mcp-endpoint-testing

Test MCP server endpoints across all transports — stdio (Python client), HTTP/StreamableHTTP (curl), SSE (curl with event-stream extractor). Transport detection, common failure patterns, and verificat

SKILL.md

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MCP Endpoint Testing

Test MCP server endpoints across all transport types. Use when onboarding a new server, diagnosing a connection failure, or verifying health during a fleet audit.

Transport Detection

Before testing, identify which transport the server uses:

Signal Transport
Config has command: + args: Stdio — test with Python client
Config has url: HTTP — test with curl
curl -v returns content-type: text/event-stream SSE — use SSE extractor pattern
curl -v returns content-type: application/json StreamableHTTP — plain JSON response
POST /mcp returns 404 but server process exists SSE (SSE does NOT respond to bare POST on /mcp)

Quick transport probe (single curl command)

curl -v -s \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","method":"initialize","id":1,"params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"probe","version":"1.0"}}}' \
  http://localhost:PORT/mcp 2>&1 | grep -E "(content-type:|event:|HTTP/)"
  • HTTP/1.1 200 OK + content-type: application/json → StreamableHTTP
  • HTTP/1.1 200 OK + content-type: text/event-stream + event: message → SSE
  • HTTP/1.1 404 → likely SSE on a different path or port
  • Connection refused / timeout → server process is DOWN

Testing Stdio MCP Servers (Python)

For stdio-based servers (command + args in config.yaml):

import asyncio
from mcp import StdioServerParameters
from mcp.client.stdio import stdio_client
from mcp import ClientSession

async def test_server():
    params = StdioServerParameters(
        command="python",
        args=["-m", "app.main"],
    )
    async with stdio_client(params) as (read, write):
        async with ClientSession(read, write) as session:
            # Initialize
            await session.initialize()
            # List tools
            result = await session.list_tools()
            print(f"Tools ({len(result.tools)}):")
            for t in result.tools:
                print(f"  {t.name}: {t.description[:80]}")
            # Call a tool
            health = await session.call_tool("health_check", {})
            print("Health:", health.content[:300])

asyncio.run(test_server())

Pre-warming for uvx/npx servers

First-run uvx/npx servers download 70MB+ of dependencies and may exceed connect_timeout. Pre-warm before testing:

uvx server-name --help 2>/dev/null || npx -y server-name --help 2>/dev/null

Testing HTTP/StreamableHTTP MCP Servers (curl)

For servers that return content-type: application/json:

# Initialize
curl -s http://localhost:PORT/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","method":"initialize","id":1,"params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}'

# List tools
curl -s http://localhost:PORT/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","method":"tools/list","id":2,"params":{}}}'

# Call a tool
curl -s http://localhost:PORT/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","method":"tools/call","id":3,"params":{"name":"tool_name","arguments":{}}}'

Key signals: initialize returns protocolVersion + serverInfo. tools/list returns tool array without error. tools/call returns content.

Testing SSE MCP Servers (curl with event-stream extractor)

SSE servers return content-type: text/event-stream and wrap JSON-RPC in:

event: message
data: {"jsonrpc":"2.0","id":1,"result":{...}}

Piping raw SSE output to json.loads() fails because the event: / data: lines are not valid JSON. Always extract the JSON payload with grep "^data: " | sed 's/^data: //':

Initialize + detect transport

curl -v -s \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","method":"initialize","id":1,"params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}' \
  http://localhost:8888/mcp 2>&1 | grep -E "(content-type:|event:)"

Expected SSE output: content-type: text/event-stream + event: message

List tools from an SSE MCP server

response=$(curl -s \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","method":"tools/list","id":2,"params":{}}' \
  http://localhost:8888/mcp)
echo "$response" | grep "^data: " | sed 's/^data: //' | python -c "
import json, sys
data = json.load(sys.stdin)
tools = data.get('result', {}).get('tools', [])
print(f'Tools count: {len(tools)}')
for t in tools:
    print(f'  - {t[\"name\"]}: {t.get(\"description\", \"\")[:80]}')
"

Call a tool on an SSE MCP server

response=$(curl -s \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","method":"tools/call","id":3,"params":{"name":"tool_name","arguments":{}}}' \
  http://localhost:8888/mcp)
echo "$response" | grep "^data: " | sed 's/^data: //' | python -c "
import json, sys
data = json.load(sys.stdin)
content = data.get('result', {}).get('content', [])
for c in content:
    print(c.get('text', str(c))[:500])
"

Key signals: grep "^data: " | sed 's/^data: //' extracts valid JSON. tools/list returns expected tools. tools/call returns meaningful content.

SSE session flow (for manual testing)

SSE servers use a two-step connection pattern:

  1. GET /mcp (SSE stream) → receive session_id from initial event
  2. POST /messages?session_id=<id> for each JSON-RPC request

Some SSE servers (like Open Coscientist) accept POST directly on /mcp and return SSE responses inline — these are hybrid implementations. Detect by testing POST directly first.

Testing WebSocket / BiDi Endpoints (Firefox remote agent, browser bridges)

Some servers expose a raw WebSocket protocol instead of HTTP JSON-RPC — notably Firefox's WebDriver BiDi remote agent (started with --remote-debugging-port) and MCP bridge servers that connect to it. Standard HTTP MCP probes do NOT apply; the liveness probe is a WebSocket upgrade handshake.

Identification signals (Firefox 129+, CDP removed)

Probe Response Meaning
GET / 200 HTML "httpd.js is up and serving requests!" BiDi remote agent ALIVE (built on Mozilla's httpd.js — this is NOT a ghost/foreign server)
GET /json/version 404 CDP endpoints removed in Firefox 129 — any CDP-based detection/readiness loop is dead
GET /session (no WS upgrade) 400 endpoint present; needs a real WebSocket handshake
ws://host:PORT/session upgrade 101 definitive BiDi-alive signal

BiDi liveness probe (Python)

import asyncio, websockets
async def bidi_alive(port=9239):
    try:
        async with websockets.connect(f'ws://127.0.0.1:{port}/session', open_timeout=8):
            return True
    except Exception:
        return False

⚠️ open_timeout, not timeout — websockets 14.0 renamed the open-handshake kwarg timeout → open_timeout; 15.0 removed the old name. Passing timeout= sweeps it into **kwargs → loop.create_connection(factory, **kwargs) → TypeError: BaseEventLoop.create_connection() got an unexpected keyword argument 'timeout'. When that call sits in try/except Exception: return False, detection fails silently forever ("Cannot detect Firefox on BiDi:PORT or CDP:PORT" while Firefox listens on the exact configured port). Always probe directly before trusting a negative detection result.

Post-connect failure modes

  • 'NoneType' object is not iterable in list/flatten — Firefox's browsingContext.getTree returns "children": null (key present, null value) for leaf contexts; ctx.get("children", []) returns None (default only applies to a MISSING key) → crash. Use (ctx.get("children") or []).
  • session.new → "Maximum number of active sessions" — one active BiDi session per browser process; hard-killing the client (taskkill /F) leaves the slot held. Reconnecting the client is NOT enough — relaunch a fresh browser (its launcher's kill_orphans clears the port first).
  • Kill-and-respawn hot-load — Hermes' MCP client auto-reconnects a killed stdio server (exponential backoff): patch the server source on disk, taskkill /F /PID <server>, and the respawned instance loads the fix — no Hermes restart. Also purges duplicate instances (venv python + hermes-runtime python can both host copies of the same server).

Full case study (ultimate-firefox-mcp bridge, Aug 2026): references/websocket-bidi-endpoint-probing.md

Diagnosing Tool Calls That Hang or Kill the Server

Symptom signature: initialize + list_tools succeed, a specific tool call then hangs forever or the server process dies mid-call (McpError: Connection closed), but the SAME code completes in seconds when run standalone. This is the classic FastMCP worker-thread context failure: mcp.server.fastmcp runs sync tool functions via anyio.to_thread, and heavy lazy-import libraries (OpenBB, pandas, numba) are not thread-safe on first use (extension auto-build / type generation deadlocks or hard-crashes the process).

Isolation ladder (cheap → expensive)

  1. In-process direct call — import the server module and call the tool function in the MAIN thread, no MCP round-trip:
    import importlib.util
spec = importlib.util.spec_from_file_location("srv", r"E:/path/to/server.py")
m = importlib.util.module_from_spec(spec); spec.loader.exec_module(m)
print(m.market_summary()[:400])   # works fast → bug is MCP/thread context, not the data layer
    • Returns JSON quickly → the data layer is fine; the hang lives in the FastMCP worker thread.
    • Hangs/errors → the tool body itself is at fault; debug the data call chain directly.
  2. Distinguish tool-vs-tool: if a DIFFERENT tool using the same stack also hangs, it's the shared context/library, not that tool's logic.
  3. Clean-env Windows note: when testing with env -i, Windows Python needs USERPROFILE (plus HOMEDRIVE/HOMEPATH/APPDATA/LOCALAPPDATA/SYSTEMROOT) — HOME alone is not enough. Omitting them yields a misleading "Could not determine home directory." error or a hang in libraries that resolve config dirs.
  4. Workarounds, in order: pre-warm the lazy library at module import (main thread) before mcp.run(); wrap the tool body in a dedicated threading.Thread with a join timeout; pin fastmcp/mcp to a known-good combo; escalate a confirmed library thread-safety defect with a repro.

Full worked example (OpenBB trading-signals: provider default-routing hang + column-oriented to_dict() extraction bug + worker-thread hang): see references/fastmcp-thread-context-hang.md.

Verification Checklist

  • Transport correctly identified (stdio vs StreamableHTTP vs SSE)
  • initialize returns protocolVersion + serverInfo without error
  • tools/list returns expected tools (count and names match docs)
  • At least one tool call returns meaningful content (not an error)
  • Server process survives multiple sequential tool calls
  • Server process exits cleanly on disconnect
  • For SSE: JSON payload successfully extracted with grep+sed extractor
  • For stdio: Python test script runs without import errors

Pitfalls

  • SSE raw pipe to json.loads() fails — always use grep "^data: " | sed 's/^data: //' to extract JSON from SSE framing before piping to Python.
  • uvx/npx first-run timeout — pre-warm with --help before testing or increase connect_timeout to 120s.
  • Same Accept header for both transports — both StreamableHTTP and SSE require Accept: application/json, text/event-stream. The response content-type tells them apart.
  • POST on /mcp returns 404 for SSE servers — some SSE servers require POST on a separate /messages?session_id=<id> endpoint. Check raw response headers first before assuming the server is dead.
  • Connection refused vs 404 — connection refused means no process is listening on the port. A 404 means the process IS running but the path is wrong. Very different fixes.
  • Stdio servers inherit filtered env — only PATH, HOME, USER, LANG, LC_ALL, TERM, SHELL, TMPDIR + XDG_* are passed. If your test works in terminal but Hermes fails, missing env vars are the most likely cause. Pass them explicitly under env:.
  • Windows which lies about .exe files — Use command -v <binary> (bash built-in) not which <binary>.
  • env -i clean tests on Windows miss USERPROFILE — Windows Python resolves the home dir from USERPROFILE, not HOME. A clean-env test (env -i PATH=... HOME=...) fails with "Could not determine home directory." or hangs in libs that resolve config dirs. Pass USERPROFILE/HOMEDRIVE/HOMEPATH/APPDATA/LOCALAPPDATA/SYSTEMROOT explicitly.

Related Skills

  • native-mcp — MCP client config reference, transport types, Hermes integration
  • mcp-fleet-audit — Fleet-wide health checks, dormant server discovery
  • mcp-server-onboarding — Full workflow for finding, fixing, and wiring servers
  • building-mcp-servers — Building MCP servers from scratch
  • mcp-server-wiring — Wiring existing Python modules as MCP tools

References

  • Open Coscientist SSE Discovery — Real-world case study: identifying SSE transport on a running MCP server, troubleshooting the raw pipe to json.loads() failure, and the grep+sed extractor pattern.
  • FastMCP Thread-Context Hang — Real-world case study: OpenBB trading-signals server whose tools hang/die over MCP stdio but work in-process; isolation technique, two OpenBB SDK quirks (provider default routing, column-oriented to_dict()), and the Windows clean-env pitfall.
  • WebSocket/BiDi Endpoint Probing — Real-world case study: ultimate-firefox-mcp bridge's week-long "Cannot detect Firefox" — websockets 15 timeout→open_timeout API break swallowed by except Exception, children: null flatten crash, BiDi session-slot exhaustion, and the kill-and-respawn hot-load pattern.

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