diff --git a/src/resources/extensions/gsd/auto.ts b/src/resources/extensions/gsd/auto.ts
index 17cb3102e..71676aa53 100644
--- a/src/resources/extensions/gsd/auto.ts
+++ b/src/resources/extensions/gsd/auto.ts
@@ -250,9 +250,9 @@ const STATE_REBUILD_MIN_INTERVAL_MS = 30_000;
 
 export function shouldUseWorktreeIsolation(): boolean {
   const prefs = loadEffectiveGSDPreferences()?.preferences?.git;
-  if (prefs?.isolation === "none") return false;
-  if (prefs?.isolation === "branch") return false;
-  return true; // default: worktree
+  if (prefs?.isolation === "worktree") return true;
+  // Default is false — worktree isolation requires explicit opt-in
+  return false;
 }
 
 /** Crash recovery prompt — set by startAuto, consumed by the main loop */
diff --git a/src/resources/extensions/gsd/doctor-checks.ts b/src/resources/extensions/gsd/doctor-checks.ts
index 20fee0fe0..0b0d05033 100644
--- a/src/resources/extensions/gsd/doctor-checks.ts
+++ b/src/resources/extensions/gsd/doctor-checks.ts
@@ -25,7 +25,7 @@ export async function checkGitHealth(
   issues: DoctorIssue[],
   fixesApplied: string[],
   shouldFix: (code: DoctorIssueCode) => boolean,
-  isolationMode: "none" | "worktree" | "branch" = "worktree",
+  isolationMode: "none" | "worktree" | "branch" = "none",
 ): Promise<void> {
   // Degrade gracefully if not a git repo
   if (!nativeIsRepo(basePath)) {
diff --git a/src/resources/extensions/gsd/doctor.ts b/src/resources/extensions/gsd/doctor.ts
index 5c301bd79..f723edd0a 100644
--- a/src/resources/extensions/gsd/doctor.ts
+++ b/src/resources/extensions/gsd/doctor.ts
@@ -360,8 +360,8 @@ export async function runGSDDoctor(basePath: string, options?: { fix?: boolean;
   // Git health checks — timed
   const t0git = Date.now();
   const isolationMode: "none" | "worktree" | "branch" = options?.isolationMode ??
-    (prefs?.preferences?.git?.isolation === "none" ? "none" :
-    prefs?.preferences?.git?.isolation === "branch" ? "branch" : "worktree");
+    (prefs?.preferences?.git?.isolation === "worktree" ? "worktree" :
+    prefs?.preferences?.git?.isolation === "branch" ? "branch" : "none");
   await checkGitHealth(basePath, issues, fixesApplied, shouldFix, isolationMode);
   const gitMs = Date.now() - t0git;
 
diff --git a/src/resources/extensions/gsd/preferences-types.ts b/src/resources/extensions/gsd/preferences-types.ts
index b57e2514f..9b0083866 100644
--- a/src/resources/extensions/gsd/preferences-types.ts
+++ b/src/resources/extensions/gsd/preferences-types.ts
@@ -34,7 +34,7 @@ export const MODE_DEFAULTS: Record<WorkflowMode, Partial<GSDPreferences>> = {
       push_branches: false,
       pre_merge_check: false,
       merge_strategy: "squash",
-      isolation: "worktree",
+      isolation: "none",
     },
     unique_milestone_ids: false,
   },
@@ -44,7 +44,7 @@ export const MODE_DEFAULTS: Record<WorkflowMode, Partial<GSDPreferences>> = {
       push_branches: true,
       pre_merge_check: true,
       merge_strategy: "squash",
-      isolation: "worktree",
+      isolation: "none",
     },
     unique_milestone_ids: true,
   },
diff --git a/src/resources/extensions/gsd/preferences.ts b/src/resources/extensions/gsd/preferences.ts
index 509ac7f61..df207d1f8 100644
--- a/src/resources/extensions/gsd/preferences.ts
+++ b/src/resources/extensions/gsd/preferences.ts
@@ -497,13 +497,17 @@ export function resolvePreDispatchHooks(): PreDispatchHookConfig[] {
 
 /**
  * Resolve the effective git isolation mode from preferences.
- * Returns "worktree" (default), "branch", or "none".
+ * Returns "none" (default), "worktree", or "branch".
+ *
+ * Default is "none" so GSD works out of the box without preferences.md.
+ * Worktree isolation requires explicit opt-in because it depends on git
+ * branch infrastructure that must be set up before use.
  */
 export function getIsolationMode(): "none" | "worktree" | "branch" {
   const prefs = loadEffectiveGSDPreferences()?.preferences?.git;
-  if (prefs?.isolation === "none") return "none";
+  if (prefs?.isolation === "worktree") return "worktree";
   if (prefs?.isolation === "branch") return "branch";
-  return "worktree"; // default
+  return "none"; // default — no isolation, work on current branch
 }
 
 export function resolveParallelConfig(prefs: GSDPreferences | undefined): import("./types.js").ParallelConfig {
diff --git a/src/resources/extensions/gsd/tests/none-mode-gates.test.ts b/src/resources/extensions/gsd/tests/none-mode-gates.test.ts
index 400288348..bdadcfc1d 100644
--- a/src/resources/extensions/gsd/tests/none-mode-gates.test.ts
+++ b/src/resources/extensions/gsd/tests/none-mode-gates.test.ts
@@ -70,18 +70,20 @@ try {
 }
 });
 
-// Test 4: shouldUseWorktreeIsolation returns true for no prefs (default)
+// Test 4: shouldUseWorktreeIsolation returns false for no prefs (default: none)
+// Worktree isolation requires explicit opt-in — default is "none" so GSD
+// works out of the box without preferences.md (#2480).
 // Skip if global prefs exist — they override the default and this test
 // cannot control ~/.gsd/preferences.md.
 
-test('shouldUseWorktreeIsolation returns true for no prefs (default)', () => {
+test('shouldUseWorktreeIsolation returns false for no prefs (default: none)', () => {
   const globalPrefsExist = existsSync(join(homedir(), ".gsd", "preferences.md"))
     || existsSync(join(homedir(), ".gsd", "PREFERENCES.md"));
   if (!globalPrefsExist) {
     try {
       removeRunnerPreferences(); // ensure no prefs file
       invalidateAllCaches();
-      assert.deepStrictEqual(shouldUseWorktreeIsolation(), true, "shouldUseWorktreeIsolation() with no prefs (default worktree)");
+      assert.deepStrictEqual(shouldUseWorktreeIsolation(), false, "shouldUseWorktreeIsolation() with no prefs (default none)");
     } finally {
       invalidateAllCaches();
     }
@@ -89,6 +91,21 @@ test('shouldUseWorktreeIsolation returns true for no prefs (default)', () => {
   }
 });
 
+// Test 5: getIsolationMode returns "none" when no preferences.md exists (#2480)
+test('getIsolationMode returns "none" with no prefs (default)', () => {
+  const globalPrefsExist = existsSync(join(homedir(), ".gsd", "preferences.md"))
+    || existsSync(join(homedir(), ".gsd", "PREFERENCES.md"));
+  if (!globalPrefsExist) {
+    try {
+      removeRunnerPreferences();
+      invalidateAllCaches();
+      assert.deepStrictEqual(getIsolationMode(), "none", "getIsolationMode() with no prefs defaults to none");
+    } finally {
+      invalidateAllCaches();
+    }
+  }
+});
+
 test('getIsolationMode returns "none" with none prefs', () => {
 try {
   writeRunnerPreferences("none");
@@ -100,6 +117,28 @@ try {
 }
 });
 
+test('getIsolationMode returns "worktree" with worktree prefs', () => {
+try {
+  writeRunnerPreferences("worktree");
+  invalidateAllCaches();
+  assert.deepStrictEqual(getIsolationMode(), "worktree", "getIsolationMode() with worktree prefs");
+} finally {
+  removeRunnerPreferences();
+  invalidateAllCaches();
+}
+});
+
+test('getIsolationMode returns "branch" with branch prefs', () => {
+try {
+  writeRunnerPreferences("branch");
+  invalidateAllCaches();
+  assert.deepStrictEqual(getIsolationMode(), "branch", "getIsolationMode() with branch prefs");
+} finally {
+  removeRunnerPreferences();
+  invalidateAllCaches();
+}
+});
+
 test('getActiveAutoWorktreeContext returns null at baseline', () => {
 assert.deepStrictEqual(getActiveAutoWorktreeContext(), null, "getActiveAutoWorktreeContext() returns null without enterAutoWorktree()");
 });
diff --git a/src/resources/extensions/gsd/tests/preferences.test.ts b/src/resources/extensions/gsd/tests/preferences.test.ts
index 26ac7261d..8c8e3d198 100644
--- a/src/resources/extensions/gsd/tests/preferences.test.ts
+++ b/src/resources/extensions/gsd/tests/preferences.test.ts
@@ -41,18 +41,16 @@ test("git.merge_to_main produces deprecation warning", () => {
 });
 
 
-test("getIsolationMode defaults to worktree when preferences have no isolation setting", () => {
+test("getIsolationMode defaults to none when preferences have no isolation setting", () => {
   // Validate the default via validatePreferences: when no isolation is set,
-  // preferences.git.isolation is undefined, and getIsolationMode returns "worktree".
-  // We test the function's logic by verifying its documented default.
+  // preferences.git.isolation is undefined, and getIsolationMode returns "none".
+  // Default changed from "worktree" to "none" so GSD works out of the box
+  // without preferences.md (#2480).
   const { preferences } = validatePreferences({});
   assert.equal(preferences.git?.isolation, undefined, "no isolation in empty prefs");
-  // The function returns "worktree" when prefs?.git?.isolation is not "none" or "branch"
-  // This is a compile-time-verifiable truth from the function body — test it directly
-  // by constructing the same conditions getIsolationMode checks.
   const isolation = preferences.git?.isolation;
-  const expected = isolation === "none" ? "none" : isolation === "branch" ? "branch" : "worktree";
-  assert.equal(expected, "worktree", "default isolation mode is worktree");
+  const expected = isolation === "worktree" ? "worktree" : isolation === "branch" ? "branch" : "none";
+  assert.equal(expected, "none", "default isolation mode is none");
 });
 
 // ── Mode defaults ────────────────────────────────────────────────────────────
@@ -63,7 +61,7 @@ test("solo mode applies correct defaults", () => {
   assert.equal(result.git?.push_branches, false);
   assert.equal(result.git?.pre_merge_check, false);
   assert.equal(result.git?.merge_strategy, "squash");
-  assert.equal(result.git?.isolation, "worktree");
+  assert.equal(result.git?.isolation, "none");
   assert.equal(result.unique_milestone_ids, false);
 });