Add ai_trace_sample_rate to sample AI observability events by trace id

Problem Statement

Teams whose AI observability volume is mostly $ai_* events have no supported way to keep a fraction of traces. The only path today is a hand-written before_send hook that hashes $ai_trace_id. Most teams do not write one. A hook that samples per event splits a trace, so the trace view shows partial trees.

Related: PostHog/posthog-js#4997 for the same option in posthog-node.

Solution Brainstorm

One client option, default off:

Client(api_key, ai_trace_sample_rate=0.1)

Env var POSTHOG_AI_TRACE_SAMPLE_RATE, same precedence rule as POSTHOG_CAPTURE_MODE (kwarg > env > default).

Behavior:

1. Applies to events named $ai_* that carry $ai_trace_id.
2. FNV-1a 32-bit hash of the trace id. Keep when hash / 2^32 < rate. Same hash in every SDK, so a mixed Python and Node stack makes one decision per trace.
3. Runs in Client._enqueue before before_send, so a customer hook sees only kept events.
4. Adds $ai_trace_sample_rate to kept events, so the server can scale totals later.
5. Events with no trace id are kept. Rate unset or 1.0 means no sampling. Out-of-range values log a warning and are ignored.

flowchart LR
  W[AI wrappers and agent processors] --> C[_capture_ai_event]
  M[Manual capture of $ai_* events] --> E[Client._enqueue]
  C --> E
  E --> S{ai_trace_sample_rate}
  S -- drop --> X[Dropped]
  S -- keep --> B[before_send] --> Q[Queue]

Alternatives considered: a helper that returns a before_send function. Rejected because before_send takes one callable, so the helper does not compose with an existing hook.

Impact

• Gives AI observability users a supported volume lever that keeps traces whole.
• Removes the need for a custom hook that is easy to get wrong.

Blast radius

• Default off. No behavior change for any existing caller.
• Touches Client.__init__ and Client._enqueue only. Wrappers do not change.
• Affects $ai_* events only. Other events never reach the sampler.
• Kept $ai_* events gain one property when the option is set.
• Public API grows by one option and one env var. references/public_api_snapshot.txt changes.

Not in this issue

Tail sampling that keeps errored traces. A per-call override. Server-side upweighting of cost dashboards.