diff --git a/ui/src/app/agent-selection.test.ts b/ui/src/app/agent-selection.test.ts
index fa3e30264d13..d9e3488193f6 100644
--- a/ui/src/app/agent-selection.test.ts
+++ b/ui/src/app/agent-selection.test.ts
@@ -138,6 +138,165 @@ describe("agent selection", () => {
     expect(selection.state).toEqual({ selectedId: "research", scopeId: "research" });
   });
 
+  it("adopts a changed default after the reconnect null phase", () => {
+    const harness = createGateway("Main");
+    const roster = createRoster();
+    const selection = createAgentSelectionCapability(harness.gateway, roster.roster);
+    roster.publish({
+      defaultId: "main",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [{ id: "main", kind: "agent" }],
+    });
+    const client = { request() {} } as unknown as GatewayBrowserClient;
+
+    harness.publish({ client, assistantAgentId: null });
+    expect(selection.state).toEqual({ selectedId: "main", scopeId: "main" });
+    harness.publish({ client, assistantAgentId: "Ops" });
+    expect(selection.state).toEqual({ selectedId: "main", scopeId: "main" });
+    roster.publish({
+      defaultId: "ops",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [
+        { id: "main", kind: "agent" },
+        { id: "ops", kind: "agent" },
+      ],
+    });
+
+    expect(selection.state).toEqual({ selectedId: "ops", scopeId: "ops" });
+  });
+
+  it("keeps following hello after the roster repairs an invalid startup default", () => {
+    const harness = createGateway("Stale");
+    const roster = createRoster();
+    roster.publish({
+      defaultId: "main",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [{ id: "main", kind: "agent" }],
+    });
+    const selection = createAgentSelectionCapability(harness.gateway, roster.roster);
+    const client = { request() {} } as unknown as GatewayBrowserClient;
+
+    expect(selection.state).toEqual({ selectedId: "main", scopeId: "main" });
+    harness.publish({ client, assistantAgentId: "Ops" });
+    roster.publish({
+      defaultId: "ops",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [
+        { id: "main", kind: "agent" },
+        { id: "ops", kind: "agent" },
+      ],
+    });
+
+    expect(selection.state).toEqual({ selectedId: "ops", scopeId: "ops" });
+  });
+
+  it("preserves an explicit owner through reconnect and a changed default", () => {
+    const harness = createGateway("Main");
+    const selection = createAgentSelectionCapability(harness.gateway, createRoster().roster);
+    selection.set("Research");
+    const client = { request() {} } as unknown as GatewayBrowserClient;
+
+    harness.publish({ client, assistantAgentId: null });
+    harness.publish({ client, assistantAgentId: "Ops" });
+
+    expect(selection.state).toEqual({ selectedId: "research", scopeId: "research" });
+  });
+
+  it("establishes explicit ownership before notifying selection subscribers", () => {
+    const harness = createGateway("Main");
+    const selection = createAgentSelectionCapability(harness.gateway, createRoster().roster);
+    const client = { request() {} } as unknown as GatewayBrowserClient;
+    selection.subscribe((state) => {
+      if (state.selectedId === "research") {
+        harness.publish({ client, assistantAgentId: "Ops" });
+      }
+    });
+
+    selection.set("Research");
+
+    expect(selection.state).toEqual({ selectedId: "research", scopeId: "research" });
+  });
+
+  it("follows hello again after the roster removes an explicit owner", () => {
+    const harness = createGateway("Main");
+    const roster = createRoster();
+    roster.publish({
+      defaultId: "main",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [
+        { id: "main", kind: "agent" },
+        { id: "research", kind: "agent" },
+      ],
+    });
+    const selection = createAgentSelectionCapability(harness.gateway, roster.roster);
+    selection.set("Research");
+
+    roster.publish({
+      defaultId: "main",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [{ id: "main", kind: "agent" }],
+    });
+    expect(selection.state).toEqual({ selectedId: "main", scopeId: "main" });
+
+    const client = { request() {} } as unknown as GatewayBrowserClient;
+    harness.publish({ client, assistantAgentId: null });
+    harness.publish({ client, assistantAgentId: "Ops" });
+    roster.publish({
+      defaultId: "ops",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [
+        { id: "main", kind: "agent" },
+        { id: "ops", kind: "agent" },
+      ],
+    });
+
+    expect(selection.state).toEqual({ selectedId: "ops", scopeId: "ops" });
+  });
+
+  it("preserves a reentrant explicit owner during roster fallback", () => {
+    const harness = createGateway("Main");
+    const roster = createRoster();
+    roster.publish({
+      defaultId: "main",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [
+        { id: "main", kind: "agent" },
+        { id: "ops", kind: "agent" },
+        { id: "research", kind: "agent" },
+      ],
+    });
+    const selection = createAgentSelectionCapability(harness.gateway, roster.roster);
+    selection.set("Research");
+    selection.subscribe((state) => {
+      if (state.selectedId === "main") {
+        selection.set("Ops");
+      }
+    });
+
+    roster.publish({
+      defaultId: "main",
+      mainKey: "main",
+      scope: "per-sender",
+      agents: [
+        { id: "main", kind: "agent" },
+        { id: "ops", kind: "agent" },
+      ],
+    });
+    const client = { request() {} } as unknown as GatewayBrowserClient;
+    harness.publish({ client, assistantAgentId: null });
+    harness.publish({ client, assistantAgentId: "Main" });
+
+    expect(selection.state).toEqual({ selectedId: "ops", scopeId: "ops" });
+  });
+
   it("self-heals an unknown cold-load selection to the roster default", () => {
     const gateway = createGateway("Main");
     const roster = createRoster();
diff --git a/ui/src/app/agent-selection.ts b/ui/src/app/agent-selection.ts
index adda21a60dab..6dbf9e231700 100644
--- a/ui/src/app/agent-selection.ts
+++ b/ui/src/app/agent-selection.ts
@@ -75,6 +75,9 @@ export function createAgentSelectionCapability(
     scopeId: resolveScopeId(initialSelectedId),
   };
   let assistantAgentId = initialId;
+  // Construction has no explicit-selection input: a roster repair still follows
+  // hello until a navigation or picker action establishes explicit ownership.
+  let followsGatewayDefault = true;
   const listeners = new Set<(next: AgentSelectionState) => void>();
 
   const publish = (next: AgentSelectionState) => {
@@ -97,13 +100,25 @@ export function createAgentSelectionCapability(
       ? normalizeAgentId(next.assistantAgentId)
       : null;
     const assistantChanged = nextAssistantAgentId !== assistantAgentId;
-    const followedPreviousDefault = state.selectedId === assistantAgentId;
     assistantAgentId = nextAssistantAgentId;
-    if (assistantChanged && (!state.selectedId || followedPreviousDefault)) {
+    // A reconnect publishes a transient null before hello. Keep the last
+    // implicit default selected until the next authoritative default arrives.
+    if (assistantChanged && followsGatewayDefault && nextAssistantAgentId) {
       publish({ selectedId: nextAssistantAgentId, scopeId: nextAssistantAgentId });
     }
   });
-  roster.subscribe(() => publish(state));
+  roster.subscribe(() => {
+    // Re-enable implicit ownership before publishing the roster fallback. A
+    // synchronous subscriber may establish a new explicit owner during publish.
+    if (!followsGatewayDefault && reconcileSelectedId(state.selectedId) !== state.selectedId) {
+      followsGatewayDefault = true;
+    }
+    if (followsGatewayDefault && assistantAgentId) {
+      publish({ selectedId: assistantAgentId, scopeId: assistantAgentId });
+    } else {
+      publish(state);
+    }
+  });
 
   return {
     get state() {
@@ -114,6 +129,8 @@ export function createAgentSelectionCapability(
       // A chip/chat switch establishes a new global page scope. The separate
       // scope field lets page controls expose all agents without losing the
       // concrete agent required by chat and new-session flows.
+      // Establish ownership before publish notifies synchronous subscribers.
+      followsGatewayDefault = reconcileSelectedId(selectedId) !== selectedId;
       publish({ selectedId, scopeId: selectedId });
     },
     setScope(agentId) {