Field Test

Context

Unit tests (add-test skill) verify handler logic with mocked context. Field testing exercises the real HTTP transport with real JSON-RPC: starts the server, calls initialize, surfaces the catalog, runs inputs, and checks what a client actually sees. It catches what unit tests miss — awkward input shapes, unhelpful errors, missing format output, drift between structuredContent and content[], edge-case surprises.

Actively call the tools. Don't read code and guess.

Transport coverage

This skill drives an HTTP server because curl + JSON-RPC is the most reliable harness for shell-based agents. The same handlers run on both transports — only the framing differs — so HTTP exercises the full functional surface. Both HTTP session modes are covered: a durable Mcp-Session-Id session, and the sessionless initialization a MCP_SESSION_MODE=stateless server performs.

Stdio coverage is a boot check only — run this before Step 1. Run bun run rebuild && bun run start:stdio < /dev/null, and confirm the startup logs look clean (banner, expected tool/resource counts, no errors/warnings, no missing-config gripes). Redirecting stdin is what ends the run: the server treats EOF as a shutdown signal, boots fully, then exits on its own, so the log also shows the graceful-shutdown path. Do not background it and reach for pkill — a pattern like pkill -f dist/index.js matches every other stdio MCP server on the machine, including the ones the calling agent's own session is connected to. Pino logs go to stderr in stdio mode (stdout is reserved for JSON-RPC), so they print straight to the terminal when you run interactively. No need to call tools over stdio — the HTTP pass already covered handler behavior.

Steps

1. Start the server

Generate a 10-character alphanumeric ID (e.g. 9DJ73-K103L) and write the helper to /tmp/<project-name>-field-test-<ID>.sh. Use that exact path in every subsequent Bash call. Two agents in the same project tree must pick different IDs — that's what keeps their helper files, server logs, and call scratch from colliding.

The helper itself is stateless — every function takes the IDs it needs (server pid, url, port, MCP sid, server log path) as positional args. mcp_start prints them; the agent threads them through every later call. No env vars, no shared state files.

# Pick your ID — example below uses 9DJ73-K103L. Substitute your own.
# (Helper path also encodes the project name so /tmp/ stays grep-friendly.)
cat > /tmp/<project-name>-field-test-9DJ73-K103L.sh <<'HELPER_EOF'
#!/bin/bash
# Field-test helper: stateless wrappers around an MCP HTTP server + JSON-RPC
# session. Every function takes the IDs it needs as positional args — the agent
# threads pid/url/port/sid/log through each call rather than relying on a state
# file or env vars (the Bash tool wipes shell state between calls, and a
# pointer file would race the same way two agents race on shared state).
# See https://github.com/cyanheads/mcp-ts-core/issues/90, #144.
#
# Surfaces failures aggressively — field test is for finding things that fail,
# so the helper auto-tails logs and prints HTTP status/body on errors instead
# of swallowing them. It also measures: every mcp_call prints a one-line
# size/latency reading on stderr, and mcp_catalog_size weighs tools/list.

# Usage: mcp_start /path/to/server [startup-timeout-seconds]   (default: 30)
# Builds, starts the HTTP server in the background, waits for the listen line,
# and prints: ready pid=<n> url=<u> port=<n> log=<path>
# Capture these — every later helper takes them as args. Raise the timeout for
# servers that build a local index at boot.
mcp_start() {
  local dir="${1:-$PWD}"
  local timeout="${2:-30}"
  local build_log; build_log=$(mktemp /tmp/mcp-field-test-build.XXXXXX)
  echo "building $dir ..." >&2
  if ! (cd "$dir" && bun run rebuild) >"$build_log" 2>&1; then
    echo "BUILD FAILED — last 30 lines of $build_log:" >&2
    tail -30 "$build_log" >&2
    return 1
  fi
  rm -f "$build_log"
  local server_log; server_log=$(mktemp /tmp/mcp-field-test-server.XXXXXX)
  echo "starting server ..." >&2
  (cd "$dir" && bun run start:http) >"$server_log" 2>&1 &
  local pid=$!
  local line=""
  local waited=0
  while [ "$waited" -lt "$((timeout * 4))" ]; do
    line=$(grep -Eo 'listening at http://[^" ]+/mcp' "$server_log" | head -1)
    [ -n "$line" ] && break
    if ! kill -0 "$pid" 2>/dev/null; then
      echo "server exited during startup — last 30 lines of $server_log:" >&2
      tail -30 "$server_log" >&2
      rm -f "$server_log"
      return 1
    fi
    sleep 0.25
    waited=$((waited + 1))
  done
  if [ -z "$line" ]; then
    echo "server failed to start within ${timeout}s — last 30 lines of $server_log:" >&2
    tail -30 "$server_log" >&2
    kill "$pid" 2>/dev/null
    rm -f "$server_log"
    return 1
  fi
  local url="${line#listening at }"
  local port; port=$(echo "$url" | sed -E 's|.*:([0-9]+)/.*|\1|')
  echo "ready pid=$pid url=$url port=$port log=$server_log"
}

# Internal: report a failed initialize with the raw exchange, then clean up.
_mcp_init_fail() {
  local msg="$1"; local body_file="$2"; local hdr="$3"
  echo "init failed — $msg" >&2
  echo "--- response body ---" >&2
  if [ -s "$body_file" ]; then cat "$body_file" >&2; else echo "(empty)" >&2; fi
  echo "--- response headers ---" >&2
  if [ -s "$hdr" ]; then cat "$hdr" >&2; else echo "(none)" >&2; fi
  rm -f "$hdr" "$body_file"
  return 1
}

# Usage: mcp_init <url>
# Runs `initialize`, sends `notifications/initialized`, prints:
#   ready sid=<id-or-empty> protocol=<negotiated-version> requested=<want> instructions=<bytes>B (HTTP <code>)
# `instructions=` is the byte size of the server's `instructions` string — it
# loads into every client session alongside tools/list, so it is the other half
# of the per-session context tax mcp_catalog_size weighs.
# The initialize *result* is what decides success — a session ID is optional.
# A server started with MCP_SESSION_MODE=stateless mints none, and the session
# header is then omitted from every later request. Capture BOTH `sid` and
# `protocol`: mcp_call takes the protocol as its 5th arg, which is what carries
# the negotiated revision when there is no session to carry it.
# A negotiated version older than the requested one means the server capped it
# — note that in the report; you are then testing an older protocol than a
# current client would use.
mcp_init() {
  local url="$1"
  [ -z "$url" ] && { echo "usage: mcp_init <url>" >&2; return 1; }
  local want="${MCP_FIELD_TEST_PROTOCOL:-2025-11-25}"
  local hdr; hdr=$(mktemp)
  local body_file; body_file=$(mktemp)
  local code curl_rc
  code=$(curl -sS -D "$hdr" -o "$body_file" -w '%{http_code}' -X POST "$url" \
    -H "Content-Type: application/json" \
    -H "Accept: application/json, text/event-stream" \
    -d "{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\",\"params\":{\"protocolVersion\":\"$want\",\"capabilities\":{},\"clientInfo\":{\"name\":\"field-test\",\"version\":\"1.0.0\"}}}")
  curl_rc=$?
  if [ "$curl_rc" -ne 0 ] || [ -z "$code" ] || [ "$code" = "000" ]; then
    _mcp_init_fail "transport failure — curl exit $curl_rc, http_code '${code:-none}'; nothing listening at $url" "$body_file" "$hdr"
    return 1
  fi
  [ "$code" -ge 400 ] && { _mcp_init_fail "HTTP $code" "$body_file" "$hdr"; return 1; }
  # Unwrap SSE framing when present; a plain JSON body is used as-is.
  local payload; payload=$(sed -n 's/^data: //p' "$body_file")
  [ -z "$payload" ] && payload=$(cat "$body_file")
  # Pick the reply frame by structure, not by substring: a server that logs to the
  # client emits `notifications/message` frames first, and a `"level":"error"` or a
  # log string containing `result` matches a text grep and gets read as the reply.
  local reply; reply=$(printf '%s\n' "$payload" | jq -c 'select(type=="object" and (has("result") or has("error")))' 2>/dev/null | tail -1)
  [ -z "$reply" ] && reply="$payload"
  if printf '%s' "$reply" | grep -q '"error"'; then
    _mcp_init_fail "server returned a JSON-RPC error" "$body_file" "$hdr"
    return 1
  fi
  if ! printf '%s' "$reply" | grep -q '"result"'; then
    _mcp_init_fail "HTTP $code but no JSON-RPC result in the body" "$body_file" "$hdr"
    return 1
  fi
  local got; got=$(printf '%s' "$reply" | grep -o '"protocolVersion":"[^"]*"' | head -1 | cut -d'"' -f4)
  if [ -z "$got" ]; then
    _mcp_init_fail "initialize result declares no protocolVersion" "$body_file" "$hdr"
    return 1
  fi
  local instr; instr=$(printf '%s' "$reply" | jq -r '.result.instructions // "" | utf8bytelength' 2>/dev/null || echo 0)
  local sid; sid=$(grep -i '^mcp-session-id:' "$hdr" | awk '{print $2}' | tr -d '\r\n')
  local init_headers=(-H "Content-Type: application/json" -H "Accept: application/json, text/event-stream" -H "MCP-Protocol-Version: $got")
  [ -n "$sid" ] && init_headers+=(-H "Mcp-Session-Id: $sid")
  curl -sS -X POST "$url" "${init_headers[@]}" \
    -d '{"jsonrpc":"2.0","method":"notifications/initialized"}' >/dev/null
  rm -f "$hdr" "$body_file"
  echo "ready sid=$sid protocol=$got requested=$want instructions=${instr}B (HTTP $code)"
}

# Internal: one stderr line per call — reply bytes, the content/structured
# split, a token estimate, wall-clock. Bytes are the reply as delivered (SSE
# framing stripped). `content` is every text block's bytes, `structured` is
# structuredContent serialized. The token figure is bytes/4 — an estimate, not
# a tokenizer. Wall-clock is curl's time_total for the whole exchange.
_mcp_measure() {
  local method="$1"; local params="$2"; local reply="$3"; local code="$4"; local secs="$5"
  local total; total=$(printf '%s' "$reply" | wc -c | tr -d ' ')
  local ms; ms=$(awk -v s="$secs" 'BEGIN { printf "%d", s * 1000 }')
  local label="$method"
  local split=""
  case "$method" in
    tools/call)
      local name; name=$(printf '%s' "$params" | jq -r '.name // empty' 2>/dev/null)
      [ -n "$name" ] && label="$method $name"
      split=$(printf '%s' "$reply" | jq -r '
        (.result // {}) as $r
        | ([$r.content[]? | select(.type == "text") | .text] | join("") | utf8bytelength) as $c
        | (if $r.structuredContent == null then "none" else ($r.structuredContent | tojson | utf8bytelength | tostring) end) as $s
        | "content \($c) · structured \($s)"' 2>/dev/null)
      ;;
    resources/read)
      split=$(printf '%s' "$reply" | jq -r '
        "text \([.result.contents[]? | .text // ""] | join("") | utf8bytelength)"' 2>/dev/null)
      ;;
  esac
  local tok; tok=$(awk -v b="$total" 'BEGIN { if (b >= 1000) printf "~%.1fk", b / 4000; else printf "~%d", b / 4 }')
  echo "⏱ $label · HTTP $code · ${total} B${split:+ ($split)} · $tok tok · ${ms} ms" >&2
}

# Usage: mcp_call <url> <sid> <method> [JSON_PARAMS] [protocol]
# Prints the JSON-RPC response. SSE framing is stripped when present, and only
# the reply is emitted (a single POST can also carry progress notifications, so
# emitting every event would break `| jq .result`). A transport failure or an
# HTTP >= 400 prints the details and returns non-zero — it never returns 0 with
# empty output. Pipe to `jq`.
# Every call also prints one measurement line on stderr, e.g.
#   ⏱ tools/call gbif_search_species · HTTP 200 · 18412 B (content 9100 · structured 8900) · ~4.6k tok · 812 ms
# Read it on every call — it is the size/latency evidence the report cites.
# `sid` may be empty ('') for a stateless server; the session header is then
# omitted. Pass the `protocol` mcp_init printed as the 5th arg — with no
# session carrying the negotiation, MCP-Protocol-Version is what tells the
# server which revision the request speaks.
mcp_call() {
  local url="$1"; local sid="$2"; local method="$3"; local params="${4:-}"; local protocol="${5:-}"
  [ -z "$url" ] || [ -z "$method" ] && { echo "usage: mcp_call <url> <sid> <method> [params] [protocol]" >&2; return 1; }
  local body
  if [ -z "$params" ]; then
    body=$(printf '{"jsonrpc":"2.0","id":%d,"method":"%s"}' "$RANDOM" "$method")
  else
    body=$(printf '{"jsonrpc":"2.0","id":%d,"method":"%s","params":%s}' "$RANDOM" "$method" "$params")
  fi
  local resp_file; resp_file=$(mktemp)
  local stats code secs curl_rc
  local headers=(-H "Content-Type: application/json" -H "Accept: application/json, text/event-stream")
  [ -n "$sid" ] && headers+=(-H "Mcp-Session-Id: $sid")
  [ -n "$protocol" ] && headers+=(-H "MCP-Protocol-Version: $protocol")
  stats=$(curl -sS -o "$resp_file" -w '%{http_code} %{time_total}' -X POST "$url" "${headers[@]}" -d "$body")
  curl_rc=$?
  read -r code secs <<< "$stats"
  if [ "$curl_rc" -ne 0 ] || [ -z "$code" ] || [ "$code" = "000" ]; then
    echo "TRANSPORT FAILURE calling $method — curl exit $curl_rc, http_code '${code:-none}'." >&2
    echo "Server not reachable at $url (check it is still running: mcp_log <log>)." >&2
    rm -f "$resp_file"
    return 1
  fi
  if [ "$code" -ge 400 ]; then
    echo "HTTP $code from $method — response:" >&2
    cat "$resp_file" >&2
    rm -f "$resp_file"
    return 1
  fi
  local reply
  local sse; sse=$(sed -n 's/^data: //p' "$resp_file")
  if [ -n "$sse" ]; then
    # Structural pick, same reason as in mcp_init: log-notification frames precede
    # the reply and can carry the literal tokens a text grep keys on.
    reply=$(printf '%s\n' "$sse" | jq -c 'select(type=="object" and (has("result") or has("error")))' 2>/dev/null | tail -1)
    reply="${reply:-$sse}"
  else
    reply=$(cat "$resp_file")
  fi
  rm -f "$resp_file"
  _mcp_measure "$method" "$params" "$reply" "$code" "$secs"
  printf '%s\n' "$reply"
}

# Usage: mcp_catalog_size <url> <sid> [protocol]
# Weighs the catalog: the bytes of the tools/list reply — what every client
# loads into context per session before a single call — then each tool's
# serialized entry, largest first, split into description / inputSchema /
# outputSchema so the row says WHERE the weight is. A fat outputSchema costs as
# much as a fat description and is the usual surprise. Prints:
#   catalog: 12 tools · 48210 B · ~12.1k tok
#   <bytes>  <~tok>  <name>  desc <b> · input <b> · output <b|none>   (one row per tool)
mcp_catalog_size() {
  local url="$1"; local sid="$2"; local protocol="${3:-}"
  [ -z "$url" ] && { echo "usage: mcp_catalog_size <url> <sid> [protocol]" >&2; return 1; }
  local reply; reply=$(mcp_call "$url" "$sid" tools/list '' "$protocol") || return 1
  printf '%s' "$reply" | jq -r '
    def tok: if . >= 1000 then "~\(. / 4000 * 10 | round / 10)k" else "~\(. / 4 | floor)" end;
    def bytes_or_none: if . == null then "none" else (tojson | utf8bytelength | tostring) end;
    (.result.tools // []) as $t
    | (. | tojson | utf8bytelength) as $total
    | "catalog: \($t | length) tools · \($total) B · \($total | tok) tok",
      ($t
       | map({name, b: (tojson | utf8bytelength),
              d: ((.description // "") | utf8bytelength),
              i: (.inputSchema | bytes_or_none),
              o: (.outputSchema | bytes_or_none)})
       | sort_by(-.b) | .[]
       | "\(.b)\t\(.b | tok)\t\(.name)\tdesc \(.d) · input \(.i) · output \(.o)")'
}

# Usage: mcp_log <server-log-path> [N]   (default: 50 lines)
# Tail the per-server log printed by mcp_start. Useful when a call surprises
# you — pino startup banner, definition lint diagnostics, request handler
# errors, upstream calls, and rate-limit warnings all land here.
mcp_log() {
  local log="$1"; local n="${2:-50}"
  [ -z "$log" ] && { echo "usage: mcp_log <log-path> [n]" >&2; return 1; }
  tail -n "$n" "$log"
}

# Usage: mcp_stop <pid> [server-log-path] [port]
# Kills the background server and the `bun run` child that actually holds the
# port (SIGKILL is not forwarded, so the child must be signalled directly or it
# survives as an orphaned listener). Pass the port from mcp_start to have the
# stop confirmed against the socket rather than against the wrapper PID.
# Removes the server log if a path is given.
mcp_stop() {
  local pid="$1"; local log="${2:-}"; local port="${3:-}"
  [ -z "$pid" ] && { echo "usage: mcp_stop <pid> [log-path] [port]" >&2; return 1; }
  local kids; kids=$(pgrep -P "$pid" 2>/dev/null)
  kill "$pid" $kids 2>/dev/null
  for _ in $(seq 1 12); do
    kill -0 "$pid" 2>/dev/null || break
    sleep 0.25
  done
  if kill -0 "$pid" 2>/dev/null; then
    echo "PID $pid didn't exit on SIGTERM — sending SIGKILL"
    kill -9 "$pid" $kids 2>/dev/null
    sleep 0.5
  fi
  local held=""
  [ -n "$port" ] && held=$(lsof -ti tcp:"$port" 2>/dev/null | tr '\n' ' ')
  if [ -n "$held" ]; then
    echo "WARNING: port $port still held by PID(s) $held after stopping $pid — kill those before re-running"
  elif kill -0 "$pid" 2>/dev/null; then
    echo "WARNING: PID $pid still alive after SIGKILL"
  else
    echo "stopped pid=$pid${port:+ (port $port free)}"
  fi
  [ -n "$log" ] && rm -f "$log"
  return 0
}
HELPER_EOF

. /tmp/<project-name>-field-test-9DJ73-K103L.sh
mcp_start /absolute/path/to/server   # replace with the target server

Capture pid, url, port, log from the mcp_start output — every later call takes them as positional args. Two agents running concurrently in the same project tree each pick their own ID, so their helper paths, server logs, and call scratch never share a name.

Notes

• MCP_HTTP_PORT is a starting port — the server auto-increments if taken. Helper parses the real URL from the log (HTTP transport listening at ...).
• If bun run rebuild fails, stop. Don't field-test broken code — fix the build first.
• Startup wait defaults to 30s. A server that builds or loads a local index at boot can need more — pass a second arg (mcp_start /path 90) rather than reading the timeout as a real startup failure.
• pid is the bun run wrapper; the process that actually holds the port is its child. mcp_stop signals both — that's why it takes the port.
• If a server is already listening on the project's port (lsof -i :<port>), confirm with the user before killing it; it may be their own session. If the user isn't available to confirm, abort the field test and surface the port conflict in your response.

2. Initialize the session

. /tmp/<project-name>-field-test-<ID>.sh
mcp_init <url-from-mcp_start>

Runs initialize, sends notifications/initialized, prints the sid and protocol to capture for mcp_call, plus instructions= — the byte size of the server's instructions string, which every client loads per session alongside the catalog (record it with the catalog total in Step 3). Success is decided by the initialize result, so a transport failure, a non-2xx status, a JSON-RPC error, a malformed body, or a result with no protocolVersion all fail loudly with the raw exchange.

The helper requests the newest initialize-negotiated revision the SDK supports (2025-11-25). If protocol= comes back older than requested=, the server capped it — every call after that exercises an older protocol than a current client would negotiate. Note it as a bug finding and check the pinned @modelcontextprotocol/server version; don't quietly test the downgraded surface. To deliberately test an older version, set MCP_FIELD_TEST_PROTOCOL.

sid= may come back empty — that is a pass, not a failure. Under MCP_SESSION_MODE=stateless the server mints no Mcp-Session-Id, and the helper then omits the session header from every later request. Thread the empty value through positionally and pass the negotiated protocol, which is what identifies the revision when no session carries it:

mcp_call <url> '' tools/list '' <protocol-from-mcp_init>

To exercise both session modes, start the server twice — once with the project's default, once with MCP_SESSION_MODE=stateless — and run the same calls against each.

Both modes exercise the 2025-era arm: initialize negotiates the revision, and the session (when there is one) carries it. The 2026-07-28 revision is a different thing from a sessionless 2025 handshake — it does not initialize at all, and is selected per request by the io.modelcontextprotocol/protocolVersion key in the request's own _meta envelope. This helper does not reach it; exercise the per-request leg from a real 2026-era client or an integration test.

3. Surface the catalog

. /tmp/<project-name>-field-test-<ID>.sh
mcp_call <url> <sid> tools/list     | jq '.result.tools[]     | {name, description, inputSchema, outputSchema}'
mcp_call <url> <sid> resources/list | jq '.result.resources[] | {uri, name, mimeType}'
mcp_call <url> <sid> prompts/list   | jq '.result.prompts[]   | {name, description, arguments}'
mcp_catalog_size <url> <sid> <protocol>

Weigh the catalog. mcp_catalog_size prints the tools/list bytes — the context every client loads per session before a single call — and each tool's entry, largest first, split into description / inputSchema / outputSchema. Record the total alongside the instructions= bytes from Step 2; together they are the per-session tax. The split says where a heavy tool's weight lives: an outputSchema narrating every field of a 60-field record is the common surprise, an over-long description the obvious one. Hand the outliers to tool-defs-analysis (its length-outliers pass) rather than trimming blind.

Present a compact catalog to the user: each definition's name + 1-line description. Flag vague or missing descriptions as you go — those feed into the report. Use this to build the test plan.

Audit every description for leaks — tool description, every parameter .describe() in inputSchema, and every field .describe() in outputSchema (the outputSchema projection above is what surfaces these; don't skim past it). Three categories:

• Implementation details — endpoint paths, API call counts, internal parameter mappings, routing logic. Describe what the tool does, not how it's wired up.
• Meta-coaching — directives about how to use the output. "Treat X as the canonical Y", "callers should…", "the LLM should…". The description sells the tool; it doesn't coach the reader.
• Consumer-aware phrasing — references to "LLM", "agent", "Claude", or any specific reader. The description shouldn't name who's reading it.

Treat any hit as a ux finding in the report. The authoring rule lives under Tool descriptions in design-mcp-server/SKILL.md — same categories, applied at review time.

4. Plan the test pass

Budget. Don't run every category against every definition — the cross-product is infeasible. Apply the universal battery to everything; apply situational categories only when the definition triggers them.

Universal battery — run on every tool

Situational — add only when triggered

Resources. Happy path, not-found URI (use a syntactically valid but non-existent ID — e.g., substitute a fake ID into the URI template), list if defined, pagination if used. Prompts. Happy path, defaults omitted, skim message quality.

Sampling for large servers. If more than 15 tools, run the universal battery on all, but pick roughly 30–40% for situational testing. Weight toward: write-shaped tools, complex schemas, external deps. List which ones you skipped in the report.

Auth & external state.

• If a tool needs real API keys and they're not set, note skipped — requires $VAR and move on. Don't fabricate inputs.
• Tools that write to real external systems (third-party APIs, shared DBs): confirm with the user before running, or use a dry-run input if one exists.

5. Execute

Use TaskCreate — one task per definition. Mark complete as you go. Don't batch.

For each call, capture: input sent, the ⏱ line (bytes, split, ms), response (trim huge payloads to files), whether isError: true appeared, anything surprising (slow response, parity drift, unhelpful text, crash).

When a call surprises you — slow, hangs, returns terse output, surfaces an unhelpful error — run . /tmp/<project-name>-field-test-<ID>.sh && mcp_log <log> to tail the server log. The pino startup banner, request handler errors, upstream API call traces, and rate-limit warnings all land in the per-server log (read via mcp_log) rather than coming back through mcp_call. Don't guess at runtime behavior from response text alone.

Interpreting responses

• content[] is an array of blocks — read all of them, never just content[0]. A success result is assembled as [...ctx.content media blocks, ...the format()/JSON domain render, ...the enrichment trailer]. Everything the handler put on ctx.enrich — empty-result notices, totals, query echoes, truncation disclosure — renders in that trailer, a separate trailing block, not inside the format() block. Quoting content[0].text and reporting those fields as absent from content[] is a false parity gap; the suggested fix (render them in format() too) would double-render them. Dump .result.content in full before claiming drift.
• Tool domain errors return {result: {content: [...], isError: true}} — they live in result, not error. Check isError, not the JSON-RPC error field.
• Tool error code/reason rides on result.structuredContent.error.{code, message, data?.reason} — inspect that, not just the text. data is only spread when the handler threw an McpError (or ZodError); plain throw new Error(...) won't populate data.reason. Use ctx.fail-thrown errors when the contract reason matters. The text in result.content[0].text mirrors the message and includes Recovery: <hint> when data.recovery.hint is present.
• Resource errors are JSON-RPC-level — they appear in the top-level error.{code, data.reason} field, not inside result. Resource handlers re-throw rather than producing an isError envelope.
• JSON-RPC error only appears for protocol issues (bad session, malformed envelope, unknown method).
• mcp_call already strips SSE framing. Pipe to jq for readability.

6. Tear down

. /tmp/<project-name>-field-test-<ID>.sh
mcp_stop <pid> <log> <port>
rm -f /tmp/<project-name>-field-test-<ID>.sh

Kills the background server and its port-holding child, removes the server log, then removes the helper script itself. Do this before writing the report so nothing leaks into the next session. Pass the port — it's what turns "the wrapper PID is gone" into "the socket is actually free." If mcp_stop warns the port is still held or the PID survived SIGKILL, note it in the report and proceed — don't block on a zombie process, but do say which PID to kill.

7. Report

Four sections. Tight. The user should be able to skim the summary, scan the numbers, read details only for what matters, and act on numbered options.

Summary (1 paragraph)

One paragraph. How many definitions exercised, how many passed clean, how many have issues, and the single most important finding. No tables, no lists.

Size & latency

Per-session tax on its own line (instructions bytes + catalog bytes, with the heaviest tool named), then one row per tool exercised, sorted by happy-path bytes descending: tool · bytes · ~tok · ms. Over 15 tools, keep every row over 24,000 B plus the three slowest and fold the rest into one line ("N more under budget, median X B"). Numbers only — what they mean goes in Findings.

Findings

Only include definitions with issues. Group by severity. Each finding is 2–4 lines unless it genuinely needs more. A parity finding cites the full content[] dump as its evidence — a quote from one index doesn't establish drift.

Format:

**<tool_name> — <bug|ux|nit>**
Input: `<short input>` → <what happened>
Expected: <what should happen>
Fix: <one sentence>

Options

Numbered, actionable, cherry-pickable. Each item maps to a concrete change.

1. Fix empty-result message in `pubmed_search_articles` — echo criteria (finding #2)
2. Add `format()` to `pubmed_lookup_mesh` — currently returns raw JSON (finding #5)
3. Tighten `ids` description in `pubmed_fetch_articles` — silent on PMID vs DOI (finding #8)

End with:

Pick by number (e.g. "do 1, 3, 5" or "expand on 2").

Checklist

• Stdio boot check completed — bun run rebuild && bun run start:stdio < /dev/null shows clean startup (banner, expected counts, no errors) and a graceful shutdown on EOF
• HTTP server built and started; real port parsed from log
• Session initialized (a stateless server returns an empty sid — still a pass); notifications/initialized sent; negotiated protocol version matches the requested one (a downgrade is a finding)
• Catalog surfaced and presented; descriptions audited for leaks (implementation details, meta-coaching, consumer-aware phrasing)
• Catalog weighed (mcp_catalog_size); total + instructions= bytes recorded for the report
• Every call's ⏱ line read; any happy-path response over 24,000 B with no disclosure + retrieval path filed as ux
• Universal battery run on every definition (happy path, parity against the full content[] array, input error)
• Situational categories applied only when triggered
• If >15 tools: sampled 30–40% for situational testing; skipped definitions listed in report
• If a tool declared an errors: [...] contract: ≥1 declared failure mode triggered; result.structuredContent.error.code and data.reason verified against the contract entry
• If a resource declared an errors: [...] contract: ≥1 declared failure mode triggered; top-level JSON-RPC error.code and error.data.reason verified against the contract entry
• If any tool truncates, caps, or spills its output: truncation forced; disclosure + a retrieval path (cursor, offset, selector, canvas handle) verified
• External-state / auth-gated tools handled explicitly (run, skip, or confirm)
• Server stopped (port confirmed free); server log and helper script removed
• Report: summary paragraph → size & latency table → grouped findings → numbered options