diff --git a/drizzle/0024_user_assistant_customization.sql b/drizzle/0024_user_assistant_customization.sql
new file mode 100644
index 0000000..e28608f
--- /dev/null
+++ b/drizzle/0024_user_assistant_customization.sql
@@ -0,0 +1,2 @@
+ALTER TABLE "users" ADD COLUMN "assistant_name" text DEFAULT 'Assistant' NOT NULL;--> statement-breakpoint
+ALTER TABLE "users" ADD COLUMN "assistant_system_prompt" text;
diff --git a/drizzle/meta/_journal.json b/drizzle/meta/_journal.json
index 04f9b73..229b5ad 100644
--- a/drizzle/meta/_journal.json
+++ b/drizzle/meta/_journal.json
@@ -169,6 +169,13 @@
       "when": 1780393000000,
       "tag": "0023_comments",
       "breakpoints": true
+    },
+    {
+      "idx": 24,
+      "version": "7",
+      "when": 1780394000000,
+      "tag": "0024_user_assistant_customization",
+      "breakpoints": true
     }
   ]
 }
\ No newline at end of file
diff --git a/src/app/api/agent/chat/route.ts b/src/app/api/agent/chat/route.ts
index 9a904f1..287c62a 100644
--- a/src/app/api/agent/chat/route.ts
+++ b/src/app/api/agent/chat/route.ts
@@ -1,7 +1,7 @@
 import { z } from "zod";
 import { apiError, apiJson } from "@/lib/api-handler";
 import { resolveApiAuth } from "@/lib/api-auth";
-import { getAssistantEnabled } from "@/lib/assistant-preference";
+import { getAssistantPreferences, resolveAssistantSystemPrompt } from "@/lib/assistant-preference";
 import { isLlmConfigured } from "@/lib/llm";
 import { encodeSseEvent } from "@/modules/agent/server/progress";
 import { runAgentChat } from "@/modules/agent/server/run";
@@ -25,11 +25,13 @@ export async function POST(request: Request) {
     return apiError("Unauthorized", 401);
   }
 
-  const assistantEnabled = await getAssistantEnabled(auth.userId);
-  if (!assistantEnabled) {
+  const assistant = await getAssistantPreferences(auth.userId);
+  if (!assistant.enabled) {
     return apiError("Assistant not enabled", 403);
   }
 
+  const systemPrompt = resolveAssistantSystemPrompt(assistant.systemPrompt);
+
   let body: unknown;
   try {
     body = await request.json();
@@ -54,6 +56,7 @@ export async function POST(request: Request) {
           const result = await runAgentChat({
             messages: parsed.data.messages,
             request,
+            systemPrompt,
             onProgress: send,
           });
 
@@ -87,6 +90,7 @@ export async function POST(request: Request) {
     const result = await runAgentChat({
       messages: parsed.data.messages,
       request,
+      systemPrompt,
     });
 
     return apiJson({
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index e91dc60..ef104b1 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -18,6 +18,7 @@ import { InstallPrompt } from "@/components/install-prompt";
 import { AppShell } from "@/components/app-shell";
 import { AppToaster } from "@/components/app-toaster";
 import { AssistantBubble } from "@/modules/agent/components/assistant-bubble";
+import { DEFAULT_ASSISTANT_NAME } from "@/lib/assistant-config";
 import { isLlmConfigured } from "@/lib/llm";
 import { DEFAULT_THEME, navStyleToDataNav } from "@/modules/_core/themes";
 import type { Palette, ThemeMode, FontPair, Density, NavStyle } from "@/modules/_core/themes";
@@ -102,6 +103,7 @@ export default async function RootLayout({ children }: { children: React.ReactNo
   let userDashboards: DashboardMeta[] = [];
   let signedIn = false;
   let assistantEnabled = false;
+  let assistantName = DEFAULT_ASSISTANT_NAME;
 
   const session = await auth();
   if (session?.user?.id) {
@@ -114,6 +116,7 @@ export default async function RootLayout({ children }: { children: React.ReactNo
         themeDensity: users.themeDensity,
         themeNavStyle: users.themeNavStyle,
         assistantEnabled: users.assistantEnabled,
+        assistantName: users.assistantName,
       })
       .from(users)
       .where(eq(users.id, session.user.id))
@@ -125,6 +128,7 @@ export default async function RootLayout({ children }: { children: React.ReactNo
       density = row.themeDensity as Density;
       navStyle = row.themeNavStyle as NavStyle;
       assistantEnabled = row.assistantEnabled;
+      assistantName = row.assistantName?.trim() || DEFAULT_ASSISTANT_NAME;
     }
     userDashboards = await db
       .select({
@@ -178,7 +182,11 @@ export default async function RootLayout({ children }: { children: React.ReactNo
           <InstallPrompt />
           <PwaRegister />
           {signedIn && assistantEnabled && session?.user?.id ? (
-            <AssistantBubble configured={isLlmConfigured()} userId={session.user.id} />
+            <AssistantBubble
+              configured={isLlmConfigured()}
+              userId={session.user.id}
+              assistantName={assistantName}
+            />
           ) : null}
           <AppToaster position="bottom-right" />
         </QuickAddProvider>
diff --git a/src/app/settings/assistant-actions.ts b/src/app/settings/assistant-actions.ts
index ea923ef..0adfe98 100644
--- a/src/app/settings/assistant-actions.ts
+++ b/src/app/settings/assistant-actions.ts
@@ -2,13 +2,58 @@
 
 import { eq } from "drizzle-orm";
 import { revalidatePath } from "next/cache";
+import { z } from "zod";
 import { db } from "@/lib/db";
+import {
+  DEFAULT_ASSISTANT_NAME,
+  MAX_ASSISTANT_NAME_LENGTH,
+  MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH,
+} from "@/lib/assistant-config";
 import { users } from "@/modules/_core/schema";
 import { getCurrentSession } from "@/lib/session";
 
+const assistantNameSchema = z
+  .string()
+  .trim()
+  .min(1, "Name is required")
+  .max(MAX_ASSISTANT_NAME_LENGTH);
+
+const assistantSystemPromptSchema = z
+  .string()
+  .trim()
+  .min(1, "Prompt cannot be empty")
+  .max(MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH);
+
+function revalidateAssistantSurfaces() {
+  revalidatePath("/settings");
+  revalidatePath("/", "layout");
+}
+
 export async function setAssistantEnabled(enabled: boolean): Promise<void> {
   const { user } = await getCurrentSession();
   await db.update(users).set({ assistantEnabled: enabled }).where(eq(users.id, user.id));
-  revalidatePath("/settings");
-  revalidatePath("/", "layout");
+  revalidateAssistantSurfaces();
+}
+
+export async function setAssistantName(name: string): Promise<void> {
+  const parsed = assistantNameSchema.parse(name);
+  const { user } = await getCurrentSession();
+  await db.update(users).set({ assistantName: parsed }).where(eq(users.id, user.id));
+  revalidateAssistantSurfaces();
+}
+
+export async function setAssistantSystemPrompt(prompt: string | null): Promise<void> {
+  const { user } = await getCurrentSession();
+  const normalized = prompt === null ? null : assistantSystemPromptSchema.parse(prompt);
+
+  await db.update(users).set({ assistantSystemPrompt: normalized }).where(eq(users.id, user.id));
+  revalidateAssistantSurfaces();
+}
+
+export async function resetAssistantName(): Promise<void> {
+  await setAssistantName(DEFAULT_ASSISTANT_NAME);
+}
+
+export async function resetAssistantSystemPrompt(): Promise<void> {
+  await setAssistantSystemPrompt(null);
 }
diff --git a/src/app/settings/page.tsx b/src/app/settings/page.tsx
index c0b3213..a495932 100644
--- a/src/app/settings/page.tsx
+++ b/src/app/settings/page.tsx
@@ -13,7 +13,8 @@ import { AvatarFallbackWithName } from "@/components/avatar-fallback";
 import { revokeShareLinkAction } from "./actions";
 import { getHouseholdApiTokenStatus } from "@/modules/_core/api-token";
 import { ApiTokenSettings } from "@/components/api-token-settings";
-import { AssistantOptIn } from "@/components/assistant-opt-in";
+import { AssistantSettings } from "@/components/assistant-settings";
+import { AGENT_SYSTEM_PROMPT } from "@/modules/agent/tools";
 import { DefaultEventRemindersSetting } from "@/components/default-event-reminders-setting";
 import { listCalendars } from "@/modules/calendar/server/queries";
 import { listLists } from "@/modules/lists/server/queries";
@@ -322,6 +323,8 @@ function AppearanceSection({
     themeCalView: string;
     themeNavStyle: string;
     assistantEnabled: boolean;
+    assistantName: string;
+    assistantSystemPrompt: string | null;
   };
 }) {
   return (
@@ -355,7 +358,12 @@ function AppearanceSection({
           <MessageCircle className="size-4 text-[var(--ink-mute)]" />
         </CardHeader>
         <CardContent>
-          <AssistantOptIn enabled={user.assistantEnabled} />
+          <AssistantSettings
+            enabled={user.assistantEnabled}
+            name={user.assistantName}
+            systemPrompt={user.assistantSystemPrompt}
+            defaultSystemPrompt={AGENT_SYSTEM_PROMPT}
+          />
         </CardContent>
       </Card>
     </>
diff --git a/src/components/assistant-opt-in.tsx b/src/components/assistant-opt-in.tsx
deleted file mode 100644
index 50a4621..0000000
--- a/src/components/assistant-opt-in.tsx
+++ /dev/null
@@ -1,35 +0,0 @@
-"use client";
-
-import { useRouter } from "next/navigation";
-import { useTransition } from "react";
-import { setAssistantEnabled } from "@/app/settings/assistant-actions";
-import { Switch } from "@/components/ui/switch";
-
-export function AssistantOptIn({ enabled }: { enabled: boolean }) {
-  const [isPending, startTransition] = useTransition();
-  const router = useRouter();
-
-  function toggle(next: boolean) {
-    startTransition(async () => {
-      await setAssistantEnabled(next);
-      router.refresh();
-    });
-  }
-
-  return (
-    <label className="flex items-center justify-between gap-4">
-      <div className="min-w-0">
-        <div className="text-sm font-medium">AI assistant</div>
-        <p className="muted text-[12px] mt-0.5">
-          Off by default. Turn on to show a chat bubble in the bottom-right corner.
-        </p>
-      </div>
-      <Switch
-        checked={enabled}
-        disabled={isPending}
-        onCheckedChange={toggle}
-        aria-label="AI assistant"
-      />
-    </label>
-  );
-}
diff --git a/src/components/assistant-settings.tsx b/src/components/assistant-settings.tsx
new file mode 100644
index 0000000..ec0ccb0
--- /dev/null
+++ b/src/components/assistant-settings.tsx
@@ -0,0 +1,185 @@
+"use client";
+
+import { useRouter } from "next/navigation";
+import { useState, useTransition } from "react";
+import {
+  resetAssistantName,
+  resetAssistantSystemPrompt,
+  setAssistantEnabled,
+  setAssistantName,
+  setAssistantSystemPrompt,
+} from "@/app/settings/assistant-actions";
+import { DEFAULT_ASSISTANT_NAME, MAX_ASSISTANT_NAME_LENGTH } from "@/lib/assistant-config";
+import { Button } from "@/components/ui/button";
+import { Input } from "@/components/ui/input";
+import { Switch } from "@/components/ui/switch";
+
+type Props = {
+  enabled: boolean;
+  name: string;
+  systemPrompt: string | null;
+  defaultSystemPrompt: string;
+};
+
+export function AssistantSettings({ enabled, name, systemPrompt, defaultSystemPrompt }: Props) {
+  const [isPending, startTransition] = useTransition();
+  const router = useRouter();
+
+  const effectivePrompt = systemPrompt ?? defaultSystemPrompt;
+  const [savedName, setSavedName] = useState(name);
+  const [draftName, setDraftName] = useState(name);
+  const [savedPrompt, setSavedPrompt] = useState(systemPrompt);
+  const [draftPrompt, setDraftPrompt] = useState(effectivePrompt);
+
+  const nameDirty = draftName.trim() !== savedName;
+  const promptDirty =
+    savedPrompt === null
+      ? draftPrompt.trim() !== defaultSystemPrompt.trim()
+      : draftPrompt.trim() !== savedPrompt.trim();
+  const usingDefaultPrompt = savedPrompt === null;
+
+  function toggle(next: boolean) {
+    startTransition(async () => {
+      await setAssistantEnabled(next);
+      router.refresh();
+    });
+  }
+
+  function saveName() {
+    const next = draftName.trim();
+    if (!next) return;
+
+    startTransition(async () => {
+      await setAssistantName(next);
+      setSavedName(next);
+      setDraftName(next);
+      router.refresh();
+    });
+  }
+
+  function savePrompt() {
+    const next = draftPrompt.trim();
+    if (!next) return;
+
+    startTransition(async () => {
+      const isDefault = next === defaultSystemPrompt.trim();
+      await setAssistantSystemPrompt(isDefault ? null : next);
+      setSavedPrompt(isDefault ? null : next);
+      setDraftPrompt(next);
+      router.refresh();
+    });
+  }
+
+  function handleResetName() {
+    startTransition(async () => {
+      await resetAssistantName();
+      setSavedName(DEFAULT_ASSISTANT_NAME);
+      setDraftName(DEFAULT_ASSISTANT_NAME);
+      router.refresh();
+    });
+  }
+
+  function handleResetPrompt() {
+    startTransition(async () => {
+      await resetAssistantSystemPrompt();
+      setSavedPrompt(null);
+      setDraftPrompt(defaultSystemPrompt);
+      router.refresh();
+    });
+  }
+
+  return (
+    <div className="flex flex-col gap-5">
+      <label className="flex items-center justify-between gap-4">
+        <div className="min-w-0">
+          <div className="text-sm font-medium">AI assistant</div>
+          <p className="muted text-[12px] mt-0.5">
+            Off by default. Turn on to show a chat bubble in the bottom-right corner.
+          </p>
+        </div>
+        <Switch
+          checked={enabled}
+          disabled={isPending}
+          onCheckedChange={toggle}
+          aria-label="AI assistant"
+        />
+      </label>
+
+      <div className="flex flex-col gap-2">
+        <div className="flex items-end justify-between gap-3">
+          <div className="min-w-0 flex-1">
+            <label htmlFor="assistant-name" className="text-sm font-medium">
+              Assistant name
+            </label>
+            <p className="muted text-[12px] mt-0.5">Shown in the chat bubble and message labels.</p>
+          </div>
+          <Button
+            type="button"
+            variant="ghost"
+            size="sm"
+            disabled={isPending || savedName === DEFAULT_ASSISTANT_NAME}
+            onClick={handleResetName}
+          >
+            Reset
+          </Button>
+        </div>
+        <Input
+          id="assistant-name"
+          value={draftName}
+          maxLength={MAX_ASSISTANT_NAME_LENGTH}
+          disabled={isPending}
+          onChange={(event) => setDraftName(event.target.value)}
+          aria-label="Assistant name"
+        />
+        <Button
+          type="button"
+          size="sm"
+          disabled={!nameDirty || isPending || !draftName.trim()}
+          onClick={saveName}
+        >
+          {isPending ? "Saving…" : "Save name"}
+        </Button>
+      </div>
+
+      <div className="flex flex-col gap-2">
+        <div className="flex items-end justify-between gap-3">
+          <div className="min-w-0 flex-1">
+            <label htmlFor="assistant-system-prompt" className="text-sm font-medium">
+              System prompt
+            </label>
+            <p className="muted text-[12px] mt-0.5">
+              Instructions sent to the model before each conversation. Customize tone, priorities,
+              or household context.
+            </p>
+          </div>
+          <Button
+            type="button"
+            variant="ghost"
+            size="sm"
+            disabled={isPending || usingDefaultPrompt}
+            onClick={handleResetPrompt}
+          >
+            Reset
+          </Button>
+        </div>
+        <textarea
+          id="assistant-system-prompt"
+          value={draftPrompt}
+          rows={10}
+          disabled={isPending}
+          onChange={(event) => setDraftPrompt(event.target.value)}
+          aria-label="Assistant system prompt"
+          className="input min-h-[180px] resize-y font-mono text-[12px] leading-relaxed"
+        />
+        <Button
+          type="button"
+          size="sm"
+          disabled={!promptDirty || isPending || !draftPrompt.trim()}
+          onClick={savePrompt}
+        >
+          {isPending ? "Saving…" : "Save prompt"}
+        </Button>
+      </div>
+    </div>
+  );
+}
diff --git a/src/lib/assistant-config.ts b/src/lib/assistant-config.ts
new file mode 100644
index 0000000..46e1a4e
--- /dev/null
+++ b/src/lib/assistant-config.ts
@@ -0,0 +1,10 @@
+import { AGENT_SYSTEM_PROMPT } from "@/modules/agent/tools";
+
+export const DEFAULT_ASSISTANT_NAME = "Assistant";
+export const MAX_ASSISTANT_NAME_LENGTH = 40;
+export const MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH = 8000;
+
+export function resolveAssistantSystemPrompt(customPrompt: string | null | undefined): string {
+  const trimmed = customPrompt?.trim();
+  return trimmed ? trimmed : AGENT_SYSTEM_PROMPT;
+}
diff --git a/src/lib/assistant-preference.ts b/src/lib/assistant-preference.ts
index be755ff..b978671 100644
--- a/src/lib/assistant-preference.ts
+++ b/src/lib/assistant-preference.ts
@@ -1,13 +1,40 @@
 import { eq } from "drizzle-orm";
 import { db } from "@/lib/db";
+import { DEFAULT_ASSISTANT_NAME } from "@/lib/assistant-config";
 import { users } from "@/modules/_core/schema";
 
-export async function getAssistantEnabled(userId: string): Promise<boolean> {
+export {
+  DEFAULT_ASSISTANT_NAME,
+  MAX_ASSISTANT_NAME_LENGTH,
+  MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH,
+  resolveAssistantSystemPrompt,
+} from "@/lib/assistant-config";
+
+export type AssistantPreferences = {
+  enabled: boolean;
+  name: string;
+  systemPrompt: string | null;
+};
+
+export async function getAssistantPreferences(userId: string): Promise<AssistantPreferences> {
   const [row] = await db
-    .select({ assistantEnabled: users.assistantEnabled })
+    .select({
+      assistantEnabled: users.assistantEnabled,
+      assistantName: users.assistantName,
+      assistantSystemPrompt: users.assistantSystemPrompt,
+    })
     .from(users)
     .where(eq(users.id, userId))
     .limit(1);
 
-  return row?.assistantEnabled ?? false;
+  return {
+    enabled: row?.assistantEnabled ?? false,
+    name: row?.assistantName?.trim() || DEFAULT_ASSISTANT_NAME,
+    systemPrompt: row?.assistantSystemPrompt ?? null,
+  };
+}
+
+export async function getAssistantEnabled(userId: string): Promise<boolean> {
+  const prefs = await getAssistantPreferences(userId);
+  return prefs.enabled;
 }
diff --git a/src/modules/_core/schema.ts b/src/modules/_core/schema.ts
index b65b171..d5d2875 100644
--- a/src/modules/_core/schema.ts
+++ b/src/modules/_core/schema.ts
@@ -36,6 +36,8 @@ export const users = pgTable("users", {
   notifInApp: boolean("notif_inapp").notNull().default(true),
   notifNtfy: boolean("notif_ntfy").notNull().default(false),
   assistantEnabled: boolean("assistant_enabled").notNull().default(false),
+  assistantName: text("assistant_name").notNull().default("Assistant"),
+  assistantSystemPrompt: text("assistant_system_prompt"),
   defaultEventReminderOffsets: jsonb("default_event_reminder_offsets")
     .notNull()
     .$type<number[]>()
diff --git a/src/modules/agent/components/assistant-bubble.tsx b/src/modules/agent/components/assistant-bubble.tsx
index b98a028..321815f 100644
--- a/src/modules/agent/components/assistant-bubble.tsx
+++ b/src/modules/agent/components/assistant-bubble.tsx
@@ -7,9 +7,10 @@ import { AssistantPanel } from "./assistant-panel";
 type Props = {
   configured: boolean;
   userId: string;
+  assistantName: string;
 };
 
-export function AssistantBubble({ configured, userId }: Props) {
+export function AssistantBubble({ configured, userId, assistantName }: Props) {
   const [open, setOpen] = useState(false);
 
   return (
@@ -18,31 +19,36 @@ export function AssistantBubble({ configured, userId }: Props) {
         <div
           className="assistant-bubble-panel"
           role="dialog"
-          aria-label="Assistant"
+          aria-label={assistantName}
           aria-modal="false"
         >
           <div className="assistant-bubble-header">
             <div>
-              <div className="serif text-[15px] font-medium tracking-tight">Assistant</div>
+              <div className="serif text-[15px] font-medium tracking-tight">{assistantName}</div>
               <div className="muted text-[11px]">Household helper</div>
             </div>
             <button
               type="button"
               className="assistant-bubble-close"
-              aria-label="Close assistant"
+              aria-label={`Close ${assistantName}`}
               onClick={() => setOpen(false)}
             >
               <X className="size-4" />
             </button>
           </div>
-          <AssistantPanel key={userId} configured={configured} userId={userId} />
+          <AssistantPanel
+            key={userId}
+            configured={configured}
+            userId={userId}
+            assistantName={assistantName}
+          />
         </div>
       ) : null}
 
       <button
         type="button"
         className="assistant-bubble-trigger"
-        aria-label={open ? "Close assistant" : "Open assistant"}
+        aria-label={open ? `Close ${assistantName}` : `Open ${assistantName}`}
         aria-expanded={open}
         onClick={() => setOpen((current) => !current)}
       >
diff --git a/src/modules/agent/components/assistant-panel.tsx b/src/modules/agent/components/assistant-panel.tsx
index 41512a7..ecde6e6 100644
--- a/src/modules/agent/components/assistant-panel.tsx
+++ b/src/modules/agent/components/assistant-panel.tsx
@@ -15,9 +15,10 @@ import {
 type Props = {
   configured: boolean;
   userId: string;
+  assistantName: string;
 };
 
-export function AssistantPanel({ configured, userId }: Props) {
+export function AssistantPanel({ configured, userId, assistantName }: Props) {
   const [messages, setMessages] = useState<AssistantChatMessage[]>(() => loadAssistantChat(userId));
   const [input, setInput] = useState("");
   const [error, setError] = useState<string | null>(null);
@@ -144,7 +145,7 @@ export function AssistantPanel({ configured, userId }: Props) {
                 style={message.role === "assistant" ? { borderColor: "var(--hair)" } : undefined}
               >
                 <div className="eyebrow mb-0.5 text-[10px]">
-                  {message.role === "user" ? "You" : "Assistant"}
+                  {message.role === "user" ? "You" : assistantName}
                 </div>
                 {message.content}
               </div>
@@ -157,7 +158,7 @@ export function AssistantPanel({ configured, userId }: Props) {
                 aria-live="polite"
                 aria-busy="true"
               >
-                <div className="eyebrow mb-1 text-[10px]">Assistant</div>
+                <div className="eyebrow mb-1 text-[10px]">{assistantName}</div>
                 <div className="flex items-center gap-2 text-[12.5px] text-muted-foreground">
                   <Loader2 className="size-3.5 shrink-0 animate-spin" />
                   <span>{activityLabel ?? "Working…"}</span>
@@ -180,9 +181,9 @@ export function AssistantPanel({ configured, userId }: Props) {
         <Input
           value={input}
           onChange={(event) => setInput(event.target.value)}
-          placeholder="Ask the assistant…"
+          placeholder={`Ask ${assistantName}…`}
           disabled={isPending}
-          aria-label="Assistant message"
+          aria-label={`Message for ${assistantName}`}
           className="h-9"
         />
         <Button type="submit" size="sm" disabled={isPending || !input.trim()} aria-label="Send">
diff --git a/src/modules/agent/server/run.ts b/src/modules/agent/server/run.ts
index 47ed67c..514e6dc 100644
--- a/src/modules/agent/server/run.ts
+++ b/src/modules/agent/server/run.ts
@@ -26,6 +26,7 @@ export type AgentProgressHandler = (event: AgentProgressEvent) => void;
 export async function runAgentChat(options: {
   messages: ClientChatMessage[];
   request: Request;
+  systemPrompt?: string;
   llm?: LlmClient;
   executeTool?: ToolExecutor;
   onProgress?: AgentProgressHandler;
@@ -33,9 +34,10 @@ export async function runAgentChat(options: {
   const llm = options.llm ?? createLlmClient();
   const executeTool = options.executeTool ?? createApiToolExecutor(options.request);
   const onProgress = options.onProgress;
+  const systemPrompt = options.systemPrompt ?? AGENT_SYSTEM_PROMPT;
 
   const transcript: ChatMessage[] = [
-    { role: "system", content: AGENT_SYSTEM_PROMPT },
+    { role: "system", content: systemPrompt },
     ...options.messages.map(
       (message): ChatMessage => ({
         role: message.role,
diff --git a/tests/e2e/assistant.spec.ts b/tests/e2e/assistant.spec.ts
index 40fdc45..eda2bcd 100644
--- a/tests/e2e/assistant.spec.ts
+++ b/tests/e2e/assistant.spec.ts
@@ -16,7 +16,7 @@ test("assistant chat smoke after opt-in", async ({ page }) => {
   await page.getByRole("button", { name: "Open assistant" }).click();
   await expect(page.getByRole("dialog", { name: "Assistant" })).toBeVisible();
 
-  await page.getByLabel("Assistant message").fill("hello assistant");
+  await page.getByLabel("Message for Assistant").fill("hello assistant");
   await page.getByRole("button", { name: "Send" }).click();
 
   await expect(page.getByText("hello assistant")).toBeVisible();
diff --git a/tests/unit/agent-chat.test.ts b/tests/unit/agent-chat.test.ts
index fab9371..18e399a 100644
--- a/tests/unit/agent-chat.test.ts
+++ b/tests/unit/agent-chat.test.ts
@@ -71,4 +71,16 @@ describe("runAgentChat", () => {
     assert.equal(result.toolCalls[0]?.name, "add_list_item");
     assert.equal(result.toolCalls[0]?.status, 201);
   });
+
+  it("accepts a custom system prompt override", async () => {
+    const result = await runAgentChat({
+      messages: [{ role: "user", content: "hello" }],
+      request: new Request("http://localhost:3000/api/agent/chat"),
+      systemPrompt: "You are a pirate.",
+      llm: createMockLlmClient(),
+    });
+
+    assert.equal(result.message.role, "assistant");
+    assert.ok(result.message.content.length > 0);
+  });
 });
diff --git a/tests/unit/assistant-preference.test.ts b/tests/unit/assistant-preference.test.ts
new file mode 100644
index 0000000..5903d14
--- /dev/null
+++ b/tests/unit/assistant-preference.test.ts
@@ -0,0 +1,27 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  DEFAULT_ASSISTANT_NAME,
+  resolveAssistantSystemPrompt,
+} from "../../src/lib/assistant-config";
+import { AGENT_SYSTEM_PROMPT } from "../../src/modules/agent/tools";
+
+describe("resolveAssistantSystemPrompt", () => {
+  it("returns the default prompt when custom is null", () => {
+    assert.equal(resolveAssistantSystemPrompt(null), AGENT_SYSTEM_PROMPT);
+  });
+
+  it("returns the default prompt when custom is blank", () => {
+    assert.equal(resolveAssistantSystemPrompt("   "), AGENT_SYSTEM_PROMPT);
+  });
+
+  it("returns trimmed custom prompt when set", () => {
+    assert.equal(resolveAssistantSystemPrompt("  Be extra cheerful.  "), "Be extra cheerful.");
+  });
+});
+
+describe("DEFAULT_ASSISTANT_NAME", () => {
+  it("is Assistant", () => {
+    assert.equal(DEFAULT_ASSISTANT_NAME, "Assistant");
+  });
+});