diff --git a/backend/__tests__/unit/routes/tasksApi.titleAssigneeNormalization.test.js b/backend/__tests__/unit/routes/tasksApi.titleAssigneeNormalization.test.js
new file mode 100644
index 000000000..bc85b2fa8
--- /dev/null
+++ b/backend/__tests__/unit/routes/tasksApi.titleAssigneeNormalization.test.js
@@ -0,0 +1,176 @@
+/**
+ * Title and assignee are single-line on every write path (TASK-354).
+ *
+ * A newline in a title breaks each single-line surface that renders it, and an
+ * assignee with an inner space or control character names a different actor to
+ * claims and the wake, which match it verbatim. Case is left alone on purpose:
+ * the name is the identity, and lowering it would merge two real names.
+ */
+const request = require('supertest');
+const express = require('express');
+
+jest.mock('../../../middleware/auth', () => (req, res, next) => {
+  req.userId = 'u1';
+  req.user = { id: 'u1', _id: 'u1' };
+  next();
+});
+jest.mock('../../../middleware/agentRuntimeAuth', () => (req, res, next) => next());
+
+jest.mock('../../../models/Pod', () => ({
+  findById: jest.fn(() => ({
+    lean: jest.fn().mockResolvedValue({ type: 'chat', members: [{ toString: () => 'u1' }] }),
+  })),
+}));
+
+const mockCreate = jest.fn();
+const mockFindOneAndUpdate = jest.fn();
+jest.mock('../../../models/Task', () => ({
+  find: jest.fn(() => ({ sort: () => ({ lean: async () => [] }) })),
+  findOne: jest.fn(() => ({
+    sort: () => ({ lean: async () => null, select: () => ({ lean: async () => null }) }),
+    select: () => ({ lean: async () => ({ updatedAt: new Date(0), assignee: null, claimedBy: null }) }),
+    lean: async () => null,
+  })),
+  findOneAndUpdate: (...args) => mockFindOneAndUpdate(...args),
+  countDocuments: jest.fn().mockResolvedValue(0),
+  create: (...args) => mockCreate(...args),
+}));
+
+jest.mock('../../../models/User', () => ({
+  findById: jest.fn(() => ({
+    select: jest.fn(() => ({ lean: jest.fn().mockResolvedValue({ username: 'alice' }) })),
+  })),
+}));
+
+const tasksApiRoutes = require('../../../routes/tasksApi');
+
+const POD = '69b7ddff0ce64c9648365fc4';
+const TASK = '69b7ddff0ce64c9648365fc5';
+
+describe('task title and assignee normalization on write (TASK-354)', () => {
+  let app;
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    mockCreate.mockImplementation(async (doc) => ({
+      ...doc,
+      toObject() { return doc; },
+    }));
+    mockFindOneAndUpdate.mockImplementation(async (_filter, update) => ({
+      ...update.$set,
+      toObject() { return update.$set; },
+    }));
+    app = express();
+    app.use(express.json());
+    app.use('/api/v1/tasks', tasksApiRoutes);
+  });
+
+  describe('POST create', () => {
+    test('a newline in the title collapses to one line before it is stored', async () => {
+      const res = await request(app)
+        .post(`/api/v1/tasks/${POD}`)
+        .send({ title: '  Fix the\r\n  board   lease\nrow  ' });
+      expect(res.status).toBe(201);
+      expect(mockCreate.mock.calls[0][0].title).toBe('Fix the board lease row');
+    });
+
+    test('a title that is only a newline is refused as empty', async () => {
+      const res = await request(app).post(`/api/v1/tasks/${POD}`).send({ title: '\n' });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toBe('title is required');
+      expect(mockCreate).not.toHaveBeenCalled();
+    });
+
+    test('the assignee is trimmed and its case is kept', async () => {
+      const res = await request(app)
+        .post(`/api/v1/tasks/${POD}`)
+        .send({ title: 'Ship it', assignee: '  Nova  ' });
+      expect(res.status).toBe(201);
+      expect(mockCreate.mock.calls[0][0].assignee).toBe('Nova');
+    });
+
+    test('an assignee with an inner space is refused before anything is stored', async () => {
+      const res = await request(app)
+        .post(`/api/v1/tasks/${POD}`)
+        .send({ title: 'Ship it', assignee: 'nova ops' });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toMatch(/assign by handle/);
+      expect(mockCreate).not.toHaveBeenCalled();
+    });
+
+    test('an assignee with a control character is refused', async () => {
+      const res = await request(app)
+        .post(`/api/v1/tasks/${POD}`)
+        .send({ title: 'Ship it', assignee: 'nova\u0007' });
+      expect(res.status).toBe(400);
+      expect(mockCreate).not.toHaveBeenCalled();
+    });
+
+    test('a control character from the C1 range is refused too', async () => {
+      const res = await request(app)
+        .post(`/api/v1/tasks/${POD}`)
+        .send({ title: 'Ship it', assignee: 'nova\u0085' });
+      expect(res.status).toBe(400);
+      expect(mockCreate).not.toHaveBeenCalled();
+    });
+
+    test('a replay looks up the exact raw multiline title before the normalized one', async () => {
+      const Task = require('../../../models/Task');
+      await request(app)
+        .post(`/api/v1/tasks/${POD}`)
+        .send({ title: 'Fix\nboard', sourceRef: 'msg:1' });
+      expect(Task.findOne.mock.calls[0][0].title).toBe('Fix\nboard');
+    });
+
+    test('a blank assignee stores as unassigned', async () => {
+      const res = await request(app)
+        .post(`/api/v1/tasks/${POD}`)
+        .send({ title: 'Ship it', assignee: '   ' });
+      expect(res.status).toBe(201);
+      expect(mockCreate.mock.calls[0][0].assignee).toBeNull();
+    });
+  });
+
+  describe('PATCH update', () => {
+    test('a newline in the title collapses to one line before the write', async () => {
+      const res = await request(app)
+        .patch(`/api/v1/tasks/${POD}/${TASK}`)
+        .send({ title: 'Renamed\nin two lines' });
+      expect(res.status).toBe(200);
+      expect(mockFindOneAndUpdate.mock.calls[0][1].$set.title).toBe('Renamed in two lines');
+    });
+
+    test('a non-string title is refused before the write, so null never reaches Mongo', async () => {
+      const res = await request(app)
+        .patch(`/api/v1/tasks/${POD}/${TASK}`)
+        .send({ title: null });
+      expect(res.status).toBe(400);
+      expect(mockFindOneAndUpdate).not.toHaveBeenCalled();
+    });
+
+    test('an assignee with an inner space is refused before the write', async () => {
+      const res = await request(app)
+        .patch(`/api/v1/tasks/${POD}/${TASK}`)
+        .send({ assignee: 'sprint impl' });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toMatch(/assign by handle/);
+      expect(mockFindOneAndUpdate).not.toHaveBeenCalled();
+    });
+
+    test('a padded assignee is trimmed on the write', async () => {
+      const res = await request(app)
+        .patch(`/api/v1/tasks/${POD}/${TASK}`)
+        .send({ assignee: ' Lily ' });
+      expect(res.status).toBe(200);
+      expect(mockFindOneAndUpdate.mock.calls[0][1].$set.assignee).toBe('Lily');
+    });
+
+    test('a title that is only whitespace is refused rather than stored blank', async () => {
+      const res = await request(app)
+        .patch(`/api/v1/tasks/${POD}/${TASK}`)
+        .send({ title: ' \n ' });
+      expect(res.status).toBe(400);
+      expect(mockFindOneAndUpdate).not.toHaveBeenCalled();
+    });
+  });
+});
diff --git a/backend/__tests__/unit/services/nativeRuntimeService.webFetch.test.js b/backend/__tests__/unit/services/nativeRuntimeService.webFetch.test.js
index 84230d7e1..f36c8d4bb 100644
--- a/backend/__tests__/unit/services/nativeRuntimeService.webFetch.test.js
+++ b/backend/__tests__/unit/services/nativeRuntimeService.webFetch.test.js
@@ -24,6 +24,11 @@ jest.mock('../../../models/Pod', () => ({
   }),
 }));
 jest.mock('../../../models/AgentRun', () => ({ create: jest.fn(), countDocuments: jest.fn() }));
+const mockTaskCreate = jest.fn();
+jest.mock('../../../models/Task', () => ({
+  countDocuments: jest.fn().mockResolvedValue(0),
+  create: (...args) => mockTaskCreate(...args),
+}));
 // The agent's own bot User (its seat). Null in the arm where it cannot be found.
 const SEAT_ID = '507f1f77bcf86cd799439077';
 const SAM_ID = '507f1f77bcf86cd7994390aa';
@@ -526,6 +531,31 @@ describe('native turn with commonly_fetch_url', () => {
     expect(taskMessage.content).toMatch(/unavailable for the rest of this turn because web results are in it/);
   });
 
+  test('commonly_create_task accepts an assignee handle and stores it (TASK-354)', async () => {
+    mockTaskCreate.mockClear();
+    mockTaskCreate.mockResolvedValue({ taskId: 'N-1', toObject: () => ({}) });
+    axios.post
+      .mockResolvedValueOnce(toolCalls(call('c1', 'commonly_create_task', { title: 'Ship it', assignee: 'nova' })))
+      .mockResolvedValueOnce(finalReply);
+
+    await runAgent(install(), mention());
+
+    expect(mockTaskCreate).toHaveBeenCalledWith(expect.objectContaining({ title: 'Ship it', assignee: 'nova' }));
+    expect(toolMessages(1)[0].content).not.toMatch(/assign by handle/);
+  });
+
+  test('commonly_create_task refuses a display-name assignee before it writes a row (TASK-354)', async () => {
+    mockTaskCreate.mockClear();
+    axios.post
+      .mockResolvedValueOnce(toolCalls(call('c1', 'commonly_create_task', { title: 'Ship it', assignee: 'Sprint Impl' })))
+      .mockResolvedValueOnce(finalReply);
+
+    await runAgent(install(), mention());
+
+    expect(toolMessages(1)[0].content).toMatch(/assign by handle/);
+    expect(mockTaskCreate).not.toHaveBeenCalled();
+  });
+
   test('a guard refusal reaches the model and does not narrow the turn', async () => {
     mockGetPage.mockRejectedValue(refusal('address 10.0.0.5 is not public'));
     axios.post
diff --git a/backend/routes/tasksApi.ts b/backend/routes/tasksApi.ts
index af5e98250..abeaafd93 100644
--- a/backend/routes/tasksApi.ts
+++ b/backend/routes/tasksApi.ts
@@ -22,6 +22,8 @@ const AgentSendFreshness = require('../services/agentSendFreshness');
 const { emitTaskUpdated, notifyPodAgents } = require('../services/taskEventService');
 // eslint-disable-next-line global-require
 const { recordTaskAttention, resolveTaskAttention } = require('../services/attentionItemService');
+// eslint-disable-next-line global-require
+const { parseAssignee, toSingleLineTitle } = require('../utils/taskFieldNormalization');
 
 /**
  * Who made this board change, for the agent fan-out.
@@ -301,8 +303,10 @@ router.post('/:podId', rateLimit({
     }
     // Materialize primitives after runtime validation so Mongo never receives
     // user-supplied query objects (for example, operator-shaped values).
-    const title = titleInput === undefined ? undefined : String(titleInput);
-    const assignee = assigneeInput === undefined ? undefined : String(assigneeInput);
+    const title = titleInput === undefined ? undefined : toSingleLineTitle(String(titleInput));
+    const assigneeParsed = assigneeInput === undefined ? undefined : parseAssignee(String(assigneeInput));
+    if (assigneeParsed?.error) return res.status(400).json({ error: assigneeParsed.error });
+    const assignee = assigneeParsed?.value ?? undefined;
     const dep = depInput === undefined ? undefined : String(depInput);
     const parentTask = parentTaskInput === undefined ? undefined : String(parentTaskInput);
     const source = sourceInput === undefined ? undefined : String(sourceInput);
@@ -316,7 +320,16 @@ router.post('/:podId', rateLimit({
       // the ask came from, the title says which ask it is. Keying on the ref
       // alone made a second ask from one source adopt the first ask's row and
       // discard the caller's title (TASK-063). Same pair = idempotent retry.
-      const existing = await Task.findOne({ podId: mongoose.Types.ObjectId.createFromHexString(podId || ''), sourceRef, title }) as { _id: unknown; title?: string; status?: string; assignee?: string; claimedAt?: Date | null; claimExpiresAt?: Date | null; notes?: string; updatedAt: Date; toObject: () => unknown } | null;
+      // Rows written before titles were collapsed hold the raw text (`Fix\nboard`);
+      // a replay of that input must still find them, or the pair index admits a twin.
+      // The exact raw title is tried first so a `Fix board` row cannot shadow it.
+      const titleCandidates = [titleInput === undefined ? undefined : String(titleInput), title].filter((t, i, all) => t !== undefined && all.indexOf(t) === i);
+      let existing = null as { _id: unknown; title?: string; status?: string; assignee?: string; claimedAt?: Date | null; claimExpiresAt?: Date | null; notes?: string; updatedAt: Date; toObject: () => unknown } | null;
+      for (const candidate of titleCandidates) {
+        // eslint-disable-next-line no-await-in-loop
+        existing = await Task.findOne({ podId: mongoose.Types.ObjectId.createFromHexString(podId || ''), sourceRef, title: candidate }) as typeof existing;
+        if (existing) break;
+      }
       if (existing) {
         if (existing.status === 'done') {
           // `sourceRef` + `title` are an idempotency key (a unique partial
@@ -959,6 +972,16 @@ router.patch('/:podId/:taskId', taskWriteRateLimit(60), auth, async (req: AuthRe
     if (typeof fieldUpdates.assignee === 'string' && fieldUpdates.assignee.trim() === '') {
       fieldUpdates.assignee = null;
     }
+    if (fieldUpdates.title !== undefined) {
+      if (typeof fieldUpdates.title !== 'string') return res.status(400).json({ error: 'title must be a string' });
+      fieldUpdates.title = toSingleLineTitle(fieldUpdates.title);
+      if (!fieldUpdates.title) return res.status(400).json({ error: 'title is required' });
+    }
+    if (typeof fieldUpdates.assignee === 'string') {
+      const parsed = parseAssignee(fieldUpdates.assignee);
+      if (parsed.error) return res.status(400).json({ error: parsed.error });
+      fieldUpdates.assignee = parsed.value;
+    }
     if (Object.keys(fieldUpdates).length === 0) return res.status(400).json({ error: 'No updatable fields provided' });
     if (fieldUpdates.status !== undefined) {
       const raw = String(fieldUpdates.status).trim().toLowerCase();
diff --git a/backend/services/nativeRuntimeService.ts b/backend/services/nativeRuntimeService.ts
index 099c043c0..5fdd6f94e 100644
--- a/backend/services/nativeRuntimeService.ts
+++ b/backend/services/nativeRuntimeService.ts
@@ -39,6 +39,7 @@ import { getFromPublicAddress, isOutboundUrlRefused } from './outboundUrlGuard';
 import { GivenAddresses } from './webAddressProvenance';
 import { nativeCueFromKernel } from './nativeKernelCue';
 import { agentComputerBroker, agentComputerEnabled } from './agentComputerBroker';
+import { parseAssignee, toSingleLineTitle } from '../utils/taskFieldNormalization';
 import { untrustedConnectorMarker } from './brokerResultMarker';
 import {
   engineForNewHostedInstall, HOSTED_ENGINE_NATIVE, hostedNativeDecline, hostedNativeRunConfig, hostedRunsNative,
@@ -289,7 +290,7 @@ export const TOOLS = [
         type: 'object',
         properties: {
           title: { type: 'string' },
-          assignee: { type: 'string', description: 'Optional assignee display name' },
+          assignee: { type: 'string', description: "Optional assignee: the agent's agentName, one token with no spaces. Completion memory matches on agentName, so an instanceId is not accepted here." },
           notes: { type: 'string' },
         },
         required: ['title'],
@@ -886,10 +887,14 @@ export async function dispatchTool(
       }
 
       case 'commonly_create_task': {
-        const title = String(args.title || '').trim();
+        const title = toSingleLineTitle(String(args.title || ''));
         if (!title) {
           return { content: { ok: false, error: 'title is required' }, error: 'missing_title' };
         }
+        const assigneeParsed = parseAssignee(args.assignee ? String(args.assignee) : '');
+        if (assigneeParsed.error) {
+          return { content: { ok: false, error: assigneeParsed.error }, error: 'invalid_assignee' };
+        }
         const Task = require('../models/Task');
         // Count existing tasks in the pod to derive a taskNum — the Task
         // schema requires a unique (podId, taskId) tuple, so we mint a
@@ -902,7 +907,7 @@ export async function dispatchTool(
           taskNum,
           taskId,
           title,
-          assignee: args.assignee ? String(args.assignee) : null,
+          assignee: assigneeParsed.value,
           notes: args.notes ? String(args.notes) : null,
           status: 'pending',
           source: 'agent',
diff --git a/backend/utils/taskFieldNormalization.ts b/backend/utils/taskFieldNormalization.ts
new file mode 100644
index 000000000..5f54ce015
--- /dev/null
+++ b/backend/utils/taskFieldNormalization.ts
@@ -0,0 +1,32 @@
+/**
+ * Task title and assignee rules, shared by every path that writes a task (the
+ * tasks API for POST and PATCH, and the native runtime's create tool).
+ *
+ * A title is one line: any run of whitespace, newlines included, collapses to a
+ * single space, so every single-line surface that shows it stays one line.
+ */
+export const toSingleLineTitle = (raw: string): string => raw.replace(/\s+/g, ' ').trim();
+
+/** C0, DEL and C1 control characters. */
+const isControlCharacter = (ch: string): boolean => {
+  const code = ch.charCodeAt(0);
+  return code < 0x20 || (code >= 0x7f && code <= 0x9f);
+};
+
+/**
+ * An assignee is one handle (an agentName or instanceId). A board wake matches
+ * that handle verbatim, so an inner space or control character names someone
+ * else, and a display name stores a row no seat is woken for. Trimmed; case is
+ * kept. Blank after trimming means unassigned.
+ */
+export const parseAssignee = (raw: string): { value: string | null; error?: string } => {
+  const value = raw.trim();
+  if (value === '') return { value: null };
+  if (/\s/.test(value) || [...value].some(isControlCharacter)) {
+    return {
+      value: null,
+      error: 'assign by handle: one word with no spaces or control characters, e.g. nova',
+    };
+  }
+  return { value };
+};
diff --git a/frontend/src/i18n/locales/en.json b/frontend/src/i18n/locales/en.json
index 552be1f34..bdf4deb8a 100644
--- a/frontend/src/i18n/locales/en.json
+++ b/frontend/src/i18n/locales/en.json
@@ -1335,7 +1335,7 @@
       "title": "Title",
       "titlePlaceholder": "What needs to happen?",
       "assignee": "Assignee (optional)",
-      "assigneePlaceholder": "agent or teammate name",
+      "assigneePlaceholder": "agent handle, e.g. nova",
       "create": "Create task",
       "creating": "Creating…"
     }
diff --git a/frontend/src/i18n/locales/zh-CN.json b/frontend/src/i18n/locales/zh-CN.json
index a390d6891..dd53a2303 100644
--- a/frontend/src/i18n/locales/zh-CN.json
+++ b/frontend/src/i18n/locales/zh-CN.json
@@ -1327,7 +1327,7 @@
       "title": "标题",
       "titlePlaceholder": "需要做什么？",
       "assignee": "负责人（可选）",
-      "assigneePlaceholder": "智能体或队友名称",
+      "assigneePlaceholder": "智能体代号",
       "create": "创建任务",
       "creating": "创建中…"
     }