diff --git a/app/api/routers/calendar.py b/app/api/routers/calendar.py
index d9b84f0..4d5d837 100644
--- a/app/api/routers/calendar.py
+++ b/app/api/routers/calendar.py
@@ -52,6 +52,7 @@ from app.schemas.calendar import (
 )
 from app.services.agent_heartbeat import get_pending_slots_for_agent
 from app.services.agent_status import (
+    AgentStatusError,
     record_heartbeat,
     transition_to_busy,
     transition_to_idle,
@@ -594,19 +595,31 @@ def update_agent_status(
 ):
     agent = _require_agent(db, payload.agent_id, payload.claw_identifier)
     target = (payload.status or '').lower().strip()
-    if target == AgentStatus.IDLE.value:
-        transition_to_idle(db, agent)
-    elif target == AgentStatus.BUSY.value:
-        transition_to_busy(db, agent, slot_type=SlotType.WORK)
-    elif target == AgentStatus.ON_CALL.value:
-        transition_to_busy(db, agent, slot_type=SlotType.ON_CALL)
-    elif target == AgentStatus.OFFLINE.value:
-        transition_to_offline(db, agent)
-    elif target == AgentStatus.EXHAUSTED.value:
-        reason = ExhaustReason.BILLING if payload.exhaust_reason == 'billing' else ExhaustReason.RATE_LIMIT
-        transition_to_exhausted(db, agent, reason=reason, recovery_at=payload.recovery_at)
-    else:
-        raise HTTPException(status_code=400, detail="Unsupported agent status")
+    # Idempotent same-state transition: a 'busy → busy' request is a
+    # no-op rather than a 500. Lets plugin status gates / cli `--set`
+    # be safe to fire-and-forget without first reading current state.
+    current = agent.status.value if hasattr(agent.status, 'value') else str(agent.status)
+    if current == target:
+        return {"ok": True, "agent_id": agent.agent_id, "status": current, "no_change": True}
+    try:
+        if target == AgentStatus.IDLE.value:
+            transition_to_idle(db, agent)
+        elif target == AgentStatus.BUSY.value:
+            transition_to_busy(db, agent, slot_type=SlotType.WORK)
+        elif target == AgentStatus.ON_CALL.value:
+            transition_to_busy(db, agent, slot_type=SlotType.ON_CALL)
+        elif target == AgentStatus.OFFLINE.value:
+            transition_to_offline(db, agent)
+        elif target == AgentStatus.EXHAUSTED.value:
+            reason = ExhaustReason.BILLING if payload.exhaust_reason == 'billing' else ExhaustReason.RATE_LIMIT
+            transition_to_exhausted(db, agent, reason=reason, recovery_at=payload.recovery_at)
+        else:
+            raise HTTPException(status_code=400, detail="Unsupported agent status")
+    except AgentStatusError as e:
+        # State-machine violation (e.g. busy → busy via wrong precondition)
+        # → 409 with the rejected transition explained, instead of a 500.
+        db.rollback()
+        raise HTTPException(status_code=409, detail=str(e))
     db.commit()
     return {"ok": True, "agent_id": agent.agent_id, "status": agent.status.value if hasattr(agent.status, 'value') else str(agent.status)}