diff --git a/apps/web/src/components/MermaidCodeBlock.tsx b/apps/web/src/components/MermaidCodeBlock.tsx
index bae7ea04..db684990 100644
--- a/apps/web/src/components/MermaidCodeBlock.tsx
+++ b/apps/web/src/components/MermaidCodeBlock.tsx
@@ -247,6 +247,9 @@ export const MermaidCodeBlock = ({ editor, node }: NodeViewProps) => {
       <NodeViewContent
         className={isMermaid ? "edgeever-code-source edgeever-mermaid-source" : "edgeever-code-source"}
         role="textbox"
+        autoCapitalize="none"
+        autoCorrect="off"
+        spellCheck={false}
         aria-label={isMermaid ? t("editorToolbar.mermaidSource") : undefined}
         aria-multiline="true"
         aria-readonly={!editor.isEditable}
diff --git a/apps/web/src/lib/code-block.test.mjs b/apps/web/src/lib/code-block.test.mjs
new file mode 100644
index 00000000..a61cc019
--- /dev/null
+++ b/apps/web/src/lib/code-block.test.mjs
@@ -0,0 +1,65 @@
+import { describe, expect, test } from "bun:test";
+import { Editor } from "@tiptap/core";
+import StarterKit from "@tiptap/starter-kit";
+import { EdgeEverCodeBlock, selectCurrentCodeBlockContent } from "./code-block.ts";
+
+const createEditor = () => new Editor({
+  extensions: [
+    StarterKit.configure({ codeBlock: false }),
+    EdgeEverCodeBlock,
+  ],
+  content: {
+    type: "doc",
+    content: [
+      { type: "paragraph", content: [{ type: "text", text: "before" }] },
+      {
+        type: "codeBlock",
+        attrs: { language: "mermaid" },
+        content: [{ type: "text", text: "flowchart TD\n  A --> B" }],
+      },
+      { type: "paragraph", content: [{ type: "text", text: "after" }] },
+    ],
+  },
+});
+
+const getCodeBlockRange = (editor) => {
+  let range = null;
+  editor.state.doc.descendants((node, pos) => {
+    if (!range && node.type.name === "codeBlock") {
+      range = { from: pos + 1, to: pos + 1 + node.content.size };
+    }
+  });
+  return range;
+};
+
+describe("code block selection", () => {
+  test("selects only the current code block on the first Mod-a", () => {
+    const editor = createEditor();
+    const range = getCodeBlockRange(editor);
+    editor.commands.setTextSelection(range.from + 2);
+
+    expect(selectCurrentCodeBlockContent(editor)).toBe(true);
+    expect({
+      from: editor.state.selection.from,
+      to: editor.state.selection.to,
+    }).toEqual(range);
+    editor.destroy();
+  });
+
+  test("allows the editor-wide Mod-a after the code block is fully selected", () => {
+    const editor = createEditor();
+    const range = getCodeBlockRange(editor);
+    editor.commands.setTextSelection(range);
+
+    expect(selectCurrentCodeBlockContent(editor)).toBe(false);
+    editor.destroy();
+  });
+
+  test("leaves Mod-a unchanged outside code blocks", () => {
+    const editor = createEditor();
+    editor.commands.setTextSelection(2);
+
+    expect(selectCurrentCodeBlockContent(editor)).toBe(false);
+    editor.destroy();
+  });
+});
diff --git a/apps/web/src/lib/code-block.ts b/apps/web/src/lib/code-block.ts
index a884f651..816af1ff 100644
--- a/apps/web/src/lib/code-block.ts
+++ b/apps/web/src/lib/code-block.ts
@@ -1,11 +1,34 @@
 import { common, createLowlight } from "lowlight";
 import CodeBlockLowlight from "@tiptap/extension-code-block-lowlight";
+import type { Editor } from "@tiptap/core";
 import { ReactNodeViewRenderer } from "@tiptap/react";
 import { MermaidCodeBlock } from "@/components/MermaidCodeBlock";
 
 export const codeBlockLowlight = createLowlight(common);
 
+export const selectCurrentCodeBlockContent = (editor: Editor) => {
+  const { $from, $to, from, to } = editor.state.selection;
+  if ($from.parent !== $to.parent || $from.parent.type.name !== "codeBlock") {
+    return false;
+  }
+
+  const blockFrom = $from.start();
+  const blockTo = $from.end();
+  if (from === blockFrom && to === blockTo) {
+    return false;
+  }
+
+  return editor.commands.setTextSelection({ from: blockFrom, to: blockTo });
+};
+
 export const EdgeEverCodeBlock = CodeBlockLowlight.extend({
+  addKeyboardShortcuts() {
+    return {
+      ...this.parent?.(),
+      "Mod-a": () => selectCurrentCodeBlockContent(this.editor),
+    };
+  },
+
   addNodeView() {
     return ReactNodeViewRenderer(MermaidCodeBlock);
   },