diff --git a/apps/web/src/components/BranchToolbar.logic.test.ts b/apps/web/src/components/BranchToolbar.logic.test.ts
index 492f080beaaa29e2ec7f5dbbbd601286ea4ded29..e940fe2c3c597f8d490e64d422e0c4335a7bf3ac 100644
--- a/apps/web/src/components/BranchToolbar.logic.test.ts
+++ b/apps/web/src/components/BranchToolbar.logic.test.ts
@@ -4,6 +4,7 @@ import {
   filterBranchPickerItems,
   dedupeRemoteBranchesWithLocalMatches,
   deriveLocalBranchNameFromRemoteRef,
+  resolveBranchSelectionTarget,
   resolveDraftEnvModeAfterBranchChange,
   resolveBranchToolbarValue,
 } from "./BranchToolbar.logic";
@@ -223,3 +224,73 @@ describe("filterBranchPickerItems", () => {
     ).toEqual(["feature/488", "__create_new_branch__:48"]);
   });
 });
+
+describe("resolveBranchSelectionTarget", () => {
+  it("reuses an existing secondary worktree for the selected branch", () => {
+    expect(
+      resolveBranchSelectionTarget({
+        activeProjectCwd: "/repo",
+        activeWorktreePath: "/repo/.t3/worktrees/feature-a",
+        branch: {
+          isDefault: false,
+          worktreePath: "/repo/.t3/worktrees/feature-b",
+        },
+      }),
+    ).toEqual({
+      checkoutCwd: "/repo/.t3/worktrees/feature-b",
+      nextWorktreePath: "/repo/.t3/worktrees/feature-b",
+      reuseExistingWorktree: true,
+    });
+  });
+
+  it("switches back to the main repo when the branch already lives there", () => {
+    expect(
+      resolveBranchSelectionTarget({
+        activeProjectCwd: "/repo",
+        activeWorktreePath: "/repo/.t3/worktrees/feature-a",
+        branch: {
+          isDefault: true,
+          worktreePath: "/repo",
+        },
+      }),
+    ).toEqual({
+      checkoutCwd: "/repo",
+      nextWorktreePath: null,
+      reuseExistingWorktree: true,
+    });
+  });
+
+  it("checks out the default branch in the main repo when leaving a secondary worktree", () => {
+    expect(
+      resolveBranchSelectionTarget({
+        activeProjectCwd: "/repo",
+        activeWorktreePath: "/repo/.t3/worktrees/feature-a",
+        branch: {
+          isDefault: true,
+          worktreePath: null,
+        },
+      }),
+    ).toEqual({
+      checkoutCwd: "/repo",
+      nextWorktreePath: null,
+      reuseExistingWorktree: false,
+    });
+  });
+
+  it("keeps checkout in the current worktree for non-default branches", () => {
+    expect(
+      resolveBranchSelectionTarget({
+        activeProjectCwd: "/repo",
+        activeWorktreePath: "/repo/.t3/worktrees/feature-a",
+        branch: {
+          isDefault: false,
+          worktreePath: null,
+        },
+      }),
+    ).toEqual({
+      checkoutCwd: "/repo/.t3/worktrees/feature-a",
+      nextWorktreePath: "/repo/.t3/worktrees/feature-a",
+      reuseExistingWorktree: false,
+    });
+  });
+});
diff --git a/apps/web/src/components/BranchToolbar.logic.ts b/apps/web/src/components/BranchToolbar.logic.ts
index 3db270eeb2852354f38924e5cbad963d36b659ee..79acb487c27517d680e34d84d332fbfa3a7cad1d 100644
--- a/apps/web/src/components/BranchToolbar.logic.ts
+++ b/apps/web/src/components/BranchToolbar.logic.ts
@@ -112,3 +112,32 @@ export function filterBranchPickerItems(input: {
     return itemValue.toLowerCase().includes(normalizedQuery);
   });
 }
+
+export function resolveBranchSelectionTarget(input: {
+  activeProjectCwd: string;
+  activeWorktreePath: string | null;
+  branch: Pick<GitBranch, "isDefault" | "worktreePath">;
+}): {
+  checkoutCwd: string;
+  nextWorktreePath: string | null;
+  reuseExistingWorktree: boolean;
+} {
+  const { activeProjectCwd, activeWorktreePath, branch } = input;
+
+  if (branch.worktreePath) {
+    return {
+      checkoutCwd: branch.worktreePath,
+      nextWorktreePath: branch.worktreePath === activeProjectCwd ? null : branch.worktreePath,
+      reuseExistingWorktree: true,
+    };
+  }
+
+  const nextWorktreePath =
+    activeWorktreePath !== null && branch.isDefault ? null : activeWorktreePath;
+
+  return {
+    checkoutCwd: nextWorktreePath ?? activeProjectCwd,
+    nextWorktreePath,
+    reuseExistingWorktree: false,
+  };
+}
diff --git a/apps/web/src/components/BranchToolbarBranchSelector.tsx b/apps/web/src/components/BranchToolbarBranchSelector.tsx
index 3869ec234a4c49b8cc6ff5c2a178b88ecb6d2120..766b284816df429dc0c46c47fd584af6f38bb206 100644
--- a/apps/web/src/components/BranchToolbarBranchSelector.tsx
+++ b/apps/web/src/components/BranchToolbarBranchSelector.tsx
@@ -27,6 +27,7 @@ import {
   deriveLocalBranchNameFromRemoteRef,
   EnvMode,
   filterBranchPickerItems,
+  resolveBranchSelectionTarget,
   resolveBranchToolbarValue,
 } from "./BranchToolbar.logic";
 import { Button } from "./ui/button";
@@ -175,10 +176,15 @@ export function BranchToolbarBranchSelector({
       return;
     }
 
+    const selectionTarget = resolveBranchSelectionTarget({
+      activeProjectCwd,
+      activeWorktreePath,
+      branch,
+    });
+
     // If the branch already lives in a worktree, point the thread there.
-    if (branch.worktreePath) {
-      const isMainWorktree = branch.worktreePath === activeProjectCwd;
-      onSetThreadBranch(branch.name, isMainWorktree ? null : branch.worktreePath);
+    if (selectionTarget.reuseExistingWorktree) {
+      onSetThreadBranch(branch.name, selectionTarget.nextWorktreePath);
       setIsBranchMenuOpen(false);
       onComposerFocusRequest?.();
       return;
@@ -194,7 +200,7 @@ export function BranchToolbarBranchSelector({
     runBranchAction(async () => {
       setOptimisticBranch(selectedBranchName);
       try {
-        await api.git.checkout({ cwd: branchCwd, branch: branch.name });
+        await api.git.checkout({ cwd: selectionTarget.checkoutCwd, branch: branch.name });
         await invalidateGitQueries(queryClient);
       } catch (error) {
         toastManager.add({
@@ -214,7 +220,7 @@ export function BranchToolbarBranchSelector({
       }
 
       setOptimisticBranch(nextBranchName);
-      onSetThreadBranch(nextBranchName, activeWorktreePath);
+      onSetThreadBranch(nextBranchName, selectionTarget.nextWorktreePath);
     });
   };