Set comment assignee from agent mentions

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/IssueChatThread.tsx

@@ -117,6 +117,7 @@ import {
   computeComposerHandoffPreview,
   extractAgentMentionIds,
   findPlainAgentNameCandidate,
+  resolveMentionedAgentAssigneeValue,
   type HandoffAgentMention,
 } from "../lib/interrupt-handoff";
 import { restoreSubmittedCommentDraft } from "../lib/comment-submit-draft";
@@ -3367,6 +3368,10 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
   const composerContainerRef = useRef<HTMLDivElement | null>(null);
   const draftTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
   const canAcceptFiles = Boolean(onImageUpload || onAttachImage);
+  const reassignOptionIds = useMemo(
+    () => new Set(reassignOptions.map((option) => option.id)),
+    [reassignOptions],
+  );
 
   function queueViewportRestore(snapshot: ReturnType<typeof captureComposerViewportSnapshot>) {
     if (!snapshot) return;
@@ -3408,6 +3413,16 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
     setReassignTarget(effectiveSuggestedAssigneeValue);
   }, [effectiveSuggestedAssigneeValue]);
 
+  const firstMentionedAssigneeValue = useMemo(
+    () => resolveMentionedAgentAssigneeValue(body, reassignOptionIds),
+    [body, reassignOptionIds],
+  );
+
+  useEffect(() => {
+    if (!enableReassign || !firstMentionedAssigneeValue) return;
+    setReassignTarget(firstMentionedAssigneeValue);
+  }, [enableReassign, firstMentionedAssigneeValue]);
+
   useEffect(() => {
     setPendingWorkMode(resolvedIssueWorkMode);
   }, [resolvedIssueWorkMode]);

ui/src/components/IssueChatThreadClassic.tsx

@@ -107,6 +107,7 @@ import { InlineEntitySelector, type InlineEntityOption } from "./InlineEntitySel
 import { IssueThreadInteractionCardClassic } from "./IssueThreadInteractionCardClassic";
 import { AgentIcon } from "./AgentIconPicker";
 import { restoreSubmittedCommentDraft } from "../lib/comment-submit-draft";
+import { resolveMentionedAgentAssigneeValue } from "../lib/interrupt-handoff";
 import {
   captureComposerViewportSnapshot,
   restoreComposerViewportSnapshot,
@@ -3311,6 +3312,10 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
   const composerContainerRef = useRef<HTMLDivElement | null>(null);
   const draftTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
   const canAcceptFiles = Boolean(onImageUpload || onAttachImage);
+  const reassignOptionIds = useMemo(
+    () => new Set(reassignOptions.map((option) => option.id)),
+    [reassignOptions],
+  );
 
   function queueViewportRestore(snapshot: ReturnType<typeof captureComposerViewportSnapshot>) {
     if (!snapshot) return;
@@ -3352,6 +3357,16 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
     setReassignTarget(effectiveSuggestedAssigneeValue);
   }, [effectiveSuggestedAssigneeValue]);
 
+  const firstMentionedAssigneeValue = useMemo(
+    () => resolveMentionedAgentAssigneeValue(body, reassignOptionIds),
+    [body, reassignOptionIds],
+  );
+
+  useEffect(() => {
+    if (!enableReassign || !firstMentionedAssigneeValue) return;
+    setReassignTarget(firstMentionedAssigneeValue);
+  }, [enableReassign, firstMentionedAssigneeValue]);
+
   useEffect(() => {
     setUnassignedConfirmed(false);
   }, [reassignTarget]);

ui/src/lib/interrupt-handoff.test.ts

@@ -9,6 +9,7 @@ import {
   extractAgentMentionIds,
   findPlainAgentNameCandidate,
   isOperatorInterruptedRun,
+  resolveMentionedAgentAssigneeValue,
   resolveRunStatusPresentation,
   type PauseAffectsIssueLike,
 } from "./interrupt-handoff";
@@ -61,6 +62,22 @@ describe("structured mention vs plain text", () => {
     expect(bodyHasAgentMention(`hey ${qaMention}`)).toBe(true);
   });
 
+  it("resolves the first available structured agent mention as the pending assignee", () => {
+    const coderHref = buildAgentMentionHref("agent-coder-2222", null);
+    const available = new Set([`agent:${QA_ID}`, "agent:agent-coder-2222"]);
+    expect(
+      resolveMentionedAgentAssigneeValue(
+        `ask ${qaMention} and [@Coder](${coderHref})`,
+        available,
+      ),
+    ).toBe(`agent:${QA_ID}`);
+  });
+
+  it("does not resolve plain text or unavailable mentioned agents as pending assignees", () => {
+    expect(resolveMentionedAgentAssigneeValue("ask QA to confirm", new Set([`agent:${QA_ID}`]))).toBeNull();
+    expect(resolveMentionedAgentAssigneeValue(`ask ${qaMention}`, new Set(["agent:other"]))).toBeNull();
+  });
+
   it("treats a plain QA name as NOT a mention", () => {
     expect(bodyHasAgentMention("QA you take the screenshot")).toBe(false);
     expect(extractAgentMentionIds("ask QA to confirm")).toEqual([]);

ui/src/lib/interrupt-handoff.ts

@@ -105,6 +105,16 @@ export function bodyHasAgentMention(body: string): boolean {
   return extractAgentMentionIds(body).length > 0;
 }
 
+export function resolveMentionedAgentAssigneeValue(
+  body: string,
+  availableAssigneeValues: ReadonlySet<string>,
+): string | null {
+  const agentId = extractAgentMentionIds(body)[0];
+  if (!agentId) return null;
+  const assigneeValue = `agent:${agentId}`;
+  return availableAssigneeValues.has(assigneeValue) ? assigneeValue : null;
+}
+
 /** Strip every markdown link so chip labels/hrefs are not mistaken for plain text. */
 function plainTextOutsideLinks(body: string): string {
   return body.replace(MARKDOWN_LINK_RE, " ");