diff --git a/.changeset/let_httpx2_choose_the_multipart_boundary.md b/.changeset/let_httpx2_choose_the_multipart_boundary.md
@@ -0,0 +1,7 @@
+---
+default: patch
+---
+
+# Let httpx2 choose the boundary for `multipart/form-data` requests
+
+Since 0.29.0, generated endpoints sent every `multipart/form-data` body with the fixed boundary `+++`, so an uploaded file containing `\r\n--+++` was cut short at that point (#1509). These bodies no longer set `Content-Type`, so httpx2 sets it with a random boundary for each request, as single-body endpoints did before 0.29.0. Endpoints with several body types still get a boundary (#947).
diff --git a/end_to_end_tests/functional_tests/generated_code_execution/test_multipart_bodies.py b/end_to_end_tests/functional_tests/generated_code_execution/test_multipart_bodies.py
@@ -0,0 +1,116 @@
+import email.parser
+import email.policy
+import io
+
+import httpx2
+
+from end_to_end_tests.functional_tests.helpers import (
+    with_generated_client_fixture,
+    with_generated_code_imports,
+)
+
+# Content that would end the part early if the request used a fixed "+++" boundary
+FILE_CONTENT = b"first line\r\n--+++\r\nstill the same file\r\n"
+
+
+def _send_and_capture(Client, sync_detailed, body) -> httpx2.Request:
+    sent: list[httpx2.Request] = []
+
+    def handler(request: httpx2.Request) -> httpx2.Response:
+        request.read()
+        sent.append(request)
+        return httpx2.Response(200)
+
+    client = Client(base_url="http://test", httpx_args={"transport": httpx2.MockTransport(handler)})
+    sync_detailed(client=client, body=body)
+    assert len(sent) == 1
+    return sent[0]
+
+
+def _parse_parts(request: httpx2.Request) -> dict[str, bytes]:
+    content_type = request.headers["content-type"]
+    assert content_type.startswith("multipart/form-data; boundary=")
+    message = email.parser.BytesParser(policy=email.policy.HTTP).parsebytes(
+        b"Content-Type: " + content_type.encode() + b"\r\n\r\n" + request.content
+    )
+    return {
+        part.get_param("name", header="content-disposition"): part.get_payload(decode=True)
+        for part in message.iter_parts()
+    }
+
+
+@with_generated_client_fixture(
+    """
+paths:
+  /upload:
+    post:
+      operationId: upload
+      requestBody:
+        required: true
+        content:
+          multipart/form-data:
+            schema:
+              type: object
+              properties:
+                file: {type: string, format: binary}
+      responses:
+        "200": {description: OK}
+"""
+)
+@with_generated_code_imports(
+    ".api.default.upload.sync_detailed",
+    ".client.Client",
+    ".models.UploadBody",
+    ".types.File",
+)
+class TestSingleMultipartBody:
+    def test_file_content_is_not_split_by_boundary(self, sync_detailed, Client, UploadBody, File):
+        body = UploadBody(file=File(payload=io.BytesIO(FILE_CONTENT), file_name="a.txt"))
+
+        request = _send_and_capture(Client, sync_detailed, body)
+
+        assert _parse_parts(request) == {"file": FILE_CONTENT}
+
+
+@with_generated_client_fixture(
+    """
+paths:
+  /upload:
+    post:
+      operationId: upload
+      requestBody:
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                name: {type: string}
+          multipart/form-data:
+            schema:
+              type: object
+              properties:
+                file: {type: string, format: binary}
+      responses:
+        "200": {description: OK}
+"""
+)
+@with_generated_code_imports(
+    ".api.default.upload.sync_detailed",
+    ".client.Client",
+    ".models.UploadJsonBody",
+    ".models.UploadFilesBody",
+    ".types.File",
+)
+class TestMultipartBodyAmongSeveralBodies:
+    def test_file_content_is_not_split_by_boundary(self, sync_detailed, Client, UploadJsonBody, UploadFilesBody, File):
+        body = UploadFilesBody(file=File(payload=io.BytesIO(FILE_CONTENT), file_name="a.txt"))
+
+        request = _send_and_capture(Client, sync_detailed, body)
+
+        assert _parse_parts(request) == {"file": FILE_CONTENT}
+
+    def test_json_body_still_sets_its_content_type(self, sync_detailed, Client, UploadJsonBody, UploadFilesBody, File):
+        request = _send_and_capture(Client, sync_detailed, UploadJsonBody(name="a"))
+
+        assert request.headers["content-type"] == "application/json"
+        assert request.content == b'{"name":"a"}'
diff --git a/...nd_tests/golden-records/my-enum-api-client/my_enum_api_client/api/tests/post_user_list.py b/...nd_tests/golden-records/my-enum-api-client/my_enum_api_client/api/tests/post_user_list.py
@@ -24,8 +24,6 @@ def _get_kwargs(
     if not isinstance(body, Unset):
         _kwargs["files"] = body.to_multipart()
 
-    headers["Content-Type"] = "multipart/form-data; boundary=+++"
-
     _kwargs["headers"] = headers
     return _kwargs
 

diff --git a/...s/golden-records/my-test-api-client/my_test_api_client/api/bodies/post_bodies_multiple.py b/...s/golden-records/my-test-api-client/my_test_api_client/api/bodies/post_bodies_multiple.py
@@ -39,8 +39,6 @@ def _get_kwargs(
         if not isinstance(body, Unset):
             _kwargs["files"] = body.to_multipart()
 
-        headers["Content-Type"] = "multipart/form-data; boundary=+++"
-
     _kwargs["headers"] = headers
     return _kwargs
 

diff --git a/...-records/my-test-api-client/my_test_api_client/api/tests/upload_file_tests_upload_post.py b/...-records/my-test-api-client/my_test_api_client/api/tests/upload_file_tests_upload_post.py
@@ -23,8 +23,6 @@ def _get_kwargs(
 
     _kwargs["files"] = body.to_multipart()
 
-    headers["Content-Type"] = "multipart/form-data; boundary=+++"
-
     _kwargs["headers"] = headers
     return _kwargs
 

diff --git a/integration-tests/integration_tests/api/body/post_body_multipart.py b/integration-tests/integration_tests/api/body/post_body_multipart.py
@@ -25,8 +25,6 @@ def _get_kwargs(
     if not isinstance(body, Unset):
         _kwargs["files"] = body.to_multipart()
 
-    headers["Content-Type"] = "multipart/form-data; boundary=+++"
-
     _kwargs["headers"] = headers
     return _kwargs
 

diff --git a/openapi_python_client/templates/endpoint_module.py.jinja b/openapi_python_client/templates/endpoint_module.py.jinja
@@ -50,18 +50,15 @@ def _get_kwargs(
 {% for body in endpoint.bodies %}
     if isinstance(body, {{body.prop.get_type_string(no_optional=True) | as_unembedded_code }}):
         {{ body_to_kwarg(body) | indent(8) }}
-        {%- if body.content_type == "multipart/form-data" %}
-        headers["Content-Type"] = "multipart/form-data; boundary=+++"
-        {% else %}
+        {#- httpx2 sets multipart/form-data with a random boundary itself, a fixed one could appear in the content #}
+        {%- if body.content_type != "multipart/form-data" %}
         headers["Content-Type"] = "{{ body.content_type | in_double_quote_literal }}"
         {% endif %}
 {% endfor %}
 {% elif endpoint.bodies | length == 1 %}
 {% set body = endpoint.bodies[0] %}
     {{ body_to_kwarg(body) | indent(4) }}
-    {%- if body.content_type == "multipart/form-data" %}
-    headers["Content-Type"] = "multipart/form-data; boundary=+++"
-    {% else %}
+    {%- if body.content_type != "multipart/form-data" %}
     headers["Content-Type"] = "{{ body.content_type | in_double_quote_literal }}"
     {% endif %}
 {% endif %}