diff --git a/spring-shell2-jcommander-adapter/src/main/java/org/springframework/shell2/jcommander/JCommanderParameterResolver.java b/spring-shell2-jcommander-adapter/src/main/java/org/springframework/shell2/jcommander/JCommanderParameterResolver.java
index 526841cc..0afa6813 100644
--- a/spring-shell2-jcommander-adapter/src/main/java/org/springframework/shell2/jcommander/JCommanderParameterResolver.java
+++ b/spring-shell2-jcommander-adapter/src/main/java/org/springframework/shell2/jcommander/JCommanderParameterResolver.java
@@ -19,15 +19,19 @@ package org.springframework.shell2.jcommander;
 import static org.springframework.shell2.Utils.unCamelify;
 
 import java.lang.annotation.Annotation;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import com.beust.jcommander.DynamicParameter;
 import com.beust.jcommander.JCommander;
 import com.beust.jcommander.Parameter;
+import com.beust.jcommander.ParameterException;
 import com.beust.jcommander.ParametersDelegate;
 
 import org.springframework.beans.BeanUtils;
@@ -92,22 +96,43 @@ public class JCommanderParameterResolver implements ParameterResolver {
 	@Override
 	public Stream<ParameterDescription> describe(MethodParameter parameter) {
 		JCommander jCommander = createJCommander(parameter);
-		com.beust.jcommander.ParameterDescription mainParameter = jCommander.getMainParameter();
-		return Stream.concat(
-			jCommander.getParameters().stream(),
-			mainParameter != null ? Stream.of(mainParameter) : Stream.empty()
-		)
+		Stream<com.beust.jcommander.ParameterDescription> jCommanderDescriptions = streamAllJCommanderDescriptions(jCommander);
+		return jCommanderDescriptions
 			.map(j -> new ParameterDescription(parameter, unCamelify(j.getParameterized().getType().getSimpleName()))
 				.keys(Arrays.asList(j.getParameter().names()))
 				.help(j.getDescription())
-				.mandatoryKey(!j.equals(mainParameter))
+				.mandatoryKey(!j.equals(jCommander.getMainParameter()))
 				// Not ideal as this does not take reverse-conversion into account, but just toString()
 				.defaultValue(j.getDefault() == null ? "" : String.valueOf(j.getDefault()))
 			);
 	}
 
+	/**
+	 * Return <em>all</em> JCommander parameter descriptions, including the "main" parameter if present.
+	 */
+	private Stream<com.beust.jcommander.ParameterDescription> streamAllJCommanderDescriptions(JCommander jCommander) {
+		return Stream.concat(
+				jCommander.getParameters().stream(),
+				jCommander.getMainParameter() != null ? Stream.of(jCommander.getMainParameter()) : Stream.empty()
+			);
+	}
+
 	@Override
 	public List<CompletionProposal> complete(MethodParameter parameter, CompletionContext context) {
-		return null;
+		JCommander jCommander = createJCommander(parameter);
+		List<String> words = context.getWords();
+		try {
+			jCommander.parseWithoutValidation(words.toArray(new String[words.size()]));
+		}
+		catch (ParameterException ignored) {
+			// Exception here certainly means current buffer is not parseable in full.
+			// Better to bail out now.
+			return Collections.emptyList();
+		}
+		return streamAllJCommanderDescriptions(jCommander)
+			.filter(p -> !p.isAssigned())
+			.flatMap(p -> Arrays.stream(p.getParameter().names()))
+			.map(CompletionProposal::new)
+			.collect(Collectors.toList());
 	}
 }
diff --git a/spring-shell2-jcommander-adapter/src/test/java/org/springframework/shell2/jcommander/JCommanderParameterResolverTest.java b/spring-shell2-jcommander-adapter/src/test/java/org/springframework/shell2/jcommander/JCommanderParameterResolverTest.java
index fa33ebc6..09195055 100644
--- a/spring-shell2-jcommander-adapter/src/test/java/org/springframework/shell2/jcommander/JCommanderParameterResolverTest.java
+++ b/spring-shell2-jcommander-adapter/src/test/java/org/springframework/shell2/jcommander/JCommanderParameterResolverTest.java
@@ -25,9 +25,12 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.stream.Stream;
 
+import org.hamcrest.Matchers;
 import org.junit.Test;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.shell2.CompletionContext;
+import org.springframework.shell2.CompletionProposal;
 import org.springframework.shell2.ParameterDescription;
 import org.springframework.shell2.Utils;
 import org.springframework.util.ReflectionUtils;
@@ -85,4 +88,26 @@ public class JCommanderParameterResolverTest {
 			.help("rest");
 		assertThat(desciptions).contains(name, level, rest);
 	}
+
+	@Test
+	public void testCanComplete() {
+		MethodParameter methodParameter = Utils.createMethodParameter(COMMAND_METHOD, 0);
+
+		CompletionContext context = new CompletionContext(Collections.emptyList(), 0, 0);
+		Stream<String> proposals = resolver.complete(methodParameter, context).stream().map(CompletionProposal::value);
+		assertThat(proposals).containsExactly("--name", "-n", "-level");
+
+		context = new CompletionContext(Arrays.asList("-n", "foo"), 0, 0);
+		proposals = resolver.complete(methodParameter, context).stream().map(CompletionProposal::value);
+		assertThat(proposals).containsExactly("-level");
+	}
+
+	@Test
+	public void testCannotComplete() {
+		MethodParameter methodParameter = Utils.createMethodParameter(COMMAND_METHOD, 0);
+
+		CompletionContext context = new CompletionContext(Arrays.asList("--name"), 0, 0);
+		Stream<String> proposals = resolver.complete(methodParameter, context).stream().map(CompletionProposal::value);
+		assertThat(proposals).isEmpty();
+	}
 }