diff --git a/.changeset/legacy-json-avoid-mutation.md b/.changeset/legacy-json-avoid-mutation.md
new file mode 100644
index 00000000..3d5d3dc2
--- /dev/null
+++ b/.changeset/legacy-json-avoid-mutation.md
@@ -0,0 +1,5 @@
+---
+'@node-core/doc-kit': patch
+---
+
+Avoids directly mutating the AST in `legacy-json`, as to ensure future generators do not run with a input different than they expect.
diff --git a/src/generators/legacy-json/types.d.ts b/src/generators/legacy-json/types.d.ts
index ed92c6ce..2d9a664d 100644
--- a/src/generators/legacy-json/types.d.ts
+++ b/src/generators/legacy-json/types.d.ts
@@ -2,14 +2,18 @@ import { ListItem } from '@types/mdast';
 import { MetadataEntry } from '../metadata/types';
 
 /**
- * Represents an entry in a hierarchical structure, extending from MetadataEntry.
- * It includes children entries organized in a hierarchy.
+ * A node in the entry hierarchy.
  */
-export interface HierarchizedEntry extends MetadataEntry {
+export interface HierarchizedEntry {
   /**
-   * List of child entries that are part of this entry's hierarchy.
+   * The metadata entry this node wraps.
    */
-  hierarchyChildren: MetadataEntry[];
+  entry: MetadataEntry;
+
+  /**
+   * Child nodes nested under this entry, based on heading depth.
+   */
+  children: HierarchizedEntry[];
 }
 
 /**
diff --git a/src/generators/legacy-json/utils/__tests__/buildHierarchy.test.mjs b/src/generators/legacy-json/utils/__tests__/buildHierarchy.test.mjs
index d6c6441a..efd936b2 100644
--- a/src/generators/legacy-json/utils/__tests__/buildHierarchy.test.mjs
+++ b/src/generators/legacy-json/utils/__tests__/buildHierarchy.test.mjs
@@ -5,14 +5,17 @@ import { findParent, buildHierarchy } from '../buildHierarchy.mjs';
 
 describe('findParent', () => {
   it('finds parent with lower depth', () => {
-    const entries = [{ heading: { depth: 1 } }, { heading: { depth: 2 } }];
-    const parent = findParent(entries[1], entries, 0);
-    assert.equal(parent, entries[0]);
+    const nodes = [
+      { entry: { heading: { depth: 1 } }, children: [] },
+      { entry: { heading: { depth: 2 } }, children: [] },
+    ];
+    const parent = findParent(nodes[1].entry, nodes, 0);
+    assert.equal(parent, nodes[0]);
   });
 
   it('throws when no parent exists', () => {
-    const entries = [{ heading: { depth: 2 } }];
-    assert.throws(() => findParent(entries[0], entries, -1));
+    const nodes = [{ entry: { heading: { depth: 2 } }, children: [] }];
+    assert.throws(() => findParent(nodes[0].entry, nodes, -1));
   });
 });
 
@@ -25,6 +28,8 @@ describe('buildHierarchy', () => {
     const entries = [{ heading: { depth: 1 } }, { heading: { depth: 1 } }];
     const result = buildHierarchy(entries);
     assert.equal(result.length, 2);
+    assert.equal(result[0].entry, entries[0]);
+    assert.equal(result[1].entry, entries[1]);
   });
 
   it('nests children under parents', () => {
@@ -32,8 +37,8 @@ describe('buildHierarchy', () => {
     const result = buildHierarchy(entries);
 
     assert.equal(result.length, 1);
-    assert.equal(result[0].hierarchyChildren.length, 1);
-    assert.equal(result[0].hierarchyChildren[0], entries[1]);
+    assert.equal(result[0].children.length, 1);
+    assert.equal(result[0].children[0].entry, entries[1]);
   });
 
   it('handles multiple levels', () => {
@@ -45,6 +50,6 @@ describe('buildHierarchy', () => {
     const result = buildHierarchy(entries);
 
     assert.equal(result.length, 1);
-    assert.equal(result[0].hierarchyChildren[0].hierarchyChildren.length, 1);
+    assert.equal(result[0].children[0].children.length, 1);
   });
 });
diff --git a/src/generators/legacy-json/utils/buildHierarchy.mjs b/src/generators/legacy-json/utils/buildHierarchy.mjs
index 0f79125a..1b17edec 100644
--- a/src/generators/legacy-json/utils/buildHierarchy.mjs
+++ b/src/generators/legacy-json/utils/buildHierarchy.mjs
@@ -1,12 +1,12 @@
 /**
- * Recursively finds the most suitable parent entry for a given `entry` based on heading depth.
+ * Recursively finds the most suitable parent node for a given `entry` based on heading depth.
  *
  * @param {import('../../metadata/types').MetadataEntry} entry
- * @param {import('../../metadata/types').MetadataEntry[]} entries
+ * @param {Array<import('../types.d.ts').HierarchizedEntry>} nodes
  * @param {number} startIdx
  * @returns {import('../types.d.ts').HierarchizedEntry}
  */
-export function findParent(entry, entries, startIdx) {
+export function findParent(entry, nodes, startIdx) {
   // Base case: if we're at the beginning of the list, no valid parent exists.
   if (startIdx < 0) {
     throw new Error(
@@ -14,17 +14,15 @@ export function findParent(entry, entries, startIdx) {
     );
   }
 
-  const candidateParent = entries[startIdx];
-  const candidateDepth = candidateParent.heading.depth;
+  const candidateParent = nodes[startIdx];
 
   // If we find a suitable parent, return it.
-  if (candidateDepth < entry.heading.depth) {
-    candidateParent.hierarchyChildren ??= [];
+  if (candidateParent.entry.heading.depth < entry.heading.depth) {
     return candidateParent;
   }
 
   // Recurse upwards to find a suitable parent.
-  return findParent(entry, entries, startIdx - 1);
+  return findParent(entry, nodes, startIdx - 1);
 }
 
 /**
@@ -37,11 +35,9 @@ export function findParent(entry, entries, startIdx) {
  *
  * If depth <= 1, it's a top-level element (aka a root).
  *
- * If it's depth is greater than the previous entry's depth, it's a child of
- * the previous entry. Otherwise (if it's less than or equal to the previous
- * entry's depth), we need to find the entry that it was the greater than. We
- * can do this by just looping through entries in reverse starting at the
- * current index - 1.
+ * Otherwise, its parent is the nearest earlier entry with a lower depth,
+ * found by looping through entries in reverse starting at the current
+ * index - 1.
  *
  * @param {Array<import('../../metadata/types').MetadataEntry>} entries
  * @returns {Array<import('../types.d.ts').HierarchizedEntry>}
@@ -49,29 +45,21 @@ export function findParent(entry, entries, startIdx) {
 export function buildHierarchy(entries) {
   const roots = [];
 
+  // Wrapper nodes, index-aligned with `entries`.
+  const nodes = entries.map(entry => ({ entry, children: [] }));
+
   // Main loop to construct the hierarchy.
-  for (let i = 0; i < entries.length; i++) {
-    const entry = entries[i];
-    const currentDepth = entry.heading.depth;
+  for (let i = 0; i < nodes.length; i++) {
+    const node = nodes[i];
 
     // Top-level entries are added directly to roots.
-    if (currentDepth <= 1) {
-      roots.push(entry);
+    if (node.entry.heading.depth <= 1) {
+      roots.push(node);
       continue;
     }
 
     // For non-root entries, find the appropriate parent.
-    const previousEntry = entries[i - 1];
-    const previousDepth = previousEntry.heading.depth;
-
-    if (currentDepth > previousDepth) {
-      previousEntry.hierarchyChildren ??= [];
-      previousEntry.hierarchyChildren.push(entry);
-    } else {
-      // Use recursive helper to find the nearest valid parent.
-      const parent = findParent(entry, entries, i - 2);
-      parent.hierarchyChildren.push(entry);
-    }
+    findParent(node.entry, nodes, i - 1).children.push(node);
   }
 
   return roots;
diff --git a/src/generators/legacy-json/utils/buildSection.mjs b/src/generators/legacy-json/utils/buildSection.mjs
index cf4992b2..c891cf16 100644
--- a/src/generators/legacy-json/utils/buildSection.mjs
+++ b/src/generators/legacy-json/utils/buildSection.mjs
@@ -32,8 +32,8 @@ export const promoteMiscChildren = (section, parent) => {
  */
 export const createSectionBuilder = () => {
   /**
-   * Creates metadata from a hierarchized entry.
-   * @param {import('../types.d.ts').HierarchizedEntry} entry - The entry to create metadata from.
+   * Creates metadata from a metadata entry.
+   * @param {import('../../metadata/types').MetadataEntry} entry - The entry to create metadata from.
    * @returns {import('../types.d.ts').Meta | undefined} The created metadata, or undefined if all fields are empty.
    */
   const createMeta = ({
@@ -73,7 +73,7 @@ export const createSectionBuilder = () => {
 
   /**
    * Creates a section from an entry and its heading.
-   * @param {import('../types.d.ts').HierarchizedEntry} entry - The AST entry.
+   * @param {import('../../metadata/types').MetadataEntry} entry - The AST entry.
    * @param {import('../../metadata/types').HeadingNode} head - The head node of the entry.
    * @returns {import('../types.d.ts').Section} The created section.
    */
@@ -98,7 +98,7 @@ export const createSectionBuilder = () => {
    * Parses stability metadata and adds it to the section.
    * @param {import('../types.d.ts').Section} section - The section to update.
    * @param {Array} nodes - The remaining AST nodes.
-   * @param {import('../types.d.ts').HierarchizedEntry} entry - The entry providing stability information.
+   * @param {import('../../metadata/types').MetadataEntry} entry - The entry providing stability information.
    */
   const parseStability = (section, nodes, { stability, content }) => {
     if (stability) {
@@ -161,26 +161,18 @@ export const createSectionBuilder = () => {
   };
 
   /**
-   * Processes children of a given entry and updates the section.
-   * @param {import('../types.d.ts').HierarchizedEntry} entry - The current entry.
-   * @param {import('../types.d.ts').Section} section - The current section.
-   */
-  const handleChildren = ({ hierarchyChildren }, section) =>
-    hierarchyChildren?.forEach(child => handleEntry(child, section));
-
-  /**
-   * Handles an entry and updates the parent section.
-   * @param {import('../types.d.ts').HierarchizedEntry} entry - The entry to process.
+   * Handles a hierarchy node and updates the parent section.
+   * @param {import('../types.d.ts').HierarchizedEntry} node - The hierarchy node to process.
    * @param {import('../types.d.ts').Section} parent - The parent section.
    */
-  const handleEntry = (entry, parent) => {
+  const handleEntry = ({ entry, children }, parent) => {
     const [headingNode, ...nodes] = entry.content.children;
     const section = createSection(entry, headingNode);
 
     parseStability(section, nodes, entry);
     parseList(section, nodes);
     addDescription(section, nodes);
-    handleChildren(entry, section);
+    children.forEach(child => handleEntry(child, section));
     addAdditionalMetadata(section, parent, headingNode);
     addToParent(section, parent);
     promoteMiscChildren(section, parent);
@@ -200,7 +192,7 @@ export const createSectionBuilder = () => {
       source: `doc/api/${head.api}.md`,
     };
 
-    buildHierarchy(entries).forEach(entry => handleEntry(entry, rootModule));
+    buildHierarchy(entries).forEach(node => handleEntry(node, rootModule));
 
     return rootModule;
   };