diff --git a/tests/aliases.test.ts b/tests/aliases.test.ts
new file mode 100644
index 0000000..6a6238e
--- /dev/null
+++ b/tests/aliases.test.ts
@@ -0,0 +1,52 @@
+import { describe, expect, it } from 'vitest';
+import { ALIAS_MAX_LENGTH, normalizeAlias, validateAlias } from '../worker/lib/aliases';
+
+describe('alias normalization', () => {
+  it('lowercases aliases', () => {
+    expect(normalizeAlias('Claude')).toBe('claude');
+  });
+
+  it('trims aliases', () => {
+    expect(normalizeAlias('  claude  ')).toBe('claude');
+  });
+
+  it('keeps hyphen and underscore', () => {
+    expect(normalizeAlias('my_link-1')).toBe('my_link-1');
+  });
+});
+
+describe('alias validation', () => {
+  it('rejects empty string with exact error', () => {
+    expect(validateAlias('')).toEqual({ ok: false, error: 'Alias is required' });
+    expect(validateAlias('   ')).toEqual({ ok: false, error: 'Alias is required' });
+  });
+
+  it('rejects path separators with exact error', () => {
+    expect(validateAlias('a/b')).toEqual({
+      ok: false,
+      error: 'Alias may contain only letters, numbers, underscore, and hyphen',
+    });
+  });
+
+  it('rejects aliases starting with hyphen or underscore', () => {
+    expect(validateAlias('-alias')).toEqual({
+      ok: false,
+      error: 'Alias may contain only letters, numbers, underscore, and hyphen',
+    });
+    expect(validateAlias('_alias')).toEqual({
+      ok: false,
+      error: 'Alias may contain only letters, numbers, underscore, and hyphen',
+    });
+  });
+
+  it('accepts length 100 and rejects length 101', () => {
+    const validAlias = 'a'.repeat(ALIAS_MAX_LENGTH);
+    const tooLongAlias = 'a'.repeat(ALIAS_MAX_LENGTH + 1);
+
+    expect(validateAlias(validAlias)).toEqual({ ok: true, value: validAlias });
+    expect(validateAlias(tooLongAlias)).toEqual({
+      ok: false,
+      error: 'Alias must be 100 characters or fewer',
+    });
+  });
+});
diff --git a/worker/lib/aliases.ts b/worker/lib/aliases.ts
new file mode 100644
index 0000000..d025c6a
--- /dev/null
+++ b/worker/lib/aliases.ts
@@ -0,0 +1,38 @@
+export const ALIAS_MAX_LENGTH = 100;
+
+const ALIAS_INVALID_ERROR = 'Alias may contain only letters, numbers, underscore, and hyphen';
+const ALIAS_PATTERN = /^[a-z0-9][a-z0-9_-]{0,99}$/;
+
+type AliasValidationSuccess = {
+  ok: true;
+  value: string;
+};
+
+type AliasValidationFailure = {
+  ok: false;
+  error: string;
+};
+
+export type ValidationResult = AliasValidationSuccess | AliasValidationFailure;
+
+export function normalizeAlias(input: string): string {
+  return input.trim().replace(/[A-Z]/g, (character) => character.toLowerCase());
+}
+
+export function validateAlias(input: string): ValidationResult {
+  const value = normalizeAlias(input);
+
+  if (value.length === 0) {
+    return { ok: false, error: 'Alias is required' };
+  }
+
+  if (value.length > ALIAS_MAX_LENGTH) {
+    return { ok: false, error: `Alias must be ${ALIAS_MAX_LENGTH} characters or fewer` };
+  }
+
+  if (!ALIAS_PATTERN.test(value)) {
+    return { ok: false, error: ALIAS_INVALID_ERROR };
+  }
+
+  return { ok: true, value };
+}