v3-notes/provider-architecture.md
Version: 3.0.0
Impact: Breaking change - MountedProvider removed
The monolithic MountedProvider was split into two focused, composable components:
FastMCPProvider: Wraps a FastMCP server, exposing its components through the Provider interfaceTransformingProvider: Wraps any provider to apply namespace prefixes and tool renamesMountedProvider was doing two things:
Separating these concerns enables:
Wraps a FastMCP server to expose it through the Provider interface:
from fastmcp.server.providers import FastMCPProvider
sub_server = FastMCP("Sub")
@sub_server.tool
def greet(name: str) -> str:
return f"Hello, {name}!"
# Wrap as provider
provider = FastMCPProvider(sub_server)
main_server.add_provider(provider)
Wraps any provider to apply transformations:
# Apply namespace to all components
provider = FastMCPProvider(server).with_namespace("api")
# "my_tool" → "api_my_tool"
# "resource://data" → "resource://api/data"
# Rename specific tools (bypasses namespace)
provider = FastMCPProvider(server).with_transforms(
namespace="api",
tool_renames={"verbose_tool_name": "short"}
)
# "verbose_tool_name" → "short"
# "other_tool" → "api_other_tool"
Transformations compose via stacking:
provider = (
FastMCPProvider(server)
.with_namespace("inner")
.with_namespace("outer")
)
# "tool" → "outer_inner_tool"
FastMCP.mount() now creates a FastMCPProvider + TransformingProvider internally:
main.mount(sub, namespace="api")
# Equivalent to:
main.add_provider(
FastMCPProvider(sub).with_namespace("api")
)
# Before (2.x)
from fastmcp.server.providers import MountedProvider
provider = MountedProvider(server, prefix="api")
# After (3.x)
from fastmcp.server.providers import FastMCPProvider
provider = FastMCPProvider(server).with_namespace("api")
# Before (deprecated)
main.mount(sub, prefix="api")
# After
main.mount(sub, namespace="api")