diff --git a/.changeset/lazy-trees-cheer.md b/.changeset/lazy-trees-cheer.md
new file mode 100644
index 0000000000..f7f1f08cd2
--- /dev/null
+++ b/.changeset/lazy-trees-cheer.md
@@ -0,0 +1,11 @@
+---
+'@redocly/openapi-core': patch
+'@redocly/cli': patch
+---
+
+Updated js-yaml from `4.2.0` to `5.2.1`.
+Fixed an issue where strings that look like numbers with underscores (for example `'12_34'`) had quotation marks removed by the `bundle` command.
+These strings stay quoted in the output.
+
+**Note**: YAML parsing is stricter: a multi-line flow collection whose closing bracket is not indented deeper than its parent key is now a parse error.
+Parse errors are reported at the offending token instead of the end of the document.
diff --git a/package-lock.json b/package-lock.json
index 07c4329a5a..9ddbcbbfa1 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -3047,13 +3047,6 @@
       "integrity": "sha512-qC4bCqYGy1y/NP7dDVr7KJarn+PbX1nSpwA7JXdu0HxT3QYjO8MJ+cntENtHFVy2dRAyBV23OZ6MxsW1AM1L8g==",
       "dev": true
     },
-    "node_modules/@types/js-yaml": {
-      "version": "4.0.9",
-      "resolved": "https://registry.npmjs.org/@types/js-yaml/-/js-yaml-4.0.9.tgz",
-      "integrity": "sha512-k4MGaQl5TGo/iipqb2UDG2UwjXziSWkh0uysQelTlJpX1qGlpUZYm8PnO4DxG1qBomtJUdYJ6qR6xdIah10JLg==",
-      "dev": true,
-      "license": "MIT"
-    },
     "node_modules/@types/json-pointer": {
       "version": "1.0.34",
       "resolved": "https://registry.npmjs.org/@types/json-pointer/-/json-pointer-1.0.34.tgz",
@@ -4875,6 +4868,7 @@
       "version": "4.2.0",
       "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-4.2.0.tgz",
       "integrity": "sha512-ePWsvanv0DWuDRsW8dnt+R4jQ31SCRCQ7hhNcPXZPsoBZiemuZNYGf7adZdqX2D86j6rvKp3RpCxVTSb8WQlOw==",
+      "dev": true,
       "funding": [
         {
           "type": "github",
@@ -8922,14 +8916,13 @@
         "colorette": "^1.2.0",
         "graphql": "^16.14.1",
         "js-levenshtein": "^1.1.6",
-        "js-yaml": "^4.2.0",
+        "js-yaml": "^5.2.1",
         "picomatch": "^4.0.4",
         "pluralize": "^8.0.0",
         "yaml-ast-parser": "0.0.43"
       },
       "devDependencies": {
         "@types/js-levenshtein": "^1.1.0",
-        "@types/js-yaml": "^4.0.3",
         "@types/picomatch": "^4.0.2",
         "@types/pluralize": "^0.0.29",
         "json-schema-to-ts": "^3.1.0",
@@ -8957,6 +8950,28 @@
         "url": "https://github.com/sponsors/epoberezkin"
       }
     },
+    "packages/core/node_modules/js-yaml": {
+      "version": "5.2.1",
+      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-5.2.1.tgz",
+      "integrity": "sha512-zfLtNfQqxVqq3uaTqSkh4x4hZw3KHobGUA0fJUj4wawW8bsQLTVqpHdXSIzidh7o+4lEW36tANuAGdaFx6Zgnw==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/puzrin"
+        },
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/nodeca"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "argparse": "^2.0.1"
+      },
+      "bin": {
+        "js-yaml": "bin/js-yaml.mjs"
+      }
+    },
     "packages/core/node_modules/picomatch": {
       "version": "4.0.4",
       "resolved": "https://registry.npmjs.org/picomatch/-/picomatch-4.0.4.tgz",
diff --git a/packages/core/package.json b/packages/core/package.json
index c8c643b7f8..d13cd05116 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -59,14 +59,13 @@
     "colorette": "^1.2.0",
     "graphql": "^16.14.1",
     "js-levenshtein": "^1.1.6",
-    "js-yaml": "^4.2.0",
+    "js-yaml": "^5.2.1",
     "picomatch": "^4.0.4",
     "pluralize": "^8.0.0",
     "yaml-ast-parser": "0.0.43"
   },
   "devDependencies": {
     "@types/js-levenshtein": "^1.1.0",
-    "@types/js-yaml": "^4.0.3",
     "@types/picomatch": "^4.0.2",
     "@types/pluralize": "^0.0.29",
     "json-schema-to-ts": "^3.1.0",
diff --git a/packages/core/src/__tests__/js-yaml.test.ts b/packages/core/src/__tests__/js-yaml.test.ts
index 71925ea8ea..93770fe6eb 100644
--- a/packages/core/src/__tests__/js-yaml.test.ts
+++ b/packages/core/src/__tests__/js-yaml.test.ts
@@ -71,4 +71,16 @@ describe('js-yaml', () => {
       'unacceptable kind of an object to dump [object Function]'
     );
   });
+
+  it('should return a nullish value instead of throwing for empty or comment-only input', () => {
+    expect(parseYaml('')).toBeUndefined();
+    expect(parseYaml('   \n  ')).toBeUndefined();
+    expect(parseYaml('# just a comment\n')).toBeNull();
+  });
+
+  it('throws when the stream contains more than one document', () => {
+    expect(() => parseYaml('a: 1\n---\nb: 2\n')).toThrow(
+      'expected a single document in the stream, but found more'
+    );
+  });
 });
diff --git a/packages/core/src/js-yaml/index.ts b/packages/core/src/js-yaml/index.ts
index 1a0b31b998..ccd53ac367 100644
--- a/packages/core/src/js-yaml/index.ts
+++ b/packages/core/src/js-yaml/index.ts
@@ -1,14 +1,34 @@
-// TODO: add a type for "types" https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/js-yaml/index.d.ts
-// eslint-disable-next-line @typescript-eslint/ban-ts-comment
-// @ts-ignore
-import { JSON_SCHEMA, types, load, dump, type LoadOptions, type DumpOptions } from 'js-yaml';
+import {
+  CORE_SCHEMA,
+  mergeTag,
+  binaryTag,
+  omapTag,
+  pairsTag,
+  setTag,
+  loadAll,
+  dump,
+  YAMLException,
+  type LoadOptions,
+  type DumpOptions,
+} from 'js-yaml';
 
-const DEFAULT_SCHEMA_WITHOUT_TIMESTAMP = JSON_SCHEMA.extend({
-  implicit: [types.merge],
-  explicit: [types.binary, types.omap, types.pairs, types.set],
-});
+const DEFAULT_SCHEMA_WITHOUT_TIMESTAMP = CORE_SCHEMA.withTags(
+  mergeTag,
+  binaryTag,
+  omapTag,
+  pairsTag,
+  setTag
+);
 
-export const parseYaml = (str: string, opts?: LoadOptions): unknown =>
-  load(str, { schema: DEFAULT_SCHEMA_WITHOUT_TIMESTAMP, ...opts });
+export const parseYaml = (str: string, opts?: LoadOptions): unknown => {
+  const documents = loadAll(str, { schema: DEFAULT_SCHEMA_WITHOUT_TIMESTAMP, ...opts });
+  if (documents.length === 0) {
+    return str.trim() === '' ? undefined : null;
+  }
+  if (documents.length > 1) {
+    throw new YAMLException('expected a single document in the stream, but found more');
+  }
+  return documents[0];
+};
 
 export const stringifyYaml = (obj: any, opts?: DumpOptions): string => dump(obj, opts);
diff --git a/packages/core/src/rules/common/__tests__/no-unresolved-refs.test.ts b/packages/core/src/rules/common/__tests__/no-unresolved-refs.test.ts
index a0f79005d4..a00c8ff61a 100644
--- a/packages/core/src/rules/common/__tests__/no-unresolved-refs.test.ts
+++ b/packages/core/src/rules/common/__tests__/no-unresolved-refs.test.ts
@@ -85,12 +85,12 @@ describe('oas3 boolean-parameter-prefixes', () => {
               "reportOnKey": false,
               "source": "fixtures/invalid-yaml.yaml",
               "start": {
-                "col": 1,
-                "line": 2,
+                "col": 8,
+                "line": 1,
               },
             },
           ],
-          "message": "Failed to parse: unexpected end of the stream within a single quoted scalar in "fixtures/invalid-yaml.yaml" (2:1)",
+          "message": "Failed to parse: unexpected end of the stream within a single quoted scalar in "fixtures/invalid-yaml.yaml" (1:8)",
           "ruleId": "no-unresolved-refs",
           "severity": "error",
           "suggest": [],
@@ -103,7 +103,7 @@ describe('oas3 boolean-parameter-prefixes', () => {
               "source": "foobar.yaml",
             },
           ],
-          "message": "Can't resolve $ref: unexpected end of the stream within a single quoted scalar in "fixtures/invalid-yaml.yaml" (2:1)",
+          "message": "Can't resolve $ref: unexpected end of the stream within a single quoted scalar in "fixtures/invalid-yaml.yaml" (1:8)",
           "reference": "https://redocly.com/docs/cli/rules/oas/no-unresolved-refs",
           "ruleId": "no-unresolved-refs",
           "severity": "error",
diff --git a/packages/core/src/rules/oas3/__tests__/no-invalid-media-type-examples.test.ts b/packages/core/src/rules/oas3/__tests__/no-invalid-media-type-examples.test.ts
index 4bc4677c53..f1959b3d89 100644
--- a/packages/core/src/rules/oas3/__tests__/no-invalid-media-type-examples.test.ts
+++ b/packages/core/src/rules/oas3/__tests__/no-invalid-media-type-examples.test.ts
@@ -635,7 +635,7 @@ describe('no-invalid-media-type-examples', () => {
                       example: {
                         "a": "test",
                         "b": "test"
-                      }
+                        }
                       schema:
                         $ref: '#/components/schemas/C'
 
diff --git a/tests/e2e/bundle/primitive-types/openapi.yaml b/tests/e2e/bundle/primitive-types/openapi.yaml
index 1e7cb6bf50..92d6cf1de8 100644
--- a/tests/e2e/bundle/primitive-types/openapi.yaml
+++ b/tests/e2e/bundle/primitive-types/openapi.yaml
@@ -37,3 +37,5 @@ paths:
                   object:
                     key1: 1
                     key2: 2
+                  underscoreNumberTransformedToString: 12_34
+                  underscoreNumberString: '12_34'
diff --git a/tests/e2e/bundle/primitive-types/snapshot.txt b/tests/e2e/bundle/primitive-types/snapshot.txt
index b1b5f89da5..a49ba63548 100644
--- a/tests/e2e/bundle/primitive-types/snapshot.txt
+++ b/tests/e2e/bundle/primitive-types/snapshot.txt
@@ -35,6 +35,8 @@ paths:
                   object:
                     key1: 1
                     key2: 2
+                  underscoreNumberTransformedToString: '12_34'
+                  underscoreNumberString: '12_34'
 components: {}
 
 bundling openapi.yaml using configuration for api 'main'...