Improved handling of FinishReason to avoid deserialization errors when Google introduces new values

src/GenerativeAI/Core/ResponseHelper.cs

@@ -73,6 +73,13 @@ internal static string FormatErrorMessage(FinishReason response)
             FinishReason.SPII => "The generation stopped because the content might include Sensitive Personally Identifiable Information (SPII).",
             FinishReason.MALFORMED_FUNCTION_CALL => "The generation stopped because a malformed or invalid function call was generated by the model.",
             FinishReason.IMAGE_SAFETY => "The generation stopped because the generated images were flagged for containing safety violations.",
+            FinishReason.MODEL_ARMOR => "The generation stopped due to model armor protection mechanisms.",
+            FinishReason.IMAGE_PROHIBITED_CONTENT => "Image generation stopped because the generated images contain prohibited content.",
+            FinishReason.IMAGE_RECITATION => "Image generation stopped because the generated images were flagged for recitation.",
+            FinishReason.IMAGE_OTHER => "Image generation stopped due to other miscellaneous issues.",
+            FinishReason.UNEXPECTED_TOOL_CALL => "The generation stopped because the model generated a tool call but no tools were enabled in the request.",
+            FinishReason.NO_IMAGE => "The generation stopped because the model was expected to generate an image, but none was generated.",
+            FinishReason.TOO_MANY_TOOL_CALLS => "The generation stopped because the model called too many tools consecutively.",
             _ => "The generation stopped for an unexpected or unhandled reason."
         };
     }

src/GenerativeAI/Types/ContentGeneration/Outputs/FinishReason.cs

@@ -1,12 +1,18 @@
 using System.Text.Json.Serialization;
+using GenerativeAI.Types.Converters;
 
 namespace GenerativeAI.Types;
 
 /// <summary>
 /// Defines the reason why the model stopped generating tokens.
 /// </summary>
+/// <remarks>
+/// This enum uses a lenient JSON converter that gracefully handles unknown values
+/// by falling back to <see cref="OTHER"/> instead of throwing an exception.
+/// This prevents crashes when Google adds new FinishReason values to their API.
+/// </remarks>
 /// <seealso href="https://ai.google.dev/api/generate-content#FinishReason">See Official API Documentation</seealso>
-[JsonConverter(typeof(JsonStringEnumConverter<FinishReason>))]
+[JsonConverter(typeof(LenientFinishReasonConverter))]
 public enum FinishReason
 {
     /// <summary>
@@ -69,4 +75,46 @@ public enum FinishReason
     /// Token generation stopped because generated images contain safety violations.
     /// </summary>
     IMAGE_SAFETY = 11,
+
+    /// <summary>
+    /// Token generation stopped due to model armor protection mechanisms.
+    /// Documented in Vertex AI (google.cloud.aiplatform.v1).
+    /// </summary>
+    MODEL_ARMOR = 12,
+
+    /// <summary>
+    /// Image generation stopped because generated images have prohibited content.
+    /// Documented in Vertex AI (google.cloud.aiplatform.v1).
+    /// </summary>
+    IMAGE_PROHIBITED_CONTENT = 13,
+
+    /// <summary>
+    /// Image generation stopped because generated images were flagged for recitation.
+    /// Documented in Vertex AI (google.cloud.aiplatform.v1).
+    /// </summary>
+    IMAGE_RECITATION = 14,
+
+    /// <summary>
+    /// Image generation stopped because of other miscellaneous issues.
+    /// Documented in Vertex AI (google.cloud.aiplatform.v1).
+    /// </summary>
+    IMAGE_OTHER = 15,
+
+    /// <summary>
+    /// Model generated a tool call but no tools were enabled in the request.
+    /// Documented in Gemini API and Vertex AI.
+    /// </summary>
+    UNEXPECTED_TOOL_CALL = 16,
+
+    /// <summary>
+    /// The model was expected to generate an image, but none was generated.
+    /// Documented in Vertex AI (google.cloud.aiplatform.v1).
+    /// </summary>
+    NO_IMAGE = 17,
+
+    /// <summary>
+    /// Model called too many tools consecutively, thus the system exited execution.
+    /// Documented in generativelanguagepb (SDK/proto contract).
+    /// </summary>
+    TOO_MANY_TOOL_CALLS = 18,
 }

src/GenerativeAI/Types/Converters/LenientFinishReasonConverter.cs

@@ -0,0 +1,45 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace GenerativeAI.Types.Converters;
+
+/// <summary>
+/// Lenient JSON converter for FinishReason that handles unknown enum values gracefully.
+/// When an unknown string value is encountered, it falls back to FinishReason.OTHER
+/// instead of throwing an exception. This prevents crashes when Google adds new
+/// FinishReason values to their API.
+/// </summary>
+public class LenientFinishReasonConverter : JsonConverter<FinishReason>
+{
+    /// <summary>
+    /// Reads and converts the JSON to FinishReason.
+    /// </summary>
+    public override FinishReason Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType != JsonTokenType.String)
+        {
+            throw new JsonException($"Expected string value for FinishReason, got {reader.TokenType}");
+        }
+
+        var value = reader.GetString();
+        if (string.IsNullOrEmpty(value))
+        {
+            return FinishReason.FINISH_REASON_UNSPECIFIED;
+        }
+
+        if (Enum.TryParse<FinishReason>(value, ignoreCase: true, out var result))
+        {
+            return result;
+        }
+
+        return FinishReason.OTHER;
+    }
+
+    /// <summary>
+    /// Writes the FinishReason value as JSON.
+    /// </summary>
+    public override void Write(Utf8JsonWriter writer, FinishReason value, JsonSerializerOptions options)
+    {
+        writer.WriteStringValue(value.ToString());
+    }
+}

tests/GenerativeAI.Tests/Converters/LenientFinishReasonConverter_Tests.cs

@@ -0,0 +1,32 @@
+using System.Text.Json;
+using GenerativeAI.Types;
+using Shouldly;
+
+namespace GenerativeAI.Tests.Converters;
+
+public class LenientFinishReasonConverter_Tests
+{
+    [Fact]
+    public void Read_KnownValues_DeserializeCorrectly()
+    {
+        // Old value
+        JsonSerializer.Deserialize<FinishReason>("\"STOP\"").ShouldBe(FinishReason.STOP);
+
+        // New value with lowercase (tests case insensitivity)
+        JsonSerializer.Deserialize<FinishReason>("\"unexpected_tool_call\"").ShouldBe(FinishReason.UNEXPECTED_TOOL_CALL);
+    }
+
+    [Fact]
+    public void Read_UnknownValue_FallsBackToOther()
+    {
+        var result = JsonSerializer.Deserialize<FinishReason>("\"UNKNOWN_FUTURE_VALUE\"");
+        result.ShouldBe(FinishReason.OTHER);
+    }
+
+    [Fact]
+    public void Write_EnumValue_SerializesCorrectly()
+    {
+        var json = JsonSerializer.Serialize(FinishReason.UNEXPECTED_TOOL_CALL);
+        json.ShouldBe("\"UNEXPECTED_TOOL_CALL\"");
+    }
+}