diff --git a/.changeset/is-hex-string-uppercase-prefix.md b/.changeset/is-hex-string-uppercase-prefix.md
new file mode 100644
index 00000000..fd4b5416
--- /dev/null
+++ b/.changeset/is-hex-string-uppercase-prefix.md
@@ -0,0 +1,5 @@
+---
+"@agentcommercekit/keys": patch
+---
+
+Accept an uppercase `0X` prefix in `isHexString`, matching `hexStringToBytes`.
diff --git a/packages/keys/src/encoding/hex.test.ts b/packages/keys/src/encoding/hex.test.ts
index c66e3b36..1cc9f2bd 100644
--- a/packages/keys/src/encoding/hex.test.ts
+++ b/packages/keys/src/encoding/hex.test.ts
@@ -40,6 +40,11 @@ describe("isHexString", () => {
     expect(isHexString("0x1234567890abcdef")).toBe(true)
   })
 
+  test("returns true for valid hex strings with uppercase 0X prefix", () => {
+    // `hexStringToBytes` already accepts this form; the guard must agree.
+    expect(isHexString("0XABCDEF")).toBe(true)
+  })
+
   test("returns true for valid hex strings without 0x prefix", () => {
     expect(isHexString("1234567890abcdef")).toBe(true)
   })
diff --git a/packages/keys/src/encoding/hex.ts b/packages/keys/src/encoding/hex.ts
index aa4de17b..bc1bd439 100644
--- a/packages/keys/src/encoding/hex.ts
+++ b/packages/keys/src/encoding/hex.ts
@@ -47,6 +47,9 @@ export function isHexString(value: unknown): value is string {
     return false
   }
 
-  const hexWithoutPrefix = value.startsWith("0x") ? value.slice(2) : value
+  // Match `hexStringToBytes`: accept both `0x` and `0X` prefixes.
+  const hexWithoutPrefix = value.toLowerCase().startsWith("0x")
+    ? value.slice(2)
+    : value
   return /^[0-9A-Fa-f]+$/.test(hexWithoutPrefix)
 }