diff --git a/src/posting/app.py b/src/posting/app.py
index 25efdd4b..5d9a6aa5 100644
--- a/src/posting/app.py
+++ b/src/posting/app.py
@@ -31,6 +31,7 @@
 from posting.collection import (
     Collection,
     Cookie,
+    Header,
     HttpRequestMethod,
     Options,
     RequestModel,
@@ -657,7 +658,21 @@ def build_request_model(self, request_options: Options) -> RequestModel:
         # We ensure elsewhere that the we can only "open" requests, not collection nodes.
         assert not isinstance(open_request, Collection)
 
+        request_editor_args = self.request_editor.to_request_model_args()
         headers = self.headers_table.to_model()
+        if request_body := request_editor_args.get("body"):
+            header_names_lower = {header.name.lower(): header for header in headers}
+            # Don't add the content type header if the user has explicitly set it.
+            if (
+                request_body.content_type is not None
+                and "content-type" not in header_names_lower
+            ):
+                headers.append(
+                    Header(
+                        name="content-type",
+                        value=request_body.content_type,
+                    )
+                )
         return RequestModel(
             name=self.request_metadata.request_name,
             path=open_request.path if open_request else None,
@@ -674,7 +689,7 @@ def build_request_model(self, request_options: Options) -> RequestModel:
                 else []
             ),
             scripts=self.request_scripts.to_model(),
-            **self.request_editor.to_request_model_args(),
+            **request_editor_args,
         )
 
     def load_request_model(self, request_model: RequestModel) -> None:
diff --git a/src/posting/collection.py b/src/posting/collection.py
index c313e9f6..c1672a94 100644
--- a/src/posting/collection.py
+++ b/src/posting/collection.py
@@ -232,17 +232,14 @@ def apply_template(self, variables: dict[str, Any]) -> None:
 
     def to_httpx(self, client: httpx.AsyncClient) -> httpx.Request:
         """Convert the request model to an httpx request."""
+        headers = httpx.Headers(
+            [(header.name, header.value) for header in self.headers if header.enabled]
+        )
         return client.build_request(
             method=self.method,
             url=self.url,
             **(self.body.to_httpx_args() if self.body else {}),
-            headers=httpx.Headers(
-                [
-                    (header.name, header.value)
-                    for header in self.headers
-                    if header.enabled
-                ]
-            ),
+            headers=headers,
             params=httpx.QueryParams(
                 [(param.name, param.value) for param in self.params if param.enabled]
             ),
diff --git a/tests/sample-collections/scripts/my_script.py b/tests/sample-collections/scripts/my_script.py
index 7b52f29e..a06faf7f 100644
--- a/tests/sample-collections/scripts/my_script.py
+++ b/tests/sample-collections/scripts/my_script.py
@@ -12,7 +12,6 @@ def setup(posting: Posting) -> None:
 
 def on_request(request: httpx.Request, posting: Posting) -> None:
     new_header = "Foo-Bar-Baz!!!!!"
-    request.headers["X-Custom-Header"] = new_header
     print(f"Set header to {new_header!r}!")
     posting.notify(
         message="Hello from my_script.py!",