DiagBridge

DiagBridge MCP is a visible Windows MCP bridge for trusted, user-authorized computer sessions.

It connects a user's local computer session to trusted MCP-capable AI agents such as ChatGPT, Codex, Claude Code, Cursor, or similar tools. It is intentionally small: it is a bridge, not a sandbox, not an RMM platform, and not a general security judge for every command.

What DiagBridge does

DiagBridge keeps only the bridge responsibilities:

• Run visibly.
• Listen on 127.0.0.1 by default.
• Use a temporary session token.
• Provide a disconnect endpoint.
• Record audit events.
• Publish honest MCP-style tool metadata.
• Avoid hidden run modes, UAC bypass, and default administrator assumptions.

What DiagBridge does not do

DiagBridge is not:

• A security platform.
• A sandbox.
• A malware analysis environment.
• A hidden remote-control tool.
• A replacement for MCP host approval.
• A system that can reliably decide whether every possible command is safe.

The MCP host is expected to handle approval prompts, automation level, allow/deny lists, and user-facing tool policy.

Transports

DiagBridge currently provides three development entry points:

Entry point	Script	Purpose
Local HTTP bridge	npm run dev	Simple local /tools and /call bridge for development.
Stdio MCP server	npm run dev:mcp	Local MCP host integration over stdio.
Dev HTTP MCP fallback	npm run dev:http-mcp	ChatGPT custom connector testing through /mcp.

The dev HTTP MCP fallback is for local connector testing. It returns text on GET /, handles OPTIONS /mcp and OPTIONS /mcp/*, and sends MCP JSON-RPC traffic through POST /mcp.

Tools

The local development bridge can expose:

Tool	Metadata	Default local enabled
system_info	read-only	Yes
list_dir	read-only	Yes
read_file	read-only	Yes
drive_inventory	read-only	Yes
junk_candidates	read-only	Yes
windows_event_summary	read-only	Yes
write_file	destructive	No
run_command	destructive + open-world	No

The ChatGPT connector HTTP fallback intentionally exposes only:

system_info
drive_inventory
junk_candidates
windows_event_summary

It does not expose read_file, write_file, or run_command by default.

run_command is a high-power capability. If enabled in a local-only development session, an agent may run local commands through the bridge. The consequences depend on the user's operating system account, the MCP host approval policy, and the command itself.

Recommended default: use only read-only tools first. Enable write_file or run_command only for trusted local sessions where the host approval settings are understood.

Windows read-only diagnostics

drive_inventory

Scans directory metadata only: path, name, type, size, modified time, and extension. It does not read file contents. It has bounded maxDepth, maxEntries, and maxSeconds controls and excludes common high-privacy or high-risk directories by default.

junk_candidates

Identifies possible junk candidates from metadata only. It does not delete, move, clean, or repair anything. Every candidate returns recommendedAction: "review_only".

windows_event_summary

Reads recent Windows Application/System error event summaries through a fixed read-only query. It focuses on application crashes, unexpected shutdowns, WHEA/hardware events, display/GPU resets, and disk/storage providers. It does not accept arbitrary command input and does not auto-elevate.

Configuration

Environment variables:

Variable	Default	Meaning
DIAGBRIDGE_HOST	127.0.0.1	Bind address for the local HTTP bridge. Keep local unless you know what you are doing.
DIAGBRIDGE_HTTP_HOST	127.0.0.1	Bind address for the dev HTTP MCP fallback.
DIAGBRIDGE_PORT	8787	Local bridge port.
DIAGBRIDGE_HTTP_PORT	8787	Dev HTTP MCP fallback port.
DIAGBRIDGE_SESSION_TOKEN	generated at startup	Session token. Protected requests without it are rejected.
DIAGBRIDGE_TOOLS	read-only local tools	Comma-separated enabled local tools.
DIAGBRIDGE_CWD	current directory	Base directory for relative file paths.
DIAGBRIDGE_AUDIT_LOG	.diagbridge-audit.jsonl	JSONL audit log path.

Development

npm install
npm run check
npm test
npm run dev
npm run dev:mcp
npm run dev:http-mcp

ChatGPT connector development

See docs/DEV_HTTP_CONNECTOR_SETUP.md.

For first-round connector testing, use the dev HTTP MCP endpoint and only test:

system_info
drive_inventory
junk_candidates
windows_event_summary

Do not enable write_file or run_command over a public tunnel.

Security expectations

Use DiagBridge only with trusted parties and a trusted MCP host. The host should provide tool approval and policy controls. DiagBridge still refuses to add hidden operation, UAC bypass, silent persistence, credential-harvesting-specific tools, or approval-bypass behavior.

See SECURITY.md and docs/threat-model.md.