René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"SSE client rejects valid `retry:` field and ignores reconnection timing (Streamable HTTP)","articleBody":"**Bug description**\n\nThe client SSE line parser in `ResponseSubscribers` rejects the standard SSE `retry:` field, throwing:\n\n```\nio.modelcontextprotocol.spec.McpTransportException: Invalid SSE response. Status code: 200 Line: retry: 500\n```\n\nPer the SSE specification, `retry:` sets the stream's reconnection time, and unknown fields MUST be ignored (never error the stream). Because of this, the Streamable HTTP client:\n1. can error a live SSE stream when the server sends a `retry:` line, and\n2. does not honor the server-provided reconnection delay, reconnecting immediately.\n\nThis is the MUST-level `client-sse-retry-timing` failure already noted as a known limitation in `conformance-tests/VALIDATION_RESULTS.md`.\n\n**Environment**\n- java-sdk `main` (2.0.1-SNAPSHOT)\n- Java 17+\n- Transport: `HttpClientStreamableHttpTransport` (client), SSE / Streamable HTTP\n\n**Steps to reproduce**\nRun the official MCP conformance `sse-retry` client scenario:\n\n```bash\n./mvnw clean package -DskipTests -pl conformance-tests/client-jdk-http-client -am\nnpx -y @modelcontextprotocol/conformance client \\\n  --command \"java -jar conformance-tests/client-jdk-http-client/target/client-jdk-http-client-2.0.1-SNAPSHOT.jar\" \\\n  --scenario sse-retry\n```\n\nObserved:\n```\nError: Invalid SSE response. Status code: 200 Line: retry: 500\n[client-sse-retry-timing ] FAILURE  Client MUST respect the retry field (reconnected ~0ms instead of 500ms)\n[client-sse-last-event-id] WARNING  Client SHOULD send Last-Event-ID on reconnection\nOVERALL: FAILED\n```\n\n**Expected behavior**\n- The SSE parser parses/ignores `retry:` (and any unknown SSE field) without erroring the stream.\n- On reconnection after a drop, the client waits the server-specified `retry` interval before reconnecting.\n\n**Minimal reproducible example**\nFeeding the SSE lines `id: e1` / `retry: 500` / `data: hello` / (blank) to the SSE line subscriber currently throws `McpTransportException` instead of yielding a single event.\n\n**Proposed scope (two parts)**\n1. Parser robustness (small, self-contained): parse `retry:` and ignore unknown fields in `ResponseSubscribers`. *(Implemented locally with unit tests.)*\n2. Reconnect timing: honor the parsed `retry` value before reconnecting in `HttpClientStreamableHttpTransport` / `DefaultMcpTransportStream`. This touches the `McpTransportStream` SPI, so I'd like to confirm the preferred approach before opening a PR.\n\nThe related `Last-Event-ID` SHOULD warning appears covered by #830, so I would keep it out of scope here.\n\nHappy to open a PR for part 1 immediately and follow up on part 2 per maintainer guidance.\n","author":{"url":"https://github.com/imankhadiv","@type":"Person","name":"imankhadiv"},"datePublished":"2026-06-30T00:31:55.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/1047/java-sdk/issues/1047"}