diff --git a/internal/gateway/methods/teams_members.go b/internal/gateway/methods/teams_members.go
index 6fc2635d..cf058e7d 100644
--- a/internal/gateway/methods/teams_members.go
+++ b/internal/gateway/methods/teams_members.go
@@ -52,10 +52,11 @@ func (m *TeamsMethods) handleAddMember(ctx context.Context, client *gateway.Clie
 		return
 	}
 
-	// Resolve agent
+	// Resolve agent — accepts agent_key or UUID. H10 fix: return i18n error instead
+	// of leaking raw store error (was: "agent: " + err.Error()).
 	ag, err := resolveAgentInfo(ctx, m.agentStore, params.Agent)
 	if err != nil {
-		client.SendResponse(protocol.NewErrorResponse(req.ID, protocol.ErrInvalidRequest, "agent: "+err.Error()))
+		client.SendResponse(protocol.NewErrorResponse(req.ID, protocol.ErrInvalidRequest, i18n.T(locale, i18n.MsgInvalidID, "agent")))
 		return
 	}
 
@@ -134,7 +135,8 @@ func (m *TeamsMethods) handleRemoveMember(ctx context.Context, client *gateway.C
 		client.SendResponse(protocol.NewErrorResponse(req.ID, protocol.ErrInvalidRequest, i18n.T(locale, i18n.MsgInvalidID, "teamId")))
 		return
 	}
-	agentID, err := uuid.Parse(params.AgentID)
+	// Phase 3 FR-1: accept agent_key or UUID via cache-aware resolver.
+	agentID, err := resolveAgentUUIDCached(ctx, m.agentRouter, m.agentStore, params.AgentID)
 	if err != nil {
 		client.SendResponse(protocol.NewErrorResponse(req.ID, protocol.ErrInvalidRequest, i18n.T(locale, i18n.MsgInvalidID, "agentId")))
 		return
diff --git a/internal/gateway/methods/teams_tasks_mutations.go b/internal/gateway/methods/teams_tasks_mutations.go
index c6c13ffd..c498c1a0 100644
--- a/internal/gateway/methods/teams_tasks_mutations.go
+++ b/internal/gateway/methods/teams_tasks_mutations.go
@@ -151,7 +151,8 @@ func (m *TeamsMethods) handleTaskAssign(ctx context.Context, client *gateway.Cli
 		client.SendResponse(protocol.NewErrorResponse(req.ID, protocol.ErrInvalidRequest, i18n.T(locale, i18n.MsgInvalidID, "taskId")))
 		return
 	}
-	agentID, err := uuid.Parse(params.AgentID)
+	// Phase 3 FR-1: accept agent_key or UUID for assignee.
+	agentID, err := resolveAgentUUIDCached(ctx, m.agentRouter, m.agentStore, params.AgentID)
 	if err != nil {
 		client.SendResponse(protocol.NewErrorResponse(req.ID, protocol.ErrInvalidRequest, i18n.T(locale, i18n.MsgInvalidID, "agentId")))
 		return