Building Agents Core

Building Agents - Core Concepts

Foundational knowledge for building goal-driven agents as Python packages.

Architecture: Python Services (Not JSON Configs)

Agents are built as Python packages:

exports/my_agent/
├── __init__.py          # Package exports
├── __main__.py          # CLI (run, info, validate, shell)
├── agent.py             # Graph construction (goal, edges, agent class)
├── nodes/__init__.py    # Node definitions (NodeSpec)
├── config.py            # Runtime config
└── README.md            # Documentation

Key Principle: Agent is visible and editable during build

• ✅ Files created immediately as components are approved
• ✅ User can watch files grow in their editor
• ✅ No session state - just direct file writes
• ✅ No "export" step - agent is ready when build completes

Core Concepts

Goal

Success criteria and constraints (written to agent.py)

goal = Goal(
    id="research-goal",
    name="Technical Research Agent",
    description="Research technical topics thoroughly",
    success_criteria=[
        SuccessCriterion(
            id="completeness",
            description="Cover all aspects of topic",
            metric="coverage_score",
            target=">=0.9",
            weight=0.4,
        ),
        # 3-5 success criteria total
    ],
    constraints=[
        Constraint(
            id="accuracy",
            description="All information must be verified",
            constraint_type="hard",
            category="quality",
        ),
        # 1-5 constraints total
    ],
)

Node

Unit of work (written to nodes/init.py)

Node Types:

• llm_generate - Text generation, parsing
• llm_tool_use - Actions requiring tools
• router - Conditional branching
• function - Deterministic operations

search_node = NodeSpec(
    id="search-web",
    name="Search Web",
    description="Search for information online",
    node_type="llm_tool_use",
    input_keys=["query"],
    output_keys=["search_results"],
    system_prompt="Search the web for: {query}",
    tools=["web_search"],
    max_retries=3,
)

Edge

Connection between nodes (written to agent.py)

Edge Conditions:

• on_success - Proceed if node succeeds
• on_failure - Handle errors
• always - Always proceed
• conditional - Based on expression

EdgeSpec(
    id="search-to-analyze",
    source="search-web",
    target="analyze-results",
    condition=EdgeCondition.ON_SUCCESS,
    priority=1,
)

Pause/Resume

Multi-turn conversations

• Pause nodes - Stop execution, wait for user input
• Resume entry points - Continue from pause with user's response

# Example pause/resume configuration
pause_nodes = ["request-clarification"]
entry_points = {
    "start": "analyze-request",
    "request-clarification_resume": "process-clarification"
}

Tool Discovery & Validation

CRITICAL: Before adding a node with tools, you MUST verify the tools exist.

Tools are provided by MCP servers. Never assume a tool exists - always discover dynamically.

Step 1: Register MCP Server (if not already done)

mcp__agent-builder__add_mcp_server(
    name="tools",
    transport="stdio",
    command="python",
    args='["mcp_server.py", "--stdio"]',
    cwd="../tools"
)

Step 2: Discover Available Tools

# List all tools from all registered servers
mcp__agent-builder__list_mcp_tools()

# Or list tools from a specific server
mcp__agent-builder__list_mcp_tools(server_name="tools")

This returns available tools with their descriptions and parameters:

{
  "success": true,
  "tools_by_server": {
    "tools": [
      {
        "name": "web_search",
        "description": "Search the web...",
        "parameters": ["query"]
      },
      {
        "name": "web_scrape",
        "description": "Scrape a URL...",
        "parameters": ["url"]
      }
    ]
  },
  "total_tools": 14
}

Step 3: Validate Before Adding Nodes

Before writing a node with tools=[...]:

1. Call list_mcp_tools() to get available tools
2. Check each tool in your node exists in the response
3. If a tool doesn't exist:
   • DO NOT proceed with the node
   • Inform the user: "The tool 'X' is not available. Available tools are: ..."
   • Ask if they want to use an alternative or proceed without the tool

Tool Validation Anti-Patterns

❌ Never assume a tool exists - always call list_mcp_tools() first ❌ Never write a node with unverified tools - validate before writing ❌ Never silently drop tools - if a tool doesn't exist, inform the user ❌ Never guess tool names - use exact names from discovery response

Example Validation Flow

# 1. User requests: "Add a node that searches the web"
# 2. Discover available tools
tools_response = mcp__agent-builder__list_mcp_tools()

# 3. Check if web_search exists
available = [t["name"] for tools in tools_response["tools_by_server"].values() for t in tools]
if "web_search" not in available:
    # Inform user and ask how to proceed
    print("❌ 'web_search' not available. Available tools:", available)
else:
    # Proceed with node creation
    # ...

Workflow Overview: Incremental File Construction

1. CREATE PACKAGE → mkdir + write skeletons
2. DEFINE GOAL → Write to agent.py + config.py
3. FOR EACH NODE:
   - Propose design
   - User approves
   - Write to nodes/__init__.py IMMEDIATELY ← FILE WRITTEN
   - (Optional) Validate with test_node ← MCP VALIDATION
   - User can open file and see it
4. CONNECT EDGES → Update agent.py ← FILE WRITTEN
   - (Optional) Validate with validate_graph ← MCP VALIDATION
5. FINALIZE → Write agent class to agent.py ← FILE WRITTEN
6. DONE - Agent ready at exports/my_agent/

Files written immediately. MCP tools optional for validation/testing bookkeeping.

The Key Difference

OLD (Bad):

MCP add_node → Session State → MCP add_node → Session State → ...
                                                                ↓
                                                     MCP export_graph
                                                                ↓
                                                       Files appear

NEW (Good):

Write node to file → (Optional: MCP test_node) → Write node to file → ...
       ↓                                               ↓
  File visible                                    File visible
  immediately                                     immediately

Bottom line: Use Write/Edit for construction, MCP for validation if needed.

When to Use This Skill

Use building-agents-core when:

• Starting a new agent project and need to understand fundamentals
• Need to understand agent architecture before building
• Want to validate tool availability before proceeding
• Learning about node types, edges, and graph execution

Next Steps:

• Ready to build? → Use building-agents-construction skill
• Need patterns and examples? → Use building-agents-patterns skill

MCP Tools for Validation

After writing files, optionally use MCP tools for validation:

test_node - Validate node configuration with mock inputs

mcp__agent-builder__test_node(
    node_id="search-web",
    test_input='{"query": "test query"}',
    mock_llm_response='{"results": "mock output"}'
)

validate_graph - Check graph structure

mcp__agent-builder__validate_graph()
# Returns: unreachable nodes, missing connections, etc.

create_session - Track session state for bookkeeping

mcp__agent-builder__create_session(session_name="my-build")

Key Point: Files are written FIRST. MCP tools are for validation only.

Related Skills

• building-agents-construction - Step-by-step building process
• building-agents-patterns - Best practices and examples
• agent-workflow - Complete workflow orchestrator
• testing-agent - Test and validate completed agents