Report malformed request bodies as 400 INVALID_REQUEST with jackson3

Fixes #1166

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

Author: Kehrlann

mcp-json-jackson3/src/main/java/io/modelcontextprotocol/json/jackson3/JacksonMcpJsonMapper.java

@@ -87,13 +87,23 @@ public <T> T readValue(byte[] content, TypeRef<T> type) throws IOException {
 
 	@Override
 	public <T> T convertValue(Object fromValue, Class<T> type) {
-		return jsonMapper.convertValue(fromValue, type);
+		try {
+			return jsonMapper.convertValue(fromValue, type);
+		}
+		catch (JacksonException ex) {
+			throw new IllegalArgumentException(ex);
+		}
 	}
 
 	@Override
 	public <T> T convertValue(Object fromValue, TypeRef<T> type) {
 		JavaType javaType = jsonMapper.getTypeFactory().constructType(type.getType());
-		return jsonMapper.convertValue(fromValue, javaType);
+		try {
+			return jsonMapper.convertValue(fromValue, javaType);
+		}
+		catch (JacksonException ex) {
+			throw new IllegalArgumentException(ex);
+		}
 	}
 
 	@Override

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

@@ -951,6 +951,26 @@ void rejectsNonJsonContentType(String contentType) throws Exception {
 		assertThat(toolCalled).isFalse();
 	}
 
+	@Test
+	void rejectsMalformedMessageAsInvalidRequest() throws Exception {
+		McpServer.sync(mcpStatelessServerTransport).build();
+
+		// Valid JSON, but "jsonrpc" is an object instead of a string, so it cannot be
+		// converted into a JSONRPCRequest
+		var request = HttpRequest.newBuilder()
+			.uri(URI.create("http://localhost:" + PORT + CUSTOM_MESSAGE_ENDPOINT))
+			.header("Content-Type", APPLICATION_JSON)
+			.header("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
+			.POST(HttpRequest.BodyPublishers.ofString("""
+					{"jsonrpc":{"a":1},"id":1,"method":"tools/list"}"""))
+			.build();
+
+		var response = HttpClient.newHttpClient().send(request, HttpResponse.BodyHandlers.ofString());
+
+		assertThat(response.statusCode()).isEqualTo(HttpServletResponse.SC_BAD_REQUEST);
+		assertThatJson(response.body()).inPath("message").isEqualTo("Invalid message format");
+	}
+
 	private double evaluateExpression(String expression) {
 		// Simple expression evaluator for testing
 		return switch (expression) {