Defense-in-Depth Validation Skill

Defense-in-Depth Validation

When fixing a bug caused by invalid data, adding validation at one place feels sufficient. But that single check can be bypassed.

Core principle: Validate at EVERY layer data passes through. Make the bug structurally impossible.

The Four Layers

Layer 1: Entry Point Validation

Reject obviously invalid input at API boundary:

function createProject(name: string, workingDirectory: string) {
  if (!workingDirectory || workingDirectory.trim() === "") {
    throw new Error("workingDirectory cannot be empty");
  }
  if (!existsSync(workingDirectory)) {
    throw new Error(`workingDirectory does not exist: ${workingDirectory}`);
  }
}

Layer 2: Business Logic Validation

Ensure data makes sense for this operation:

function initializeWorkspace(projectDir: string, sessionId: string) {
  if (!projectDir) {
    throw new Error("projectDir required for workspace initialization");
  }
}

Layer 3: Environment Guards

Prevent dangerous operations in specific contexts:

async function gitInit(directory: string) {
  if (process.env.NODE_ENV === "test") {
    const normalized = normalize(resolve(directory));
    const tmpDir = normalize(resolve(tmpdir()));

    if (!normalized.startsWith(tmpDir)) {
      throw new Error(`Refusing git init outside temp dir during tests`);
    }
  }
}

Layer 4: Debug Instrumentation

Capture context for forensics:

async function gitInit(directory: string) {
  logger.debug("About to git init", {
    directory,
    cwd: process.cwd(),
    stack: new Error().stack,
  });
}

Applying the Pattern

When fixing a bug:

Trace data flow - Where does bad value originate? Where used?
Map checkpoints - List every point data passes through
Add validation at each layer - Entry, business, environment, debug
Test each layer - Try to bypass layer 1, verify layer 2 catches it

Why Multiple Layers

Single validation: "We fixed the bug" Multiple layers: "We made the bug impossible"

Different layers catch different cases:

Entry validation catches most bugs
Business logic catches edge cases
Environment guards prevent context-specific dangers
Debug logging helps when other layers fail

Integration

Referenced by systematic-debugging after root cause is found. Add validation at each layer the data passed through.

Agent Skills: Defense-in-Depth Validation

Install this agent skill to your local

Skill Files