diff --git a/tests/templates.test.ts b/tests/templates.test.ts
new file mode 100644
index 0000000..85bfb72
--- /dev/null
+++ b/tests/templates.test.ts
@@ -0,0 +1,103 @@
+import { describe, expect, it } from 'vitest';
+import { resolveTemplateUrl, TemplateResolutionError } from '../worker/lib/templates';
+
+function params(query = ''): URLSearchParams {
+  return new URLSearchParams(query);
+}
+
+describe('resolveTemplateUrl', () => {
+  it('returns a URL without template placeholders unchanged', () => {
+    expect(
+      resolveTemplateUrl({
+        targetUrl: 'https://example.com/static/path?x=1#hash',
+        query: params(),
+      }),
+    ).toBe('https://example.com/static/path?x=1#hash');
+  });
+
+  it('uses a default value when no override is provided', () => {
+    expect(
+      resolveTemplateUrl({
+        targetUrl: 'https://google.com/search?q={query,default=hello}',
+        query: params(),
+      }),
+    ).toBe('https://google.com/search?q=hello');
+  });
+
+  it('uses pathParam for the first template variable', () => {
+    expect(
+      resolveTemplateUrl({
+        targetUrl: 'https://en.wikipedia.org/wiki/{topic,default=Python}',
+        pathParam: 'Django',
+        query: params(),
+      }),
+    ).toBe('https://en.wikipedia.org/wiki/Django');
+  });
+
+  it('lets matching query params override pathParam and defaults', () => {
+    expect(
+      resolveTemplateUrl({
+        targetUrl: 'https://example.com/search?q={query,default=hello}',
+        pathParam: 'from-path',
+        query: params('query=from query'),
+      }),
+    ).toBe('https://example.com/search?q=from%20query');
+  });
+
+  it('resolves multiple params from query/default values', () => {
+    expect(
+      resolveTemplateUrl({
+        targetUrl: 'https://example.com/{locale,default=en}/search?q={query}&page={page,default=1}',
+        query: params('query=Django tips&page=2'),
+      }),
+    ).toBe('https://example.com/en/search?q=Django%20tips&page=2');
+  });
+
+  it('throws a typed error for a missing required param', () => {
+    expect(() =>
+      resolveTemplateUrl({
+        targetUrl: 'https://example.com/search?q={query}',
+        query: params(),
+      }),
+    ).toThrow(TemplateResolutionError);
+    expect(() =>
+      resolveTemplateUrl({
+        targetUrl: 'https://example.com/search?q={query}',
+        query: params(),
+      }),
+    ).toThrow('No value provided for template parameter "query"');
+  });
+
+  it('throws a typed error for invalid parameter names', () => {
+    expect(() =>
+      resolveTemplateUrl({
+        targetUrl: 'https://example.com/search?q={bad-name,default=hello}',
+        query: params(),
+      }),
+    ).toThrow(TemplateResolutionError);
+    expect(() =>
+      resolveTemplateUrl({
+        targetUrl: 'https://example.com/search?q={bad-name,default=hello}',
+        query: params(),
+      }),
+    ).toThrow('Invalid template parameter name "bad-name"');
+  });
+
+  it('preserves static Chinese text around params and encodes only substituted values', () => {
+    expect(
+      resolveTemplateUrl({
+        targetUrl: 'https://chatgpt.com/?q=翻译: {query,default=hello} 保证真实性',
+        query: params('query=cool beans'),
+      }),
+    ).toBe('https://chatgpt.com/?q=翻译: cool%20beans 保证真实性');
+  });
+
+  it('splits defaults only on the first comma', () => {
+    expect(
+      resolveTemplateUrl({
+        targetUrl: 'https://google.com/search?q={query,default=hello,world}',
+        query: params(),
+      }),
+    ).toBe('https://google.com/search?q=hello%2Cworld');
+  });
+});
diff --git a/worker/lib/templates.ts b/worker/lib/templates.ts
new file mode 100644
index 0000000..981af87
--- /dev/null
+++ b/worker/lib/templates.ts
@@ -0,0 +1,100 @@
+export type TemplateInput = {
+  targetUrl: string;
+  pathParam?: string;
+  query: URLSearchParams;
+};
+
+export type TemplateParameter = {
+  name: string;
+  defaultValue?: string;
+  raw: string;
+  index: number;
+};
+
+export class TemplateResolutionError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'TemplateResolutionError';
+  }
+}
+
+const TEMPLATE_PATTERN = /\{([^{}]*)\}/g;
+const PARAMETER_NAME_PATTERN = /^[A-Za-z_][A-Za-z0-9_]*$/;
+const DEFAULT_PREFIX = 'default=';
+
+export function parseTemplateParameters(targetUrl: string): TemplateParameter[] {
+  return Array.from(targetUrl.matchAll(TEMPLATE_PATTERN), ([raw, content], index) => {
+    const { name, defaultValue } = parseParameterContent(content, raw);
+
+    return { name, defaultValue, raw, index };
+  });
+}
+
+export function resolveTemplateUrl(input: TemplateInput): string {
+  const parameters = parseTemplateParameters(input.targetUrl);
+
+  if (parameters.length === 0) {
+    return input.targetUrl;
+  }
+
+  let parameterIndex = 0;
+
+  return input.targetUrl.replace(TEMPLATE_PATTERN, () => {
+    const parameter = parameters[parameterIndex];
+    parameterIndex += 1;
+
+    const value = resolveParameterValue(parameter, input);
+    return encodeURIComponent(value);
+  });
+}
+
+function parseParameterContent(content: string, raw: string): Pick<TemplateParameter, 'name' | 'defaultValue'> {
+  const commaIndex = content.indexOf(',');
+  const rawName = commaIndex === -1 ? content : content.slice(0, commaIndex);
+  const name = rawName.trim();
+
+  if (!PARAMETER_NAME_PATTERN.test(name)) {
+    throw new TemplateResolutionError(`Invalid template parameter name "${name}" in placeholder ${raw}`);
+  }
+
+  if (commaIndex === -1) {
+    return { name };
+  }
+
+  const defaultPart = content.slice(commaIndex + 1).trim();
+  if (!defaultPart.startsWith(DEFAULT_PREFIX)) {
+    throw new TemplateResolutionError(`Invalid default for template parameter "${name}" in placeholder ${raw}`);
+  }
+
+  return { name, defaultValue: unquoteDefault(defaultPart.slice(DEFAULT_PREFIX.length).trim()) };
+}
+
+function unquoteDefault(value: string): string {
+  if (value.length >= 2) {
+    const first = value[0];
+    const last = value[value.length - 1];
+
+    if ((first === '"' && last === '"') || (first === "'" && last === "'")) {
+      return value.slice(1, -1);
+    }
+  }
+
+  return value;
+}
+
+function resolveParameterValue(parameter: TemplateParameter, input: TemplateInput): string {
+  const queryValue = input.query.get(parameter.name);
+  if (queryValue !== null) {
+    return queryValue;
+  }
+
+  if (parameter.index === 0 && input.pathParam !== undefined) {
+    return input.pathParam;
+  }
+
+  if (parameter.defaultValue !== undefined) {
+    return parameter.defaultValue;
+  }
+
+  throw new TemplateResolutionError(`No value provided for template parameter "${parameter.name}"`);
+}