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UseCase Service

UseCaseService lets you define business logic as service classes that serve both MCP and web frameworks — one definition, two presentations.

Design Philosophy

UseCaseService subclass ──┬── MCP server (AI agents, four-layer progressive discovery)
                         └── FastAPI routes (REST API, OpenAPI docs)

Defining a UseCaseService

UseCaseService subclasses declare async classmethod methods. The metaclass auto-discovers public methods:

from sqlmodel_nexus.use_case import UseCaseService

class SprintService(UseCaseService):
    """Sprint management service."""

    @classmethod
    async def list_sprints(cls) -> list[SprintSummary]:
        """Get all sprints with their task counts."""
        stmt = build_dto_select(SprintSummary)
        async with async_session() as session:
            rows = (await session.exec(stmt)).all()
        dtos = [SprintSummary(**dict(row._mapping)) for row in rows]
        return await Resolver().resolve(dtos)

    @classmethod
    async def get_sprint(cls, sprint_id: int) -> SprintSummary | None:
        """Get a sprint by ID."""
        stmt = build_dto_select(SprintSummary, where=Sprint.id == sprint_id)
        async with async_session() as session:
            rows = (await session.exec(stmt)).all()
        if not rows:
            return None
        dto = SprintSummary(**dict(rows[0]._mapping))
        return await Resolver().resolve(dto)

Exposing to MCP

Four-layer progressive discovery: app discovery → service list → method details → execution.

from sqlmodel_nexus.use_case import UseCaseAppConfig, create_use_case_mcp_server

mcp = create_use_case_mcp_server(
    apps=[
        UseCaseAppConfig(
            name="project",
            services=[SprintService, TaskService],
            description="Project management",
        ),
    ],
    name="Project UseCase API",
)
mcp.run()  # stdio mode

MCP Tools

Tool Purpose
list_apps() Discover available apps
list_services(app_name) List services and method counts in an app
describe_service(app_name, service_name) Method signatures (SDL format) + DTO type definitions
call_use_case(app_name, service_name, method_name, params) Execute method

describe_service Output

{
  "name": "SprintService",
  "methods": [
    {"name": "list_sprints", "signature_sdl": "list_sprints(): [SprintSummary!]!"},
    {"name": "get_sprint", "signature_sdl": "get_sprint(sprint_id: Int!): SprintSummary"}
  ],
  "types": "type SprintSummary {\n  id: Int\n  name: String!\n  tasks: [TaskSummary!]!\n}"
}

Next Steps