fix(web): new threads keep the viewed thread's model

apps/web/src/hooks/useHandleNewThread.ts

@@ -104,9 +104,10 @@ export function useNewThreadHandler() {
       } = useComposerDraftStore.getState();
       const currentRouteTarget = getCurrentRouteTarget();
       // A new thread carries the user's working mode from the thread being
-      // viewed. The target project's configured model still wins; runtime and
-      // interaction modes carry independently. Branch, worktree, and env mode
-      // come from configured defaults unless the caller passes them explicitly.
+      // viewed: model, runtime mode, and interaction mode. The target project's
+      // default model only fills in when there is nothing to carry. Branch,
+      // worktree, and env mode come from configured defaults unless the caller
+      // passes them explicitly.
       const carrySourceShell =
         currentRouteTarget?.kind === "server"
           ? readThreadShell(currentRouteTarget.threadRef)
@@ -304,8 +305,8 @@ export function useNewThreadHandler() {
           }
           // Model intent: an explicit human pick always stands. Seeds and
           // legacy entries alike re-resolve here — sticky first, mirroring
-          // the mint-fresh path, then the project default or carried
-          // selection on top. This runs even when the draft is already open:
+          // the mint-fresh path, then the carried selection or project
+          // default on top. This runs even when the draft is already open:
           // without it, a changed pin could never reach the draft the user
           // is looking at, because explicit picks are the only thing the
           // flag protects.
@@ -457,8 +458,8 @@ export function useNewThreadHandler() {
         applyStickyState(draftId);
         const modelSelectionOverride = resolveModelSelectionOverride(draftId);
         if (modelSelectionOverride) {
-          // Project defaults and carried selections both outrank global sticky
-          // state. The project default wins when both are present.
+          // Carried selections and project defaults both outrank global sticky
+          // state. The carried selection wins when both are present.
           setModelSelection(draftId, modelSelectionOverride, { replaceOptions: true });
         }
         carryComposerContentTo(draftId);

apps/web/src/lib/chatThreadActions.test.ts

@@ -59,14 +59,33 @@ describe("chatThreadActions", () => {
     ).toEqual(CARRIED_SELECTION);
   });
 
-  it("keeps the project default above any carried selection", () => {
+  it("keeps the carried selection above the project default", () => {
     expect(
       resolveNewThreadModelSelectionOverride({
         projectDefaultSelection: PROJECT_DEFAULT_SELECTION,
         carrySelection: CARRIED_SELECTION,
         carrySourceDraftId: "draft-a",
         destinationDraftId: "draft-b",
       }),
+    ).toEqual(CARRIED_SELECTION);
+  });
+
+  it("falls back to the project default when there is nothing to carry", () => {
+    expect(
+      resolveNewThreadModelSelectionOverride({
+        projectDefaultSelection: PROJECT_DEFAULT_SELECTION,
+        carrySelection: null,
+        carrySourceDraftId: null,
+        destinationDraftId: "draft-b",
+      }),
+    ).toEqual(PROJECT_DEFAULT_SELECTION);
+    expect(
+      resolveNewThreadModelSelectionOverride({
+        projectDefaultSelection: PROJECT_DEFAULT_SELECTION,
+        carrySelection: CARRIED_SELECTION,
+        carrySourceDraftId: "draft-a",
+        destinationDraftId: "draft-a",
+      }),
     ).toEqual(PROJECT_DEFAULT_SELECTION);
   });
 

apps/web/src/lib/chatThreadActions.ts

@@ -39,16 +39,19 @@ export function resolveNewDraftStartFromOrigin(input: {
   return input.envMode === "worktree" && input.newWorktreesStartFromOrigin;
 }
 
+// The model the user is looking at carries into the new thread. The project
+// default only fills in when there is nothing to carry: every project gets a
+// default seeded at creation, so letting it outrank the carried selection
+// would reset the model on every new thread.
 export function resolveNewThreadModelSelectionOverride(input: {
   readonly projectDefaultSelection: ModelSelection | null;
   readonly carrySelection: ModelSelection | null;
   readonly carrySourceDraftId: string | null;
   readonly destinationDraftId: string;
 }): ModelSelection | null {
-  return (
-    input.projectDefaultSelection ??
-    (input.carrySourceDraftId === input.destinationDraftId ? null : input.carrySelection)
-  );
+  const carrySelection =
+    input.carrySourceDraftId === input.destinationDraftId ? null : input.carrySelection;
+  return carrySelection ?? input.projectDefaultSelection;
 }
 
 export function resolveThreadActionProjectRef(