feat(vertexai): add support for token-based usage metrics

packages/firebase_vertexai/firebase_vertexai/lib/src/api.dart

@@ -19,7 +19,8 @@ import 'schema.dart';
 /// Response for Count Tokens
 final class CountTokensResponse {
   /// Constructor
-  CountTokensResponse(this.totalTokens, {this.totalBillableCharacters});
+  CountTokensResponse(this.totalTokens,
+      {this.totalBillableCharacters, this.promptTokensDetails});
 
   /// The number of tokens that the `model` tokenizes the `prompt` into.
   ///
@@ -30,6 +31,9 @@ final class CountTokensResponse {
   ///
   /// Always non-negative.
   final int? totalBillableCharacters;
+
+  /// List of modalities that were processed in the request input.
+  final List<ModalityTokenCount>? promptTokensDetails;
 }
 
 /// Response from the model; supports multiple candidates.
@@ -128,11 +132,12 @@ final class PromptFeedback {
 /// Metadata on the generation request's token usage.
 final class UsageMetadata {
   /// Constructor
-  UsageMetadata._({
-    this.promptTokenCount,
-    this.candidatesTokenCount,
-    this.totalTokenCount,
-  });
+  UsageMetadata._(
+      {this.promptTokenCount,
+      this.candidatesTokenCount,
+      this.totalTokenCount,
+      this.promptTokensDetails,
+      this.candidatesTokensDetails});
 
   /// Number of tokens in the prompt.
   final int? promptTokenCount;
@@ -142,6 +147,12 @@ final class UsageMetadata {
 
   /// Total token count for the generation request (prompt + candidates).
   final int? totalTokenCount;
+
+  /// List of modalities that were processed in the request input.
+  final List<ModalityTokenCount>? promptTokensDetails;
+
+  /// List of modalities that were returned in the response.
+  final List<ModalityTokenCount>? candidatesTokensDetails;
 }
 
 /// Response candidate generated from a [GenerativeModel].
@@ -481,6 +492,62 @@ enum FinishReason {
   String toString() => name;
 }
 
+/// Represents token counting info for a single modality.
+final class ModalityTokenCount {
+  /// Constructor
+  ModalityTokenCount(this.modality, this.tokenCount);
+
+  /// The modality associated with this token count.
+  final ContentModality modality;
+
+  /// The number of tokens counted.
+  final int tokenCount;
+}
+
+/// Content part modality.
+enum ContentModality {
+  /// Unspecified modality.
+  unspecified('MODALITY_UNSPECIFIED'),
+
+  /// Plain text.
+  text('TEXT'),
+
+  /// Image.
+  image('IMAGE'),
+
+  /// Video.
+  video('VIDEO'),
+
+  /// Audio.
+  audio('AUDIO'),
+
+  /// Document, e.g. PDF.
+  document('DOCUMENT');
+
+  const ContentModality(this._jsonString);
+
+  static ContentModality _parseValue(Object jsonObject) {
+    return switch (jsonObject) {
+      'MODALITY_UNSPECIFIED' => ContentModality.unspecified,
+      'TEXT' => ContentModality.text,
+      'IMAGE' => ContentModality.image,
+      'video' => ContentModality.video,
+      'audio' => ContentModality.audio,
+      'document' => ContentModality.document,
+      _ =>
+        throw FormatException('Unhandled ContentModality format', jsonObject),
+    };
+  }
+
+  final String _jsonString;
+
+  @override
+  String toString() => name;
+
+  /// Convert to json format.
+  Object toJson() => _jsonString;
+}
+
 /// Safety setting, affecting the safety-blocking behavior.
 ///
 /// Passing a safety setting for a category changes the allowed probability that
@@ -696,16 +763,28 @@ GenerateContentResponse parseGenerateContentResponse(Object jsonObject) {
 /// Parse the json to [CountTokensResponse]
 CountTokensResponse parseCountTokensResponse(Object jsonObject) {
   if (jsonObject case {'error': final Object error}) throw parseError(error);
-  if (jsonObject case {'totalTokens': final int totalTokens}) {
-    if (jsonObject
-        case {'totalBillableCharacters': final int totalBillableCharacters}) {
-      return CountTokensResponse(totalTokens,
-          totalBillableCharacters: totalBillableCharacters);
-    } else {
-      return CountTokensResponse(totalTokens);
-    }
+
+  if (jsonObject is! Map) {
+    throw unhandledFormat('CountTokensResponse', jsonObject);
   }
-  throw unhandledFormat('CountTokensResponse', jsonObject);
+
+  final totalTokens = jsonObject['totalTokens'] as int;
+  final totalBillableCharacters = switch (jsonObject) {
+    {'totalBillableCharacters': final int totalBillableCharacters} =>
+      totalBillableCharacters,
+    _ => null,
+  };
+  final promptTokensDetails = switch (jsonObject) {
+    {'promptTokensDetails': final List<Object?> promptTokensDetails} =>
+      promptTokensDetails.map(_parseModalityTokenCount).toList(),
+    _ => null,
+  };
+
+  return CountTokensResponse(
+    totalTokens,
+    totalBillableCharacters: totalBillableCharacters,
+    promptTokensDetails: promptTokensDetails,
+  );
 }
 
 Candidate _parseCandidate(Object? jsonObject) {
@@ -777,10 +856,30 @@ UsageMetadata _parseUsageMetadata(Object jsonObject) {
     {'totalTokenCount': final int totalTokenCount} => totalTokenCount,
     _ => null,
   };
+  final promptTokensDetails = switch (jsonObject) {
+    {'promptTokensDetails': final List<Object?> promptTokensDetails} =>
+      promptTokensDetails.map(_parseModalityTokenCount).toList(),
+    _ => null,
+  };
+  final candidatesTokensDetails = switch (jsonObject) {
+    {'candidatesTokensDetails': final List<Object?> candidatesTokensDetails} =>
+      candidatesTokensDetails.map(_parseModalityTokenCount).toList(),
+    _ => null,
+  };
   return UsageMetadata._(
       promptTokenCount: promptTokenCount,
       candidatesTokenCount: candidatesTokenCount,
-      totalTokenCount: totalTokenCount);
+      totalTokenCount: totalTokenCount,
+      promptTokensDetails: promptTokensDetails,
+      candidatesTokensDetails: candidatesTokensDetails);
+}
+
+ModalityTokenCount _parseModalityTokenCount(Object? jsonObject) {
+  if (jsonObject is! Map) {
+    throw unhandledFormat('ModalityTokenCount', jsonObject);
+  }
+  return ModalityTokenCount(ContentModality._parseValue(jsonObject['modality']),
+      jsonObject['tokenCount'] as int);
 }
 
 SafetyRating _parseSafetyRating(Object? jsonObject) {

packages/firebase_vertexai/firebase_vertexai/test/response_parsing_test.dart

@@ -654,6 +654,81 @@ void main() {
       );
     });
 
+    test('response including usage metadata', () async {
+      const response = '''
+{
+  "candidates": [{
+    "content": {
+      "role": "model",
+      "parts": [{
+        "text": "Here is a description of the image:"
+      }]
+    },
+    "finishReason": "STOP"
+  }],
+  "usageMetadata": {
+    "promptTokenCount": 1837,
+    "candidatesTokenCount": 76,
+    "totalTokenCount": 1913,
+    "promptTokensDetails": [{
+      "modality": "TEXT",
+      "tokenCount": 76
+    }, {
+      "modality": "IMAGE",
+      "tokenCount": 1806
+    }],
+    "candidatesTokensDetails": [{
+      "modality": "TEXT",
+      "tokenCount": 76
+    }]
+  }
+}
+        ''';
+      final decoded = jsonDecode(response) as Object;
+      final generateContentResponse = parseGenerateContentResponse(decoded);
+      expect(
+          generateContentResponse.text, 'Here is a description of the image:');
+      expect(generateContentResponse.usageMetadata?.totalTokenCount, 1913);
+      expect(
+          generateContentResponse
+              .usageMetadata?.promptTokensDetails?[1].modality,
+          ContentModality.image);
+      expect(
+          generateContentResponse
+              .usageMetadata?.promptTokensDetails?[1].tokenCount,
+          1806);
+      expect(
+          generateContentResponse
+              .usageMetadata?.candidatesTokensDetails?.first.modality,
+          ContentModality.text);
+      expect(
+          generateContentResponse
+              .usageMetadata?.candidatesTokensDetails?.first.tokenCount,
+          76);
+    });
+
+    test('countTokens with modality fields returned', () async {
+      const response = '''
+{
+  "totalTokens": 1837,
+  "totalBillableCharacters": 117,
+  "promptTokensDetails": [{
+    "modality": "IMAGE",
+    "tokenCount": 1806
+  }, {
+    "modality": "TEXT",
+    "tokenCount": 31
+  }]
+}
+        ''';
+      final decoded = jsonDecode(response) as Object;
+      final countTokensResponse = parseCountTokensResponse(decoded);
+      expect(countTokensResponse.totalTokens, 1837);
+      expect(countTokensResponse.promptTokensDetails?.first.modality,
+          ContentModality.image);
+      expect(countTokensResponse.promptTokensDetails?.first.tokenCount, 1806);
+    });
+
     test('text getter joins content', () async {
       const response = '''
 {