mcp_transcoder: add sse server response support. by guoyilin42 · Pull Request #45374 · envoyproxy/envoy

guoyilin42 · 2026-06-01T22:02:46Z

Commit Message: mcp_transcoder: add sse server response support.
Additional Description:
This PR introduces Server-Sent Events (SSE) server response support to the MCP JSON REST bridge filter. When the upstream server responds with a text/event-stream content type, the filter will now parse the stream and wrap each extracted complete event payload into a distinct text content item within the JSON-RPC content array on the fly.

Key Changes:

Added SseResponseExtractor: Introduced sse_response_extractor.cc and its header to process incoming chunks of an SSE stream and return completed event payloads.
Updated McpJsonRestBridgeFilter: Modified the encoding path to check for the text/event-stream content type. It now delegates chunk processing to the new SSE extractor and formats the results into the JSON-RPC streaming envelope.
Testing: Added unit tests in sse_response_extractor_test.cc to verify the handling of single/multiple events, multiline data, incomplete chunked events, and CRLF line endings. Also added ToolsCallSseStreamingTranscoding to integration tests.

Risk Level: Low
Testing: Unit and Integration tests.
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

Signed-off-by: Yilin Guo <guoyilin@google.com>

repokitteh-read-only · 2026-06-01T22:02:57Z

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @wbpcode
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #45374 was opened by guoyilin42.

see: more, trace.

guoyilin42 · 2026-06-01T22:03:44Z

/gemini review

cc @paulhong01

#44146

gemini-code-assist

Code Review

This pull request introduces support for Server-Sent Events (SSE) responses in the MCP JSON-RPC REST bridge filter. It adds an SseResponseExtractor to parse incoming text/event-stream chunks on the fly and wrap completed event payloads into distinct text items within the JSON-RPC content array. The review feedback highlights two critical robustness and security concerns: first, nlohmann::json::dump() could throw an exception and crash Envoy if it encounters invalid UTF-8 in the untrusted upstream payload, which should be mitigated by using a safe error-handling replacement strategy; second, the unbounded buffering of incomplete SSE events poses a memory exhaustion (DoS) risk, necessitating a maximum buffer size limit.

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gemini-code-assist · 2026-06-01T22:05:48Z

+        json content_item;
+        content_item[McpConstants::TYPE_FIELD] = McpConstants::TEXT_FIELD;
+        content_item[McpConstants::TEXT_FIELD] = event_payload;
+        std::string serialized_item = content_item.dump();


By default, nlohmann::json::dump() throws an exception if it encounters invalid UTF-8 characters. Since event_payload is parsed from the upstream server's response (which is untrusted external data), receiving malformed UTF-8 could cause an uncaught exception and crash the Envoy process. To prevent this, use json::error_handler_t::replace to safely replace invalid UTF-8 sequences with the replacement character.

Suggested change

std::string serialized_item = content_item.dump();

std::string serialized_item = content_item.dump(-1, ' ', false, json::error_handler_t::replace);

Waiting reviewer's opinion.

Can we use JsonEscaper::escapeString?

gemini-code-assist · 2026-06-01T22:05:48Z

+std::vector<std::string> SseResponseExtractor::processChunk(absl::string_view chunk,
+                                                            bool end_stream) {
+  std::vector<std::string> event_payloads;
+  buffer_.append(chunk.data(), chunk.size());


The buffer_ member variable accumulates incoming chunks of the SSE stream until a complete event (terminated by a double newline) is found. If a misbehaving or malicious upstream server sends an infinite stream of data without any double newlines, buffer_ will grow indefinitely, potentially leading to memory exhaustion (OOM) and a Denial of Service (DoS). Consider enforcing a maximum limit on the size of buffer_ and closing the connection or returning an error if the limit is exceeded.

Waiting reviewer's opinion.

I think it is better to apply the max_response_body_size limitation.

paulhong01 · 2026-06-02T17:10:57Z

+std::vector<std::string> SseResponseExtractor::processChunk(absl::string_view chunk,
+                                                            bool end_stream) {
+  std::vector<std::string> event_payloads;
+  buffer_.append(chunk.data(), chunk.size());


I think it is better to apply the max_response_body_size limitation.

paulhong01 · 2026-06-02T17:12:11Z

+
+  while (!buffer_view.empty()) {
+    // Safely handles chunk boundaries and all line-ending formats
+    auto result = Http::Sse::SseParser::findEventEnd(buffer_view, end_stream);


minor nit: Since this type is not very obvious, consider avoid using auto here.

paulhong01 · 2026-06-02T17:15:57Z

+        json content_item;
+        content_item[McpConstants::TYPE_FIELD] = McpConstants::TEXT_FIELD;
+        content_item[McpConstants::TEXT_FIELD] = event_payload;
+        std::string serialized_item = content_item.dump();


Can we use JsonEscaper::escapeString?

paulhong01 · 2026-06-02T17:21:52Z

  // When enabled, the response body is streamed directly to the client without buffering. Each
  // chunk is JSON escaped as it arrives and wrapped with a pre-built JSON-RPC prefix and suffix.
  //
+  // For Server-Sent Events (SSE) responses (i.e. ``text/event-stream`` content type), Envoy parses


Are we going to support SSE case when text_content_streaming_enabled: False in the follow-up change?

paulhong01 · 2026-06-02T17:30:37Z

+
+  // Processes an incoming chunk of an SSE stream and returns any completed event payloads.
+  // Set end_stream to true if this is the final chunk of the response.
+  std::vector<std::string> processChunk(absl::string_view chunk, bool end_stream = false);


nit: Default it to false doesn't seem to be needed since the current logic is passing end_stream from encodeData

paulhong01 · 2026-06-02T17:32:11Z

-    std::string escaped_chunk = JsonEscaper::escapeString(chunk, JsonEscaper::extraSpace(chunk));
+
+    std::string output_to_add;
+    if (is_sse_response_) {


nit/optional: consider wrapping this logic into a utility function since encodeData has contained more logic and long.

mcp_transcoder: add sse server response support.

8e5bcf2

Signed-off-by: Yilin Guo <guoyilin@google.com>

guoyilin42 requested review from botengyao, tyxia and yanavlasov as code owners June 1, 2026 22:02

repokitteh-read-only Bot added the api label Jun 1, 2026

repokitteh-read-only Bot assigned wbpcode Jun 1, 2026

gemini-code-assist Bot reviewed Jun 1, 2026

View reviewed changes

paulhong01 reviewed Jun 2, 2026

View reviewed changes

	std::string serialized_item = content_item.dump();
	std::string serialized_item = content_item.dump(-1, ' ', false, json::error_handler_t::replace);

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guoyilin42 commented Jun 1, 2026

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