feat: Add Langchain agent with OpenAI & A2A endpoints (refs #262)

Signed-off-by: Vicky <vicky.kuo.contact@gmail.com>

Author: Vicky

agent_runtimes/langchain_agent/README.md

@@ -1,3 +1,4 @@
+# MCP Langchain Agent
 A configurable Langchain agent that supports MCP and integrates with the MCP Gateway via streamable HTTP + Auth.
 
 Tools can be specified as a CSV list.
@@ -11,3 +12,154 @@ Endpoints for:
 /list_tools
 
 etc. are provided.
+
+## Features update (2025-Aug-17)
+- **Dynamic Tool Discovery**: Automatically discovers tools from MCP Gateway (`GET /tools`)
+- **OpenAI-Compatible API**: Standard `/v1/chat/completions` endpoint with streaming support
+- **A2A Communication**: JSON-RPC `/a2a` endpoint for gateway-to-gateway communication
+- **Langchain Integration**: Full Langchain agent with function calling support
+- **Health Monitoring**: `/health`, `/ready`, `/list_tools` endpoints
+
+## User Stories Implemented
+[x] **Dynamic Tool Discovery** - Auto-discovers from gateway or uses allowlist  
+[x] **Dual Endpoint Exposure** - OpenAI + A2A JSON-RPC endpoints  
+[x] **Parameterised Tool Allow-List** - `TOOLS=` environment variable  
+[x] **Tool Schema Introspection** - JSON schema parsing and validation
+
+Structure:
+```
+agent_runtimes/langchain_agent/
+├── app.py                    # FastAPI application incl. /v1/chat/completions and /a2a
+├── agent_langchain.py    # Core Langchain agent
+├── mcp_client.py            # MCP Gateway client
+├── models.py                # Pydantic models
+├── config.py                # Configuration management
+├── start_agent.py           # Startup script
+├── requirements.txt         # Dependencies
+└── README.md               # This file
+```
+
+### Configuration (env vars)
+- OPENAI_API_KEY – required
+- MCPGATEWAY_BEARER_TOKEN – JWT for the gateway
+
+
+### Installation
+Install dependencies:
+```bash
+cd agent_runtimes/langchain_agent
+pip install -r requirements.txt
+```
+
+
+### Quick Start
+1) Start the MCP Gateway (from project root):
+```bash
+make serve
+```
+
+2) Start the Langchain Agent (in another terminal):
+```bash
+# Set environment variables
+export OPENAI_API_KEY=your-openai-api-key
+export GATEWAY_BEARER_TOKEN=$(python3 -m mcpgateway.utils.create_jwt_token -u admin --secret my-test-key)
+
+# Optional: Tool allowlist (if unset, all tools discovered)
+export TOOLS="tool1,tool2,tool3" # Replace with actual tool names
+
+python -m agent_runtimes.langchain_agent.start_agent
+```
+
+3) Test the agent (in new terminal):
+
+Basic Health Checks
+```bash
+# Health check
+curl http://localhost:8000/health
+
+# Readiness check  
+curl http://localhost:8000/ready
+
+# List available tools
+curl http://localhost:8000/list_tools
+```
+
+Basic Health Checks
+```bash
+# Basic chat completion
+curl -X POST http://localhost:8000/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "gpt-4o-mini",
+    "messages": [
+      {"role": "user", "content": "What tools do you have available?"}
+    ]
+  }'
+
+# Streaming chat completion
+curl -X POST http://localhost:8000/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "gpt-4o-mini",
+    "messages": [
+      {"role": "user", "content": "Hello!"}
+    ],
+    "stream": true
+  }'
+```
+
+Test A2A communication with a working tool (books-search)
+- Step 1: Find a tool to test
+```bash
+# Get your available tools and pick one
+curl http://localhost:8000/list_tools | jq '.tools[0]'
+```
+
+- Step 2: Test A2A with your tool
+```bash
+# Replace "YOUR_TOOL_NAME" with an actual tool from step 1
+curl -X POST http://localhost:8000/a2a \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": "1",
+    "method": "invoke",
+    "params": {
+      "tool": "YOUR_TOOL_NAME",
+      "args": {}
+    }
+  }'
+
+# For tools that need parameters, check the tool schema:
+curl -X POST http://localhost:8000/a2a \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0", 
+    "id": "1",
+    "method": "invoke",
+    "params": {
+      "tool": "YOUR_TOOL_NAME",
+      "args": {
+        "param1": "value1",
+        "param2": "value2"
+      }
+    }
+  }'
+```
+Expected Success Response:
+```json
+{
+  "jsonrpc": "2.0",
+  "id": "1",
+  "result": {
+    "success": true,
+    "result": {
+      "tool_id": "your-tool-id",
+      "executed_via": "direct_rest_fallback",
+      "status_code": 200,
+      "result": "...actual tool output..."
+    }
+  }
+}
+```
+![alt text](image.png)

agent_runtimes/langchain_agent/__init__.py

@@ -0,0 +1,12 @@
+"""
+MCP Langchain Agent Package
+
+A configurable Langchain agent that supports MCP and integrates with the MCP Gateway
+via streamable HTTP + Auth. Exposes an OpenAI compatible API.
+"""
+from .app import app
+from .agent_langchain import LangchainMCPAgent
+from .mcp_client import MCPClient
+from .config import get_settings
+
+__all__ = []