fix(keys): accept a bare 0x prefix in isHexString

.changeset/is-hex-string-empty-body.md

@@ -0,0 +1,5 @@
+---
+"@agentcommercekit/keys": patch
+---
+
+`isHexString` now returns `true` for a bare `0x` prefix with an empty body, matching its documented behavior. The check required at least one hex digit after the prefix, so `isHexString("0x")` returned `false` even though the JSDoc example states it returns `true`. A bare empty string with no prefix (`""`) continues to return `false`.

packages/keys/src/encoding/hex.test.ts

@@ -44,6 +44,14 @@ describe("isHexString", () => {
     expect(isHexString("1234567890abcdef")).toBe(true)
   })
 
+  test("returns true for a bare 0x prefix with an empty body", () => {
+    expect(isHexString("0x")).toBe(true)
+  })
+
+  test("returns false for an empty string with no prefix", () => {
+    expect(isHexString("")).toBe(false)
+  })
+
   test("returns false for invalid hex strings", () => {
     expect(isHexString("0x1234567890abcdefg")).toBe(false)
     expect(isHexString("not hex")).toBe(false)

packages/keys/src/encoding/hex.ts

@@ -47,6 +47,14 @@ export function isHexString(value: unknown): value is string {
     return false
   }
 
-  const hexWithoutPrefix = value.startsWith("0x") ? value.slice(2) : value
+  const hasPrefix = value.startsWith("0x")
+  const hexWithoutPrefix = hasPrefix ? value.slice(2) : value
+
+  // A bare "0x" prefix has an empty body and is a valid (zero-length) hex
+  // string, as documented above. An empty string with no prefix is not.
+  if (hexWithoutPrefix.length === 0) {
+    return hasPrefix
+  }
+
   return /^[0-9A-Fa-f]+$/.test(hexWithoutPrefix)
 }