From e6e57764f4db3a602f6292087422824d2adefc0c Mon Sep 17 00:00:00 2001
From: Tatsunori Uchino <tats.u@live.jp>
Date: Mon, 29 Jun 2026 21:34:47 +0900
Subject: [PATCH] Cap ordered list mark at 999,999,999 (#19351)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: fisker <lionkay@gmail.com>
---
 changelog_unreleased/markdown/19351.md        | 32 +++++++++++++
 src/language-markdown/print/list.js           | 10 +++-
 .../__snapshots__/format.test.js.snap         | 48 +++++++++++++++++++
 .../list/huge-first-number/format.test.js     |  5 ++
 .../huge-first-number/huge-first-number.md    | 18 +++++++
 5 files changed, 111 insertions(+), 2 deletions(-)
 create mode 100644 changelog_unreleased/markdown/19351.md
 create mode 100644 tests/format/markdown/list/huge-first-number/__snapshots__/format.test.js.snap
 create mode 100644 tests/format/markdown/list/huge-first-number/format.test.js
 create mode 100644 tests/format/markdown/list/huge-first-number/huge-first-number.md

diff --git a/changelog_unreleased/markdown/19351.md b/changelog_unreleased/markdown/19351.md
new file mode 100644
index 000000000..8341bf584
--- /dev/null
+++ b/changelog_unreleased/markdown/19351.md
@@ -0,0 +1,32 @@
+#### Cap ordered list mark at 999,999,999 (#19351 by @tats-u)
+
+CommonMark parsers only support ordered list item numbers up to 999,999,999.
+
+With this change, Prettier now caps the ordered list item number at 999,999,999 to ensure that the output is correctly parsed as an ordered list by CommonMark parsers. Numbers larger than 999,999,999 are not parsed as list item numbers and are left unchanged in the output:
+
+<!-- prettier-ignore -->
+```markdown
+<!-- Input -->
+999999998. text
+999999998. text
+999999998. text
+999999998. text
+
+1234567890123456789012) text
+
+<!-- Prettier stable -->
+999999998. text
+999999999. text
+1000000000. text
+1000000001. text
+
+1234567890123456789012) text
+
+<!-- Prettier main -->
+999999998. text
+999999999. text
+999999999. text
+999999999. text
+
+1234567890123456789012) text
+```
diff --git a/src/language-markdown/print/list.js b/src/language-markdown/print/list.js
index 1a56bff36..911325b6f 100644
--- a/src/language-markdown/print/list.js
+++ b/src/language-markdown/print/list.js
@@ -10,6 +10,10 @@ import { printChildren } from "./children.js";
  * @import {Doc} from "../../document/index.js"
  */
 
+// 999,999,999 is the maximum number that can be parsed as a list item number as intended in CommonMark
+// https://spec.commonmark.org/0.31.2/#ordered-list-marker
+const MAXIMUM_ORDERED_LIST_MARKER = 999_999_999;
+
 /**
  * @param {AstPath} path
  * @param {*} options
@@ -55,8 +59,10 @@ function printList(path, options, print) {
               ? node.start
               : isGitDiffFriendlyOrderedList
                 ? 1
-                : node.start + path.index) +
-            (nthSiblingIndex % 2 === 0 ? ". " : ") ")
+                : Math.min(
+                    node.start + path.index,
+                    MAXIMUM_ORDERED_LIST_MARKER,
+                  )) + (nthSiblingIndex % 2 === 0 ? ". " : ") ")
           : nthSiblingIndex % 2 === 0
             ? "- "
             : "* ";
diff --git a/tests/format/markdown/list/huge-first-number/__snapshots__/format.test.js.snap b/tests/format/markdown/list/huge-first-number/__snapshots__/format.test.js.snap
new file mode 100644
index 000000000..0ab60ba04
--- /dev/null
+++ b/tests/format/markdown/list/huge-first-number/__snapshots__/format.test.js.snap
@@ -0,0 +1,48 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`huge-first-number.md format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+999999998. text
+999999998. text
+999999998. text
+999999998. text
+
+---
+
+999999998. text
+1. text
+1. text
+1. text
+
+---
+
+999999998. text
+1. text
+2. text
+3. text
+
+=====================================output=====================================
+999999998. text
+999999999. text
+999999999. text
+999999999. text
+
+---
+
+999999998. text
+1. text
+1. text
+1. text
+
+---
+
+999999998. text
+1. text
+1. text
+1. text
+
+================================================================================
+`;
diff --git a/tests/format/markdown/list/huge-first-number/format.test.js b/tests/format/markdown/list/huge-first-number/format.test.js
new file mode 100644
index 000000000..c1fda6633
--- /dev/null
+++ b/tests/format/markdown/list/huge-first-number/format.test.js
@@ -0,0 +1,5 @@
+runFormatTest(import.meta, [
+  "markdown",
+  // Add this parser when closing https://github.com/prettier/prettier/issues/19339
+  // "mdx",
+]);
diff --git a/tests/format/markdown/list/huge-first-number/huge-first-number.md b/tests/format/markdown/list/huge-first-number/huge-first-number.md
new file mode 100644
index 000000000..603db3fb2
--- /dev/null
+++ b/tests/format/markdown/list/huge-first-number/huge-first-number.md
@@ -0,0 +1,18 @@
+999999998. text
+999999998. text
+999999998. text
+999999998. text
+
+---
+
+999999998. text
+1. text
+1. text
+1. text
+
+---
+
+999999998. text
+1. text
+2. text
+3. text