Reject listRoots when client lacks roots capability

Fail fast with an IllegalStateException instead of sending a roots/list
request the client cannot handle, matching the existing guards in
createMessage and createElicitation.

Fixes #1067

Author: nikita-kibitkin

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

@@ -225,6 +225,13 @@ public Mono<McpSchema.ListRootsResult> listRoots() {
 	 * @return A Mono that emits the list of roots result containing
 	 */
 	public Mono<McpSchema.ListRootsResult> listRoots(String cursor) {
+		if (this.clientCapabilities == null) {
+			return Mono
+				.error(new IllegalStateException("Client must be initialized. Call the initialize method first!"));
+		}
+		if (this.clientCapabilities.roots() == null) {
+			return Mono.error(new IllegalStateException("Client must be configured with roots capabilities"));
+		}
 		return this.session.sendRequest(McpSchema.METHOD_ROOTS_LIST, new McpSchema.PaginatedRequest(cursor),
 				LIST_ROOTS_RESULT_TYPE_REF);
 	}

mcp-core/src/test/java/io/modelcontextprotocol/server/McpAsyncServerExchangeTests.java

@@ -170,6 +170,40 @@ void testListRootsWithError() {
 		});
 	}
 
+	@Test
+	void testListRootsWithNullCapabilities() {
+		// Given - Create exchange with null capabilities
+		McpAsyncServerExchange exchangeWithNullCapabilities = new McpAsyncServerExchange("testSessionId", mockSession,
+				null, clientInfo, McpTransportContext.EMPTY);
+
+		StepVerifier.create(exchangeWithNullCapabilities.listRoots()).verifyErrorSatisfies(error -> {
+			assertThat(error).isInstanceOf(IllegalStateException.class)
+				.hasMessage("Client must be initialized. Call the initialize method first!");
+		});
+
+		// Verify that sendRequest was never called due to null capabilities
+		verify(mockSession, never()).sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), any(), any(TypeRef.class));
+	}
+
+	@Test
+	void testListRootsWithoutRootsCapabilities() {
+		// Given - Create exchange without roots capabilities
+		McpSchema.ClientCapabilities capabilitiesWithoutRoots = McpSchema.ClientCapabilities.builder()
+			.sampling()
+			.build();
+
+		McpAsyncServerExchange exchangeWithoutRoots = new McpAsyncServerExchange("testSessionId", mockSession,
+				capabilitiesWithoutRoots, clientInfo, McpTransportContext.EMPTY);
+
+		StepVerifier.create(exchangeWithoutRoots.listRoots()).verifyErrorSatisfies(error -> {
+			assertThat(error).isInstanceOf(IllegalStateException.class)
+				.hasMessage("Client must be configured with roots capabilities");
+		});
+
+		// Verify that sendRequest was never called due to missing roots capabilities
+		verify(mockSession, never()).sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), any(), any(TypeRef.class));
+	}
+
 	@Test
 	void testListRootsUnmodifiabilityAfterAccumulation() {
 

mcp-core/src/test/java/io/modelcontextprotocol/server/McpSyncServerExchangeTests.java

@@ -169,6 +169,39 @@ void testListRootsWithError() {
 		assertThatThrownBy(() -> exchange.listRoots()).isInstanceOf(RuntimeException.class).hasMessage("Network error");
 	}
 
+	@Test
+	void testListRootsWithNullCapabilities() {
+		// Given - Create exchange with null capabilities
+		McpAsyncServerExchange asyncExchangeWithNullCapabilities = new McpAsyncServerExchange("testSessionId",
+				mockSession, null, clientInfo, McpTransportContext.EMPTY);
+		McpSyncServerExchange exchangeWithNullCapabilities = new McpSyncServerExchange(
+				asyncExchangeWithNullCapabilities);
+
+		assertThatThrownBy(() -> exchangeWithNullCapabilities.listRoots()).isInstanceOf(IllegalStateException.class)
+			.hasMessage("Client must be initialized. Call the initialize method first!");
+
+		// Verify that sendRequest was never called due to null capabilities
+		verify(mockSession, never()).sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), any(), any(TypeRef.class));
+	}
+
+	@Test
+	void testListRootsWithoutRootsCapabilities() {
+		// Given - Create exchange without roots capabilities
+		McpSchema.ClientCapabilities capabilitiesWithoutRoots = McpSchema.ClientCapabilities.builder()
+			.sampling()
+			.build();
+
+		McpAsyncServerExchange asyncExchangeWithoutRoots = new McpAsyncServerExchange("testSessionId", mockSession,
+				capabilitiesWithoutRoots, clientInfo, McpTransportContext.EMPTY);
+		McpSyncServerExchange exchangeWithoutRoots = new McpSyncServerExchange(asyncExchangeWithoutRoots);
+
+		assertThatThrownBy(() -> exchangeWithoutRoots.listRoots()).isInstanceOf(IllegalStateException.class)
+			.hasMessage("Client must be configured with roots capabilities");
+
+		// Verify that sendRequest was never called due to missing roots capabilities
+		verify(mockSession, never()).sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), any(), any(TypeRef.class));
+	}
+
 	@Test
 	void testListRootsUnmodifiabilityAfterAccumulation() {
 

mcp-test/src/main/java/io/modelcontextprotocol/AbstractMcpClientServerIntegrationTests.java

@@ -1056,12 +1056,10 @@ void testRootsWithoutCapability() {
 			assertThat(mcpClient.initialize()).isNotNull();
 
 			// Attempt to list roots should fail
-			try {
-				mcpClient.callTool(McpSchema.CallToolRequest.builder("tool1").arguments(Map.of()).build());
-			}
-			catch (McpError e) {
-				assertThat(e).isInstanceOf(McpError.class).hasMessage("Roots not supported");
-			}
+			assertThatThrownBy(
+					() -> mcpClient.callTool(McpSchema.CallToolRequest.builder("tool1").arguments(Map.of()).build()))
+				.isInstanceOf(McpError.class)
+				.hasMessage("Client must be configured with roots capabilities");
 		}
 		finally {
 			mcpServer.closeGracefully();