diff --git a/tests/gmail/test_draft_gmail_message.py b/tests/gmail/test_draft_gmail_message.py
new file mode 100644
index 0000000..b5def51
--- /dev/null
+++ b/tests/gmail/test_draft_gmail_message.py
@@ -0,0 +1,251 @@
+import base64
+import os
+import sys
+from unittest.mock import Mock
+
+import pytest
+
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "../..")))
+
+from core.utils import UserInputError
+from gmail.gmail_tools import draft_gmail_message
+
+
+def _unwrap(tool):
+    """Unwrap FunctionTool + decorators to the original async function."""
+    fn = tool.fn if hasattr(tool, "fn") else tool
+    while hasattr(fn, "__wrapped__"):
+        fn = fn.__wrapped__
+    return fn
+
+
+def _thread_response(*message_ids):
+    return {
+        "messages": [
+            {
+                "payload": {
+                    "headers": [{"name": "Message-ID", "value": message_id}],
+                }
+            }
+            for message_id in message_ids
+        ]
+    }
+
+
+@pytest.mark.asyncio
+async def test_draft_gmail_message_reports_actual_attachment_count(
+    tmp_path, monkeypatch
+):
+    monkeypatch.setenv("ALLOWED_FILE_DIRS", str(tmp_path))
+    attachment_path = tmp_path / "sample.txt"
+    attachment_path.write_text("hello attachment", encoding="utf-8")
+
+    mock_service = Mock()
+    mock_service.users().drafts().create().execute.return_value = {"id": "draft123"}
+
+    result = await _unwrap(draft_gmail_message)(
+        service=mock_service,
+        user_google_email="user@example.com",
+        to="recipient@example.com",
+        subject="Attachment test",
+        body="Please see attached.",
+        attachments=[{"path": str(attachment_path)}],
+        include_signature=False,
+    )
+
+    assert "Draft created with 1 attachment(s)! Draft ID: draft123" in result
+
+    create_kwargs = (
+        mock_service.users.return_value.drafts.return_value.create.call_args.kwargs
+    )
+    raw_message = create_kwargs["body"]["message"]["raw"]
+    raw_bytes = base64.urlsafe_b64decode(raw_message)
+
+    assert b"Content-Disposition: attachment;" in raw_bytes
+    assert b"sample.txt" in raw_bytes
+
+
+@pytest.mark.asyncio
+async def test_draft_gmail_message_raises_when_no_attachments_are_added(
+    tmp_path, monkeypatch
+):
+    monkeypatch.setenv("ALLOWED_FILE_DIRS", str(tmp_path))
+    missing_path = tmp_path / "missing.txt"
+
+    mock_service = Mock()
+    mock_service.users().drafts().create().execute.return_value = {"id": "draft123"}
+
+    with pytest.raises(UserInputError, match="No valid attachments were added"):
+        await _unwrap(draft_gmail_message)(
+            service=mock_service,
+            user_google_email="user@example.com",
+            to="recipient@example.com",
+            subject="Attachment test",
+            body="Please see attached.",
+            attachments=[{"path": str(missing_path)}],
+            include_signature=False,
+        )
+
+
+@pytest.mark.asyncio
+async def test_draft_gmail_message_appends_gmail_signature_html():
+    mock_service = Mock()
+    mock_service.users().drafts().create().execute.return_value = {"id": "draft_sig"}
+    mock_service.users().settings().sendAs().list().execute.return_value = {
+        "sendAs": [
+            {
+                "sendAsEmail": "user@example.com",
+                "isPrimary": True,
+                "signature": "<div>Best,<br>Alice</div>",
+            }
+        ]
+    }
+
+    result = await _unwrap(draft_gmail_message)(
+        service=mock_service,
+        user_google_email="user@example.com",
+        to="recipient@example.com",
+        subject="Signature test",
+        body="<p>Hello</p>",
+        body_format="html",
+        include_signature=True,
+    )
+
+    assert "Draft created! Draft ID: draft_sig" in result
+
+    create_kwargs = (
+        mock_service.users.return_value.drafts.return_value.create.call_args.kwargs
+    )
+    raw_message = create_kwargs["body"]["message"]["raw"]
+    raw_text = base64.urlsafe_b64decode(raw_message).decode("utf-8", errors="ignore")
+
+    assert "<p>Hello</p>" in raw_text
+    assert "Best,<br>Alice" in raw_text
+
+
+@pytest.mark.asyncio
+async def test_draft_gmail_message_autofills_reply_headers_from_thread():
+    mock_service = Mock()
+    mock_service.users().drafts().create().execute.return_value = {"id": "draft_reply"}
+    mock_service.users().threads().get().execute.return_value = _thread_response(
+        "<msg1@example.com>",
+        "<msg2@example.com>",
+        "<msg3@example.com>",
+    )
+
+    result = await _unwrap(draft_gmail_message)(
+        service=mock_service,
+        user_google_email="user@example.com",
+        to="recipient@example.com",
+        subject="Meeting tomorrow",
+        body="Thanks for the update.",
+        thread_id="thread123",
+        include_signature=False,
+    )
+
+    assert "Draft created! Draft ID: draft_reply" in result
+
+    create_kwargs = (
+        mock_service.users.return_value.drafts.return_value.create.call_args.kwargs
+    )
+    raw_message = create_kwargs["body"]["message"]["raw"]
+    raw_text = base64.urlsafe_b64decode(raw_message).decode("utf-8", errors="ignore")
+
+    assert "In-Reply-To: <msg3@example.com>" in raw_text
+    assert (
+        "References: <msg1@example.com> <msg2@example.com> <msg3@example.com>"
+        in raw_text
+    )
+    assert create_kwargs["body"]["message"]["threadId"] == "thread123"
+
+
+@pytest.mark.asyncio
+async def test_draft_gmail_message_uses_explicit_in_reply_to_when_filling_references():
+    mock_service = Mock()
+    mock_service.users().drafts().create().execute.return_value = {"id": "draft_reply"}
+    mock_service.users().threads().get().execute.return_value = _thread_response(
+        "<msg1@example.com>",
+        "<msg2@example.com>",
+        "<msg3@example.com>",
+    )
+
+    await _unwrap(draft_gmail_message)(
+        service=mock_service,
+        user_google_email="user@example.com",
+        to="recipient@example.com",
+        subject="Meeting tomorrow",
+        body="Replying to an earlier message.",
+        thread_id="thread123",
+        in_reply_to="<msg2@example.com>",
+        include_signature=False,
+    )
+
+    create_kwargs = (
+        mock_service.users.return_value.drafts.return_value.create.call_args.kwargs
+    )
+    raw_message = create_kwargs["body"]["message"]["raw"]
+    raw_text = base64.urlsafe_b64decode(raw_message).decode("utf-8", errors="ignore")
+
+    assert "In-Reply-To: <msg2@example.com>" in raw_text
+    assert "References: <msg1@example.com> <msg2@example.com>" in raw_text
+    assert "<msg3@example.com>" not in raw_text
+
+
+@pytest.mark.asyncio
+async def test_draft_gmail_message_uses_explicit_references_when_filling_in_reply_to():
+    mock_service = Mock()
+    mock_service.users().drafts().create().execute.return_value = {"id": "draft_reply"}
+    mock_service.users().threads().get().execute.return_value = _thread_response(
+        "<msg1@example.com>",
+        "<msg2@example.com>",
+        "<msg3@example.com>",
+    )
+
+    await _unwrap(draft_gmail_message)(
+        service=mock_service,
+        user_google_email="user@example.com",
+        to="recipient@example.com",
+        subject="Meeting tomorrow",
+        body="Replying to an earlier message.",
+        thread_id="thread123",
+        references="<msg1@example.com> <msg2@example.com>",
+        include_signature=False,
+    )
+
+    create_kwargs = (
+        mock_service.users.return_value.drafts.return_value.create.call_args.kwargs
+    )
+    raw_message = create_kwargs["body"]["message"]["raw"]
+    raw_text = base64.urlsafe_b64decode(raw_message).decode("utf-8", errors="ignore")
+
+    assert "In-Reply-To: <msg2@example.com>" in raw_text
+    assert "References: <msg1@example.com> <msg2@example.com>" in raw_text
+    assert "<msg3@example.com>" not in raw_text
+
+
+@pytest.mark.asyncio
+async def test_draft_gmail_message_gracefully_degrades_when_thread_fetch_fails():
+    mock_service = Mock()
+    mock_service.users().drafts().create().execute.return_value = {"id": "draft_reply"}
+    mock_service.users().threads().get().execute.side_effect = RuntimeError("boom")
+
+    result = await _unwrap(draft_gmail_message)(
+        service=mock_service,
+        user_google_email="user@example.com",
+        to="recipient@example.com",
+        subject="Meeting tomorrow",
+        body="Thanks for the update.",
+        thread_id="thread123",
+        include_signature=False,
+    )
+
+    assert "Draft created! Draft ID: draft_reply" in result
+
+    create_kwargs = (
+        mock_service.users.return_value.drafts.return_value.create.call_args.kwargs
+    )
+    raw_message = create_kwargs["body"]["message"]["raw"]
+    raw_text = base64.urlsafe_b64decode(raw_message).decode("utf-8", errors="ignore")
+
+    assert "In-Reply-To:" not in raw_text
+    assert "References:" not in raw_text