Ensure the Ollama clients validate HTTP status codes.

src/Libraries/Microsoft.Extensions.AI.Ollama/OllamaChatClient.cs

@@ -87,6 +87,11 @@ public async Task<ChatCompletion> CompleteAsync(IList<ChatMessage> chatMessages,
             JsonContext.Default.OllamaChatRequest,
             cancellationToken).ConfigureAwait(false);
 
+        if (!httpResponse.IsSuccessStatusCode)
+        {
+            await OllamaUtilities.ThrowUnsuccessfulOllamaResponseAsync(httpResponse, cancellationToken).ConfigureAwait(false);
+        }
+
         var response = (await httpResponse.Content.ReadFromJsonAsync(
             JsonContext.Default.OllamaChatResponse,
             cancellationToken).ConfigureAwait(false))!;
@@ -117,6 +122,12 @@ public async IAsyncEnumerable<StreamingChatCompletionUpdate> CompleteStreamingAs
             Content = JsonContent.Create(ToOllamaChatRequest(chatMessages, options, stream: true), JsonContext.Default.OllamaChatRequest)
         };
         using var httpResponse = await _httpClient.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
+
+        if (!httpResponse.IsSuccessStatusCode)
+        {
+            await OllamaUtilities.ThrowUnsuccessfulOllamaResponseAsync(httpResponse, cancellationToken).ConfigureAwait(false);
+        }
+
         using var httpResponseStream = await httpResponse.Content
 #if NET
             .ReadAsStreamAsync(cancellationToken)

src/Libraries/Microsoft.Extensions.AI.Ollama/OllamaEmbeddingGenerator.cs

@@ -110,6 +110,11 @@ public async Task<GeneratedEmbeddings<Embedding<float>>> GenerateAsync(
             JsonContext.Default.OllamaEmbeddingRequest,
             cancellationToken).ConfigureAwait(false);
 
+        if (!httpResponse.IsSuccessStatusCode)
+        {
+            await OllamaUtilities.ThrowUnsuccessfulOllamaResponseAsync(httpResponse, cancellationToken).ConfigureAwait(false);
+        }
+
         var response = (await httpResponse.Content.ReadFromJsonAsync(
             JsonContext.Default.OllamaEmbeddingResponse,
             cancellationToken).ConfigureAwait(false))!;

src/Libraries/Microsoft.Extensions.AI.Ollama/OllamaUtilities.cs

@@ -2,8 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Net.Http;
+using System.Text.Json;
 using System.Threading;
+using System.Threading.Tasks;
 
 namespace Microsoft.Extensions.AI;
 
@@ -31,4 +35,38 @@ public static void TransferNanosecondsTime<TResponse>(TResponse response, Func<T
             }
         }
     }
+
+    [DoesNotReturn]
+    public static async ValueTask ThrowUnsuccessfulOllamaResponseAsync(HttpResponseMessage response, CancellationToken cancellationToken)
+    {
+        Debug.Assert(!response.IsSuccessStatusCode, "must only be invoked for unsuccessful responses.");
+
+        // Read the entire response content into a string.
+        string errorContent =
+#if NET
+            await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
+#else
+            await response.Content.ReadAsStringAsync().ConfigureAwait(false);
+#endif
+
+        // The response content *could* be JSON formatted, try to extract the error field.
+
+#pragma warning disable CA1031 // Do not catch general exception types
+        try
+        {
+            using JsonDocument document = JsonDocument.Parse(errorContent);
+            if (document.RootElement.TryGetProperty("error", out JsonElement errorElement) &&
+                errorElement.ValueKind is JsonValueKind.String)
+            {
+                errorContent = errorElement.GetString()!;
+            }
+        }
+        catch
+        {
+            // Ignore JSON parsing errors.
+        }
+#pragma warning restore CA1031 // Do not catch general exception types
+
+        throw new InvalidOperationException($"Ollama error: {errorContent}");
+    }
 }

test/Libraries/Microsoft.Extensions.AI.Ollama.Tests/OllamaChatClientIntegrationTests.cs

@@ -87,6 +87,24 @@ public async Task PromptBasedFunctionCalling_WithArgs()
         Assert.False(didCallIrrelevantTool);
     }
 
+    [ConditionalFact]
+    public async Task InvalidModelParameter_ThrowsInvalidOperationException()
+    {
+        SkipIfNotEnabled();
+
+        var endpoint = IntegrationTestHelpers.GetOllamaUri();
+        Assert.NotNull(endpoint);
+
+        using var chatClient = new OllamaChatClient(endpoint, modelId: "inexistent-model");
+
+        InvalidOperationException ex;
+        ex = await Assert.ThrowsAsync<InvalidOperationException>(() => chatClient.CompleteAsync("Hello, world!"));
+        Assert.Contains("inexistent-model", ex.Message);
+
+        ex = await Assert.ThrowsAsync<InvalidOperationException>(() => chatClient.CompleteStreamingAsync("Hello, world!").ToChatCompletionAsync());
+        Assert.Contains("inexistent-model", ex.Message);
+    }
+
     private sealed class AssertNoToolsDefinedChatClient(IChatClient innerClient) : DelegatingChatClient(innerClient)
     {
         public override Task<ChatCompletion> CompleteAsync(

test/Libraries/Microsoft.Extensions.AI.Ollama.Tests/OllamaEmbeddingGeneratorIntegrationTests.cs

@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Threading.Tasks;
+using Microsoft.TestUtilities;
+using Xunit;
 
 namespace Microsoft.Extensions.AI;
 
@@ -11,4 +14,19 @@ public class OllamaEmbeddingGeneratorIntegrationTests : EmbeddingGeneratorIntegr
         IntegrationTestHelpers.GetOllamaUri() is Uri endpoint ?
             new OllamaEmbeddingGenerator(endpoint, "all-minilm") :
             null;
+
+    [ConditionalFact]
+    public async Task InvalidModelParameter_ThrowsInvalidOperationException()
+    {
+        SkipIfNotEnabled();
+
+        var endpoint = IntegrationTestHelpers.GetOllamaUri();
+        Assert.NotNull(endpoint);
+
+        using var generator = new OllamaEmbeddingGenerator(endpoint, modelId: "inexistent-model");
+
+        InvalidOperationException ex;
+        ex = await Assert.ThrowsAsync<InvalidOperationException>(() => generator.GenerateAsync(["Hello, world!"]));
+        Assert.Contains("inexistent-model", ex.Message);
+    }
 }