diff --git a/.fernignore b/.fernignore
index 950b552..206f5a6 100644
--- a/.fernignore
+++ b/.fernignore
@@ -2,3 +2,8 @@
 
 src/agentmail/core/query_encoder.py
 tests/utils/test_query_encoding.py
+
+# Frozen to preserve the Idempotency-Key auto-mint patch (see idempotency.py).
+src/agentmail/core/idempotency.py
+src/agentmail/core/http_client.py
+tests/utils/test_idempotency.py
diff --git a/src/agentmail/core/http_client.py b/src/agentmail/core/http_client.py
index ee93758..b20d74e 100644
--- a/src/agentmail/core/http_client.py
+++ b/src/agentmail/core/http_client.py
@@ -11,6 +11,7 @@
 import httpx
 from .file import File, convert_file_dict_to_httpx_tuples
 from .force_multipart import FORCE_MULTIPART
+from .idempotency import maybe_mint_idempotency_key
 from .jsonable_encoder import jsonable_encoder
 from .logging import LogConfig, Logger, create_logger
 from .query_encoder import encode_query
@@ -304,6 +305,20 @@ def request(
         omit: typing.Optional[typing.Any] = None,
         force_multipart: typing.Optional[bool] = None,
     ) -> httpx.Response:
+        # Auto-mint an Idempotency-Key for send endpoints. This runs ONCE per
+        # logical call (retries == 0, before the retry recursion below) so every
+        # internal retry reuses the same key. The minted key is stored on the
+        # `headers` local, which is re-passed as headers=headers in the recursion.
+        if retries == 0:
+            _minted_headers = maybe_mint_idempotency_key(
+                method=method,
+                path=path,
+                headers=headers,
+                additional_headers=(request_options.get("additional_headers") if request_options is not None else None),
+            )
+            if _minted_headers is not None:
+                headers = _minted_headers
+
         base_url = self.get_base_url(base_url)
         timeout = (
             request_options.get("timeout_in_seconds")
@@ -564,6 +579,20 @@ async def request(
         omit: typing.Optional[typing.Any] = None,
         force_multipart: typing.Optional[bool] = None,
     ) -> httpx.Response:
+        # Auto-mint an Idempotency-Key for send endpoints. This runs ONCE per
+        # logical call (retries == 0, before the retry recursion below) so every
+        # internal retry reuses the same key. The minted key is stored on the
+        # `headers` local, which is re-passed as headers=headers in the recursion.
+        if retries == 0:
+            _minted_headers = maybe_mint_idempotency_key(
+                method=method,
+                path=path,
+                headers=headers,
+                additional_headers=(request_options.get("additional_headers") if request_options is not None else None),
+            )
+            if _minted_headers is not None:
+                headers = _minted_headers
+
         base_url = self.get_base_url(base_url)
         timeout = (
             request_options.get("timeout_in_seconds")
diff --git a/src/agentmail/core/idempotency.py b/src/agentmail/core/idempotency.py
new file mode 100644
index 0000000..2605997
--- /dev/null
+++ b/src/agentmail/core/idempotency.py
@@ -0,0 +1,74 @@
+# This file is a hand-written AgentMail patch (NOT Fern-generated). It is pinned
+# in .fernignore so Fern regeneration cannot clobber it.
+#
+# Purpose: auto-mint an `Idempotency-Key` header for send endpoints so that the
+# HttpClient's internal auto-retries (429/5xx/timeouts) all carry the SAME key.
+# The AgentMail API makes SENDS idempotent via this header: a retry with the same
+# key returns the original message (no duplicate email); reuse with a different
+# payload is a 409. Minting once per logical call (before the retry loop) keeps
+# every attempt idempotent.
+
+import re
+import typing
+import uuid
+
+IDEMPOTENCY_KEY_HEADER = "Idempotency-Key"
+
+# The 5 POST send endpoints, matched on the path suffix (after the /v0 prefix and
+# ignoring any query string). {segment} is a single non-slash path segment.
+#
+# Anchored to the END of the path ($) so that ONLY the exact final segments
+# /send, /reply, /reply-all, /forward match. This deliberately does NOT match the
+# create-draft endpoints (/messages/{seg}/draft-reply, /draft-reply-all,
+# /draft-forward), which end in different segments.
+_SEND_PATH_REGEX = re.compile(
+    r"(?:^|/)(?:"
+    r"messages/send"
+    r"|messages/[^/]+/reply"
+    r"|messages/[^/]+/reply-all"
+    r"|messages/[^/]+/forward"
+    r"|drafts/[^/]+/send"
+    r")$"
+)
+
+
+def _is_send_path(path: typing.Optional[str]) -> bool:
+    if not path:
+        return False
+    # Ignore any query string / fragment; strip a trailing slash before anchoring.
+    clean = path.split("?", 1)[0].split("#", 1)[0].rstrip("/")
+    return _SEND_PATH_REGEX.search(clean) is not None
+
+
+def _has_idempotency_key(headers: typing.Optional[typing.Mapping[str, typing.Any]]) -> bool:
+    if not headers:
+        return False
+    return any(k.lower() == IDEMPOTENCY_KEY_HEADER.lower() for k in headers.keys())
+
+
+def maybe_mint_idempotency_key(
+    *,
+    method: str,
+    path: typing.Optional[str],
+    headers: typing.Optional[typing.Dict[str, typing.Any]],
+    additional_headers: typing.Optional[typing.Mapping[str, typing.Any]],
+) -> typing.Optional[typing.Dict[str, typing.Any]]:
+    """Return a headers dict carrying an auto-minted Idempotency-Key, or None.
+
+    Mints (UUID4) only when: the method is POST, the path is one of the send
+    endpoints, and no caller-supplied `Idempotency-Key` header is already present
+    (case-insensitive) in either `headers` or the request_options
+    `additional_headers`. Returns a NEW dict (never mutates the caller's) that the
+    request method should thread through its retry recursion so all attempts share
+    the same key. Returns None when nothing should change.
+    """
+    if method.upper() != "POST":
+        return None
+    if not _is_send_path(path):
+        return None
+    if _has_idempotency_key(headers) or _has_idempotency_key(additional_headers):
+        return None
+
+    minted = dict(headers) if headers else {}
+    minted[IDEMPOTENCY_KEY_HEADER] = str(uuid.uuid4())
+    return minted
diff --git a/tests/utils/test_idempotency.py b/tests/utils/test_idempotency.py
new file mode 100644
index 0000000..5ade74d
--- /dev/null
+++ b/tests/utils/test_idempotency.py
@@ -0,0 +1,172 @@
+# Tests for the hand-written Idempotency-Key auto-mint patch. Pinned in
+# .fernignore so Fern regeneration cannot clobber it.
+
+import uuid
+from typing import List
+
+import httpx
+import pytest
+
+from agentmail.core.http_client import HttpClient
+from agentmail.core.request_options import RequestOptions
+
+
+def _is_valid_uuid4(value: str) -> bool:
+    try:
+        parsed = uuid.UUID(value)
+    except (ValueError, TypeError, AttributeError):
+        return False
+    return parsed.version == 4 and str(parsed) == value.lower()
+
+
+def _make_client(handler: httpx.MockTransport) -> HttpClient:
+    return HttpClient(
+        httpx_client=httpx.Client(transport=handler),
+        base_timeout=lambda: None,
+        base_headers=lambda: {},
+        base_url=lambda: "https://api.agentmail.to/v0",
+    )
+
+
+def _capture_transport(captured: List[httpx.Request], status_codes: List[int]) -> httpx.MockTransport:
+    """MockTransport that records each request and returns the next status code."""
+    calls = {"i": 0}
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        captured.append(request)
+        idx = min(calls["i"], len(status_codes) - 1)
+        calls["i"] += 1
+        return httpx.Response(status_codes[idx], json={})
+
+    return httpx.MockTransport(handle)
+
+
+def test_send_path_mints_uuid4_idempotency_key() -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    client.request(path="messages/send", method="POST", json={"to": "a@b.com"}, request_options=None)
+
+    assert len(captured) == 1
+    key = captured[0].headers.get("Idempotency-Key")
+    assert key is not None
+    assert _is_valid_uuid4(key)
+
+
+def test_caller_supplied_key_is_preserved() -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    options: RequestOptions = {"additional_headers": {"Idempotency-Key": "caller-key-123"}}
+    client.request(path="messages/send", method="POST", json={"to": "a@b.com"}, request_options=options)
+
+    assert len(captured) == 1
+    assert captured[0].headers.get("Idempotency-Key") == "caller-key-123"
+
+
+def test_retry_reuses_identical_key() -> None:
+    captured: List[httpx.Request] = []
+    # 503 once, then 200 -> the client should retry and both attempts share the key.
+    client = _make_client(_capture_transport(captured, [503, 200]))
+
+    client.request(
+        path="messages/send",
+        method="POST",
+        json={"to": "a@b.com"},
+        request_options={"max_retries": 2},
+    )
+
+    assert len(captured) == 2
+    keys = [r.headers.get("Idempotency-Key") for r in captured]
+    assert keys[0] is not None
+    assert _is_valid_uuid4(keys[0])
+    assert keys[0] == keys[1]
+
+
+def test_non_send_post_is_not_injected() -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    client.request(path="inboxes", method="POST", json={"username": "x"}, request_options=None)
+
+    assert len(captured) == 1
+    assert "Idempotency-Key" not in captured[0].headers
+
+
+def test_two_calls_mint_different_keys() -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    client.request(path="messages/send", method="POST", json={"to": "a@b.com"}, request_options=None)
+    client.request(path="messages/send", method="POST", json={"to": "c@d.com"}, request_options=None)
+
+    assert len(captured) == 2
+    key1 = captured[0].headers.get("Idempotency-Key")
+    key2 = captured[1].headers.get("Idempotency-Key")
+    assert key1 is not None and key2 is not None
+    assert key1 != key2
+
+
+def test_create_draft_path_is_not_injected() -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    # /messages/{id}/draft-reply is a create-draft endpoint, NOT a send.
+    client.request(path="messages/msg_123/draft-reply", method="POST", json={}, request_options=None)
+
+    assert len(captured) == 1
+    assert "Idempotency-Key" not in captured[0].headers
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "messages/send",
+        "messages/msg_1/reply",
+        "messages/msg_1/reply-all",
+        "messages/msg_1/forward",
+        "drafts/dft_1/send",
+    ],
+)
+def test_all_send_paths_are_injected(path: str) -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    client.request(path=path, method="POST", json={}, request_options=None)
+
+    assert "Idempotency-Key" in captured[0].headers, f"expected injection for {path}"
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "messages/msg_1/draft-reply",
+        "messages/msg_1/draft-reply-all",
+        "messages/msg_1/draft-forward",
+    ],
+)
+def test_create_draft_paths_are_not_injected(path: str) -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    client.request(path=path, method="POST", json={}, request_options=None)
+
+    assert "Idempotency-Key" not in captured[0].headers, f"unexpected injection for {path}"
+
+
+def test_send_path_with_query_string_is_injected() -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    client.request(path="messages/send?foo=bar", method="POST", json={}, request_options=None)
+
+    assert "Idempotency-Key" in captured[0].headers
+
+
+def test_get_on_send_path_is_not_injected() -> None:
+    captured: List[httpx.Request] = []
+    client = _make_client(_capture_transport(captured, [200]))
+
+    client.request(path="messages/send", method="GET", request_options=None)
+
+    assert "Idempotency-Key" not in captured[0].headers