diff --git a/roboco/services/gateway/content_actions.py b/roboco/services/gateway/content_actions.py
index 2dbaf00e..a6738bde 100644
--- a/roboco/services/gateway/content_actions.py
+++ b/roboco/services/gateway/content_actions.py
@@ -29,6 +29,26 @@ _VALID_NOTE_SCOPES: frozenset[str] = frozenset(
 _TASK_ID_PREFIX_RE = re.compile(r"^\s*\[[a-zA-Z0-9_-]+\]\s*")
 
 
+def _ownership_violation(task_id: UUID) -> Envelope:
+    """Standard envelope for Gate Set D ownership violations.
+
+    Pre-gateway, agents could not even see tasks they didn't own; the
+    gateway exposes task_id parameters so the explicit gate is required.
+    """
+    return Envelope.not_authorized(
+        message=(
+            f"you are not the assignee of {task_id}; "
+            "cannot post content to it"
+        ),
+        remediate=(
+            "only the task's assignee may attach content (commit/note/say/"
+            "dm/evidence) to it. Use a different task_id or omit task_id "
+            "for off-task channel posts (say/dm only)."
+        ),
+        context_briefing={},
+    )
+
+
 @dataclass(frozen=True)
 class ContentActionsDeps:
     """Service deps for ContentActions; bundled to keep init signature flat."""
@@ -111,6 +131,25 @@ class ContentActions:
             context_briefing={},
         )
 
+    async def _verify_explicit_task_ownership(
+        self, agent_id: UUID, task_id: UUID
+    ) -> Envelope | None:
+        """Gate Set D: refuse content posts on tasks the caller does not own.
+
+        Only call this for *explicit* task_id (caller passed it themselves).
+        Auto-fill from get_active_task_for_agent is implicitly self-owned
+        and does not need a re-check.
+
+        Allows ``assigned_to=None`` (post-handoff transient state) so QA /
+        documenter can still inspect tasks between reassignments.
+        """
+        t = await self.task.get(task_id)
+        if t is None:
+            return Envelope.not_found(message=f"task {task_id} not found")
+        if t.assigned_to is not None and t.assigned_to != agent_id:
+            return _ownership_violation(task_id)
+        return None
+
     async def note(
         self,
         *,
@@ -126,7 +165,10 @@ class ContentActions:
                 remediate=f"scope must be one of: {sorted(_VALID_NOTE_SCOPES)}",
                 context_briefing={},
             )
-        if task_id is None:
+        if task_id is not None:
+            if reject := await self._verify_explicit_task_ownership(agent_id, task_id):
+                return reject
+        else:
             t = await self.task.get_active_task_for_agent(agent_id)
             if t is not None:
                 task_id = t.id
@@ -154,7 +196,10 @@ class ContentActions:
         task_id: UUID | None = None,
     ) -> Envelope:
         """Post to a channel. task_id auto-injected if you have an active task."""
-        if task_id is None:
+        if task_id is not None:
+            if reject := await self._verify_explicit_task_ownership(agent_id, task_id):
+                return reject
+        else:
             t = await self.task.get_active_task_for_agent(agent_id)
             if t is not None:
                 task_id = t.id
@@ -181,7 +226,10 @@ class ContentActions:
         skill: str | None = None,
     ) -> Envelope:
         """A2A direct message. Requires task_id (active or explicit)."""
-        if task_id is None:
+        if task_id is not None:
+            if reject := await self._verify_explicit_task_ownership(agent_id, task_id):
+                return reject
+        else:
             t = await self.task.get_active_task_for_agent(agent_id)
             if t is not None:
                 task_id = t.id
@@ -214,10 +262,15 @@ class ContentActions:
         """Inspect a task's PR diff, commits, files.
 
         Fetches dev branch into the agent's workspace before diffing.
+        Allows inspection when caller is assignee OR task is unassigned
+        (post-handoff transient state) — strict ownership only blocks
+        cross-agent inspection of an actively-owned task.
         """
         t = await self.task.get(task_id)
         if t is None:
             return Envelope.not_found(message=f"task {task_id} not found")
+        if t.assigned_to is not None and t.assigned_to != agent_id:
+            return _ownership_violation(task_id)
         if t.branch_name and t.work_session_id:
             await self.workspace.fetch_branch_for_inspection(
                 agent_id=agent_id, branch_name=t.branch_name
diff --git a/tests/unit/gateway/test_content_actions.py b/tests/unit/gateway/test_content_actions.py
index 4023d2f1..ece79859 100644
--- a/tests/unit/gateway/test_content_actions.py
+++ b/tests/unit/gateway/test_content_actions.py
@@ -147,6 +147,10 @@ async def test_note_reflect_scope_succeeds() -> None:
     task_id = uuid4()
     task_svc = AsyncMock()
     task_svc.get_active_task_for_agent.return_value = None
+    # When task_id is explicit, ownership is verified — agent must be assignee.
+    task_svc.get.return_value = MagicMock(
+        id=task_id, assigned_to=agent_id, status="in_progress"
+    )
     journal_svc = AsyncMock()
 
     deps = _make_deps(task=task_svc, journal=journal_svc)
@@ -325,6 +329,8 @@ async def test_evidence_valid_task_returns_ok_with_pr_diff() -> None:
     task_obj = MagicMock(
         id=task_id,
         status="awaiting_qa",
+        # assigned_to=None covers post-handoff inspection (QA reviewing dev work)
+        assigned_to=None,
         branch_name="feature/backend/abc",
         work_session_id=ws_id,
         commits=["sha1"],
diff --git a/tests/unit/gateway/test_content_actions_ownership.py b/tests/unit/gateway/test_content_actions_ownership.py
new file mode 100644
index 00000000..676f313f
--- /dev/null
+++ b/tests/unit/gateway/test_content_actions_ownership.py
@@ -0,0 +1,354 @@
+"""Gate Set D: content-tool ownership guards in ContentActions.
+
+When a caller passes a ``task_id`` to commit / say / dm / note / evidence,
+the gateway must verify ``task.assigned_to == caller_agent_id`` before
+allowing the side effect.
+
+Pre-gateway equivalent: agents could only act on tasks they owned because
+the MCP handlers resolved task from session context. With the gateway
+exposing the task_id parameter, the ownership check must be explicit.
+
+Exception: ``say`` and ``dm`` without a task_id (channel-only / off-task
+A2A) are exempt — used for channel announcements.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import AsyncMock, MagicMock
+from uuid import uuid4
+
+import pytest
+from roboco.services.gateway.content_actions import ContentActions, ContentActionsDeps
+
+
+def _make_deps(**overrides: AsyncMock) -> ContentActionsDeps:
+    if "task" in overrides:
+        task = overrides["task"]
+    else:
+        task = AsyncMock()
+        task.get_active_task_for_agent.return_value = None
+
+    if "git" in overrides:
+        git = overrides["git"]
+    else:
+        git = AsyncMock()
+        git.commit.return_value = {"sha": "abc12345"}
+        git.diff.return_value = ""
+
+    messaging = overrides.get("messaging", AsyncMock())
+    a2a = overrides.get("a2a", AsyncMock())
+    journal = overrides.get("journal", AsyncMock())
+    workspace = overrides.get("workspace", AsyncMock())
+    return ContentActionsDeps(
+        task=task,
+        git=git,
+        messaging=messaging,
+        a2a=a2a,
+        journal=journal,
+        workspace=workspace,
+    )
+
+
+# ---------------------------------------------------------------------------
+# commit — uses get_active_task_for_agent which is already a self-scoped query.
+# But if a caller had explicit task_id semantics, we'd guard them. The current
+# commit verb uses active task; tests below verify it cannot reach a task that
+# isn't theirs (active task query handles this naturally).
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_commit_active_task_returned_must_be_caller_owned() -> None:
+    """get_active_task_for_agent already self-scopes; verify call signature."""
+    agent_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(
+        id=task_id,
+        status="in_progress",
+        assigned_to=agent_id,
+        branch_name="feature/backend/abc",
+    )
+    task_svc = AsyncMock()
+    task_svc.get_active_task_for_agent.return_value = task_obj
+    git_svc = AsyncMock()
+    git_svc.commit.return_value = {"sha": "deadbeef1234"}
+
+    deps = _make_deps(task=task_svc, git=git_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.commit(
+        agent_id=agent_id,
+        message="feat(api): add /healthz endpoint",
+    )
+    assert env.error is None
+    task_svc.get_active_task_for_agent.assert_awaited_once_with(agent_id)
+
+
+# ---------------------------------------------------------------------------
+# note — explicit task_id ownership
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_note_with_task_id_blocks_when_not_assignee() -> None:
+    """note(task_id=X) where X is owned by someone else is rejected."""
+    agent_id = uuid4()
+    other_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(id=task_id, status="in_progress", assigned_to=other_id)
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    journal_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, journal=journal_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.note(
+        agent_id=agent_id,
+        text="I am putting a note on someone else's task",
+        scope="reflect",
+        task_id=task_id,
+    )
+    body = env.as_dict()
+    assert body["error"] == "not_authorized"
+    assert str(task_id) in body["message"] or "assignee" in body["message"]
+    journal_svc.write_entry.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_note_with_task_id_allows_when_assignee() -> None:
+    """note(task_id=X) where X is owned by caller succeeds."""
+    agent_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(id=task_id, status="in_progress", assigned_to=agent_id)
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    task_svc.get_active_task_for_agent.return_value = None
+    journal_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, journal=journal_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.note(
+        agent_id=agent_id,
+        text="Reflecting on my own task",
+        scope="reflect",
+        task_id=task_id,
+    )
+    assert env.error is None
+    journal_svc.write_entry.assert_awaited_once()
+
+
+@pytest.mark.asyncio
+async def test_note_without_task_id_skips_ownership_check() -> None:
+    """note without task_id is fine (might be a general entry)."""
+    agent_id = uuid4()
+    task_svc = AsyncMock()
+    task_svc.get_active_task_for_agent.return_value = None
+    journal_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, journal=journal_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.note(
+        agent_id=agent_id,
+        text="A general reflection note with enough length",
+        scope="reflect",
+    )
+    assert env.error is None
+    journal_svc.write_entry.assert_awaited_once()
+
+
+# ---------------------------------------------------------------------------
+# say — explicit task_id ownership (only when task_id provided)
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_say_with_task_id_blocks_when_not_assignee() -> None:
+    """say(task_id=X) where X is owned by someone else is rejected."""
+    agent_id = uuid4()
+    other_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(id=task_id, status="in_progress", assigned_to=other_id)
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    messaging_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, messaging=messaging_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.say(
+        agent_id=agent_id,
+        channel="backend-cell",
+        text="Posting on someone else's task",
+        task_id=task_id,
+    )
+    body = env.as_dict()
+    assert body["error"] == "not_authorized"
+    messaging_svc.post_to_channel.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_say_without_task_id_is_exempt() -> None:
+    """say() with NO task_id and no active task: channel announcement, allowed."""
+    agent_id = uuid4()
+    task_svc = AsyncMock()
+    task_svc.get_active_task_for_agent.return_value = None
+    messaging_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, messaging=messaging_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.say(
+        agent_id=agent_id, channel="all-hands", text="General announcement"
+    )
+    assert env.error is None
+    messaging_svc.post_to_channel.assert_awaited_once()
+
+
+@pytest.mark.asyncio
+async def test_say_with_explicit_task_id_owned_by_caller_succeeds() -> None:
+    """say(task_id=X) when caller owns X: allowed."""
+    agent_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(id=task_id, status="in_progress", assigned_to=agent_id)
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    messaging_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, messaging=messaging_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.say(
+        agent_id=agent_id,
+        channel="backend-cell",
+        text="Working on my task",
+        task_id=task_id,
+    )
+    assert env.error is None
+    messaging_svc.post_to_channel.assert_awaited_once()
+
+
+# ---------------------------------------------------------------------------
+# dm — explicit task_id ownership
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_dm_with_task_id_blocks_when_not_assignee() -> None:
+    """dm(task_id=X) where X is owned by someone else is rejected."""
+    agent_id = uuid4()
+    other_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(id=task_id, status="in_progress", assigned_to=other_id)
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    a2a_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, a2a=a2a_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.dm(
+        agent_id=agent_id,
+        recipient="be-qa-1",
+        text="DM about someone else's task",
+        task_id=task_id,
+    )
+    body = env.as_dict()
+    assert body["error"] == "not_authorized"
+    a2a_svc.send.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_dm_with_task_id_owned_by_caller_succeeds() -> None:
+    agent_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(id=task_id, status="in_progress", assigned_to=agent_id)
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    a2a_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, a2a=a2a_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.dm(
+        agent_id=agent_id,
+        recipient="be-qa-1",
+        text="DM about my own task",
+        task_id=task_id,
+    )
+    assert env.error is None
+    a2a_svc.send.assert_awaited_once()
+
+
+# ---------------------------------------------------------------------------
+# evidence — explicit task_id ownership
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_evidence_blocks_when_not_assignee() -> None:
+    """evidence(task_id=X) where X is owned by someone else is rejected.
+
+    Exception: QA/documenter agents legitimately review tasks they don't
+    own. We allow the read when assigned_to is None (post-handoff state)
+    or matches the caller. The strict gate is for posting/writing only.
+
+    For this test we use a task with another developer as assignee — the
+    inspect call is denied because there's no role-based exemption logic
+    yet at the gateway layer for content-tools.
+    """
+    agent_id = uuid4()
+    other_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(
+        id=task_id,
+        status="in_progress",
+        assigned_to=other_id,
+        branch_name="feature/backend/abc",
+        work_session_id=uuid4(),
+        commits=["sha1"],
+        pr_number=8,
+        pr_url="x",
+        dev_notes="",
+        acceptance_criteria_status=[],
+    )
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    git_svc = AsyncMock()
+    git_svc.diff.return_value = ""
+    workspace_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, git=git_svc, workspace=workspace_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.evidence(agent_id=agent_id, task_id=task_id)
+    body = env.as_dict()
+    assert body["error"] == "not_authorized"
+    workspace_svc.fetch_branch_for_inspection.assert_not_awaited()
+    git_svc.diff.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_evidence_unassigned_task_allows_inspection() -> None:
+    """A task with assigned_to=None (between handoffs) may be inspected.
+
+    This handles the QA-after-dev-submit case where assigned_to is briefly
+    None until reassigned. Strict ownership only applies to write-side.
+    """
+    agent_id = uuid4()
+    task_id = uuid4()
+    task_obj = MagicMock(
+        id=task_id,
+        status="awaiting_qa",
+        assigned_to=None,
+        branch_name="feature/backend/abc",
+        work_session_id=uuid4(),
+        commits=["sha1"],
+        pr_number=8,
+        pr_url="x",
+        dev_notes="",
+        acceptance_criteria_status=[],
+    )
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task_obj
+    git_svc = AsyncMock()
+    git_svc.diff.return_value = "diff content"
+    workspace_svc = AsyncMock()
+    deps = _make_deps(task=task_svc, git=git_svc, workspace=workspace_svc)
+    ca = ContentActions(deps)
+
+    env = await ca.evidence(agent_id=agent_id, task_id=task_id)
+    assert env.error is None