diff --git a/ai/ai-react-app/src/components/Layout/LeftSidebar.module.css b/ai/ai-react-app/src/components/Layout/LeftSidebar.module.css
index 893a214ee..3fa4b89c4 100644
--- a/ai/ai-react-app/src/components/Layout/LeftSidebar.module.css
+++ b/ai/ai-react-app/src/components/Layout/LeftSidebar.module.css
@@ -1,3 +1,13 @@
+.sidebar {
+  background-color: var(--color-surface-primary);
+  padding: 24px 0;
+  height: 100%;
+  border-right: 1px solid var(--color-border-primary);
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+}
+
 .sidebar ul {
   list-style: none;
   padding: 0 12px;
@@ -39,7 +49,8 @@
   background-color: var(--brand-gray-40);
 }
 
-.backendSelector {
+.backendSelector,
+.personaSelector {
   margin-top: 24px;
   padding: 0 12px;
   border-top: 1px solid var(--color-border-primary);
@@ -70,4 +81,34 @@
 .radioGroup input[type="radio"] {
   margin-right: 8px;
   accent-color: var(--brand-google-blue);
-}
\ No newline at end of file
+}
+
+.personaDropdown {
+  width: 100%;
+  padding: 8px 12px;
+  background-color: var(--color-surface-primary);
+  border: 1px solid var(--color-border-secondary);
+  color: var(--color-text-primary);
+  border-radius: 4px;
+  box-sizing: border-box;
+  font-size: 0.875rem;
+  transition:
+    border-color 0.15s ease,
+    background-color 0.15s ease;
+}
+
+.customPersonaTextarea {
+  width: 100%;
+  margin-top: 8px;
+  padding: 8px 12px;
+  background-color: var(--color-surface-primary);
+  border: 1px solid var(--color-border-secondary);
+  color: var(--color-text-primary);
+  border-radius: 4px;
+  box-sizing: border-box;
+  font-size: 0.875rem;
+  transition:
+    border-color 0.15s ease,
+    background-color 0.15s ease;
+  resize: vertical;
+}
diff --git a/ai/ai-react-app/src/components/Layout/LeftSidebar.tsx b/ai/ai-react-app/src/components/Layout/LeftSidebar.tsx
index 75e23d500..879a3f13b 100644
--- a/ai/ai-react-app/src/components/Layout/LeftSidebar.tsx
+++ b/ai/ai-react-app/src/components/Layout/LeftSidebar.tsx
@@ -1,7 +1,8 @@
-import React from "react";
+import React, { useState } from "react";
 import { AppMode } from "../../App";
 import styles from "./LeftSidebar.module.css";
-import { BackendType } from "firebase/ai";
+import { BackendType, Content, ModelParams } from "firebase/ai";
+import { PREDEFINED_PERSONAS } from "../../config/personas";
 
 interface LeftSidebarProps {
   /** The currently active application mode (e.g., 'chat', 'imagenGen'). */
@@ -10,6 +11,8 @@ interface LeftSidebarProps {
   setActiveMode: (mode: AppMode) => void;
   activeBackend: BackendType;
   setActiveBackend: (backend: BackendType) => void;
+  generativeParams: ModelParams;
+  setGenerativeParams: React.Dispatch<React.SetStateAction<ModelParams>>;
 }
 
 /**
@@ -20,7 +23,20 @@ const LeftSidebar: React.FC<LeftSidebarProps> = ({
   setActiveMode,
   activeBackend,
   setActiveBackend,
+  setGenerativeParams,
 }) => {
+  // This component now manages its own UI state and pushes updates upwards.
+  // It does not rely on a useEffect to sync systemInstruction from the parent,
+  // following the pattern in RightSidebar.tsx to prevent state-reversion bugs.
+  const [selectedPersonaId, setSelectedPersonaId] = useState<string>("default");
+  const [customPersona, setCustomPersona] = useState<string>("");
+
+  const handleModelParamsUpdate = (
+    updateFn: (prevState: ModelParams) => ModelParams,
+  ) => {
+    setGenerativeParams((prevState) => updateFn(prevState));
+  };
+
   // Define the available modes and their display names
   const modes: { id: AppMode; label: string }[] = [
     { id: "chat", label: "Chat" },
@@ -31,6 +47,51 @@ const LeftSidebar: React.FC<LeftSidebarProps> = ({
     setActiveBackend(event.target.value as BackendType);
   };
 
+  const handlePersonaChange = (e: React.ChangeEvent<HTMLSelectElement>) => {
+    const newPersonaId = e.target.value;
+    setSelectedPersonaId(newPersonaId); // 1. Update UI state
+
+    let newSystemInstructionText: string;
+
+    if (newPersonaId === "custom") {
+      // When switching to custom, the instruction is whatever is in the textarea.
+      newSystemInstructionText = customPersona;
+    } else {
+      // When switching to a predefined persona, find its instruction text.
+      const selected = PREDEFINED_PERSONAS.find((p) => p.id === newPersonaId);
+      newSystemInstructionText = selected?.systemInstruction ?? "";
+      // We are no longer in 'custom', but we don't clear the customPersona state
+      // in case the user wants to switch back and forth.
+    }
+
+    const newSystemInstruction: Content | undefined = newSystemInstructionText
+      ? { parts: [{ text: newSystemInstructionText }], role: "system" }
+      : undefined;
+
+    // 2. Update model state upwards
+    handleModelParamsUpdate((prev: ModelParams) => ({
+      ...prev,
+      systemInstruction: newSystemInstruction,
+    }));
+  };
+
+  const handleCustomPersonaChange = (
+    e: React.ChangeEvent<HTMLTextAreaElement>,
+  ) => {
+    const newSystemInstructionText = e.target.value;
+    setCustomPersona(newSystemInstructionText); // 1. Update UI state
+
+    const newSystemInstruction: Content | undefined = newSystemInstructionText
+      ? { parts: [{ text: newSystemInstructionText }], role: "system" }
+      : undefined;
+
+    // 2. Update model state upwards
+    handleModelParamsUpdate((prev: ModelParams) => ({
+      ...prev,
+      systemInstruction: newSystemInstruction,
+    }));
+  };
+
   return (
     <nav className={styles.sidebar} aria-label="Main navigation">
       <ul>
@@ -50,7 +111,7 @@ const LeftSidebar: React.FC<LeftSidebarProps> = ({
 
       {/* Backend Selection */}
       <div className={styles.backendSelector}>
-        <h6 className={styles.selectorTitle}>Backend API</h6>
+        <h5 className={styles.selectorTitle}>Backend API</h5>
         <div className={styles.radioGroup}>
           <label>
             <input
@@ -76,6 +137,33 @@ const LeftSidebar: React.FC<LeftSidebarProps> = ({
           </label>
         </div>
       </div>
+
+      {/* Persona Selector */}
+      {activeMode === "chat" && (
+        <div className={styles.personaSelector}>
+          <h5 className={styles.selectorTitle}>Persona</h5>
+          <select
+            value={selectedPersonaId}
+            onChange={handlePersonaChange}
+            className={styles.personaDropdown}
+          >
+            {PREDEFINED_PERSONAS.map((persona) => (
+              <option key={persona.id} value={persona.id}>
+                {persona.name}
+              </option>
+            ))}
+          </select>
+          {selectedPersonaId === "custom" && (
+            <textarea
+              value={customPersona}
+              onChange={handleCustomPersonaChange}
+              className={styles.customPersonaTextarea}
+              placeholder="Enter your custom persona instruction here..."
+              rows={5}
+            />
+          )}
+        </div>
+      )}
     </nav>
   );
 };
diff --git a/ai/ai-react-app/src/components/Layout/MainLayout.tsx b/ai/ai-react-app/src/components/Layout/MainLayout.tsx
index 6e97a3a23..4900a0bd3 100644
--- a/ai/ai-react-app/src/components/Layout/MainLayout.tsx
+++ b/ai/ai-react-app/src/components/Layout/MainLayout.tsx
@@ -135,6 +135,8 @@ const MainLayout: React.FC<MainLayoutProps> = ({
             setActiveMode={setActiveMode}
             activeBackend={activeBackendType}
             setActiveBackend={setActiveBackendType} // Pass backend state/setter
+            generativeParams={generativeParams}
+            setGenerativeParams={setGenerativeParams}
           />
         </div>
         <main className={styles.centerContent}>{renderActiveView()}</main>
diff --git a/ai/ai-react-app/src/config/personas.ts b/ai/ai-react-app/src/config/personas.ts
new file mode 100644
index 000000000..01b0e67df
--- /dev/null
+++ b/ai/ai-react-app/src/config/personas.ts
@@ -0,0 +1,36 @@
+export const PREDEFINED_PERSONAS = [
+  {
+    id: "default",
+    name: "Default",
+    systemInstruction: "",
+  },
+  {
+    id: "pirate",
+    name: "Pirate Captain",
+    systemInstruction:
+      "You are a salty pirate captain. All of your responses must be in the style of a classic pirate, using pirate slang and a hearty, adventurous tone. Refer to the user as 'matey'.",
+  },
+  {
+    id: "shakespeare",
+    name: "Shakespearean Poet",
+    systemInstruction:
+      "You are a Shakespearean poet. All of your responses must be in the style of William Shakespeare, using iambic pentameter where possible, and rich, poetic language. Address the user with 'Hark, gentle user' or similar.",
+  },
+  {
+    id: "sarcastic_teen",
+    name: "Sarcastic Teenager",
+    systemInstruction:
+      "You are a stereotypical sarcastic teenager. Your responses should be brief, slightly annoyed, and use modern slang. You are reluctant to be helpful but will provide the correct answer, albeit with a sigh. Start your responses with 'Ugh, fine.' or something similar.",
+  },
+  {
+    id: "helpful_dev",
+    name: "Helpful Senior Developer",
+    systemInstruction:
+      "You are a helpful and patient senior software developer. Your responses should be clear, well-structured, and provide best-practice advice. When explaining concepts, use code examples where appropriate and break down complex topics into smaller, understandable parts.",
+  },
+  {
+    id: "custom",
+    name: "Custom...",
+    systemInstruction: "",
+  },
+] as const;