Implementing MCP tools

Read the full guide at docs/published/handbook/engineering/ai/implementing-mcp-tools.md.

Quick workflow

# 1. Scaffold a starter YAML with all operations disabled.
#    --product discovers endpoints via x-explicit-tags (priority 1) then
#    URL substring match (fallback). ViewSets in products/<name>/backend/
#    are auto-tagged. ViewSets elsewhere need @extend_schema(tags=["<product>"]).
pnpm --filter=@posthog/mcp run scaffold-yaml -- --product your_product \
    --output ../../products/your_product/mcp/tools.yaml

# 2. Configure the YAML — enable tools, add scopes, annotations, descriptions
#    Place in products/<product>/mcp/*.yaml (preferred) or services/mcp/definitions/*.yaml

# 3. Add a HogQL system table in posthog/hogql/database/schema/system.py
#    and a model reference in products/posthog_ai/skills/query-examples/references/

# 4. Generate handlers and schemas
hogli build:openapi

Before you scaffold: fix the backend first

The codegen pipeline can only generate correct tools if the Django backend exposes correct types. Read the type system guide for the full picture.

Before scaffolding YAML, verify:

1. Serializers have explicit field types and help_text — these flow all the way to Zod .describe() in the generated tool. Missing descriptions = agents guessing at parameters. Use ListField(child=serializers.CharField()) instead of bare ListField(), and @extend_schema_field(PydanticModel) on JSONField subclasses to get typed Zod output (see posthog/api/alert.py for the pattern).
2. Plain ViewSet methods have @extend_schema(request=...) — without it, drf-spectacular can't discover the request body and the generated tool gets z.object({}) (zero parameters). ModelViewSet with a serializer_class is fine; plain ViewSet with manual validation is not.
3. Query parameters use @validated_request or @extend_schema with a query serializer — otherwise boolean and array query params may produce type mismatches in the generated code.

If a generated tool has an empty or wrong schema, the fix is almost always on the Django side, not in the YAML config. For a full audit checklist and before/after examples, use the improving-drf-endpoints skill.

When to add MCP tools

When a product exposes API endpoints that agents should be able to call. MCP tools are atomic capabilities (list, get, create, update, delete) — not workflows.

If you're adding a new endpoint, check whether it should be agent-accessible. If yes, add a YAML definition and generate the tool.

Tool design

Tools should be basic capabilities — atomic CRUD operations and simple actions. Agents compose these primitives into higher-level workflows.

Good: "List feature flags", "Get experiment by ID", "Create a survey". Bad: "Search for session recordings of an experiment" — bundles multiple concerns.

YAML definitions

YAML files configure which operations are exposed as MCP tools. See existing definitions for patterns:

• products/<product>/mcp/*.yaml — preferred, keeps config close to the code
• services/mcp/definitions/*.yaml — fallback for functionality without a product folder

The build pipeline discovers YAML files from both paths.

Key fields

category: Human readable name
feature: snake_case_name # should match the product folder name (used for runtime filtering)
url_prefix: /path # frontend app route, used for enrich_url links
tools:
  your-tool-name: # kebab-case
    operation: operationId_from_openapi
    enabled: true
    scopes:
      - your_product:read
    annotations:
      readOnly: true
      destructive: false
      idempotent: true
    # Optional:
    mcp_version: 1 # 2 for create/update/delete ops, 1 for read/list if available via HogQL
    title: List things
    description: >
      Human-friendly description for the LLM.
    list: true
    enrich_url: '{id}'
    param_overrides:
      name:
        description: Custom description for the LLM

Unknown keys are rejected at build time (Zod .strict()).

Syncing after endpoint changes

pnpm --filter=@posthog/mcp run scaffold-yaml -- --sync-all

Idempotent and non-destructive — adds new operations as enabled: false, removes stale ones.

Serializer descriptions

Descriptions flow through the entire pipeline:

Django serializer field → OpenAPI spec → Zod schema → MCP tool description

These descriptions are what agents read to understand tool parameters.

• Use help_text on serializer fields — it becomes the OpenAPI description.
• Use param_overrides in YAML to override generated descriptions with imperative instructions.
• Be specific about formats, constraints, and valid values.
• Avoid jargon that an LLM wouldn't understand without context.

HogQL system tables

Every list/get endpoint should have a corresponding HogQL system table in posthog/hogql/database/schema/system.py. This lets agents query data via SQL in v2 of the MCP.

Each system table must include a team_id column for data isolation.

Use mcp_version: 1 on read/list YAML tools when a system table covers the same data — v2 agents use SQL instead.

When adding a system table, also add a model reference file (models-<domain>.md) in products/posthog_ai/skills/query-examples/references/ and register it in products/posthog_ai/skills/query-examples/SKILL.md under Data Schema.

Two MCP versions

• v1 (legacy): all CRUD tools exposed, for clients without skill support.
• v2 (SQL-first): read/list tools replaced by HogQL, create/update/delete tools kept. For coding agents.

Control per-tool availability with mcp_version: 1/2 in the YAML definition.