diff --git a/src/index.tsx b/src/index.tsx
index 0ccf188..6a85fe9 100644
--- a/src/index.tsx
+++ b/src/index.tsx
@@ -161,14 +161,37 @@ export class Hype<
   get defaults() {
     this.registerRoutes()
 
+    const isDev = process.env.NODE_ENV !== 'production'
+    let port = process.env.PORT ? Number(process.env.PORT) : 3000
+
+    if (isDev) {
+      port = findAvailablePortSync(port)
+    }
+
     return {
-      port: process.env.PORT ?? '3000',
+      port,
       fetch: this.fetch,
       idleTimeout: 255
     }
   }
 }
 
+/**
+ * Synchronously find an available port starting from the given port
+ */
+function findAvailablePortSync(startPort: number, maxAttempts = 100): number {
+  for (let port = startPort; port < startPort + maxAttempts; port++) {
+    try {
+      const server = Bun.serve({ port, fetch: () => new Response() })
+      server.stop(true)
+      return port
+    } catch {
+      continue
+    }
+  }
+  throw new Error(`Could not find an available port after ${maxAttempts} attempts starting from ${startPort}`)
+}
+
 function render404(c: Context) {
   return c.text('File not found', 404)
 }
\ No newline at end of file
diff --git a/src/port-selection.test.ts b/src/port-selection.test.ts
new file mode 100644
index 0000000..2625ce3
--- /dev/null
+++ b/src/port-selection.test.ts
@@ -0,0 +1,44 @@
+import { test, expect } from "bun:test"
+import { Hype } from "./index.tsx"
+
+test("defaults returns next available port in dev mode", () => {
+  // Start a server on port 4000 to block it
+  const blocker = Bun.serve({
+    port: 4000,
+    fetch: () => new Response("blocker"),
+  })
+
+  // Set NODE_ENV to development and PORT to 4000
+  const originalNodeEnv = process.env.NODE_ENV
+  const originalPort = process.env.PORT
+  process.env.NODE_ENV = "development"
+  process.env.PORT = "4000"
+
+  const app = new Hype({})
+  const config = app.defaults
+
+  // Should have found port 4001 since 4000 is taken
+  expect(config.port).toBe(4001)
+
+  // Clean up
+  blocker.stop(true)
+  process.env.NODE_ENV = originalNodeEnv
+  process.env.PORT = originalPort
+})
+
+test("defaults uses exact port in production mode", () => {
+  const originalNodeEnv = process.env.NODE_ENV
+  const originalPort = process.env.PORT
+  process.env.NODE_ENV = "production"
+  process.env.PORT = "4002"
+
+  const app = new Hype({})
+  const config = app.defaults
+
+  // Should use exact port in production
+  expect(config.port).toBe(4002)
+
+  // Clean up
+  process.env.NODE_ENV = originalNodeEnv
+  process.env.PORT = originalPort
+})