The GraphQL Schema Stitcher combines multiple GraphQL subgraph schemas into a unified federated API gateway. It implements Apollo Federation v2 composition using @apollo/composition to merge subgraph SDL definitions with proper entity resolution, @key directives, and shared type handling. The tool also supports legacy schema stitching via graphql-tools stitchSchemas for non-federated architectures, automatically generating type merging configuration and delegating resolvers. Before composition, it validates individual subgraph schemas using graphql-js validateSchema and checks federation compliance with rover graph check against Apollo Studio. The stitcher detects conflicting type definitions, missing entity resolvers, and circular reference issues across subgraphs. It generates a composition report showing type ownership, shared fields, and potential breaking changes. For development workflows, it supports hot-reloading of subgraph schemas from local SDL files or introspection endpoints, with automatic recomposition when changes are detected using chokidar file watchers.