Support SSE + streaming with MCP notifications. #1025
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adwsingh
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| // Check if it's a notification by looking for "method" field | ||
| if (jsonData.contains("\"method\"")) { |
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Parse into a Document and then extract the "method" field. You're looking for a top-level JSON key named "method" but the string "method" can ostensibly appear anywhere in the payload.
mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/HttpMcpProxy.java
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| StringBuilder dataBuffer = new StringBuilder(); | ||
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| for (String line : lines) { | ||
| if (line.startsWith("data: ")) { |
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The space after data: is optional
adwsingh
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Jan 31, 2026
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| String responseSessionId = response.headers().firstValue("Mcp-Session-Id"); | ||
| if (responseSessionId != null) { | ||
| sessionId = responseSessionId; | ||
| LOG.debug("Stored session ID from response: {}", responseSessionId); | ||
| } |
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This is only allowed during initialize but we are accepting it during any responses.
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| if (isNotification(jsonData)) { | ||
| // This is a notification - parse as JsonRpcRequest and forward | ||
| JsonRpcRequest notification = | ||
| JSON_CODEC.deserializeShape(jsonData, JsonRpcRequest.builder()); |
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We are double parsing the JSON here. You can convert the Document to JsonRpcRequest.
mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/HttpMcpProxy.java
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added 2 commits
February 2, 2026 11:44
- Respond to list/changed notifications by invalidation currently cached tools. - Track session ID header values when response recieved from initialize method call. - Forward notifications to notificaiton writers. - Handle SSE format (when handling text/event-stream), read line by line.
adwsingh
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rhernandez35
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Feb 2, 2026
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| for (String line : lines) { | ||
| if (line.startsWith("data:")) { | ||
| dataBuffer.append(line.substring(5).replaceFirst("^ ", "")); |
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replaceFirst compiles and evaluates a regular expression, which is expensive. Can we do a cheaper if (line.length() > 6 && Charater.isWhitespace(line.charAt(5)) check instead?
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I took a different approach. We can't use Charater.isWhitespace because the SSE spec only allows a U+0020 space. https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream
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mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/HttpMcpProxy.java
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rhernandez35
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adwsingh
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The proxy implementations do not currently support SSE formatting.
Description of changes:
Added support for reading streamed HTTP responses with SSE formatting (e.g. data: prefix). Responses are read from the buffer, line by line and notifications processed.
Notifications are passed to a new notification listener for JsonRPCRequest. The current one used JsonRPCRequest.
When a notification/list-changed response is received, we invalidate the local cache and make sure the proxies to do a full refresh the next time list/tools is called.
If an MCP server includes Mcp-Session-Id in its response we cache this and provide it forward for subsequent requests. This is the spec for this https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#session-management.
Tests pass and run locally to ensure proxy behaviour works as expected.