feat: interpolate template variables in custom connection headers

Admins can now use {{chat_id}}, {{message_id}}, {{user_id}}, and
{{user_name}} as dynamic placeholders in custom header values for
OpenAI-compatible connections.

Before merging custom headers into the outgoing request,
get_headers_and_cookies() performs a simple string-replace pass
using values already available in the metadata dict and user object.
Header values with no {{...}} tokens pass through unchanged, and
unknown tokens are left as-is, so the change is fully backwards
compatible.

Use-case: providers like Manifest that use a session-scoped header
(e.g. x-session-key) to route or isolate conversations can now be
configured with {"x-session-key": "{{chat_id}}"} so every request
automatically carries the current chat UUID.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/open_webui/routers/openai.py

@@ -215,7 +215,19 @@ async def get_headers_and_cookies(
         headers['Authorization'] = f'Bearer {token}'
 
     if config.get('headers') and isinstance(config.get('headers'), dict):
-        headers = {**headers, **config.get('headers')}
+        template_vars = {
+            '{{chat_id}}': (metadata or {}).get('chat_id', '') or '',
+            '{{message_id}}': (metadata or {}).get('message_id', '') or '',
+            '{{user_id}}': (user.id if user else '') or '',
+            '{{user_name}}': (user.name if user else '') or '',
+        }
+        custom_headers = {}
+        for k, v in config.get('headers').items():
+            if isinstance(v, str):
+                for token, value in template_vars.items():
+                    v = v.replace(token, value)
+            custom_headers[k] = v
+        headers = {**headers, **custom_headers}
 
     return headers, cookies
 

src/lib/components/AddConnectionModal.svelte

@@ -444,7 +444,7 @@
 									<div class="flex-1">
 										<Tooltip
 											content={$i18n.t(
-												'Enter additional headers in JSON format (e.g. {"X-Custom-Header": "value"}'
+												'Enter additional headers in JSON format (e.g. {"X-Custom-Header": "value"}). Dynamic values supported: {{chat_id}}, {{message_id}}, {{user_id}}, {{user_name}}'
 											)}
 										>
 											<Textarea