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fix: only show server stderr in debug mode #247

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fix: only show server stderr in debug mode #247

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19 changes: 7 additions & 12 deletions src/client.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -244,14 +244,16 @@ export async function connectToServer(
transport = createStdioTransport(config);

// Capture stderr for debugging - attach BEFORE connect
// Always stream stderr immediately so auth prompts are visible
const stderrStream = transport.stderr;
if (stderrStream) {
stderrStream.on('data', (chunk: Buffer) => {
const text = chunk.toString();
stderrChunks.push(text);
// Always stream stderr immediately so users can see auth prompts
process.stderr.write(`[${serverName}] ${text}`);

// Only forward live stderr in debug mode to avoid noisy token burn.
if (process.env.MCP_DEBUG) {
process.stderr.write(`[${serverName}] ${text}`);
}
});
}
}
Expand All @@ -268,15 +270,8 @@ export async function connectToServer(
throw error;
}

// For successful connections, forward stderr to console
if (!isHttpServer(config)) {
const stderrStream = (transport as StdioClientTransport).stderr;
if (stderrStream) {
stderrStream.on('data', (chunk: Buffer) => {
process.stderr.write(chunk);
});
}
}
// For successful connections, stderr has already been captured above.
// It is only forwarded live when MCP_DEBUG is enabled.

return {
client,
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6 changes: 6 additions & 0 deletions src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -129,6 +129,10 @@ function parseArgs(args: string[]): ParsedArgs {
result.command = 'version';
return result;

case '--debug':
process.env.MCP_DEBUG = '1';
break;

case '-d':
case '--with-descriptions':
result.withDescriptions = true;
Expand Down Expand Up @@ -367,6 +371,7 @@ Options:
-v, --version Show version number
-d, --with-descriptions Include tool descriptions
-c, --config <path> Path to mcp_servers.json config file
--debug Show MCP server stderr/debug output

Output:
mcp-cli/info/grep Human-readable text to stdout
Expand All @@ -385,6 +390,7 @@ Examples:
Environment Variables:
MCP_NO_DAEMON=1 Disable connection caching (force fresh connections)
MCP_DAEMON_TIMEOUT=N Set daemon idle timeout in seconds (default: 60)
MCP_DEBUG=1 Show MCP server stderr/debug output

Config File:
The CLI looks for mcp_servers.json in:
Expand Down