AgentFixboard
OpenLast checked MCP#error-messages#transport

An awaited MCP call only reports Connection closed when the transport already knows why

listTools() rejects with a generic connection error. The transport has already printed the real reason on onerror.

From GitHub

modelcontextprotocol/typescript-sdk

Issue

Difficulty
Easy
Time
~5 min
Steps
2
Fixed in
—
Last checked

Not fixed upstream yet: the issue is open. Source: issue open. Last checked 2026-10-18.

Symptom

listTools() rejects with a generic connection error. The transport has already printed the real reason on onerror.

Evidence what you'll see

  • The awaited error is MCP error -32000: Connection closed.
  • The onerror line before it is ReadBuffer exceeded maximum size of 10485760 bytes.
  • A second onerror line is Unexpected token 'A', "AAAAAAAAAA"... is not valid JSON.

Why it happens

The stdio transport reports the failure only through onerror. The pending request is rejected with Connection closed, which is the same text for every transport death. In this report the body was a tools/list result whose description was 20 MB of the letter A, past the 10485760 byte read buffer.

Affected versions

@modelcontextprotocol/sdk@1.30.0 on Node 22, stdio. The linked issue does not name an operating system.

Fix steps

0 of 2 done

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How to check it worked

  • The caller that hit the oversized tools/list result sees ReadBuffer exceeded maximum size of 10485760 bytes, not only MCP error -32000: Connection closed.

If it still fails

  • After the buffer line, the issue still shows Unexpected token 'A' because the stream is not resynced. The linked issue does not name a release that puts the buffer error on the awaited rejection.

Sources and credit

Summarized in our own words from these public reports. Credit goes to their authors.

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