Fix OpenAIEmbeddingGenerator to handle missing usage data (dotnet#7074)

Author: stephentoub

src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIEmbeddingGenerator.cs

@@ -68,18 +68,22 @@ public async Task<GeneratedEmbeddings<Embedding<float>>> GenerateAsync(IEnumerab
             _embeddingClient.GenerateEmbeddingsAsync(values, openAIOptions, cancellationToken);
         var embeddings = (await t.ConfigureAwait(false)).Value;
 
+        UsageDetails? usage = embeddings.Usage is not null ?
+            new()
+            {
+                InputTokenCount = embeddings.Usage.InputTokenCount,
+                TotalTokenCount = embeddings.Usage.TotalTokenCount
+            } :
+            null;
+
         return new(embeddings.Select(e =>
                 new Embedding<float>(e.ToFloats())
                 {
                     CreatedAt = DateTimeOffset.UtcNow,
                     ModelId = embeddings.Model,
                 }))
         {
-            Usage = new()
-            {
-                InputTokenCount = embeddings.Usage.InputTokenCount,
-                TotalTokenCount = embeddings.Usage.TotalTokenCount
-            },
+            Usage = usage,
         };
     }
 

test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIEmbeddingGeneratorTests.cs

@@ -216,6 +216,91 @@ public async Task EmbeddingGenerationOptions_DoNotOverwrite_NotNullPropertiesInR
         }
     }
 
+    [Fact]
+    public async Task EmbeddingGenerationOptions_MissingUsage_Ignored()
+    {
+        const string Input = """
+            {
+              "input":["hello, world!"],
+              "model":"text-embedding-3-small",
+              "encoding_format":"base64"
+            }
+            """;
+
+        const string Output = """
+            {
+              "object": "list",
+              "data": [
+                {
+                  "object": "embedding",
+                  "index": 0,
+                  "embedding": "AAAAAA=="
+                }
+              ],
+              "model": "text-embedding-3-small"
+            }
+            """;
+
+        using VerbatimHttpHandler handler = new(Input, Output);
+        using HttpClient httpClient = new(handler);
+        using IEmbeddingGenerator<string, Embedding<float>> generator = CreateEmbeddingGenerator(httpClient, "text-embedding-3-small");
+
+        var response = await generator.GenerateAsync(["hello, world!"]);
+        Assert.NotNull(response);
+        Assert.Single(response);
+        Assert.Null(response.Usage);
+
+        foreach (Embedding<float> e in response)
+        {
+            Assert.Equal("text-embedding-3-small", e.ModelId);
+            Assert.NotNull(e.CreatedAt);
+            Assert.Equal(1, e.Vector.Length);
+        }
+    }
+
+    [Fact]
+    public async Task EmbeddingGenerationOptions_NullUsage_Ignored()
+    {
+        const string Input = """
+            {
+              "input":["hello, world!"],
+              "model":"text-embedding-3-small",
+              "encoding_format":"base64"
+            }
+            """;
+
+        const string Output = """
+            {
+              "object": "list",
+              "data": [
+                {
+                  "object": "embedding",
+                  "index": 0,
+                  "embedding": "AAAAAA=="
+                }
+              ],
+              "model": "text-embedding-3-small",
+              "usage": null
+            }
+            """;
+
+        using VerbatimHttpHandler handler = new(Input, Output);
+        using HttpClient httpClient = new(handler);
+        using IEmbeddingGenerator<string, Embedding<float>> generator = CreateEmbeddingGenerator(httpClient, "text-embedding-3-small");
+
+        var response = await generator.GenerateAsync(["hello, world!"]);
+        Assert.NotNull(response);
+        Assert.Single(response);
+        Assert.Null(response.Usage);
+
+        foreach (Embedding<float> e in response)
+        {
+            Assert.Equal("text-embedding-3-small", e.ModelId);
+            Assert.NotNull(e.CreatedAt);
+            Assert.Equal(1, e.Vector.Length);
+        }
+    }
+
     [Fact]
     public async Task RequestHeaders_UserAgent_ContainsMEAI()
     {