diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 86f1ba2e4..6e6521b41 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -509,6 +509,7 @@ jobs:
           tests/boundary/providers/native/artifactProvider.extractionCompleteness.test.ts
           tests/boundary/filesystem/setupUnavailableProvider.test.ts
           tests/boundary/filesystem/windowsNpxRegistration.test.ts
+          tests/acceptance/setup/homeDirectoryFallback.test.ts
           tests/boundary/filesystem/webScriptManifestPaths.test.ts
           tests/boundary/process/ProcessEvidence.host.test.ts
           tests/acceptance/webModuleCli.test.ts
diff --git a/experiments/analysis-efficiency/windows-empty-profile.md b/experiments/analysis-efficiency/windows-empty-profile.md
new file mode 100644
index 000000000..f8e5558e0
--- /dev/null
+++ b/experiments/analysis-efficiency/windows-empty-profile.md
@@ -0,0 +1,69 @@
+# Windows empty-profile startup verification
+
+Follow-up to [#2345](https://github.com/morluto/rea/pull/2345), including
+@gefrezim's diagnosis of the import-time Windows home lookup.
+
+## Source and boundary
+
+Baseline: main `172407787f1842dedbcd27b5385177595f786ce4`. Verification used
+the real Windows 11 10.0.22631.0 host, official Windows x64 Node 24.18.0,
+locked production dependencies and REA's compiled public CLI / SDK 2.3.1 MCP
+server. No IDA or Ghidra engine was used. Runtime copies and compact receipts
+live outside the checkout; no account profile path or ambient environment is
+recorded here.
+
+The tested dispatcher SHA-256 is
+`526105418604cc6e5a202a6275b700814e27450bfaffd45e7510530ba669de8d`.
+Only the dispatcher changed in the deployed runtime. It treats an empty Windows
+`USERPROFILE` as unset before dependency imports. Nonempty selections continue
+to reach the existing home resolver; no additional lookup or fallback owner is
+introduced.
+
+## Real Windows observations
+
+With `HOME` empty in each case, the baseline produced:
+
+| USERPROFILE selection       | `--version`             | `--help`                | MCP discovery |
+| --------------------------- | ----------------------- | ----------------------- | ------------- |
+| Unset                       | Exit 0                  | Exit 0                  | Healthy       |
+| Empty                       | Exit 1, `uv_os_homedir` | Exit 1, `uv_os_homedir` | Healthy       |
+| Nonempty test-owned profile | Exit 0                  | Exit 0                  | Healthy       |
+
+The corrected CLI passed all six invocations with no stderr. Version output
+remained 6 bytes and help output 15,497 bytes. Real setup dry runs with empty
+and nonempty profile selections both exited 0, planned their configuration
+under the expected home, preserved existing configuration bytes and created no
+configuration under the working directory. No installation was performed.
+
+MCP was already healthy in the baseline; this change does not claim to repair
+an MCP startup failure. Complete baseline discovery was 187 tools and 2,545,790
+serialized bytes, SHA-256
+`1e6bbca7f62852134eb703ea098f5a302d1fe333b8433475429f6fc1b1aba18f`.
+All three corrected MCP controls also passed with no stderr and the same
+catalogue bytes and digest.
+
+The existing `homeDirectoryFallback` acceptance test exercises the import-time
+failure through the public setup dry run. Adding it to the Windows curated lane
+makes that regression execute on the affected platform. No duplicate startup
+test was retained. Nine related tests passed, including the existing MCP doctor
+and curated-lane path checks; Linux passing alone does not prove Windows behavior.
+Review caught a missing package-scope registration for the newly listed test;
+the original CI shard failed that ownership invariant. The path is now also
+registered in `scripts/ci/scopes.mjs`, and all 84 scope checks plus the two
+curated-path checks pass. Runtime source and its tested digest are unchanged.
+
+## Efficiency and resource limits
+
+Heavy host probes ran serially at BelowNormal priority, with one CPU and a
+512 MiB CLI/server heap setting (256 MiB for the probe parent). Heap settings do
+not bound total process memory. An exit-time `process.resourceUsage()` observer
+recorded individual corrected CLI invocations: CPU time ranged from 7.968 to
+9.874 seconds and peak working set from 299,956 to 301,852 KiB. These figures
+exclude the probe parent and any observer overhead outside the CLI process.
+The six observed wall times ranged from 8.656 to 10.961 seconds.
+
+The baseline did not use that resource observer, and cold/warm caches were not
+normalized. These observations establish functional behavior and bounded probe
+scope; no CPU, memory or latency improvement is claimed. The fix adds one
+constant-time environment check, no filesystem lookup, engine call, retry or
+cache. Output bytes are not model tokens; actual model usage was not measured.
diff --git a/scripts/ci/scopes.mjs b/scripts/ci/scopes.mjs
index 497dfeed0..4493fe512 100644
--- a/scripts/ci/scopes.mjs
+++ b/scripts/ci/scopes.mjs
@@ -55,6 +55,7 @@ export const SCOPES = {
     "tests/boundary/providers/native/artifactProvider.extractionCompleteness.test.ts",
     "tests/boundary/filesystem/setupUnavailableProvider.test.ts",
     "tests/boundary/filesystem/windowsNpxRegistration.test.ts",
+    "tests/acceptance/setup/homeDirectoryFallback.test.ts",
     "tests/boundary/filesystem/webScriptManifestPaths.test.ts",
     "tests/boundary/process/ProcessEvidence.host.test.ts",
     "tests/acceptance/webModuleCli.test.ts",
diff --git a/scripts/rea.mjs b/scripts/rea.mjs
index 78ffbdd21..75ee552c5 100755
--- a/scripts/rea.mjs
+++ b/scripts/rea.mjs
@@ -16,6 +16,12 @@ const isMcpMode =
 const isMcpDoctorMode = args[0] === "mcp" && args[1] === "doctor";
 const packageRoot = resolve(dirname(fileURLToPath(import.meta.url)), "..");
 
+// Windows home lookup throws for an empty USERPROFILE. Dependencies resolve
+// home during import, before REA's own resolver can apply its account fallback.
+if (process.platform === "win32" && process.env.USERPROFILE === "") {
+  delete process.env.USERPROFILE;
+}
+
 // Playwright's registry assumes one of its three desktop host platforms when
 // it chooses the browser cache directory. Termux reports `android`, although
 // its Node runtime and filesystem are POSIX-compatible. Keep the cache path