diff --git a/front/components/assistant/details/AgentDetailsButtonBar.tsx b/front/components/assistant/details/AgentDetailsButtonBar.tsx
index 6766fdf2b1bc..8cd78667c5d6 100644
--- a/front/components/assistant/details/AgentDetailsButtonBar.tsx
+++ b/front/components/assistant/details/AgentDetailsButtonBar.tsx
@@ -38,14 +38,12 @@ interface AgentDetailsButtonBarProps {
   agentConfiguration: LightAgentConfigurationType;
   owner: WorkspaceType;
   isAgentConfigurationValidating: boolean;
-  onClose: () => void;
 }
 
 export function AgentDetailsButtonBar({
   agentConfiguration,
   isAgentConfigurationValidating,
   owner,
-  onClose,
 }: AgentDetailsButtonBarProps) {
   const { user, providersHealth } = useAuth();
   const router = useAppRouter();
@@ -76,7 +74,8 @@ export function AgentDetailsButtonBar({
   const agentIsFavorite = agentConfiguration.userFavorite || isFavoriteDisabled;
 
   const handleNewConversation = async () => {
-    onClose();
+    // Navigate only — closing the sheet first does a separate router.push that
+    // races this navigation when opening a new conversation with ?agent=.
     await router.push(
       getConversationRoute(owner.sId, "new", `agent=${agentConfiguration.sId}`)
     );
diff --git a/front/components/assistant/details/AgentDetailsSheet.tsx b/front/components/assistant/details/AgentDetailsSheet.tsx
index 92b66aa10a4b..51581b6b03ef 100644
--- a/front/components/assistant/details/AgentDetailsSheet.tsx
+++ b/front/components/assistant/details/AgentDetailsSheet.tsx
@@ -267,7 +267,6 @@ export function AgentDetailsSheet({
             owner={owner}
             agentConfiguration={agentConfiguration}
             isAgentConfigurationValidating={isAgentConfigurationValidating}
-            onClose={onClose}
           />
         )}
 
diff --git a/front/components/editor/input_bar/useHandleMentions.tsx b/front/components/editor/input_bar/useHandleMentions.tsx
index 5dd5265734e1..06f37970a272 100644
--- a/front/components/editor/input_bar/useHandleMentions.tsx
+++ b/front/components/editor/input_bar/useHandleMentions.tsx
@@ -1,6 +1,7 @@
 import type { PendingInputText } from "@app/components/assistant/conversation/input_bar/InputBarContext";
 import { InputBarContext } from "@app/components/assistant/conversation/input_bar/InputBarContext";
 import type { EditorService } from "@app/components/editor/input_bar/useCustomEditor";
+import { useSearchParam } from "@app/lib/platform";
 import type { LightAgentConfigurationType } from "@app/types/assistant/agent";
 import { GLOBAL_AGENTS_SID } from "@app/types/assistant/assistant";
 import type { ConversationWithoutContentType } from "@app/types/assistant/conversation";
@@ -49,6 +50,8 @@ const useHandleMentions = ({
 }: UseHandleMentionsOptions) => {
   const stickyMentionsTextContent = useRef<string | null>(null);
   const { setSelectedSingleAgent } = useContext(InputBarContext);
+  // When present, useAgentFromSearchParam owns the selection on the new-conversation page.
+  const agentSearchParam = useSearchParam("agent");
 
   // Priority: draft > sticky mentions > @dust fallback.
   // Also resets when the conversation changes so stale state doesn't leak.
@@ -72,6 +75,13 @@ const useHandleMentions = ({
       return;
     }
 
+    // New conversation with ?agent= in the URL: leave selection to
+    // useAgentFromSearchParam / the selectedAgent effect. Applying draft, sticky,
+    // or @dust here races that path and can overwrite the custom agent in the URL.
+    if (agentSearchParam && !conversation && !isAgentBuilder) {
+      return;
+    }
+
     // Agent builder: wait for the draft agent to arrive via stickyMentions.
     // Clear any stale selectedSingleAgent from the previous page while waiting,
     // so the old agent doesn't flash in the input bar.
@@ -116,6 +126,7 @@ const useHandleMentions = ({
       }
     }
   }, [
+    agentSearchParam,
     isAgentBuilder,
     conversation,
     stickyMentions,
diff --git a/front/hooks/useAgentFromSearchParam.test.ts b/front/hooks/useAgentFromSearchParam.test.ts
index 95934c4e1217..d70bf34fa008 100644
--- a/front/hooks/useAgentFromSearchParam.test.ts
+++ b/front/hooks/useAgentFromSearchParam.test.ts
@@ -143,8 +143,10 @@ describe("useAgentFromSearchParam", () => {
     selectedSingleAgentHolder.current = makeMention("agent_1");
 
     const { rerender } = renderHook(() => useAgentFromSearchParam("w_1"));
+    await waitFor(() => expect(setSelectedAgent).toHaveBeenCalledTimes(1));
     expect(replaceMock).not.toHaveBeenCalled();
 
+    setSelectedAgent.mockClear();
     selectedSingleAgentHolder.current = makeMention("agent_2");
     rerender();
 
@@ -161,7 +163,9 @@ describe("useAgentFromSearchParam", () => {
     selectedSingleAgentHolder.current = makeMention("agent_1");
 
     const { rerender } = renderHook(() => useAgentFromSearchParam("w_1"));
+    await waitFor(() => expect(setSelectedAgent).toHaveBeenCalledTimes(1));
 
+    setSelectedAgent.mockClear();
     setUrl("?agent=agent_2");
     searchParamHolder.current = "agent_2";
     agentConfigurationHolder.current = makeAgentConfiguration("agent_2");
@@ -221,4 +225,49 @@ describe("useAgentFromSearchParam", () => {
 
     expect(replaceMock).not.toHaveBeenCalled();
   });
+
+  it("re-pushes a matching URL agent when entering a new conversation", async () => {
+    activeConversationIdHolder.current = "conv_1";
+    setUrl("?agent=agent_1");
+    searchParamHolder.current = "agent_1";
+    selectedSingleAgentHolder.current = makeMention("agent_1");
+
+    const { rerender } = renderHook(() => useAgentFromSearchParam("w_1"));
+    await waitFor(() => expect(setSelectedAgent).toHaveBeenCalledTimes(1));
+    setSelectedAgent.mockClear();
+
+    // Navigate to /conversation/new with the same custom agent still selected.
+    activeConversationIdHolder.current = null;
+    rerender();
+
+    await waitFor(() => expect(setSelectedAgent).toHaveBeenCalledTimes(1));
+    expect(setSelectedAgent).toHaveBeenCalledWith(
+      expect.objectContaining({ id: "agent_1" })
+    );
+    expect(replaceMock).not.toHaveBeenCalled();
+  });
+
+  it("does not mirror a default overwrite while the URL agent is pending after entering new", async () => {
+    activeConversationIdHolder.current = "conv_1";
+    setUrl("?agent=agent_1");
+    searchParamHolder.current = "agent_1";
+    selectedSingleAgentHolder.current = makeMention("agent_1");
+
+    const { rerender } = renderHook(() => useAgentFromSearchParam("w_1"));
+    await waitFor(() => expect(setSelectedAgent).toHaveBeenCalledTimes(1));
+    setSelectedAgent.mockClear();
+
+    // Enter new conversation, then simulate the homepage default winning the race
+    // before the URL agent is re-applied.
+    activeConversationIdHolder.current = null;
+    selectedSingleAgentHolder.current = makeMention("dust");
+    agentConfigurationHolder.current = makeAgentConfiguration("agent_1");
+    rerender();
+
+    await waitFor(() => expect(setSelectedAgent).toHaveBeenCalled());
+    expect(setSelectedAgent).toHaveBeenCalledWith(
+      expect.objectContaining({ id: "agent_1" })
+    );
+    expect(replaceMock).not.toHaveBeenCalled();
+  });
 });
diff --git a/front/hooks/useAgentFromSearchParam.ts b/front/hooks/useAgentFromSearchParam.ts
index 985f843ac1bc..c20719fe397e 100644
--- a/front/hooks/useAgentFromSearchParam.ts
+++ b/front/hooks/useAgentFromSearchParam.ts
@@ -15,49 +15,85 @@ export function useAgentFromSearchParam(workspaceId: string) {
   const agent = useSearchParam("agent");
   const activeConversationId = useActiveConversationId();
   const { selectedSingleAgent, setSelectedAgent } = useContext(InputBarContext);
-  const syncedParamRef = useRef<string | null>(null);
+  // Last ?agent= value whose selection we have observed in the composer (isSynced).
+  // Intentionally not set when we merely call setSelectedAgent — that would let a
+  // still-stale selection be mirrored into the URL as a fake picker change.
+  const appliedParamRef = useRef<string | null>(null);
+  const prevConversationIdRef = useRef(activeConversationId);
+  // Bumped when entering /conversation/new so the URL→composer effect re-runs after
+  // appliedParamRef is cleared (refs alone do not invalidate effects).
+  const newConversationVisitRef = useRef(0);
+
+  // Entering /conversation/new from an existing conversation remounts the homepage
+  // InputBar. Clear the applied marker so we re-push ?agent= through setSelectedAgent;
+  // otherwise an already-matching selection looks "synced" without that push and can
+  // lose to the @dust default, which then gets mirrored back into the URL.
+  if (prevConversationIdRef.current !== activeConversationId) {
+    const enteredNewConversation =
+      activeConversationId === null && prevConversationIdRef.current !== null;
+    prevConversationIdRef.current = activeConversationId;
+    if (enteredNewConversation) {
+      appliedParamRef.current = null;
+      newConversationVisitRef.current += 1;
+    }
+  }
+  const newConversationVisit = newConversationVisitRef.current;
 
   const isSynced = !!agent && selectedSingleAgent?.id === agent;
+  const isUrlAgentPending = !!agent && appliedParamRef.current !== agent;
 
   const { agentConfiguration, isAgentConfigurationError } =
     useAgentConfiguration({
       workspaceId,
       agentConfigurationId: agent,
-      disabled: !agent || isSynced,
+      disabled: !isUrlAgentPending,
     });
 
-  // URL to composer. When url param "agent" names an agent other than the selected one, fetch it
-  // and select it. syncedParamRef tracks the last param the selection has matched, so a
-  // param that is still being applied is not mistaken for a picker change by the effect
-  // below.
+  // URL to composer. When url param "agent" is not yet reflected in the composer,
+  // push it via setSelectedAgent. Re-push even when selection already matches so a
+  // remounted InputBar can mark it as an external/URL selection.
   useEffect(() => {
-    if (!agent || !agentConfiguration || syncedParamRef.current === agent) {
+    // Read the visit counter so entering /conversation/new re-runs this effect after
+    // appliedParamRef is cleared.
+    void newConversationVisit;
+
+    if (!agent || appliedParamRef.current === agent) {
+      return;
+    }
+
+    if (selectedSingleAgent?.id === agent) {
+      setSelectedAgent(selectedSingleAgent);
+      return;
+    }
+
+    if (!agentConfiguration) {
       return;
     }
 
     setSelectedAgent(toRichAgentMentionType(agentConfiguration));
-  }, [agent, agentConfiguration, setSelectedAgent]);
+  }, [
+    agent,
+    agentConfiguration,
+    newConversationVisit,
+    selectedSingleAgent,
+    setSelectedAgent,
+  ]);
 
   // Composer to URL. On a new conversation, once the URL agent has been applied, a picker
   // change is mirrored into the url param "agent" so the address bar always reflects the selected agent.
   useEffect(() => {
     if (isSynced) {
-      syncedParamRef.current = agent;
+      appliedParamRef.current = agent;
     }
 
     const isNewConversation = activeConversationId === null;
     const isUrlAgentNotFound =
       isAgentConfigurationError?.error?.type ===
       "agent_configuration_not_found";
-    const isUrlAgentPending =
-      !!agent && syncedParamRef.current !== agent && !isUrlAgentNotFound;
+    const isPending =
+      !!agent && appliedParamRef.current !== agent && !isUrlAgentNotFound;
 
-    if (
-      isSynced ||
-      !isNewConversation ||
-      !selectedSingleAgent ||
-      isUrlAgentPending
-    ) {
+    if (isSynced || !isNewConversation || !selectedSingleAgent || isPending) {
       return;
     }