diff --git a/extensions/ringcentral/src/api.ts b/extensions/ringcentral/src/api.ts
index 851642c64..ff711f6a7 100644
--- a/extensions/ringcentral/src/api.ts
+++ b/extensions/ringcentral/src/api.ts
@@ -1,5 +1,6 @@
 import type { ResolvedRingCentralAccount } from "./accounts.js";
 import { getRingCentralPlatform } from "./auth.js";
+import { toRingCentralMarkdown } from "./markdown.js";
 import type {
   RingCentralChat,
   RingCentralPost,
@@ -20,7 +21,7 @@ export async function sendRingCentralMessage(params: {
   const platform = await getRingCentralPlatform(account);
 
   const body: Record<string, unknown> = {};
-  if (text) body.text = text;
+  if (text) body.text = toRingCentralMarkdown(text);
   if (attachments && attachments.length > 0) {
     body.attachments = attachments;
   }
@@ -41,7 +42,7 @@ export async function updateRingCentralMessage(params: {
 
   const response = await platform.patch(
     `${TM_API_BASE}/chats/${chatId}/posts/${postId}`,
-    { text },
+    { text: toRingCentralMarkdown(text) },
   );
   const result = (await response.json()) as RingCentralPost;
   return { postId: result.id };
diff --git a/extensions/ringcentral/src/markdown.test.ts b/extensions/ringcentral/src/markdown.test.ts
new file mode 100644
index 000000000..93faa8bc3
--- /dev/null
+++ b/extensions/ringcentral/src/markdown.test.ts
@@ -0,0 +1,121 @@
+import { describe, expect, it } from "vitest";
+import { toRingCentralMarkdown, needsMarkdownConversion } from "./markdown.js";
+
+describe("toRingCentralMarkdown", () => {
+  it("converts single underscore italic to asterisk", () => {
+    expect(toRingCentralMarkdown("_italic_")).toBe("*italic*");
+    expect(toRingCentralMarkdown("This is _italic_ text")).toBe("This is *italic* text");
+  });
+
+  it("preserves double asterisk bold", () => {
+    expect(toRingCentralMarkdown("**bold**")).toBe("**bold**");
+    expect(toRingCentralMarkdown("This is **bold** text")).toBe("This is **bold** text");
+  });
+
+  it("converts double underscore to bold", () => {
+    expect(toRingCentralMarkdown("__bold__")).toBe("**bold**");
+  });
+
+  it("removes strikethrough", () => {
+    expect(toRingCentralMarkdown("~~strikethrough~~")).toBe("strikethrough");
+    expect(toRingCentralMarkdown("This is ~~deleted~~ text")).toBe("This is deleted text");
+  });
+
+  it("removes code blocks", () => {
+    expect(toRingCentralMarkdown("```\ncode\n```")).toBe("code");
+    expect(toRingCentralMarkdown("```javascript\nconst x = 1;\n```")).toBe("const x = 1;");
+  });
+
+  it("removes inline code", () => {
+    expect(toRingCentralMarkdown("`code`")).toBe("code");
+    expect(toRingCentralMarkdown("Use `npm install` to install")).toBe("Use npm install to install");
+  });
+
+  it("converts headings to bold", () => {
+    expect(toRingCentralMarkdown("# Heading 1")).toBe("**Heading 1**");
+    expect(toRingCentralMarkdown("## Heading 2")).toBe("**Heading 2**");
+    expect(toRingCentralMarkdown("### Heading 3")).toBe("**Heading 3**");
+  });
+
+  it("normalizes bullet lists", () => {
+    expect(toRingCentralMarkdown("- item")).toBe("* item");
+    expect(toRingCentralMarkdown("+ item")).toBe("* item");
+    expect(toRingCentralMarkdown("* item")).toBe("* item");
+  });
+
+  it("preserves links", () => {
+    expect(toRingCentralMarkdown("[link](https://example.com)")).toBe("[link](https://example.com)");
+  });
+
+  it("preserves blockquotes", () => {
+    expect(toRingCentralMarkdown("> quote")).toBe("> quote");
+  });
+
+  it("preserves numbered lists", () => {
+    expect(toRingCentralMarkdown("1. first\n2. second")).toBe("1. first\n2. second");
+  });
+
+  it("handles complex markdown", () => {
+    const input = `# Title
+
+This is _italic_ and **bold** text.
+
+- Item 1
+- Item 2
+
+\`\`\`
+code block
+\`\`\`
+
+Use \`inline code\` here.`;
+
+    const expected = `**Title**
+
+This is *italic* and **bold** text.
+
+* Item 1
+* Item 2
+
+code block
+
+Use inline code here.`;
+
+    expect(toRingCentralMarkdown(input)).toBe(expected);
+  });
+
+  it("does not convert underscore in URLs or paths", () => {
+    // URLs with underscores should be preserved in links
+    expect(toRingCentralMarkdown("[link](https://example.com/path_with_underscore)"))
+      .toBe("[link](https://example.com/path_with_underscore)");
+  });
+});
+
+describe("needsMarkdownConversion", () => {
+  it("returns true for text with underscore italic", () => {
+    expect(needsMarkdownConversion("_italic_")).toBe(true);
+  });
+
+  it("returns true for text with strikethrough", () => {
+    expect(needsMarkdownConversion("~~strike~~")).toBe(true);
+  });
+
+  it("returns true for text with code blocks", () => {
+    expect(needsMarkdownConversion("```code```")).toBe(true);
+  });
+
+  it("returns true for text with inline code", () => {
+    expect(needsMarkdownConversion("`code`")).toBe(true);
+  });
+
+  it("returns true for text with headings", () => {
+    expect(needsMarkdownConversion("# Heading")).toBe(true);
+  });
+
+  it("returns false for plain text", () => {
+    expect(needsMarkdownConversion("plain text")).toBe(false);
+  });
+
+  it("returns false for already compatible markdown", () => {
+    expect(needsMarkdownConversion("*italic* and **bold**")).toBe(false);
+  });
+});
diff --git a/extensions/ringcentral/src/markdown.ts b/extensions/ringcentral/src/markdown.ts
new file mode 100644
index 000000000..9df29d5d4
--- /dev/null
+++ b/extensions/ringcentral/src/markdown.ts
@@ -0,0 +1,87 @@
+/**
+ * Convert standard Markdown to RingCentral Mini-Markdown format.
+ * 
+ * RingCentral Mini-Markdown differences:
+ * - `_text_` = underline (not italic)
+ * - `*text*` = italic
+ * - `**text**` = bold
+ * - `[text](url)` = link
+ * - `> quote` = blockquote
+ * - `* item` = bullet list
+ * 
+ * Not supported: strikethrough (~~), code blocks (```), headings (#), etc.
+ */
+
+/**
+ * Convert standard markdown to RingCentral mini-markdown
+ */
+export function toRingCentralMarkdown(text: string): string {
+  let result = text;
+
+  // 1. Preserve links - temporarily replace them to avoid modifying URLs
+  const linkPlaceholders: string[] = [];
+  result = result.replace(/\[([^\]]+)\]\(([^)]+)\)/g, (match) => {
+    const idx = linkPlaceholders.length;
+    linkPlaceholders.push(match);
+    return `\x00LINK_${idx}\x00`;
+  });
+
+  // 2. Convert __text__ (some markdown bold) to **text**
+  result = result.replace(/__([^_]+)__/g, "**$1**");
+
+  // 3. Convert single _text_ to *text* for italic
+  //    Match _text_ but not inside words (e.g., snake_case_name)
+  result = result.replace(/(?<=^|[\s\p{P}])_([^_\n]+)_(?=$|[\s\p{P}])/gu, "*$1*");
+
+  // 4. Remove strikethrough ~~text~~ (not supported)
+  result = result.replace(/~~([^~]+)~~/g, "$1");
+
+  // 5. Convert code blocks to plain text (not well supported)
+  // Remove triple backtick code blocks
+  result = result.replace(/```[\s\S]*?```/g, (match) => {
+    // Extract content without the backticks and language identifier
+    const content = match.replace(/```\w*\n?/, "").replace(/\n?```$/, "");
+    return content;
+  });
+
+  // Convert inline code `text` to plain text
+  result = result.replace(/`([^`]+)`/g, "$1");
+
+  // 6. Convert headings to bold (# Heading -> **Heading**)
+  result = result.replace(/^#{1,6}\s+(.+)$/gm, "**$1**");
+
+  // 7. Convert horizontal rules (---, ***, ___) to simple separator
+  result = result.replace(/^[-*_]{3,}$/gm, "---");
+
+  // 8. Normalize bullet lists (-, +, *) to * for consistency
+  result = result.replace(/^(\s*)[-+]\s+/gm, "$1* ");
+
+  // 9. Restore links
+  result = result.replace(/\x00LINK_(\d+)\x00/g, (_match, idx) => {
+    return linkPlaceholders[Number(idx)];
+  });
+
+  // 10. Keep numbered lists as-is (1. item)
+  // 11. Keep blockquotes as-is (> quote)
+
+  return result;
+}
+
+/**
+ * Check if text contains markdown that needs conversion
+ */
+export function needsMarkdownConversion(text: string): boolean {
+  // Check for patterns that need conversion
+  return (
+    // Single underscore italic
+    /(?<![\\*_])_[^_\n]+_(?![_])/.test(text) ||
+    // Strikethrough
+    /~~[^~]+~~/.test(text) ||
+    // Code blocks
+    /```[\s\S]*?```/.test(text) ||
+    // Inline code
+    /`[^`]+`/.test(text) ||
+    // Headings
+    /^#{1,6}\s+.+$/m.test(text)
+  );
+}