AgenticWire test record: fastmcp-vs-mcp-python-sdk
Article: https://www.agenticwire.news/article/fastmcp-vs-mcp-python-sdk
Machine and versions: see results.json (machine field).
results.json is the October 5, 2026 run. results-2026-08.json is the original August run, kept unchanged
so readers can see what moved (FastMCP 3.4.5 bundling mcp 1.29.0 then; 4.0.11 bundling mcp 2.3.0 now).

Harness script (experiment/run.sh):

#!/usr/bin/env bash
# fastmcp vs the official MCP Python SDK: what a plain install actually gives you.
#
# Self-contained: builds its own venvs, assumes only python3 + uv on PATH.
# Emits one {"metric","value",...} JSON line per measurement on stdout; narration goes to stderr.
#
# Re-run:  node blog-app/scripts/pipeline/bench.mjs drafts/fastmcp-vs-mcp-python-sdk
set -uo pipefail
cd "$(dirname "$0")"
log() { echo "$@" >&2; }
emit() { echo "$1"; }

WORK="${TMPDIR:-/tmp}/fastmcp-bench-$$"
mkdir -p "$WORK"
trap 'rm -rf "$WORK"' EXIT

log "building venvs in $WORK"
uv venv --python 3.13 "$WORK/fastmcp" >&2 2>/dev/null
uv venv --python 3.13 "$WORK/sdk" >&2 2>/dev/null
VIRTUAL_ENV="$WORK/fastmcp" uv pip install fastmcp >&2 2>/dev/null
VIRTUAL_ENV="$WORK/sdk" uv pip install "mcp[cli]" >&2 2>/dev/null

PY_F="$WORK/fastmcp/bin/python"
PY_S="$WORK/sdk/bin/python"
SP_F="$WORK/fastmcp/lib/python3.13/site-packages"
SP_S="$WORK/sdk/lib/python3.13/site-packages"

# --- A1: what a plain install resolves to (no --prerelease flag anywhere above) ---
V_FASTMCP=$($PY_F -c "import importlib.metadata as m; print(m.version('fastmcp'))")
V_MCP=$($PY_S -c "import importlib.metadata as m; print(m.version('mcp'))")
emit "{\"metric\":\"fastmcp_version_resolved\",\"value\":\"$V_FASTMCP\",\"note\":\"uv pip install fastmcp, no flags\"}"
emit "{\"metric\":\"mcp_sdk_version_resolved\",\"value\":\"$V_MCP\",\"note\":\"uv pip install 'mcp[cli]', no prerelease flag\"}"

# --- A2: transitive pin — which mcp does fastmcp actually bundle? ---
V_BUNDLED=$($PY_F -c "import importlib.metadata as m; print(m.version('mcp'))")
emit "{\"metric\":\"mcp_version_bundled_by_fastmcp\",\"value\":\"$V_BUNDLED\",\"note\":\"transitive dep inside the fastmcp venv\"}"

# --- A3: API surface — does the widely-documented import path still exist? ---
if $PY_S -c "import mcp.server.fastmcp" 2>/dev/null; then R=true; else R=false; fi
emit "{\"metric\":\"sdk_mcp_server_fastmcp_importable\",\"value\":$R,\"note\":\"from mcp.server.fastmcp import FastMCP\"}"
if $PY_S -c "from mcp.server import MCPServer" 2>/dev/null; then R=true; else R=false; fi
emit "{\"metric\":\"sdk_mcpserver_importable\",\"value\":$R,\"note\":\"from mcp.server import MCPServer\"}"
if $PY_F -c "import mcp.server.fastmcp" 2>/dev/null; then R=true; else R=false; fi
emit "{\"metric\":\"fastmcp_venv_v1_path_importable\",\"value\":$R,\"note\":\"old path still alive under fastmcp\"}"

# --- A4: error behaviour on the obvious mistake (bare vs called decorator) ---
$PY_S - >/dev/null 2>&1 <<'PY'
from mcp.server.mcpserver import MCPServer
m = MCPServer("b")
@m.tool
def f(a: int) -> int:
    "doc"
    return a
PY
[ $? -ne 0 ] && R=true || R=false
emit "{\"metric\":\"sdk_rejects_bare_tool_decorator\",\"value\":$R,\"note\":\"@mcp.tool without parens raises TypeError\"}"
$PY_F - >/dev/null 2>&1 <<'PY'
from fastmcp import FastMCP
m = FastMCP("b")
@m.tool
def f(a: int) -> int:
    "doc"
    return a
@m.tool()
def g(a: int) -> int:
    "doc"
    return a
PY
[ $? -eq 0 ] && R=true || R=false
emit "{\"metric\":\"fastmcp_accepts_both_decorator_forms\",\"value\":$R,\"note\":\"bare and called both work\"}"

# --- A5: install footprint ---
emit "{\"metric\":\"fastmcp_site_packages_mb\",\"value\":$(du -sm "$SP_F" | cut -f1),\"unit\":\"MB\"}"
emit "{\"metric\":\"sdk_site_packages_mb\",\"value\":$(du -sm "$SP_S" | cut -f1),\"unit\":\"MB\"}"
emit "{\"metric\":\"fastmcp_package_count\",\"value\":$(ls -1 "$SP_F" | grep -c 'dist-info$')}"
emit "{\"metric\":\"sdk_package_count\",\"value\":$(ls -1 "$SP_S" | grep -c 'dist-info$')}"

# --- A9: cold import, median of 15, interpreter startup excluded ---
bench() {
  "$1" - "$2" <<'PY'
import sys, time, statistics, subprocess
py, mod = sys.executable, sys.argv[1]
code = f"import time,importlib; t=time.perf_counter(); importlib.import_module('{mod}'); print((time.perf_counter()-t)*1000)"
xs = []
for _ in range(15):
    r = subprocess.run([py, "-c", code], capture_output=True, text=True)
    if r.returncode == 0 and r.stdout.strip():
        xs.append(float(r.stdout))
print(round(statistics.median(xs), 1) if xs else "null")
PY
}
emit "{\"metric\":\"fastmcp_cold_import_median\",\"value\":$(bench "$PY_F" fastmcp),\"unit\":\"ms\",\"n\":15}"
emit "{\"metric\":\"sdk_cold_import_median\",\"value\":$(bench "$PY_S" mcp.server.mcpserver),\"unit\":\"ms\",\"n\":15}"

# --- A7: equivalent 3-tool server, non-blank LOC ---
emit "{\"metric\":\"fastmcp_server_loc\",\"value\":$(grep -cve '^[[:space:]]*$' srv_fastmcp.py),\"note\":\"3-tool server\"}"
emit "{\"metric\":\"sdk_server_loc\",\"value\":$(grep -cve '^[[:space:]]*$' srv_sdk.py),\"note\":\"3-tool server\"}"

# both servers must actually register their tools, or the LOC comparison is meaningless
CNT=$(cd "$(pwd)" && $PY_F -c "
import asyncio,sys; sys.path.insert(0,'.')
from srv_fastmcp import mcp; print(len(asyncio.run(mcp.list_tools())))" 2>/dev/null || echo 0)
emit "{\"metric\":\"fastmcp_tools_registered\",\"value\":$CNT}"
CNT=$($PY_S -c "
import asyncio,sys; sys.path.insert(0,'.')
from srv_sdk import mcp; print(len(asyncio.run(mcp.list_tools())))" 2>/dev/null || echo 0)
emit "{\"metric\":\"sdk_tools_registered\",\"value\":$CNT}"
log "done"