diff --git a/internal/ai/runtime/executor/context_builders.go b/internal/ai/runtime/executor/context_builders.go
index e250106..8ef1fcf 100644
--- a/internal/ai/runtime/executor/context_builders.go
+++ b/internal/ai/runtime/executor/context_builders.go
@@ -44,7 +44,7 @@ func appendRetrievedContext(ctx context.Context, req RunInput, summary *RunResul
 	retrieveOptions.QueryPreview = preview(req.UserMessage.Content, 120)
 	retrieveResult, retrieveErr := retriever.RetrieveContextByOptions(ctx, retrieveOptions, strings.TrimSpace(req.UserMessage.Content))
 	if retrieveErr != nil || retrieveResult == nil {
-		return knowledgeGuardDecision{}
+		return buildKnowledgeUnavailableDecision(req.AIAgent, retriever.KnowledgeBaseIDs())
 	}
 	if summary != nil {
 		summary.RetrieverCount = len(retrieveResult.Hits)
diff --git a/internal/ai/runtime/executor/knowledge_guard.go b/internal/ai/runtime/executor/knowledge_guard.go
index aecf3a4..34eca15 100644
--- a/internal/ai/runtime/executor/knowledge_guard.go
+++ b/internal/ai/runtime/executor/knowledge_guard.go
@@ -15,12 +15,19 @@ type knowledgeGuardDecision struct {
 	Instructions  []*schema.Message
 }
 
+func buildKnowledgeUnavailableDecision(aiAgent models.AIAgent, knowledgeBaseIDs []int64) knowledgeGuardDecision {
+	if len(knowledgeBaseIDs) == 0 {
+		return knowledgeGuardDecision{}
+	}
+	return knowledgeGuardDecision{FallbackReply: resolveKnowledgeFallbackReply(aiAgent)}
+}
+
 func buildKnowledgeGuardDecision(aiAgent models.AIAgent, retrieveResult *retrievers.KnowledgeRetrieveResult) knowledgeGuardDecision {
 	if retrieveResult == nil || len(retrieveResult.KnowledgeBaseIDs) == 0 {
 		return knowledgeGuardDecision{}
 	}
 	fallbackReply := resolveKnowledgeFallbackReply(aiAgent)
-	if len(retrieveResult.Hits) == 0 {
+	if len(retrieveResult.Hits) == 0 || strings.TrimSpace(retrieveResult.ContextText) == "" {
 		return knowledgeGuardDecision{FallbackReply: fallbackReply}
 	}
 	instruction := buildKnowledgeRuntimeInstruction(retrieveResult.AnswerMode, fallbackReply)
@@ -50,7 +57,7 @@ func buildKnowledgeRuntimeInstruction(answerMode enums.KnowledgeAnswerMode, fall
 		fallbackReply = "当前知识库暂无明确信息。"
 	}
 	if answerMode == enums.KnowledgeAnswerModeAssist {
-		return "知识库回答约束：优先依据后续提供的知识片段回答，可以做轻度归纳，但不要编造片段中未提供的事实。若知识片段不足以直接支持答案，必须明确回复：" + fallbackReply
+		return "知识库回答约束：优先依据后续提供的知识片段回答，可以做轻度归纳，但不要编造片段中未提供的事实。回答中的具体事实、步骤、承诺必须能被知识片段直接支持；若知识片段不足以直接支持答案，必须明确回复：" + fallbackReply
 	}
-	return "知识库回答约束：本轮只能依据后续提供的知识片段回答，不得使用模型常识补充未提供的事实。若知识片段不足以支持回答，必须明确回复：" + fallbackReply
+	return "知识库回答约束：本轮只能依据后续提供的知识片段回答，不得使用模型常识补充未提供的事实，不得输出知识片段外的具体事实、步骤、承诺或建议。若知识片段不足以支持回答，必须明确回复：" + fallbackReply
 }
diff --git a/internal/ai/runtime/executor/knowledge_guard_test.go b/internal/ai/runtime/executor/knowledge_guard_test.go
index 1059833..6446aca 100644
--- a/internal/ai/runtime/executor/knowledge_guard_test.go
+++ b/internal/ai/runtime/executor/knowledge_guard_test.go
@@ -44,7 +44,8 @@ func TestBuildKnowledgeGuardDecisionInjectsStrictInstructionOnHit(t *testing.T)
 		Hits: []rag.RetrieveResult{
 			{KnowledgeBaseID: 1, Score: 0.88},
 		},
-		AnswerMode: enums.KnowledgeAnswerModeStrict,
+		ContextText: "知识库上下文",
+		AnswerMode:  enums.KnowledgeAnswerModeStrict,
 	})
 
 	if decision.FallbackReply != "" {
@@ -62,6 +63,43 @@ func TestBuildKnowledgeGuardDecisionInjectsStrictInstructionOnHit(t *testing.T)
 	}
 }
 
+func TestBuildKnowledgeGuardDecisionFallsBackWhenHitHasNoContext(t *testing.T) {
+	agent := newKnowledgeGuardAgentFixture()
+	agent.FallbackMessage = "我暂时没有找到足够准确的信息。"
+	decision := buildKnowledgeGuardDecision(agent, &retrievers.KnowledgeRetrieveResult{
+		KnowledgeBaseIDs: []int64{1},
+		Hits: []rag.RetrieveResult{
+			{KnowledgeBaseID: 1, Score: 0.88},
+		},
+		AnswerMode: enums.KnowledgeAnswerModeStrict,
+	})
+
+	if decision.FallbackReply != "我暂时没有找到足够准确的信息。" {
+		t.Fatalf("expected fallback on empty context, got %q", decision.FallbackReply)
+	}
+	if len(decision.Instructions) != 0 {
+		t.Fatalf("expected no instructions on empty context, got %d", len(decision.Instructions))
+	}
+}
+
+func TestBuildKnowledgeUnavailableDecisionFallsBackWhenAgentHasKnowledge(t *testing.T) {
+	agent := newKnowledgeGuardAgentFixture()
+	agent.FallbackMessage = "知识库暂时不可用。"
+	decision := buildKnowledgeUnavailableDecision(agent, []int64{1})
+
+	if decision.FallbackReply != "知识库暂时不可用。" {
+		t.Fatalf("expected fallback when knowledge unavailable, got %q", decision.FallbackReply)
+	}
+}
+
+func TestBuildKnowledgeUnavailableDecisionSkipsWhenAgentHasNoKnowledge(t *testing.T) {
+	decision := buildKnowledgeUnavailableDecision(newKnowledgeGuardAgentFixture(), nil)
+
+	if decision.FallbackReply != "" {
+		t.Fatalf("expected no fallback without knowledge bases, got %q", decision.FallbackReply)
+	}
+}
+
 func newKnowledgeGuardAgentFixture() models.AIAgent {
 	return models.AIAgent{
 		FallbackMode: enums.AIAgentFallbackModeNoAnswer,