diff --git a/docs/content/guides/http2.md b/docs/content/guides/http2.md
index 902d0d0c..34f3d230 100644
--- a/docs/content/guides/http2.md
+++ b/docs/content/guides/http2.md
@@ -225,7 +225,7 @@ server {
 
 !!! note
     For nginx to forward 103 Early Hints from upstream, you need nginx 1.29+
-    and the `early_hints` directive.
+    and the [`early_hints`](https://nginx.org/en/docs/http/ngx_http_core_module.html#early_hints) directive.
 
 ### Direct TLS Termination
 
@@ -312,10 +312,10 @@ http2_max_concurrent_streams = 200  # Increase from default 100
 
 ```bash
 # Check HTTP/2 support
-curl -v --http2 https://localhost:8443/
+curl -v --http2 https://localhost:443/
 
 # Force HTTP/2
-curl --http2-prior-knowledge https://localhost:8443/
+curl --http2-prior-knowledge https://localhost:443/
 ```
 
 ### Using Python
diff --git a/gunicorn/http/wsgi.py b/gunicorn/http/wsgi.py
index f26555d6..9db7d443 100644
--- a/gunicorn/http/wsgi.py
+++ b/gunicorn/http/wsgi.py
@@ -107,7 +107,7 @@ def proxy_environ(req):
     }
 
 
-def _make_early_hints_callback(req, sock):
+def _make_early_hints_callback(req, sock, resp):
     """Create a wsgi.early_hints callback for sending 103 Early Hints.
 
     This allows WSGI applications to send 103 Early Hints responses
@@ -116,6 +116,7 @@ def _make_early_hints_callback(req, sock):
     Args:
         req: The request object
         sock: The socket to write to
+        resp: The Response object to check if headers have been sent
 
     Returns:
         A callback function that accepts a list of (name, value) header tuples
@@ -125,6 +126,7 @@ def _make_early_hints_callback(req, sock):
         - Early hints are only sent for HTTP/1.1 or later clients
         - HTTP/1.0 clients will silently ignore the callback
         - Multiple calls are allowed (sending multiple 103 responses)
+        - Calls after response has started are silently ignored
     """
     def send_early_hints(headers):
         """Send 103 Early Hints response.
@@ -133,6 +135,10 @@ def _make_early_hints_callback(req, sock):
             headers: List of (name, value) header tuples, typically Link headers
                      Example: [('Link', '</style.css>; rel=preload; as=style')]
         """
+        # Don't send after response has started - would break framing
+        if resp.headers_sent:
+            return
+
         # Don't send to HTTP/1.0 clients - they don't support 1xx responses
         if req.version < (1, 1):
             return
@@ -242,7 +248,7 @@ def create(req, sock, client, server, cfg):
     environ.update(proxy_environ(req))
 
     # Add wsgi.early_hints callback for sending 103 Early Hints
-    environ['wsgi.early_hints'] = _make_early_hints_callback(req, sock)
+    environ['wsgi.early_hints'] = _make_early_hints_callback(req, sock, resp)
 
     return resp, environ
 
diff --git a/gunicorn/http2/request.py b/gunicorn/http2/request.py
index b1c2ac40..50683c48 100644
--- a/gunicorn/http2/request.py
+++ b/gunicorn/http2/request.py
@@ -133,8 +133,10 @@ class HTTP2Request:
             # Convert to uppercase for WSGI compatibility
             self.headers.append((name.upper(), value))
 
-        # Add Host header if not present (from :authority)
-        if authority and not any(h[0] == 'HOST' for h in self.headers):
+        # Set Host header from :authority (RFC 9113 section 8.3.1)
+        # :authority MUST take precedence over Host header
+        if authority:
+            self.headers = [(n, v) for n, v in self.headers if n != 'HOST']
             self.headers.append(('HOST', authority))
 
         # Trailers (if any)