diff --git a/lib/entry-points.js b/lib/entry-points.js
index f385af4a9..7a739846f 100644
--- a/lib/entry-points.js
+++ b/lib/entry-points.js
@@ -147520,7 +147520,7 @@ var object = {
   validate: isObject,
   required: true
 };
-function optional(validator) {
+function optionalOrNull(validator) {
   return {
     validate: (val) => {
       return val === void 0 || val === null || validator.validate(val);
@@ -147528,7 +147528,7 @@ function optional(validator) {
     required: false
   };
 }
-function undefinable(validator) {
+function optional(validator) {
   return {
     validate: (val) => {
       return val === void 0 || validator.validate(val);
@@ -153679,8 +153679,8 @@ function isVersionInfo(x) {
   return isObject(x) && validateSchema(
     {
       version: string,
-      features: undefinable(object),
-      overlayVersion: undefinable(number)
+      features: optional(object),
+      overlayVersion: optional(number)
     },
     x
   );
@@ -160094,14 +160094,14 @@ var toolcache4 = __toESM(require_tool_cache());
 // src/start-proxy/types.ts
 var usernameSchema = {
   /** The username needed to authenticate to the package registry, if any. */
-  username: optional(string)
+  username: optionalOrNull(string)
 };
 function hasUsername(config) {
   return "username" in config;
 }
 var usernamePasswordSchema = {
   /** The password needed to authenticate to the package registry, if any. */
-  password: optional(string),
+  password: optionalOrNull(string),
   ...usernameSchema
 };
 function hasUsernameAndPassword(config) {
@@ -160109,7 +160109,7 @@ function hasUsernameAndPassword(config) {
 }
 var tokenSchema = {
   /** The token needed to authenticate to the package registry, if any. */
-  token: optional(string),
+  token: optionalOrNull(string),
   ...usernameSchema
 };
 function hasToken(config) {
@@ -160131,15 +160131,15 @@ var awsConfigSchema = {
   "role-name": string,
   domain: string,
   "domain-owner": string,
-  audience: optional(string)
+  audience: optionalOrNull(string)
 };
 function isAWSConfig(config) {
   return validateSchema(awsConfigSchema, config);
 }
 var jfrogConfigSchema = {
   "jfrog-oidc-provider-name": string,
-  audience: optional(string),
-  "identity-mapping-name": optional(string)
+  audience: optionalOrNull(string),
+  "identity-mapping-name": optionalOrNull(string)
 };
 function isJFrogConfig(config) {
   return validateSchema(jfrogConfigSchema, config);
@@ -160154,8 +160154,8 @@ function isCloudsmithConfig(config) {
 }
 var gcpConfigSchema = {
   "workload-identity-provider": string,
-  "service-account": optional(string),
-  audience: optional(string)
+  "service-account": optionalOrNull(string),
+  audience: optionalOrNull(string)
 };
 function isGCPConfig(config) {
   return validateSchema(gcpConfigSchema, config);
diff --git a/src/codeql.ts b/src/codeql.ts
index f960ce023..c7c0b4e9d 100644
--- a/src/codeql.ts
+++ b/src/codeql.ts
@@ -240,8 +240,8 @@ export function isVersionInfo(x: unknown): x is VersionInfo {
     json.validateSchema(
       {
         version: json.string,
-        features: json.undefinable(json.object),
-        overlayVersion: json.undefinable(json.number),
+        features: json.optional(json.object),
+        overlayVersion: json.optional(json.number),
       },
       x,
     )
diff --git a/src/json/index.test.ts b/src/json/index.test.ts
index d37a86c37..3e96d7639 100644
--- a/src/json/index.test.ts
+++ b/src/json/index.test.ts
@@ -10,8 +10,8 @@ const testSchema = {
   requiredKey: json.string,
 };
 
-const optionalSchema = {
-  optionalKey: json.optional(json.string),
+const optionalOrNullSchema = {
+  optionalKey: json.optionalOrNull(json.string),
 };
 
 test("validateSchema - required properties are required", async (t) => {
@@ -30,11 +30,34 @@ test("validateSchema - required properties are required", async (t) => {
 
 test("validateSchema - optional properties are optional", async (t) => {
   // Optional fields may be absent
-  t.true(json.validateSchema(optionalSchema, {}));
-  t.true(json.validateSchema(optionalSchema, { optionalKey: undefined }));
-  t.true(json.validateSchema(optionalSchema, { optionalKey: null }));
+  t.true(json.validateSchema(optionalOrNullSchema, {}));
+  t.true(json.validateSchema(optionalOrNullSchema, { optionalKey: undefined }));
+  t.true(json.validateSchema(optionalOrNullSchema, { optionalKey: null }));
 
   // But, if present, should have the expected type
+  t.false(json.validateSchema(optionalOrNullSchema, { optionalKey: 0 }));
+  t.false(json.validateSchema(optionalOrNullSchema, { optionalKey: 123 }));
+  t.false(json.validateSchema(optionalOrNullSchema, { optionalKey: false }));
+  t.false(json.validateSchema(optionalOrNullSchema, { optionalKey: true }));
+  t.false(json.validateSchema(optionalOrNullSchema, { optionalKey: [] }));
+  t.false(json.validateSchema(optionalOrNullSchema, { optionalKey: {} }));
+  t.true(json.validateSchema(optionalOrNullSchema, { optionalKey: "" }));
+  t.true(json.validateSchema(optionalOrNullSchema, { optionalKey: "foo" }));
+});
+
+const optionalSchema = {
+  optionalKey: json.optional(json.string),
+};
+
+test("validateSchema - optional properties may be absent or undefined, but reject null", async (t) => {
+  // Optional fields may be absent or explicitly undefined
+  t.true(json.validateSchema(optionalSchema, {}));
+  t.true(json.validateSchema(optionalSchema, { optionalKey: undefined }));
+
+  // But should reject null
+  t.false(json.validateSchema(optionalSchema, { optionalKey: null }));
+
+  // And, if present, should have the expected type
   t.false(json.validateSchema(optionalSchema, { optionalKey: 0 }));
   t.false(json.validateSchema(optionalSchema, { optionalKey: 123 }));
   t.false(json.validateSchema(optionalSchema, { optionalKey: false }));
@@ -44,26 +67,3 @@ test("validateSchema - optional properties are optional", async (t) => {
   t.true(json.validateSchema(optionalSchema, { optionalKey: "" }));
   t.true(json.validateSchema(optionalSchema, { optionalKey: "foo" }));
 });
-
-const undefinableSchema = {
-  optionalKey: json.undefinable(json.string),
-};
-
-test("validateSchema - undefinable properties are optional but reject null", async (t) => {
-  // Optional fields may be absent or explicitly undefined
-  t.true(json.validateSchema(undefinableSchema, {}));
-  t.true(json.validateSchema(undefinableSchema, { optionalKey: undefined }));
-
-  // But, unlike `optional`, `null` is rejected
-  t.false(json.validateSchema(undefinableSchema, { optionalKey: null }));
-
-  // And, if present, should have the expected type
-  t.false(json.validateSchema(undefinableSchema, { optionalKey: 0 }));
-  t.false(json.validateSchema(undefinableSchema, { optionalKey: 123 }));
-  t.false(json.validateSchema(undefinableSchema, { optionalKey: false }));
-  t.false(json.validateSchema(undefinableSchema, { optionalKey: true }));
-  t.false(json.validateSchema(undefinableSchema, { optionalKey: [] }));
-  t.false(json.validateSchema(undefinableSchema, { optionalKey: {} }));
-  t.true(json.validateSchema(undefinableSchema, { optionalKey: "" }));
-  t.true(json.validateSchema(undefinableSchema, { optionalKey: "foo" }));
-});
diff --git a/src/json/index.ts b/src/json/index.ts
index 4a3f65947..66adc606b 100644
--- a/src/json/index.ts
+++ b/src/json/index.ts
@@ -76,7 +76,7 @@ export const object = {
  * Transforms a validator to be optional, accepting `undefined` or `null` for an
  * absent value.
  */
-export function optional<T>(validator: Validator<T>) {
+export function optionalOrNull<T>(validator: Validator<T>) {
   return {
     validate: (val: unknown) => {
       return val === undefined || val === null || validator.validate(val);
@@ -87,9 +87,9 @@ export function optional<T>(validator: Validator<T>) {
 
 /**
  * Transforms a validator to be optional, accepting `undefined` for an absent
- * value but, unlike `optional`, rejecting `null`.
+ * value but, unlike `optionalOrNull`, rejecting `null`.
  */
-export function undefinable<T>(validator: Validator<T>) {
+export function optional<T>(validator: Validator<T>) {
   return {
     validate: (val: unknown): val is T | undefined => {
       return val === undefined || validator.validate(val);
diff --git a/src/start-proxy/types.ts b/src/start-proxy/types.ts
index 13a4ce0e8..13369edbf 100644
--- a/src/start-proxy/types.ts
+++ b/src/start-proxy/types.ts
@@ -12,7 +12,7 @@ export type RawCredential = UnvalidatedObject<Credential>;
 /** A schema for credential objects with a username. */
 export const usernameSchema = {
   /** The username needed to authenticate to the package registry, if any. */
-  username: json.optional(json.string),
+  username: json.optionalOrNull(json.string),
 } as const satisfies json.Schema;
 
 /** Usernames may be present for both authentication with tokens or passwords. */
@@ -29,7 +29,7 @@ export function hasUsername(config: AuthConfig): config is Username {
 /** A schema for credential objects with a username and password. */
 export const usernamePasswordSchema = {
   /** The password needed to authenticate to the package registry, if any. */
-  password: json.optional(json.string),
+  password: json.optionalOrNull(json.string),
   ...usernameSchema,
 } as const satisfies json.Schema;
 
@@ -52,7 +52,7 @@ export function hasUsernameAndPassword(
 /** A schema for credential objects for token-based authentication. */
 export const tokenSchema = {
   /** The token needed to authenticate to the package registry, if any. */
-  token: json.optional(json.string),
+  token: json.optionalOrNull(json.string),
   ...usernameSchema,
 } as const satisfies json.Schema;
 
@@ -100,7 +100,7 @@ export const awsConfigSchema = {
   "role-name": json.string,
   domain: json.string,
   "domain-owner": json.string,
-  audience: json.optional(json.string),
+  audience: json.optionalOrNull(json.string),
 } as const satisfies json.Schema;
 
 /** Configuration for AWS OIDC. */
@@ -116,8 +116,8 @@ export function isAWSConfig(
 /** A schema for JFrog OIDC configurations. */
 export const jfrogConfigSchema = {
   "jfrog-oidc-provider-name": json.string,
-  audience: json.optional(json.string),
-  "identity-mapping-name": json.optional(json.string),
+  audience: json.optionalOrNull(json.string),
+  "identity-mapping-name": json.optionalOrNull(json.string),
 } as const satisfies json.Schema;
 
 /** Configuration for JFrog OIDC. */
@@ -150,8 +150,8 @@ export function isCloudsmithConfig(
 /** A schema for GCP OIDC configurations. */
 export const gcpConfigSchema = {
   "workload-identity-provider": json.string,
-  "service-account": json.optional(json.string),
-  audience: json.optional(json.string),
+  "service-account": json.optionalOrNull(json.string),
+  audience: json.optionalOrNull(json.string),
 } as const satisfies json.Schema;
 
 /** Configuration for GCP OIDC. */