diff --git a/plugins/platforms/email/adapter.py b/plugins/platforms/email/adapter.py
index d983fd81c79d5..0ea0423d8d703 100644
--- a/plugins/platforms/email/adapter.py
+++ b/plugins/platforms/email/adapter.py
@@ -33,6 +33,7 @@
 from gateway.config import Platform, PlatformConfig
 from utils import is_truthy_value
 from gateway.platforms._shared import get_scoped_secret as _get_secret, coerce_port, decode_json_list_literal, send_error
+from plugins.platforms.email.markdown_html import markdown_to_email_html
 
 logger = logging.getLogger(__name__)
 
@@ -436,6 +437,14 @@ def _extract_attachments(msg: email_lib.message.Message, skip_attachments: bool
     return attachments
 
 
+def _body_alternative(body: str) -> MIMEMultipart:
+    """Plain part first, HTML second: a client that cannot render HTML keeps the original text."""
+    alternative = MIMEMultipart("alternative")
+    alternative.attach(MIMEText(body, "plain", "utf-8"))
+    alternative.attach(MIMEText(markdown_to_email_html(body), "html", "utf-8"))
+    return alternative
+
+
 def _attach_file(msg: MIMEMultipart, path: Path, filename: str) -> None:
     """Attach *path* to *msg* as base64 application/octet-stream."""
     with open(path, "rb") as f:
@@ -871,7 +880,7 @@ def _new_reply(self, to_addr: str, body: str, reply_to_msg_id: Optional[str] = N
                            ("Date", formatdate(localtime=True)), ("Message-ID", msg_id)):
             msg[key] = value
         if body or attach_empty_body:
-            msg.attach(MIMEText(body, "plain", "utf-8"))
+            msg.attach(_body_alternative(body))
         return msg, msg_id, subject
 
     def _smtp_send(self, msg: MIMEMultipart) -> None:
@@ -972,7 +981,7 @@ async def _standalone_send(pconfig, chat_id, message, *, thread_id=None, media_f
     if not all([address, password, smtp_host]):
         return send_error("Email not configured (EMAIL_ADDRESS, EMAIL_PASSWORD, EMAIL_SMTP_HOST required)")
     try:
-        msg = MIMEText(message, "plain", "utf-8")
+        msg = _body_alternative(message)
         for key, value in (("From", address), ("To", chat_id), ("Subject", t("platform.email.standalone_subject")), ("Date", formatdate(localtime=True))):
             msg[key] = value
         server = _open_smtp(smtp_host, smtp_port, smtp_security, _tls_context(smtp_tls_verify, smtp_host), smtplib.SMTP, smtplib.SMTP_SSL)
diff --git a/plugins/platforms/email/markdown_html.py b/plugins/platforms/email/markdown_html.py
new file mode 100644
index 0000000000000..c5460ebf78f62
--- /dev/null
+++ b/plugins/platforms/email/markdown_html.py
@@ -0,0 +1,115 @@
+"""Render agent Markdown as inline-styled email HTML.
+
+The email adapter sends ``text/plain`` only, so Markdown the agent writes for chat
+(``**bold**``, ``## Headings``, bullets) reaches a mail reader as literal asterisks and
+hashes. This converts the subset the agent actually emits into simple HTML, which the
+adapter attaches as a ``text/html`` alternative *beside* the unchanged plain part:
+HTML-capable clients render it, and every other client still gets the original text.
+"""
+
+from __future__ import annotations
+
+import html
+import re
+
+# Anchored block markers. A heading requires its space, so "#hashtag" stays prose.
+_HEADING = re.compile(r"^(#{1,6})\s+(.*)$")
+_BULLET = re.compile(r"^[-*+]\s+(.*)$")
+_NUMBERED = re.compile(r"^\d+[.)]\s+(.*)$")
+
+_BOLD = re.compile(r"\*\*(.+?)\*\*")
+_ITALIC = re.compile(r"(?<!\*)\*([^*\n]+)\*(?!\*)")
+_CODE = re.compile(r"`([^`\n]+)`")
+_LINK = re.compile(r"\[([^\]]+)\]\((https?://[^)\s]+)\)")
+# The lookbehind skips a URL already inside an href or a wrapped parenthetical.
+_BARE_URL = re.compile(r"""(?<!["'=(])(https?://[^\s<>()]+)""")
+
+_SENTENCE_END = ".,;:!?"
+_HEADING_STYLE = {
+    2: "margin:18px 0 8px;font-size:1.25em;font-weight:600",
+    3: "margin:16px 0 8px;font-size:1.15em;font-weight:600",
+    4: "margin:14px 0 6px;font-size:1.05em;font-weight:600",
+    5: "margin:12px 0 6px;font-size:1em;font-weight:600",
+    6: "margin:12px 0 6px;font-size:1em;font-weight:600",
+}
+_PARAGRAPH_STYLE = "margin:0 0 12px"
+_LIST_STYLE = "margin:0 0 12px;padding-left:22px"
+_ITEM_STYLE = "margin:0 0 4px"
+
+
+def _linkify(match: re.Match) -> str:
+    """Wrap a bare URL, leaving sentence punctuation outside the anchor."""
+    url, trailing = match.group(1), ""
+    while url and url[-1] in _SENTENCE_END:
+        trailing = url[-1] + trailing
+        url = url[:-1]
+    return f'<a href="{url}">{url}</a>{trailing}' if url else match.group(0)
+
+
+def _inline(text: str) -> str:
+    """Escape, then apply inline Markdown. Escaping first means content cannot inject markup."""
+    text = html.escape(text)
+    text = _CODE.sub(r"<code>\1</code>", text)
+    text = _BOLD.sub(r"<strong>\1</strong>", text)
+    text = _ITALIC.sub(r"<em>\1</em>", text)
+    text = _LINK.sub(r'<a href="\2">\1</a>', text)
+    return _BARE_URL.sub(_linkify, text)
+
+
+def markdown_to_email_html(text: str) -> str:
+    """Convert the Markdown subset the agent emits into inline-styled email HTML."""
+    if not text.strip():
+        return ""
+    blocks: list[str] = []
+    paragraph: list[str] = []
+    items: list[str] = []
+    list_tag = ""
+
+    def flush_paragraph() -> None:
+        if paragraph:
+            blocks.append(f'<p style="{_PARAGRAPH_STYLE}">{_inline(" ".join(paragraph))}</p>')
+            paragraph.clear()
+
+    def flush_list() -> None:
+        nonlocal list_tag
+        if items:
+            rendered = "".join(f'<li style="{_ITEM_STYLE}">{item}</li>' for item in items)
+            blocks.append(f'<{list_tag} style="{_LIST_STYLE}">{rendered}</{list_tag}>')
+            items.clear()
+        list_tag = ""
+
+    def start_list(tag: str) -> None:
+        nonlocal list_tag
+        if list_tag != tag:
+            flush_list()
+            list_tag = tag
+
+    for line in text.splitlines():
+        stripped = line.strip()
+        if not stripped:
+            flush_paragraph()
+            flush_list()
+            continue
+        if heading := _HEADING.match(stripped):
+            flush_paragraph()
+            flush_list()
+            # h1 stays reserved for the message subject line.
+            tag = min(len(heading.group(1)) + 1, 6)
+            blocks.append(f'<h{tag} style="{_HEADING_STYLE[tag]}">{_inline(heading.group(2))}</h{tag}>')
+            continue
+        if bullet := _BULLET.match(stripped):
+            flush_paragraph()
+            start_list("ul")
+            items.append(_inline(bullet.group(1)))
+            continue
+        if numbered := _NUMBERED.match(stripped):
+            flush_paragraph()
+            start_list("ol")
+            items.append(_inline(numbered.group(1)))
+            continue
+        flush_list()
+        paragraph.append(stripped)
+
+    flush_paragraph()
+    flush_list()
+    return "\n".join(blocks)
diff --git a/tests/gateway/test_email.py b/tests/gateway/test_email.py
index 9b272d4fa3024..f45ce03007da1 100644
--- a/tests/gateway/test_email.py
+++ b/tests/gateway/test_email.py
@@ -1451,7 +1451,10 @@ def test_oversized_cron_output_is_delivered_as_one_whole_email():
         adapter = EmailAdapter(PlatformConfig(enabled=True))
     sent = []
     smtp = MagicMock()
-    smtp.send_message.side_effect = lambda msg: sent.append(msg.get_payload()[0].get_payload(decode=True).decode())
+    # The body rides inside a multipart/alternative, so walk to the plain part.
+    smtp.send_message.side_effect = lambda msg: sent.append(
+        next(p.get_payload(decode=True).decode() for p in msg.walk() if p.get_content_type() == "text/plain")
+    )
     adapter._connect_smtp = lambda: smtp
     content = "\n\n".join(f"line {i} " + "x" * 200 for i in range(60))
     payload = DeliveryRouter(GatewayConfig())._cap_oversized_output(adapter, content, "job")
diff --git a/tests/gateway/test_email_html_body.py b/tests/gateway/test_email_html_body.py
new file mode 100644
index 0000000000000..fb05ee2445dd9
--- /dev/null
+++ b/tests/gateway/test_email_html_body.py
@@ -0,0 +1,121 @@
+"""Outgoing email carries a ``text/html`` alternative, and the plain part is unchanged.
+
+The adapter sent ``text/plain`` only, so Markdown the agent writes for chat
+(``**bold**``, ``## Headings``) arrived in a mail reader as literal asterisks and hashes.
+Both send paths must now attach an HTML alternative, and a client that cannot render HTML
+must still receive the byte-identical plain text it received before.
+"""
+
+import asyncio
+import os
+import tempfile
+import unittest
+from unittest.mock import patch, MagicMock
+
+# Markdown the agent emits, plus untrusted markup that must not survive as live HTML.
+_BODY = """## Bottom line
+
+The **PJM** order stands, per [the filing](https://example.gov/pjm).
+
+## Major stories
+
+1. DOE issues an emergency order
+2. FERC opens a docket
+
+- Watch the Senate markup
+- Track the <script>alert(1)</script> filing
+"""
+
+_SMTP_ENV = {
+    "EMAIL_ADDRESS": "hermes@test.com",
+    "EMAIL_PASSWORD": "secret",
+    "EMAIL_IMAP_HOST": "imap.test.com",
+    "EMAIL_SMTP_HOST": "smtp.test.com",
+}
+
+
+def _assert_alternative(test: unittest.TestCase, msg) -> None:
+    """The contract both send paths share: an HTML alternative beside an unchanged plain part."""
+    test.assertTrue(
+        any(p.get_content_type() == "multipart/alternative" for p in msg.walk()),
+        f"no multipart/alternative in {msg.get_content_type()}",
+    )
+    bodies = {
+        part.get_content_type(): part.get_payload(decode=True).decode("utf-8")
+        for part in msg.walk()
+        if part.get_content_type() in ("text/plain", "text/html")
+    }
+
+    # Clients that cannot render HTML keep the exact text they got before the fix.
+    test.assertEqual(bodies["text/plain"], _BODY)
+
+    html = bodies["text/html"]
+    test.assertIn("<h3", html)                     # "## " became a heading
+    test.assertIn("<strong>PJM</strong>", html)    # "**PJM**" became bold
+    test.assertNotIn("##", html)                   # no literal Markdown left behind
+    test.assertNotIn("**", html)
+    # Content is escaped, so a body can never inject live markup.
+    test.assertNotIn("<script>", html)
+    test.assertIn("&lt;script&gt;", html)
+
+
+class TestReplyCarriesHtmlAlternative(unittest.TestCase):
+    """The in-process send path (replies, attachments, images, documents)."""
+
+    def _adapter(self):
+        from gateway.config import PlatformConfig
+        from plugins.platforms.email.adapter import EmailAdapter
+        with patch.dict(os.environ, _SMTP_ENV):
+            return EmailAdapter(PlatformConfig(enabled=True))
+
+    def _send(self, adapter, fn, *args):
+        with patch("smtplib.SMTP") as mock_smtp:
+            server = MagicMock()
+            mock_smtp.return_value = server
+            fn(*args)
+        return server.send_message.call_args[0][0]
+
+    def test_reply_carries_html_alternative(self):
+        adapter = self._adapter()
+        sent = self._send(adapter, adapter._send_email, "user@test.com", _BODY, None)
+        _assert_alternative(self, sent)
+
+    def test_attachment_rides_beside_the_alternative(self):
+        """An attachment is a sibling of the alternative, not a part inside it."""
+        adapter = self._adapter()
+        with tempfile.NamedTemporaryFile(suffix=".txt", delete=False) as f:
+            f.write(b"attachment bytes")
+            path = f.name
+        try:
+            sent = self._send(adapter, adapter._send_email_with_attachment,
+                              "user@test.com", _BODY, path)
+        finally:
+            os.unlink(path)
+
+        self.assertEqual(sent.get_content_type(), "multipart/mixed")
+        _assert_alternative(self, sent)
+        self.assertTrue(any(
+            "attachment" in str(p.get("Content-Disposition", "")) for p in sent.walk()
+        ))
+
+
+class TestStandaloneSenderCarriesHtmlAlternative(unittest.TestCase):
+    """The out-of-process path cron delivery uses — the sibling call path."""
+
+    def test_standalone_send_carries_html_alternative(self):
+        from gateway.config import PlatformConfig
+        from plugins.platforms.email import adapter as email_adapter
+
+        with patch.dict(os.environ, _SMTP_ENV):
+            with patch.object(email_adapter, "_open_smtp") as open_smtp:
+                server = MagicMock()
+                open_smtp.return_value = server
+                result = asyncio.run(
+                    email_adapter._standalone_send(PlatformConfig(enabled=True), "user@test.com", _BODY)
+                )
+
+        self.assertTrue(result.get("success"), result)
+        sent = server.send_message.call_args[0][0]
+        # Nothing to attach, so the alternative is the top level — not wrapped in a mixed part.
+        self.assertEqual(sent.get_content_type(), "multipart/alternative")
+        _assert_alternative(self, sent)