diff --git a/packages/api/src/__tests__/notification-procedure-support.test.ts b/packages/api/src/__tests__/notification-procedure-support.test.ts
index 51019bf..e73f703 100644
--- a/packages/api/src/__tests__/notification-procedure-support.test.ts
+++ b/packages/api/src/__tests__/notification-procedure-support.test.ts
@@ -82,6 +82,24 @@ describe("notification procedure support", () => {
     }
   });
 
+  it("rewrites assignee foreign-key errors to a not found TRPC error", () => {
+    const error = {
+      code: "P2003",
+      meta: { field_name: "Notification_assigneeId_fkey" },
+    };
+
+    try {
+      rethrowNotificationReferenceError(error);
+      throw new Error("expected notification reference error");
+    } catch (caught) {
+      expect(caught).toBeInstanceOf(TRPCError);
+      expect(caught).toMatchObject<Partial<TRPCError>>({
+        code: "NOT_FOUND",
+        message: "Assignee user not found",
+      });
+    }
+  });
+
   it("rewrites broadcast source foreign-key errors to a not found TRPC error", () => {
     const error = {
       code: "P2003",
diff --git a/packages/api/src/__tests__/notification-router.test.ts b/packages/api/src/__tests__/notification-router.test.ts
index 1d4679f..4fbadea 100644
--- a/packages/api/src/__tests__/notification-router.test.ts
+++ b/packages/api/src/__tests__/notification-router.test.ts
@@ -336,6 +336,45 @@ describe("notification.createBroadcast", () => {
     );
   });
 
+  it("rejects immediate broadcasts when transactional persistence support is unavailable", async () => {
+    resolveRecipientsMock.mockResolvedValue(["user_a", "user_b"]);
+
+    const create = vi.fn();
+    const update = vi.fn();
+    const createNotification = vi.fn();
+    const db = {
+      notificationBroadcast: {
+        create,
+        update,
+      },
+      notification: {
+        create: createNotification,
+      },
+    };
+
+    const caller = createManagerCaller(db);
+
+    await expect(caller.createBroadcast({
+      title: "Ops update",
+      targetType: "all",
+    })).rejects.toMatchObject({
+      code: "INTERNAL_SERVER_ERROR",
+      message: "Immediate broadcasts require transactional persistence support.",
+    });
+
+    expect(resolveRecipientsMock).toHaveBeenCalledWith(
+      "all",
+      undefined,
+      db,
+      "user_mgr",
+    );
+    expect(create).not.toHaveBeenCalled();
+    expect(update).not.toHaveBeenCalled();
+    expect(createNotification).not.toHaveBeenCalled();
+    expect(emitNotificationCreated).not.toHaveBeenCalled();
+    expect(emitTaskAssigned).not.toHaveBeenCalled();
+  });
+
   it("rejects broadcasts with no recipients before opening a broadcast transaction", async () => {
     resolveRecipientsMock.mockResolvedValue([]);
 
@@ -1294,6 +1333,38 @@ describe("notification.assignTask", () => {
     });
     expect(emitTaskAssigned).toHaveBeenCalledWith("user_4", "task_9");
   });
+
+  it("returns NOT_FOUND when assigning a task to a missing assignee", async () => {
+    const findUnique = vi.fn().mockResolvedValue({
+      id: "task_9",
+      category: "TASK",
+      assigneeId: "user_2",
+    });
+    const update = vi.fn().mockRejectedValue({
+      code: "P2003",
+      message: "Foreign key constraint failed",
+      meta: { field_name: "Notification_assigneeId_fkey" },
+    });
+    const db = {
+      notification: {
+        findUnique,
+        update,
+      },
+    };
+
+    const caller = createManagerCaller(db);
+    await expect(caller.assignTask({ id: "task_9", assigneeId: "user_missing" })).rejects.toMatchObject({
+      code: "NOT_FOUND",
+      message: "Assignee user not found",
+    });
+
+    expect(findUnique).toHaveBeenCalledWith({ where: { id: "task_9" } });
+    expect(update).toHaveBeenCalledWith({
+      where: { id: "task_9" },
+      data: { assigneeId: "user_missing" },
+    });
+    expect(emitTaskAssigned).not.toHaveBeenCalledWith("user_missing", "task_9");
+  });
 });
 
 // ─── reminders ──────────────────────────────────────────────────────────────
diff --git a/packages/api/src/router/notification-broadcast-procedure-support.ts b/packages/api/src/router/notification-broadcast-procedure-support.ts
index a5490e5..060d716 100644
--- a/packages/api/src/router/notification-broadcast-procedure-support.ts
+++ b/packages/api/src/router/notification-broadcast-procedure-support.ts
@@ -31,6 +31,19 @@ function hasTaskLikeBroadcastMetadata(input: z.infer<typeof CreateBroadcastInput
     || input.dueDate !== undefined;
 }
 
+function requireImmediateBroadcastTransaction(
+  db: BroadcastPersistenceDb,
+): NonNullable<BroadcastPersistenceDb["$transaction"]> {
+  if (typeof db.$transaction !== "function") {
+    throw new TRPCError({
+      code: "INTERNAL_SERVER_ERROR",
+      message: "Immediate broadcasts require transactional persistence support.",
+    });
+  }
+
+  return db.$transaction.bind(db);
+}
+
 function buildBroadcastCreateData(
   senderId: string,
   input: z.infer<typeof CreateBroadcastInputSchema>,
@@ -178,10 +191,9 @@ export async function createBroadcast(
   let notificationIds: BroadcastRecipientNotification[] = [];
 
   try {
-    const transactionResult = typeof ctx.db.$transaction === "function"
-      ? await ctx.db.$transaction((tx) =>
-          persistImmediateBroadcast(tx as typeof ctx.db, senderId, input, recipientIds))
-      : await persistImmediateBroadcast(ctx.db, senderId, input, recipientIds);
+    const transaction = requireImmediateBroadcastTransaction(ctx.db);
+    const transactionResult = await transaction((tx) =>
+      persistImmediateBroadcast(tx as typeof ctx.db, senderId, input, recipientIds));
 
     persistedBroadcast = transactionResult.broadcast;
     notificationIds = transactionResult.notificationIds;
diff --git a/packages/api/src/router/notification-procedure-base.ts b/packages/api/src/router/notification-procedure-base.ts
index 6a584e2..cbf13a2 100644
--- a/packages/api/src/router/notification-procedure-base.ts
+++ b/packages/api/src/router/notification-procedure-base.ts
@@ -93,6 +93,18 @@ export function rethrowNotificationReferenceError(error: unknown): never {
       ? candidate.meta.modelName.toLowerCase()
       : "";
 
+    if (
+      typeof candidate.code === "string"
+      && (candidate.code === "P2003" || candidate.code === "P2025")
+      && fieldName.includes("assignee")
+    ) {
+      throw new TRPCError({
+        code: "NOT_FOUND",
+        message: "Assignee user not found",
+        cause: error,
+      });
+    }
+
     if (
       typeof candidate.code === "string"
       && (candidate.code === "P2003" || candidate.code === "P2025")
diff --git a/packages/api/src/router/notification-task-procedure-support.ts b/packages/api/src/router/notification-task-procedure-support.ts
index acc3cca..81281e7 100644
--- a/packages/api/src/router/notification-task-procedure-support.ts
+++ b/packages/api/src/router/notification-task-procedure-support.ts
@@ -15,6 +15,7 @@ import {
   ListNotificationTasksInputSchema,
   NotificationIdInputSchema,
   type NotificationProcedureContext,
+  rethrowNotificationReferenceError,
   requireNotificationDbUser,
   resolveUserId,
   sendNotificationEmail,
@@ -312,10 +313,15 @@ export async function assignTask(
     });
   }
 
-  const updated = await ctx.db.notification.update({
-    where: { id: input.id },
-    data: { assigneeId: input.assigneeId },
-  });
+  let updated;
+  try {
+    updated = await ctx.db.notification.update({
+      where: { id: input.id },
+      data: { assigneeId: input.assigneeId },
+    });
+  } catch (error) {
+    rethrowNotificationReferenceError(error);
+  }
 
   emitTaskAssigned(input.assigneeId, updated.id);
   return updated;