mcpGraph

# mcpGraph Design Document ## Overview This document memorializes the high-level design and tooling recommendations for mcpGraph, a product similar to n8n that surfaces an MCP interface and internally implements a directed graph of MCP server calls. The system enables filtering and reformatting data between nodes, makes routing decisions based on node output, and maintains a declarative, observable configuration without embedding a full programming language. ## Core Architecture: The Orchestrator ### Recommended Platform: TypeScript (Node.js) **Rationale:** - The MCP SDK is most mature in TypeScript - Frontend tools for visual graph editing (like React Flow) are industry standard - Strong ecosystem for both backend orchestration and frontend visualization ### Graph Execution Engine The system implements a custom graph execution engine that orchestrates the directed graph of MCP server calls. **Design Approach:** - **Custom Execution Loop**: A lightweight, sequential execution loop that provides full control over execution flow - **Simple Architecture**: Direct mapping from YAML node definitions to execution, avoiding abstraction layers - **Data Flow**: Execution context maintains state and data flow between nodes - **Control Flow**: Supports conditional routing via switch nodes; cycles are supported for future loop constructs - **Observability**: Built-in execution history tracking for debugging and introspection **Implementation:** The YAML configuration is parsed into a graph structure (`Graph` class) and executed by a custom `GraphExecutor` that: - Starts at the tool's entry node - Executes nodes sequentially based on the graph structure - Tracks execution state and history - Routes conditionally via switch nodes - Continues until the exit node is reached ### MCP Integration - Use `@modelcontextprotocol/sdk` - The system exposes an MCP server interface - The YAML configuration defines the MCP server metadata and the tools it exposes - Each tool definition includes standard MCP tool metadata (name, description, input/output parameters) - Each tool's graph has an explicit entry node that receives tool arguments and initializes execution - Each tool's graph has an explicit exit node that returns the final result to the MCP tool caller - Execution flows through the graph from entry node to exit node ## Declarative Logic & Data Transformation To avoid embedding a full programming language while maintaining declarative, observable configurations, the system uses standardized expression engines. ### Data Reformatting: JSONata **Why JSONata:** - Declarative query and transformation language for JSON - Single string expression enables clear observability - Can log input, expression, and output to debug transformations **Resources:** [JSONata Documentation](https://jsonata.org/) **YAML Example:** ```yaml transform: expr: "$merge([payload, {'timestamp': $now()}])" ``` ### Routing Decisions: JSON Logic **Why JSON Logic:** - Allows complex rules (e.g., "if price > 100 and status == 'active'") as pure JSON objects - Declarative and observable - No embedded code execution - **Note:** `var` operations in JSON Logic rules are evaluated using JSONata, allowing full JSONata expression support (including `$previousNode()` function) **Resources:** [JSON Logic Documentation](https://jsonlogic.com/) **YAML Example:** ```yaml condition: and: - ">": [{ var: "entry.price" }, 100] - "==": [{ var: "entry.status" }, "active"] ``` **Advanced Example with JSONata:** ```yaml condition: ">": [{ var: "$previousNode().count" }, 10] ``` ## High-Level Design: The Graph Runner The system exposes an MCP server that reads a YAML configuration file. When a tool is called on this MCP server, it executes the corresponding graph starting at the tool's entry node and continuing until the exit node is reached. ### The Workflow Lifecycle 1. **Parse**: Read the YAML configuration into a structure containing MCP server metadata, tool definitions, and the directed graph of nodes 2. **Initialize MCP Server**: Expose the MCP server with the tools defined in the configuration 3. **Tool Invocation**: When a tool is called: - Receive tool arguments from the MCP client - Start graph execution at the tool's entry node - Entry node receives tool arguments and initializes the execution context 4. **Execute Node**: - **Entry Node**: Receives tool arguments, initializes execution context, passes to next node - **Pre-transform**: Apply JSONata to the incoming data to format the tool arguments (if node is an MCP tool call) - **Call Tool**: Use the MCP SDK to `callTool` on the target server (if node is an MCP tool call) - **Transform**: Apply JSONata expressions to transform data (if node is a transform node) - **Route**: Evaluate JSON Logic against the current data state to decide which edge to follow next (if node is a switch/conditional) - **Track History**: Record node execution with inputs and outputs for observability 5. **Exit Node**: When execution reaches the exit node, extract the final result and return it to the MCP tool caller ## Visual Tooling & Observability ### Component Recommendations | Component | Tooling Recommendation | |-----------|----------------------| | **Visual Editor** | **React Flow** - Industry standard for node-based UIs, used by companies like Stripe and Typeform. Provides customizable nodes, edges, zooming, panning, and built-in components like MiniMap and Controls. [React Flow Documentation](https://reactflow.dev/) | | **Observability** | OpenTelemetry - wrap each node execution in a "Span" to see the "Trace" of data through the graph in tools like Jaeger | ## Implementation Strategy: The YAML Standard Graph definitions should feel like Kubernetes manifests or GitHub Actions - declarative and version-controlled. ### Configuration Structure The YAML configuration centers around MCP server and tool definitions: 1. **MCP Server Metadata**: Defines the MCP server information (name, version, description) 2. **Tools**: Array of tool definitions, each containing: - Standard MCP tool metadata (name, description) - Input parameters schema (MCP tool parameter definitions) - Output schema (what the tool returns) - Note: Entry and exit nodes are defined in the nodes section with a `tool` field indicating which tool they belong to 3. **Nodes**: The directed graph of nodes that execute when tools are called. Node types include: - **`entry`**: Entry point for a tool's graph execution. Receives tool arguments and initializes execution context. - **`mcp`**: Calls an MCP tool on an internal or external MCP server using `callTool` - **`transform`**: Applies JSONata expressions to transform data between nodes - **`switch`**: Uses JSON Logic to conditionally route to different nodes based on data - **`exit`**: Exit point for a tool's graph execution. Extracts and returns the final result to the MCP tool caller ### Example YAML Structure: count_files Tool This example defines a `count_files` tool that takes a directory, lists its contents using the filesystem MCP server, counts the files, and returns the count: ```yaml version: "1.0" # MCP Server Metadata server: name: "fileUtils" version: "1.0.0" description: "File utilities" # Tool Definitions tools: - name: "count_files" description: "Counts the number of files in a directory" inputSchema: type: "object" properties: directory: type: "string" description: "The directory path to count files in" required: - directory outputSchema: type: "object" properties: count: type: "number" description: "The number of files in the directory" # MCP Servers used by the graph servers: filesystem: command: "npx" args: - "-y" - "@modelcontextprotocol/server-filesystem" - "./tests/files" # Graph Nodes nodes: # Entry node: Receives tool arguments - id: "entry_count_files" type: "entry" tool: "count_files" next: "list_directory_node" # List directory contents - id: "list_directory_node" type: "mcp" server: "filesystem" tool: "list_directory" args: path: "$.entry_count_files.directory" next: "count_files_node" # Transform and count files - id: "count_files_node" type: "transform" transform: expr: | { "count": $count($split($.list_directory_node, "\n")) } next: "exit_count_files" # Exit node: Returns the count - id: "exit_count_files" type: "exit" tool: "count_files" ``` ## Key Design Principles 1. **Declarative Configuration**: All logic expressed in YAML using standard expression languages (JSONata, JSON Logic) 2. **Observability**: Every transformation and decision is traceable and loggable 3. **No Embedded Code**: Avoid full programming languages to maintain clarity and safety 4. **Standard-Based**: Favor existing standards (JSONata, JSON Logic) over custom solutions 5. **Visual First**: Graph should be viewable and editable through a visual interface 6. **Execution Transparency**: Ability to observe graph execution in real-time ## System Components 1. **Executable**: Exposes an MCP server to run the graph (`mcpgraph` CLI) 2. **Programmatic API**: Exports `McpGraphApi` class for programmatic use (e.g., by visualizer applications) 3. **Graph Executor**: Core orchestration engine that executes the directed graph sequentially 4. **Execution Context**: Tracks execution state, data flow, and history 5. **Visual Editor**: Tools to visually view and edit the graph (future) 6. **Execution Observer**: Ability to observe and debug graph execution (future - see `docs/future-introspection-debugging.md`)