fix(mcp): wait for client event stream before server->client requests#41559
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pavelfeldman merged 1 commit intoJul 1, 2026
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Over the Streamable HTTP transport, server->client requests (e.g. listRoots during initialization) can only be delivered once the client has opened its standalone GET SSE event stream. Thread a transportInitialized promise through connect()/createServer() and await it before issuing listRoots. It resolves when the client issues the GET event-stream request, or after a 5s fallback timeout so clients that never open the stream are not blocked.
yury-s
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Jun 30, 2026
| const capabilities = server.getClientCapabilities(); | ||
| let clientRoots: Root[] = []; | ||
| if (capabilities?.roots) { | ||
| await transportInitialized; |
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should it just resolve to false in this case, so that we don't need to send listRoots at all?
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I'm trying to be extra conservative here: wait for up to 5 seconds, relative to what was happening before the change, to not affect customers.
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Test results for "MCP"7461 passed, 1132 skipped Merge workflow run. |
yury-s
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Jul 9, 2026
## What's New ### New Tools - **`browser_find`** — Search the accessibility snapshot of the current page for text or a regular expression. Returns matching snapshot nodes with a few lines of surrounding context (like search snippets), which is cheaper than capturing the whole snapshot when you only need to locate an element and its ref ([#41605](microsoft/playwright#41605), [#41654](microsoft/playwright#41654)) ### Tool Improvements - **Less verbose snapshots** — Accessibility snapshots are distilled to reduce noise and token usage ([#41604](microsoft/playwright#41604)) - **Mobile & device emulation** — New `--mobile` flag and `--device` option to emulate mobile devices from the CLI ([#41657](microsoft/playwright#41657)) - **Navigation status** — `browser_navigate` now surfaces non-2xx navigation status codes ([#41589](microsoft/playwright#41589)) ## Bug Fixes - Enable the chromium sandbox for the default browser ([#41652](microsoft/playwright#41652)) - Close the isolated HTTP client context on disconnect ([#41560](microsoft/playwright#41560)) - Wait for the client event stream before sending server→client requests ([#41559](microsoft/playwright#41559))
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Summary
listRootsduring init) can only be delivered after the client opens its standalone GET SSE event stream.transportInitializedpromise throughconnect()/createServer()and await it before issuinglistRoots.