diff --git a/packages/lib-common/package.json b/packages/lib-common/package.json
index 42549c1381..395d2ed797 100644
--- a/packages/lib-common/package.json
+++ b/packages/lib-common/package.json
@@ -29,7 +29,7 @@
     "@types/tinycolor2": "^1.4.6",
     "cross-spawn": "^7.0.6",
     "fast-check": "^4.8.0",
-    "js-yaml": "^4.2.0",
+    "js-yaml": "^5.2.1",
     "web-tree-sitter": "^0.26.10"
   }
 }
diff --git a/packages/lib-common/src/testUtil/serialize.ts b/packages/lib-common/src/testUtil/serialize.ts
index 870476879a..3342402833 100644
--- a/packages/lib-common/src/testUtil/serialize.ts
+++ b/packages/lib-common/src/testUtil/serialize.ts
@@ -1,70 +1,82 @@
-// From https://github.com/nodeca/js-yaml/issues/586#issuecomment-814310104
 // This file ensures that simple objects and arrays (ie without array or object
 // children) will be serialized inline, and also ensures that "fullTargets" will be inlined as well
 
-import { dump, Type, DEFAULT_SCHEMA } from "js-yaml";
-import type { DumpOptions } from "js-yaml";
+import type { Document, MappingNode, Node, SequenceNode } from "js-yaml";
+import { CORE_SCHEMA, dump, visit, YAML11_SCHEMA } from "js-yaml";
 
-class CustomDump {
-  constructor(
-    private readonly data: unknown,
-    private readonly opts: DumpOptions,
-  ) {}
-
-  represent(): string {
-    let result = dump(this.data, { replacer, schema, ...this.opts });
-    result = result.trim();
-    if (result.includes("\n")) {
-      result = `\n${result}`;
-    }
-    return result;
-  }
+export function serialize(obj: unknown): string {
+  return dump(toSerializableObject(obj), {
+    noRefs: true,
+    quoteStyle: "double",
+    // CORE_SCHEMA preserves existing fixture output for numeric-looking strings
+    // like `1_01_001`, which js-yaml's default schema would quote.
+    schema: CORE_SCHEMA,
+    transform: inlineSimpleCollections,
+  });
 }
 
-const customDumpType = new Type("!format", {
-  kind: "scalar",
-  resolve: () => false,
-  instanceOf: CustomDump,
-  represent: (d: unknown) => (d as CustomDump).represent(),
-});
-
-const schema = DEFAULT_SCHEMA.extend({ implicit: [customDumpType] });
-
-const isObject = (value: unknown): value is object =>
-  typeof value === "object" && value != null;
-
-function hasSimpleChildren(value: unknown): boolean {
-  if (isObject(value)) {
-    return Object.values(value).every(
-      (value) => !isObject(value) && !Array.isArray(value),
-    );
-  }
+function toSerializableObject(value: unknown): unknown {
   if (Array.isArray(value)) {
-    return value.every((value) => !isObject(value) && !Array.isArray(value));
+    return value.map(toSerializableObject);
   }
-  return false;
-}
 
-function replacer(key: string, value: unknown): unknown {
-  // top-level, don't change this
-  if (key === "") {
+  if (value == null || typeof value !== "object") {
     return value;
   }
 
-  if (hasSimpleChildren(value)) {
-    return new CustomDump(value, { flowLevel: 0 });
-  }
+  return Object.fromEntries(
+    Object.entries(value).map(([key, value]) => [
+      key,
+      toSerializableObject(value),
+    ]),
+  );
+}
+
+function inlineSimpleCollections(documents: Document[]): void {
+  visit(documents, (node: Node, { depth, isKey, parent }) => {
+    // Keep nested simple objects and arrays in flow style, eg `{line: 0}` and
+    // `[default.a]`, while leaving top-level mappings in block style.
+    if (depth > 0 && isCollectionNode(node) && hasOnlyScalarChildren(node)) {
+      node.style.flow = true;
+    }
 
-  // default
-  return value;
+    // Existing fixtures use single quotes for flow scalar values that need
+    // quoting, eg `{character: ']'}`. Do not apply this to keys, non-string
+    // scalars, block scalars, or multiline strings.
+    if (
+      !isKey &&
+      parent != null &&
+      parent.style.flow &&
+      node.kind === "scalar" &&
+      node.tag === "tag:yaml.org,2002:str" &&
+      !node.value.includes("\n") &&
+      scalarNeedsQuotesInFlow(node.value)
+    ) {
+      node.style.singleQuoted = true;
+    }
+  });
 }
 
-export function serialize(obj: unknown): string {
-  const dump = new CustomDump(obj, {
-    noRefs: true,
-    quotingType: '"',
-  })
-    .represent()
-    .trim();
-  return `${dump}\n`;
+function isCollectionNode(node: Node): node is MappingNode | SequenceNode {
+  return node.kind === "mapping" || node.kind === "sequence";
+}
+
+function hasOnlyScalarChildren(node: MappingNode | SequenceNode): boolean {
+  return node.kind === "mapping"
+    ? node.items.every((item) => item.value.kind === "scalar")
+    : node.items.every((item) => item.kind === "scalar");
+}
+
+function scalarNeedsQuotesInFlow(value: string): boolean {
+  // Dump the value inside a flow array so js-yaml applies the same scalar
+  // rules it would use for an inline collection. If the result starts with
+  // `["`, the value cannot be emitted plainly in flow style, so we switch the
+  // actual fixture value to single quotes to match the existing fixtures.
+  return dump([value], {
+    flowLevel: 0,
+    quoteStyle: "double",
+    // Use YAML 1.1 schema to preserve existing fixtures where legacy
+    // boolean-like strings such as `y` and `n` are quoted in flow collections.
+    schema: YAML11_SCHEMA,
+  }).startsWith('["');
 }
diff --git a/packages/lib-node-common/package.json b/packages/lib-node-common/package.json
index f8fb76609a..e42fca566a 100644
--- a/packages/lib-node-common/package.json
+++ b/packages/lib-node-common/package.json
@@ -21,6 +21,6 @@
   "devDependencies": {
     "@types/js-yaml": "^4.0.9",
     "@types/lodash-es": "^4.17.12",
-    "js-yaml": "^4.2.0"
+    "js-yaml": "^5.2.1"
   }
 }
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 546b0fd0c9..bd374483ae 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -397,8 +397,8 @@ importers:
         specifier: ^4.8.0
         version: 4.8.0
       js-yaml:
-        specifier: ^4.2.0
-        version: 4.3.0
+        specifier: ^5.2.1
+        version: 5.2.1
       web-tree-sitter:
         specifier: ^0.26.10
         version: 0.26.10
@@ -516,8 +516,8 @@ importers:
         specifier: ^4.17.12
         version: 4.17.12
       js-yaml:
-        specifier: ^4.2.0
-        version: 4.3.0
+        specifier: ^5.2.1
+        version: 5.2.1
 
   packages/lib-sentence-parser:
     devDependencies:
@@ -7743,6 +7743,10 @@ packages:
     resolution: {integrity: sha512-1td788aAnnZ5qs7V2QIRl1owjtYpbKt749Y3xauqQgwIIGF/xXWz1wMTEBx5O3LK3lXLVuqXPdPxj2BoFHaW9Q==}
     hasBin: true
 
+  js-yaml@5.2.1:
+    resolution: {integrity: sha512-zfLtNfQqxVqq3uaTqSkh4x4hZw3KHobGUA0fJUj4wawW8bsQLTVqpHdXSIzidh7o+4lEW36tANuAGdaFx6Zgnw==}
+    hasBin: true
+
   jsdom@26.1.0:
     resolution: {integrity: sha512-Cvc9WUhxSMEo4McES3P7oK3QaXldCfNWp7pl2NNeiIFlCoLr3kfq9kb1fxftiwk1FLV7CvpvDfonxtzUDeSOPg==}
     engines: {node: '>=18'}
@@ -20059,6 +20063,10 @@ snapshots:
     dependencies:
       argparse: 2.0.1
 
+  js-yaml@5.2.1:
+    dependencies:
+      argparse: 2.0.1
+
   jsdom@26.1.0:
     dependencies:
       cssstyle: 4.6.0