diff --git a/test/routes/ws-routes.test.ts b/test/routes/ws-routes.test.ts
new file mode 100644
index 00000000..a20c8775
--- /dev/null
+++ b/test/routes/ws-routes.test.ts
@@ -0,0 +1,362 @@
+/**
+ * @fileoverview Tests for WebSocket terminal I/O route.
+ *
+ * Unlike other route tests that use app.inject(), WebSocket testing requires
+ * a real listening server since inject() doesn't support upgrade requests.
+ * Uses the `ws` package (transitive dep of @fastify/websocket) as the client.
+ *
+ * Port: 3170 (ws-routes tests)
+ */
+
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import Fastify, { type FastifyInstance } from 'fastify';
+import fastifyWebsocket from '@fastify/websocket';
+import WebSocket from 'ws';
+import { createMockRouteContext, type MockRouteContext } from '../mocks/index.js';
+import { registerWsRoutes } from '../../src/web/routes/ws-routes.js';
+
+const PORT = 3170;
+
+/** Helper: open a WebSocket connection and wait for it to reach OPEN state. */
+function connectWs(path: string): Promise<WebSocket> {
+  return new Promise((resolve, reject) => {
+    const ws = new WebSocket(`ws://127.0.0.1:${PORT}${path}`);
+    ws.on('open', () => resolve(ws));
+    ws.on('error', reject);
+  });
+}
+
+/** Helper: wait for the next WS message, parsed as JSON. */
+function nextMessage(ws: WebSocket, timeoutMs = 2000): Promise<unknown> {
+  return new Promise((resolve, reject) => {
+    const timer = setTimeout(() => reject(new Error('WS message timeout')), timeoutMs);
+    ws.once('message', (raw) => {
+      clearTimeout(timer);
+      resolve(JSON.parse(String(raw)));
+    });
+  });
+}
+
+/** Helper: wait for WS close event and return { code, reason }. */
+function waitForClose(ws: WebSocket, timeoutMs = 2000): Promise<{ code: number; reason: string }> {
+  return new Promise((resolve, reject) => {
+    const timer = setTimeout(() => reject(new Error('WS close timeout')), timeoutMs);
+    ws.on('close', (code, reason) => {
+      clearTimeout(timer);
+      resolve({ code, reason: reason.toString() });
+    });
+  });
+}
+
+describe('ws-routes', () => {
+  let app: FastifyInstance;
+  let ctx: MockRouteContext;
+
+  beforeEach(async () => {
+    app = Fastify({ logger: false });
+    await app.register(fastifyWebsocket);
+
+    ctx = createMockRouteContext({ sessionId: 'ws-test-session' });
+    registerWsRoutes(app, ctx as never);
+
+    await app.listen({ port: PORT, host: '127.0.0.1' });
+  });
+
+  afterEach(async () => {
+    await app.close();
+  });
+
+  // ========== Session not found ==========
+
+  describe('session not found', () => {
+    it('closes with 4004 when session does not exist', async () => {
+      const ws = new WebSocket(`ws://127.0.0.1:${PORT}/ws/sessions/nonexistent/terminal`);
+      const { code, reason } = await waitForClose(ws);
+      expect(code).toBe(4004);
+      expect(reason).toBe('Session not found');
+    });
+  });
+
+  // ========== Terminal output ==========
+
+  describe('terminal output', () => {
+    it('receives terminal output via WS with DEC 2026 sync markers', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        // Emit terminal data from mock session
+        session.emit('terminal', 'hello world');
+
+        // Wait for the micro-batched message (8ms batch interval + margin)
+        const msg = (await nextMessage(ws)) as { t: string; d: string };
+        expect(msg.t).toBe('o');
+        // Should contain DEC 2026 sync markers wrapping the data
+        expect(msg.d).toContain('hello world');
+        expect(msg.d).toMatch(/^\x1b\[\?2026h/); // starts with DEC 2026 start
+        expect(msg.d).toMatch(/\x1b\[\?2026l$/); // ends with DEC 2026 end
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('sends clearTerminal event as {"t":"c"}', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        ctx._session.emit('clearTerminal');
+
+        const msg = (await nextMessage(ws)) as { t: string };
+        expect(msg.t).toBe('c');
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('sends needsRefresh event as {"t":"r"}', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        ctx._session.emit('needsRefresh');
+
+        const msg = (await nextMessage(ws)) as { t: string };
+        expect(msg.t).toBe('r');
+      } finally {
+        ws.close();
+      }
+    });
+  });
+
+  // ========== Client input ==========
+
+  describe('client input', () => {
+    it('forwards input messages to session.write()', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+        session.writeBuffer = [];
+
+        ws.send(JSON.stringify({ t: 'i', d: 'ls -la\r' }));
+
+        // Give the message handler time to process
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('ls -la\r');
+        });
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('ignores input exceeding MAX_INPUT_LENGTH', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+        session.writeBuffer = [];
+
+        // MAX_INPUT_LENGTH is 64KB
+        const hugeInput = 'x'.repeat(65 * 1024);
+        ws.send(JSON.stringify({ t: 'i', d: hugeInput }));
+
+        // Send a valid message after to confirm the connection still works
+        ws.send(JSON.stringify({ t: 'i', d: 'ok' }));
+
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('ok');
+        });
+
+        // The oversized input should not have been written
+        expect(session.writeBuffer).not.toContain(hugeInput);
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('ignores malformed JSON messages', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+        session.writeBuffer = [];
+
+        ws.send('not-json{{{');
+
+        // Send valid input to verify connection still alive
+        ws.send(JSON.stringify({ t: 'i', d: 'after-bad' }));
+
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('after-bad');
+        });
+
+        // Only 'after-bad' should be in the buffer
+        expect(session.writeBuffer).toHaveLength(1);
+      } finally {
+        ws.close();
+      }
+    });
+  });
+
+  // ========== Resize validation ==========
+
+  describe('resize validation', () => {
+    it('accepts valid resize within bounds', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: 120, r: 40 }));
+
+        await vi.waitFor(() => {
+          expect(session.resize).toHaveBeenCalledWith(120, 40);
+        });
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('accepts resize at minimum bounds (1x1)', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: 1, r: 1 }));
+
+        await vi.waitFor(() => {
+          expect(session.resize).toHaveBeenCalledWith(1, 1);
+        });
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('accepts resize at maximum bounds (500x200)', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: 500, r: 200 }));
+
+        await vi.waitFor(() => {
+          expect(session.resize).toHaveBeenCalledWith(500, 200);
+        });
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('rejects resize with cols out of bounds (0 cols)', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: 0, r: 40 }));
+
+        // Send a valid message to confirm processing continues
+        ws.send(JSON.stringify({ t: 'i', d: 'sentinel' }));
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('sentinel');
+        });
+
+        expect(session.resize).not.toHaveBeenCalled();
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('rejects resize with cols exceeding 500', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: 501, r: 40 }));
+
+        ws.send(JSON.stringify({ t: 'i', d: 'sentinel' }));
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('sentinel');
+        });
+
+        expect(session.resize).not.toHaveBeenCalled();
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('rejects resize with rows exceeding 200', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: 80, r: 201 }));
+
+        ws.send(JSON.stringify({ t: 'i', d: 'sentinel' }));
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('sentinel');
+        });
+
+        expect(session.resize).not.toHaveBeenCalled();
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('rejects resize with non-integer values', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: 80.5, r: 24 }));
+
+        ws.send(JSON.stringify({ t: 'i', d: 'sentinel' }));
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('sentinel');
+        });
+
+        expect(session.resize).not.toHaveBeenCalled();
+      } finally {
+        ws.close();
+      }
+    });
+
+    it('rejects resize with negative values', async () => {
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+      try {
+        const session = ctx._session;
+
+        ws.send(JSON.stringify({ t: 'z', c: -1, r: 24 }));
+
+        ws.send(JSON.stringify({ t: 'i', d: 'sentinel' }));
+        await vi.waitFor(() => {
+          expect(session.writeBuffer).toContain('sentinel');
+        });
+
+        expect(session.resize).not.toHaveBeenCalled();
+      } finally {
+        ws.close();
+      }
+    });
+  });
+
+  // ========== Connection cleanup ==========
+
+  describe('connection cleanup', () => {
+    it('removes session event listeners on close', async () => {
+      const session = ctx._session;
+      const listenersBefore = session.listenerCount('terminal');
+
+      const ws = await connectWs('/ws/sessions/ws-test-session/terminal');
+
+      // A listener was added for 'terminal'
+      expect(session.listenerCount('terminal')).toBe(listenersBefore + 1);
+      expect(session.listenerCount('clearTerminal')).toBeGreaterThanOrEqual(1);
+      expect(session.listenerCount('needsRefresh')).toBeGreaterThanOrEqual(1);
+
+      // Close the WS connection
+      ws.close();
+      await waitForClose(ws);
+
+      // Give the server-side close handler time to run
+      await new Promise((resolve) => setTimeout(resolve, 50));
+
+      // Listeners should be cleaned up
+      expect(session.listenerCount('terminal')).toBe(listenersBefore);
+      expect(session.listenerCount('clearTerminal')).toBe(0);
+      expect(session.listenerCount('needsRefresh')).toBe(0);
+    });
+  });
+});