Implementations of the MCP catalog's functions (see A2UICore.Catalog.MCP).

• callMcpTool calls a tool through the host's A2UICore.MCP.Client and returns the tool's CallToolResult. The A2UI messages the result carries — in the UI resources it names (_meta.ui.resourceUri, else those its tool descriptor declares) and embedded in its content — come back as {:messages, %{messages: messages, source: :resource | :inline}} effects for the renderer to apply; messages without a version get the catalog's (v0.9 or v1.0). With targetDataPath (v1.0), the MCP App a resource holds is written there as %{"uri", "htmlContent", "csp", "permissions", "toolInput", "toolResult"} (a {:write, ...} effect), ready for an McpApp component to bind.
• jmespath evaluates a JMESPath expression (A2UICore.JMESPath).
• split, regexCapture and regexReplace work on a string, or on each string of an array. Patterns are RE2 expressions, run by PCRE with RE2's meaning of $ (end of text) and ASCII \d, \w, \b.
• updateDataModel asks for its updates to be written: one {:write, %{path: path, value: value}} effect per entry, the path resolved against the calling scope.

Effects reach the renderer from an action (see A2UICore.Resolution.NodeResolver.invoke/5); a renderer applies them in order. Failures are errors with category :expression.