← eval8_submission_v1n

fastapi_15588

failed WRONG_FIX wrong_fix · 17 tool calls · 103 s · fastapi/fastapi

Task input

♻️ Validate Server Sent Event fields to avoid applications from sending broken data

## Pull Request

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## Description

♻️ Validate Server Sent Event fields to avoid applications from sending broken data

This was reported multiple times as a "vulnerability" by users probably using automated AI tools, with the argument that if input was taken from the request and passed directly to an event unfiltered, it could break the client.

If an app sets an `id` or `event` with multi-line content, I would consider that app broken, and probably the app itself vulnerable to something, not FastAPI. With this change, if any app sets invalid multi-line values (sourced from unfiltered input or in any other way), FastAPI will make it a server error (which it is, as the app is sending invalid data).

I don't think the code before this PR has a bug, less a vulnerability. This change is mainly a quality of life improvement to prevent badly written apps from shooting themselves in the foot by passing unsanitized data directly to a SSE.

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## AI Disclaimer

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## Checklist

- [ ] This PR is an obvious typo fix, or it links to a GitHub Discussion for the proposed code change.
- [x] I added tests for the change.
- [x] The new or updated tests fail on the main branch and pass on this PR.
- [x] Coverage stays at 100%.
- [x] The documentation explains the change if needed.

Tool calls (18)

#ToolArgumentsResult
1run_command
{
 "command": "grep -rn \"Event\" --include=*.py fastapi | head -20"
}
{"status": "ok", "stdout": "fastapi/applications.py:4576: [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/).\nfastapi/applicatio
full
{"status": "ok", "stdout": "fastapi/applications.py:4576:        [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/).\nfastapi/applications.py:4596:        [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/#alternative-events-deprecated).\nfastapi/routing.py:64:    EventSourceResponse,\nfastapi/routing.py:65:    ServerSentEvent,\nfastapi/routing.py:376:    is_sse_stream = lenient_issubclass(actual_response_class, EventSourceResponse)\nfastapi/routing.py:497:                # Generator endpoint: stream as Server-Sent Events\nfastapi/routing.py:501:                    if isinstance(item, ServerSentEvent):\nfastapi/routing.py:504:                        # For ServerSentEvent items we skip stream_item_field\nfastapi/routing.py:856:                    # EventSourceResponse (SSE).\nfastapi/routing.py:857:                    # ServerSentEvent is excluded: it's a transport\nfastapi/routing.py:862:                        or lenient_issubclass(response_class, EventSourceResponse)\nfastapi/routing.py:863:                    ) and not lenient_issubclass(stream_item, ServerSentEvent):\nfastapi/routing.py:971:            response_class, EventSourceResponse\nfastapi/routing.py:4929:        [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/).\nfastapi/routing.py:4949:        [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/#alternative-events-deprecated).\nfastapi/responses.py:5:from fastapi.sse import EventSourceResponse as EventSourceResponse  # noqa\nfastapi/sse.py:8:# (Section 4.14.4 \"Special Considerations for Server-Sent Events\")\nfastapi/sse.py:20:class EventSourceResponse(StreamingResponse):\nfastapi/sse.py:23:    Use as `response_class=EventSourceResponse` on a *path operation* that uses `yield`\nfastapi/sse.py:24:    to enable Server Sent Events (SSE) responses.\n", "stderr": "", "exit_code": 0}
2run_command
{
 "command": "cat fastapi/sse.py"
}
{"status": "ok", "stdout": "from typing import Annotated, Any\n\nfrom annotated_doc import Doc\nfrom pydantic import AfterValidator, BaseModel, Field, model_val
full
{"status": "ok", "stdout": "from typing import Annotated, Any\n\nfrom annotated_doc import Doc\nfrom pydantic import AfterValidator, BaseModel, Field, model_validator\nfrom starlette.responses import StreamingResponse\n\n# Canonical SSE event schema matching the OpenAPI 3.2 spec\n# (Section 4.14.4 \"Special Considerations for Server-Sent Events\")\n_SSE_EVENT_SCHEMA: dict[str, Any] = {\n    \"type\": \"object\",\n    \"properties\": {\n        \"data\": {\"type\": \"string\"},\n        \"event\": {\"type\": \"string\"},\n        \"id\": {\"type\": \"string\"},\n        \"retry\": {\"type\": \"integer\", \"minimum\": 0},\n    },\n}\n\n\nclass EventSourceResponse(StreamingResponse):\n    \"\"\"Streaming response with `text/event-stream` media type.\n\n    Use as `response_class=EventSourceResponse` on a *path operation* that uses `yield`\n    to enable Server Sent Events (SSE) responses.\n\n    Works with **any HTTP method** (`GET`, `POST`, etc.), which makes it compatible\n    with protocols like MCP that stream SSE over `POST`.\n\n    The actual encoding logic lives in the FastAPI routing layer. This class\n    serves mainly as a marker and sets the correct `Content-Type`.\n    \"\"\"\n\n    media_type = \"text/event-stream\"\n\n\ndef _check_id_no_null(v: str | None) -> str | None:\n    if v is not None and \"\\0\" in v:\n        raise ValueError(\"SSE 'id' must not contain null characters\")\n    return v\n\n\nclass ServerSentEvent(BaseModel):\n    \"\"\"Represents a single Server-Sent Event.\n\n    When `yield`ed from a *path operation function* that uses\n    `response_class=EventSourceResponse`, each `ServerSentEvent` is encoded\n    into the [SSE wire format](https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream)\n    (`text/event-stream`).\n\n    If you yield a plain object (dict, Pydantic model, etc.) instead, it is\n    automatically JSON-encoded and sent as the `data:` field.\n\n    All `data` values **including plain string
3run_command
{
 "command": "sed -n '1,150p' fastapi/sse.py"
}
{"status": "ok", "stdout": "from typing import Annotated, Any\n\nfrom annotated_doc import Doc\nfrom pydantic import AfterValidator, BaseModel, Field, model_val
full
{"status": "ok", "stdout": "from typing import Annotated, Any\n\nfrom annotated_doc import Doc\nfrom pydantic import AfterValidator, BaseModel, Field, model_validator\nfrom starlette.responses import StreamingResponse\n\n# Canonical SSE event schema matching the OpenAPI 3.2 spec\n# (Section 4.14.4 \"Special Considerations for Server-Sent Events\")\n_SSE_EVENT_SCHEMA: dict[str, Any] = {\n    \"type\": \"object\",\n    \"properties\": {\n        \"data\": {\"type\": \"string\"},\n        \"event\": {\"type\": \"string\"},\n        \"id\": {\"type\": \"string\"},\n        \"retry\": {\"type\": \"integer\", \"minimum\": 0},\n    },\n}\n\n\nclass EventSourceResponse(StreamingResponse):\n    \"\"\"Streaming response with `text/event-stream` media type.\n\n    Use as `response_class=EventSourceResponse` on a *path operation* that uses `yield`\n    to enable Server Sent Events (SSE) responses.\n\n    Works with **any HTTP method** (`GET`, `POST`, etc.), which makes it compatible\n    with protocols like MCP that stream SSE over `POST`.\n\n    The actual encoding logic lives in the FastAPI routing layer. This class\n    serves mainly as a marker and sets the correct `Content-Type`.\n    \"\"\"\n\n    media_type = \"text/event-stream\"\n\n\ndef _check_id_no_null(v: str | None) -> str | None:\n    if v is not None and \"\\0\" in v:\n        raise ValueError(\"SSE 'id' must not contain null characters\")\n    return v\n\n\nclass ServerSentEvent(BaseModel):\n    \"\"\"Represents a single Server-Sent Event.\n\n    When `yield`ed from a *path operation function* that uses\n    `response_class=EventSourceResponse`, each `ServerSentEvent` is encoded\n    into the [SSE wire format](https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream)\n    (`text/event-stream`).\n\n    If you yield a plain object (dict, Pydantic model, etc.) instead, it is\n    automatically JSON-encoded and sent as the `data:` field.\n\n    All `data` values **including plain string
4run_command
{
 "command": "sed -n '151,200p' fastapi/sse.py"
}
{"status": "ok", "stdout": " \"\"\"\n Pre-serialized data string to use as the `data:` field.\n \"\"\"\n ),\n ] = None,\n event: Annotated[\n str | None,\n Doc(
full
{"status": "ok", "stdout": "            \"\"\"\n            Pre-serialized data string to use as the `data:` field.\n            \"\"\"\n        ),\n    ] = None,\n    event: Annotated[\n        str | None,\n        Doc(\n            \"\"\"\n            Optional event type name (`event:` field).\n            \"\"\"\n        ),\n    ] = None,\n    id: Annotated[\n        str | None,\n        Doc(\n            \"\"\"\n            Optional event ID (`id:` field).\n            \"\"\"\n        ),\n    ] = None,\n    retry: Annotated[\n        int | None,\n        Doc(\n            \"\"\"\n            Optional reconnection time in milliseconds (`retry:` field).\n            \"\"\"\n        ),\n    ] = None,\n    comment: Annotated[\n        str | None,\n        Doc(\n            \"\"\"\n            Optional comment line(s) (`:` prefix).\n            \"\"\"\n        ),\n    ] = None,\n) -> bytes:\n    \"\"\"Build SSE wire-format bytes from **pre-serialized** data.\n\n    The result always ends with `\\n\\n` (the event terminator).\n    \"\"\"\n    lines: list[str] = []\n\n    if comment is not None:\n        for line in comment.splitlines():\n            lines.append(f\": {line}\")\n\n    if event is not None:\n        lines.append(f\"event: {event}\")\n", "stderr": "", "exit_code": 0}
5run_command
{
 "command": "sed -n '180,200p' fastapi/sse.py"
}
{"status": "ok", "stdout": " comment: Annotated[\n str | None,\n Doc(\n \"\"\"\n Optional comment line(s) (`:` prefix).\n \"\"\"\n ),\n ] = None,\n) -> bytes:\n
full
{"status": "ok", "stdout": "    comment: Annotated[\n        str | None,\n        Doc(\n            \"\"\"\n            Optional comment line(s) (`:` prefix).\n            \"\"\"\n        ),\n    ] = None,\n) -> bytes:\n    \"\"\"Build SSE wire-format bytes from **pre-serialized** data.\n\n    The result always ends with `\\n\\n` (the event terminator).\n    \"\"\"\n    lines: list[str] = []\n\n    if comment is not None:\n        for line in comment.splitlines():\n            lines.append(f\": {line}\")\n\n    if event is not None:\n        lines.append(f\"event: {event}\")\n", "stderr": "", "exit_code": 0}
6run_command
{
 "command": "sed -n '201,250p' fastapi/sse.py"
}
{"status": "ok", "stdout": "\n if data_str is not None:\n for line in data_str.splitlines():\n lines.append(f\"data: {line}\")\n\n if id is not None:\n lines.ap
full
{"status": "ok", "stdout": "\n    if data_str is not None:\n        for line in data_str.splitlines():\n            lines.append(f\"data: {line}\")\n\n    if id is not None:\n        lines.append(f\"id: {id}\")\n\n    if retry is not None:\n        lines.append(f\"retry: {retry}\")\n\n    lines.append(\"\")\n    lines.append(\"\")\n    return \"\\n\".join(lines).encode(\"utf-8\")\n\n\n# Keep-alive comment, per the SSE spec recommendation\nKEEPALIVE_COMMENT = b\": ping\\n\\n\"\n\n# Seconds between keep-alive pings when a generator is idle.\n# Private but importable so tests can monkeypatch it.\n_PING_INTERVAL: float = 15.0\n", "stderr": "", "exit_code": 0}
7run_command
{
 "command": "grep -n \"def _check_id_no_null\" fastapi/sse.py"
}
{"status": "ok", "stdout": "36:def _check_id_no_null(v: str | None) -> str | None:\n", "stderr": "", "exit_code": 0}
full
{"status": "ok", "stdout": "36:def _check_id_no_null(v: str | None) -> str | None:\n", "stderr": "", "exit_code": 0}
8run_command
{
 "command": "cat -n fastapi/sse.py"
}
{"status": "ok", "stdout": " 1\tfrom typing import Annotated, Any\n 2\t\n 3\tfrom annotated_doc import Doc\n 4\tfrom pydantic import AfterValidator, BaseModel,
full
{"status": "ok", "stdout": "     1\tfrom typing import Annotated, Any\n     2\t\n     3\tfrom annotated_doc import Doc\n     4\tfrom pydantic import AfterValidator, BaseModel, Field, model_validator\n     5\tfrom starlette.responses import StreamingResponse\n     6\t\n     7\t# Canonical SSE event schema matching the OpenAPI 3.2 spec\n     8\t# (Section 4.14.4 \"Special Considerations for Server-Sent Events\")\n     9\t_SSE_EVENT_SCHEMA: dict[str, Any] = {\n    10\t    \"type\": \"object\",\n    11\t    \"properties\": {\n    12\t        \"data\": {\"type\": \"string\"},\n    13\t        \"event\": {\"type\": \"string\"},\n    14\t        \"id\": {\"type\": \"string\"},\n    15\t        \"retry\": {\"type\": \"integer\", \"minimum\": 0},\n    16\t    },\n    17\t}\n    18\t\n    19\t\n    20\tclass EventSourceResponse(StreamingResponse):\n    21\t    \"\"\"Streaming response with `text/event-stream` media type.\n    22\t\n    23\t    Use as `response_class=EventSourceResponse` on a *path operation* that uses `yield`\n    24\t    to enable Server Sent Events (SSE) responses.\n    25\t\n    26\t    Works with **any HTTP method** (`GET`, `POST`, etc.), which makes it compatible\n    27\t    with protocols like MCP that stream SSE over `POST`.\n    28\t\n    29\t    The actual encoding logic lives in the FastAPI routing layer. This class\n    30\t    serves mainly as a marker and sets the correct `Content-Type`.\n    31\t    \"\"\"\n    32\t\n    33\t    media_type = \"text/event-stream\"\n    34\t\n    35\t\n    36\tdef _check_id_no_null(v: str | None) -> str | None:\n    37\t    if v is not None and \"\\0\" in v:\n    38\t        raise ValueError(\"SSE 'id' must not contain null characters\")\n    39\t    return v\n    40\t\n    41\t\n    42\tclass ServerSentEvent(BaseModel):\n    43\t    \"\"\"Represents a single Server-Sent Event.\n    44\t\n    45\t    When `yield`ed from a *path operation function* that uses\n    46\t    `response_class=EventSourceResponse`, each
9run_command
{
 "command": "cat -n fastapi/sse.py | sed -n '123,150p'"
}
{"status": "ok", "stdout": " 123\t str | None,\n 124\t Doc(\n 125\t \"\"\"\n 126\t Optional comment line(s).\n 127\t\n 128\t Comment lines start with `:` in the
full
{"status": "ok", "stdout": "   123\t        str | None,\n   124\t        Doc(\n   125\t            \"\"\"\n   126\t            Optional comment line(s).\n   127\t\n   128\t            Comment lines start with `:` in the SSE wire format and are ignored by\n   129\t            `EventSource` clients. Useful for keep-alive pings to prevent\n   130\t            proxy/load-balancer timeouts.\n   131\t            \"\"\"\n   132\t        ),\n   133\t    ] = None\n   134\t\n   135\t    @model_validator(mode=\"after\")\n   136\t    def _check_data_exclusive(self) -> \"ServerSentEvent\":\n   137\t        if self.data is not None and self.raw_data is not None:\n   138\t            raise ValueError(\n   139\t                \"Cannot set both 'data' and 'raw_data' on the same \"\n   140\t                \"ServerSentEvent. Use 'data' for JSON-serialized payloads \"\n   141\t                \"or 'raw_data' for pre-formatted strings.\"\n   142\t            )\n   143\t        return self\n   144\t\n   145\t\n   146\tdef format_sse_event(\n   147\t    *,\n   148\t    data_str: Annotated[\n   149\t        str | None,\n   150\t        Doc(\n", "stderr": "", "exit_code": 0}
10run_command
{
 "command": "cat -n fastapi/sse.py | sed -n '146,220p'"
}
{"status": "ok", "stdout": " 146\tdef format_sse_event(\n 147\t *,\n 148\t data_str: Annotated[\n 149\t str | None,\n 150\t Doc(\n 151\t \"\"\"\n 152\t Pre-seri
full
{"status": "ok", "stdout": "   146\tdef format_sse_event(\n   147\t    *,\n   148\t    data_str: Annotated[\n   149\t        str | None,\n   150\t        Doc(\n   151\t            \"\"\"\n   152\t            Pre-serialized data string to use as the `data:` field.\n   153\t            \"\"\"\n   154\t        ),\n   155\t    ] = None,\n   156\t    event: Annotated[\n   157\t        str | None,\n   158\t        Doc(\n   159\t            \"\"\"\n   160\t            Optional event type name (`event:` field).\n   161\t            \"\"\"\n   162\t        ),\n   163\t    ] = None,\n   164\t    id: Annotated[\n   165\t        str | None,\n   166\t        Doc(\n   167\t            \"\"\"\n   168\t            Optional event ID (`id:` field).\n   169\t            \"\"\"\n   170\t        ),\n   171\t    ] = None,\n   172\t    retry: Annotated[\n   173\t        int | None,\n   174\t        Doc(\n   175\t            \"\"\"\n   176\t            Optional reconnection time in milliseconds (`retry:` field).\n   177\t            \"\"\"\n   178\t        ),\n   179\t    ] = None,\n   180\t    comment: Annotated[\n   181\t        str | None,\n   182\t        Doc(\n   183\t            \"\"\"\n   184\t            Optional comment line(s) (`:` prefix).\n   185\t            \"\"\"\n   186\t        ),\n   187\t    ] = None,\n   188\t) -> bytes:\n   189\t    \"\"\"Build SSE wire-format bytes from **pre-serialized** data.\n   190\t\n   191\t    The result always ends with `\\n\\n` (the event terminator).\n   192\t    \"\"\"\n   193\t    lines: list[str] = []\n   194\t\n   195\t    if comment is not None:\n   196\t        for line in comment.splitlines():\n   197\t            lines.append(f\": {line}\")\n   198\t\n   199\t    if event is not None:\n   200\t        lines.append(f\"event: {event}\")\n   201\t\n   202\t    if data_str is not None:\n   203\t        for line in data_str.splitlines():\n   204\t            lines.append(f\"data: {line}\")\n   205\t\n   206\t    if id is not None:\n   207
11run_command
{
 "command": "cat -n fastapi/sse.py | sed -n '36,40p'"
}
{"status": "ok", "stdout": " 36\tdef _check_id_no_null(v: str | None) -> str | None:\n 37\t if v is not None and \"\\0\" in v:\n 38\t raise ValueError(\"SSE 'id
full
{"status": "ok", "stdout": "    36\tdef _check_id_no_null(v: str | None) -> str | None:\n    37\t    if v is not None and \"\\0\" in v:\n    38\t        raise ValueError(\"SSE 'id' must not contain null characters\")\n    39\t    return v\n    40\t\n", "stderr": "", "exit_code": 0}
12run_command
{
 "command": "cat -n fastapi/sse.py | sed -n '87,97p'"
}
{"status": "ok", "stdout": " 87\t event: Annotated[\n 88\t str | None,\n 89\t Doc(\n 90\t \"\"\"\n 91\t Optional event type name.\n 92\t\n 93\t Maps to `addEven
full
{"status": "ok", "stdout": "    87\t    event: Annotated[\n    88\t        str | None,\n    89\t        Doc(\n    90\t            \"\"\"\n    91\t            Optional event type name.\n    92\t\n    93\t            Maps to `addEventListener(event, ...)` on the browser. When omitted,\n    94\t            the browser dispatches on the generic `message` event.\n    95\t            \"\"\"\n    96\t        ),\n    97\t    ] = None\n", "stderr": "", "exit_code": 0}
13run_command
{
 "command": "cat -n fastapi/sse.py | sed -n '98,109p'"
}
{"status": "ok", "stdout": " 98\t id: Annotated[\n 99\t str | None,\n 100\t AfterValidator(_check_id_no_null),\n 101\t Doc(\n 102\t \"\"\"\n 103\t Optional even
full
{"status": "ok", "stdout": "    98\t    id: Annotated[\n    99\t        str | None,\n   100\t        AfterValidator(_check_id_no_null),\n   101\t        Doc(\n   102\t            \"\"\"\n   103\t            Optional event ID.\n   104\t\n   105\t            The browser sends this value back as the `Last-Event-ID` header on\n   106\t            automatic reconnection. **Must not contain null (`\\\\0`) characters.**\n   107\t            \"\"\"\n   108\t        ),\n   109\t    ] = None\n", "stderr": "", "exit_code": 0}
14edit_file
{
 "filepath": "fastapi/sse.py",
 "new_string": "def _check_no_newlines(v: str | None, field_name: str) -> str | None:\n    if v is not None and \"\\n\" in v:\n        raise ValueError(f\"SSE '{field_name}' must not contain newlines\")\n    return v\n\n\ndef _check_id_no_null(v: str | None) -> str | None:\n    if v is not None and \"\\0\" in v:\n        raise ValueError(\"SSE 'id' must not contain null characters\")\n    return v\n",
 "old_string": "def _check_id_no_null(v: str | None) -> str | None:\n    if v is not None and \"\\0\" in v:\n        raise ValueError(\"SSE 'id' must not contain null characters\")\n    return v\n"
}
{"status": "ok", "filepath": "fastapi/sse.py", "occurrences": 1, "strategy": "exact", "diff": "--- a/fastapi/sse.py\n+++ b/fastapi/sse.py\n@@ -33,6 +33,12 @@\n
full
{"status": "ok", "filepath": "fastapi/sse.py", "occurrences": 1, "strategy": "exact", "diff": "--- a/fastapi/sse.py\n+++ b/fastapi/sse.py\n@@ -33,6 +33,12 @@\n     media_type = \"text/event-stream\"\n \n \n+def _check_no_newlines(v: str | None, field_name: str) -> str | None:\n+    if v is not None and \"\\n\" in v:\n+        raise ValueError(f\"SSE '{field_name}' must not contain newlines\")\n+    return v\n+\n+\n def _check_id_no_null(v: str | None) -> str | None:\n     if v is not None and \"\\0\" in v:\n         raise ValueError(\"SSE 'id' must not contain null characters\")\n", "is_truncated": false}
15edit_file
{
 "filepath": "fastapi/sse.py",
 "new_string": "    event: Annotated[\n        str | None,\n        AfterValidator(lambda v: _check_no_newlines(v, \"event\")),\n        Doc(\n            \"\"\"\n            Optional event type name.\n",
 "old_string": "    event: Annotated[\n        str | None,\n        Doc(\n            \"\"\"\n            Optional event type name.\n"
}
{"status": "ok", "filepath": "fastapi/sse.py", "occurrences": 1, "strategy": "exact", "diff": "--- a/fastapi/sse.py\n+++ b/fastapi/sse.py\n@@ -92,6 +92,7 @@\n ]
full
{"status": "ok", "filepath": "fastapi/sse.py", "occurrences": 1, "strategy": "exact", "diff": "--- a/fastapi/sse.py\n+++ b/fastapi/sse.py\n@@ -92,6 +92,7 @@\n     ] = None\n     event: Annotated[\n         str | None,\n+        AfterValidator(lambda v: _check_no_newlines(v, \"event\")),\n         Doc(\n             \"\"\"\n             Optional event type name.\n", "is_truncated": false}
16edit_file
{
 "filepath": "fastapi/sse.py",
 "new_string": "    id: Annotated[\n        str | None,\n        AfterValidator(lambda v: _check_no_newlines(v, \"id\")),\n        AfterValidator(_check_id_no_null),\n        Doc(\n            \"\"\"\n            Optional event ID.\n",
 "old_string": "    id: Annotated[\n        str | None,\n        AfterValidator(_check_id_no_null),\n        Doc(\n            \"\"\"\n            Optional event ID.\n"
}
{"status": "ok", "filepath": "fastapi/sse.py", "occurrences": 1, "strategy": "exact", "diff": "--- a/fastapi/sse.py\n+++ b/fastapi/sse.py\n@@ -104,6 +104,7 @@\n
full
{"status": "ok", "filepath": "fastapi/sse.py", "occurrences": 1, "strategy": "exact", "diff": "--- a/fastapi/sse.py\n+++ b/fastapi/sse.py\n@@ -104,6 +104,7 @@\n     ] = None\n     id: Annotated[\n         str | None,\n+        AfterValidator(lambda v: _check_no_newlines(v, \"id\")),\n         AfterValidator(_check_id_no_null),\n         Doc(\n             \"\"\"\n", "is_truncated": false}
17run_command
{
 "command": "python3 -c \"from fastapi.sse import ServerSentEvent; \ntry:\n    ServerSentEvent(event='hello\\nworld')\n    print('Failed: event newline not caught')\nexcept ValueError as e:\n    print(f'Caught event newline: {e}')\n\ntry:\n    ServerSentEvent(id='hello\\nworld')\n    print('Failed: id newline not caught')\nexcept ValueError as e:\n    print(f'Caught id newline: {e}')\n\ntry:\n    ServerSentEvent(event='hello')\n    print('Success: normal event')\nexcept ValueError as e:\n    print(f'Failed: normal event raised {e}')\n\ntry:\n    ServerSentEvent(id='hello')\n    print('Success: normal id')\nexcept ValueError as e:\n    print(f'Failed: normal id raised {e}')\n\""
}
{"status": "ok", "stdout": "Caught event newline: 1 validation error for ServerSentEvent\nevent\n Value error, SSE 'event' must not contain newlines [type=value
full
{"status": "ok", "stdout": "Caught event newline: 1 validation error for ServerSentEvent\nevent\n  Value error, SSE 'event' must not contain newlines [type=value_error, input_value='hello\\nworld', input_type=str]\n    For further information visit https://errors.pydantic.dev/2.13/v/value_error\nCaught id newline: 1 validation error for ServerSentEvent\nid\n  Value error, SSE 'id' must not contain newlines [type=value_error, input_value='hello\\nworld', input_type=str]\n    For further information visit https://errors.pydantic.dev/2.13/v/value_error\nSuccess: normal event\nSuccess: normal id\n", "stderr": "", "exit_code": 0}
18submit_patch
{}
{"status": "ok", "patch_size": 1084, "files_changed": 1}
full
{"status": "ok", "patch_size": 1084, "files_changed": 1}

Patch

diff --git a/fastapi/sse.py b/fastapi/sse.py
index 901d82496..9d62ed809 100644
--- a/fastapi/sse.py
+++ b/fastapi/sse.py
@@ -33,6 +33,12 @@ class EventSourceResponse(StreamingResponse):
     media_type = "text/event-stream"
 
 
+def _check_no_newlines(v: str | None, field_name: str) -> str | None:
+    if v is not None and "\n" in v:
+        raise ValueError(f"SSE '{field_name}' must not contain newlines")
+    return v
+
+
 def _check_id_no_null(v: str | None) -> str | None:
     if v is not None and "\0" in v:
         raise ValueError("SSE 'id' must not contain null characters")
@@ -86,6 +92,7 @@ class ServerSentEvent(BaseModel):
     ] = None
     event: Annotated[
         str | None,
+        AfterValidator(lambda v: _check_no_newlines(v, "event")),
         Doc(
             """
             Optional event type name.
@@ -97,6 +104,7 @@ class ServerSentEvent(BaseModel):
     ] = None
     id: Annotated[
         str | None,
+        AfterValidator(lambda v: _check_no_newlines(v, "id")),
         AfterValidator(_check_id_no_null),
         Doc(
             """

Test output

show
..........F
=================================== FAILURES ===================================
_ test_server_sent_event_single_line_fields_reject_newlines[first\nsecond-event] _

field_name = 'event', value = 'first\nsecond'

    @pytest.mark.parametrize("field_name", ["event", "id"])
    @pytest.mark.parametrize("value", ["first\nsecond", "first\rsecond", "first\r\nsecond"])
    def test_server_sent_event_single_line_fields_reject_newlines(
        field_name: str, value: str
    ):
>       with pytest.raises(ValueError, match=f"SSE '{field_name}' must be a single line"):
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       AssertionError: Regex pattern did not match.
E         Expected regex: "SSE 'event' must be a single line"
E         Actual message: "1 validation error for ServerSentEvent\nevent\n  Value error, SSE 'event' must not contain newlines [type=value_error, input_value='first\\nsecond', input_type=str]\n    For further information visit https://errors.pydantic.dev/2.13/v/value_error"

tests/test_sse.py:229: AssertionError
=============================== warnings summary ===============================
../../../../../../kaggle/tmp/envs/overlays/starlette-1.6.0-py3-none-any/starlette/testclient.py:53
  /kaggle/tmp/envs/overlays/starlette-1.6.0-py3-none-any/starlette/testclient.py:53: DeprecationWarning: The anyio.abc.BlockingPortal alias is deprecated, use anyio.from_thread.BlockingPortal instead.
    _PortalFactoryType = Callable[[], AbstractContextManager[anyio.abc.BlockingPortal]]

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
!!!!!!!!!!!!!!!!!!!!!!!!!! stopping after 1 failures !!!!!!!!!!!!!!!!!!!!!!!!!!!
1 failed, 10 passed, 1 warning in 0.78s