Fix: WP_CLI::line() in serve command corrupts JSON-RPC stdout stream

[graham73may]

Problem

When wp mcp-adapter serve is called without a --server flag, it falls back to the first registered server and calls WP_CLI::line() to report the selection:

\WP_CLI::line( sprintf( 'Using server: %s', $server_id ) );

WP_CLI::line() writes to stdout. The serve command communicates with MCP clients over STDIO using the JSON-RPC 2.0 protocol, so anything written to stdout before the first { is treated as part of the protocol stream. This causes the client's JSON parser to fail immediately.

Real-world impact: Any MCP client using STDIO transport (Claude Code, VS Code Copilot, Cursor, etc.) that omits --server will fail to connect. Workarounds require piping output through grep '^{' to strip the non-JSON line.

Fix

Change WP_CLI::line() to WP_CLI::debug(). Debug output goes to stderr and is suppressed by default, so:

• Normal usage is unaffected
• The message is still visible when running with --debug
• The JSON-RPC stream on stdout is clean

Change

includes/Cli/McpCommand.php line 78:

- \WP_CLI::line( sprintf( 'Using server: %s', $server_id ) );
+ \WP_CLI::debug( sprintf( 'Using server: %s', $server_id ) );

[github-actions]

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Co-authored-by: webmyc <urbankidro@git.wordpress.org>

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[Copilot]

Pull request overview

Fixes a protocol-breaking stdout write in the wp mcp-adapter serve command when no --server flag is provided, ensuring the JSON-RPC 2.0 STDIO stream remains clean for MCP clients.

Changes:

• Replace WP_CLI::line() (stdout) with WP_CLI::debug() (stderr) when auto-selecting the first registered server.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.85%. Comparing base (7cc42a0) to head (00a02e5).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #194      +/-   ##
============================================
- Coverage     88.45%   87.85%   -0.60%     
  Complexity     1243     1243              
============================================
  Files            53       53              
  Lines          4035     4035              
============================================
- Hits           3569     3545      -24     
- Misses          466      490      +24     

Flag	Coverage Δ
unit	87.85% <100.00%> (-0.60%)	⬇️

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[webmyc]

Thanks for this. Inhale (the wp.org settings page i shipped on top of the Abilities API + MCP Adapter) had users hitting the corrupted-stream bug on every fresh wp mcp-adapter serve connection. The one-line stderr move is exactly right for STDIO.