Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/posthog/posthog-foss/isolating-product-facade-contractsnpx skills add PostHog/posthog-foss --skill isolating-product-facade-contractsgit clone --depth 1 https://github.com/PostHog/posthog-fossWhat it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00076 | $0.08662 |
| Opus 5 | $0.00038 | $0.04331 |
| Sonnet 5 | $0.00015 | $0.01732 |
| Haiku 4.5 | $0.00008 | $0.00866 |
Grade A, and why
isolating-product-facade-contracts scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured today.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 565 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Isolating a product with facade and contracts
Before you choose a product to isolate, check things already tried. It records which product was the first candidate, and why the field test moved to another one.
Use this skill to migrate an existing product to the isolated architecture used by Visual review. Optimize for short calendar exposure, not small diffs: authoring is cheap and the verification chain catches mechanical breakage, while human review latency and a fast-moving master are the real bottlenecks. Default to one PR structured in reviewable commits; a separate facade-first PR exists only as the merge base for team-sliced sweeps — per the PR strategy below.
Prerequisite: the product must already live under products/<name>/. This skill does not cover moving code out of posthog/, ee/, or other shared directories — do that first.
Core docs to load first
Read these before changing code:
- products/architecture.md
- products/README.md
- docs/internal/monorepo-layout.md
- posthog/models/team/README.md (team extension model rule)
- docs/published/handbook/engineering/type-system.md (serializer/OpenAPI type flow)
- docs/published/handbook/engineering/ai/implementing-mcp-tools.md (schema quality and team isolation expectations)
- .agents/security.md (SQL/HogQL security guidelines)
Use Visual review as the concrete reference implementation:
- products/visual_review/backend/facade/contracts.py
- products/visual_review/backend/facade/api.py
- products/visual_review/backend/presentation/views.py
- products/visual_review/backend/presentation/serializers.py
- products/visual_review/backend/logic/
- products/visual_review/backend/tests/test_api.py
- products/visual_review/backend/tests/test_presentation.py
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- today Changed · +29 lines e5ff34b2d024
- yesterday First seen · 536 lines · 76 tokens per session scan A 78abc720332d
isolating-product-facade-contracts is a skill published in the GitHub repository PostHog/posthog-foss (713 stars, last pushed today), licensed MIT. It adds 76 tokens to every session and 8,662 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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