diff --git a/packages/modal-infra/src/sandbox/manager.py b/packages/modal-infra/src/sandbox/manager.py
index 199bfc06..772db1c0 100644
--- a/packages/modal-infra/src/sandbox/manager.py
+++ b/packages/modal-infra/src/sandbox/manager.py
@@ -199,6 +199,25 @@ async def _resolve_and_setup_tunnels(
         ttyd_url = resolved.pop(TTYD_PROXY_PORT, None)
         extra_urls = resolved if resolved else None
 
+        # Write tunnel URLs into the sandbox so the coding agent can discover them
+        if extra_urls:
+            lines = [f"{port} {url}" for port, url in sorted(extra_urls.items())]
+            content = "\n".join(lines) + "\n"
+            try:
+                proc = await sandbox.exec.aio(
+                    "bash",
+                    "-c",
+                    f"cat > /workspace/.tunnel-urls << 'EOF'\n{content}EOF",
+                )
+                await proc.wait.aio()
+                log.info(
+                    "tunnel.urls_written",
+                    sandbox_id=sandbox_id,
+                    ports=list(extra_urls.keys()),
+                )
+            except Exception as e:
+                log.warn("tunnel.urls_write_failed", sandbox_id=sandbox_id, exc=e)
+
         return code_server_url, ttyd_url, extra_urls
 
     @staticmethod
diff --git a/packages/modal-infra/tests/test_tunnel_ports.py b/packages/modal-infra/tests/test_tunnel_ports.py
index 89eb7119..a87b109e 100644
--- a/packages/modal-infra/tests/test_tunnel_ports.py
+++ b/packages/modal-infra/tests/test_tunnel_ports.py
@@ -93,7 +93,10 @@ async def test_returns_none_none_none_for_no_ports(self):
     async def test_resolves_extra_ports(self):
         tunnel_urls = {3000: "https://tunnel-3000.example.com"}
 
+        proc = AsyncMock()
         sandbox = MagicMock()
+        sandbox.exec = MagicMock()
+        sandbox.exec.aio = AsyncMock(return_value=proc)
         with patch.object(
             SandboxManager,
             "_resolve_tunnels",
@@ -115,7 +118,10 @@ async def test_splits_code_server_from_extra_ports(self):
             3000: "https://tunnel-3000.example.com",
         }
 
+        proc = AsyncMock()
         sandbox = MagicMock()
+        sandbox.exec = MagicMock()
+        sandbox.exec.aio = AsyncMock(return_value=proc)
 
         with patch.object(
             SandboxManager,
@@ -131,6 +137,114 @@ async def test_splits_code_server_from_extra_ports(self):
         assert ttyd_url is None
         assert extra == {3000: "https://tunnel-3000.example.com"}
 
+    @pytest.mark.asyncio
+    async def test_writes_tunnel_urls_to_sandbox_filesystem(self):
+        """Tunnel URLs should be written to /workspace/.tunnel-urls inside the sandbox."""
+        tunnel_urls = {
+            3000: "https://tunnel-3000.example.com",
+            3001: "https://tunnel-3001.example.com",
+        }
+
+        proc = AsyncMock()
+        sandbox = MagicMock()
+        sandbox.exec = MagicMock()
+        sandbox.exec.aio = AsyncMock(return_value=proc)
+
+        with patch.object(
+            SandboxManager,
+            "_resolve_tunnels",
+            new_callable=AsyncMock,
+            return_value=tunnel_urls,
+        ):
+            await SandboxManager._resolve_and_setup_tunnels(
+                sandbox, "sb-1", False, False, [3000, 3001]
+            )
+
+        sandbox.exec.aio.assert_called_once()
+        args = sandbox.exec.aio.call_args
+        cmd = args[0][2]  # the bash -c argument
+        assert "3000 https://tunnel-3000.example.com" in cmd
+        assert "3001 https://tunnel-3001.example.com" in cmd
+        assert "/workspace/.tunnel-urls" in cmd
+        proc.wait.aio.assert_awaited_once()
+
+    @pytest.mark.asyncio
+    async def test_does_not_write_file_when_no_extra_urls(self):
+        """No file should be written when there are no extra tunnel URLs."""
+        sandbox = MagicMock()
+        sandbox.exec = MagicMock()
+        sandbox.exec.aio = AsyncMock()
+
+        with patch.object(
+            SandboxManager,
+            "_resolve_tunnels",
+            new_callable=AsyncMock,
+            return_value={},
+        ):
+            _, _, extra = await SandboxManager._resolve_and_setup_tunnels(
+                sandbox, "sb-1", False, False, [3000]
+            )
+
+        assert extra is None
+        sandbox.exec.aio.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_tunnel_file_write_failure_does_not_raise(self):
+        """If writing the tunnel file fails, it should log a warning but not raise."""
+        tunnel_urls = {3000: "https://tunnel-3000.example.com"}
+
+        sandbox = MagicMock()
+        sandbox.exec = MagicMock()
+        sandbox.exec.aio = AsyncMock(side_effect=Exception("exec failed"))
+
+        with (
+            patch.object(
+                SandboxManager,
+                "_resolve_tunnels",
+                new_callable=AsyncMock,
+                return_value=tunnel_urls,
+            ),
+            patch("src.sandbox.manager.log") as mock_log,
+        ):
+            _cs_url, _ttyd_url, extra = await SandboxManager._resolve_and_setup_tunnels(
+                sandbox, "sb-1", False, False, [3000]
+            )
+
+        # Should still return the URLs despite the write failure
+        assert extra == {3000: "https://tunnel-3000.example.com"}
+        mock_log.warn.assert_called_once()
+        assert mock_log.warn.call_args[0][0] == "tunnel.urls_write_failed"
+
+    @pytest.mark.asyncio
+    async def test_tunnel_file_wait_failure_does_not_raise(self):
+        """If proc.wait raises after exec succeeds, it should log a warning but not raise."""
+        tunnel_urls = {3000: "https://tunnel-3000.example.com"}
+
+        proc = AsyncMock()
+        proc.wait.aio = AsyncMock(side_effect=Exception("wait failed"))
+        sandbox = MagicMock()
+        sandbox.exec = MagicMock()
+        sandbox.exec.aio = AsyncMock(return_value=proc)
+
+        with (
+            patch.object(
+                SandboxManager,
+                "_resolve_tunnels",
+                new_callable=AsyncMock,
+                return_value=tunnel_urls,
+            ),
+            patch("src.sandbox.manager.log") as mock_log,
+        ):
+            _cs_url, _ttyd_url, extra = await SandboxManager._resolve_and_setup_tunnels(
+                sandbox, "sb-1", False, False, [3000]
+            )
+
+        # Should still return the URLs despite the wait failure
+        assert extra == {3000: "https://tunnel-3000.example.com"}
+        sandbox.exec.aio.assert_called_once()
+        mock_log.warn.assert_called_once()
+        assert mock_log.warn.call_args[0][0] == "tunnel.urls_write_failed"
+
 
 class TestCollectExposedPorts:
     """SandboxManager._collect_exposed_ports tests."""