diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index f167521..90a1464 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -29,6 +29,20 @@ jobs:
       artifact_path: reports/
     secrets: inherit
 
+  test-windows:
+    name: Run tests on Windows
+    uses: ./.github/workflows/python-job.yml
+    with:
+      runner: windows-latest
+      run: |
+        # tests/installer.py is a symlink to ../dk-installer.py. A Windows checkout
+        # materializes it as a plain file holding the target path unless core.symlinks is
+        # enabled, which turns `from tests.installer import ...` into a SyntaxError. Copy
+        # over it when that is what we got; a real symlink is left alone.
+        if [ ! -L tests/installer.py ]; then cp dk-installer.py tests/installer.py; fi
+        pytest
+    secrets: inherit
+
   report-coverage:
     name: Report tests coverage
     runs-on: ubuntu-latest
diff --git a/dk-installer.py b/dk-installer.py
index f04768f..53c5782 100755
--- a/dk-installer.py
+++ b/dk-installer.py
@@ -343,7 +343,12 @@ def stream_iterator(proc: subprocess.Popen, stream_name: str, file_path: pathlib
         "stdout": (0, "output"),
         "stderr": (1, "stderr"),
     }[stream_name]
-    buffer = io.TextIOWrapper(io.BytesIO())
+    # Explicit utf-8: the default is ``locale.getpreferredencoding()``, which is cp1252 on
+    # US Windows. That mangles every non-ASCII byte a command writes (docker compose and uv
+    # both draw progress with box-drawing and check marks) and, worse, silently defeats the
+    # partial-character handling below -- cp1252 maps almost any byte to *something*, so an
+    # incomplete utf-8 sequence decodes to garbage instead of raising UnicodeDecodeError.
+    buffer = io.TextIOWrapper(io.BytesIO(), encoding="utf-8")
 
     def _iter():
         proc_exited = False
diff --git a/tests/conftest.py b/tests/conftest.py
index 41b737a..85486c3 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,4 +1,5 @@
 import json
+import os
 from argparse import Namespace
 from contextlib import contextmanager
 from pathlib import Path
@@ -25,7 +26,13 @@ def _no_real_process_group_signals():
     pgid 1 (init) and ``os.killpg(1, SIGTERM)`` from a root container actually
     signals init → CI runner shutdown. Tests that need to assert on these
     explicitly override the patches inside their own ``with patch(...)``.
+
+    Nothing to guard where process groups do not exist: Windows has no ``os.killpg`` to
+    call by accident, and the tests that would reach it are skipped there.
     """
+    if not hasattr(os, "killpg"):
+        yield Mock()
+        return
     with (
         patch("tests.installer.os.killpg") as killpg_mock,
         patch("tests.installer.os.getpgid", return_value=99999),
diff --git a/tests/test_obs_install.py b/tests/test_obs_install.py
index 604e7ea..a717359 100644
--- a/tests/test_obs_install.py
+++ b/tests/test_obs_install.py
@@ -1,3 +1,4 @@
+import json
 from functools import partial
 from itertools import count
 from pathlib import Path
@@ -66,7 +67,7 @@ def _stdout_side_effect():
 @pytest.mark.integration
 def test_obs_existing_install_abort(obs_install_action, compose_path, stdout_mock):
     stdout_mock.side_effect = [
-        [f'[{{"Name":"test-project","Status":"running(4)","ConfigFiles":"{compose_path}"}}]'],
+        [json.dumps([{"Name": "test-project", "Status": "running(4)", "ConfigFiles": str(compose_path)}])],
         [],
     ]
     with patch.object(obs_install_action, "steps", new=[ComposeVerifyExistingInstallStep]):
diff --git a/tests/test_tg_install.py b/tests/test_tg_install.py
index 85228d8..d4d9b87 100644
--- a/tests/test_tg_install.py
+++ b/tests/test_tg_install.py
@@ -1,3 +1,4 @@
+import json
 from functools import partial
 from pathlib import Path
 from unittest.mock import call, patch
@@ -50,24 +51,26 @@ def test_tg_install(tg_install_action, start_cmd_mock, stdout_mock, tmp_data_fol
     assert Path(tmp_data_folder).joinpath("dk-tg-credentials.txt").stat().st_size > 0
     marker = Path(tmp_data_folder).joinpath("dk-tg-install.json")
     assert marker.exists()
-    import json as _json
-
-    assert _json.loads(marker.read_text())["install_mode"] == "docker"
+    assert json.loads(marker.read_text())["install_mode"] == "docker"
 
 
 @pytest.mark.integration
 @pytest.mark.parametrize(
     "stdout_effect",
     (
-        [['[{"Name":"test-project","Status":"running(2)","ConfigFiles":"<COMPOSE>"}]'], []],
-        [[], ['{"Labels":"com.docker.compose.project=test-project,", "Status":"N/A"}']],
+        lambda compose_path: [
+            [json.dumps([{"Name": "test-project", "Status": "running(2)", "ConfigFiles": str(compose_path)}])],
+            [],
+        ],
+        lambda compose_path: [
+            [],
+            [json.dumps({"Labels": "com.docker.compose.project=test-project,", "Status": "N/A"})],
+        ],
     ),
     ids=("container", "volume"),
 )
 def test_tg_existing_install_abort(stdout_effect, tg_install_action, stdout_mock, compose_path):
-    stdout_mock.side_effect = [
-        [line.replace("<COMPOSE>", str(compose_path)) for line in output] for output in stdout_effect
-    ]
+    stdout_mock.side_effect = stdout_effect(compose_path)
     compose_path.touch()
 
     with patch.object(tg_install_action, "steps", new=[ComposeVerifyExistingInstallStep]):
diff --git a/tests/test_tg_pip_delete.py b/tests/test_tg_pip_delete.py
index f44e85d..ab7f0bf 100644
--- a/tests/test_tg_pip_delete.py
+++ b/tests/test_tg_pip_delete.py
@@ -1,3 +1,4 @@
+import platform
 from functools import partial
 from pathlib import Path
 from unittest.mock import MagicMock, patch
@@ -72,7 +73,9 @@ def test_pip_delete_removes_uv_tool_and_home(pip_delete_action, start_cmd_mock,
 def test_pip_delete_removes_installer_local_uv(pip_delete_action, start_cmd_mock, tmp_data_folder, tmp_path):
     local_bin = Path(tmp_data_folder) / "bin"
     local_bin.mkdir()
-    local_uv = local_bin / "uv"
+    # ``resolve_uv_path`` looks for uv.exe on Windows: create the name this platform's
+    # installer will actually look for, so the test exercises the real branch on both.
+    local_uv = local_bin / ("uv.exe" if platform.system() == "Windows" else "uv")
     local_uv.write_bytes(b"#!/bin/sh\n")
 
     fake_tg_home = tmp_path / ".testgen"
diff --git a/tests/test_tg_start.py b/tests/test_tg_start.py
index 90795d7..3379a88 100644
--- a/tests/test_tg_start.py
+++ b/tests/test_tg_start.py
@@ -1,3 +1,4 @@
+import os
 import signal
 from pathlib import Path
 from unittest.mock import MagicMock, patch
@@ -18,6 +19,14 @@
 )
 
 
+# The POSIX stop path signals a process group, which Windows has no equivalent of --
+# ``supports_graceful_stop`` keeps the installer off that branch there entirely. These are
+# skipped rather than shimmed: synthesizing ``os.killpg``/``signal.SIGKILL`` would also make
+# a future capability-based ``supports_graceful_stop`` report True on Windows and quietly
+# send the Windows tests down the wrong branch.
+posix_only = pytest.mark.skipif(not hasattr(os, "killpg"), reason="needs POSIX process groups")
+
+
 # --- start_testgen_app helper -------------------------------------------------
 
 
@@ -55,6 +64,10 @@ def test_start_testgen_app_happy_path(app_action, args_mock, proc_running_then_s
         patch("tests.installer.resolve_testgen_path", return_value="/bin/testgen"),
         patch("tests.installer.subprocess.Popen", return_value=proc_running_then_stops) as popen_mock,
         patch("tests.installer.wait_for_tcp_port", return_value=True) as port_mock,
+        # The ``finally`` cleanup is not this test's subject, and on Windows it shells out
+        # via ``subprocess.run`` -- which goes through the Popen patched right above and
+        # would count as a second app launch.
+        patch("tests.installer.stop_app_tree"),
     ):
         start_testgen_app(app_action, args_mock)
 
@@ -120,6 +133,9 @@ def test_start_testgen_app_handles_keyboard_interrupt(app_action, args_mock, con
         patch("tests.installer.resolve_testgen_path", return_value="/bin/testgen"),
         patch("tests.installer.subprocess.Popen", return_value=proc),
         patch("tests.installer.wait_for_tcp_port", return_value=True),
+        # Pinned: the grace-period messages below are POSIX-only behaviour, and the
+        # Windows counterpart is test_start_testgen_app_makes_no_grace_promise_on_windows.
+        patch("tests.installer.supports_graceful_stop", return_value=True),
         patch("tests.installer.stop_app_tree", return_value=True) as stop_mock,
     ):
         start_testgen_app(app_action, args_mock)
@@ -148,6 +164,8 @@ def test_start_testgen_app_warns_when_grace_period_expires(app_action, args_mock
         patch("tests.installer.resolve_testgen_path", return_value="/bin/testgen"),
         patch("tests.installer.subprocess.Popen", return_value=proc),
         patch("tests.installer.wait_for_tcp_port", return_value=True),
+        # Pinned: Windows never promises a grace period, so it never warns one expired.
+        patch("tests.installer.supports_graceful_stop", return_value=True),
         patch("tests.installer.stop_app_tree", return_value=False),
     ):
         start_testgen_app(app_action, args_mock)
@@ -185,10 +203,16 @@ def test_start_testgen_app_makes_no_grace_promise_on_windows(app_action, args_mo
 
 @pytest.mark.unit
 def test_stop_app_tree_no_op_when_proc_already_exited():
+    """POSIX only. Windows deliberately does not take this shortcut: a console Ctrl+C
+    kills the parent along with the installer, so a dead parent there says nothing about
+    its children -- see test_stop_app_tree_windows_uses_taskkill_tree."""
     proc = MagicMock()
     proc.poll.return_value = 0  # already exited
 
-    with patch("tests.installer.subprocess.run") as run_mock:
+    with (
+        patch("tests.installer.supports_graceful_stop", return_value=True),
+        patch("tests.installer.subprocess.run") as run_mock,
+    ):
         stop_app_tree(proc)
 
     run_mock.assert_not_called()
@@ -215,6 +239,7 @@ def test_stop_app_tree_windows_uses_taskkill_tree():
 
 
 @pytest.mark.unit
+@posix_only
 def test_stop_app_tree_posix_signals_process_group():
     proc = MagicMock()
     proc.poll.return_value = None
@@ -234,6 +259,7 @@ def test_stop_app_tree_posix_signals_process_group():
 
 
 @pytest.mark.unit
+@posix_only
 def test_stop_app_tree_falls_through_to_kill_on_timeout():
     """If SIGTERM doesn't take, escalate to SIGKILL / proc.kill()."""
     import subprocess as sp
@@ -255,6 +281,7 @@ def test_stop_app_tree_falls_through_to_kill_on_timeout():
 
 
 @pytest.mark.unit
+@posix_only
 def test_stop_app_tree_reports_graceful_stop():
     """Exiting within the grace period is the signal the caller uses to decide
     whether a running job got to checkpoint."""
@@ -272,6 +299,7 @@ def test_stop_app_tree_reports_graceful_stop():
 
 
 @pytest.mark.unit
+@posix_only
 def test_stop_app_tree_swallows_second_interrupt():
     """A second Ctrl+C while we wait means 'stop now'. It must force-kill here rather
     than escaping to the caller's ``finally``, which would re-signal the tree — TestGen
@@ -311,6 +339,7 @@ def test_stop_app_tree_windows_stop_is_always_forced():
 
 
 @pytest.mark.unit
+@posix_only
 def test_force_kill_sweeps_orphans_outside_the_process_group():
     """``run-app all`` starts its ui/scheduler children in their own sessions, so killpg
     on the parent leaves them holding the port and the data dir — breaking the next
@@ -331,6 +360,7 @@ def test_force_kill_sweeps_orphans_outside_the_process_group():
 
 
 @pytest.mark.unit
+@posix_only
 def test_second_interrupt_still_sweeps_orphans():
     """The second-Ctrl+C path force-kills, so it owes the same cleanup."""
     proc = MagicMock()
@@ -350,6 +380,7 @@ def test_second_interrupt_still_sweeps_orphans():
 
 
 @pytest.mark.unit
+@posix_only
 def test_graceful_stop_does_not_sweep_orphans():
     """A tree that stopped on its own has nothing left behind — don't reach for pkill."""
     proc = MagicMock()