diff --git a/ui/src/components/IssueChatThread.test.tsx b/ui/src/components/IssueChatThread.test.tsx
index ab28a875fba..429ff516f0d 100644
--- a/ui/src/components/IssueChatThread.test.tsx
+++ b/ui/src/components/IssueChatThread.test.tsx
@@ -13,6 +13,8 @@ import {
   resolveAssistantMessageFoldedState,
   resolveIssueChatHumanAuthor,
 } from "./IssueChatThread";
+import { ToastProvider } from "../context/ToastContext";
+import { ToastViewport } from "./ToastViewport";
 import type {
   AskUserQuestionsInteraction,
   RequestConfirmationInteraction,
@@ -1617,6 +1619,127 @@ describe("IssueChatThread", () => {
     });
   });
 
+  it("warns once before sending a reply with no assignee selected", async () => {
+    const root = createRoot(container);
+
+    act(() => {
+      root.render(
+        <ToastProvider>
+          <ToastViewport />
+          <MemoryRouter>
+            <IssueChatThread
+              comments={[]}
+              linkedRuns={[]}
+              timelineEvents={[]}
+              liveRuns={[]}
+              onAdd={async () => {}}
+              enableReassign
+              reassignOptions={[
+                { id: "", label: "No assignee" },
+                { id: "agent:agent-1", label: "Agent 1" },
+              ]}
+              currentAssigneeValue=""
+              suggestedAssigneeValue=""
+              enableLiveTranscriptPolling={false}
+            />
+          </MemoryRouter>
+        </ToastProvider>,
+      );
+    });
+
+    const editor = container.querySelector('textarea[aria-label="Issue chat editor"]') as HTMLTextAreaElement | null;
+    const submitButton = Array.from(container.querySelectorAll("button")).find(
+      (element) => element.textContent === "Send",
+    ) as HTMLButtonElement | undefined;
+    expect(editor).not.toBeNull();
+    expect(submitButton).toBeDefined();
+
+    act(() => {
+      const valueSetter = Object.getOwnPropertyDescriptor(
+        window.HTMLTextAreaElement.prototype,
+        "value",
+      )?.set;
+      valueSetter?.call(editor, "Reply without assignee");
+      editor?.dispatchEvent(new Event("input", { bubbles: true }));
+    });
+
+    await act(async () => {
+      submitButton?.click();
+    });
+
+    expect(appendMock).not.toHaveBeenCalled();
+    expect(document.body.textContent).toContain("No assignee selected");
+
+    await act(async () => {
+      submitButton?.click();
+    });
+
+    expect(appendMock).toHaveBeenCalledTimes(1);
+    expect(appendMock).toHaveBeenCalledWith(
+      expect.objectContaining({
+        content: [{ type: "text", text: "Reply without assignee" }],
+      }),
+    );
+
+    act(() => {
+      root.unmount();
+    });
+  });
+
+  it("does not warn when sending a reply with an assignee selected", async () => {
+    const root = createRoot(container);
+
+    act(() => {
+      root.render(
+        <ToastProvider>
+          <ToastViewport />
+          <MemoryRouter>
+            <IssueChatThread
+              comments={[]}
+              linkedRuns={[]}
+              timelineEvents={[]}
+              liveRuns={[]}
+              onAdd={async () => {}}
+              enableReassign
+              reassignOptions={[
+                { id: "", label: "No assignee" },
+                { id: "agent:agent-1", label: "Agent 1" },
+              ]}
+              currentAssigneeValue="agent:agent-1"
+              suggestedAssigneeValue="agent:agent-1"
+              enableLiveTranscriptPolling={false}
+            />
+          </MemoryRouter>
+        </ToastProvider>,
+      );
+    });
+
+    const editor = container.querySelector('textarea[aria-label="Issue chat editor"]') as HTMLTextAreaElement | null;
+    const submitButton = Array.from(container.querySelectorAll("button")).find(
+      (element) => element.textContent === "Send",
+    ) as HTMLButtonElement | undefined;
+
+    act(() => {
+      const valueSetter = Object.getOwnPropertyDescriptor(
+        window.HTMLTextAreaElement.prototype,
+        "value",
+      )?.set;
+      valueSetter?.call(editor, "Reply with assignee");
+      editor?.dispatchEvent(new Event("input", { bubbles: true }));
+    });
+
+    await act(async () => {
+      submitButton?.click();
+    });
+
+    expect(appendMock).toHaveBeenCalledTimes(1);
+    expect(document.body.textContent).not.toContain("No assignee selected");
+
+    act(() => {
+      root.unmount();
+    });
+  });
+
   it("exposes a composer focus handle that forwards to the editor", () => {
     const root = createRoot(container);
     const composerRef = createRef<{ focus: () => void; restoreDraft: (submittedBody: string) => void }>();
diff --git a/ui/src/components/IssueChatThread.tsx b/ui/src/components/IssueChatThread.tsx
index 5f647e79861..6d61a0d8fb6 100644
--- a/ui/src/components/IssueChatThread.tsx
+++ b/ui/src/components/IssueChatThread.tsx
@@ -90,6 +90,7 @@ import {
   shouldPreserveComposerViewport,
 } from "../lib/issue-chat-scroll";
 import { formatAssigneeUserLabel } from "../lib/assignees";
+import { useOptionalToastActions } from "../context/ToastContext";
 import type { CompanyUserProfile } from "../lib/company-members";
 import { timeAgo } from "../lib/timeAgo";
 import {
@@ -546,6 +547,10 @@ function shouldImplicitlyReopenComment(issueStatus: string | undefined, assignee
   return resumesToTodo && assigneeValue.startsWith("agent:");
 }
 
+function isUnassignedReassignValue(value: string): boolean {
+  return !value || value === "__none__";
+}
+
 const WEEK_MS = 7 * 24 * 60 * 60 * 1000;
 
 function commentDateLabel(date: Date | string | undefined): string {
@@ -2375,6 +2380,7 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
   issueStatus,
 }, forwardedRef) {
   const api = useAui();
+  const toastActions = useOptionalToastActions();
   const [body, setBody] = useState("");
   const [submitting, setSubmitting] = useState(false);
   const [attaching, setAttaching] = useState(false);
@@ -2383,6 +2389,7 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
   const dragDepthRef = useRef(0);
   const effectiveSuggestedAssigneeValue = suggestedAssigneeValue ?? currentAssigneeValue;
   const [reassignTarget, setReassignTarget] = useState(effectiveSuggestedAssigneeValue);
+  const [unassignedConfirmed, setUnassignedConfirmed] = useState(false);
   const attachInputRef = useRef<HTMLInputElement | null>(null);
   const editorRef = useRef<MarkdownEditorRef>(null);
   const composerContainerRef = useRef<HTMLDivElement | null>(null);
@@ -2429,6 +2436,10 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
     setReassignTarget(effectiveSuggestedAssigneeValue);
   }, [effectiveSuggestedAssigneeValue]);
 
+  useEffect(() => {
+    setUnassignedConfirmed(false);
+  }, [reassignTarget]);
+
   useImperativeHandle(forwardedRef, () => ({
     focus: focusComposer,
     restoreDraft: (submittedBody: string) => {
@@ -2446,6 +2457,22 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
     const trimmed = body.trim();
     if (!trimmed || submitting) return;
 
+    const composerHasAssigneePicker = enableReassign && reassignOptions.length > 0;
+    if (
+      composerHasAssigneePicker
+      && isUnassignedReassignValue(reassignTarget)
+      && !unassignedConfirmed
+    ) {
+      toastActions?.pushToast({
+        title: "No assignee selected",
+        body: "Pick an assignee or click Send again to post without one.",
+        tone: "warn",
+        dedupeKey: `issue-chat-no-assignee:${draftKey ?? ""}`,
+      });
+      setUnassignedConfirmed(true);
+      return;
+    }
+
     const hasReassignment = enableReassign && reassignTarget !== currentAssigneeValue;
     const reassignment = hasReassignment ? parseReassignment(reassignTarget) : undefined;
     const reopen = shouldImplicitlyReopenComment(
@@ -2457,6 +2484,7 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
 
     setSubmitting(true);
     setBody("");
+    setUnassignedConfirmed(false);
     try {
       const appendPromise = api.thread().append({
         role: "user",
diff --git a/ui/src/context/ToastContext.tsx b/ui/src/context/ToastContext.tsx
index 77720d31776..0374c065ed6 100644
--- a/ui/src/context/ToastContext.tsx
+++ b/ui/src/context/ToastContext.tsx
@@ -186,6 +186,10 @@ export function useToastActions() {
   return context;
 }
 
+export function useOptionalToastActions() {
+  return useContext(ToastActionsContext);
+}
+
 export function useToast() {
   const toasts = useToastState();
   const actions = useToastActions();