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WebMCP tools execute inside a page, while a consumer may run in an iframe, browser extension, userscript, or desktop MCP client. A transport carries tool discovery, calls, results, and lifecycle notifications across that boundary without changing where the tool runs. @mcp-b/transports implements low-level tab, iframe, extension, and userscript paths. @mcp-b/mcp-iframe wraps iframe forwarding. @mcp-b/webmcp-local-relay connects browser tools to desktop clients through localhost and stdio. Upstream chrome-devtools-mcp uses the Chrome DevTools Protocol for coding-agent workflows. Each bridge establishes a security boundary. Origin validation, connection identity, extension permissions, and relay exposure must be configured in the corresponding package. Tool execution remains in the page. Start with the @mcp-b/transports reference.