fix: Return empty prompt completion result when prompt has no arguments

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/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();
+	}
+
 }