Reject queryType=semantic with 400

AzureSearchEmulator.IntegrationTests/SemanticQueryIntegrationTests.cs

@@ -0,0 +1,44 @@
+using System.Net;
+using System.Text;
+using Azure.Search.Documents.Indexes.Models;
+using Xunit;
+
+namespace AzureSearchEmulator.IntegrationTests;
+
+/// <summary>
+/// Semantic ranking depends on a hosted reranker, so the emulator refuses it with a 400 rather
+/// than silently returning lexical results.
+/// </summary>
+public class SemanticQueryIntegrationTests(EmulatorFactory factory)
+    : IClassFixture<EmulatorFactory>
+{
+    [Fact]
+    public async Task SemanticQueryType_IsRejectedWithBadRequest()
+    {
+        var indexName = "semantic-" + Guid.NewGuid().ToString("N")[..8];
+        var cancellationToken = TestContext.Current.CancellationToken;
+
+        await factory.CreateSearchIndexClient().CreateIndexAsync(
+            new SearchIndex(indexName)
+            {
+                Fields =
+                {
+                    new SearchField("id", SearchFieldDataType.String) { IsKey = true },
+                    new SearchField("name", SearchFieldDataType.String) { IsSearchable = true },
+                }
+            },
+            cancellationToken);
+
+        using var http = factory.CreateHttpClient();
+
+        var post = await http.PostAsync(
+            $"/indexes/{indexName}/docs/search?api-version=2024-07-01",
+            new StringContent("""{ "search": "x", "queryType": "semantic" }""", Encoding.UTF8, "application/json"),
+            cancellationToken);
+        Assert.Equal(HttpStatusCode.BadRequest, post.StatusCode);
+        Assert.Contains("semantic", await post.Content.ReadAsStringAsync(cancellationToken));
+
+        var get = await http.GetAsync($"/indexes/{indexName}/docs?search=x&queryType=semantic&api-version=2024-07-01", cancellationToken);
+        Assert.Equal(HttpStatusCode.BadRequest, get.StatusCode);
+    }
+}

AzureSearchEmulator/Controllers/DocumentSearchingController.cs

@@ -127,6 +127,13 @@ public async Task<IActionResult> SearchPost(string indexKey, [FromBody] SearchRe
             ModelState.AddModelError(nameof(request.Skip), "Skip must be between 0 and 100,000");
         }
 
+        // Semantic ranking needs a Microsoft-hosted reranker that cannot be emulated. Refusing is
+        // better than quietly returning lexical results a caller would take for reranked ones.
+        if (string.Equals(request.QueryType, "semantic", StringComparison.OrdinalIgnoreCase))
+        {
+            ModelState.AddModelError(nameof(request.QueryType), "queryType 'semantic' is not supported by the emulator.");
+        }
+
         if (!ModelState.IsValid)
         {
             return BadRequest(ModelState);