diff --git a/tests/unit/test_state_machine.py b/tests/unit/test_state_machine.py
new file mode 100644
index 0000000..0be7567
--- /dev/null
+++ b/tests/unit/test_state_machine.py
@@ -0,0 +1,301 @@
+"""P0 单元测试：状态机转换 VALID_TRANSITIONS 全覆盖。"""
+
+import pytest
+
+pytestmark = pytest.mark.unit
+
+from src.blackboard.db import VALID_TRANSITIONS
+
+ALL_STATES = set(VALID_TRANSITIONS.keys())
+
+
+class TestCompleteness:
+    """完整性校验：测试覆盖的状态集合 == VALID_TRANSITIONS 的 key 集合。"""
+
+    def test_all_states_are_tested(self):
+        tested = {
+            "pending", "claimed", "working", "paused", "review",
+            "blocked", "failed", "escalated", "waiting_human",
+            "done", "reviewing", "cancelled",
+        }
+        assert tested == ALL_STATES
+
+
+class TestPendingTransitions:
+    """pending 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["claimed", "paused", "blocked", "cancelled"])
+    def test_valid_from_pending(self, target):
+        assert target in VALID_TRANSITIONS["pending"]
+
+    def test_invalid_from_pending_to_working(self):
+        assert "working" not in VALID_TRANSITIONS["pending"]
+
+    def test_invalid_from_pending_to_done(self):
+        assert "done" not in VALID_TRANSITIONS["pending"]
+
+    def test_invalid_from_pending_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["pending"]
+
+    def test_invalid_from_pending_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["pending"]
+
+    def test_invalid_from_pending_to_escalated(self):
+        assert "escalated" not in VALID_TRANSITIONS["pending"]
+
+    def test_invalid_from_pending_to_waiting_human(self):
+        assert "waiting_human" not in VALID_TRANSITIONS["pending"]
+
+
+class TestClaimedTransitions:
+    """claimed 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["working", "paused", "pending", "cancelled"])
+    def test_valid_from_claimed(self, target):
+        assert target in VALID_TRANSITIONS["claimed"]
+
+    def test_invalid_from_claimed_to_done(self):
+        assert "done" not in VALID_TRANSITIONS["claimed"]
+
+    def test_invalid_from_claimed_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["claimed"]
+
+    def test_invalid_from_claimed_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["claimed"]
+
+    def test_invalid_from_claimed_to_blocked(self):
+        assert "blocked" not in VALID_TRANSITIONS["claimed"]
+
+    def test_invalid_from_claimed_to_escalated(self):
+        assert "escalated" not in VALID_TRANSITIONS["claimed"]
+
+
+class TestWorkingTransitions:
+    """working 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize(
+        "target",
+        ["review", "done", "blocked", "failed", "paused", "escalated", "waiting_human", "cancelled", "pending"],
+    )
+    def test_valid_from_working(self, target):
+        assert target in VALID_TRANSITIONS["working"]
+
+    def test_working_to_pending_is_valid(self):
+        """重点：working → pending，Mail spawn 失败回退。"""
+        assert "pending" in VALID_TRANSITIONS["working"]
+
+    def test_invalid_from_working_to_claimed(self):
+        assert "claimed" not in VALID_TRANSITIONS["working"]
+
+
+class TestPausedTransitions:
+    """paused 状态的合法/非法转换——可恢复到多个状态。"""
+
+    @pytest.mark.parametrize("target", ["working", "claimed", "review", "escalated", "waiting_human", "cancelled"])
+    def test_valid_from_paused(self, target):
+        assert target in VALID_TRANSITIONS["paused"]
+
+    def test_paused_can_resume_to_multiple_states(self):
+        """重点：paused 可以恢复到 working / claimed / review。"""
+        resume_targets = {"working", "claimed", "review"}
+        assert resume_targets.issubset(VALID_TRANSITIONS["paused"])
+
+    def test_invalid_from_paused_to_done(self):
+        assert "done" not in VALID_TRANSITIONS["paused"]
+
+    def test_invalid_from_paused_to_pending(self):
+        assert "pending" not in VALID_TRANSITIONS["paused"]
+
+    def test_invalid_from_paused_to_blocked(self):
+        assert "blocked" not in VALID_TRANSITIONS["paused"]
+
+    def test_invalid_from_paused_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["paused"]
+
+
+class TestReviewTransitions:
+    """review 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["done", "pending", "failed", "paused", "escalated", "waiting_human", "cancelled"])
+    def test_valid_from_review(self, target):
+        assert target in VALID_TRANSITIONS["review"]
+
+    def test_invalid_from_review_to_working(self):
+        assert "working" not in VALID_TRANSITIONS["review"]
+
+    def test_invalid_from_review_to_claimed(self):
+        assert "claimed" not in VALID_TRANSITIONS["review"]
+
+    def test_invalid_from_review_to_blocked(self):
+        assert "blocked" not in VALID_TRANSITIONS["review"]
+
+
+class TestBlockedTransitions:
+    """blocked 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["pending", "escalated", "cancelled"])
+    def test_valid_from_blocked(self, target):
+        assert target in VALID_TRANSITIONS["blocked"]
+
+    def test_invalid_from_blocked_to_working(self):
+        assert "working" not in VALID_TRANSITIONS["blocked"]
+
+    def test_invalid_from_blocked_to_done(self):
+        assert "done" not in VALID_TRANSITIONS["blocked"]
+
+    def test_invalid_from_blocked_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["blocked"]
+
+    def test_invalid_from_blocked_to_paused(self):
+        assert "paused" not in VALID_TRANSITIONS["blocked"]
+
+    def test_invalid_from_blocked_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["blocked"]
+
+
+class TestFailedTransitions:
+    """failed 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["pending", "escalated", "cancelled"])
+    def test_valid_from_failed(self, target):
+        assert target in VALID_TRANSITIONS["failed"]
+
+    def test_invalid_from_failed_to_working(self):
+        assert "working" not in VALID_TRANSITIONS["failed"]
+
+    def test_invalid_from_failed_to_done(self):
+        assert "done" not in VALID_TRANSITIONS["failed"]
+
+    def test_invalid_from_failed_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["failed"]
+
+    def test_invalid_from_failed_to_paused(self):
+        assert "paused" not in VALID_TRANSITIONS["failed"]
+
+    def test_invalid_from_failed_to_claimed(self):
+        assert "claimed" not in VALID_TRANSITIONS["failed"]
+
+
+class TestEscalatedTransitions:
+    """escalated 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["working", "pending", "paused", "cancelled"])
+    def test_valid_from_escalated(self, target):
+        assert target in VALID_TRANSITIONS["escalated"]
+
+    def test_invalid_from_escalated_to_done(self):
+        assert "done" not in VALID_TRANSITIONS["escalated"]
+
+    def test_invalid_from_escalated_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["escalated"]
+
+    def test_invalid_from_escalated_to_blocked(self):
+        assert "blocked" not in VALID_TRANSITIONS["escalated"]
+
+    def test_invalid_from_escalated_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["escalated"]
+
+    def test_invalid_from_escalated_to_claimed(self):
+        assert "claimed" not in VALID_TRANSITIONS["escalated"]
+
+
+class TestWaitingHumanTransitions:
+    """waiting_human 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["working", "done", "paused", "cancelled"])
+    def test_valid_from_waiting_human(self, target):
+        assert target in VALID_TRANSITIONS["waiting_human"]
+
+    def test_invalid_from_waiting_human_to_pending(self):
+        assert "pending" not in VALID_TRANSITIONS["waiting_human"]
+
+    def test_invalid_from_waiting_human_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["waiting_human"]
+
+    def test_invalid_from_waiting_human_to_claimed(self):
+        assert "claimed" not in VALID_TRANSITIONS["waiting_human"]
+
+    def test_invalid_from_waiting_human_to_blocked(self):
+        assert "blocked" not in VALID_TRANSITIONS["waiting_human"]
+
+    def test_invalid_from_waiting_human_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["waiting_human"]
+
+
+class TestDoneTransitions:
+    """done 状态的合法/非法转换——只有 cancelled 和 reviewing。"""
+
+    @pytest.mark.parametrize("target", ["cancelled", "reviewing"])
+    def test_valid_from_done(self, target):
+        assert target in VALID_TRANSITIONS["done"]
+
+    def test_done_to_reviewing_is_valid(self):
+        """重点：done → reviewing，审查回溯。"""
+        assert "reviewing" in VALID_TRANSITIONS["done"]
+
+    def test_invalid_from_done_to_working(self):
+        assert "working" not in VALID_TRANSITIONS["done"]
+
+    def test_invalid_from_done_to_pending(self):
+        assert "pending" not in VALID_TRANSITIONS["done"]
+
+    def test_invalid_from_done_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["done"]
+
+    def test_invalid_from_done_to_paused(self):
+        assert "paused" not in VALID_TRANSITIONS["done"]
+
+    def test_invalid_from_done_to_blocked(self):
+        assert "blocked" not in VALID_TRANSITIONS["done"]
+
+
+class TestReviewingTransitions:
+    """reviewing 状态的合法/非法转换。"""
+
+    @pytest.mark.parametrize("target", ["done", "working", "cancelled"])
+    def test_valid_from_reviewing(self, target):
+        assert target in VALID_TRANSITIONS["reviewing"]
+
+    def test_invalid_from_reviewing_to_pending(self):
+        assert "pending" not in VALID_TRANSITIONS["reviewing"]
+
+    def test_invalid_from_reviewing_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["reviewing"]
+
+    def test_invalid_from_reviewing_to_paused(self):
+        assert "paused" not in VALID_TRANSITIONS["reviewing"]
+
+    def test_invalid_from_reviewing_to_blocked(self):
+        assert "blocked" not in VALID_TRANSITIONS["reviewing"]
+
+    def test_invalid_from_reviewing_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["reviewing"]
+
+
+class TestCancelledTransitions:
+    """cancelled 状态的合法/非法转换——唯一出路是回到 pending。"""
+
+    def test_valid_from_cancelled_to_pending(self):
+        """重点：cancelled → pending，唯一能从 cancelled 出来的路径。"""
+        assert "pending" in VALID_TRANSITIONS["cancelled"]
+
+    def test_cancelled_only_has_pending(self):
+        assert VALID_TRANSITIONS["cancelled"] == {"pending"}
+
+    def test_invalid_from_cancelled_to_working(self):
+        assert "working" not in VALID_TRANSITIONS["cancelled"]
+
+    def test_invalid_from_cancelled_to_done(self):
+        assert "done" not in VALID_TRANSITIONS["cancelled"]
+
+    def test_invalid_from_cancelled_to_review(self):
+        assert "review" not in VALID_TRANSITIONS["cancelled"]
+
+    def test_invalid_from_cancelled_to_failed(self):
+        assert "failed" not in VALID_TRANSITIONS["cancelled"]
+
+    def test_invalid_from_cancelled_to_paused(self):
+        assert "paused" not in VALID_TRANSITIONS["cancelled"]
+
+    def test_invalid_from_cancelled_to_claimed(self):
+        assert "claimed" not in VALID_TRANSITIONS["cancelled"]