The important model:

Gmail MCP server owns Google login and token files.
MCP Boundary owns the MCP execution boundary.

For the tested downstream-managed stdio Gmail path, the Google OAuth client type was Web application because the server used a local browser callback URL. Redirect URI host, port, and path must match exactly; localhost and 127.0.0.1 are not interchangeable.

Setup Shape

Install the Gmail MCP server.
Configure its Google OAuth client exactly as its README requires.
Run the server's own auth flow.
Confirm the server works directly.
Add it to MCP Boundary with auth_mode=downstream_managed.
Start with one read/list/metadata tool.
Open draft/send/trash tools only when you intentionally test them.

Example:

powershell

.\mcpboundary.exe add-server gmail `

--mode guarded `

--auth-mode downstream_managed `

--policy-config C:\mcpboundary\policies\gmail.policy.json `

-- npx tsx C:\path\to\advanced-gmail-mcp\src\server.ts

From an extracted Linux package, use the package-local binary:

./mcpboundary add-server gmail \

--mode guarded \

--auth-mode downstream_managed \

--policy-config "$HOME/.config/mcpboundary/policies/gmail.policy.json" \

-- npx tsx /path/to/advanced-gmail-mcp/src/server.ts

If MCP Boundary is installed on your PATH, the command may start with bare mcpboundary. Package-first examples use ./mcpboundary.

First Policy

Start narrow:

json

{

"version": "mcp-adapter-host-policy/v1",

"servers": [

{

"server_id": "gmail",

"tools": [

{

"downstream_tool_name": "get_labels",

"exposure": "visible",

"handling_mode": "generic_guarded",

"policy_input_mode": "allow",

"result_limits": {

"max_result_bytes": 16384

}

{

"downstream_tool_name": "send_email",

"exposure": "hidden",

"handling_mode": "generic_guarded",

"policy_input_mode": "block"

}

]

}

]

}

Copy exact native downstream tool names from your Gmail MCP server. The client-safe names shown to the agent may replace dots with underscores, but the Policy still uses the native name in downstream_tool_name. Another server may use different names.

What To Test First

First:

list labels
search messages
read metadata

Later:

create one controlled draft
send one controlled test email
apply one controlled label/trash action

Do not start by making every Gmail tool requestable.

Common Gmail Auth Issue

If Google shows redirect_uri_mismatch, add the exact URI shown by Google to your OAuth client. Match host, port, and path exactly.

A longer guide also ships inside the downloaded ZIP/tarball at docs/mcpboundary/publish/guides/real-gmail.md.