\n\nQuick Guide: Use Mercurius as a Fastify plugin for GraphQL APIs with built-in loader batching (solves N+1), JIT query compilation, subscriptions via WebSocket, and federation support. Register with
\napp.register(mercurius, { schema, resolvers, loaders }). Loaders are Mercurius's killer feature: define them per-type to batch field resolution automatically. Usejit: 1to enable query compilation for production performance.
<critical_requirements>
\n\n\nAll code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering,
\nimport type, named constants)
(You MUST define loaders for any field that fetches related data — loaders solve the N+1 problem automatically through batching)
\n(You MUST use named constants for all numeric values — JIT thresholds, query depth limits, port numbers)
\n(You MUST return an array from loaders matching the exact length and order of the queries parameter)
(You MUST use fastify.graphql.pubsub.publish() inside mutations to trigger subscriptions — not external pubsub directly)
</critical_requirements>
\nAuto-detection: Mercurius, mercurius, app.graphql, fastify.graphql, mercurius loaders, mercurius subscription, pubsub.publish, pubsub.subscribe, @mercuriusjs/federation, @mercuriusjs/gateway, mercurius-codegen, MercuriusContext, graphql-jit, withFilter, preParsing, preValidation, preExecution, onResolution
\nWhen to use:
\n@mercuriusjs/federationWhen NOT to use:
\nKey patterns covered:
\nwithFilterMercuriusContext augmentationDetailed Resources:
\n<red_flags>
\nUser.posts, Post.author) should use a loader, not inline resolver queries. Without loaders, you get the classic N+1 problem.queries array by index. Returning fewer/more items or in wrong order corrupts the response silently.__resolveReference as resolver instead of loader in federation — Causes N+1 on entity resolution across services. The docs strongly recommend defining it as a loader.jit: 0 means no JIT compilation. Set jit: 1 for production workloads with repeated queries.context function for per-request data — Accessing request headers or auth tokens requires a context builder function, not Fastify decorators alonegraphiql: false or conditionally disable based on NODE_ENVqueryDepth limit — Without depth limiting, deeply nested queries can exhaust server resourcespreExecution — Disables JIT compilation for that query executionsubscription: true in registration — Subscriptions are disabled by default; subscription resolvers silently fail without this optionpayload in pubsub.publish() must match the subscription field name exactly (e.g., { notificationAdded: data } for a notificationAdded subscription)addHook before app.ready() — GraphQL hooks must be registered after app.ready() or inside a Fastify plugin that ensures readiness\n\nDetailed anti-pattern code examples: reference.md
\n
opts: { cache: false } when data changes mid-requestpreValidation is skipped for cached queries — If a query is parsed from cache, validation hooks do not firesubscription.context option for custom subscription contextconnection_init payload goes into request.headers — During WebSocket handshake, properties from the client's connection_init payload are copied into request headers automaticallyschema, resolvers, or loaders on the gateway instancequeryDepth must be at least 7 for GraphiQL — GraphiQL's introspection query requires depth 7+; lower values break the IDE</red_flags>
\n<critical_reminders>
\n\n\nAll code must follow project conventions in CLAUDE.md
\n
(You MUST define loaders for any field that fetches related data — loaders solve the N+1 problem automatically through batching)
\n(You MUST use named constants for all numeric values — JIT thresholds, query depth limits, port numbers)
\n(You MUST return an array from loaders matching the exact length and order of the queries parameter)
(You MUST use fastify.graphql.pubsub.publish() inside mutations to trigger subscriptions — not external pubsub directly)
Failure to follow these rules will cause N+1 performance problems, corrupted GraphQL responses, and broken subscriptions.
\n</critical_reminders>
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