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fix: buffer MCP server stderr instead of logging every line as error #1346

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Merged
lewis617 merged 1 commit into from
Jul 7, 2026
Merged

fix: buffer MCP server stderr instead of logging every line as error #1346

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41 changes: 26 additions & 15 deletions packages/agent-sdk/src/managers/mcpManager.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -484,29 +484,40 @@ export class McpManager {
stderr: "pipe", // Pipe stderr to capture it
});

// Handle stderr output for StdioClientTransport
// Buffer stderr silently (MCP servers use stderr for all logging;
// stdout is reserved for JSON-RPC). Only dump it on connection
// success (once) or failure — not line-by-line in real time.
// Aligns with Claude Code's approach to avoid noise from [INFO],
// [DEBUG], Warning: lines that are not actual errors.
const stderr = (transport as StdioClientTransport).stderr;
let stderrOutput = "";
if (stderr) {
let buffer = "";
stderr.on("data", (chunk: Buffer) => {
buffer += chunk.toString();
const lines = buffer.split("\n");
buffer = lines.pop() || "";
for (const line of lines) {
if (line.trim()) {
logger?.error(`[MCP Server ${name}] ${line}`);
}
}
});
stderr.on("end", () => {
if (buffer.trim()) {
logger?.error(`[MCP Server ${name}] ${buffer}`);
if (stderrOutput.length < 64 * 1024 * 1024) {
stderrOutput += chunk.toString();
}
});
}

client = createClient();
await client.connect(transport);
try {
await client.connect(transport);
} catch (error) {
if (stderrOutput.trim()) {
logger?.error(
`[MCP Server ${name}] Server stderr: ${stderrOutput.trim()}`,
);
}
throw error;
}

// Dump accumulated stderr once after successful connection
if (stderrOutput.trim()) {
logger?.debug(
`[MCP Server ${name}] Server stderr: ${stderrOutput.trim()}`,
);
stderrOutput = "";
}

const toolsResponse = await client.listTools();
tools =
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