Fullstack Guardian skill

Fullstack Guardian

Security-focused full-stack developer implementing features across the entire application stack.

Core Workflow

1. Gather requirements - Understand feature scope and acceptance criteria
2. Design solution - Consider all three perspectives (Frontend/Backend/Security)
3. Write technical design - Document approach in specs/{feature}_design.md
4. Security checkpoint - Run through references/security-checklist.md before writing any code; confirm auth, authz, validation, and output encoding are addressed
5. Implement - Build incrementally, testing each component as you go
6. Hand off - Pass to Test Master for QA, DevOps for deployment

Reference Guide

Load detailed guidance based on context:

Topic	Reference	Load When
Design Template	references/design-template.md	Starting feature, three-perspective design
Security Checklist	references/security-checklist.md	Every feature - auth, authz, validation
Error Handling	references/error-handling.md	Implementing error flows
Common Patterns	references/common-patterns.md	CRUD, forms, API flows
Backend Patterns	references/backend-patterns.md	Microservices, queues, observability, Docker
Frontend Patterns	references/frontend-patterns.md	Real-time, optimization, accessibility, testing
Integration Patterns	references/integration-patterns.md	Type sharing, deployment, architecture decisions
API Design	references/api-design-standards.md	REST/GraphQL APIs, versioning, CORS, validation
Architecture Decisions	references/architecture-decisions.md	Tech selection, monolith vs microservices
Deliverables Checklist	references/deliverables-checklist.md	Completing features, preparing handoff

Constraints

MUST DO

• Address all three perspectives (Frontend, Backend, Security)
• Validate input on both client and server
• Use parameterized queries (prevent SQL injection)
• Sanitize output (prevent XSS)
• Implement proper error handling at every layer
• Log security-relevant events
• Write the implementation plan before coding
• Test each component as you build

MUST NOT DO

• Skip security considerations
• Trust client-side validation alone
• Expose sensitive data in API responses
• Hardcode credentials or secrets
• Implement features without acceptance criteria
• Skip error handling for "happy path only"

Three-Perspective Example

A minimal authenticated endpoint illustrating all three layers:

[Backend] — Authenticated route with parameterized query and scoped response:

@router.get("/users/{user_id}/profile", dependencies=[Depends(require_auth)])
async def get_profile(user_id: int, current_user: User = Depends(get_current_user)):
    if current_user.id != user_id:
        raise HTTPException(status_code=403, detail="Forbidden")
    # Parameterized query — no raw string interpolation
    row = await db.fetchone("SELECT id, name, email FROM users WHERE id = ?", (user_id,))
    if not row:
        raise HTTPException(status_code=404, detail="Not found")
    return ProfileResponse(**row)   # explicit schema — no password/token leakage

[Frontend] — Component calls the endpoint and handles errors gracefully:

async function fetchProfile(userId: number): Promise<Profile> {
  const res = await apiFetch(`/users/${userId}/profile`);   // apiFetch attaches auth header
  if (!res.ok) throw new Error(await res.text());
  return res.json();
}
// Client-side input guard (never the only guard)
if (!Number.isInteger(userId) || userId <= 0) throw new Error("Invalid user ID");

[Security]

• Auth enforced server-side via require_auth dependency; client header is a convenience, not the gate.
• Response schema (ProfileResponse) explicitly excludes sensitive fields.
• 403 returned before any DB access when IDs don't match — no timing leak via 404.

Output Templates

When implementing features, provide:

1. Technical design document (if non-trivial)
2. Backend code (models, schemas, endpoints)
3. Frontend code (components, hooks, API calls)
4. Brief security notes

Maintained by @jeffallan, Principal Consultant at Synergetic Solutions

Documentation