diff --git a/.github/workflows/typecheck.yml b/.github/workflows/typecheck.yml
new file mode 100644
index 000000000..3e614acde
--- /dev/null
+++ b/.github/workflows/typecheck.yml
@@ -0,0 +1,43 @@
+name: Type check
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  mypy:
+    # `--platform` is what selects the branches mypy resolves, so all three run
+    # on Linux. Checking out on three runner OSes would cost three times as much
+    # and still check whatever platform each runner happened to be.
+    strategy:
+      fail-fast: false
+      matrix:
+        platform:
+          - linux
+          - darwin
+          - win32
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262  # v4.4.0
+        with:
+          persist-credentials: false
+
+      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5.6.0
+        with:
+          python-version: '3.12'
+
+      - uses: astral-sh/setup-uv@d4b2f3b6ecc6e67c4457f6d3e41ec42d3d0fcb86  # v5.4.2
+
+      - name: Install dependencies
+        run: uv sync --frozen
+
+      - name: mypy --platform ${{ matrix.platform }}
+        run: uv run mypy --platform ${{ matrix.platform }} strix/
diff --git a/Makefile b/Makefile
index 05038f799..2d6c306bc 100644
--- a/Makefile
+++ b/Makefile
@@ -46,8 +46,11 @@ lint:
 	@echo "✅ Linting complete!"
 
 type-check:
-	@echo "🔍 Type checking with mypy..."
-	uv run mypy strix/
+	@echo "🔍 Type checking with mypy (linux, darwin, win32)..."
+	@for platform in linux darwin win32; do \
+		echo "  → $$platform"; \
+		uv run mypy --platform $$platform strix/ || exit 1; \
+	done
 	@echo "🔍 Type checking with pyright..."
 	uv run pyright strix/
 	@echo "✅ Type checking complete!"
diff --git a/strix/config/codex.py b/strix/config/codex.py
index 9f81ff6db..b171f48ee 100644
--- a/strix/config/codex.py
+++ b/strix/config/codex.py
@@ -16,6 +16,7 @@ import hashlib
 import json
 import logging
 import secrets
+import sys
 import threading
 import time
 import urllib.parse
@@ -103,27 +104,52 @@ def logout() -> None:
         AUTH_PATH.unlink()
 
 
+# `fcntl` exists only on POSIX. Resolve it once behind a platform guard, which
+# mypy narrows on every `--platform` it is asked to check, rather than at the
+# call sites where it would need a per-platform ignore.
+if sys.platform != "win32":
+    import fcntl
+
+    _HAS_FILE_LOCKING = True
+
+    def _lock_exclusive(descriptor: int) -> None:
+        fcntl.flock(descriptor, fcntl.LOCK_EX)
+
+    def _unlock(descriptor: int) -> None:
+        fcntl.flock(descriptor, fcntl.LOCK_UN)
+
+else:
+    _HAS_FILE_LOCKING = False
+
+    def _lock_exclusive(descriptor: int) -> None:
+        """Windows has no advisory file lock; the in-process lock still holds."""
+
+    def _unlock(descriptor: int) -> None:
+        """Windows has no advisory file lock; the in-process lock still holds."""
+
+
 @contextlib.contextmanager
 def _refresh_guard() -> Iterator[None]:
     """Serialize token refresh within (lock) and across (flock) Strix processes,
     so concurrent runs can't both spend the single-use refresh token."""
     with _refresh_lock:
+        if not _HAS_FILE_LOCKING:
+            yield
+            return
         try:
-            import fcntl
-
             lock_path = AUTH_PATH.with_suffix(".lock")
             lock_path.parent.mkdir(parents=True, exist_ok=True)
             handle = lock_path.open("w")
-        except (ImportError, OSError):
+        except OSError:
             yield
             return
         try:
             with contextlib.suppress(OSError):
-                fcntl.flock(handle.fileno(), fcntl.LOCK_EX)
+                _lock_exclusive(handle.fileno())
             yield
         finally:
             with contextlib.suppress(OSError):
-                fcntl.flock(handle.fileno(), fcntl.LOCK_UN)
+                _unlock(handle.fileno())
             handle.close()
 
 
diff --git a/strix/runtime/session_manager.py b/strix/runtime/session_manager.py
index 3c95d31ae..379231acd 100644
--- a/strix/runtime/session_manager.py
+++ b/strix/runtime/session_manager.py
@@ -40,15 +40,19 @@ _PROTECTED_METADATA_NAMES = (".git", ".agents", ".codex")
 
 
 def _host_identity_env() -> dict[str, str]:
-    # Read the platform through a local so it is not narrowed to whichever OS is
-    # type-checking: comparing sys.platform directly makes one of these branches
-    # statically dead, and which one flips between Linux and macOS.
-    platform_name: str = sys.platform
-    if platform_name != "linux":
-        return {}
-    # Bind-mount ownership only needs mapping on Linux, where the container uid
-    # must match the host's.
-    return {"STRIX_HOST_UID": str(os.getuid()), "STRIX_HOST_GID": str(os.getgid())}
+    # The outer guard is a real platform check, so mypy narrows it under every
+    # --platform it is asked about and `os.getuid` resolves where it exists.
+    if sys.platform != "win32":
+        # Inside it, read the platform through a local so it is not narrowed to
+        # whichever OS is type-checking: comparing sys.platform directly makes
+        # one of these branches statically dead, and which one flips between
+        # Linux and macOS.
+        platform_name: str = sys.platform
+        if platform_name == "linux":
+            # Bind-mount ownership only needs mapping on Linux, where the
+            # container uid must match the host's.
+            return {"STRIX_HOST_UID": str(os.getuid()), "STRIX_HOST_GID": str(os.getgid())}
+    return {}
 
 
 def build_bind_mounts(local_sources: list[dict[str, Any]]) -> list[dict[str, Any]]: