PTC

Programmatic Tool Calling for Claude Code via MCP.

Claude Code on subscription plans lacks the Anthropic API's programmatic tool calling (PTC) feature, where Claude can write Python scripts that call multiple tools in a single execution. Without it, every tool invocation is a full model round-trip — intermediate results enter the context window, consuming tokens and adding latency.

PTC-MCP fixes this. It's an MCP server that exposes three tools:

• list_callable_tools — Returns a JSON list of all available tool names. Use this to discover what's callable before writing a script.
• inspect_tool — Returns the schema and description of a specific tool, including its outputSchema if the upstream server defines one.
• execute_program — Runs a Python script with MCP tools injected as async functions. Only stdout comes back. Intermediate tool results stay in the Python runtime and never enter the conversation.

How it works

flowchart TD
    A[Claude Code] -->|list_callable_tools| B[PTC-MCP Server]
    A -->|inspect_tool| B
    A -->|execute_program| B

    B --> C[Tool Registry]
    B --> D[Execution Engine]

    C -->|Connects at startup,<br/>applies allow/block filters| E[Downstream MCP Servers]
    D -->|Runs script with tools<br/>as async functions| C
    D -->|stdout only| A

At startup, PTC-MCP connects to your configured MCP servers as a client, discovers their tools, and makes them callable as mcp__<server>__<tool>() async functions inside scripts. Claude can call list_callable_tools to discover available tools, inspect_tool to understand a tool's schema, and then execute_program to run a script using those tools. Tool calls proxy to the real MCP servers, results stay local, and only print() output goes back.

Tools

list_callable_tools

Takes no arguments. Returns a JSON array of sorted namespaced tool names:

["mcp__financial_data__query_financials", "mcp__internal_apis__get_resource"]

inspect_tool

Takes a tool_name string. Returns the tool's schema, description, and outputSchema (if available):

{
  "name": "mcp__financial_data__query_financials",
  "description": "Query financial statements for a given ticker.",
  "inputSchema": { "type": "object", "properties": { "ticker": { "type": "string" } }, "required": ["ticker"] },
  "outputSchema": null,
  "note": "No output schema defined by the upstream server. Inspect the return value in your script."
}

Note: outputSchema is populated when the downstream MCP server defines one on its tools per the MCP tool output schema specification. Downstream servers that declare output schemas improve discoverability — Claude can understand return types before writing a script. Without one, inspect_tool returns null for outputSchema and suggests inspecting return values at runtime instead.

execute_program

Takes a code string. Runs the Python script with all registered tools available as async functions. Returns stdout prefixed with a status line.

Example

Claude decides comparing three tickers benefits from batched execution:

execute_program(code="""
tickers = ["AMZN", "MSFT", "GOOG"]
for t in tickers:
    data = await mcp__financial_data__query_financials(
        ticker=t, statement="income", period="quarter", limit=4
    )
    revenues = [q["revenue"] for q in data]
    trend = " → ".join(f"${r/1e9:.1f}B" for r in revenues)
    print(f"{t}: {trend}")
""")

Three tool calls happen inside the script. Claude sees only:

[Script executed successfully]
AMZN: $170.0B → $165.3B → $158.9B → $149.2B
MSFT: $65.6B → $62.0B → $59.1B → $56.5B
GOOG: $96.5B → $88.3B → $85.0B → $80.5B

Setup

Requires Python 3.11+.

uv venv && uv pip install -e ".[dev]"

Configuration

Create a config.yaml (or set PTC_MCP_CONFIG to point elsewhere):

servers:
  - name: financial-data
    transport: stdio
    command: node
    args: ["./financial-data-mcp/dist/index.js"]

  - name: internal-apis
    transport: sse
    url: "http://localhost:8080/mcp"

tools:
  block:
    - "mcp__internal_apis__delete_resource"

execution:
  timeout_seconds: 120
  max_output_bytes: 65536

• servers — MCP servers to bridge. Supports stdio and sse transports.
• tools.allow / tools.block — Whitelist or blacklist namespaced tool names (mutually exclusive). Omit both to allow everything.
• execution — Timeout and output size limits for execute_program.

The server starts fine with no config file or an empty servers list.

Running

# Directly
uv run python -m ptc_mcp

# Or via the installed entry point
ptc-mcp

The server communicates over stdio (JSON-RPC). Add it to your Claude Code MCP settings to use it.

Testing

uv run pytest tests/ -v

Tests include unit tests for config parsing, the execution engine, registry filtering/namespacing, and end-to-end integration tests that spin up a real mock MCP server.