#!/usr/bin/env bash
# Extension capabilities management for MCP Apps and other protocol extensions.
# Tracks which extensions the client supports during capability negotiation.
set -euo pipefail
# Track client extension support (initialized in mcp_extensions_init)
# Using 0/1 instead of true/false for consistency with elicitation.sh
MCPBASH_CLIENT_SUPPORTS_UI="${MCPBASH_CLIENT_SUPPORTS_UI:-0}"
# Extension data storage (bash 3.2 compatible - uses state files instead of associative array)
_MCP_UI_EXTENSION_DATA=""
# UI extension identifier per MCP Apps spec (SEP-1865)
MCP_UI_EXTENSION_ID="io.modelcontextprotocol/ui"
# Supported MIME types for UI extension
MCP_UI_SUPPORTED_MIMETYPES='["text/html;profile=mcp-app"]'
# --- State file paths for subprocess access ---
mcp_extensions_ui_support_flag_path() {
printf '%s/extensions.ui.support' "${MCPBASH_STATE_DIR}"
}
mcp_extensions_ui_mimetypes_path() {
printf '%s/extensions.ui.mimetypes' "${MCPBASH_STATE_DIR}"
}
mcp_extensions_write_ui_support_flag() {
local value="$1"
printf '%s' "${value}" >"$(mcp_extensions_ui_support_flag_path 2>/dev/null)" 2>/dev/null || true
}
mcp_extensions_write_ui_mimetypes() {
local mimetypes="$1"
printf '%s' "${mimetypes}" >"$(mcp_extensions_ui_mimetypes_path 2>/dev/null)" 2>/dev/null || true
}
# --- Initialization ---
# Initialize extension capabilities from client capabilities JSON
# Called from handlers/lifecycle.sh during initialize
mcp_extensions_init() {
local client_caps="${1:-{}}"
MCPBASH_CLIENT_SUPPORTS_UI=0
# Check for io.modelcontextprotocol/ui extension
# Client advertises via: capabilities.extensions["io.modelcontextprotocol/ui"]
local ui_ext=""
if [ "${MCPBASH_JSON_TOOL:-none}" != "none" ]; then
ui_ext="$(printf '%s' "${client_caps}" | "${MCPBASH_JSON_TOOL_BIN}" -c '.extensions["io.modelcontextprotocol/ui"] // empty' 2>/dev/null || true)"
fi
if [ -n "${ui_ext}" ] && [ "${ui_ext}" != "null" ]; then
MCPBASH_CLIENT_SUPPORTS_UI=1
_MCP_UI_EXTENSION_DATA="${ui_ext}"
# Extract supported MIME types from client (for future use)
local client_mimetypes
client_mimetypes="$(printf '%s' "${ui_ext}" | "${MCPBASH_JSON_TOOL_BIN}" -c '.mimeTypes // []' 2>/dev/null || printf '[]')"
mcp_extensions_write_ui_mimetypes "${client_mimetypes}"
else
# Fallback for environments without JSON tooling: string match
case "${client_caps}" in
*\"io.modelcontextprotocol/ui\"*)
MCPBASH_CLIENT_SUPPORTS_UI=1
;;
esac
fi
mcp_extensions_write_ui_support_flag "${MCPBASH_CLIENT_SUPPORTS_UI}"
}
# --- Query functions ---
# Check if client supports UI extension
# Returns: 0 if supported, 1 if not
mcp_client_supports_ui() {
if [ "${MCPBASH_CLIENT_SUPPORTS_UI}" = "1" ]; then
return 0
fi
# Check state file for subprocess context
local flag
flag="$(cat "$(mcp_extensions_ui_support_flag_path 2>/dev/null)" 2>/dev/null || true)"
[ "${flag}" = "1" ]
}
# Check if client supports a specific extension by ID
# Usage: mcp_client_supports_extension "io.modelcontextprotocol/ui"
# Returns: 0 if supported, 1 if not
mcp_client_supports_extension() {
local extension_id="$1"
case "${extension_id}" in
"${MCP_UI_EXTENSION_ID}")
mcp_client_supports_ui
return $?
;;
*)
# Currently only UI extension is supported
return 1
;;
esac
}
# Get extension data for a supported extension
# Returns: JSON string of extension capabilities, or empty string
mcp_client_extension_data() {
local extension_id="$1"
case "${extension_id}" in
"${MCP_UI_EXTENSION_ID}")
printf '%s' "${_MCP_UI_EXTENSION_DATA:-}"
;;
*)
printf ''
;;
esac
}
# --- Server capability building ---
# Build server extension capabilities for initialize response
# Only includes extensions that the client also supports (capability negotiation)
mcp_extensions_build_server_capabilities() {
local extensions="{}"
# Only advertise UI extension if client supports it
if mcp_client_supports_ui; then
if [ "${MCPBASH_JSON_TOOL:-none}" != "none" ]; then
extensions="$(printf '%s' "${extensions}" | "${MCPBASH_JSON_TOOL_BIN}" -c \
--argjson mimetypes "${MCP_UI_SUPPORTED_MIMETYPES}" \
'.["io.modelcontextprotocol/ui"] = {mimeTypes: $mimetypes}')"
else
# Minimal mode fallback
extensions='{"io.modelcontextprotocol/ui":{"mimeTypes":["text/html;profile=mcp-app"]}}'
fi
fi
printf '%s' "${extensions}"
}
# Merge extension capabilities into full server capabilities JSON
# Usage: mcp_extensions_merge_capabilities '{"tools":{},...}'
# Returns: Capabilities JSON with extensions field added
mcp_extensions_merge_capabilities() {
local base_capabilities="$1"
local extensions
extensions="$(mcp_extensions_build_server_capabilities)"
# Only add extensions field if non-empty
if [ "${extensions}" = "{}" ]; then
printf '%s' "${base_capabilities}"
return
fi
if [ "${MCPBASH_JSON_TOOL:-none}" != "none" ]; then
printf '%s' "${base_capabilities}" | "${MCPBASH_JSON_TOOL_BIN}" -c \
--argjson ext "${extensions}" \
'. + {extensions: $ext}'
else
# Minimal mode: simple string manipulation (assumes base_capabilities ends with })
local without_brace="${base_capabilities%\}}"
printf '%s,"extensions":%s}' "${without_brace}" "${extensions}"
fi
}
