Fusion Backend Dev

Fusion Backend Consumption

When to use

Use when needing to understand Fusion backend services, available APIs, integration patterns, or architectural decisions.

Typical triggers:

• "How do I call the People API?"
• "Show an example of how the authorization pattern works"
• "What's the contract for the Org service?"
• "How do services handle validation errors?"
• "What async/messaging patterns does Fusion use?"
• "Can I see a reference implementation of a CQRS handler?"
• "How do services integrate with external APIs?"
• "What authentication/authorization requirements do I need?"
• "Show how events flow through the system"
• "What's the pattern for cross-service calls?"
• "How should I structure my API client?"
• "Where should I call the Context API?"
• "What's the difference between a command and a query in Fusion?"

Implicit triggers:

• Building frontend/client app needing to understand backend contracts
• Integrating with Fusion APIs and need patterns
• Designing architecture needing backend best practices
• Learning from existing Fusion service implementations

When not to use

• Creating or modifying backend services — use service-specific repo skill
• Adding new endpoints or API operations — backend development
• Database schema changes or migrations — backend development
• Authorization requirement definitions — backend development (this skill shows what exists, not new requirements)
• Pure architecture discussions without code references — use fusion-research or ADR-focused skills
• Selecting between Fusion Framework alternatives — use fusion-research or fusion-app-react-dev

Required inputs

Mandatory

• What you're trying to do: clear description of integration point, use case, or pattern
• Your role/context: building a frontend app? Integrating externally? Designing architecture?

Conditional

• When comparing patterns: which two options you're deciding between
• When consuming an API: what operation/scenario (CRUD, async, real-time, etc.)
• When integrating: external system name and direction of flow (calling out vs being called)

Instructions

Step 1 — Clarify consumption context

Before searching for code, understand what you need:

1. Integration point: Calling a backend API? Reading event messages? Implementing a webhook? Integrating with external system?
2. Your boundaries: Frontend developer? Backend developer in another service? External integrator? Architect?
3. Scope: Single API contract? Full pattern? Reference implementation? Architectural tradeoffs?

Use assets/follow-up-questions.md if user intent is unclear.

Step 2 — Search for reference implementation

Use mcp_fusion_search_backend_code to locate existing patterns:

1. Call with high-level intent: "How People service exposes authorization" or "Cross-service API integration patterns"
2. Start with top: 3-5 results
3. Capture metadata.repository, metadata.service, metadata.filePath
4. Extract minimal code snippets showing the pattern (method signature, type contract, authorization check)
5. If results are unclear, refine once:
   • Add specific service name or interface
   • Narrow to specific layer (controller, handler, client interface)
   • Try a different phrase focusing on outcome rather than implementation details

Step 3 — Explain the pattern

Use evidence from Step 2:

1. State the pattern clearly: What does the service do? What contract does it expose?
2. Show the reference code: Quote relevant snippet with file path and line range
3. Explain the constraints: Preconditions? Authorization? Error handling? Async behavior?
4. Relate to your use case: How to apply this pattern?
5. Surface tradeoffs or alternatives if they exist

Step 4 — Verify completeness

Before ending, check:

• User understands the contract (inputs, outputs, errors)
• User sees a real code reference (not invented)
• User knows where the code lives (repository, service, file path)
• User knows prerequisites (authentication, configuration, dependencies)
• User has enough context to implement or integrate

If uncertainty remains, flag it explicitly.

Reference guides

See references/ for deeper pattern documentation:

• api-contracts.md — Fusion service API contracts and versioning
• authorization-patterns.md — Authentication, authorization requirements, role-based access
• validation-patterns.md — Input validation, error responses, business rules
• async-patterns.md — Events, service bus, domain notifications, eventual consistency
• integration-patterns.md — Cross-service calls, external APIs, webhook handling
• cqrs-reference.md — CQRS handlers, commands, queries, notifications structure

Assets

• assets/follow-up-questions.md — Clarifying questions for ambiguous requests
• references/integration-patterns.md — Common integration scenarios and which patterns apply

Safety & constraints

Never:

• Describe real backend API behavior as fact unless verifiable in retrieved source code or cited repo docs
• Claim a pattern exists when search returns no evidence
• Present illustrative pseudo-code as retrieved source code
• Suggest modifying a backend service — that's out of scope

Always:

• Label illustrative examples as examples/pseudo-code when explanatory rather than retrieved
• Capture and cite repository, file path, and line references for real code
• State which repository the pattern comes from
• Note when a pattern exists in one service but not others
• Offer to escalate to the fusion-services-developer agent if user wants to implement changes
• For setting up or deploying a new standalone backend API (app registration, Roles V2, database, Radix/Kubernetes deployment, observability), point to the New Backend Service Checklist on fusion-docs rather than improvising the sequence