docs: update model to gpt-4.1 and add LiquidJS reference

- Changed all examples from gpt-4o-mini to gpt-4.1
- Added note explaining that Vapi uses LiquidJS for variable substitution
- Clarified that {{ }} syntax follows Liquid template language conventions

VAP-11219

Co-Authored-By: Claude <[email protected]>

Author: vapi-tasker[bot]

fern/chat/variable-substitution.mdx

@@ -10,6 +10,10 @@ When using sessions with the Chat API, understanding how variable substitution w
 
 **Key concept:** Variables are substituted at session creation time and "baked into" the stored assistant configuration. Template placeholders like `{{name}}` are replaced with actual values and no longer exist in the session.
 
+<Note>
+Vapi uses [LiquidJS](https://liquidjs.com/) for variable substitution. The `{{ }}` syntax follows Liquid template language conventions, giving you access to filters, conditionals, and other Liquid features beyond simple variable replacement.
+</Note>
+
 ---
 
 ## How variable substitution works
@@ -19,7 +23,7 @@ When using sessions with the Chat API, understanding how variable substitution w
 When you create a session with `assistantOverrides.variableValues`, the system:
 
 1. Takes your assistant's template variables (e.g., `"Hello {{name}} from {{company}}"`)
-2. Substitutes all `{{}}` placeholders with provided values
+2. Substitutes all `{{ }}` placeholders using LiquidJS
 3. Stores the **pre-substituted assistant** in the session
 4. Saves the original variable values in `session.metadata.variableValues` for reference
 
@@ -69,7 +73,7 @@ If your assistant's system prompt was `"You are a helpful assistant for {{name}}
 When you send a chat request with a `sessionId`:
 
 1. The system loads the session's pre-substituted assistant
-2. Any `variableValues` in the chat request are processed, but **there are no `{{}}` placeholders left** to substitute
+2. Any `variableValues` in the chat request are processed, but **there are no `{{ }}` placeholders left** to substitute
 3. New variable values have **no effect** on already-substituted text
 
 ---
@@ -166,7 +170,7 @@ The assistant still responds with "John" and "Acme Corp" because the original te
 
 ### Provide fresh templates to use new values
 
-To use different variable values mid-session, provide a new template with `{{}}` placeholders along with the new values:
+To use different variable values mid-session, provide a new template with `{{ }}` placeholders along with the new values:
 
 <CodeBlocks>
 ```bash title="cURL"
@@ -179,7 +183,7 @@ curl -X POST https://api.vapi.ai/chat \
     "assistantOverrides": {
       "model": {
         "provider": "openai",
-        "model": "gpt-4o-mini",
+        "model": "gpt-4.1",
         "systemPrompt": "You are a helpful assistant for {{name}} at {{company}}. Be very formal."
       },
       "variableValues": {
@@ -196,7 +200,7 @@ const chat = await vapi.chat.create({
   assistantOverrides: {
     model: {
       provider: "openai",
-      model: "gpt-4o-mini",
+      model: "gpt-4.1",
       systemPrompt: "You are a helpful assistant for {{name}} at {{company}}. Be very formal."
     },
     variableValues: {
@@ -214,7 +218,7 @@ chat = vapi.chat.create(
     assistant_overrides={
         "model": {
             "provider": "openai",
-            "model": "gpt-4o-mini",
+            "model": "gpt-4.1",
             "systemPrompt": "You are a helpful assistant for {{name}} at {{company}}. Be very formal."
         },
         "variableValues": {
@@ -237,8 +241,8 @@ Now the assistant responds with "Jane" and "Wayne Enterprises" because fresh tem
 |----------|-------------------|-----|
 | Session creation with `variableValues` | ✅ Yes | Templates exist, substitution happens |
 | Chat with just `sessionId` | ✅ Session values persist | Pre-substituted assistant is used |
-| Chat with `sessionId` + new `variableValues` | ❌ No effect | No `{{}}` placeholders left to substitute |
-| Chat with `sessionId` + new template with `{{}}` + new `variableValues` | ✅ New values applied | Fresh templates provided |
+| Chat with `sessionId` + new `variableValues` | ❌ No effect | No `{{ }}` placeholders left to substitute |
+| Chat with `sessionId` + new template with `{{ }}` + new `variableValues` | ✅ New values applied | Fresh templates provided |
 
 ---
 
@@ -287,15 +291,15 @@ Choose one of these approaches:
     ```
   </Accordion>
   <Accordion title="Option B: Override with fresh templates">
-    Include a new system prompt (or other text field) with `{{}}` placeholders plus new `variableValues` in your chat request.
+    Include a new system prompt (or other text field) with `{{ }}` placeholders plus new `variableValues` in your chat request.
 
     ```typescript
     const chat = await vapi.chat.create({
       sessionId: "existing-session-id",
       assistantOverrides: {
         model: {
           provider: "openai",
-          model: "gpt-4o-mini",
+          model: "gpt-4.1",
           systemPrompt: "You assist {{customerName}} with their {{accountType}} account."
         },
         variableValues: {