diff --git a/README.md b/README.md
index e2dba84..48e80c4 100644
--- a/README.md
+++ b/README.md
@@ -74,6 +74,7 @@ Open `about:addons`, select this extension, and choose the **Options** tab
   guarantee.
 - **Open in** — a new tab, a new background tab, or a new window.
 - **Prompt template** — wraps your text, for example `Explain simply: {query}`.
+  Up to 1,000 characters.
 
 Restore defaults resets extension preferences to Simple mode with translation
 hidden; it does not change Firefox's search engine or shortcut settings.
diff --git a/lib/defaults.js b/lib/defaults.js
index fcf5e57..fc8e59c 100644
--- a/lib/defaults.js
+++ b/lib/defaults.js
@@ -3,6 +3,12 @@
 
 export const OPEN_IN_VALUES = ["new-tab", "background-tab", "new-window"];
 
+// In UTF-16 units, as maxlength counts. Keeps the template far inside
+// storage.sync's 8192-byte item quota even when every character is a pasted
+// control character (six bytes once JSON-escaped), and far beyond any template
+// the 8,000-character URL budget could send.
+export const MAX_TEMPLATE_CHARS = 1000;
+
 export const DEFAULT_SETTINGS = {
   contextMenuMode: "simple",
   showTranslate: false,
diff --git a/lib/settings.js b/lib/settings.js
index 978b926..068a1ba 100644
--- a/lib/settings.js
+++ b/lib/settings.js
@@ -1,4 +1,4 @@
-import { DEFAULT_SETTINGS, OPEN_IN_VALUES } from "./defaults.js";
+import { DEFAULT_SETTINGS, MAX_TEMPLATE_CHARS, OPEN_IN_VALUES } from "./defaults.js";
 
 export function normalizeLanguage(value) {
   if (typeof value !== "string") return "";
@@ -28,7 +28,7 @@ export function mergeSettings(stored) {
   if (OPEN_IN_VALUES.includes(stored.openIn)) {
     merged.openIn = stored.openIn;
   }
-  if (typeof stored.promptTemplate === "string") {
+  if (typeof stored.promptTemplate === "string" && stored.promptTemplate.length <= MAX_TEMPLATE_CHARS) {
     merged.promptTemplate = stored.promptTemplate;
   }
   return merged;
diff --git a/options/options.html b/options/options.html
index 127b569..0b852e5 100644
--- a/options/options.html
+++ b/options/options.html
@@ -80,8 +80,8 @@
 
   <div class="field">
     <label for="promptTemplate">Prompt template</label>
-    <input type="text" id="promptTemplate" spellcheck="false" />
-    <p class="hint"><code>{query}</code> is replaced with your selected text. Used by Ask normally, Simple mode, the toolbar button, and the keyboard shortcut. Other advanced actions use their own prompts.</p>
+    <input type="text" id="promptTemplate" spellcheck="false" maxlength="1000" />
+    <p class="hint"><code>{query}</code> is replaced with your selected text. Used by Ask normally, Simple mode, the toolbar button, and the keyboard shortcut. Other advanced actions use their own prompts. Up to 1,000 characters.</p>
   </div>
 
   <div class="actions">
diff --git a/test/settings.test.js b/test/settings.test.js
index 58b6e13..0f7fb34 100644
--- a/test/settings.test.js
+++ b/test/settings.test.js
@@ -1,7 +1,9 @@
 import test from "node:test";
 import assert from "node:assert/strict";
 
-import { DEFAULT_SETTINGS } from "../lib/defaults.js";
+import { readFileSync } from "node:fs";
+
+import { DEFAULT_SETTINGS, MAX_TEMPLATE_CHARS } from "../lib/defaults.js";
 import { mergeSettings } from "../lib/settings.js";
 
 test("empty object yields the defaults", () => {
@@ -67,6 +69,30 @@ test("wrong types fall back to the default", () => {
   assert.equal(merged.promptTemplate, DEFAULT_SETTINGS.promptTemplate);
 });
 
+test("a prompt template over the length limit falls back to the default", () => {
+  assert.equal(typeof MAX_TEMPLATE_CHARS, "number");
+  const atLimit = "x".repeat(MAX_TEMPLATE_CHARS);
+  assert.equal(mergeSettings({ promptTemplate: atLimit }).promptTemplate, atLimit);
+  assert.equal(mergeSettings({ promptTemplate: `${atLimit}x` }).promptTemplate,
+    DEFAULT_SETTINGS.promptTemplate);
+});
+
+// storage.sync rejects any item over 8192 bytes (key plus JSON value), and the
+// options page writes every field in one set(), so one oversized template would
+// make every later save fail. The worst character is a pasted control
+// character, which JSON escapes to six bytes.
+test("a template at the limit fits storage.sync's per-item quota in the worst case", () => {
+  const worst = "\u0001".repeat(MAX_TEMPLATE_CHARS);
+  const bytes = new TextEncoder().encode("promptTemplate" + JSON.stringify(worst)).length;
+  assert.ok(bytes <= 8192, `${bytes} bytes`);
+});
+
+test("the options page enforces the same template limit", () => {
+  const html = readFileSync(new URL("../options/options.html", import.meta.url), "utf8");
+  const input = html.match(/<input[^>]*id="promptTemplate"[^>]*>/)?.[0] ?? "";
+  assert.equal(input.match(/maxlength="(\d+)"/)?.[1], String(MAX_TEMPLATE_CHARS));
+});
+
 test("an out-of-range openIn falls back to the default", () => {
   assert.equal(mergeSettings({ openIn: "teleport" }).openIn, DEFAULT_SETTINGS.openIn);
   assert.equal(mergeSettings({ openIn: "" }).openIn, DEFAULT_SETTINGS.openIn);