diff --git a/packages/downgrader/README.md b/packages/downgrader/README.md
index 094c8b7..e206646 100644
--- a/packages/downgrader/README.md
+++ b/packages/downgrader/README.md
@@ -61,6 +61,8 @@ The input is never changed, and the output shares no plain objects or arrays wit
 
 External `$ref`s are left as written, except in a 3.2 `content` map, where 3.1 allows none and the entry becomes `{}`, which takes any payload. The files they point at are not converted, so bundle a multi-file document into one first.
 
+Extensions pass through unconverted, so a `$ref` into one at the document root, in `components`, or right under a component schema, as Stoplight writes when it bundles files into `x-ext`, is replaced by its converted target.
+
 ## 3.2 → 3.1
 
 ### Spec (`downgradeSpecV32ToV31`)
@@ -265,7 +267,7 @@ Accepts the default OAS dialect, which adds `discriminator`, `xml`, `externalDoc
 - An `anyOf` or `oneOf` of `{ type: "null" }` and a branch that a `$ref` points into stays as written, so the `$ref` still resolves, and code generators read it as a union.
 - `$ref`s into a keyword 3.0 removes, such as `prefixItems/0` or `propertyNames`, into a `oneOf` that becomes `anyOf`, into a `not` removed over a schema that lost a restriction, into an `anyOf` removed for a branch that holds only `contentMediaType`, or into the `items` or `properties` of a schema whose `type` list names more than one type besides `"null"`, which move into the branches of the `anyOf` or `oneOf` that takes the place of the list, are left as written and dangle. A `$ref` into the `allOf` of a schema that also holds a `$ref` resolves one member off, since that `$ref` becomes the first member of the `allOf`.
 - Recursion through `$defs` is cut to `{}` after one level. Where several `$ref`s enter the same cycle, the first copy made is reused, so a later one can be cut sooner. Such a cut `$ref` tells no `oneOf` branch apart, and counts as taking null, wherever its copy is reused.
-- Each `$ref` into `$defs` repeats its target once serialized, so definitions that reference each other in many layers make a large document, or one too large to serialize: SchemaStore's Kestra flow schema, 4 MB as compact JSON, would serialize to about 84 TB. In a document, move such definitions to `components/schemas`, where `$ref`s stay as written.
+- Each `$ref` into `$defs` or an extension repeats its target once serialized, so definitions that reference each other in many layers make a large document, or one too large to serialize: SchemaStore's Kestra flow schema, 4 MB as compact JSON, would serialize to about 84 TB. In a document, move such definitions to `components/schemas`, where `$ref`s stay as written.
 - Older drafts are not supported. `definitions` is read as `$defs`, but other keywords that only draft-07 or 2019-09 define, such as `dependencies` and `additionalItems`, become extensions unconverted, so `$ref`s into them dangle, except right under a component schema, and array-form `items` passes through. Convert such schemas to 2020-12 first. oRPC 1.x's Valibot converter writes draft-07 unless it is given `{ target: 'draft-2020-12' }`. The Rust crate aide writes draft-07 through schemars in its 3.1 documents, so a Rust tuple or tuple enum variant becomes array-form `items`, unless `aide::generate::in_context` sets its schema generator to `SchemaSettings::draft2020_12()`, with `definitions_path` kept at `#/components/schemas/`. TypeBox, up to 1.4, also writes `Type.Tuple` as array-form `items`, as Elysia's OpenAPI plugin does in its 3.1 documents.
 - `patternProperties` that matches every name, as TypeBox emits for a record, gives `additionalProperties: true`, so its value schema is lost.
 - A record keyed by a union of literals, which Zod and Valibot write as an `anyOf` of `const` branches in `propertyNames`, loses its key names, so 3.0 allows any key. Key it by an enum instead, such as `z.enum` or `v.picklist`.
diff --git a/packages/downgrader/src/v3.1-to-v3.0.ts b/packages/downgrader/src/v3.1-to-v3.0.ts
index 6c8baac..9a88085 100644
--- a/packages/downgrader/src/v3.1-to-v3.0.ts
+++ b/packages/downgrader/src/v3.1-to-v3.0.ts
@@ -297,7 +297,9 @@ const DOCUMENT_FIELDS = defineFields({
 // `#/components/<type>/` is a name, so a component named `items` is not one.
 function isInlined(ref: string, ctx: Context): boolean {
   let keywords = ref
-  let renamed = false
+  // Extensions pass through unconverted, so a target kept in one, as Stoplight
+  // bundles files into `x-ext`, is converted where a `$ref` uses it.
+  let unconverted = ref.startsWith('#/x-') || ref.startsWith('#/components/x-')
   if (ref.startsWith('#/components/')) {
     const name = ref.indexOf('/', 13) + 1
     const end = ref.indexOf('/', name)
@@ -307,12 +309,11 @@ function isInlined(ref: string, ctx: Context): boolean {
     if (keywords !== '' && ref.startsWith('#/components/schemas/')) {
       const next = keywords.indexOf('/', 1)
       const key = next === -1 ? keywords.slice(1) : keywords.slice(1, next)
-      renamed =
-        !V30_SCHEMA_KEYWORDS.has(key) && !REMOVED_KEYWORDS.includes(key) && !key.startsWith('x-')
+      unconverted = !V30_SCHEMA_KEYWORDS.has(key) && !REMOVED_KEYWORDS.includes(key)
     }
   }
   return (
-    (renamed ||
+    (unconverted ||
       ref.includes('%') ||
       keywords.includes('/$defs/') ||
       keywords.includes('/definitions/') ||
diff --git a/packages/downgrader/tests/v3.1-to-v3.0.test.ts b/packages/downgrader/tests/v3.1-to-v3.0.test.ts
index b24e9b2..ddcbc3f 100644
--- a/packages/downgrader/tests/v3.1-to-v3.0.test.ts
+++ b/packages/downgrader/tests/v3.1-to-v3.0.test.ts
@@ -271,6 +271,38 @@ describe('downgradeSpecV31ToV30', () => {
     })
   })
 
+  describe('extensions', () => {
+    it('inlines $refs into an extension, whose content passes through unconverted, as Stoplight bundles into x-ext', () => {
+      const ext = {
+        tag: { const: 'a' },
+        loop: { type: 'object', properties: { next: { $ref: '#/x-ext/loop' } } },
+        query: { name: 'q', in: 'query', schema: { type: ['string', 'null'] } },
+      }
+      const out = convert({
+        'paths': { '/a': { get: { parameters: [{ $ref: '#/x-ext/query' }], responses } } },
+        'components': {
+          'schemas': {
+            Tag: { oneOf: [{ $ref: '#/x-ext/tag' }, { type: 'null' }] },
+            Loop: { $ref: '#/x-ext/loop' },
+            Kind: { $ref: '#/components/x-schemas/Kind' },
+          },
+          'x-schemas': { Kind: { const: 'b' } },
+        },
+        'x-ext': ext,
+      })
+      expect(out.paths['/a']?.get?.parameters).toEqual([
+        { name: 'q', in: 'query', schema: { type: 'string', nullable: true } },
+      ])
+      // A 3.0 reader would ignore `const`, so null would match both branches.
+      expect(out.components?.schemas).toEqual({
+        Tag: { oneOf: [{ type: 'string', enum: ['a'] }, onlyNull] },
+        Loop: { type: 'object', properties: { next: {} } },
+        Kind: { type: 'string', enum: ['b'] },
+      })
+      expect(out['x-ext']).toEqual(ext)
+    })
+  })
+
   describe('mutual TLS', () => {
     it('drops mutualTLS schemes, and the $refs that resolve to them', () => {
       const out = convert({
@@ -2968,7 +3000,7 @@ describe('downgradeSchemaV31ToV30', () => {
           FieldOptions: {
             'Text': { type: 'object', properties: { type: { const: 'Text' } } },
             'Number': { type: ['number', 'null'] },
-            'x-note': { type: 'string' },
+            'x-note': { const: 'note' },
             'prefixItems': [{ type: 'string' }],
           },
           Field: {
@@ -2979,7 +3011,7 @@ describe('downgradeSchemaV31ToV30', () => {
             },
           },
           Nested: ref('FieldOptions/Text/properties/type'),
-          Kept: ref('FieldOptions/x-note'),
+          Extension: ref('FieldOptions/x-note'),
           Removed: ref('FieldOptions/prefixItems/0'),
           // A key 3.0 keeps, and a key of another component type, are not renamed.
           Negated: { not: { type: 'string' } },
@@ -2992,7 +3024,7 @@ describe('downgradeSchemaV31ToV30', () => {
     // The discriminator goes, as its mapping would dangle.
     expect(out.components?.schemas?.Field).not.toHaveProperty('discriminator')
     expect(out.components?.schemas).toMatchObject({
-      FieldOptions: { 'x-Text': {}, 'x-Number': {} },
+      FieldOptions: { 'x-Text': {}, 'x-Number': {}, 'x-note': { const: 'note' } },
       Field: {
         oneOf: [
           { type: 'object', properties: { type: { type: 'string', enum: ['Text'] } } },
@@ -3000,7 +3032,7 @@ describe('downgradeSchemaV31ToV30', () => {
         ],
       },
       Nested: { type: 'string', enum: ['Text'] },
-      Kept: ref('FieldOptions/x-note'),
+      Extension: { type: 'string', enum: ['note'] },
       Removed: ref('FieldOptions/prefixItems/0'),
       Negated: { not: { type: 'string' } },
     })