Fresh App Debugging: AI-Optimized Technical Reference

Critical Configuration Requirements

Import System Configuration

Production Requirements:

• Fresh 2.0 requires JSR imports: jsr:@fresh/core@2.0.0-beta.1/
• Old deno.land/x URLs break with 90% frequency
• Import maps require trailing slashes for directory imports
• Cache corruption occurs weekly in active development

Working Import Map (deno.json):

{
  "imports": {
    "$fresh/": "jsr:@fresh/core@2.0.0-beta.1/",
    "preact": "https://esm.sh/preact@10.19.2",
    "preact/": "https://esm.sh/preact@10.19.2/"
  }
}

Cache Reset Process (fixes 90% of import issues):

rm -rf ~/.cache/deno .deno
deno cache --reload main.ts

Middleware Signature Breaking Changes

Fresh 1.x to 2.0 Migration Impact:

• All middleware signatures changed from (req: Request, ctx: FreshContext) to (ctx: FreshContext)
• Request object now accessed via ctx.req
• Zero backward compatibility - requires manual rewrite

Migration Pattern:

// Before (Fresh 1.x)
export const handler = (req: Request, ctx: FreshContext) => {
  const userAgent = req.headers.get("user-agent");
  return ctx.next();
};

// After (Fresh 2.0)
export const handler = (ctx: FreshContext) => {
  const userAgent = ctx.req.headers.get("user-agent");
  return ctx.next();
};

Database Connection Requirements

Connection Pooling (Critical for Production)

Problem: New connections take 100-200ms each request
Solution Impact: Reduces response time by 80-90%

Implementation Requirements:

import { Pool } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const pool = new Pool({
  hostname: Deno.env.get("DB_HOST"),
  database: Deno.env.get("DB_NAME"),
  username: Deno.env.get("DB_USER"),
  password: Deno.env.get("DB_PASSWORD"),
}, 3, true); // 3 connections, lazy connect

export const handler: Handlers = {
  async GET() {
    const client = await pool.connect();
    try {
      const result = await client.queryArray("SELECT * FROM users");
      return Response.json(result.rows);
    } finally {
      client.release(); // Critical: Always release
    }
  }
};

Edge Database Requirements

Performance Thresholds:

• US East to Europe: 300ms+ connection time
• Connection timeout default: 30 seconds
• Memory limit (Deno Deploy free): 128MB

Recommended Edge Databases:

• Neon: PostgreSQL with connection pooling
• PlanetScale: MySQL at edge locations
• Turso: SQLite distributed globally
• Deno KV: Built-in key-value store

Environment Variable Issues

Deno Deploy Limitations

Critical Timing Issue: Environment variables sync takes 30+ seconds
Debugging Impact: Causes false positive "missing variable" errors
Validation Requirements:

const requiredEnvVars = ["DATABASE_URL", "API_KEY", "SECRET_KEY"];
const missing = requiredEnvVars.filter(key => !Deno.env.get(key));

if (missing.length > 0) {
  console.error(`Missing environment variables: ${missing.join(", ")}`);
  Deno.exit(1);
}

Island Component Constraints

Server-Only Import Restrictions

Failure Patterns:

• File system access (readFileSync, Deno.readTextFile)
• Database clients in island components
• Node.js APIs (node:fs, node:path)
• Deno namespace access

Hydration Mismatch Prevention:

// ❌ Breaks hydration
import { readFileSync } from "node:fs";

export default function UserProfile() {
  const config = readFileSync("./config.json"); // Server-only
  return <div>{config}</div>;
}

// ✅ Proper pattern
export const handler: Handlers = {
  async GET(ctx) {
    const config = await Deno.readTextFile("./config.json");
    return ctx.render({ config });
  }
};

export default function ProfilePage({ data }: PageProps) {
  return <UserProfile config={data.config} />;
}

Fresh 2.0 Specific Failures

Plugin Compatibility Issues

Breaking Change Impact: All Fresh 1.x plugins incompatible
Error Patterns:

• plugin.render is not a function
• Module not found "https://deno.land/x/fresh@1.6.8/plugin.ts"
• Multiple copies of preact are used at the same time

Migration Strategy:

// Fresh 1.x plugin (broken)
const authPlugin: Plugin = {
  name: "auth",
  middlewares: [/* complex plugin object */]
};

// Fresh 2.0 approach (working)
function authPlugin(app: FreshApp) {
  app.use(async (ctx) => {
    const token = ctx.req.headers.get("authorization");
    if (!token && ctx.url.pathname.startsWith("/dashboard")) {
      return Response.redirect("/login");
    }
    if (token) {
      ctx.state.user = await verifyToken(token);
    }
    return ctx.next();
  });
}

Vite Integration Problems

September 2025 Issues:

• Bundle size limits on Cloudflare Workers
• WebSocket upgrade request failures
• Base path configuration errors

Required Vite Configuration:

// vite.config.ts
import { defineConfig } from "vite";
import { fresh } from "@fresh/plugin-vite";

export default defineConfig({
  plugins: [fresh()],
  optimizeDeps: {
    include: ["preact", "preact/hooks"] // Explicit inclusion required
  }
});

Performance Debugging Requirements

Slow Route Detection

Threshold: Routes >100ms considered slow
User Impact: Routes >200ms cause user abandonment

Implementation:

export const performanceMiddleware = async (ctx: FreshContext) => {
  const start = performance.now();
  const response = await ctx.next();
  const duration = performance.now() - start;

  if (duration > 100) {
    console.warn(`SLOW ROUTE: ${ctx.req.method} ${ctx.url.pathname} took ${duration.toFixed(2)}ms`);
  }

  return response;
};

Memory Leak Detection

Critical Threshold: 100MB RSS usage
Deno Deploy Limit: 128MB kills process

Monitoring Pattern:

setInterval(() => {
  const memUsage = Deno.memoryUsage();
  if (memUsage.rss > 100 * 1024 * 1024) { // 100MB
    console.warn(`HIGH MEMORY: ${(memUsage.rss / 1024 / 1024).toFixed(2)}MB`);
  }
}, 30000); // Every 30 seconds

Error Handling Patterns

Global Error Capture

Production Requirements:

globalThis.addEventListener("error", (event) => {
  console.error("Global error:", {
    message: event.error?.message,
    stack: event.error?.stack,
    filename: event.filename,
    lineno: event.lineno,
    timestamp: new Date().toISOString()
  });
});

globalThis.addEventListener("unhandledrejection", (event) => {
  console.error("Unhandled promise rejection:", {
    reason: event.reason,
    timestamp: new Date().toISOString()
  });
});

Common Failure Scenarios

Import Resolution (90% of Issues)

Root Causes by Frequency:

1. Fresh 2.0 JSR migration (40%)
2. Import map conflicts (25%)
3. Cache corruption (15%)
4. Case sensitivity (Linux deployment) (10%)

Resolution Priority:

1. Clear all caches
2. Fix import map syntax
3. Update Fresh version references
4. Check file path case sensitivity

Database Timeouts (Production Reality)

Failure Conditions:

• Edge function to database >200ms latency
• New connection per request pattern
• Connection pool exhaustion
• Environment variable sync delays

Mitigation Requirements:

• Use connection pooling (mandatory)
• Choose edge-optimized databases
• Implement connection retry logic
• Monitor connection counts

Development vs Production Differences

Common Disparities:

• Local database (1ms) vs production (300ms)
• Memory unlimited vs 128MB limit
• Environment variable availability
• Deno version mismatches
• Module cache differences

Debugging Tool Effectiveness

Tool	Success Rate	Setup Time	Production Safe	Best Use Case
console.log	90%	0 minutes	Yes	Initial debugging
Chrome DevTools	95%	15 minutes	No	Complex issues
Cache clearing	80%	1 minute	Yes	Import problems
Connection pooling	85%	30 minutes	Yes	Database issues
Performance middleware	70%	15 minutes	Yes	Speed optimization

Version Compatibility Matrix

Fresh Version	Deno Version	Breaking Changes	Migration Effort
1.x	1.x	N/A	N/A
2.0	2.0+	Middleware signatures, imports	High (2-3 days)
2.0 + Vite	2.0+	Additional bundle issues	Very High (1 week)

Critical Resource Requirements

Minimum Production Specs:

• Memory: 128MB (Deno Deploy limit)
• Database connections: 3-5 pool size
• Response time target: <200ms
• Error rate threshold: <1%

Performance Benchmarks:

• Route response: <100ms (good), <200ms (acceptable)
• Database query: <50ms (good), <100ms (acceptable)
• Memory usage: <100MB (safe), >120MB (critical)
• Import resolution: <1s (acceptable), >5s (broken)

Emergency Recovery Procedures

Complete Cache Reset

rm -rf ~/.cache/deno ~/.deno .deno
deno cache --reload main.ts
deno task start

Minimal Fresh Test

deno run -Ar jsr:@fresh/init@2.0.0-beta.1 test-app
cd test-app
deno task start

Fresh 1.x Rollback

git log --oneline | head -10
git reset --hard <working-commit>
# Manually revert JSR imports to deno.land/x

This technical reference provides AI-consumable debugging intelligence for Fresh applications, focusing on actionable solutions for common production failures.