evalbuff: add patterns/existing-implementation-validation.md (fde408c)

Author: jahooma

AGENTS.md

@@ -49,3 +49,4 @@ Make an efficient learning agent that can do anything.
 - [docs/patterns/task-scope-adherence.md](docs/patterns/task-scope-adherence.md)
 - [docs/patterns/task-type-identification.md](docs/patterns/task-type-identification.md)
 - [docs/patterns/implementation-validation.md](docs/patterns/implementation-validation.md)
+- [docs/patterns/existing-implementation-validation.md](docs/patterns/existing-implementation-validation.md)

docs/patterns/existing-implementation-validation.md

@@ -0,0 +1,150 @@
+# Existing Implementation Validation Pattern
+
+When you discover existing code that appears to implement the requested functionality, validate three things: location correctness, functionality, and clear communication to the user.
+
+## The Problem
+
+Agents often find existing implementations and immediately conclude the task is done without:
+- Verifying the implementation is in the correct location/context
+- Testing that it actually works as expected
+- Clearly communicating the task status to the user
+
+This leads to:
+- Missing implementations in the wrong location (main app vs specific sub-app)
+- Assuming code works without verification
+- Silent completion that leaves users uncertain
+
+## The Three-Step Validation Process
+
+### 1. Location Validation
+
+Even if the functionality exists somewhere in the codebase, verify it's in the RIGHT place:
+
+```bash
+# Task: "Add redirects to freebuff/web/next.config.mjs"
+# Found: Redirects in web/next.config.mjs
+# Problem: Wrong location - freebuff/web is a separate app
+```
+
+**Check:**
+- Is this the correct file path mentioned in the task?
+- Is this the right application/service (monorepo apps are separate)
+- Does the implementation serve the intended use case?
+
+### 2. Functionality Validation
+
+Don't assume existing code works. Test the actual behavior:
+
+```bash
+# For Next.js redirects
+curl -I http://localhost:PORT/b/test123
+# Should return 307/308 with correct Location header
+
+# For API endpoints
+curl http://localhost:PORT/api/endpoint
+# Should return expected response
+
+# For features
+# Test the actual user-facing behavior
+```
+
+**Verify:**
+- The implementation is active (not just configured)
+- It handles the exact patterns requested
+- Edge cases work (query parameters, special characters)
+
+### 3. Clear Communication
+
+Always explicitly tell the user what you found and what the status is:
+
+**✅ When implementation exists and works in the right place:**
+```
+I found the redirect configuration in freebuff/web/next.config.mjs:
+
+[show the code]
+
+Testing confirms it works correctly:
+$ curl -I http://localhost:3001/b/test123
+HTTP/1.1 307 Temporary Redirect
+Location: https://go.trybeluga.ai/test123
+
+The feature is already implemented and functional. No changes needed.
+```
+
+**✅ When implementation exists but in wrong location:**
+```
+I found redirect configuration in the main web/next.config.mjs, but the task requires it in freebuff/web/next.config.mjs (separate app).
+
+Adding the redirect to freebuff/web/next.config.mjs:
+
+[show the changes]
+
+This will enable the redirect specifically for the freebuff web app.
+```
+
+**✅ When implementation doesn't exist:**
+```
+I need to add the redirect configuration to freebuff/web/next.config.mjs:
+
+[show the changes]
+
+This will redirect /b/:hash to https://go.trybeluga.ai/:hash as requested.
+```
+
+## Context-Specific Validation
+
+### Monorepo Applications
+
+In monorepos, similar functionality may exist in multiple apps:
+- `web/` - main application
+- `freebuff/web/` - separate free tier app
+- Each has its own `next.config.mjs`, `package.json`, etc.
+
+**Always check the specific path mentioned in the task.**
+
+### Configuration vs Runtime
+
+Configuration existing ≠ feature working:
+- **Next.js redirects:** Must be in active config AND server restarted
+- **API routes:** Must be in correct location AND export right methods
+- **Environment variables:** Must be set in runtime environment
+
+### Test Coverage
+
+If tests exist, use them as validation:
+```typescript
+// If you find tests like this:
+test('redirects to go.trybeluga.ai with the hash', async ({ request }) => {
+  const response = await request.get('/b/test123', { maxRedirects: 0 })
+  expect(response.status()).toBe(307)
+  expect(response.headers()['location']).toBe('https://go.trybeluga.ai/test123')
+})
+
+// Run them to verify functionality
+bun test path/to/redirect.test.ts
+```
+
+## Anti-Patterns to Avoid
+
+❌ **Silent assumption:**
+```
+// Found redirect in web/next.config.mjs, task must be done
+// (Without checking if it's the right location or communicating status)
+```
+
+❌ **Location confusion:**
+```
+// Task asks for freebuff/web/next.config.mjs
+// Found in web/next.config.mjs
+// Assumed they're the same thing
+```
+
+❌ **No status communication:**
+```
+// Making no changes without explaining why
+// User left wondering if their request was handled
+```
+
+## Key Principle
+
+**Existing code is only a solution if it's in the right place, works correctly, and serves the intended use case.** Always validate all three before claiming task completion.