diff --git a/src/config/homeDirectory.ts b/src/config/homeDirectory.ts
index 4e135a1686..ee958a21c3 100644
--- a/src/config/homeDirectory.ts
+++ b/src/config/homeDirectory.ts
@@ -1,17 +1,29 @@
 import { userInfo } from "node:os";
 
+const firstNonempty = (...values: readonly (string | undefined)[]) =>
+  values.find(
+    (value): value is string => value !== undefined && value.length > 0,
+  );
+
 /** Resolve the caller's selected home before consulting the host account. */
 export const homeDirectoryFromEnvironment = (
   environment: Readonly<NodeJS.ProcessEnv>,
   platform: NodeJS.Platform,
 ): string => {
+  const windowsDriveHome =
+    environment.HOMEDRIVE !== undefined &&
+    environment.HOMEDRIVE.length > 0 &&
+    environment.HOMEPATH !== undefined &&
+    environment.HOMEPATH.length > 0
+      ? `${environment.HOMEDRIVE}${environment.HOMEPATH}`
+      : undefined;
   const selected =
     platform === "win32"
-      ? (environment.USERPROFILE ??
-        (environment.HOMEDRIVE !== undefined &&
-        environment.HOMEPATH !== undefined
-          ? `${environment.HOMEDRIVE}${environment.HOMEPATH}`
-          : environment.HOME))
-      : (environment.HOME ?? environment.USERPROFILE);
+      ? firstNonempty(
+          environment.USERPROFILE,
+          windowsDriveHome,
+          environment.HOME,
+        )
+      : firstNonempty(environment.HOME, environment.USERPROFILE);
   return selected ?? userInfo().homedir;
 };
diff --git a/src/homeDirectory.test.ts b/src/homeDirectory.test.ts
new file mode 100644
index 0000000000..076fdfc143
--- /dev/null
+++ b/src/homeDirectory.test.ts
@@ -0,0 +1,64 @@
+import { userInfo } from "node:os";
+
+import { describe, expect, it } from "vitest";
+
+import { homeDirectoryFromEnvironment } from "./config/homeDirectory.js";
+
+describe("home directory environment selection", () => {
+  it("skips an empty HOME before selecting USERPROFILE on POSIX", () => {
+    expect(
+      homeDirectoryFromEnvironment(
+        { HOME: "", USERPROFILE: "relative/profile" },
+        "linux",
+      ),
+    ).toBe("relative/profile");
+  });
+
+  it("falls back to the account home when POSIX home variables are empty", () => {
+    expect(
+      homeDirectoryFromEnvironment({ HOME: "", USERPROFILE: "" }, "linux"),
+    ).toBe(userInfo().homedir);
+  });
+
+  it("preserves a nonempty relative Windows profile ahead of drive variables", () => {
+    expect(
+      homeDirectoryFromEnvironment(
+        {
+          USERPROFILE: "relative/profile",
+          HOMEDRIVE: "D:",
+          HOMEPATH: "\\Users\\account",
+          HOME: "other/home",
+        },
+        "win32",
+      ),
+    ).toBe("relative/profile");
+  });
+
+  it("uses the Windows drive pair when USERPROFILE is empty", () => {
+    expect(
+      homeDirectoryFromEnvironment(
+        {
+          USERPROFILE: "",
+          HOMEDRIVE: "D:",
+          HOMEPATH: "\\Users\\account",
+          HOME: "other/home",
+        },
+        "win32",
+      ),
+    ).toBe("D:\\Users\\account");
+  });
+
+  it("falls back to the account home when all Windows home values are empty", () => {
+    expect(
+      homeDirectoryFromEnvironment(
+        {
+          USERPROFILE: "",
+          HOMEDRIVE: "",
+          HOMEPATH: "",
+          HOME: "",
+        },
+        "win32",
+      ),
+    ).toBe(userInfo().homedir);
+  });
+});
diff --git a/tests/acceptance/setup/homeDirectoryFallback.test.ts b/tests/acceptance/setup/homeDirectoryFallback.test.ts
new file mode 100644
index 0000000000..ccdf168a88
--- /dev/null
+++ b/tests/acceptance/setup/homeDirectoryFallback.test.ts
@@ -0,0 +1,139 @@
+import { access, mkdir, readFile } from "node:fs/promises";
+import { userInfo } from "node:os";
+import { isAbsolute, join, relative, sep } from "node:path";
+import { parse } from "jsonc-parser";
+
+import { expect } from "vitest";
+
+import { createTestTempDirectory } from "../../fixtures/temporaryDirectory.js";
+import { cliTest } from "../../support/cli/cliFixture.js";
+
+cliTest(
+  "setup dry run resolves empty home values without writing to the cwd",
+  async ({ cli }) => {
+    const directory = await createTestTempDirectory("rea-empty-home-dry-run-");
+    const workingDirectory = join(directory, "cwd");
+    await mkdir(workingDirectory);
+    const hostHome = userInfo().homedir;
+    const result = await cli.run({
+      arguments: [
+        "setup",
+        "--client",
+        "cursor",
+        "--skill=false",
+        "--dry-run",
+        "--json",
+      ],
+      cwd: workingDirectory,
+      environment: {
+        HOME: "",
+        USERPROFILE: "",
+        HOMEDRIVE: "",
+        HOMEPATH: "",
+        XDG_CONFIG_HOME: "",
+      },
+    });
+
+    expect(result.exitCode, result.stdout).toBe(0);
+    const output = result.json as {
+      plannedActions?: Array<{ kind?: unknown; target?: unknown }>;
+      status?: unknown;
+    };
+    expect(["planned", "ready"]).toContain(output.status);
+    const configTargets = (output.plannedActions ?? [])
+      .filter((action) => action.kind === "configure_client")
+      .map((action) => action.target);
+    for (const target of configTargets) {
+      expect(typeof target).toBe("string");
+      if (typeof target !== "string") continue;
+      expect(isAbsolute(target)).toBe(true);
+      const relativeTarget = relative(hostHome, target);
+      expect(
+        relativeTarget === "" ||
+          (!relativeTarget.startsWith(`..${sep}`) && relativeTarget !== ".."),
+      ).toBe(true);
+    }
+    expect(configTargets).not.toContain(
+      join(workingDirectory, ".cursor", "mcp.json"),
+    );
+    await expect(
+      access(join(workingDirectory, ".cursor", "mcp.json")),
+    ).rejects.toMatchObject({
+      code: "ENOENT",
+    });
+  },
+);
+
+cliTest.skipIf(process.platform === "win32")(
+  "setup, doctor, and uninstall use the nonempty fallback home when HOME is empty",
+  async ({ cli }) => {
+    const directory = await createTestTempDirectory(
+      "rea-empty-home-lifecycle-",
+    );
+    const home = join(directory, "fallback-home");
+    const workingDirectory = join(directory, "cwd");
+    await mkdir(home);
+    await mkdir(workingDirectory);
+    const environment = {
+      HOME: "",
+      USERPROFILE: home,
+      XDG_CONFIG_HOME: join(home, ".config"),
+    };
+    const run = (arguments_: readonly string[]) =>
+      cli.run({
+        arguments: arguments_,
+        cwd: workingDirectory,
+        environment,
+      });
+
+    const setup = await run(["setup", "--client", "cursor", "--yes", "--json"]);
+    expect(setup.exitCode, setup.stderr).toBe(0);
+    expect(setup.json).toMatchObject({ status: "ready" });
+    await access(join(home, ".cursor", "mcp.json"));
+    await access(
+      join(home, ".agents", "skills", "reverse-engineer-anything", "SKILL.md"),
+    );
+    await expect(
+      access(join(workingDirectory, ".cursor", "mcp.json")),
+    ).rejects.toMatchObject({
+      code: "ENOENT",
+    });
+    await expect(
+      access(join(workingDirectory, ".agents", "skills")),
+    ).rejects.toMatchObject({ code: "ENOENT" });
+
+    const doctor = await run([
+      "doctor",
+      "--client",
+      "cursor",
+      "--skill",
+      "--json",
+    ]);
+    expect(doctor.exitCode, doctor.stdout).toBe(0);
+    expect(doctor.json).toMatchObject({ healthy: true });
+
+    const uninstall = await run(["uninstall", "--json"]);
+    expect(uninstall.exitCode, uninstall.stderr).toBe(0);
+    expect(uninstall.json).toMatchObject({ status: "complete" });
+    const remainingConfig = parse(
+      await readFile(join(home, ".cursor", "mcp.json"), "utf8"),
+    );
+    expect(remainingConfig).not.toHaveProperty("mcpServers.rea");
+    await expect(
+      access(
+        join(
+          home,
+          ".agents",
+          "skills",
+          "reverse-engineer-anything",
+          "SKILL.md",
+        ),
+      ),
+    ).rejects.toMatchObject({ code: "ENOENT" });
+    await expect(
+      access(join(workingDirectory, ".cursor", "mcp.json")),
+    ).rejects.toMatchObject({
+      code: "ENOENT",
+    });
+  },
+);