From 9277ead4c1fae1b43cc646cf160953213150baff Mon Sep 17 00:00:00 2001
From: Bruno Gale <bruno.gale97@gmail.com>
Date: Sun, 27 Sep 2026 10:43:02 +0200
Subject: [PATCH] fix(functions): send the caller's content-type instead of
 dropping it

---
 .../Functions.Tests/ClientContractTests.cs       | 11 +++++++++++
 packages/Functions/Functions/Client.cs           | 16 +++++++++++-----
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/packages/Functions/Functions.Tests/ClientContractTests.cs b/packages/Functions/Functions.Tests/ClientContractTests.cs
index 6ae0fc36..bea1aa1e 100644
--- a/packages/Functions/Functions.Tests/ClientContractTests.cs
+++ b/packages/Functions/Functions.Tests/ClientContractTests.cs
@@ -171,6 +171,17 @@ public async Task Invoke_ShouldPreferOptionHeadersOverDynamicHeaders()
         this.HeaderOf("x-shared").Should().Be("from-options");
     }
 
+    [TestMethod]
+    public async Task Invoke_ShouldSendOptionContentType_GivenLowercaseHeaderName()
+    {
+        this.RespondWith(200, "ok");
+        await this.client.Invoke(FunctionName, options: new InvokeFunctionOptions
+        {
+            Headers = new Dictionary<string, string> { { "content-type", "text/plain" } }
+        });
+        this.HeaderOf("Content-Type").Should().Be("text/plain");
+    }
+
     [TestMethod]
     public async Task Invoke_ShouldThrowFunctionsException_GivenServerError()
     {
diff --git a/packages/Functions/Functions/Client.cs b/packages/Functions/Functions/Client.cs
index 70f2c865..89cb2bba 100644
--- a/packages/Functions/Functions/Client.cs
+++ b/packages/Functions/Functions/Client.cs
@@ -285,14 +285,20 @@ private static Uri BuildUri(string url)
 
     private static HttpRequestMessage BuildRequestMessage(InvokeFunctionOptions options, Uri uri)
     {
-        var request = new HttpRequestMessage(options.HttpMethod, uri)
-        {
-            Content = new StringContent(JsonSerializer.Serialize(options.Body, SerializerOptions), Encoding.UTF8, "application/json")
-        };
+        var content = new StringContent(JsonSerializer.Serialize(options.Body, SerializerOptions), Encoding.UTF8, "application/json");
+        var request = new HttpRequestMessage(options.HttpMethod, uri) { Content = content };
 
         foreach (var kvp in options.Headers)
         {
-            request.Headers.TryAddWithoutValidation(kvp.Key, kvp.Value);
+            if (string.Equals(kvp.Key, "Content-Type", StringComparison.OrdinalIgnoreCase))
+            {
+                content.Headers.Remove("Content-Type");
+                content.Headers.TryAddWithoutValidation("Content-Type", kvp.Value);
+            }
+            else
+            {
+                request.Headers.TryAddWithoutValidation(kvp.Key, kvp.Value);
+            }
         }
 
         return request;