diff --git a/src/Libraries/Microsoft.Extensions.AI.AzureAIInference/AzureAIInferenceChatClient.cs b/src/Libraries/Microsoft.Extensions.AI.AzureAIInference/AzureAIInferenceChatClient.cs
index 4a373baf069..becb73c9606 100644
--- a/src/Libraries/Microsoft.Extensions.AI.AzureAIInference/AzureAIInferenceChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.AzureAIInference/AzureAIInferenceChatClient.cs
@@ -377,7 +377,8 @@ private ChatCompletionsOptions ToAzureAIOptions(IEnumerable<ChatMessage> chatCon
                             ["required"] = BinaryData.FromBytes(JsonSerializer.SerializeToUtf8Bytes(tool.Required, JsonContext.Default.ListString)),
                             ["additionalProperties"] = _falseString,
                         },
-                        json.SchemaDescription);
+                        json.SchemaDescription,
+                        jsonSchemaIsStrict: true);
                 }
                 else
                 {
diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIAssistantClient.cs b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIAssistantClient.cs
index 3dec5920e22..1e6a29ad80e 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIAssistantClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIAssistantClient.cs
@@ -217,10 +217,11 @@ private static (RunCreationOptions RunOptions, List<FunctionResultContent>? Tool
                     switch (tool)
                     {
                         case AIFunction aiFunction:
-                            bool? strict =
-                                aiFunction.AdditionalProperties.TryGetValue("Strict", out object? strictObj) &&
-                                strictObj is bool strictValue ?
-                                strictValue : null;
+                            // Default strict to true, but allow to be overridden by an additional Strict property.
+                            bool strict =
+                                !aiFunction.AdditionalProperties.TryGetValue("Strict", out object? strictObj) ||
+                                strictObj is not bool strictValue ||
+                                strictValue;
 
                             var functionParameters = BinaryData.FromBytes(
                                 JsonSerializer.SerializeToUtf8Bytes(
@@ -267,7 +268,8 @@ strictObj is bool strictValue ?
                     AssistantResponseFormat.CreateJsonSchemaFormat(
                         jsonFormat.SchemaName ?? "json_schema",
                         BinaryData.FromBytes(JsonSerializer.SerializeToUtf8Bytes(jsonSchema, OpenAIJsonContext.Default.JsonElement)),
-                        jsonFormat.SchemaDescription) :
+                        jsonFormat.SchemaDescription,
+                        strictSchemaEnabled: true) :
                     AssistantResponseFormat.JsonObject;
             }
         }
diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIModelMapper.ChatCompletion.cs b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIModelMapper.ChatCompletion.cs
index c16ad7fe543..9edcac55c5e 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIModelMapper.ChatCompletion.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIModelMapper.ChatCompletion.cs
@@ -402,7 +402,8 @@ public static ChatCompletionOptions ToOpenAIOptions(ChatOptions? options)
                         jsonFormat.SchemaName ?? "json_schema",
                         BinaryData.FromBytes(
                             JsonSerializer.SerializeToUtf8Bytes(jsonSchema, OpenAIJsonContext.Default.JsonElement)),
-                        jsonFormat.SchemaDescription) :
+                        jsonFormat.SchemaDescription,
+                        jsonSchemaIsStrict: true) :
                     OpenAI.Chat.ChatResponseFormat.CreateJsonObjectFormat();
             }
         }
@@ -443,10 +444,11 @@ private sealed class MetadataOnlyAIFunction(string name, string description, Jso
     /// <summary>Converts an Extensions function to an OpenAI chat tool.</summary>
     private static ChatTool ToOpenAIChatTool(AIFunction aiFunction)
     {
-        bool? strict =
-            aiFunction.AdditionalProperties.TryGetValue("Strict", out object? strictObj) &&
-            strictObj is bool strictValue ?
-            strictValue : null;
+        // Default strict to true, but allow to be overridden by an additional Strict property.
+        bool strict =
+            !aiFunction.AdditionalProperties.TryGetValue("Strict", out object? strictObj) ||
+            strictObj is not bool strictValue ||
+            strictValue;
 
         // Map to an intermediate model so that redundant properties are skipped.
         var tool = JsonSerializer.Deserialize(aiFunction.JsonSchema, OpenAIJsonContext.Default.OpenAIChatToolJson)!;
diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponseChatClient.cs b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponseChatClient.cs
index d54440902aa..1944f652c9c 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponseChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponseChatClient.cs
@@ -412,7 +412,8 @@ private static ResponseCreationOptions ToOpenAIResponseCreationOptions(ChatOptio
                     ResponseTextFormat.CreateJsonSchemaFormat(
                         jsonFormat.SchemaName ?? "json_schema",
                         BinaryData.FromBytes(JsonSerializer.SerializeToUtf8Bytes(jsonSchema, OpenAIJsonContext.Default.JsonElement)),
-                        jsonFormat.SchemaDescription) :
+                        jsonFormat.SchemaDescription,
+                        jsonSchemaIsStrict: true) :
                     ResponseTextFormat.CreateJsonObjectFormat();
             }
         }
diff --git a/test/Libraries/Microsoft.Extensions.AI.AzureAIInference.Tests/AzureAIInferenceChatClientTests.cs b/test/Libraries/Microsoft.Extensions.AI.AzureAIInference.Tests/AzureAIInferenceChatClientTests.cs
index d0167c8778b..c3e2c74cb4c 100644
--- a/test/Libraries/Microsoft.Extensions.AI.AzureAIInference.Tests/AzureAIInferenceChatClientTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.AzureAIInference.Tests/AzureAIInferenceChatClientTests.cs
@@ -408,7 +408,8 @@ public async Task ResponseFormat_JsonSchema_NonStreaming()
                             "required":["description"],
                             "additionalProperties":false
                          },
-                         "description":"An object with a description"
+                         "description":"An object with a description",
+                         "strict":true
                     }
                 }
             }
diff --git a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
index 8a3e158041e..4097491399f 100644
--- a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
@@ -691,6 +691,7 @@ public async Task FunctionCallContent_NonStreaming()
                         "function": {
                             "description": "Gets the age of the specified person.",
                             "name": "GetPersonAge",
+                            "strict":true,
                             "parameters": {
                                 "type": "object",
                                 "required": [
@@ -811,6 +812,7 @@ public async Task FunctionCallContent_Streaming()
                         "function": {
                             "description": "Gets the age of the specified person.",
                             "name": "GetPersonAge",
+                            "strict":true,
                             "parameters": {
                                 "type": "object",
                                 "required": [