diff --git a/apps/web/src/composerDraftStore.test.ts b/apps/web/src/composerDraftStore.test.ts
index ce88deb494d11b0d9054ce92b747cb3d0d21fd24..c2dd5c1240b706f0634df523c6c2d6e02aaae061 100644
--- a/apps/web/src/composerDraftStore.test.ts
+++ b/apps/web/src/composerDraftStore.test.ts
@@ -275,6 +275,24 @@ describe("composerDraftStore project draft thread mapping", () => {
       worktreePath: "/tmp/feature-next",
     });
   });
+
+  it("preserves existing branch and worktree when setProjectDraftThreadId receives undefined", () => {
+    const store = useComposerDraftStore.getState();
+    store.setProjectDraftThreadId(projectId, threadId, {
+      branch: "main",
+      worktreePath: "/tmp/main-worktree",
+    });
+    store.setProjectDraftThreadId(projectId, threadId, {
+      branch: undefined,
+      worktreePath: undefined,
+    });
+
+    expect(useComposerDraftStore.getState().getDraftThread(threadId)).toMatchObject({
+      projectId,
+      branch: "main",
+      worktreePath: "/tmp/main-worktree",
+    });
+  });
 });
 
 describe("composerDraftStore setModel", () => {
diff --git a/apps/web/src/composerDraftStore.ts b/apps/web/src/composerDraftStore.ts
index a5002f200c07398255d19e4d7c8609e6724257c5..1606f2aeb1af30d84d44c8f0897686afa0a5b836 100644
--- a/apps/web/src/composerDraftStore.ts
+++ b/apps/web/src/composerDraftStore.ts
@@ -460,13 +460,13 @@ export const useComposerDraftStore = create<ComposerDraftStoreState>()(
             projectId,
             createdAt: options?.createdAt ?? existingThread?.createdAt ?? new Date().toISOString(),
             branch:
-              options && "branch" in options
-                ? (options.branch ?? null)
-                : (existingThread?.branch ?? null),
+              options?.branch === undefined
+                ? (existingThread?.branch ?? null)
+                : (options.branch ?? null),
             worktreePath:
-              options && "worktreePath" in options
-                ? (options.worktreePath ?? null)
-                : (existingThread?.worktreePath ?? null),
+              options?.worktreePath === undefined
+                ? (existingThread?.worktreePath ?? null)
+                : (options.worktreePath ?? null),
           };
           const hasSameProjectMapping = previousThreadIdForProject === threadId;
           const hasSameDraftThread =