diff --git a/src/test/java/br/com/arquivolivre/myjavagenie/service/QueryServiceGuardrailTest.java b/src/test/java/br/com/arquivolivre/myjavagenie/service/QueryServiceGuardrailTest.java
new file mode 100644
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--- /dev/null
+++ b/src/test/java/br/com/arquivolivre/myjavagenie/service/QueryServiceGuardrailTest.java
@@ -0,0 +1,99 @@
+package br.com.arquivolivre.myjavagenie.service;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.Mockito.lenient;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import br.com.arquivolivre.myjavagenie.config.ModelConfig;
+import br.com.arquivolivre.myjavagenie.config.QueryConfig;
+import br.com.arquivolivre.myjavagenie.model.DocumentChunk;
+import br.com.arquivolivre.myjavagenie.model.DocumentMetadata;
+import br.com.arquivolivre.myjavagenie.model.GenerationResponse;
+import br.com.arquivolivre.myjavagenie.model.QueryResponse;
+import br.com.arquivolivre.myjavagenie.model.ScoredDocument;
+import java.util.List;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for the grounding guardrail in {@link QueryService}: a grounded answer keeps its
+ * source references, while the fixed out-of-scope reply suppresses them.
+ */
+class QueryServiceGuardrailTest {
+
+  private RetrievalEngine retrievalEngine;
+  private LanguageModelProvider languageModel;
+  private QueryService queryService;
+
+  @BeforeEach
+  void setUp() {
+    retrievalEngine = mock(RetrievalEngine.class);
+    languageModel = mock(LanguageModelProvider.class);
+    TokenUsageTracker tokenTracker = mock(TokenUsageTracker.class);
+
+    QueryConfig queryConfig = new QueryConfig(5, 0.5, 30, false, 0);
+    ModelConfig modelConfig = new ModelConfig("openai", null, null, null, null, 0.7, 512);
+
+    lenient().when(languageModel.getProviderName()).thenReturn("test-provider");
+
+    queryService =
+        new QueryService(
+            retrievalEngine,
+            languageModel,
+            new PromptBuilder(),
+            tokenTracker,
+            queryConfig,
+            modelConfig,
+            null,
+            null);
+  }
+
+  private void mockRetrieval() {
+    DocumentChunk chunk =
+        new DocumentChunk("Records are immutable data carriers.", new DocumentMetadata(), 10);
+    when(retrievalEngine.retrieveRelevantChunks(anyString()))
+        .thenReturn(List.of(new ScoredDocument(chunk, 0.8)));
+  }
+
+  @Test
+  void groundedAnswerKeepsSources() {
+    mockRetrieval();
+    when(languageModel.generate(any()))
+        .thenReturn(new GenerationResponse("Records are described in the JLS.", 100, 50, 150));
+
+    QueryResponse response = queryService.processQuery("What are records?");
+
+    assertThat(response.getAnswer()).contains("Records");
+    assertThat(response.getSources()).hasSize(1);
+  }
+
+  @Test
+  void outOfScopeReplySuppressesSources() {
+    mockRetrieval();
+    when(languageModel.generate(any()))
+        .thenReturn(new GenerationResponse(PromptBuilder.OUT_OF_SCOPE_ANSWER, 100, 30, 130));
+
+    QueryResponse response = queryService.processQuery("Best chocolate chip cookie recipe?");
+
+    assertThat(response.getAnswer()).isEqualTo(PromptBuilder.OUT_OF_SCOPE_ANSWER);
+    assertThat(response.getSources()).isEmpty();
+  }
+
+  @Test
+  void promptCarriesGroundingInstructionsAndContext() {
+    PromptBuilder builder = new PromptBuilder();
+    DocumentChunk chunk =
+        new DocumentChunk("Pattern matching for switch.", new DocumentMetadata(), 8);
+
+    String prompt = builder.buildPrompt("What changed in switch?", List.of(chunk));
+
+    assertThat(prompt)
+        .contains("Use ONLY the information in the Context section")
+        .contains(PromptBuilder.OUT_OF_SCOPE_ANSWER)
+        .contains("Pattern matching for switch.")
+        .contains("Question: What changed in switch?");
+  }
+}