MCP: Audit and harden native MCP server implementation

[MichaelFisher1997]

Summary

A comprehensive audit and hardening of the native MCP (Model Context Protocol) server module (modules/mcp/). The module was functional but had several bugs, spec compliance gaps, and unimplemented actions advertised in the tool schemas.

Bugfixes

• Capture crash: Null-deref in capture action when viewport returns no image (headless/first-frame) or PNG encoding fails
• Double-wrapped errors: Pre-init tools/* requests returned a JSON-RPC error nested inside a result envelope instead of a top-level error. Fixed via a process_string override that gates pre-init requests and notifications

Protocol compliance

• Updated protocol version from 2024-11-05 to 2025-06-18 with proper version negotiation (echoes client version if supported)
• Moved initialized flag from initialize response to notifications/initialized per spec lifecycle
• Declared tools.listChanged capability explicitly

Security & robustness

• Hardened validate_path: segment-based .. traversal check, applied consistently to all path-accepting actions (previously instance_path, resource paths, and code_intel paths were unvalidated)
• Fixed bridge concurrency: update() no longer drops an active connection mid-command
• Improved send_command: chunked reads via CharString accumulation with single-pass UTF-8 decode

New functionality

• code_intel search action: recursive .gd file content search
• project_config add_input action: writes InputEventKey to project.godot with runtime InputMap registration
• project_config add_autoload action: writes autoload entry to project.godot

Quality

• MCP tool annotations (readOnlyHint, destructiveHint, etc.) on all 5 tools per 2025-06-18 spec
• Clarified project_config schema descriptions (which params each action uses)
• Improved _ensure_callback_exists word-boundary detection (handles static func, annotation prefixes)
• Added signal-exists validation before connect() in scene_action
• Filled in class reference docs for MCPServer, MCPBridge, MCPProtocol
• Removed dead code (_make_text_content)

Testing

All changes manually tested end-to-end with a live MCP handshake (--mcp-server --headless) across all 5 tools:

Tool	Actions verified
code_intel	get_symbols, get_docs, validate, search
game_control	wait, capture, inspect_live, click, trigger_action, type
project_config	get_info, list_files, run, stop, output, read_file_res, create_file_res, set_setting, add_input, add_autoload
resource_action	inspect, inspect_asset, create, modify, duplicate
scene_action	get_node, create, add, set_prop, instance, reparent, remove, connect

Pre-init gating verified: single requests return JSON-RPC error, notifications are silently dropped.

Notes

• AI tools were used during development, in accordance with the project AI Policy. All changes have been manually tested and reviewed.
• Built and tested on Linux (linuxbsd platform, editor target).

Checklist

• Code compiles (scons platform=linuxbsd target=editor)
• Manually tested all advertised tool actions
• Class reference documentation updated
• Pre-commit hooks passed (clang-format, codespell, copyright-headers, header-guards, file-format)
• CodeRabbit review addressed

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