diff --git a/app/MindWork AI Studio/Provider/BaseProvider.cs b/app/MindWork AI Studio/Provider/BaseProvider.cs
index 325b4ce2..98a34c74 100644
--- a/app/MindWork AI Studio/Provider/BaseProvider.cs	
+++ b/app/MindWork AI Studio/Provider/BaseProvider.cs	
@@ -1245,6 +1245,7 @@ public abstract class BaseProvider : IProvider, ISecretId
     /// <param name="requestPath">The request path, relative to the provider base URL.</param>
     /// <param name="headersAction">Optional additional headers to add.</param>
     /// <param name="mayAskForSequentialToolCalls">Whether a request which offers tools may ask for one call at a time. False for a provider which rejects the parallel_tool_calls parameter.</param>
+    /// <param name="enforcesStrictToolSchemas">Whether the provider binds the model's tool calls to a strict schema. Only then are tools offered in strict mode; everywhere else a strict schema would only tell the model that every argument is required.</param>
     /// <param name="token">The cancellation token.</param>
     /// <typeparam name="TRequest">The request DTO type.</typeparam>
     /// <typeparam name="TDelta">The delta stream line type.</typeparam>
@@ -1262,6 +1263,7 @@ public abstract class BaseProvider : IProvider, ISecretId
         string requestPath = "chat/completions",
         Action<HttpRequestHeaders>? headersAction = null,
         bool mayAskForSequentialToolCalls = true,
+        bool enforcesStrictToolSchemas = false,
         [EnumeratorCancellation] CancellationToken token = default)
         where TRequest : ChatCompletionAPIRequest
         where TDelta : IResponseStreamLine
@@ -1300,7 +1302,7 @@ public abstract class BaseProvider : IProvider, ISecretId
             if (runnableTools.Count > 0)
             {
                 var adapter = new ChatCompletionToolCallingAdapter<TRequest>(requestFactory, systemPrompt, apiParameters,
-                    runnableTools.Select(x => ProviderToolAdapters.ToChatCompletionTool(x.Definition)).ToList(), mayAskForSequentialToolCalls, runnableTools,
+                    runnableTools.Select(x => ProviderToolAdapters.ToChatCompletionTool(x.Definition, enforcesStrictToolSchemas)).ToList(), mayAskForSequentialToolCalls, runnableTools,
                     (requestDto, requestToken) => this.StreamChatCompletionRequest(requestDto, providerName, requestPath, requestedSecret, headersAction, requestToken),
                     ChatCompletionSourceReader.Read<TDelta, TAnnotation>,
                     this.logger);
diff --git a/app/MindWork AI Studio/Provider/OpenAI/ProviderOpenAI.cs b/app/MindWork AI Studio/Provider/OpenAI/ProviderOpenAI.cs
index f6e81b2a..1f524e97 100644
--- a/app/MindWork AI Studio/Provider/OpenAI/ProviderOpenAI.cs	
+++ b/app/MindWork AI Studio/Provider/OpenAI/ProviderOpenAI.cs	
@@ -168,6 +168,16 @@ public sealed class ProviderOpenAI() : BaseProvider(LLMProviders.OPEN_AI, new Ur
                                },
                                systemPromptRole: systemPromptRole,
                                requestPath: "chat/completions",
+
+                               //
+                               // OpenAI binds a strict function's arguments to its schema -- read on
+                               // 2026-09-24 at https://developers.openai.com/api/docs/guides/function-calling.
+                               // Most other hosts of this API do not: Groq applies strict mode to
+                               // response formats only (https://console.groq.com/docs/structured-outputs,
+                               // same day) and merely validates a tool call afterward, so it rejects
+                               // every call that leaves out an argument the strict schema requires:
+                               //
+                               enforcesStrictToolSchemas: true,
                                token: token))
                 yield return content;
 
diff --git a/app/MindWork AI Studio/Provider/ProviderToolAdapters.cs b/app/MindWork AI Studio/Provider/ProviderToolAdapters.cs
index 80a60b85..d8016bb2 100644
--- a/app/MindWork AI Studio/Provider/ProviderToolAdapters.cs	
+++ b/app/MindWork AI Studio/Provider/ProviderToolAdapters.cs	
@@ -10,33 +10,47 @@ namespace AIStudio.Provider;
 /// <remarks>
 /// The definitions state a tool once, in plain JSON Schema. What differs per API is not only the
 /// field names but how an optional argument is expressed, which is why the OpenAI shapes convert
-/// the schema while Anthropic takes it as written.
+/// the schema while Anthropic takes it as written.<br/><br/>
+/// Strict mode is a promise of the host, not of the definition: only a host which binds the
+/// model's output to the schema keeps it. Everywhere else the model merely reads a schema that
+/// calls every argument required, and then either invents a value for an argument it meant to
+/// leave out, or the host rejects the call for lacking one. That is why the Chat Completions
+/// shape, which reaches every OpenAI-compatible host, only goes strict where the host says so.
 /// </remarks>
 public static class ProviderToolAdapters
 {
     /// <summary>
     /// Builds the nested function tool shape used by Chat Completions compatible APIs.
     /// </summary>
-    public static object ToChatCompletionTool(ToolDefinition definition) => new
+    /// <param name="definition">The tool to describe.</param>
+    /// <param name="hostEnforcesStrict">Whether the host binds the model's tool calls to a strict schema. Only then is the tool sent in strict mode.</param>
+    public static object ToChatCompletionTool(ToolDefinition definition, bool hostEnforcesStrict)
     {
-        type = "function",
-        function = new
+        var isStrict = definition.Function.Strict && hostEnforcesStrict;
+        return new
         {
-            name = definition.Function.Name,
-            description = definition.Function.DescriptionForLLM,
-            parameters = ToOpenAIParameters(definition),
-            strict = definition.Function.Strict,
-        }
-    };
+            type = "function",
+            function = new
+            {
+                name = definition.Function.Name,
+                description = definition.Function.DescriptionForLLM,
+                parameters = ToOpenAIParameters(definition, isStrict),
+                strict = isStrict,
+            }
+        };
+    }
 
     /// <summary>
     /// Builds the flat function tool shape used by the OpenAI Responses API.
     /// </summary>
+    /// <remarks>
+    /// Only OpenAI speaks this API, and it enforces strict mode, so the definition alone decides.
+    /// </remarks>
     public static ResponsesFunctionTool ToResponsesTool(ToolDefinition definition) => new()
     {
         Name = definition.Function.Name,
         Description = definition.Function.DescriptionForLLM,
-        Parameters = ToOpenAIParameters(definition),
+        Parameters = ToOpenAIParameters(definition, definition.Function.Strict),
         Strict = definition.Function.Strict,
     };
 
@@ -59,7 +73,7 @@ public static class ProviderToolAdapters
     /// <summary>
     /// The parameter schema for the OpenAI APIs, converted only when strict mode asks for it.
     /// </summary>
-    private static System.Text.Json.JsonElement ToOpenAIParameters(ToolDefinition definition) => definition.Function.Strict
+    private static System.Text.Json.JsonElement ToOpenAIParameters(ToolDefinition definition, bool isStrict) => isStrict
         ? OpenAIStrictToolSchema.FromToolParameters(definition.Function.Parameters)
         : definition.Function.Parameters;
 }
diff --git a/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolFunctionDefinition.cs b/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolFunctionDefinition.cs
index 7557413e..6d8f36a0 100644
--- a/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolFunctionDefinition.cs	
+++ b/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolFunctionDefinition.cs	
@@ -8,6 +8,14 @@ public sealed class ToolFunctionDefinition
 
     public string DescriptionForLLM { get; init; } = string.Empty;
 
+    /// <summary>
+    /// Whether this tool may be offered in strict mode.
+    /// </summary>
+    /// <remarks>
+    /// The host has a say as well: a tool goes strict only where the host binds the model's calls
+    /// to the schema, see ProviderToolAdapters. Setting this to false keeps a tool out of strict
+    /// mode everywhere, for a schema which strict mode cannot express.
+    /// </remarks>
     public bool Strict { get; init; } = true;
 
     public JsonElement Parameters { get; init; }
diff --git a/app/Tests/Provider/ToolCalling/OpenAIStrictToolSchemaTests.cs b/app/Tests/Provider/ToolCalling/OpenAIStrictToolSchemaTests.cs
new file mode 100644
index 00000000..9bf71d9b
--- /dev/null
+++ b/app/Tests/Provider/ToolCalling/OpenAIStrictToolSchemaTests.cs
@@ -0,0 +1,107 @@
+using System.Text.Json.Nodes;
+
+using AIStudio.Provider.OpenAI;
+using AIStudio.Tools.ToolCallingSystem;
+
+namespace AIStudio.Tests.Provider.ToolCalling;
+
+/// <summary>
+/// Checks how a tool's parameter schema is translated into the form OpenAI's strict mode requires.
+/// </summary>
+/// <remarks>
+/// Strict mode wants every argument required and says "may be left out" by allowing null instead.
+/// The translation has to keep what the schema means: an argument the tool can do without stays
+/// one the model can leave empty, and an argument the tool needs stays exactly as it was.
+/// </remarks>
+[TestFixture]
+public sealed class OpenAIStrictToolSchemaTests
+{
+    [Test]
+    public void AnOptionalArgumentMayBeNullInstead()
+    {
+        var properties = Converted(ToolParameterSchemaBuilder.Create()
+            .RequiredString("query", "The search query.")
+            .OptionalString("language", "The language.")
+            .OptionalInteger("page", "The page."))["properties"]!;
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(Types(properties["language"]!), Is.EqualTo(new[] { "string", "null" }));
+            Assert.That(Types(properties["page"]!), Is.EqualTo(new[] { "integer", "null" }));
+        });
+    }
+
+    [Test]
+    public void AnOptionalChoiceOffersNullAmongItsValues()
+    {
+        var timeRange = Converted(ToolParameterSchemaBuilder.Create()
+            .RequiredString("query", "The search query.")
+            .OptionalEnum("time_range", "The time range.", "day", "month", "year"))["properties"]!["time_range"]!;
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(Types(timeRange), Is.EqualTo(new[] { "string", "null" }));
+            Assert.That(timeRange["enum"]!.AsArray().Select(value => value?.GetValue<string>()), Is.EqualTo(new[] { null, "day", "month", "year" }));
+        });
+    }
+
+    [Test]
+    public void ARequiredArgumentStaysAsItIs()
+    {
+        var query = Converted(ToolParameterSchemaBuilder.Create()
+            .RequiredString("query", "The search query.")
+            .OptionalString("language", "The language."))["properties"]!["query"]!;
+
+        Assert.That(query["type"]!.GetValue<string>(), Is.EqualTo("string"));
+    }
+
+    [Test]
+    public void EveryArgumentIsRequiredInTheOrderOfTheProperties()
+    {
+        //
+        // The order has to be stable across requests, because prompt caching depends on it. The
+        // optional argument comes first here, so an order taken from the old required list would
+        // show.
+        //
+        var schema = Converted(ToolParameterSchemaBuilder.Create()
+            .OptionalString("language", "The language.")
+            .RequiredString("query", "The search query."));
+
+        Assert.That(schema["required"]!.AsArray().Select(name => name!.GetValue<string>()), Is.EqualTo(new[] { "language", "query" }));
+    }
+
+    [Test]
+    public void ExtraArgumentsStayRefused()
+    {
+        var schema = Converted(ToolParameterSchemaBuilder.Create()
+            .RequiredString("query", "The search query.")
+            .OptionalString("language", "The language."));
+
+        Assert.That(schema["additionalProperties"]!.GetValue<bool>(), Is.False);
+    }
+
+    [Test]
+    public void ASchemaWithoutOptionalArgumentsIsLeftUntouched()
+    {
+        var parameters = ToolParameterSchemaBuilder.Create()
+            .RequiredString("url", "The address of the page.")
+            .Build();
+
+        var converted = OpenAIStrictToolSchema.FromToolParameters(parameters);
+        Assert.That(JsonNode.DeepEquals(JsonNode.Parse(converted.GetRawText()), JsonNode.Parse(parameters.GetRawText())), Is.True);
+    }
+
+    /// <summary>
+    /// Builds the schema and returns it the way strict mode receives it.
+    /// </summary>
+    private static JsonNode Converted(ToolParameterSchemaBuilder builder) => JsonNode.Parse(OpenAIStrictToolSchema.FromToolParameters(builder.Build()).GetRawText())!;
+
+    /// <summary>
+    /// Reads the types a property allows, whether it names one or several.
+    /// </summary>
+    private static string[] Types(JsonNode property) => property["type"] switch
+    {
+        JsonArray types => types.Select(type => type!.GetValue<string>()).ToArray(),
+        var type => [type!.GetValue<string>()],
+    };
+}
\ No newline at end of file
diff --git a/app/Tests/Provider/ToolCalling/ProviderToolAdaptersTests.cs b/app/Tests/Provider/ToolCalling/ProviderToolAdaptersTests.cs
new file mode 100644
index 00000000..5428d0ae
--- /dev/null
+++ b/app/Tests/Provider/ToolCalling/ProviderToolAdaptersTests.cs
@@ -0,0 +1,85 @@
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+using AIStudio.Provider;
+using AIStudio.Tools.ToolCallingSystem;
+
+namespace AIStudio.Tests.Provider.ToolCalling;
+
+/// <summary>
+/// Checks when a tool reaches a Chat Completions host in strict mode, and when as written.
+/// </summary>
+/// <remarks>
+/// Strict mode only holds where the host binds the model's calls to the schema. Anywhere else, the
+/// model just reads a schema which calls every argument required: Groq then rejects each call that
+/// leaves one out, and other models fill the gap with a placeholder the tool has to refuse. Such a
+/// host therefore gets the schema the way the tool wrote it.
+/// </remarks>
+[TestFixture]
+public sealed class ProviderToolAdaptersTests
+{
+    [Test]
+    public void AHostWhichDoesNotEnforceStrictModeGetsTheSchemaAsWritten()
+    {
+        var definition = WebSearchLike();
+        var function = Sent(definition, hostEnforcesStrict: false);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(function["strict"]!.GetValue<bool>(), Is.False);
+            Assert.That(JsonNode.DeepEquals(function["parameters"], JsonNode.Parse(definition.Function.Parameters.GetRawText())), Is.True);
+        });
+    }
+
+    [Test]
+    public void AHostWhichEnforcesStrictModeGetsTheConvertedSchema()
+    {
+        var function = Sent(WebSearchLike(), hostEnforcesStrict: true);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(function["strict"]!.GetValue<bool>(), Is.True);
+            Assert.That(function["parameters"]!["required"]!.AsArray().Select(name => name!.GetValue<string>()), Is.EqualTo(new[] { "query", "page" }));
+        });
+    }
+
+    [Test]
+    public void AToolWhichOptsOutStaysOutOfStrictModeEverywhere()
+    {
+        var definition = WebSearchLike(isStrict: false);
+        var function = Sent(definition, hostEnforcesStrict: true);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(function["strict"]!.GetValue<bool>(), Is.False);
+            Assert.That(JsonNode.DeepEquals(function["parameters"], JsonNode.Parse(definition.Function.Parameters.GetRawText())), Is.True);
+        });
+    }
+
+    /// <summary>
+    /// A tool with one required and one optional argument, the shape the web search has.
+    /// </summary>
+    private static ToolDefinition WebSearchLike(bool isStrict = true) => new()
+    {
+        Id = "web_search",
+        Function = new()
+        {
+            Name = "web_search",
+            DescriptionForLLM = "Search the web.",
+            Strict = isStrict,
+            Parameters = ToolParameterSchemaBuilder.Create()
+                .RequiredString("query", "The search query.")
+                .OptionalInteger("page", "The page, starting at 1.")
+                .Build(),
+        },
+    };
+
+    /// <summary>
+    /// Returns the function object of the tool as it goes over the wire.
+    /// </summary>
+    private static JsonNode Sent(ToolDefinition definition, bool hostEnforcesStrict)
+    {
+        var tool = ProviderToolAdapters.ToChatCompletionTool(definition, hostEnforcesStrict);
+        return JsonNode.Parse(JsonSerializer.Serialize(tool, ProviderJsonOptions.OPTIONS))!["function"]!;
+    }
+}
\ No newline at end of file