diff --git a/src/bot/PlannerBot.ts b/src/bot/PlannerBot.ts
index c9883f1..58aae74 100644
--- a/src/bot/PlannerBot.ts
+++ b/src/bot/PlannerBot.ts
@@ -316,22 +316,50 @@ export class PlannerBot extends TeamsActivityHandler {
       ...appliedExplicitNames,
     ]);
 
-    // --- Assignee name resolution ----------------------------------------
-    // We accept any id that's either in the plan's group members OR already
-    // assigned on the task — Planner sometimes carries assignees that aren't
-    // direct group members (guests, ex-members, cross-tenant). Names get
-    // resolved via Graph /users/{id} when the group listing didn't surface
-    // them, and cached on the bot instance so we don't re-hit per turn.
+    // --- Assignment (self-only project rule) -----------------------------
+    // Project rule: every create_task MUST be assigned, and only the signed-in
+    // user can be assigned. If the LLM tried to assign anyone else, we silently
+    // coerce to self and surface that on the preview card. update_task only
+    // touches assigneeUserIds if the LLM intended a change at all.
     const memberById = new Map(plan.members.map((m) => [m.userId, m]));
     const knownAssigneeIds = new Set<string>([
       ...plan.members.map((m) => m.userId),
       ...(task?.assigneeIds ?? []),
     ]);
-    const validAssigneeIds = (action.assigneeUserIds ?? []).filter((id) =>
-      knownAssigneeIds.has(id),
-    );
     const resolveName = (id: string) => this.resolveAssigneeName(planner, memberById, id);
-    const newAssigneeNames = await Promise.all(validAssigneeIds.map(resolveName));
+
+    const selfId = plan.members.find((m) => m.isMe)?.userId;
+    const llmRequestedAssignees = action.assigneeUserIds;
+    const llmRequestedNonSelf = (llmRequestedAssignees ?? []).filter(
+      (id) => id !== selfId,
+    );
+
+    let assignmentCoercedToSelf = false;
+    let attemptedNonSelfNames: string[] | undefined;
+    if (llmRequestedNonSelf.length > 0) {
+      assignmentCoercedToSelf = true;
+      const validNonSelf = llmRequestedNonSelf.filter((id) =>
+        knownAssigneeIds.has(id),
+      );
+      if (validNonSelf.length) {
+        attemptedNonSelfNames = await Promise.all(validNonSelf.map(resolveName));
+      }
+    }
+
+    // What we'll actually write to Planner.
+    //  - create_task: always [selfId] — assignment is mandatory
+    //  - update_task: leave undefined unless the LLM signaled an assignment
+    //                 change, in which case force [selfId]
+    let finalAssigneeIds: string[] | undefined;
+    if (action.type === "create_task") {
+      finalAssigneeIds = selfId ? [selfId] : [];
+    } else if (llmRequestedAssignees !== undefined) {
+      finalAssigneeIds = selfId ? [selfId] : [];
+    }
+    const newAssigneeNames =
+      finalAssigneeIds && finalAssigneeIds.length
+        ? await Promise.all(finalAssigneeIds.map(resolveName))
+        : [];
 
     // --- Build ActionContext for preview / result -------------------------
     const ctx: ActionContext = {
@@ -339,6 +367,8 @@ export class PlannerBot extends TeamsActivityHandler {
       appliedLabelNames: appliedLabelNames.length ? appliedLabelNames : undefined,
       droppedInferredLabels: dropped.length ? dropped : undefined,
       missingExplicitLabels: missing.length ? missing : undefined,
+      assignmentCoercedToSelf: assignmentCoercedToSelf || undefined,
+      attemptedNonSelfNames,
     };
 
     if (action.type === "create_task") {
@@ -358,7 +388,7 @@ export class PlannerBot extends TeamsActivityHandler {
       }
       ctx.currentPercent = task.percentComplete;
       ctx.currentPriority = task.priority;
-      if (action.assigneeUserIds) {
+      if (llmRequestedAssignees !== undefined) {
         ctx.currentAssigneeNames = await Promise.all(
           (task.assigneeIds ?? []).map(resolveName),
         );
@@ -372,9 +402,11 @@ export class PlannerBot extends TeamsActivityHandler {
     const pending: PendingAction = {
       action: {
         ...action,
-        ...(action.type === "create_task" ? { assigneeUserIds: validAssigneeIds } : {}),
-        ...(action.type === "update_task" && action.assigneeUserIds
-          ? { assigneeUserIds: validAssigneeIds }
+        ...(action.type === "create_task"
+          ? { assigneeUserIds: finalAssigneeIds }
+          : {}),
+        ...(action.type === "update_task" && llmRequestedAssignees !== undefined
+          ? { assigneeUserIds: finalAssigneeIds }
           : {}),
       } as Exclude<ClassifiedAction, { type: "ask_clarification" }>,
       ctx,
diff --git a/src/cards/confirmationCard.ts b/src/cards/confirmationCard.ts
index 426f312..d5d1aa5 100644
--- a/src/cards/confirmationCard.ts
+++ b/src/cards/confirmationCard.ts
@@ -19,6 +19,11 @@ export interface ActionContext {
   // Assignment diff (update_task)
   currentAssigneeNames?: string[];
   newAssigneeNames?: string[];
+  /** Project rule: only self can be assigned. True when the LLM tried to assign
+   *  someone other than the signed-in user and the bot silently coerced it. */
+  assignmentCoercedToSelf?: boolean;
+  /** displayName 목록 — coerce 된 비-본인 후보들. preview 안내에 사용. */
+  attemptedNonSelfNames?: string[];
 
   // Update-only "before" values (for diff display)
   currentProgressLabel?: string;
@@ -145,6 +150,15 @@ function actionDetailLines(a: ClassifiedAction, ctx: ActionContext): { text: str
   }
 
   // Side-effect / advisory lines — apply to both create_task & update_task.
+  if (ctx.assignmentCoercedToSelf) {
+    const tried = ctx.attemptedNonSelfNames && ctx.attemptedNonSelfNames.length
+      ? ` (요청한 ${ctx.attemptedNonSelfNames.join(", ")} 은(는) 무시)`
+      : "";
+    lines.push({
+      text: `🔒 이 봇은 본인에게만 할당할 수 있어요 — 본인으로 자동 할당했습니다${tried}.`,
+      emphasis: "warning",
+    });
+  }
   if (ctx.droppedInferredLabels && ctx.droppedInferredLabels.length) {
     lines.push({
       text: `ℹ️ 추론 라벨 중 이 Plan에 없어 무시됨: ${ctx.droppedInferredLabels.join(", ")}`,
diff --git a/src/llm/prompt.ts b/src/llm/prompt.ts
index 42f8e6d..36e3648 100644
--- a/src/llm/prompt.ts
+++ b/src/llm/prompt.ts
@@ -36,11 +36,12 @@ export const SYSTEM_PROMPT = `당신은 사용자의 한국어 자연어 작업
 - 새 작업이면 checklistItems, 기존 작업에 덧붙이는 거면 addChecklistItems 를 사용.
 - 최대 20개. 그보다 많으면 앞 20개만.
 
-## 할당 (assigneeUserIds)
-- 반드시 컨텍스트의 plan members 목록 안에서 displayName 부분 매칭으로 사용자를 찾고, 그 userId 를 채웁니다.
-- "나/내가/제가" → members 중 isMe=true 항목의 userId.
-- 발화에 이름이 있는데 members 에 일치하는 사람이 없으면 ask_clarification.
-- update_task 에서는 assigneeUserIds 가 전체 교체로 동작합니다. "X 도 추가" 처럼 누적 의도가 분명하면 기존 assigneeIds + 새 사람을 모두 채워서 넣으세요.
+## 할당 (assigneeUserIds) — 본인만 할당 가능, 그리고 필수
+- **이 봇은 본인(members 중 isMe=true) 외에는 누구도 할당하지 못합니다. 무조건 본인 userId 한 개만 사용하세요.**
+- **모든 create_task 에 반드시 isMe=true 인 본인 userId 한 개를 assigneeUserIds 에 넣으세요.** 누락 금지(빈 배열·undefined 금지).
+- update_task 에서 할당을 바꾸려는 의도가 명확할 때(예: "이거 나한테 줘", "내가 할게")만 assigneeUserIds 에 본인 userId 한 개를 넣으세요. 변경 의도가 없으면 assigneeUserIds 를 통째로 생략해서 기존 할당을 유지합니다.
+- 발화에 다른 사람 이름이 나와도 무시하고 본인 userId 만 넣으세요. ask_clarification 으로 되묻지 마세요 — 봇이 preview 카드에서 "본인으로 자동 할당" 안내를 해 줍니다.
+- members 의 다른 사람 userId 를 절대 assigneeUserIds 에 넣지 마세요.
 
 ## 라벨 (inferredLabels vs explicitLabels)
 - **inferredLabels**: 사용자가 직접 라벨을 언급하지 않았지만 발화 의미가 plan 의 기존 라벨 중 하나와 명확히 매칭될 때만 채웁니다. 자신 없으면 비워두세요. plan 에 없는 이름은 절대 넣지 마세요.