diff --git a/scripts/lib/plan-canvas/markdown.js b/scripts/lib/plan-canvas/markdown.js
index 84799a3c..94167141 100644
--- a/scripts/lib/plan-canvas/markdown.js
+++ b/scripts/lib/plan-canvas/markdown.js
@@ -132,8 +132,12 @@ function renderListItem(text) {
   return '<li>' + renderInline(text) + '</li>';
 }
 
+function listTag(marker) {
+  return /^\d/.test(marker) ? 'ol' : 'ul';
+}
+
 function buildList(items, start, indent) {
-  const tag = /^\d/.test(items[start].marker) ? 'ol' : 'ul';
+  const tag = listTag(items[start].marker);
   const parts = [];
   let i = start;
   while (i < items.length && items[i].indent >= indent) {
@@ -148,6 +152,10 @@ function buildList(items, start, indent) {
       }
       i = nested.end;
     } else {
+      // A marker-type change at the same indent starts a new list
+      // (CommonMark); stop here so the caller renders the next run with
+      // its own tag instead of absorbing it into this one.
+      if (listTag(items[i].marker) !== tag) break;
       parts.push(renderListItem(items[i].text));
       i += 1;
     }
@@ -155,6 +163,17 @@ function buildList(items, start, indent) {
   return { html: '<' + tag + '>\n' + parts.join('\n') + '\n</' + tag + '>', end: i };
 }
 
+function buildListBlock(items) {
+  let lists = [];
+  let i = 0;
+  while (i < items.length) {
+    const list = buildList(items, i, items[i].indent);
+    lists = [...lists, list.html];
+    i = list.end;
+  }
+  return lists.join('\n');
+}
+
 function startsBlock(line, nextLine) {
   return /^```/.test(line) ||
     /^#{1,6}\s/.test(line) ||
@@ -257,7 +276,10 @@ function renderMarkdown(text) {
         items.push({ indent: m[1].length, marker: m[2], text: m[3] });
         i += 1;
       }
-      out.push(buildList(items, 0, items[0].indent).html);
+      // An outdent below the first item's indentation ends that list. Render
+      // the remaining run as a sibling list so malformed indentation cannot
+      // silently drop content or create an empty parent item.
+      out.push(buildListBlock(items));
       continue;
     }
 
diff --git a/tests/lib/plan-canvas-markdown.test.js b/tests/lib/plan-canvas-markdown.test.js
index 5b401ff4..d0953814 100644
--- a/tests/lib/plan-canvas-markdown.test.js
+++ b/tests/lib/plan-canvas-markdown.test.js
@@ -152,6 +152,44 @@ function runTests() {
     assert.strictEqual(renderMarkdown('* a\n* b'), '<ul>\n<li>a</li>\n<li>b</li>\n</ul>');
   })) passed++; else failed++;
 
+  if (test('preserves items that outdent below the first item', () => {
+    assert.strictEqual(
+      renderMarkdown('  - alpha\n- beta\n- gamma'),
+      '<ul>\n<li>alpha</li>\n</ul>\n<ul>\n<li>beta</li>\n<li>gamma</li>\n</ul>'
+    );
+  })) passed++; else failed++;
+
+  if (test('uses each outdented run marker for its list type', () => {
+    assert.strictEqual(
+      renderMarkdown('  - prep\n1. phase one\n2. phase two'),
+      '<ul>\n<li>prep</li>\n</ul>\n<ol>\n<li>phase one</li>\n<li>phase two</li>\n</ol>'
+    );
+  })) passed++; else failed++;
+
+  if (test('marker type change at the same indent starts a new list', () => {
+    assert.strictEqual(
+      renderMarkdown('- prep\n1. phase one\n2. phase two'),
+      '<ul>\n<li>prep</li>\n</ul>\n<ol>\n<li>phase one</li>\n<li>phase two</li>\n</ol>'
+    );
+  })) passed++; else failed++;
+
+  if (test('switching back to bullets after a numbered run starts a third list', () => {
+    assert.strictEqual(
+      renderMarkdown('1. one\n- bullet\n2. two'),
+      '<ol>\n<li>one</li>\n</ol>\n<ul>\n<li>bullet</li>\n</ul>\n<ol>\n<li>two</li>\n</ol>'
+    );
+  })) passed++; else failed++;
+
+  if (test('renders repeated outdents without empty parent items', () => {
+    const out = renderMarkdown('    - deep one\n    - deep two\n  - middle\n- shallow');
+    assert.strictEqual(
+      out,
+      '<ul>\n<li>deep one</li>\n<li>deep two</li>\n</ul>\n' +
+      '<ul>\n<li>middle</li>\n</ul>\n<ul>\n<li>shallow</li>\n</ul>'
+    );
+    assert.ok(!out.includes('<li>\n<ul>'), `No empty parent item expected, got ${out}`);
+  })) passed++; else failed++;
+
   // Table tests
   console.log('\nTables:');