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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Streamable HTTP client ignores SSE messages when event name is omitted","articleBody":"Disclaimer: I am not a native English speaker. This issue was drafted with AI translation assistance. I apologize for any awkward phrasing.\n\n## Bug description\n\nWhen using the Streamable HTTP transport, the Java client receives an SSE frame whose payload contains a valid JSON-RPC response, but the SSE event name is omitted by the server.\n\nIn standard SSE semantics, if the `event:` field is omitted, the event type defaults to `message`. However, the current Java SDK only parses the frame if `event.event()` is explicitly equal to `\"message\"`. If the event name is missing and Spring/WebFlux exposes it as `null`, the client drops the frame instead of treating it as a default `message` event.\n\nAs a result, the `initialize` response is ignored even though the client has already received the JSON-RPC result, and initialization eventually times out with:\n\n`Client failed to initialize by explicit API call`\n\nIn my case, the server is `rmcp` over Streamable HTTP.\n\n## Environment\n\n- Spring AI: `2.0.0-M3`\n- MCP Java SDK: `1.1.0`\n- Transport: `WebClientStreamableHttpTransport` (via `spring-ai-starter-mcp-client-webflux`)\n- Spring Boot: `4.0.4`\n- Java: `25.0.2`\n- Reactor Netty: `1.3.4`\n- MCP server: `rmcp 1.2.0` (Rust, Streamable HTTP)\n- OS: macOS\n\n## Steps to reproduce\n\n1. Run an MCP server over Streamable HTTP.\n2. Make the server return a valid JSON-RPC `initialize` result as `text/event-stream`.\n3. The SSE frame should contain only `data: ...` and omit the `event:` field. For example:\n\n```text\n   data: {\"jsonrpc\":\"2.0\",\"id\":\"1\",\"result\":{\"protocolVersion\":\"2025-06-18\",\"capabilities\":{\"tools\":{}},\"serverInfo\":{\"name\":\"test\",\"version\":\"1.0.0\"}}}\n````\n\n4. Start a Spring AI / MCP Java SDK client using `WebClientStreamableHttpTransport`.\n5. Call `client.initialize()` (or let Spring AI auto-initialize the MCP client during startup).\n\n## Expected behavior\n\nThe client should treat an SSE frame without an explicit `event:` field as a default `message` event and parse the JSON-RPC payload normally.\n\nSo the `initialize` response above should be accepted, and client initialization should complete successfully.\n\n## Actual behavior\n\nThe client receives the SSE payload, but the event type is exposed as `null` and is not parsed as a JSON-RPC message.\n\nAs a result, the initialize response is effectively ignored, and initialization times out.\n\nFrom debug logs:\n\n```text\nReceived SSE event with type: ServerSentEvent [id = 'null', event='null', retry=null, comment='null', data={\"jsonrpc\":\"2.0\",\"id\":\"9642da98-0\",\"result\":{\"protocolVersion\":\"2025-06-18\",\"capabilities\":{\"tools\":{}},\"serverInfo\":{\"name\":\"rmcp\",\"version\":\"1.2.0\"},\"instructions\":\"...\"}}]\n...\nCaused by: java.util.concurrent.TimeoutException: Did not observe any item or terminal signal within 20000ms\n...\nClient failed to initialize by explicit API call","author":{"url":"https://github.com/nympheon","@type":"Person","name":"nympheon"},"datePublished":"2026-03-27T06:54:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/885/java-sdk/issues/885"}