diff --git a/backend/apps/agents/manager/prompt/compose_turn_system_prompt.py b/backend/apps/agents/manager/prompt/compose_turn_system_prompt.py
index 4c949788e..51afac498 100644
--- a/backend/apps/agents/manager/prompt/compose_turn_system_prompt.py
+++ b/backend/apps/agents/manager/prompt/compose_turn_system_prompt.py
@@ -59,10 +59,13 @@ def compose_turn_system_prompt(
         tz_name = tz_name or "UTC"
         now_local = datetime.now(ZoneInfo(tz_name))
         tz_abbr = now_local.strftime("%Z") or tz_name
+        # No `%-d` / `%-I`: those are glibc extensions. On Windows strftime raises ValueError on
+        # them, the except below swallowed it, and the agent shipped without a clock on that OS.
+        hour = now_local.strftime("%I:%M %p").lstrip("0")
         time_ctx = (
             "<current_time>\n"
-            f"Today is {now_local.strftime('%A, %B %-d, %Y')}.\n"
-            f"Local time: {now_local.strftime('%-I:%M %p')} {tz_abbr} ({tz_name}).\n"
+            f"Today is {now_local.strftime('%A, %B')} {now_local.day}, {now_local.year}.\n"
+            f"Local time: {hour} {tz_abbr} ({tz_name}).\n"
             "Use this as ground truth for any date/time/day-of-week question. The timezone also "
             "gives the user's coarse region; when they say 'here' or 'near me' without a place, "
             "infer the likely city from it (say you inferred it) instead of claiming you can't know.\n"
diff --git a/backend/apps/outputs/publish_capability.py b/backend/apps/outputs/publish_capability.py
index eee884777..5ec326d9a 100644
--- a/backend/apps/outputs/publish_capability.py
+++ b/backend/apps/outputs/publish_capability.py
@@ -69,7 +69,7 @@ def p_api_callers(root: str) -> List[str]:
                     continue
                 with open(full, "r", encoding="utf-8", errors="replace") as fh:
                     if P_API_CALL.search(fh.read()):
-                        hits.append(os.path.relpath(full, root))
+                        hits.append(os.path.relpath(full, root).replace(os.sep, "/"))
             except OSError:
                 continue
     return sorted(hits)
diff --git a/backend/apps/skills/skills.py b/backend/apps/skills/skills.py
index 878252fe8..ca3872c20 100644
--- a/backend/apps/skills/skills.py
+++ b/backend/apps/skills/skills.py
@@ -503,7 +503,8 @@ async def list_skill_files(skill_id: str):
         dirs[:] = [d for d in dirs if not d.startswith(".")]
         for n in sorted(names):
             path = os.path.join(root, n)
-            rel = os.path.relpath(path, base_abs)
+            # API paths are posix on every OS: the frontend joins them with "/" and the SKILL.md-first sort compares them as strings.
+            rel = os.path.relpath(path, base_abs).replace(os.sep, "/")
             if n.startswith(".") or os.path.getsize(path) > 512_000:
                 continue
             try:
diff --git a/backend/apps/swarm/entities/skills.py b/backend/apps/swarm/entities/skills.py
index f8134a3cf..161156e7b 100644
--- a/backend/apps/swarm/entities/skills.py
+++ b/backend/apps/swarm/entities/skills.py
@@ -100,7 +100,9 @@ def p_read_supporting_files(skill_dir: str) -> dict[str, bytes]:
     for root, p_dirs, names in os.walk(skill_dir):
         for n in names:
             full = os.path.join(root, n)
-            rel = os.path.relpath(full, skill_dir)
+            # Forward slashes: these keys travel inside .swarm bundles, and the importer joins them
+            # onto the destination on whatever OS opens the bundle.
+            rel = os.path.relpath(full, skill_dir).replace(os.sep, "/")
             if rel == "SKILL.md" or n.startswith("."):
                 continue
             try:
diff --git a/backend/tests/test_app_export_no_stale_files.py b/backend/tests/test_app_export_no_stale_files.py
index 00aeb247c..90e457fdc 100644
--- a/backend/tests/test_app_export_no_stale_files.py
+++ b/backend/tests/test_app_export_no_stale_files.py
@@ -34,7 +34,7 @@ def p_make_app(ws_root, *, workspace: bool) -> str:
     if wsid:
         folder = ws_root / wsid
         folder.mkdir()
-        (folder / "app.py").write_text("print('v1')\n")
+        (folder / "app.py").write_text("print('v1')\n", newline="\n")
     o = Output(
         name="Demo", description="", icon="view_quilt",
         input_schema={"type": "object", "properties": {}, "required": []},
@@ -50,8 +50,9 @@ def test_workspace_app_export_omits_stale_inline_files(p_ws_root):
     o = workspace_io.load_output(oid)
     folder = p_ws_root / o.workspace_id
     # agent edit -> v2 on disk: modify app.py + add new.py (output.files stays v1)
-    (folder / "app.py").write_text("print('v2 EDITED')\n")
-    (folder / "new.py").write_text("print('v2 NEW')\n")
+    # newline="\n": the export is compared byte-for-byte below, and text mode would write CRLF on Windows.
+    (folder / "app.py").write_text("print('v2 EDITED')\n", newline="\n")
+    (folder / "new.py").write_text("print('v2 NEW')\n", newline="\n")
 
     exp = AppExportable.load(oid)
     payload = exp.serialize(P_Ctx())
diff --git a/backend/tests/test_browser_cookies_windows.py b/backend/tests/test_browser_cookies_windows.py
index b275d94e0..576bc2229 100644
--- a/backend/tests/test_browser_cookies_windows.py
+++ b/backend/tests/test_browser_cookies_windows.py
@@ -64,8 +64,10 @@ def test_truncated_ciphertext_degrades(on_windows):
     assert bc.decrypt_cookie_value(b"v10" + NONCE, KEY) is None
 
 
-def test_the_mac_branch_does_not_try_gcm():
+def test_the_mac_branch_does_not_try_gcm(monkeypatch):
     """Same bytes, no Windows flag: the CBC branch must not accidentally accept a GCM blob."""
+    # Pin the flag rather than the host: on a Windows runner IS_WIN is naturally True and the GCM branch would (correctly) decrypt.
+    monkeypatch.setattr(bc, "IS_WIN", False)
     assert bc.decrypt_cookie_value(p_win_blob(b"sessionid=abc123"), KEY) is None
 
 
diff --git a/backend/tests/test_browser_login_handoff.py b/backend/tests/test_browser_login_handoff.py
index 81a34c1e1..11a2b5479 100644
--- a/backend/tests/test_browser_login_handoff.py
+++ b/backend/tests/test_browser_login_handoff.py
@@ -47,9 +47,12 @@ def test_first_seen_preserved_last_login_advances():
     assert second["last_login"] >= first["last_login"]
 
 
-def test_record_login_is_fail_open(monkeypatch):
+def test_record_login_is_fail_open(monkeypatch, tmp_path):
     # an unwritable path must not raise; the run just treats it as a fresh sign-in next time
-    monkeypatch.setattr(h, "P_STORE_PATH", "/nonexistent-dir-xyz/authenticated_domains.json")
+    # (a path UNDER a regular file is unwritable on every OS; a bare "/nonexistent-dir" is creatable on Windows)
+    blocker = tmp_path / "not-a-dir"
+    blocker.write_text("x", encoding="utf-8")
+    monkeypatch.setattr(h, "P_STORE_PATH", str(blocker / "authenticated_domains.json"))
     h.record_login("x.com")  # no exception
     assert h.is_authenticated("x.com") is False
 
diff --git a/backend/tests/test_browser_metrics.py b/backend/tests/test_browser_metrics.py
index c02d06873..43cc72fbc 100644
--- a/backend/tests/test_browser_metrics.py
+++ b/backend/tests/test_browser_metrics.py
@@ -119,6 +119,7 @@ def test_task_secrets_are_scrubbed_from_tasks_jsonl(tmp_path, monkeypatch):
     line = open(p_os.path.join(str(tmp_path), "tasks.jsonl")).read()
     assert "hunter2" not in line and "sk-abc" not in line
     assert "password [redacted]" in line
-    # owner-only file perms
-    mode = p_os.stat(p_os.path.join(str(tmp_path), "tasks.jsonl")).st_mode & 0o777
-    assert mode == 0o600
+    # owner-only file perms (POSIX mode bits; Windows has no equivalent to assert on)
+    if p_os.name == "posix":
+        mode = p_os.stat(p_os.path.join(str(tmp_path), "tasks.jsonl")).st_mode & 0o777
+        assert mode == 0o600
diff --git a/backend/tests/test_browser_provider_agnostic.py b/backend/tests/test_browser_provider_agnostic.py
index 0ee14a9df..44ce9f1a6 100644
--- a/backend/tests/test_browser_provider_agnostic.py
+++ b/backend/tests/test_browser_provider_agnostic.py
@@ -27,7 +27,7 @@
 def p_model_ids_in_code(path: pathlib.Path):
     """Every concrete model id appearing in a STRING literal, with its line number."""
     hits = []
-    with path.open() as f:
+    with path.open(encoding="utf-8") as f:
         for tok in tokenize.generate_tokens(f.readline):
             if tok.type != tokenize.STRING:
                 continue
@@ -74,7 +74,7 @@ def test_every_aux_call_asks_for_a_tier():
     call_re = re.compile(r"preferred_tier\s*=\s*\"([^\"]+)\"")
     bad = []
     for path in sorted(P_BROWSER_DIR.glob("*.py")):
-        for i, line in enumerate(path.read_text().splitlines(), 1):
+        for i, line in enumerate(path.read_text(encoding="utf-8").splitlines(), 1):
             for m in call_re.finditer(line):
                 if m.group(1) not in ("haiku", "sonnet", "opus"):
                     bad.append(f"{path.name}:{i} -> {m.group(1)}")
diff --git a/backend/tests/test_bundled_extracted_modules.py b/backend/tests/test_bundled_extracted_modules.py
index 21d6ed21b..ae04ea5c4 100644
--- a/backend/tests/test_bundled_extracted_modules.py
+++ b/backend/tests/test_bundled_extracted_modules.py
@@ -42,7 +42,10 @@ def test_warm_cache_is_complete_requires_the_sentinel(tmp_path):
     assert vt.warm_cache_is_complete(str(nm)) is False
     bindir = nm / ".bin"
     bindir.mkdir()
-    (bindir / "vite").symlink_to("../vite/bin/vite.js")
+    # A plain file, not a symlink: on Windows a relative symlink target with forward slashes does not
+    # resolve (exists() is False), and npm writes .bin shims as files there anyway. The check under test
+    # is os.path.exists, which the two are equivalent for.
+    (bindir / "vite").write_text("#!/usr/bin/env node\n")
     assert vt.warm_cache_is_complete(str(nm)) is False
     (tmp_path / ".install-complete").write_text("digest")
     assert vt.warm_cache_is_complete(str(nm)) is True
diff --git a/backend/tests/test_credential_store.py b/backend/tests/test_credential_store.py
index df305bdfa..c32960ec4 100644
--- a/backend/tests/test_credential_store.py
+++ b/backend/tests/test_credential_store.py
@@ -121,6 +121,7 @@ async def test_restoring_a_refresh_token_round_trips(p_router):
 
 
 @pytest.mark.asyncio
+@pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits; Windows ACLs do not surface through st_mode")
 async def test_written_db_is_owner_only(p_router):
     await store.apply_to_connection("conn-1", changes={}, drop=["refreshToken"])
     mode = stat.S_IMODE(os.stat(store.db_path()).st_mode)
diff --git a/backend/tests/test_disk_resilience.py b/backend/tests/test_disk_resilience.py
index e94c2b023..9cb973359 100644
--- a/backend/tests/test_disk_resilience.py
+++ b/backend/tests/test_disk_resilience.py
@@ -79,9 +79,11 @@ def tracking_replace(src, dst):
 
 
 def test_atomic_write_fsyncs_directory_after_rename(tmp_path, monkeypatch):
-    # POSIX only: the rename is only durable once the parent dir is fsync'd.
-    if not hasattr(os, "O_RDONLY"):
-        pytest.skip("directory fsync not applicable on this platform")
+    # POSIX only: the rename is only durable once the parent dir is fsync'd. Windows exposes
+    # os.O_RDONLY but cannot open a directory with it, and json_store skips the dir fsync there
+    # by design, so the old hasattr guard never fired where it was needed.
+    if os.name == "nt":
+        pytest.skip("directory fsync is not possible on Windows; json_store skips it there")
     from backend.config import json_store
 
     fsync_targets = []
diff --git a/backend/tests/test_reap_ghost_runtimes.py b/backend/tests/test_reap_ghost_runtimes.py
index ec9647fa6..560087d52 100644
--- a/backend/tests/test_reap_ghost_runtimes.py
+++ b/backend/tests/test_reap_ghost_runtimes.py
@@ -7,6 +7,8 @@
 import os
 from unittest.mock import patch
 
+import pytest
+
 from backend.apps.outputs import reap_ghost_runtimes as mod
 
 
@@ -163,6 +165,7 @@ class P_Out:
     assert rg.find_ghost_runtime_pids() == [], "a live backend's own app runtime must never be killed"
 
 
+@pytest.mark.skipif(os.name == "nt", reason="SIGSTOP/SIGCONT freezing is POSIX-only; the reaper never pauses runtimes on Windows")
 def test_a_frozen_ghost_is_thawed_before_being_signalled(monkeypatch):
     """Idle app runtimes are parked with SIGSTOP, and a STOPPED process never handles SIGTERM: it
     queues it and lives forever. Found live as a frozen `bash run.sh` that had survived every reap
diff --git a/backend/tests/test_service.py b/backend/tests/test_service.py
index bc9085fbe..9e443c197 100644
--- a/backend/tests/test_service.py
+++ b/backend/tests/test_service.py
@@ -36,9 +36,16 @@ def patch_settings(tmp_path):
     }))
     import backend.apps.settings.store as settings_mod
     old = settings_mod.SETTINGS_FILE
+    old_dir = settings_mod.DATA_DIR
     settings_mod.SETTINGS_FILE = str(sf)
+    # The atomic writer stages its temp file in DATA_DIR and os.replace()s it onto SETTINGS_FILE, so
+    # the two must live together as they do in production. Left at the real data dir, the temp file
+    # lands in the repo's backend/data and, on a runner where that is another drive than tmp_path,
+    # the replace fails with WinError 17 (and the writer's caller quietly falls back).
+    settings_mod.DATA_DIR = str(tmp_path)
     yield
     settings_mod.SETTINGS_FILE = old
+    settings_mod.DATA_DIR = old_dir
 
 
 @pytest.fixture(autouse=True)
diff --git a/backend/tests/test_shutdown_fuse_disarms.py b/backend/tests/test_shutdown_fuse_disarms.py
index f81e4eef8..342527500 100644
--- a/backend/tests/test_shutdown_fuse_disarms.py
+++ b/backend/tests/test_shutdown_fuse_disarms.py
@@ -9,8 +9,15 @@
 skip: a green-looking run that never executed.
 """
 
+import os
+
+import pytest
+
 from backend.apps.service.shutdown_fuse import arm_shutdown_fuse, disarm_shutdown_fuse, fuse_armed
 
+# The fuse walks the process tree with pgrep and is deliberately a no-op on Windows (arm returns without a timer), so there is nothing to stand down there.
+pytestmark = pytest.mark.skipif(os.name == "nt", reason="the shutdown fuse is POSIX-only; arm is a no-op on Windows")
+
 
 def test_arming_then_disarming_leaves_no_live_timer() -> None:
     arm_shutdown_fuse()
diff --git a/backend/tests/test_skill_replay_vs_send_script.py b/backend/tests/test_skill_replay_vs_send_script.py
index d3f142c71..21797ed3a 100644
--- a/backend/tests/test_skill_replay_vs_send_script.py
+++ b/backend/tests/test_skill_replay_vs_send_script.py
@@ -61,7 +61,7 @@ def test_the_agent_uses_the_predicate_rather_than_reinventing_the_condition():
     """The whole point of extracting it. If someone re-inlines the check, these tests would keep
     passing while the live path regressed, which is exactly how this bug survived the first time."""
     src = (bs.__file__).replace("browser_skills.py", "browser_agent.py")
-    with open(src) as f:
+    with open(src, encoding="utf-8") as f:
         text = f.read()
     assert "browser_skills.replay_owns_nav(" in text
     assert "p_skip_prestage_for_skill = bool(" not in text, "the condition was re-inlined"
diff --git a/backend/tests/test_system_prompt.py b/backend/tests/test_system_prompt.py
index 2b431f768..58a1e5293 100644
--- a/backend/tests/test_system_prompt.py
+++ b/backend/tests/test_system_prompt.py
@@ -3,6 +3,8 @@
 App Builder skill. The context builders are mocked to None so the test is deterministic and
 doesn't depend on dashboard/tool disk state."""
 
+import re
+
 import pytest
 from unittest.mock import patch
 
@@ -26,6 +28,10 @@ def test_base_composition_includes_default_and_time_pin():
     out = p_compose(session)
     assert "You are a helpful agent." in out
     assert "<current_time>" in out  # the wall-clock pin is always appended
+    # The pin carries a real date and time on every OS (the block is built with portable strftime
+    # codes; a formatting error would be swallowed and the whole pin would vanish).
+    assert re.search(r"Today is [A-Z][a-z]+, [A-Z][a-z]+ [1-9]\d?, \d{4}\.", out), out
+    assert re.search(r"Local time: [1-9]\d?:\d{2} [AP]M ", out), out
 
 
 def test_view_builder_appends_live_skill_block():