diff --git a/app/manage-breast-screening/2026/03/my-amazing-post/ed-forgot-to-add-a-file-extension b/app/manage-breast-screening/2026/03/my-amazing-post/ed-forgot-to-add-a-file-extension
new file mode 100644
index 000000000..2604ac846
--- /dev/null
+++ b/app/manage-breast-screening/2026/03/my-amazing-post/ed-forgot-to-add-a-file-extension
@@ -0,0 +1,3 @@
+---
+title: This is my great post
+---
diff --git a/eleventy.config.js b/eleventy.config.js
index 300ae630b..9bfd3cc02 100644
--- a/eleventy.config.js
+++ b/eleventy.config.js
@@ -2,6 +2,8 @@ import fs from 'node:fs/promises'
 
 import { nhsukEleventyPlugin } from '@x-govuk/nhsuk-eleventy-plugin'
 
+import { findFileWithoutExtension } from './lib/find-file-without-extension.js'
+
 const serviceName = 'Digital prevention services design history'
 
 export default function (eleventyConfig) {
@@ -174,6 +176,17 @@ export default function (eleventyConfig) {
 
   // Reset contents of output directory before each build
   eleventyConfig.on('eleventy.before', async ({ directories, runMode }) => {
+    // Check for files without extensions in the app directory
+    const fileWithoutExtension = await findFileWithoutExtension(
+      directories.input
+    )
+
+    if (fileWithoutExtension) {
+      throw new Error(
+        `Found file without extension: ${fileWithoutExtension}\n\nDid you forget to add an .md to it?`
+      )
+    }
+
     if (runMode === 'build') {
       await fs.rm(directories.output, {
         force: true,
diff --git a/lib/find-file-without-extension.js b/lib/find-file-without-extension.js
new file mode 100644
index 000000000..dd20da71d
--- /dev/null
+++ b/lib/find-file-without-extension.js
@@ -0,0 +1,38 @@
+import fs from 'node:fs/promises'
+import path from 'node:path'
+
+/**
+ * Recursively find the first file without an extension in a directory
+ *
+ * @param {string} dir - Directory to search
+ * @returns {Promise<string|null>} - Path of first file without extension, or null
+ */
+export async function findFileWithoutExtension(dir) {
+  async function scan(currentDir) {
+    const entries = await fs.readdir(currentDir, { withFileTypes: true })
+
+    for (const entry of entries) {
+      const fullPath = path.join(currentDir, entry.name)
+
+      if (entry.isDirectory()) {
+        // Skip directories starting with underscore (e.g. _layouts, _components)
+        if (!entry.name.startsWith('_')) {
+          const found = await scan(fullPath)
+          if (found) return found
+        }
+      } else if (entry.isFile()) {
+        // Skip hidden files (e.g. .gitkeep)
+        if (!entry.name.startsWith('.')) {
+          // Check if the file has no extension
+          const ext = path.extname(entry.name)
+          if (ext === '') {
+            return fullPath
+          }
+        }
+      }
+    }
+    return null
+  }
+
+  return scan(dir)
+}