feat(ph-ai): PostHog properties dict in GenerationMetadata

[tatoalo]

Problem

PostHog AI team needs to support billing certain LLM generations based on centralized logic that will live in the posthog/hogai repository.

The core idea is simple, add a $ai_billable metadata property that would be signaling, at generation time, wether we should flag it as billable or not.

Changes

• billable field throughout the LangChain callback handler
• Metadata extraction
• Event property capture

Tests

• unit tests
• integration tests locally with posthog repo

[greptile-apps]

_{4 files reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

syntax: Typo: should be ai_billable not ai_billing to match the actual property name used in the code ($ai_billable).

Suggested change

	- feat(ph-ai): ai_billing metadata per-generation
	- feat(ph-ai): ai_billable metadata per-generation

Prompt To Fix With AI

This is a comment left during a code review.
Path: CHANGELOG.md
Line: 3:3

Comment:
**syntax:** Typo: should be `ai_billable` not `ai_billing` to match the actual property name used in the code (`$ai_billable`).

```suggestion
- feat(ph-ai): ai_billable metadata per-generation
```

How can I resolve this? If you propose a fix, please make it concise.

[skoob13]

Should we make it useful for customers and for us?

More details: https://github.com/PostHog/posthog/pull/40881/files#r2493990657

[skoob13]

I still don't understand why we don't allow setting a dictionary?

[tatoalo]

Why would we need it to be? Previously I did that but I don't see a big reason, we are still looking into the metadata but populating the single field on the client side. Having a dict would either be for all metadata or I don't see a point introducing it for this field honestly.

[skoob13]

Taking the current path, we add our internal implementation details to the SDK, which has little value and is limiting for customers. You can achieve the desired result by allowing a dictionary to be passed in LangChain's metadata, so anyone else who wants to set similar data, but not the billing props, won't have the same problem you're currently having.

[skoob13]

Thanks, looks good. Left a nit, which is up to you.

[Twixes]

Generalized version looks good