diff --git a/lib/build-url.js b/lib/build-url.js
new file mode 100644
index 0000000..e27e356
--- /dev/null
+++ b/lib/build-url.js
@@ -0,0 +1,100 @@
+// Pure URL construction. No browser APIs, so node:test imports this directly.
+//
+// Every ChatGPT parameter below is undocumented by OpenAI and has broken before.
+// This module's contract is only that it emits a well-formed URL; whether
+// ChatGPT honours model / temporary-chat / hints is out of our control, which is
+// why each is independently omittable and every label is hint-strength.
+//
+// The root path is mandatory: chatgpt.com/search began returning 404 in
+// mid-2025, and the root form is what OpenAI's own extension uses.
+
+const BASE = "https://chatgpt.com/";
+
+export const MAX_URL_CHARS = 8000;
+
+// Fraction of the kept text within which a whitespace cut is preferred.
+const WHITESPACE_WINDOW = 0.9;
+
+function applyTemplate(template, query) {
+  if (query === "") {
+    return "";
+  }
+  const tpl = typeof template === "string" ? template : "";
+  if (tpl.includes("{query}")) {
+    return tpl.replaceAll("{query}", query);
+  }
+  const trimmed = tpl.trim();
+  return trimmed === "" ? query : `${trimmed} ${query}`;
+}
+
+function addOptionalParams(url, settings) {
+  const model = typeof settings.model === "string" ? settings.model.trim() : "";
+  if (model !== "") {
+    url.searchParams.set("model", model);
+  }
+  if (settings.temporaryChat) {
+    url.searchParams.set("temporary-chat", "true");
+  }
+  if (settings.webSearch) {
+    url.searchParams.set("hints", "search");
+  }
+  return url;
+}
+
+function lastWhitespaceIndex(text) {
+  for (let i = text.length - 1; i >= 0; i -= 1) {
+    if (/\s/.test(text[i])) {
+      return i;
+    }
+  }
+  return -1;
+}
+
+// Returns the longest prefix of `prompt` whose finished URL fits the budget.
+// Slicing happens by CODE POINT on the decoded string, so a surrogate pair can
+// never be split and a percent-escape can never be severed (encoding happens
+// afterwards, via URLSearchParams).
+function fitToBudget(prompt, settings) {
+  const codePoints = Array.from(prompt);
+
+  const urlLengthFor = (count) => {
+    const url = new URL(BASE);
+    url.searchParams.set("q", codePoints.slice(0, count).join(""));
+    addOptionalParams(url, settings);
+    return url.href.length;
+  };
+
+  if (urlLengthFor(codePoints.length) <= MAX_URL_CHARS) {
+    return prompt;
+  }
+
+  let low = 0;
+  let high = codePoints.length;
+  while (low < high) {
+    const mid = Math.floor((low + high + 1) / 2);
+    if (urlLengthFor(mid) <= MAX_URL_CHARS) {
+      low = mid;
+    } else {
+      high = mid - 1;
+    }
+  }
+
+  const kept = codePoints.slice(0, low).join("");
+  const boundary = Math.floor(kept.length * WHITESPACE_WINDOW);
+  const cut = lastWhitespaceIndex(kept);
+  return cut >= boundary && cut > 0 ? kept.slice(0, cut) : kept;
+}
+
+export function buildChatGptUrl(settings, rawQuery) {
+  const query = String(rawQuery ?? "").trim();
+  const prompt = fitToBudget(applyTemplate(settings.promptTemplate, query), settings);
+
+  // q first, matching the most-cited working examples. Total serialized length
+  // is order-independent, so measuring without q above is sound.
+  const url = new URL(BASE);
+  if (prompt !== "") {
+    url.searchParams.set("q", prompt);
+  }
+  addOptionalParams(url, settings);
+  return url.href;
+}
diff --git a/test/build-url.test.js b/test/build-url.test.js
new file mode 100644
index 0000000..aa3202f
--- /dev/null
+++ b/test/build-url.test.js
@@ -0,0 +1,135 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { DEFAULT_SETTINGS } from "../lib/defaults.js";
+import { buildChatGptUrl, MAX_URL_CHARS } from "../lib/build-url.js";
+
+const withSettings = (overrides) => ({ ...DEFAULT_SETTINGS, ...overrides });
+const queryOf = (url) => new URL(url).searchParams.get("q");
+const paramsOf = (url) => [...new URL(url).searchParams.keys()];
+
+test("defaults produce only the q parameter on the root path", () => {
+  assert.equal(
+    buildChatGptUrl(DEFAULT_SETTINGS, "hello"),
+    "https://chatgpt.com/?q=hello",
+  );
+});
+
+test("an empty or whitespace-only query omits q entirely", () => {
+  for (const raw of ["", "   ", "\n\t", null, undefined]) {
+    const url = buildChatGptUrl(DEFAULT_SETTINGS, raw);
+    assert.equal(paramsOf(url).includes("q"), false, `raw=${JSON.stringify(raw)}`);
+    assert.equal(url, "https://chatgpt.com/");
+  }
+});
+
+test("model is sent only when non-empty after trimming", () => {
+  assert.equal(queryOf(buildChatGptUrl(withSettings({ model: "auto" }), "hi")), "hi");
+  assert.equal(new URL(buildChatGptUrl(withSettings({ model: "auto" }), "hi")).searchParams.get("model"), "auto");
+  assert.equal(new URL(buildChatGptUrl(withSettings({ model: "  auto  " }), "hi")).searchParams.get("model"), "auto");
+  for (const model of ["", "   "]) {
+    assert.equal(paramsOf(buildChatGptUrl(withSettings({ model }), "hi")).includes("model"), false);
+  }
+});
+
+test("temporaryChat and webSearch each appear only when enabled", () => {
+  assert.equal(paramsOf(buildChatGptUrl(DEFAULT_SETTINGS, "hi")).includes("temporary-chat"), false);
+  assert.equal(paramsOf(buildChatGptUrl(DEFAULT_SETTINGS, "hi")).includes("hints"), false);
+
+  const temp = new URL(buildChatGptUrl(withSettings({ temporaryChat: true }), "hi"));
+  assert.equal(temp.searchParams.get("temporary-chat"), "true");
+
+  const search = new URL(buildChatGptUrl(withSettings({ webSearch: true }), "hi"));
+  assert.equal(search.searchParams.get("hints"), "search");
+});
+
+test("all four URL-affecting settings combine, q first", () => {
+  const url = buildChatGptUrl(
+    withSettings({
+      model: "auto",
+      temporaryChat: true,
+      webSearch: true,
+      promptTemplate: "Explain: {query}",
+    }),
+    "entropy",
+  );
+  assert.deepEqual(paramsOf(url), ["q", "model", "temporary-chat", "hints"]);
+  assert.equal(queryOf(url), "Explain: entropy");
+});
+
+test("openIn never influences the URL", () => {
+  const base = buildChatGptUrl(DEFAULT_SETTINGS, "hi");
+  for (const openIn of ["new-tab", "background-tab", "new-window"]) {
+    assert.equal(buildChatGptUrl(withSettings({ openIn }), "hi"), base);
+  }
+});
+
+test("special characters survive encoding round-trip", () => {
+  const raw = 'a & b # c ? d = e + f "g" <h> 100% 😀 日本語\nnewline';
+  const url = buildChatGptUrl(DEFAULT_SETTINGS, raw);
+  assert.equal(queryOf(url), raw);
+  assert.equal(url.includes(" "), false);
+  assert.equal(url.includes("#"), false);
+});
+
+test("the template replaces every {query} occurrence", () => {
+  const url = buildChatGptUrl(withSettings({ promptTemplate: "{query} / {query}" }), "x");
+  assert.equal(queryOf(url), "x / x");
+});
+
+test("a template without {query} is treated as a prefix, never dropping the query", () => {
+  const url = buildChatGptUrl(withSettings({ promptTemplate: "Be concise:" }), "why");
+  assert.equal(queryOf(url), "Be concise: why");
+});
+
+test("an empty or whitespace-only template yields the bare query", () => {
+  for (const promptTemplate of ["", "   "]) {
+    assert.equal(queryOf(buildChatGptUrl(withSettings({ promptTemplate }), "why")), "why");
+  }
+});
+
+test("a non-string template degrades to the bare query", () => {
+  assert.equal(queryOf(buildChatGptUrl(withSettings({ promptTemplate: null }), "why")), "why");
+});
+
+test("an over-long prompt is truncated to fit the URL budget", () => {
+  const url = buildChatGptUrl(DEFAULT_SETTINGS, "x".repeat(20000));
+  assert.ok(url.length <= MAX_URL_CHARS, `length was ${url.length}`);
+  assert.ok(queryOf(url).length > 0);
+  assert.ok(queryOf(url).length < 20000);
+});
+
+test("truncation accounts for the other parameters", () => {
+  const settings = withSettings({ model: "some-long-model-slug", temporaryChat: true, webSearch: true });
+  const url = buildChatGptUrl(settings, "x".repeat(20000));
+  assert.ok(url.length <= MAX_URL_CHARS, `length was ${url.length}`);
+  assert.equal(new URL(url).searchParams.get("model"), "some-long-model-slug");
+  assert.equal(new URL(url).searchParams.get("hints"), "search");
+});
+
+test("truncation never splits a surrogate pair", () => {
+  const url = buildChatGptUrl(DEFAULT_SETTINGS, "😀".repeat(9000));
+  assert.ok(url.length <= MAX_URL_CHARS);
+  const lonelySurrogate = /[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/;
+  assert.equal(lonelySurrogate.test(queryOf(url)), false);
+});
+
+test("truncation never leaves a severed percent-escape", () => {
+  const url = buildChatGptUrl(DEFAULT_SETTINGS, "😀 ".repeat(9000));
+  assert.equal(/%[0-9A-Fa-f]?$/.test(url), false);
+  assert.doesNotThrow(() => decodeURIComponent(new URL(url).search.slice(3)));
+});
+
+test("truncation prefers a whitespace boundary, so no word is cut in half", () => {
+  const url = buildChatGptUrl(DEFAULT_SETTINGS, "lorem ipsum ".repeat(2000));
+  const tokens = queryOf(url).trim().split(/\s+/);
+  assert.ok(tokens.length > 1);
+  for (const token of tokens) {
+    assert.ok(token === "lorem" || token === "ipsum", `truncated mid-word: ${token}`);
+  }
+});
+
+test("a query needing no truncation is left byte-identical", () => {
+  const raw = "a modest question about thermodynamics";
+  assert.equal(queryOf(buildChatGptUrl(DEFAULT_SETTINGS, raw)), raw);
+});