diff --git a/backend/app/api/board_onboarding.py b/backend/app/api/board_onboarding.py
index 5ef21e63..ae7c70ba 100644
--- a/backend/app/api/board_onboarding.py
+++ b/backend/app/api/board_onboarding.py
@@ -168,6 +168,29 @@ async def start_onboarding(
         .first(session)
     )
     if onboarding:
+        last_user_content: str | None = None
+        messages = onboarding.messages or []
+        if messages:
+            last_message = messages[-1]
+            if isinstance(last_message, dict):
+                last_role = last_message.get("role")
+                content = last_message.get("content")
+                if last_role == "user" and isinstance(content, str) and content:
+                    last_user_content = content
+
+        if last_user_content:
+            # Retrigger the agent when the session is waiting on a response.
+            dispatcher = BoardOnboardingMessagingService(session)
+            await dispatcher.dispatch_answer(
+                board=board,
+                onboarding=onboarding,
+                answer_text=last_user_content,
+                correlation_id=f"onboarding.resume:{board.id}:{onboarding.id}",
+            )
+            onboarding.updated_at = utcnow()
+            session.add(onboarding)
+            await session.commit()
+            await session.refresh(onboarding)
         return onboarding
 
     dispatcher = BoardOnboardingMessagingService(session)
diff --git a/backend/tests/test_board_onboarding_start_api.py b/backend/tests/test_board_onboarding_start_api.py
new file mode 100644
index 00000000..5edc5c57
--- /dev/null
+++ b/backend/tests/test_board_onboarding_start_api.py
@@ -0,0 +1,175 @@
+# ruff: noqa: INP001, S101
+"""Tests for board onboarding start-session restart behavior."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from types import SimpleNamespace
+from typing import Any
+from uuid import uuid4
+
+import pytest
+
+from app.api import board_onboarding
+from app.core.time import utcnow
+from app.models.board_onboarding import BoardOnboardingSession
+from app.schemas.board_onboarding import BoardOnboardingStart
+
+
+@dataclass
+class _FakeScalarResult:
+    value: object | None
+
+    def first(self) -> object | None:
+        return self.value
+
+
+@dataclass
+class _FakeSession:
+    first_value: object | None
+    added: list[object] = field(default_factory=list)
+    committed: int = 0
+    refreshed: list[object] = field(default_factory=list)
+
+    async def exec(self, _statement: object) -> _FakeScalarResult:
+        return _FakeScalarResult(self.first_value)
+
+    def add(self, value: object) -> None:
+        self.added.append(value)
+
+    async def commit(self) -> None:
+        self.committed += 1
+
+    async def refresh(self, value: object) -> None:
+        self.refreshed.append(value)
+
+
+@pytest.mark.asyncio
+async def test_start_onboarding_redispatches_when_last_message_is_user(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    board_id = uuid4()
+    onboarding = BoardOnboardingSession(
+        board_id=board_id,
+        session_key="session-key",
+        status="active",
+        messages=[
+            {
+                "role": "user",
+                "content": "I prefer concise updates.",
+                "timestamp": utcnow().isoformat(),
+            },
+        ],
+    )
+    session: Any = _FakeSession(first_value=onboarding)
+    board = SimpleNamespace(id=board_id, name="Roadmap", description="Build v1")
+    captured_calls: list[dict[str, object]] = []
+
+    class _FakeMessagingService:
+        def __init__(self, _session: object) -> None:
+            self._session = _session
+
+        async def dispatch_answer(
+            self,
+            *,
+            board: object,
+            onboarding: object,
+            answer_text: str,
+            correlation_id: str,
+        ) -> None:
+            captured_calls.append(
+                {
+                    "board": board,
+                    "onboarding": onboarding,
+                    "answer_text": answer_text,
+                    "correlation_id": correlation_id,
+                },
+            )
+
+    monkeypatch.setattr(
+        board_onboarding,
+        "BoardOnboardingMessagingService",
+        _FakeMessagingService,
+    )
+
+    before = onboarding.updated_at
+    result = await board_onboarding.start_onboarding(
+        _payload=BoardOnboardingStart(),
+        board=board,
+        session=session,
+    )
+
+    assert result is onboarding
+    assert len(captured_calls) == 1
+    assert captured_calls[0]["answer_text"] == "I prefer concise updates."
+    assert str(captured_calls[0]["correlation_id"]).startswith("onboarding.resume:")
+    assert onboarding.updated_at >= before
+    assert session.added == [onboarding]
+    assert session.committed == 1
+    assert session.refreshed == [onboarding]
+
+
+@pytest.mark.asyncio
+async def test_start_onboarding_does_not_redispatch_when_waiting_for_user(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    board_id = uuid4()
+    onboarding = BoardOnboardingSession(
+        board_id=board_id,
+        session_key="session-key",
+        status="active",
+        messages=[
+            {
+                "role": "user",
+                "content": "I prefer concise updates.",
+                "timestamp": utcnow().isoformat(),
+            },
+            {
+                "role": "assistant",
+                "content": '{"question":"What is your timezone?","options":[{"id":"1","label":"UTC"}]}',
+                "timestamp": utcnow().isoformat(),
+            },
+        ],
+    )
+    session: Any = _FakeSession(first_value=onboarding)
+    board = SimpleNamespace(id=board_id, name="Roadmap", description="Build v1")
+    captured_calls: list[dict[str, object]] = []
+
+    class _FakeMessagingService:
+        def __init__(self, _session: object) -> None:
+            self._session = _session
+
+        async def dispatch_answer(
+            self,
+            *,
+            board: object,
+            onboarding: object,
+            answer_text: str,
+            correlation_id: str,
+        ) -> None:
+            captured_calls.append(
+                {
+                    "board": board,
+                    "onboarding": onboarding,
+                    "answer_text": answer_text,
+                    "correlation_id": correlation_id,
+                },
+            )
+
+    monkeypatch.setattr(
+        board_onboarding,
+        "BoardOnboardingMessagingService",
+        _FakeMessagingService,
+    )
+
+    result = await board_onboarding.start_onboarding(
+        _payload=BoardOnboardingStart(),
+        board=board,
+        session=session,
+    )
+
+    assert result is onboarding
+    assert captured_calls == []
+    assert session.added == []
+    assert session.committed == 0
+    assert session.refreshed == []