diff --git a/src/utils/case.ts b/src/utils/case.ts
index 5178b64af..d03f91d87 100644
--- a/src/utils/case.ts
+++ b/src/utils/case.ts
@@ -83,6 +83,20 @@ function camelToSnake(key: string): string {
   )
 }
 
+/**
+ * Whether a single character earns an underscore in front of it, i.e. whether
+ * {@link camelToSnake} would emit one.
+ *
+ * `Ch extends Uppercase<Ch>` is the wrong test for this. `Uppercase` is the
+ * identity on digits and on anything else without a case, so `"1" extends
+ * Uppercase<"1">` is true and every digit read as an uppercase letter. Comparing
+ * against `Lowercase` instead is true for exactly the characters `Lowercase`
+ * actually rewrites, which is the set the runtime `/[A-Z]/` matches.
+ */
+type IsUppercaseLetter<Ch extends string> = Ch extends Lowercase<Ch>
+  ? false
+  : true
+
 /**
  * Maps a camelCase string literal to snake_case at the type level.
  * `"isNsfw"` → `"is_nsfw"`. The inverse of {@link Camelize}.
@@ -93,13 +107,13 @@ function camelToSnake(key: string): string {
  */
 type CamelToSnakeInner<S extends string> =
   S extends `${infer Head}${infer Tail}`
-    ? Head extends Uppercase<Head>
+    ? IsUppercaseLetter<Head> extends true
       ? `_${Lowercase<Head>}${CamelToSnakeInner<Tail>}`
       : `${Head}${CamelToSnakeInner<Tail>}`
     : S
 
 type CamelToSnake<S extends string> = S extends `${infer Head}${infer Tail}`
-  ? Head extends Uppercase<Head>
+  ? IsUppercaseLetter<Head> extends true
     ? `${Lowercase<Head>}${CamelToSnakeInner<Tail>}`
     : `${Head}${CamelToSnakeInner<Tail>}`
   : S
diff --git a/test/utils/case.spec.ts b/test/utils/case.spec.ts
index 6734d5a20..278005b81 100644
--- a/test/utils/case.spec.ts
+++ b/test/utils/case.spec.ts
@@ -174,6 +174,33 @@ describe("Snakeize<T>", () => {
     type Out = Snakeize<In>
     expectTypeOf<Out>().toEqualTypeOf<{ my_key: number }>()
   })
+
+  test("no underscore before a digit, agreeing with the runtime", () => {
+    // `Uppercase<"1">` is `"1"`, so the old guard read every digit as an
+    // uppercase letter and the type promised `chain_2_id` where the runtime
+    // writes `chain2_id`. Reading `body.chain_2_id` then type-checked and came
+    // back undefined. `expectTypeOf` is only checked by `check-types`; the
+    // runtime assertion below is what vitest runs.
+    type In = { chain2Id: string; erc721TokenId: string }
+    type Out = Snakeize<In>
+    expectTypeOf<Out>().toEqualTypeOf<{
+      chain2_id: string
+      erc721_token_id: string
+    }>()
+
+    expect(snakeizeKeysDeep({ chain2Id: "1", erc721TokenId: "2" })).toEqual({
+      chain2_id: "1",
+      erc721_token_id: "2",
+    })
+  })
+
+  test("a leading underscore is not an uppercase letter either", () => {
+    // Same root cause: `Uppercase<"_">` is `"_"`. The runtime's `/[A-Z]/`
+    // leaves the key alone; the type used to add a second underscore.
+    type In = { _private: number }
+    expectTypeOf<Snakeize<In>>().toEqualTypeOf<{ _private: number }>()
+    expect(snakeizeKeysDeep({ _private: 1 })).toEqual({ _private: 1 })
+  })
 })
 
 describe("Camelize<T>", () => {