diff --git a/Sources/OrreryCore/Models/Tool.swift b/Sources/OrreryCore/Models/Tool.swift
index f4815ad..a413aaf 100644
--- a/Sources/OrreryCore/Models/Tool.swift
+++ b/Sources/OrreryCore/Models/Tool.swift
@@ -17,7 +17,9 @@ public enum Tool: String, Codable, CaseIterable, Sendable {
 
     /// The system default config directory (when not using Orrery).
     public var defaultConfigDir: URL {
-        let home = FileManager.default.homeDirectoryForCurrentUser
+        // userHomeURL() is overridable (ORRERY_USER_HOME) so tests can isolate
+        // ~/.claude etc. from the developer's real home; unchanged in production.
+        let home = userHomeURL()
         switch self {
         case .claude: return home.appendingPathComponent(".claude")
         case .codex:  return home.appendingPathComponent(".codex")
diff --git a/Sources/OrreryCore/Storage/OrreryHome.swift b/Sources/OrreryCore/Storage/OrreryHome.swift
index 3e372e0..fffa6f6 100644
--- a/Sources/OrreryCore/Storage/OrreryHome.swift
+++ b/Sources/OrreryCore/Storage/OrreryHome.swift
@@ -1,11 +1,26 @@
 import Foundation
 
+/// The current user's home directory. Single source of truth for "the home dir"
+/// used to build `~/.claude` etc., so tests can isolate those paths from the
+/// developer's real home by setting `ORRERY_USER_HOME` (see `withIsolatedHome`).
+///
+/// A dedicated override — not `$HOME` — is used deliberately: setting the OS
+/// `$HOME` would break macOS Keychain resolution (it locates the login keychain
+/// via `$HOME`). In production `ORRERY_USER_HOME` is unset, so this is exactly
+/// `homeDirectoryForCurrentUser` and behavior is unchanged.
+public func userHomeURL() -> URL {
+    if let override = ProcessInfo.processInfo.environment["ORRERY_USER_HOME"],
+       !override.isEmpty {
+        return URL(fileURLWithPath: override)
+    }
+    return FileManager.default.homeDirectoryForCurrentUser
+}
+
 /// Resolves the orrery home directory (`$ORRERY_HOME`, else `~/.orrery`).
 /// Single source of truth shared by EnvironmentStore and AccountStore.
 public func orreryHomeURL() -> URL {
     if let custom = ProcessInfo.processInfo.environment["ORRERY_HOME"] {
         return URL(fileURLWithPath: custom)
     }
-    return FileManager.default.homeDirectoryForCurrentUser
-        .appendingPathComponent(".orrery")
+    return userHomeURL().appendingPathComponent(".orrery")
 }
diff --git a/Tests/OrreryTests/TestHelpers.swift b/Tests/OrreryTests/TestHelpers.swift
index 11009ba..c4c3e98 100644
--- a/Tests/OrreryTests/TestHelpers.swift
+++ b/Tests/OrreryTests/TestHelpers.swift
@@ -23,7 +23,14 @@ func withIsolatedHome(_ body: () throws -> Void) rethrows {
 
     let savedHome = ProcessInfo.processInfo.environment["ORRERY_HOME"]
     let savedActiveEnv = ProcessInfo.processInfo.environment["ORRERY_ACTIVE_ENV"]
+    // ORRERY_USER_HOME is redirected too: `Tool.defaultConfigDir` (and other
+    // home-relative paths) resolve via `userHomeURL()`, which honors it. Without
+    // this a test that triggers origin-takeover code would symlink/write into the
+    // developer's real ~/.claude even though ORRERY_HOME was isolated. We use a
+    // dedicated var, NOT $HOME — setting $HOME breaks macOS Keychain resolution.
+    let savedUserHome = ProcessInfo.processInfo.environment["ORRERY_USER_HOME"]
     setenv("ORRERY_HOME", tmpDir.path, 1)
+    setenv("ORRERY_USER_HOME", tmpDir.path, 1)
     unsetenv("ORRERY_ACTIVE_ENV")
     defer {
         if let savedHome {
@@ -31,6 +38,11 @@ func withIsolatedHome(_ body: () throws -> Void) rethrows {
         } else {
             unsetenv("ORRERY_HOME")
         }
+        if let savedUserHome {
+            setenv("ORRERY_USER_HOME", savedUserHome, 1)
+        } else {
+            unsetenv("ORRERY_USER_HOME")
+        }
         if let savedActiveEnv {
             setenv("ORRERY_ACTIVE_ENV", savedActiveEnv, 1)
         } else {
diff --git a/Tests/OrreryTests/TestIsolationTests.swift b/Tests/OrreryTests/TestIsolationTests.swift
new file mode 100644
index 0000000..5082b2a
--- /dev/null
+++ b/Tests/OrreryTests/TestIsolationTests.swift
@@ -0,0 +1,30 @@
+import Foundation
+import Testing
+@testable import OrreryCore
+
+/// Guards against the test-isolation defect where the suite touched the real
+/// ~/.claude: `Tool.defaultConfigDir` used `homeDirectoryForCurrentUser` (which
+/// ignores $HOME), so origin-takeover code — which symlinks at
+/// `tool.defaultConfigDir` — hijacked the developer's real ~/.claude even though
+/// `withIsolatedHome` had redirected ORRERY_HOME.
+@Suite("test isolation")
+struct TestIsolationTests {
+
+    @Test("withIsolatedHome isolates tool.defaultConfigDir from the real home")
+    func isolatesDefaultConfigDir() {
+        // Captured OUTSIDE isolation: homeDirectoryForCurrentUser ignores $HOME,
+        // so this is always the developer's real home.
+        let realClaude = FileManager.default.homeDirectoryForCurrentUser
+            .appendingPathComponent(".claude").path
+
+        withIsolatedHome {
+            let isolated = Tool.claude.defaultConfigDir.path
+            #expect(isolated != realClaude,
+                "defaultConfigDir must NOT resolve to the real ~/.claude inside withIsolatedHome")
+
+            let tmpHome = ProcessInfo.processInfo.environment["ORRERY_HOME"] ?? ""
+            #expect(!tmpHome.isEmpty && isolated.hasPrefix(tmpHome),
+                "defaultConfigDir should resolve under the isolated temp home")
+        }
+    }
+}