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Tools let agents call your Python functions. Decorate, register, serve. Tools are the core building blocks that allow an MCP client to invoke your Python functions via the MCP protocol:
  1. A client discovers tools via tools/list (each tool includes inputSchema and optional outputSchema).
  2. A client calls a tool via tools/call with arguments matching the schema.
  3. The server executes your callable.
  4. The server returns a CallToolResult containing content (and optionally structuredContent) per MCP Spec.

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Basic tool

from dedalus_mcp import MCPServer, tool

@tool(description="Add two numbers")
def add(a: int, b: int) -> int:
    return a + b

server = MCPServer("math")
server.collect(add)
The description tells the LLM what the tool does. Type hints become JSON Schema.

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Async tools

import anyio
from dedalus_mcp import tool

@tool(description="Fetch user data (simulated I/O)")
async def get_user(user_id: str) -> dict:
    await anyio.sleep(0.1)
    return {"user_id": user_id, "status": "ok"}
Prefer async for I/O. Important: in Dedalus MCP, sync tools run inline (they are not automatically moved to a thread pool). If you need concurrency for blocking work, use async def and offload explicitly.

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Type inference

Type hints become JSON Schema automatically: 
from typing import Literal
from pydantic import BaseModel
from dedalus_mcp import tool

class SearchFilters(BaseModel):
    category: str | None = None
    min_price: float = 0.0

@tool(description="Search products")
def search(
    query: str,
    limit: int = 10,
    sort: Literal["relevance", "price", "date"] = "relevance",
    filters: SearchFilters | None = None,
) -> list[dict]:
    return [{"query": query, "limit": limit, "sort": sort, "filters": filters.model_dump() if filters else None}]
Supported: primitives, list, dict, Literal, Enum, optionals/unions, Pydantic models, dataclasses, nested models. Required parameters have no default. Optional parameters have one.

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Decorator options

from dedalus_mcp import tool

@tool(
    name="find_products",           # Override tool name
    description="Search catalog",   # Tool description
    tags={"search", "catalog"},     # For filtering/metadata
)
def search_products_impl(query: str) -> list[dict]:
    return [{"id": "p_1", "name": "Widget", "query": query}]

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Structured returns

Return JSON-serializable values: 
from dedalus_mcp import tool

@tool(description="Analyze text")
def analyze(text: str) -> dict:
    return {"word_count": len(text.split()), "char_count": len(text)}
For explicit control, return CallToolResult: 
from dedalus_mcp import tool
from dedalus_mcp.types import CallToolResult, TextContent

@tool(description="Custom result")
def custom() -> CallToolResult:
    return CallToolResult(
        content=[TextContent(type="text", text="Custom message")],
        isError=False,
    )

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Context access

Logging and progress via get_context(): 
import anyio
from dedalus_mcp import tool, get_context

@tool(description="Process files with progress reporting")
async def process_files(paths: list[str]) -> dict:
    ctx = get_context()
    await ctx.info("Starting", data={"count": len(paths)})

    processed = 0
    try:
        async with ctx.progress(total=len(paths)) as tracker:
            for path in paths:
                # Simulate work; cancellation is delivered as task cancellation
                await anyio.sleep(0.01)
                processed += 1
                await tracker.advance(1)
    except anyio.get_cancelled_exc_class():
        await ctx.warning("Cancelled", data={"processed": processed})
        raise

    return {"processed": processed}

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Allow-lists

Restrict visible tools: 
from dedalus_mcp import MCPServer, tool

@tool(description="Add")
def add(a: int, b: int) -> int:
    return a + b

@tool(description="Multiply")
def multiply(a: int, b: int) -> int:
    return a * b

server = MCPServer("gated")
server.collect(add, multiply)
server.allow_tools({"add"})
Calling a hidden tool returns an error CallToolResult indicating the tool is not available.

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Error handling

Raise exceptions normally: 
from dedalus_mcp import tool

@tool(description="Divide")
def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

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Testing

Test tools as normal functions: 
def test_add():
    assert add(2, 3) == 5
For tools using context, test the core logic separately (or use an integration-style harness).
Last modified on January 27, 2026