fix(markdown): require double-tilde strikethrough delimiters

packages/coding-agent/src/core/export-html/template.js

@@ -1448,9 +1448,23 @@
       }
 
       // Configure marked with syntax highlighting and HTML escaping for text
+      const strictStrikethroughRegex = /^(~~)(?=[^\s~])((?:\\.|[^\\])*?(?:\\.|[^\s~\\]))\1(?=[^~]|$)/;
+
       marked.use({
         breaks: true,
         gfm: true,
+        tokenizer: {
+          del(src) {
+            const match = strictStrikethroughRegex.exec(src);
+            if (!match) return undefined;
+            return {
+              type: 'del',
+              raw: match[0],
+              text: match[2],
+              tokens: this.lexer.inlineTokens(match[2])
+            };
+          }
+        },
         renderer: {
           // Code blocks: syntax highlight, no HTML escaping
           code(token) {

packages/tui/src/components/markdown.ts

@@ -1,8 +1,32 @@
-import { marked, type Token } from "marked";
+import { Marked, type Token, Tokenizer, type Tokens } from "marked";
 import { isImageLine } from "../terminal-image.js";
 import type { Component } from "../tui.js";
 import { applyBackgroundToLine, visibleWidth, wrapTextWithAnsi } from "../utils.js";
 
+const STRICT_STRIKETHROUGH_REGEX = /^(~~)(?=[^\s~])((?:\\.|[^\\])*?(?:\\.|[^\s~\\]))\1(?=[^~]|$)/;
+
+class StrictStrikethroughTokenizer extends Tokenizer {
+	override del(src: string): Tokens.Del | undefined {
+		const match = STRICT_STRIKETHROUGH_REGEX.exec(src);
+		if (!match) {
+			return undefined;
+		}
+
+		const text = match[2];
+		return {
+			type: "del",
+			raw: match[0],
+			text,
+			tokens: this.lexer.inlineTokens(text),
+		};
+	}
+}
+
+const markdownParser = new Marked();
+markdownParser.setOptions({
+	tokenizer: new StrictStrikethroughTokenizer(),
+});
+
 /**
  * Default text styling for markdown content.
  * Applied to all text unless overridden by markdown formatting.
@@ -112,7 +136,7 @@ export class Markdown implements Component {
 		const normalizedText = this.text.replace(/\t/g, "   ");
 
 		// Parse markdown to HTML-like tokens
-		const tokens = marked.lexer(normalizedText);
+		const tokens = markdownParser.lexer(normalizedText);
 
 		// Convert tokens to styled terminal output
 		const renderedLines: string[] = [];

packages/tui/test/markdown.test.ts

@@ -1061,6 +1061,31 @@ bar`,
 		});
 	});
 
+	describe("Strikethrough syntax", () => {
+		it("should render ~~text~~ as strikethrough", () => {
+			const markdown = new Markdown("Use ~~strikethrough~~ here", 0, 0, defaultMarkdownTheme);
+
+			const lines = markdown.render(80);
+			const joinedOutput = lines.join("\n");
+			const joinedPlain = lines.map((line) => line.replace(/\x1b\[[0-9;]*m/g, "")).join(" ");
+
+			assert.ok(joinedOutput.includes("\x1b[9m"), "Should apply strikethrough styling");
+			assert.ok(joinedPlain.includes("strikethrough"), "Should include struck text content");
+			assert.ok(!joinedPlain.includes("~~strikethrough~~"), "Should not render delimiters as text");
+		});
+
+		it("should keep ~text~ as plain text", () => {
+			const markdown = new Markdown("Use ~strikethrough~ literally", 0, 0, defaultMarkdownTheme);
+
+			const lines = markdown.render(80);
+			const joinedOutput = lines.join("\n");
+			const joinedPlain = lines.map((line) => line.replace(/\x1b\[[0-9;]*m/g, "")).join(" ");
+
+			assert.ok(joinedPlain.includes("~strikethrough~"), "Single-tilde delimiters should remain visible");
+			assert.ok(!joinedOutput.includes("\x1b[9m"), "Single-tilde text should not use strikethrough styling");
+		});
+	});
+
 	describe("Links", () => {
 		it("should not duplicate URL for autolinked emails", () => {
 			const markdown = new Markdown("Contact user@example.com for help", 0, 0, defaultMarkdownTheme);