()` precedence logic.
+- `posthog/capture_compression.py` — the `CaptureCompression` enum and `_resolve_capture_compression()` precedence logic (with `gzip` fallback).
+- `posthog/capture_v1.py` — pure transforms (`_to_v1_event`, `_build_v1_batch_body`) and transport (`_post_v1`, `_compress_v1`, `_parse_v1_response`, `_send_v1_batch`, `CaptureV1Error`).
+- Public API surface (enforced by `references/public_api_snapshot.txt`): `CaptureMode`, `CaptureCompression` (both re-exported from `posthog`), `CaptureV1Error`, and the two env var names. Everything else in these modules is underscore-private plumbing.
+- Routing: `Consumer._send_analytics` (async) and `Client._enqueue` (sync) pick the analytics submitter by `capture_mode`. The dedicated `$ai_*` endpoint has no v1 form and always uses the legacy submitter.
+
+v1-specific behavior to preserve when editing: sentinel `$`-properties are lifted into `options` (coerced to native JSON types or omitted — a wrong type 400s the whole batch); top-level `$set`/`$set_once` are relocated into `properties`; only events the server tags `retry` are resent (stable `PostHog-Request-Id`/`created_at`, incrementing `PostHog-Attempt`); a server `drop` is a terminal per-event rejection — drops are accumulated across attempts and surfaced via `CaptureV1Error`/`on_error` even on a 2xx with no retries (a success status is not full delivery); `Retry-After` is a *minimum*, not a replacement (the client waits `max(configured_backoff, min(Retry-After, _MAX_BACKOFF_SECONDS))`); `_MAX_BACKOFF_SECONDS` (30s) is the single ceiling for both the exponential backoff and the `Retry-After` clamp; `429` is terminal.
+
+Retry blocking matches v0: in the default async mode retries happen on the background consumer thread, but with `sync_mode=True` the partial-retry loop (including its backoff sleeps) runs inline on the calling thread, so a slow/erroring endpoint blocks the caller until retries are exhausted.
+
 ## Validation
 
 Useful checks: