diff --git a/apps/desktop/src/app/DesktopAppIdentity.test.ts b/apps/desktop/src/app/DesktopAppIdentity.test.ts
index 5c39ff304b3b67445db5a46aaa8ba13d3bcb6574..71bcf5f7aef1c4b5753d2882e325180b9fdfd797 100644
--- a/apps/desktop/src/app/DesktopAppIdentity.test.ts
+++ b/apps/desktop/src/app/DesktopAppIdentity.test.ts
@@ -1,3 +1,4 @@
+import * as NodePath from "@effect/platform-node/NodePath";
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import { assert, describe, it } from "@effect/vitest";
 import * as Effect from "effect/Effect";
@@ -88,6 +89,7 @@ const makeEnvironmentLayer = (overrides: TestEnvironmentInput = {}) => {
     Layer.provide(
       Layer.mergeAll(
         NodeServices.layer,
+        NodePath.layerPosix,
         DesktopConfig.layerTest({
           ...env,
         }),
diff --git a/apps/desktop/src/app/DesktopAssets.test.ts b/apps/desktop/src/app/DesktopAssets.test.ts
index bb118d43d29a9d77d5685a4cc120d9851a12cf8f..78819d06e6cc9e4d644e73e06935f2beab4a0069 100644
--- a/apps/desktop/src/app/DesktopAssets.test.ts
+++ b/apps/desktop/src/app/DesktopAssets.test.ts
@@ -1,3 +1,4 @@
+import * as NodePath from "@effect/platform-node/NodePath";
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import { assert, describe, it } from "@effect/vitest";
 import * as Effect from "effect/Effect";
@@ -20,7 +21,11 @@ const environmentLayer = DesktopEnvironment.layer({
   isPackaged: true,
   resourcesPath: "/Applications/T3 Code.app/Contents/Resources",
   runningUnderArm64Translation: false,
-}).pipe(Layer.provide(Layer.mergeAll(NodeServices.layer, DesktopConfig.layerTest({}))));
+}).pipe(
+  Layer.provide(
+    Layer.mergeAll(NodeServices.layer, NodePath.layerPosix, DesktopConfig.layerTest({})),
+  ),
+);
 
 describe("DesktopAssets", () => {
   it.effect("uses canonical source-tree icons for unpackaged development", () =>
@@ -39,6 +44,7 @@ describe("DesktopAssets", () => {
         Layer.provide(
           Layer.mergeAll(
             NodeServices.layer,
+            NodePath.layerPosix,
             DesktopConfig.layerTest({ VITE_DEV_SERVER_URL: "http://localhost:5733" }),
           ),
         ),
diff --git a/apps/desktop/src/app/DesktopEnvironment.test.ts b/apps/desktop/src/app/DesktopEnvironment.test.ts
index 218e2c3e4ba2dc62b5bc1f25ccd90dfdf0170db6..89cc592831a7bc67626cc964267239210cec504f 100644
--- a/apps/desktop/src/app/DesktopEnvironment.test.ts
+++ b/apps/desktop/src/app/DesktopEnvironment.test.ts
@@ -1,3 +1,4 @@
+import * as NodePath from "@effect/platform-node/NodePath";
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import { assert, describe, it } from "@effect/vitest";
 import * as Effect from "effect/Effect";
@@ -26,7 +27,11 @@ const makeEnvironmentLayer = (
   DesktopEnvironment.layer({
     ...defaultInput,
     ...overrides,
-  }).pipe(Layer.provide(Layer.mergeAll(NodeServices.layer, DesktopConfig.layerTest(env))));
+  }).pipe(
+    Layer.provide(
+      Layer.mergeAll(NodeServices.layer, NodePath.layerPosix, DesktopConfig.layerTest(env)),
+    ),
+  );
 
 const makeEnvironment = (
   overrides: Partial<DesktopEnvironment.MakeDesktopEnvironmentInput> = {},
diff --git a/apps/server/src/auth/ServerSecretStore.test.ts b/apps/server/src/auth/ServerSecretStore.test.ts
index d4411fb9f3b7b8e9b1f5253ec405bd9b02d84a62..6b7850aa363bddcee46ea4fb22159fe315daebb0 100644
--- a/apps/server/src/auth/ServerSecretStore.test.ts
+++ b/apps/server/src/auth/ServerSecretStore.test.ts
@@ -109,7 +109,7 @@ const ConcurrentReadMissFileSystemLayer = Layer.effect(
     return {
       ...fileSystem,
       readFile: (path) =>
-        String(path).endsWith("/session-signing-key.bin")
+        /[\\/]session-signing-key\.bin$/.test(String(path))
           ? Ref.updateAndGet(readCountRef, (count) => count + 1).pipe(
               Effect.flatMap((count) => {
                 if (count > 2) {
@@ -219,7 +219,7 @@ it.layer(NodeServices.layer)("ServerSecretStore.layer", (it) => {
       yield* secretStore.set("session-signing-key", Uint8Array.from([1, 2, 3]));
 
       assert.isTrue(
-        chmodCalls.some((call) => call.mode === 0o700 && call.path.endsWith("/secrets")),
+        chmodCalls.some((call) => call.mode === 0o700 && /[\\/]secrets$/.test(call.path)),
       );
       assert.isAtLeast(chmodCalls.filter((call) => call.mode === 0o600).length, 2);
     }).pipe(Effect.provide(NodeServices.layer)),
diff --git a/apps/server/src/project/ProjectFaviconResolver.test.ts b/apps/server/src/project/ProjectFaviconResolver.test.ts
index 2c7b0f7bdc62c9382dc46e549db943bce2f2aab4..b8d6b46da088921eebed968e9193d28ec3b66fa0 100644
--- a/apps/server/src/project/ProjectFaviconResolver.test.ts
+++ b/apps/server/src/project/ProjectFaviconResolver.test.ts
@@ -54,11 +54,12 @@ it.layer(TestLayer)("ProjectFaviconResolverLive", (it) => {
     it.effect("serves repeated resolves from cache instead of re-walking candidates", () =>
       Effect.gen(function* () {
         const resolver = yield* ProjectFaviconResolver.ProjectFaviconResolver;
+        const path = yield* Path.Path;
         const cwd = yield* makeTempDir;
         yield* writeTextFile(cwd, "public/favicon.svg", "<svg>public</svg>");
 
         const resolved = yield* resolver.resolvePath(cwd);
-        expect(resolved?.endsWith("public/favicon.svg")).toBe(true);
+        expect(resolved?.endsWith(path.join("public", "favicon.svg"))).toBe(true);
 
         // `favicon.svg` outranks `public/favicon.svg`, so a resolver that walked
         // the candidate list again would switch to it. Staying on the original
@@ -71,7 +72,7 @@ it.layer(TestLayer)("ProjectFaviconResolverLive", (it) => {
 
         yield* TestClock.adjust(Duration.minutes(11));
 
-        expect((yield* resolver.resolvePath(cwd))?.endsWith("/favicon.svg")).toBe(true);
+        expect(yield* resolver.resolvePath(cwd)).toBe(path.join(cwd, "favicon.svg"));
         expect(yield* resolver.resolvePath(cwd)).not.toBe(resolved);
       }).pipe(Effect.provide(TestClock.layer())),
     );
diff --git a/apps/server/src/provider/Drivers/ClaudeSkills.test.ts b/apps/server/src/provider/Drivers/ClaudeSkills.test.ts
index 3e46ba94df03f174ffb86aea01b91a5d42023529..d126a15c12b8a9757f30c395aab846d12cc8e212 100644
--- a/apps/server/src/provider/Drivers/ClaudeSkills.test.ts
+++ b/apps/server/src/provider/Drivers/ClaudeSkills.test.ts
@@ -1,3 +1,4 @@
+import * as NodePath from "@effect/platform-node/NodePath";
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import { assert, it } from "@effect/vitest";
 import * as Effect from "effect/Effect";
@@ -492,7 +493,11 @@ it.layer(NodeServices.layer)("discoverClaudeSkills", (it) => {
 
   it.effect("lets the administrator's managed policy outrank every other settings file", () =>
     Effect.gen(function* () {
-      const path = yield* Path.Path;
+      // The function is pure on its `path` argument, so hand it the
+      // implementation matching each platform under test rather than the
+      // host's.
+      const path = yield* Path.Path.pipe(Effect.provide(NodePath.layerPosix));
+      const win32Path = yield* Path.Path.pipe(Effect.provide(NodePath.layerWin32));
 
       for (const [platform, expected] of [
         ["darwin", "/Library/Application Support/ClaudeCode/managed-settings.json"],
@@ -508,14 +513,15 @@ it.layer(NodeServices.layer)("discoverClaudeSkills", (it) => {
       }
 
       assert.deepEqual(
-        skillOverrideSettingsPaths(path, "/home/.claude", undefined, "win32", {
-          PROGRAMDATA: "C:/ProgramData",
+        skillOverrideSettingsPaths(win32Path, "C:\\Users\\me\\.claude", undefined, "win32", {
+          PROGRAMDATA: "C:\\ProgramData",
         }).at(-1),
-        "C:/ProgramData/ClaudeCode/managed-settings.json",
+        "C:\\ProgramData\\ClaudeCode\\managed-settings.json",
+      );
+      assert.deepEqual(
+        skillOverrideSettingsPaths(win32Path, "C:\\Users\\me\\.claude", undefined, "win32", {}),
+        ["C:\\Users\\me\\.claude\\settings.json"],
       );
-      assert.deepEqual(skillOverrideSettingsPaths(path, "/home/.claude", undefined, "win32", {}), [
-        "/home/.claude/settings.json",
-      ]);
 
       // Only the repository root's local file joins in, after the
       // workspace's own local file so it wins.
diff --git a/packages/ssh/src/config.test.ts b/packages/ssh/src/config.test.ts
index 0446370337ad8a0bae63838e8391f8a94c89fd4f..4fd3b7636d3f67c2ac238747f2e8234f6e5d742d 100644
--- a/packages/ssh/src/config.test.ts
+++ b/packages/ssh/src/config.test.ts
@@ -1,3 +1,4 @@
+import * as NodePath from "@effect/platform-node/NodePath";
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import { assert, describe, it } from "@effect/vitest";
 import * as Effect from "effect/Effect";
@@ -133,6 +134,6 @@ describe("ssh config", () => {
         "/tmp/home",
       );
       assert.equal(pattern, "/tmp/home/.ssh/config.d/*.conf");
-    }).pipe(Effect.provide(NodeServices.layer)),
+    }).pipe(Effect.provide(NodePath.layerPosix)),
   );
 });
diff --git a/packages/tailscale/src/tailscale.test.ts b/packages/tailscale/src/tailscale.test.ts
index 09d9def2106645b776fb386bfbfa08a848fce0e8..102153f6e2046898314d0f35504452e49b56adcc 100644
--- a/packages/tailscale/src/tailscale.test.ts
+++ b/packages/tailscale/src/tailscale.test.ts
@@ -8,6 +8,7 @@ import * as Sink from "effect/Sink";
 import * as Stream from "effect/Stream";
 import * as TestClock from "effect/testing/TestClock";
 import { ChildProcessSpawner } from "effect/unstable/process";
+import { HostProcessPlatform } from "@t3tools/shared/hostProcess";
 
 import {
   buildTailscaleHttpsBaseUrl,
@@ -98,14 +99,22 @@ function neverFinishingMockHandle() {
   });
 }
 
+// The executable name depends on the host platform (`tailscale.exe` on
+// Windows), so pin it: these tests assert the posix spelling.
+function spawnerLayer(spawner: ChildProcessSpawner.ChildProcessSpawner["Service"]) {
+  return Layer.merge(
+    Layer.succeed(ChildProcessSpawner.ChildProcessSpawner, spawner),
+    Layer.succeed(HostProcessPlatform, "linux"),
+  );
+}
+
 function mockSpawnerLayer(
   handler: (
     command: string,
     args: ReadonlyArray<string>,
   ) => { stdout?: string; stderr?: string; code?: number },
 ) {
-  return Layer.succeed(
-    ChildProcessSpawner.ChildProcessSpawner,
+  return spawnerLayer(
     ChildProcessSpawner.make((command) => {
       const childProcess = command as unknown as {
         readonly command: string;
@@ -194,10 +203,7 @@ describe("tailscale", () => {
       method: "spawn",
       cause: systemCause,
     });
-    const layer = Layer.succeed(
-      ChildProcessSpawner.ChildProcessSpawner,
-      ChildProcessSpawner.make(() => Effect.fail(cause)),
-    );
+    const layer = spawnerLayer(ChildProcessSpawner.make(() => Effect.fail(cause)));
 
     return Effect.gen(function* () {
       const error = yield* readTailscaleStatus.pipe(Effect.flip, Effect.provide(layer));
@@ -218,8 +224,7 @@ describe("tailscale", () => {
     // inside an `Effect.callback` registration, so that throw arrives as a
     // defect rather than a typed error - the shape reproduced here.
     const defect = Object.assign(new Error("spawn tailscale ENOTDIR"), { code: "ENOTDIR" });
-    const layer = Layer.succeed(
-      ChildProcessSpawner.ChildProcessSpawner,
+    const layer = spawnerLayer(
       ChildProcessSpawner.make(() =>
         Effect.callback<never, never>(() => {
           throw defect;
@@ -297,10 +302,7 @@ describe("tailscale", () => {
   it.effect("times out tailscale status through TestClock", () => {
     const layer = Layer.merge(
       TestClock.layer(),
-      Layer.succeed(
-        ChildProcessSpawner.ChildProcessSpawner,
-        ChildProcessSpawner.make(() => Effect.succeed(neverFinishingMockHandle())),
-      ),
+      spawnerLayer(ChildProcessSpawner.make(() => Effect.succeed(neverFinishingMockHandle()))),
     );
 
     return Effect.gen(function* () {
diff --git a/scripts/mobile-showcase.test.ts b/scripts/mobile-showcase.test.ts
index 2fd4c1dd8edd3233e309a744404f703e1cc20cfc..a3f11e3a37c7997ecfa9d116455e52d2ea795146 100644
--- a/scripts/mobile-showcase.test.ts
+++ b/scripts/mobile-showcase.test.ts
@@ -1,3 +1,5 @@
+// @effect-diagnostics nodeBuiltinImport:off - expectations mirror the host path joins the script under test performs.
+import * as NodePath from "node:path";
 import { assert, it } from "@effect/vitest";
 import { PNG } from "pngjs";
 
@@ -167,8 +169,14 @@ it("expands both appearances into independent upload-ready directories", () => {
       directory: showcaseCaptureDirectory("/captures", capture),
     })),
     [
-      { appearance: "light", directory: "/captures/apple/iphone-test/light/t3-code" },
-      { appearance: "dark", directory: "/captures/apple/iphone-test/dark/t3-code" },
+      {
+        appearance: "light",
+        directory: NodePath.join("/captures", "apple", "iphone-test", "light", "t3-code"),
+      },
+      {
+        appearance: "dark",
+        directory: NodePath.join("/captures", "apple", "iphone-test", "dark", "t3-code"),
+      },
     ],
   );
 });
@@ -192,10 +200,10 @@ it("expands themes into independent upload-ready directories per appearance", ()
       showcaseCaptureDirectory("/captures", capture),
     ),
     [
-      "/captures/apple/iphone-test/light/ocean",
-      "/captures/apple/iphone-test/light/ember",
-      "/captures/apple/iphone-test/dark/ocean",
-      "/captures/apple/iphone-test/dark/ember",
+      NodePath.join("/captures", "apple", "iphone-test", "light", "ocean"),
+      NodePath.join("/captures", "apple", "iphone-test", "light", "ember"),
+      NodePath.join("/captures", "apple", "iphone-test", "dark", "ocean"),
+      NodePath.join("/captures", "apple", "iphone-test", "dark", "ember"),
     ],
   );
 });
diff --git a/scripts/mobile-showcase.ts b/scripts/mobile-showcase.ts
index d6ced75d5db2152cff4ad845f18da9aed74afa9c..6b6adebeb779edf75a690be7ecfd936b33c1b0b7 100644
--- a/scripts/mobile-showcase.ts
+++ b/scripts/mobile-showcase.ts
@@ -53,7 +53,8 @@ export function resolveAndroidSdkRoot(
   const configured = environment.ANDROID_HOME ?? environment.ANDROID_SDK_ROOT;
   if (configured) return configured;
   const home = environment.HOME ?? environment.USERPROFILE ?? "";
-  return NodePath.join(home, platform === "darwin" ? "Library/Android/sdk" : "Android/Sdk");
+  const path = platform === "win32" ? NodePath.win32 : NodePath.posix;
+  return path.join(home, platform === "darwin" ? "Library/Android/sdk" : "Android/Sdk");
 }
 
 const ANDROID_SDK_ROOT = resolveAndroidSdkRoot(NodeProcess.env);