diff --git a/spring-ai-model/src/test/java/org/springframework/ai/aot/ToolRuntimeHintsTests.java b/spring-ai-model/src/test/java/org/springframework/ai/aot/ToolRuntimeHintsTests.java
index 30683356203..622a5c678b9 100644
--- a/spring-ai-model/src/test/java/org/springframework/ai/aot/ToolRuntimeHintsTests.java
+++ b/spring-ai-model/src/test/java/org/springframework/ai/aot/ToolRuntimeHintsTests.java
@@ -47,4 +47,36 @@ void registerHintsWithNullClassLoader() {
 		assertThatCode(() -> toolRuntimeHints.registerHints(runtimeHints, null)).doesNotThrowAnyException();
 	}
 
+	@Test
+	void registerHintsWithCustomClassLoader() {
+		RuntimeHints runtimeHints = new RuntimeHints();
+		ToolRuntimeHints toolRuntimeHints = new ToolRuntimeHints();
+		ClassLoader customClassLoader = Thread.currentThread().getContextClassLoader();
+
+		toolRuntimeHints.registerHints(runtimeHints, customClassLoader);
+
+		assertThat(runtimeHints).matches(reflection().onType(DefaultToolCallResultConverter.class));
+	}
+
+	@Test
+	void registerHintsMultipleTimes() {
+		RuntimeHints runtimeHints = new RuntimeHints();
+		ToolRuntimeHints toolRuntimeHints = new ToolRuntimeHints();
+
+		toolRuntimeHints.registerHints(runtimeHints, null);
+		toolRuntimeHints.registerHints(runtimeHints, null);
+
+		assertThat(runtimeHints).matches(reflection().onType(DefaultToolCallResultConverter.class));
+	}
+
+	@Test
+	void toolRuntimeHintsInstanceCreation() {
+		assertThatCode(() -> new ToolRuntimeHints()).doesNotThrowAnyException();
+
+		ToolRuntimeHints hints1 = new ToolRuntimeHints();
+		ToolRuntimeHints hints2 = new ToolRuntimeHints();
+
+		assertThat(hints1).isNotSameAs(hints2);
+	}
+
 }
diff --git a/spring-ai-model/src/test/java/org/springframework/ai/metadata/UsageTests.java b/spring-ai-model/src/test/java/org/springframework/ai/metadata/UsageTests.java
index 72b1c6cc169..029e2e9b0c5 100644
--- a/spring-ai-model/src/test/java/org/springframework/ai/metadata/UsageTests.java
+++ b/spring-ai-model/src/test/java/org/springframework/ai/metadata/UsageTests.java
@@ -87,4 +87,28 @@ void totalTokensEqualsPromptTokensPlusGenerationTokens() {
 		verifyUsage(usage);
 	}
 
+	@Test
+	void totalTokensHandlesZeroPromptTokens() {
+		Usage usage = mockUsage(0, 1);
+
+		assertThat(usage.getTotalTokens()).isEqualTo(1);
+		verifyUsage(usage);
+	}
+
+	@Test
+	void totalTokensHandlesZeroCompletionTokens() {
+		Usage usage = mockUsage(1, 0);
+
+		assertThat(usage.getTotalTokens()).isEqualTo(1);
+		verifyUsage(usage);
+	}
+
+	@Test
+	void totalTokensHandlesBothZeroTokens() {
+		Usage usage = mockUsage(0, 0);
+
+		assertThat(usage.getTotalTokens()).isZero();
+		verifyUsage(usage);
+	}
+
 }
diff --git a/spring-ai-model/src/test/java/org/springframework/ai/model/transformer/KeywordMetadataEnricherTest.java b/spring-ai-model/src/test/java/org/springframework/ai/model/transformer/KeywordMetadataEnricherTest.java
index a708671d405..690909f06df 100644
--- a/spring-ai-model/src/test/java/org/springframework/ai/model/transformer/KeywordMetadataEnricherTest.java
+++ b/spring-ai-model/src/test/java/org/springframework/ai/model/transformer/KeywordMetadataEnricherTest.java
@@ -165,4 +165,106 @@ private String getDefaultTemplatePromptText(int keywordCount, String documentCon
 		return prompt.getContents();
 	}
 
+	@Test
+	void testApplyWithEmptyDocumentsList() {
+		List<Document> emptyDocuments = List.of();
+		KeywordMetadataEnricher keywordMetadataEnricher = new KeywordMetadataEnricher(chatModel, 3);
+
+		keywordMetadataEnricher.apply(emptyDocuments);
+
+		verify(chatModel, never()).call(any(Prompt.class));
+	}
+
+	@Test
+	void testApplyWithSingleDocument() {
+		List<Document> documents = List.of(new Document("single content"));
+		given(chatModel.call(any(Prompt.class))).willReturn(new ChatResponse(
+				List.of(new Generation(new AssistantMessage("single, keyword, test, document, content")))));
+
+		KeywordMetadataEnricher keywordMetadataEnricher = new KeywordMetadataEnricher(chatModel, 5);
+		keywordMetadataEnricher.apply(documents);
+
+		verify(chatModel, times(1)).call(promptCaptor.capture());
+		assertThat(documents.get(0).getMetadata()).containsEntry(EXCERPT_KEYWORDS_METADATA_KEY,
+				"single, keyword, test, document, content");
+	}
+
+	@Test
+	void testApplyWithDocumentContainingExistingMetadata() {
+		Document document = new Document("content with existing metadata");
+		document.getMetadata().put("existing_key", "existing_value");
+		List<Document> documents = List.of(document);
+		given(chatModel.call(any(Prompt.class)))
+			.willReturn(new ChatResponse(List.of(new Generation(new AssistantMessage("new, keywords")))));
+
+		KeywordMetadataEnricher keywordMetadataEnricher = new KeywordMetadataEnricher(chatModel, 2);
+		keywordMetadataEnricher.apply(documents);
+
+		assertThat(documents.get(0).getMetadata()).containsEntry("existing_key", "existing_value");
+		assertThat(documents.get(0).getMetadata()).containsEntry(EXCERPT_KEYWORDS_METADATA_KEY, "new, keywords");
+	}
+
+	@Test
+	void testApplyWithEmptyStringResponse() {
+		List<Document> documents = List.of(new Document("content"));
+		given(chatModel.call(any(Prompt.class)))
+			.willReturn(new ChatResponse(List.of(new Generation(new AssistantMessage("")))));
+
+		KeywordMetadataEnricher keywordMetadataEnricher = new KeywordMetadataEnricher(chatModel, 3);
+		keywordMetadataEnricher.apply(documents);
+
+		assertThat(documents.get(0).getMetadata()).containsEntry(EXCERPT_KEYWORDS_METADATA_KEY, "");
+	}
+
+	@Test
+	void testApplyWithWhitespaceOnlyResponse() {
+		List<Document> documents = List.of(new Document("content"));
+		given(chatModel.call(any(Prompt.class)))
+			.willReturn(new ChatResponse(List.of(new Generation(new AssistantMessage("   \n\t   ")))));
+
+		KeywordMetadataEnricher keywordMetadataEnricher = new KeywordMetadataEnricher(chatModel, 3);
+		keywordMetadataEnricher.apply(documents);
+
+		assertThat(documents.get(0).getMetadata()).containsEntry(EXCERPT_KEYWORDS_METADATA_KEY, "   \n\t   ");
+	}
+
+	@Test
+	void testApplyOverwritesExistingKeywords() {
+		Document document = new Document("content");
+		document.getMetadata().put(EXCERPT_KEYWORDS_METADATA_KEY, "old, keywords");
+		List<Document> documents = List.of(document);
+		given(chatModel.call(any(Prompt.class)))
+			.willReturn(new ChatResponse(List.of(new Generation(new AssistantMessage("new, keywords")))));
+
+		KeywordMetadataEnricher keywordMetadataEnricher = new KeywordMetadataEnricher(chatModel, 2);
+		keywordMetadataEnricher.apply(documents);
+
+		assertThat(documents.get(0).getMetadata()).containsEntry(EXCERPT_KEYWORDS_METADATA_KEY, "new, keywords");
+	}
+
+	@Test
+	void testBuilderWithBothKeywordCountAndTemplate() {
+		PromptTemplate customTemplate = new PromptTemplate(CUSTOM_TEMPLATE);
+
+		KeywordMetadataEnricher enricher = builder(chatModel).keywordCount(5).keywordsTemplate(customTemplate).build();
+
+		assertThat(enricher.getKeywordsTemplate()).isEqualTo(customTemplate);
+	}
+
+	@Test
+	void testApplyWithSpecialCharactersInContent() {
+		List<Document> documents = List.of(new Document("Content with special chars: @#$%^&*()"));
+		given(chatModel.call(any(Prompt.class))).willReturn(
+				new ChatResponse(List.of(new Generation(new AssistantMessage("special, characters, content")))));
+
+		KeywordMetadataEnricher keywordMetadataEnricher = new KeywordMetadataEnricher(chatModel, 3);
+		keywordMetadataEnricher.apply(documents);
+
+		verify(chatModel, times(1)).call(promptCaptor.capture());
+		assertThat(promptCaptor.getValue().getUserMessage().getText())
+			.contains("Content with special chars: @#$%^&*()");
+		assertThat(documents.get(0).getMetadata()).containsEntry(EXCERPT_KEYWORDS_METADATA_KEY,
+				"special, characters, content");
+	}
+
 }