feat(go): refactored formatters + added support for formatting streams

go/ai/action_test.go

@@ -136,13 +136,15 @@ func TestGenerateAction(t *testing.T) {
 					t.Fatalf("action failed: %v", err)
 				}
 
-				if diff := cmp.Diff(tc.ExpectChunks, chunks); diff != "" {
+				if diff := cmp.Diff(tc.ExpectChunks, chunks, cmp.Options{
+					cmpopts.IgnoreFields(ModelResponseChunk{}, "formatHandler"),
+				}); diff != "" {
 					t.Errorf("chunks mismatch (-want +got):\n%s", diff)
 				}
 
 				if diff := cmp.Diff(tc.ExpectResponse, resp, cmp.Options{
 					cmpopts.EquateEmpty(),
-					cmpopts.IgnoreFields(ModelResponse{}, "LatencyMs"),
+					cmpopts.IgnoreFields(ModelResponse{}, "LatencyMs", "formatHandler"),
 					cmpopts.IgnoreFields(GenerationUsage{}, "InputCharacters", "OutputCharacters"),
 					cmpopts.IgnoreFields(ToolDefinition{}, "Metadata"),
 				}); diff != "" {
@@ -156,7 +158,7 @@ func TestGenerateAction(t *testing.T) {
 
 				if diff := cmp.Diff(tc.ExpectResponse, resp, cmp.Options{
 					cmpopts.EquateEmpty(),
-					cmpopts.IgnoreFields(ModelResponse{}, "LatencyMs"),
+					cmpopts.IgnoreFields(ModelResponse{}, "LatencyMs", "formatHandler"),
 					cmpopts.IgnoreFields(GenerationUsage{}, "InputCharacters", "OutputCharacters"),
 					cmpopts.IgnoreFields(ToolDefinition{}, "Metadata"),
 				}); diff != "" {

go/ai/format_array.go

@@ -16,9 +16,7 @@ package ai
 
 import (
 	"encoding/json"
-	"errors"
 	"fmt"
-	"strings"
 
 	"github.com/firebase/genkit/go/internal/base"
 )
@@ -45,6 +43,7 @@ func (a arrayFormatter) Handler(schema map[string]any) (FormatHandler, error) {
 	handler := &arrayHandler{
 		instructions: instructions,
 		config: ModelOutputConfig{
+			Constrained: true,
 			Format:      OutputFormatArray,
 			Schema:      schema,
 			ContentType: "application/json",
@@ -55,58 +54,49 @@ func (a arrayFormatter) Handler(schema map[string]any) (FormatHandler, error) {
 }
 
 type arrayHandler struct {
-	instructions string
-	config       ModelOutputConfig
+	instructions    string
+	config          ModelOutputConfig
+	accumulatedText string
+	currentIndex    int
+	cursor          int
 }
 
 // Instructions returns the instructions for the formatter.
-func (a arrayHandler) Instructions() string {
+func (a *arrayHandler) Instructions() string {
 	return a.instructions
 }
 
 // Config returns the output config for the formatter.
-func (a arrayHandler) Config() ModelOutputConfig {
+func (a *arrayHandler) Config() ModelOutputConfig {
 	return a.config
 }
 
-// ParseMessage parses the message and returns the formatted message.
-func (a arrayHandler) ParseMessage(m *Message) (*Message, error) {
-	if a.config.Format == OutputFormatArray {
-		if m == nil {
-			return nil, errors.New("message is empty")
-		}
-		if len(m.Content) == 0 {
-			return nil, errors.New("message has no content")
-		}
-
-		var nonTextParts []*Part
-		accumulatedText := strings.Builder{}
+// ParseOutput parses the final message and returns the parsed array.
+func (a *arrayHandler) ParseOutput(m *Message) (any, error) {
+	result := base.ExtractItems(m.Text(), 0)
+	return result.Items, nil
+}
 
-		for _, part := range m.Content {
-			if !part.IsText() {
-				nonTextParts = append(nonTextParts, part)
-			} else {
-				accumulatedText.WriteString(part.Text)
-			}
-		}
+// ParseChunk processes a streaming chunk and returns parsed output.
+func (a *arrayHandler) ParseChunk(chunk *ModelResponseChunk) (any, error) {
+	if chunk.Index != a.currentIndex {
+		a.accumulatedText = ""
+		a.currentIndex = chunk.Index
+		a.cursor = 0
+	}
 
-		var newParts []*Part
-		lines := base.GetJSONObjectLines(accumulatedText.String())
-		for _, line := range lines {
-			var schemaBytes []byte
-			schemaBytes, err := json.Marshal(a.config.Schema["items"])
-			if err != nil {
-				return nil, fmt.Errorf("expected schema is not valid: %w", err)
-			}
-			if err = base.ValidateRaw([]byte(line), schemaBytes); err != nil {
-				return nil, err
-			}
-
-			newParts = append(newParts, NewJSONPart(line))
+	for _, part := range chunk.Content {
+		if part.IsText() {
+			a.accumulatedText += part.Text
 		}
-
-		m.Content = append(newParts, nonTextParts...)
 	}
 
+	result := base.ExtractItems(a.accumulatedText, a.cursor)
+	a.cursor = result.Cursor
+	return result.Items, nil
+}
+
+// ParseMessage parses the message and returns the formatted message.
+func (a *arrayHandler) ParseMessage(m *Message) (*Message, error) {
 	return m, nil
 }

go/ai/format_enum.go

@@ -20,6 +20,8 @@ import (
 	"regexp"
 	"slices"
 	"strings"
+
+	"github.com/firebase/genkit/go/core"
 )
 
 type enumFormatter struct{}
@@ -33,14 +35,15 @@ func (e enumFormatter) Name() string {
 func (e enumFormatter) Handler(schema map[string]any) (FormatHandler, error) {
 	enums := objectEnums(schema)
 	if schema == nil || len(enums) == 0 {
-		return nil, fmt.Errorf("schema is not valid JSON enum")
+		return nil, core.NewError(core.INVALID_ARGUMENT, "schema must be an object with an 'enum' property for enum format")
 	}
 
 	instructions := fmt.Sprintf("Output should be ONLY one of the following enum values. Do not output any additional information or add quotes.\n\n```%s```", strings.Join(enums, "\n"))
 
 	handler := &enumHandler{
 		instructions: instructions,
 		config: ModelOutputConfig{
+			Constrained: true,
 			Format:      OutputFormatEnum,
 			Schema:      schema,
 			ContentType: "text/enum",
@@ -52,23 +55,49 @@ func (e enumFormatter) Handler(schema map[string]any) (FormatHandler, error) {
 }
 
 type enumHandler struct {
-	instructions string
-	config       ModelOutputConfig
-	enums        []string
+	instructions    string
+	config          ModelOutputConfig
+	enums           []string
+	accumulatedText string
+	currentIndex    int
 }
 
 // Instructions returns the instructions for the formatter.
-func (e enumHandler) Instructions() string {
+func (e *enumHandler) Instructions() string {
 	return e.instructions
 }
 
 // Config returns the output config for the formatter.
-func (e enumHandler) Config() ModelOutputConfig {
+func (e *enumHandler) Config() ModelOutputConfig {
 	return e.config
 }
 
+// ParseOutput parses the final message and returns the enum value.
+func (e *enumHandler) ParseOutput(m *Message) (any, error) {
+	return e.parseEnum(m.Text())
+}
+
+// ParseChunk processes a streaming chunk and returns parsed output.
+func (e *enumHandler) ParseChunk(chunk *ModelResponseChunk) (any, error) {
+	if chunk.Index != e.currentIndex {
+		e.accumulatedText = ""
+		e.currentIndex = chunk.Index
+	}
+
+	for _, part := range chunk.Content {
+		if part.IsText() {
+			e.accumulatedText += part.Text
+		}
+	}
+
+	// Ignore error since we are doing best effort parsing.
+	enum, _ := e.parseEnum(e.accumulatedText)
+
+	return enum, nil
+}
+
 // ParseMessage parses the message and returns the formatted message.
-func (e enumHandler) ParseMessage(m *Message) (*Message, error) {
+func (e *enumHandler) ParseMessage(m *Message) (*Message, error) {
 	if e.config.Format == OutputFormatEnum {
 		if m == nil {
 			return nil, errors.New("message is empty")
@@ -107,23 +136,63 @@ func (e enumHandler) ParseMessage(m *Message) (*Message, error) {
 	return m, nil
 }
 
-// Get enum strings from json schema
+// Get enum strings from json schema.
+// Supports both top-level enum (e.g. {"type": "string", "enum": ["a", "b"]})
+// and nested property enum (e.g. {"properties": {"value": {"enum": ["a", "b"]}}}).
 func objectEnums(schema map[string]any) []string {
-	var enums []string
+	if enums := extractEnumStrings(schema["enum"]); len(enums) > 0 {
+		return enums
+	}
 
 	if properties, ok := schema["properties"].(map[string]any); ok {
 		for _, propValue := range properties {
 			if propMap, ok := propValue.(map[string]any); ok {
-				if enumSlice, ok := propMap["enum"].([]any); ok {
-					for _, enumVal := range enumSlice {
-						if enumStr, ok := enumVal.(string); ok {
-							enums = append(enums, enumStr)
-						}
-					}
+				if enums := extractEnumStrings(propMap["enum"]); len(enums) > 0 {
+					return enums
 				}
 			}
 		}
 	}
 
-	return enums
+	return nil
+}
+
+// Extracts string values from an enum field, supporting both []any (from JSON) and []string (from Go code).
+func extractEnumStrings(v any) []string {
+	if v == nil {
+		return nil
+	}
+
+	if strs, ok := v.([]string); ok {
+		return strs
+	}
+
+	if slice, ok := v.([]any); ok {
+		enums := make([]string, 0, len(slice))
+		for _, val := range slice {
+			if s, ok := val.(string); ok {
+				enums = append(enums, s)
+			}
+		}
+		return enums
+	}
+
+	return nil
+}
+
+// parseEnum is the shared parsing logic used by both ParseOutput and ParseChunk.
+func (e *enumHandler) parseEnum(text string) (string, error) {
+	if text == "" {
+		return "", nil
+	}
+
+	re := regexp.MustCompile(`['"]`)
+	clean := re.ReplaceAllString(text, "")
+	trimmed := strings.TrimSpace(clean)
+
+	if !slices.Contains(e.enums, trimmed) {
+		return "", fmt.Errorf("message %s not in list of valid enums: %s", trimmed, strings.Join(e.enums, ", "))
+	}
+
+	return trimmed, nil
 }