fix: Return empty prompt completion result when prompt has no arguments (#934)

Recent changes don't coerce null completion arguments to empty lists so we have
to check for null when handling prompt completions.

Resolves #932

Signed-off-by: Dariusz Jędrzejczyk <2554306+chemicL@users.noreply.github.com>

Author: chemicL

mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java

@@ -999,12 +999,9 @@ private McpRequestHandler<McpSchema.CompleteResult> completionCompleteRequestHan
 						.message("Prompt not found: " + promptReference.name())
 						.build());
 				}
-				if (!promptSpec.prompt()
-					.arguments()
-					.stream()
-					.filter(arg -> arg.name().equals(argumentName))
-					.findFirst()
-					.isPresent()) {
+				List<McpSchema.PromptArgument> arguments = promptSpec.prompt().arguments();
+				if (arguments == null
+						|| !arguments.stream().filter(arg -> arg.name().equals(argumentName)).findFirst().isPresent()) {
 
 					logger.warn("Argument not found: {} in prompt: {}", argumentName, promptReference.name());
 

mcp-core/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java

@@ -744,12 +744,9 @@ private McpStatelessRequestHandler<McpSchema.CompleteResult> completionCompleteR
 						.message("Prompt not found: " + promptReference.name())
 						.build());
 				}
-				if (!promptSpec.prompt()
-					.arguments()
-					.stream()
-					.filter(arg -> arg.name().equals(argumentName))
-					.findFirst()
-					.isPresent()) {
+				List<McpSchema.PromptArgument> arguments = promptSpec.prompt().arguments();
+				if (arguments == null
+						|| !arguments.stream().filter(arg -> arg.name().equals(argumentName)).findFirst().isPresent()) {
 
 					logger.warn("Argument not found: {} in prompt: {}", argumentName, promptReference.name());
 

mcp-test/src/test/java/io/modelcontextprotocol/server/HttpServletStatelessIntegrationTests.java

@@ -184,10 +184,10 @@ void testCompletionShouldReturnExpectedSuggestions(String clientType) {
 				true // hasMore
 		));
 
-		AtomicReference<CompleteRequest> samplingRequest = new AtomicReference<>();
+		AtomicReference<CompleteRequest> completeRequest = new AtomicReference<>();
 		BiFunction<McpTransportContext, CompleteRequest, CompleteResult> completionHandler = (transportContext,
 				request) -> {
-			samplingRequest.set(request);
+			completeRequest.set(request);
 			return completionResponse;
 		};
 
@@ -214,9 +214,9 @@ void testCompletionShouldReturnExpectedSuggestions(String clientType) {
 
 			assertThat(result).isNotNull();
 
-			assertThat(samplingRequest.get().argument().name()).isEqualTo("language");
-			assertThat(samplingRequest.get().argument().value()).isEqualTo("py");
-			assertThat(samplingRequest.get().ref().type()).isEqualTo(PromptReference.TYPE);
+			assertThat(completeRequest.get().argument().name()).isEqualTo("language");
+			assertThat(completeRequest.get().argument().value()).isEqualTo("py");
+			assertThat(completeRequest.get().ref().type()).isEqualTo(PromptReference.TYPE);
 		}
 		finally {
 			mcpServer.close();

mcp-test/src/test/java/io/modelcontextprotocol/server/McpCompletionTests.java

@@ -13,11 +13,9 @@
 import org.apache.catalina.LifecycleState;
 import org.apache.catalina.startup.Tomcat;
 
-import static org.assertj.core.api.Assertions.assertThat;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
-import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
 import io.modelcontextprotocol.client.McpClient;
 import io.modelcontextprotocol.client.transport.HttpClientSseClientTransport;
@@ -37,6 +35,9 @@
 import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
 import io.modelcontextprotocol.spec.McpError;
 
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
 /**
  * Tests for completion functionality with context support.
  *
@@ -324,4 +325,35 @@ void testCompletionErrorOnMissingContext() {
 		mcpServer.close();
 	}
 
+	@Test
+	void testPromptWithoutArgumentsCompletionForArgument() {
+		BiFunction<McpSyncServerExchange, CompleteRequest, CompleteResult> completionHandler = (exchange,
+				request) -> new CompleteResult(new CompleteResult.CompleteCompletion(List.of("test"), 1, false));
+
+		McpSchema.Prompt prompt = new Prompt("test-prompt", "this is a test prompt", null);
+
+		var mcpServer = McpServer.sync(mcpServerTransportProvider)
+			.capabilities(ServerCapabilities.builder().completions().build())
+			.prompts(new McpServerFeatures.SyncPromptSpecification(prompt,
+					(mcpSyncServerExchange, getPromptRequest) -> null))
+			.completions(new McpServerFeatures.SyncCompletionSpecification(
+					new PromptReference(PromptReference.TYPE, "test-prompt"), completionHandler))
+			.build();
+
+		try (var mcpClient = clientBuilder.clientInfo(new McpSchema.Implementation("Sample " + "client", "0.0.0"))
+			.build()) {
+			InitializeResult initResult = mcpClient.initialize();
+			assertThat(initResult).isNotNull();
+
+			// try completing an argument knowing that the prompt is not parameterized
+			CompleteRequest request = new CompleteRequest(new PromptReference(PromptReference.TYPE, "test-prompt"),
+					new CompleteRequest.CompleteArgument("arg", "val"));
+
+			CompleteResult completeResult = mcpClient.completeCompletion(request);
+			assertThat(completeResult.completion().values()).isEmpty();
+		}
+
+		mcpServer.close();
+	}
+
 }