mcp: large stdio tool response (~11 MB) closes the connection with "Connection closed" #51092

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1. opencodejr commented on Oct 28, 2026

   

   opencodejrBot

   on Oct 28, 2026

   Contributor

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   Thanks for the detailed report and the minimal server. I could reproduce this.

   I ran your big_mcp.py as a local MCP server big on 2.0.16 and had the agent call big with a 12 MB response. The call fails with Connection closed and the TUI shows "MCP server failed: big". The same call with a 1 MB response works. On the server side, the write of the large line fails with BrokenPipeError, so OpenCode closes the pipe while the response is still arriving. The server itself is fine.

   It still happens on the latest dev (fa1260d). The error is now more detailed, but it's misleading: Connection closed: MCP server process exited with code 1: … BrokenPipeError, which makes it look like the server crashed.

   A likely cause, from reading the code (not confirmed with a fix yet): the stdio transport has its own 16 MiB per-message limit, but the MCP SDK's read buffer it uses has a default limit of 10 MiB. A message over 10 MiB, like your ~11.6 MB one, makes it throw. That ends the whole transport instead of failing just the one call, and the error never mentions size.

   The recording and screenshots below show it (on dev and on 2.0.16), and the script below reproduces it. Until this is addressed, keeping responses under about 10 MiB avoids it. Possibly related, but a different problem: #50725.

   recording.mp4

   

   

   Reproduction script (drive.ts)

   // Reproduces #51092: a ~12 MiB stdio MCP tool result closes the MCP connection.
   // Needs the issue's big_mcp.py at /workspace/repro/big_mcp.py (or BIG_MCP); size in MB via BIG_MB (default 12).
   import { Effect, Stream } from "effect"
   import { Llm, OpenCodeDriver } from "opencode-drive"
   
   const MB = process.env.BIG_MB ?? "12"
   const server = process.env.BIG_MCP ?? "/workspace/repro/big_mcp.py"
   
   export default OpenCodeDriver.use(
     {
       ...(process.env.OC_DEV ? { opencode: { dev: process.env.OC_DEV } } : {}),
       tui: { recording: true },
       config: {
         mcp: { big: { type: "local", command: ["python3", server, MB], enabled: true } },
       },
     } as any,
     ({ ui, llm }) =>
       Effect.gen(function* () {
         yield* Effect.sleep(4000)
         yield* llm.serve((request: any, index: number) => {
           // The first request without a tool result gets the tool call (request order can vary by version).
           if (!JSON.stringify(request).includes('"role":"tool"'))
             return Stream.make(
               Llm.toolCall({ index: 0, id: "call_big", name: "execute", input: { code: "return await tools.big.big({})" } }),
               Llm.finish("tool-calls"),
             )
           // Echo what OpenCode sent back as the tool result (the error text, or its size on success).
           const body = JSON.stringify(request)
           const m = body.match(/(Connection closed[^"\\]*|MCP stdio[^"\\]{0,200}|xxxxxxxxxx)/)
           const summary = m ? m[0] : `tool result received (${body.length} bytes of request)`
           console.log("TOOL RESULT SEEN BY MODEL:", summary)
           return Stream.make(Llm.text(`done: ${summary}`))
         })
         yield* ui.submit(`Call the big tool (${MB} MB)`)
         yield* ui.waitFor("done", { timeout: 60000 } as any).pipe(Effect.ignore)
         yield* Effect.sleep(2000)
         yield* ui.screenshot(`big-${MB}mb`)
         const frame = yield* ui.capture()
         const text = (frame as any).lines.map((l: any) => l.spans.map((s: any) => s.text).join("")).join("\n")
         console.log(text)
       }),
   )

   Run it with opencode-drive: opencode-drive run ./drive.ts. It drives opencode from your PATH; set OC_DEV to the path of an OpenCode checkout to run that from source instead.
2. opencodejr

   added

   reproduced

   on Oct 28, 2026
3. rekram1-node

   closed this as completedin #60694on Oct 30, 2026