Improving DRF Endpoints

Overview

Serializer fields are the source of truth for PostHog's entire type pipeline:

Django serializer → drf-spectacular → OpenAPI JSON → Orval → Zod schemas → MCP tools

Every help_text, every field type, every @extend_schema annotation flows downstream. A missing help_text means an agent guessing at parameters. A bare ListField() means z.unknown() in the generated Zod schema. Getting the serializer right means every consumer — frontend types, MCP tools, API docs — gets correct types and descriptions automatically.

When to use

• Editing or reviewing any file that defines a Serializer or ViewSet
• Fixing OpenAPI spec warnings or generated type issues
• Preparing an endpoint for MCP tool exposure
• Code review of API changes

Audit checklist

Triage: check the generated output first

Before diving into Python, look at the committed generated types to see what's broken. Find the generated files for the endpoint's product:

• Core API: frontend/src/generated/core/
• Product APIs: products/<product>/frontend/generated/

Each has two files:

• `api.schemas.ts` — TypeScript interfaces derived from serializers. Search for the serializer name and look for unknown types (bare ListField/JSONField), missing JSDoc descriptions (missing help_text), or overly generic Record<string, unknown> shapes.
• `api.ts` — API client functions. Check if the endpoint's operation exists at all — if missing, the viewset method likely lacks @extend_schema.

This tells you exactly which fields and endpoints to prioritize.

Serializer fields

Work through this list for every serializer and viewset you touch.

1. Every field has `help_text` — describes purpose, format, constraints, valid values
2. No bare `ListField()` or `DictField()` — always specify child= with a typed serializer or field
3. No bare `JSONField()` — create a custom field class with @extend_schema_field(TypedSchema)
4. `SerializerMethodField` has `@extend_schema_field` on its get_* method
5. `ChoiceField` has explicit `choices=` with all valid values listed
6. Avoid collision-prone enum field names — format, type, status, kind, level, mode, state, platform, provider clash with existing choices and fail CI under --fail-on-warn; pick a specific name or add an ENUM_NAME_OVERRIDES entry up front (see serializer-fields.md)
7. Read vs write serializers are separate when input shape differs from output
8. Every success response is backed by a serializer — returning raw dicts or untyped lists means no generated types downstream

See serializer-fields.md for patterns and examples.

Viewset and action annotations

1. Every custom `@action` has `@extend_schema` or `@validated_request` — without it, drf-spectacular discovers zero parameters
2. Plain `ViewSet` methods have schema annotations — ModelViewSet with serializer_class is auto-discovered; plain ViewSet is not
3. `@extend_schema` is on the actual method (get, post, create, list), not on a helper or the class itself
4. Error responses are typed — use OpenApiResponse(response=ErrorSerializer), not OpenApiTypes.OBJECT
5. List endpoints declare pagination — reset with pagination_class=None on custom actions that don't paginate
6. Prefer `@validated_request` over manual serializer.is_valid() + @extend_schema — it handles both in one decorator
7. ViewSets outside `products/` need `@extend_schema(extensions={"x-product": "<product>"})` — ViewSets in products/<name>/backend/ are auto-attributed via module path; ViewSets in posthog/api/ or ee/ aren't and must declare attribution explicitly via the x-product extension. Accepts a plain string ("product_analytics") or ProductKey.X enum (kebab values are normalized). Don't use tags=["<product>"] to influence codegen routing — tags is for Swagger UI display only. Without x-product, the MCP scaffold and frontend type generator can't route the endpoint to the right product
8. `partial_update` `request=` override must be a superset of runtime write fields — extend_schema(request=CustomSerializer) replaces drf-spectacular's inference from serializer_class; omitted fields disappear from OpenAPI, frontend types, and MCP tool schemas even when the runtime serializer still accepts them. After changing the override, run hogli build:openapi and verify generated MCP tool schemas still expose every OpenAPI body field

Streaming endpoints: For SSE or streaming responses, use @extend_schema(request=InputSerializer, responses={(200, "text/event-stream"): OpenApiTypes.STR}) to document the request schema even though the response can't be fully typed.

See viewset-annotations.md for patterns and examples.

URL routing — where to register new team-nested endpoints

PostHog briefly split projects and environments as separate concepts then rolled the split back. `/api/projects/:team_id/...` is the canonical path for any team-nested endpoint. /api/environments/:team_id/... is a backward-compat alias preserved only for clients that integrated against it during the split.

For a new team-nested endpoint, register it under routers.projects. Routes live in each product's own products/<name>/backend/routes.py, in a register_routes(routers) function:

# products/<name>/backend/routes.py
from posthog.api.routing import RouterRegistry


def register_routes(routers: RouterRegistry) -> None:
    routers.projects.register(r"my_thing", MyThingViewSet, "project_my_thing", ["team_id"])

Product routes are auto-discovered — posthog/api/__init__.py iterates INSTALLED_APPS and calls register_routes(routers) on every products.* app that has a routes.py. Adding a product needs no edit to core: create products/<name>/backend/routes.py and make sure the product is in PRODUCTS_APPS (posthog/settings/web.py). Only core, non-product viewsets still register directly in __init__.py.

Why core discovers and calls the product (not the product calling core). Core registers the four parents (root + projects/environments/organizations) first, then runs the discovery loop. Products only nest onto those parents and never onto each other, so discovery order is irrelevant. The registration is kept eager (it runs when posthog.api is first imported, i.e. on the first request) and deliberately _not_ moved into AppConfig.ready(): ready() runs inside django.setup() in every process, and registering a route imports its viewset, so that would pull the whole API into setup() everywhere — regressing the laziness that keeps the API out of Celery workers and management commands. See the RouterRegistry docstring and the discovery loop in posthog/api/__init__.py for the full reasoning.

Do not register new endpoints under environments_router. Do not use the dual-route helper (routers.register_legacy_dual_route, or register_legacy_dual_route_team_nested_viewset in __init__.py) — it exists only for endpoints already exposed on both /api/projects/ and /api/environments/ before the rollback.

If existing clients need /api/environments/... too, the OpenAPI postprocess hook at posthog.api.documentation.preprocess_exclude_path_format auto-marks the env-side path as deprecated: true whenever both routes exist.

Facade products (DataclassSerializer)

For products using the facade pattern (e.g., visual_review) with DataclassSerializer wrapping frozen dataclasses from contracts.py:

• Field types are auto-derived from the dataclass — fewer typing issues by design
• Focus on `help_text` (dataclass fields don't carry it; add it on the serializer field overrides)
• `@validated_request` is already the standard pattern — verify response serializers are declared
• @extend_schema tags and descriptions still need to be set on viewset methods

Decision flowchart

digraph audit {
    rankdir=TB
    node [shape=diamond fontsize=10]
    edge [fontsize=9]

    start [label="Serializer or\nViewSet file?" shape=box]
    is_model [label="ModelViewSet with\nserializer_class?"]
    is_plain [label="Plain ViewSet or\ncustom @action?"]
    is_facade [label="DataclassSerializer\n(facade product)?"]

    check_fields [label="Check fields:\nhelp_text, ListField,\nJSONField, ChoiceField" shape=box]
    add_schema [label="Add @validated_request\nor @extend_schema to\nevery method" shape=box]
    check_help [label="Focus on help_text\nand response declarations" shape=box]
    check_responses [label="Check response types,\npagination, error schemas" shape=box]

    start -> is_model
    is_model -> check_fields [label="yes"]
    is_model -> is_plain [label="no"]
    is_plain -> add_schema [label="yes"]
    is_plain -> is_facade [label="no"]
    is_facade -> check_help [label="yes"]
    check_fields -> check_responses
    add_schema -> check_fields
    check_help -> check_responses
}

Quick reference

See quick-reference-table.md for a scannable "I see X, do Y" lookup.

See common-anti-patterns.md for before/after code pairs.

Canonical examples in the codebase

• JSONField + @extend_schema_field: posthog/api/alert.py
• @validated_request: products/tasks/backend/api.py
• help_text + typed responses: products/llm_analytics/backend/api/evaluation_summary.py
• Facade product: products/visual_review/backend/presentation/views.py

Related

• Downstream: After fixing serializers, use the implementing-mcp-tools skill to scaffold MCP tools
• Pipeline docs: docs/published/handbook/engineering/type-system.md
• Mixins: posthog/api/mixins.py (@validated_request source)
• drf-spectacular config: posthog/settings/web.py (SPECTACULAR_SETTINGS)
• Enum collision diagnostic: python manage.py find_enum_collisions — finds unresolved collisions and suggests overrides