diff --git a/app/SourceCodeRules/SourceCodeRules/AnalyzerReleases.Shipped.md b/app/SourceCodeRules/SourceCodeRules/AnalyzerReleases.Shipped.md
index 308d6c626..716a5fb13 100644
--- a/app/SourceCodeRules/SourceCodeRules/AnalyzerReleases.Shipped.md
+++ b/app/SourceCodeRules/SourceCodeRules/AnalyzerReleases.Shipped.md
@@ -18,3 +18,4 @@
  MWAIS0012 | Usage    | Error    | DirectI18NGetTextAnalyzer
  MWAIS0013 | Usage    | Error    | ModelPatternLiteralAnalyzer
  MWAIS0014 | Usage    | Error    | MissingInterpolationAnalyzer
+ MWAIS0015 | Usage    | Error    | JsonSourceGenerationAnalyzer
diff --git a/app/SourceCodeRules/SourceCodeRules/Identifier.cs b/app/SourceCodeRules/SourceCodeRules/Identifier.cs
index 007684364..38cf392b3 100644
--- a/app/SourceCodeRules/SourceCodeRules/Identifier.cs
+++ b/app/SourceCodeRules/SourceCodeRules/Identifier.cs
@@ -16,4 +16,5 @@ public static class Identifier
     public const string DIRECT_I18N_GET_TEXT_ANALYZER = $"{Tools.ID_PREFIX}0012";
     public const string MODEL_PATTERN_LITERAL_ANALYZER = $"{Tools.ID_PREFIX}0013";
     public const string MISSING_INTERPOLATION_ANALYZER = $"{Tools.ID_PREFIX}0014";
+    public const string JSON_SOURCE_GENERATION_ANALYZER = $"{Tools.ID_PREFIX}0015";
 }
\ No newline at end of file
diff --git a/app/SourceCodeRules/SourceCodeRules/UsageAnalyzers/JsonSourceGenerationAnalyzer.cs b/app/SourceCodeRules/SourceCodeRules/UsageAnalyzers/JsonSourceGenerationAnalyzer.cs
new file mode 100644
index 000000000..c0df67f5b
--- /dev/null
+++ b/app/SourceCodeRules/SourceCodeRules/UsageAnalyzers/JsonSourceGenerationAnalyzer.cs
@@ -0,0 +1,146 @@
+using System.Collections.Immutable;
+using System.Linq;
+
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+
+namespace SourceCodeRules.UsageAnalyzers;
+
+/// <summary>
+/// Reports JSON which System.Text.Json would read or write with contracts it builds by reflection.
+/// </summary>
+/// <remarks>
+/// The options of AI Studio name a JsonSerializerContext as their TypeInfoResolver, so that the contracts of
+/// their types are generated while compiling. A contract which System.Text.Json builds by reflection instead
+/// may lack a property or fail altogether in a published build, since the trimmer removes code which only
+/// reflection reaches. Neither a debug build nor the tests show that. Switching reflection off for the whole
+/// app is no option, because Blazor Server needs it for JSON of its own. Hence, this analyzer refuses every
+/// way into reflection within the code of AI Studio: a call without options, the options
+/// JsonSerializerOptions.Default and .Web, a DefaultJsonTypeInfoResolver, and new options without a
+/// TypeInfoResolver.
+/// </remarks>
+#pragma warning disable RS1038
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+#pragma warning restore RS1038
+public sealed class JsonSourceGenerationAnalyzer : DiagnosticAnalyzer
+{
+    private const string DIAGNOSTIC_ID = Identifier.JSON_SOURCE_GENERATION_ANALYZER;
+    private const string CATEGORY = "Usage";
+
+    private const string JSON_NAMESPACE = "System.Text.Json";
+    private const string SERIALIZATION_NAMESPACE = "System.Text.Json.Serialization";
+    private const string METADATA_NAMESPACE = "System.Text.Json.Serialization.Metadata";
+    private const string OPTIONS_TYPE = "JsonSerializerOptions";
+    private const string TYPE_INFO_RESOLVER = "TypeInfoResolver";
+
+    private const string TITLE = "JSON must use the generated contracts of its options";
+
+    private const string MESSAGE_FORMAT = "{0} A contract which System.Text.Json builds by reflection may break in a published build, since the trimmer removes what reflection needs";
+
+    private const string DESCRIPTION = "AI Studio reads and writes JSON with options whose TypeInfoResolver is a JsonSerializerContext, so that every contract is generated while compiling. A call without options, the options JsonSerializerOptions.Default and .Web, a DefaultJsonTypeInfoResolver, and new options without a TypeInfoResolver all make System.Text.Json build the contracts by reflection, which only a published build shows to fail.";
+
+    /// <summary>
+    /// The types whose methods read or write JSON, and which fall back to reflection without options.
+    /// </summary>
+    private static readonly ImmutableHashSet<string> SERIALIZING_TYPES =
+    [
+        "System.Text.Json.JsonSerializer",
+        "System.Net.Http.Json.HttpClientJsonExtensions",
+        "System.Net.Http.Json.HttpContentJsonExtensions",
+        "System.Net.Http.Json.JsonContent",
+    ];
+
+    private static readonly DiagnosticDescriptor RULE = new(DIAGNOSTIC_ID, TITLE, MESSAGE_FORMAT, CATEGORY, DiagnosticSeverity.Error, isEnabledByDefault: true, description: DESCRIPTION);
+
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => [RULE];
+
+    public override void Initialize(AnalysisContext context)
+    {
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+        context.EnableConcurrentExecution();
+        context.RegisterOperationAction(AnalyzeInvocation, OperationKind.Invocation);
+        context.RegisterOperationAction(AnalyzePropertyReference, OperationKind.PropertyReference);
+        context.RegisterOperationAction(AnalyzeObjectCreation, OperationKind.ObjectCreation);
+    }
+
+    /// <summary>
+    /// Reports reading or writing JSON without options, or with options which are null.
+    /// </summary>
+    private static void AnalyzeInvocation(OperationAnalysisContext context)
+    {
+        var invocation = (IInvocationOperation)context.Operation;
+        var method = invocation.TargetMethod;
+        if (!SERIALIZING_TYPES.Contains(method.ContainingType.ToDisplayString()))
+            return;
+
+        //
+        // A method which gets the contract itself, or the context which holds it, needs no options:
+        //
+        if (method.Parameters.Any(parameter => IsContract(parameter.Type)))
+            return;
+
+        var options = invocation.Arguments.FirstOrDefault(argument => Is(argument.Parameter?.Type, JSON_NAMESPACE, OPTIONS_TYPE));
+        if (options is not null && options.ArgumentKind is not ArgumentKind.DefaultValue && !IsNull(options.Value))
+            return;
+
+        context.ReportDiagnostic(Diagnostic.Create(RULE, invocation.Syntax.GetLocation(), $"{method.ContainingType.Name}.{method.Name} gets no options, so it builds the contract of its type by reflection: pass the options whose context knows the type."));
+    }
+
+    /// <summary>
+    /// Reports the shared options of System.Text.Json, which build every contract by reflection.
+    /// </summary>
+    private static void AnalyzePropertyReference(OperationAnalysisContext context)
+    {
+        var reference = (IPropertyReferenceOperation)context.Operation;
+        if (reference.Property is not { IsStatic: true, Name: "Default" or "Web" } property || !Is(property.ContainingType, JSON_NAMESPACE, OPTIONS_TYPE))
+            return;
+
+        context.ReportDiagnostic(Diagnostic.Create(RULE, reference.Syntax.GetLocation(), $"JsonSerializerOptions.{property.Name} builds every contract by reflection: use the options whose context knows the type."));
+    }
+
+    /// <summary>
+    /// Reports a resolver which builds contracts by reflection, and new options which name no resolver.
+    /// </summary>
+    private static void AnalyzeObjectCreation(OperationAnalysisContext context)
+    {
+        var creation = (IObjectCreationOperation)context.Operation;
+        if (IsReflectionResolver(creation.Type))
+        {
+            context.ReportDiagnostic(Diagnostic.Create(RULE, creation.Syntax.GetLocation(), $"{creation.Type!.Name} builds contracts by reflection: register the type in the JsonSerializerContext of its options instead."));
+            return;
+        }
+
+        if (!Is(creation.Type, JSON_NAMESPACE, OPTIONS_TYPE) || NamesTypeInfoResolver(creation.Initializer))
+            return;
+
+        context.ReportDiagnostic(Diagnostic.Create(RULE, creation.Syntax.GetLocation(), "These options name no TypeInfoResolver, so System.Text.Json builds their contracts by reflection: set it to the JsonSerializerContext which knows their types, right in the object initializer."));
+    }
+
+    /// <summary>
+    /// Whether a parameter takes a contract, as a JsonTypeInfo, or the context which holds the contracts.
+    /// </summary>
+    private static bool IsContract(ITypeSymbol type) =>
+        Is(type, METADATA_NAMESPACE, "JsonTypeInfo") ||
+        Is(type, SERIALIZATION_NAMESPACE, "JsonSerializerContext");
+
+    private static bool IsReflectionResolver(ITypeSymbol? type)
+    {
+        for (var current = type; current is not null; current = current.BaseType)
+            if (Is(current, METADATA_NAMESPACE, "DefaultJsonTypeInfoResolver"))
+                return true;
+
+        return false;
+    }
+
+    private static bool NamesTypeInfoResolver(IObjectOrCollectionInitializerOperation? initializer) =>
+        initializer is not null &&
+        initializer.Initializers.Any(item => item is ISimpleAssignmentOperation { Target: IPropertyReferenceOperation { Property.Name: TYPE_INFO_RESOLVER } } assignment && !IsNull(assignment.Value));
+
+    private static bool IsNull(IOperation value) => value.ConstantValue is { HasValue: true, Value: null };
+
+    private static bool Is(ITypeSymbol? type, string namespaceName, string typeName) =>
+        type is not null &&
+        type.Name == typeName &&
+        type.ContainingNamespace?.ToDisplayString() == namespaceName;
+}
diff --git a/app/Tests/Analyzers/JsonSourceGenerationAnalyzerTests.cs b/app/Tests/Analyzers/JsonSourceGenerationAnalyzerTests.cs
new file mode 100644
index 000000000..e100c1793
--- /dev/null
+++ b/app/Tests/Analyzers/JsonSourceGenerationAnalyzerTests.cs
@@ -0,0 +1,106 @@
+using AIStudio.Tests.Models.Generation;
+
+using Microsoft.CodeAnalysis;
+
+using SourceCodeRules.UsageAnalyzers;
+
+namespace AIStudio.Tests.Analyzers;
+
+/// <summary>
+/// Checks that every way into JSON contracts built by reflection is refused while compiling.
+/// </summary>
+/// <remarks>
+/// The pre-release v26.10.1 failed because a published build lacked what reflection needed for the JSON of
+/// AI Studio. Since then, the options of AI Studio name a JsonSerializerContext, whose contracts are generated
+/// while compiling. A single call without options brings reflection back, and only a published build shows
+/// it, so MWAIS0015 turns such a call into a build error. These tests say which code it refuses and which it
+/// leaves alone.
+/// </remarks>
+[TestFixture]
+public sealed class JsonSourceGenerationAnalyzerTests
+{
+    [TestCase("JsonSerializer.Serialize(meeting)", "JsonSerializer.Serialize gets no options", TestName = "Writing without options")]
+    [TestCase("JsonSerializer.Serialize(meeting, options: null)", "JsonSerializer.Serialize gets no options", TestName = "Writing with options which are null")]
+    [TestCase("JsonSerializer.Deserialize<Meeting>(json)", "JsonSerializer.Deserialize gets no options", TestName = "Reading without options")]
+    [TestCase("element.Deserialize<Meeting>()", "JsonSerializer.Deserialize gets no options", TestName = "Reading a JSON element without options")]
+    [TestCase("client.GetFromJsonAsync<Meeting>(\"/meeting\")", "HttpClientJsonExtensions.GetFromJsonAsync gets no options", TestName = "Reading a web answer without options")]
+    [TestCase("client.PostAsJsonAsync(\"/meeting\", meeting)", "HttpClientJsonExtensions.PostAsJsonAsync gets no options", TestName = "Sending a web request without options")]
+    [TestCase("content.ReadFromJsonAsync<Meeting>()", "HttpContentJsonExtensions.ReadFromJsonAsync gets no options", TestName = "Reading web content without options")]
+    [TestCase("JsonContent.Create(meeting)", "JsonContent.Create gets no options", TestName = "Creating web content without options")]
+    [TestCase("JsonSerializer.Serialize(meeting, JsonSerializerOptions.Default)", "JsonSerializerOptions.Default builds every contract by reflection", TestName = "The default options")]
+    [TestCase("JsonSerializerOptions.Web", "JsonSerializerOptions.Web builds every contract by reflection", TestName = "The options for the web")]
+    [TestCase("new DefaultJsonTypeInfoResolver()", "DefaultJsonTypeInfoResolver builds contracts by reflection", TestName = "The resolver which uses reflection")]
+    [TestCase("new JsonSerializerOptions { TypeInfoResolver = new DefaultJsonTypeInfoResolver() }", "DefaultJsonTypeInfoResolver builds contracts by reflection", TestName = "Options with the resolver which uses reflection")]
+    [TestCase("new JsonSerializerOptions { WriteIndented = true }", "These options name no TypeInfoResolver", TestName = "Options without a resolver")]
+    [TestCase("new JsonSerializerOptions(JsonSerializerDefaults.Web)", "These options name no TypeInfoResolver", TestName = "Options for the web without a resolver")]
+    [TestCase("new JsonSerializerOptions { TypeInfoResolver = null }", "These options name no TypeInfoResolver", TestName = "Options with a resolver which is null")]
+    public async Task AWayIntoReflectionIsRefused(string expression, string expectedMessage)
+    {
+        var reported = await AnalyzeAsync(expression);
+
+        using (Assert.EnterMultipleScope())
+        {
+            Assert.That(reported, Has.Count.EqualTo(1));
+            Assert.That(reported.FirstOrDefault()?.Id, Is.EqualTo("MWAIS0015"));
+            Assert.That(reported.FirstOrDefault()?.GetMessage(), Does.StartWith(expectedMessage));
+        }
+    }
+
+    [Test]
+    public async Task OptionsCreatedByATargetTypedNewWithoutAResolverAreRefused()
+    {
+        var reported = await AnalyzeAsync("OTHER_OPTIONS", "private static readonly JsonSerializerOptions OTHER_OPTIONS = new() { WriteIndented = true };");
+
+        using (Assert.EnterMultipleScope())
+        {
+            Assert.That(reported, Has.Count.EqualTo(1));
+            Assert.That(reported.FirstOrDefault()?.GetMessage(), Does.StartWith("These options name no TypeInfoResolver"));
+        }
+    }
+
+    [TestCase("JsonSerializer.Serialize(meeting, OPTIONS)", TestName = "Writing with options")]
+    [TestCase("JsonSerializer.Serialize(meeting, contract)", TestName = "Writing with the contract")]
+    [TestCase("element.Deserialize(contract)", TestName = "Reading a JSON element with the contract")]
+    [TestCase("client.GetFromJsonAsync<Meeting>(\"/meeting\", OPTIONS)", TestName = "Reading a web answer with options")]
+    [TestCase("content.ReadFromJsonAsync(contract)", TestName = "Reading web content with the contract")]
+    [TestCase("JsonContent.Create(meeting, contract)", TestName = "Creating web content with the contract")]
+    [TestCase("new JsonSerializerOptions { WriteIndented = true, TypeInfoResolver = resolver }", TestName = "Options with a resolver")]
+    [TestCase("new JsonSerializerOptions(JsonSerializerDefaults.Web) { TypeInfoResolver = resolver }", TestName = "Options for the web with a resolver")]
+    [TestCase("JsonNode.Parse(json)", TestName = "A tree of JSON nodes")]
+    public async Task JsonWithGeneratedContractsIsLeftAlone(string expression)
+    {
+        var reported = await AnalyzeAsync(expression);
+
+        Assert.That(reported, Is.Empty);
+    }
+
+    private static async Task<IReadOnlyList<Diagnostic>> AnalyzeAsync(string expression, string otherMembers = "")
+    {
+        var source =
+            $$"""
+              using System.Net.Http;
+              using System.Net.Http.Json;
+              using System.Text.Json;
+              using System.Text.Json.Nodes;
+              using System.Text.Json.Serialization.Metadata;
+
+              namespace Sample;
+
+              public sealed class Meeting
+              {
+                  public string Location { get; set; } = "Cologne";
+              }
+
+              public static class MeetingJson
+              {
+                  private static readonly JsonSerializerOptions OPTIONS = new() { TypeInfoResolver = JsonTypeInfoResolver.Combine() };
+
+                  {{otherMembers}}
+
+                  public static object? Use(Meeting meeting, string json, JsonElement element, HttpClient client, HttpContent content, JsonTypeInfo<Meeting> contract, IJsonTypeInfoResolver resolver) => {{expression}};
+              }
+              """;
+
+        return await CompilationHarness.AnalyzeAsync(source, new JsonSourceGenerationAnalyzer());
+    }
+}
\ No newline at end of file