Fix polyglot AppHost builder and callback generation

src/Aspire.Hosting.CodeGeneration.Go/AtsGoCodeGenerator.cs

@@ -1401,6 +1401,28 @@ private void EmitCallbackRegistration(string indent, AtsParameterInfo p, string
             // Legacy untyped callback returning any — preserve return value.
             WriteLine($"{indent}\t\treturn {callExpr}");
         }
+        else if (p.CallbackParameters is { Count: > 0 } callbackParameters && callbackParameters.Any(cp => cp.Type.Category == AtsTypeCategory.Dto))
+        {
+            var argNames = new List<string>(callbackParameters.Count);
+            for (var i = 0; i < callbackParameters.Count; i++)
+            {
+                var argName = $"arg{i}";
+                argNames.Add(argName);
+                var goType = MapTypeRefToGo(callbackParameters[i].Type, false);
+                WriteLine($"{indent}\t\t{argName} := callbackArg[{goType}](args, {i})");
+            }
+
+            WriteLine($"{indent}\t\tcb({string.Join(", ", argNames)})");
+            WriteLine($"{indent}\t\treturn map[string]any{{");
+            for (var i = 0; i < callbackParameters.Count; i++)
+            {
+                if (callbackParameters[i].Type.Category == AtsTypeCategory.Dto)
+                {
+                    WriteLine($"{indent}\t\t\t\"p{i}\": serializeValue({argNames[i]}),");
+                }
+            }
+            WriteLine($"{indent}\t\t}}");
+        }
         else
         {
             WriteLine($"{indent}\t\t{callExpr}");
@@ -1840,9 +1862,15 @@ private void GenerateCreateBuilder(AtsContext context)
             WriteLine("\t}");
         }
         WriteLine("\tif _, ok := resolved[\"Args\"]; !ok { resolved[\"Args\"] = os.Args[1:] }");
-        WriteLine("\tif _, ok := resolved[\"ProjectDirectory\"]; !ok {");
+        WriteLine("\tif projectDirectory, ok := resolved[\"ProjectDirectory\"].(string); !ok || projectDirectory == \"\" {");
         WriteLine("\t\tif pwd, err := os.Getwd(); err == nil { resolved[\"ProjectDirectory\"] = pwd }");
         WriteLine("\t}");
+        WriteLine("\tif appHostFilePath, ok := resolved[\"AppHostFilePath\"].(string); !ok || appHostFilePath == \"\" {");
+        WriteLine("\t\tif appHostFilePath := os.Getenv(\"ASPIRE_APPHOST_FILEPATH\"); appHostFilePath != \"\" { resolved[\"AppHostFilePath\"] = appHostFilePath }");
+        WriteLine("\t}");
+        WriteLine("\tif dashboardApplicationName, ok := resolved[\"DashboardApplicationName\"].(string); ok && dashboardApplicationName == \"\" {");
+        WriteLine("\t\tdelete(resolved, \"DashboardApplicationName\")");
+        WriteLine("\t}");
         WriteLine();
         WriteLine($"\tresult, err := c.invokeCapability(context.Background(), \"{AtsConstants.CreateBuilderCapability}\", map[string]any{{\"argsOrOptions\": resolved}})");
         WriteLine("\tif err != nil { return nil, err }");

src/Aspire.Hosting.CodeGeneration.Go/Resources/base.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
+	"strings"
 	"sync"
 )
 
@@ -712,11 +713,71 @@ func decodeAs[T any](raw any) (T, error) {
 	}
 	var out T
 	if err := json.Unmarshal(bytes, &out); err != nil {
+		if decoded, ok := decodeStructFields[T](raw); ok {
+			return decoded, nil
+		}
 		return zero, err
 	}
 	return out, nil
 }
 
+func decodeStructFields[T any](raw any) (T, bool) {
+	var zero T
+	rawMap, ok := raw.(map[string]any)
+	if !ok {
+		return zero, false
+	}
+
+	targetType := reflect.TypeOf((*T)(nil)).Elem()
+	isPointer := targetType.Kind() == reflect.Ptr
+	if isPointer {
+		targetType = targetType.Elem()
+	}
+	if targetType.Kind() != reflect.Struct {
+		return zero, false
+	}
+
+	targetValue := reflect.New(targetType)
+	structValue := targetValue.Elem()
+	for i := 0; i < targetType.NumField(); i++ {
+		fieldInfo := targetType.Field(i)
+		fieldValue := structValue.Field(i)
+		if !fieldValue.CanSet() {
+			continue
+		}
+
+		fieldName := fieldInfo.Name
+		if tag := fieldInfo.Tag.Get("json"); tag != "" {
+			name, _, _ := strings.Cut(tag, ",")
+			if name == "-" {
+				continue
+			}
+			if name != "" {
+				fieldName = name
+			}
+		}
+
+		rawFieldValue, ok := rawMap[fieldName]
+		if !ok {
+			continue
+		}
+
+		bytes, err := json.Marshal(rawFieldValue)
+		if err != nil {
+			continue
+		}
+		if err := json.Unmarshal(bytes, fieldValue.Addr().Interface()); err != nil {
+			continue
+		}
+	}
+
+	if isPointer {
+		return targetValue.Interface().(T), true
+	}
+
+	return structValue.Interface().(T), true
+}
+
 // ── deepUpdate ───────────────────────────────────────────────────────────────
 //
 // Used by generated code to merge variadic Options structs:

src/Aspire.Hosting.CodeGeneration.Java/AtsJavaCodeGenerator.cs

@@ -527,7 +527,7 @@ private void GenerateDtoTypes(IReadOnlyList<AtsDtoTypeInfo> dtoTypes)
 
             var dtoName = _dtoNames[dto.TypeId];
             WriteLine($"/** {dto.Name} DTO. */");
-            WriteLine($"class {dtoName} {{");
+            WriteLine($"class {dtoName} implements JsonSerializable {{");
 
             // Fields
             foreach (var property in dto.Properties)

src/Aspire.Hosting.CodeGeneration.Java/Resources/Transport.java

@@ -80,6 +80,10 @@ void onCancel(Runnable listener) {
     }
 }
 
+interface JsonSerializable {
+    Map<String, Object> toMap();
+}
+
 /**
  * AspireClient handles JSON-RPC communication with the AppHost server.
  */
@@ -314,7 +318,7 @@ private String readLine() throws IOException {
     private void handleServerRequest(Map<String, Object> request) throws IOException {
         String method = (String) request.get("method");
         Object idObj = request.get("id");
-        Map<String, Object> params = (Map<String, Object>) request.get("params");
+        Object params = request.get("params");
 
         debug("Received server request: " + method);
 
@@ -323,25 +327,33 @@ private void handleServerRequest(Map<String, Object> request) throws IOException
 
         try {
             if ("invokeCallback".equals(method)) {
-                String callbackId = (String) params.get("callbackId");
-                List<Object> args = (List<Object>) params.get("args");
-
-                Function<Object[], Object> callback = callbacks.get(callbackId);
-                if (callback != null) {
-                    Object[] unwrappedArgs = args.stream()
-                        .map(this::unwrapResult)
-                        .toArray();
-                    result = awaitValue(callback.apply(unwrappedArgs));
+                String callbackId = getCallbackId(params);
+                if (callbackId == null) {
+                    error = createError(-32602, "Invalid params: callbackId is required.");
                 } else {
-                    error = createError(-32601, "Callback not found: " + callbackId);
+                    List<Object> args = getCallbackArgs(params);
+
+                    Function<Object[], Object> callback = callbacks.get(callbackId);
+                    if (callback != null) {
+                        Object[] unwrappedArgs = args.stream()
+                            .map(this::unwrapResult)
+                            .toArray();
+                        result = awaitValue(callback.apply(unwrappedArgs));
+                    } else {
+                        error = createError(-32601, "Callback not found: " + callbackId);
+                    }
                 }
             } else if ("cancel".equals(method)) {
-                String cancellationId = (String) params.get("cancellationId");
-                Consumer<Void> handler = cancellations.get(cancellationId);
-                if (handler != null) {
-                    handler.accept(null);
+                String cancellationId = getCancellationId(params);
+                if (cancellationId == null) {
+                    error = createError(-32602, "Invalid params: cancellationId is required.");
+                } else {
+                    Consumer<Void> handler = cancellations.get(cancellationId);
+                    if (handler != null) {
+                        handler.accept(null);
+                    }
+                    result = true;
                 }
-                result = true;
             } else {
                 error = createError(-32601, "Unknown method: " + method);
             }
@@ -362,6 +374,64 @@ private void handleServerRequest(Map<String, Object> request) throws IOException
         sendMessage(response);
     }
 
+    @SuppressWarnings("unchecked")
+    private String getCallbackId(Object params) {
+        if (params instanceof List<?> list && !list.isEmpty()) {
+            return asString(list.get(0));
+        }
+
+        if (params instanceof Map<?, ?> map) {
+            return asString(map.get("callbackId"));
+        }
+
+        return null;
+    }
+
+    @SuppressWarnings("unchecked")
+    private List<Object> getCallbackArgs(Object params) {
+        Object args = null;
+        if (params instanceof List<?> list && list.size() > 1) {
+            args = list.get(1);
+        } else if (params instanceof Map<?, ?> map) {
+            args = map.get("args");
+        }
+
+        if (args instanceof Map<?, ?> map) {
+            List<Object> positionalArgs = new ArrayList<>();
+            for (var i = 0; ; i++) {
+                var key = "p" + i;
+                if (map.containsKey(key)) {
+                    positionalArgs.add(map.get(key));
+                } else {
+                    break;
+                }
+            }
+            return positionalArgs;
+        }
+
+        if (args instanceof List<?> list) {
+            return (List<Object>) list;
+        }
+
+        return args == null ? List.of() : List.of(args);
+    }
+
+    private String getCancellationId(Object params) {
+        if (params instanceof List<?> list && !list.isEmpty()) {
+            return asString(list.get(0));
+        }
+
+        if (params instanceof Map<?, ?> map) {
+            return asString(map.get("cancellationId"));
+        }
+
+        return null;
+    }
+
+    private String asString(Object value) {
+        return value instanceof String string ? string : null;
+    }
+
     private Map<String, Object> createError(int code, String message) {
         Map<String, Object> error = new HashMap<>();
         error.put("code", code);
@@ -462,6 +532,9 @@ public static Object serializeValue(Object value) {
         if (value instanceof AspireUnion union) {
             return serializeValue(union.getValue());
         }
+        if (value instanceof JsonSerializable jsonSerializable) {
+            return jsonSerializable.toMap();
+        }
         if (value instanceof Map) {
             @SuppressWarnings("unchecked")
             Map<String, Object> map = (Map<String, Object>) value;

src/Aspire.Hosting.CodeGeneration.Python/AtsPythonCodeGenerator.cs

@@ -4,6 +4,7 @@
 using System.Globalization;
 using System.Reflection;
 using System.Text;
+using System.Text.Json;
 using System.Text.Json.Nodes;
 using Aspire.TypeSystem;
 
@@ -933,23 +934,7 @@ private static string ToSnakeCase(string name)
             return name;
         }
 
-        var result = new System.Text.StringBuilder();
-        result.Append(char.ToLowerInvariant(name[0]));
-
-        for (int i = 1; i < name.Length; i++)
-        {
-            var c = name[i];
-            if (char.IsUpper(c))
-            {
-                result.Append('_');
-                result.Append(char.ToLowerInvariant(c));
-            }
-            else
-            {
-                result.Append(c);
-            }
-        }
-        var resultStr = result.ToString();
+        var resultStr = JsonNamingPolicy.SnakeCaseLower.ConvertName(name);
         resultStr = resultStr.Replace("environment", "env");
         resultStr = resultStr.Replace("configuration", "config");
         resultStr = resultStr.Replace("application", "app");

src/Aspire.Hosting.CodeGeneration.Python/PythonModuleBuilder.cs

@@ -1795,6 +1795,7 @@ def create_builder(
             *,
             args: typing.Iterable[str] | None = None,
             project_directory: str | None = None,
+            app_host_file_path: str | None = None,
             container_registry_override: str | None = None,
             disable_dashboard: bool | None = None,
             dashboard_application_name: str | None = None,
@@ -1813,6 +1814,8 @@ def create_builder(
                     passed to the Aspire command line (arguments specified after '--'). Specifying them here will override that default.
                 project_directory (str): The directory containing the AppHost project file. By default, this will  use the ASPIRE_PROJECT_DIRECTORY
                     environment variable if set, otherwise it will use the current working directory.
+                app_host_file_path (str): The path to the AppHost source file. By default, this will use the ASPIRE_APPHOST_FILEPATH
+                    environment variable if set.
                 container_registry_override (str): When containers are used, use this value to override the container registry.
                 disable_dashboard (bool): Determines whether the dashboard is disabled.
                 dashboard_application_name (str): The application name to display in the dashboard.
@@ -1842,6 +1845,12 @@ elif not effective_options.get('Args'):
                 effective_options['ProjectDirectory'] = project_directory
             elif not effective_options.get('ProjectDirectory'):
                 effective_options['ProjectDirectory'] = os.environ.get('ASPIRE_PROJECT_DIRECTORY', os.getcwd())
+            if app_host_file_path is not None:
+                effective_options['AppHostFilePath'] = app_host_file_path
+            elif not effective_options.get('AppHostFilePath'):
+                app_host_file_path = os.environ.get('ASPIRE_APPHOST_FILEPATH')
+                if app_host_file_path:
+                    effective_options['AppHostFilePath'] = app_host_file_path
             if container_registry_override is not None:
                 effective_options['ContainerRegistryOverride'] = container_registry_override
             if disable_dashboard is not None:

src/Aspire.Hosting.RemoteHost/Ats/AtsCallbackProxyFactory.cs

@@ -157,7 +157,8 @@ private Expression BuildMarshalArgs(ParameterExpression[] paramExprs, ParameterI
             var marshalCall = Expression.Call(
                 Expression.Constant(this),
                 marshalMethod,
-                Expression.Convert(paramExpr, typeof(object)));
+                Expression.Convert(paramExpr, typeof(object)),
+                Expression.Constant(param.ParameterType, typeof(Type)));
 
             // Use positional key (p0, p1, p2, ...) instead of param.Name
             var addCall = Expression.Call(jsonObjVar, addMethod!, Expression.Constant($"p{paramIndex}"), marshalCall);
@@ -169,9 +170,9 @@ private Expression BuildMarshalArgs(ParameterExpression[] paramExprs, ParameterI
         return Expression.Block(new[] { jsonObjVar }, expressions);
     }
 
-    private JsonNode? MarshalArg(object? value)
+    private JsonNode? MarshalArg(object? value, Type declaredType)
     {
-        return _marshaller.MarshalToJson(value);
+        return _marshaller.MarshalToJson(value, declaredType);
     }
 
     private Expression BuildSyncVoidCall(string callbackId, Expression? argsExpr, Expression? ctExpr, int ctParamIndex)