mcptest

pytest for MCP agents. A testing framework for Model Context Protocol (MCP) agents that lets you mock MCP servers with YAML fixtures, run agents against them in isolation, and assert against the resulting tool-call trajectories.

pip install mcptest
mcptest init
mcptest run

Why

Building an agent that talks to real MCP servers means:

• Cost — every test run spends tokens and may hit paid APIs.
• Flakiness — external services go down, rate-limit, or return non-deterministic data.
• Slow feedback — end-to-end runs take minutes, not milliseconds.
• No regression safety — change a prompt or swap a model and you have no way to know if the agent's behavior changed until something breaks in production.

mcptest gives MCP agents what pytest gave Python code: fast, hermetic, asserted tests and a regression safety net.

Core features

• Mock MCP servers from YAML — declare tools, responses, and error scenarios in a fixture file. No code required for the common case.
• Full MCP protocol — mocks speak real MCP over stdio (and SSE), so your agent connects to them the same way it connects to production servers.
• Trajectory assertions — assert which tools were called, in what order, with what parameters, how many times, and how quickly.
• Error injection — trigger named error scenarios to test your agent's recovery paths.
• Metric-gated assertions & scorecards — use any quality metric as a YAML assertion gate (metric_above, metric_below), compose assertions with boolean combinators (all_of, any_of, none_of, weighted_score), and generate a weighted quality report card with mcptest scorecard for model comparison and prompt tuning.
• Regression diffing — snapshot an agent's trajectory and detect drift when prompts, models, or MCP servers change.
• Watch mode — mcptest watch monitors your test files and fixtures, automatically re-running only the affected tests when anything changes. Smart dependency tracking means only the tests that reference a changed fixture are re-run.
• pytest integration — use YAML files or write Python tests with fixtures.
• CI/CD ready — GitHub Action + PR comment bot for regression gating.
• Inline docs — mcptest explain <name> shows Rich-formatted terminal docs for any assertion, metric, or check. mcptest docs build generates a full MkDocs site with auto-generated reference pages that stay in sync with code automatically.

Capture — tests write themselves

The fastest way to get started is mcptest capture. Point it at any MCP server and it auto-discovers tools, samples responses, and writes both fixture YAML and test-spec YAML — no hand-writing required.

# 1. Install
pip install mcptest

# 2. Capture a live server → auto-generate fixture + tests
mcptest capture "python my_server.py" --output fixtures/ --generate-tests

# Generated files:
#   fixtures/my-server.yaml   ← fixture with real responses
#   fixtures/my-server-tests.yaml  ← ready-to-run test suite

# 3. Run the generated tests
mcptest run fixtures/my-server-tests.yaml

# 4. Watch mode — auto-run on save
mcptest watch --watch-extra src/

Options:

Flag	Default	Description
--output / -o	.	Directory where files are written
--generate-tests	off	Also write a test-spec YAML
--samples-per-tool	3	Argument variations tried per tool
--dry-run	off	Preview without writing files
--agent	python agent.py	Agent command embedded in test suites

Quickstart (manual)

# 1. Install
pip install mcptest

# 2. Scaffold a project
mcptest init

# 3. Edit fixtures/example.yaml and tests/example.yaml

# 4. Run
mcptest run

# 5. Watch mode — auto-run on save
mcptest watch --watch-extra src/

Example fixture

# fixtures/github.yaml
server:
  name: mock-github

tools:
  - name: create_issue
    description: Create a GitHub issue
    input_schema:
      type: object
      properties:
        repo: { type: string }
        title: { type: string }
      required: [repo, title]
    responses:
      - match: { repo: acme/api }
        return:
          issue_number: 42
          url: https://github.com/acme/api/issues/42
      - default: true
        return:
          issue_number: 1

errors:
  - name: rate_limited
    tool: create_issue
    error_code: -32000
    message: GitHub API rate limit exceeded

Example test

# tests/issue_triage.yaml
name: Issue triage agent
fixtures:
  - fixtures/github.yaml
agent:
  command: python examples/issue_agent.py
cases:
  - name: Creates issue for bug report
    input: "File a bug: login page 500 error on Safari"
    assertions:
      - tool_called: create_issue
      - param_matches:
          tool: create_issue
          param: title
          contains: "500"
      - max_tool_calls: 3

Metric-gated assertions

Use any of the 7 built-in quality metrics directly as YAML assertion gates:

assertions:
  # Agent must be efficient (≥80% unique tool usage)
  - metric_above: {metric: tool_efficiency, threshold: 0.8}
  # Agent must not be repetitive (non-redundancy score ≥0.9)
  - metric_above: {metric: redundancy, threshold: 0.9}
  # Gate on a weighted composite quality score
  - weighted_score:
      threshold: 0.75
      weights:
        tool_efficiency: 0.3
        redundancy: 0.2
        error_recovery_rate: 0.5

Boolean combinators for complex logic:

assertions:
  - all_of:
      - tool_called: create_issue
      - max_tool_calls: 5
  - any_of:
      - tool_called: create_issue
      - output_contains: created
  - none_of:
      - tool_called: delete_all
      - output_contains: ERROR

Agent scorecard

Generate a weighted quality report card from any saved trace:

# Render a human-readable table (exit 1 if composite score < 0.75)
mcptest scorecard trace.json

# Override the threshold
mcptest scorecard trace.json --fail-under 0.8

# Custom weights from a YAML config
mcptest scorecard trace.json --config scorecard.yaml

# Machine-readable JSON output (for CI pipelines)
mcptest scorecard trace.json --json

Example scorecard.yaml:

composite_threshold: 0.75
default_threshold: 0.7
thresholds:
  tool_efficiency: 0.8
  error_recovery_rate: 0.9
weights:
  tool_efficiency: 2.0
  redundancy: 1.0
  error_recovery_rate: 3.0

Conformance testing

Verify that any MCP server implementation correctly implements the protocol. 19 checks across 5 sections, each tagged with RFC 2119 severity (MUST / SHOULD / MAY).

Quick start

# Test a server subprocess over stdio
mcptest conformance "python my_server.py"

# Test in-process using a fixture YAML (fast, no subprocess)
mcptest conformance --fixture fixtures/my_server.yaml

# Filter to a specific section
mcptest conformance --fixture fixtures/my_server.yaml --section initialization

# Only run MUST checks (CI gate — fail only on hard violations)
mcptest conformance --fixture fixtures/my_server.yaml --severity must

# Also fail on SHOULD violations
mcptest conformance --fixture fixtures/my_server.yaml --fail-on-should

# Machine-readable output for CI pipelines
mcptest conformance --fixture fixtures/my_server.yaml --json

Check catalogue

ID	Section	Severity	Description
INIT-001	initialization	MUST	Server provides non-empty name
INIT-002	initialization	MUST	Server info includes version string
INIT-003	initialization	MUST	Server reports capabilities object
INIT-004	initialization	SHOULD	Capabilities includes tools when server has tools
TOOL-001	tool_listing	MUST	list_tools() returns a list
TOOL-002	tool_listing	MUST	Each tool has name and inputSchema fields
TOOL-003	tool_listing	MUST	All tool names are unique
TOOL-004	tool_listing	SHOULD	Each inputSchema has type: "object" at root
CALL-001	tool_calling	MUST	Calling a valid tool with matching arguments returns result
CALL-002	tool_calling	MUST	Result contains content list
CALL-003	tool_calling	MUST	Successful result has isError absent or False
CALL-004	tool_calling	MUST	Calling unknown tool name returns error
CALL-005	tool_calling	SHOULD	Error response sets isError to True
ERR-001	error_handling	MUST	Error result contains text content with message
ERR-002	error_handling	SHOULD	Server handles empty arguments dict without crashing
ERR-003	error_handling	SHOULD	Server handles None arguments without crashing
RES-001	resources	MUST	list_resources() returns a list
RES-002	resources	MUST	Each resource has uri and name fields
RES-003	resources	MUST	Resource URIs are unique

Resource checks (RES-*) are automatically skipped when the server has no resources capability.

CI integration

# .github/workflows/conformance.yml
- name: MCP conformance
  run: mcptest conformance --fixture fixtures/server.yaml --severity must --json > conformance.json

Exit code is 1 when any MUST check fails (or any SHOULD check fails with --fail-on-should).

Programmatic usage

import anyio
from mcptest.conformance import ConformanceRunner, InProcessServer, Severity
from mcptest.fixtures.loader import load_fixture
from mcptest.mock_server.server import MockMCPServer

fixture = load_fixture("fixtures/my_server.yaml")
mock = MockMCPServer(fixture)
server = InProcessServer(mock=mock, fixture=fixture)

runner = ConformanceRunner(server=server, severities=[Severity.MUST])
results = anyio.run(runner.run)
must_failures = [r for r in results if not r.passed and not r.skipped]

Documentation

Full reference documentation is auto-generated from live registries so it never goes stale.

# Look up any assertion, metric, or check inline
mcptest explain tool_called
mcptest explain tool_efficiency
mcptest explain INIT-001

# List all available assertions, metrics, and checks
mcptest docs list

# Generate a full MkDocs documentation site
mcptest docs build --output ./site
cd site && mkdocs serve

The generated site includes:

• Getting Started — capture-first 5-minute quickstart
• Assertions Reference — all 19 assertions with YAML examples
• Metrics Reference — 7 quality metrics with score interpretation
• Conformance Checks Reference — 19 protocol checks with severity
• CLI Reference — every command with full option tables

Configuration

Place a mcptest.yaml file in your project root (or any parent directory) to set defaults for all CLI flags. mcptest walks up from the current directory to find it — the same discovery strategy git uses for .gitignore.

# mcptest.yaml
test_paths: ["tests/"]
fixture_paths: ["fixtures/"]
baseline_dir: .mcptest/baselines

retry: 3          # default retry count for every case
tolerance: 0.8    # default pass-rate tolerance (0.0–1.0)
parallel: 4       # default worker count (-j flag)
fail_fast: false  # stop at first failure
fail_under: 0.0   # coverage gate (mcptest coverage --threshold)

# Per-metric thresholds used by mcptest scorecard
thresholds:
  tool_efficiency: 0.7
  redundancy: 0.3

# Plugins to load at startup (dotted module name or file path)
plugins:
  - my_company.mcptest_extensions
  - ./custom_assertions.py

# Cloud settings
cloud:
  url: https://mcptest.example.com
  api_key_env: MCPTEST_API_KEY

CLI flags always override config-file values. To inspect the resolved configuration and loaded plugins:

mcptest config

Benchmarking

mcptest bench runs the same test suite against multiple agent profiles and produces a side-by-side quality comparison. Use it to quantify which model or prompt strategy performs best before migrating, or to add a regression gate to CI.

Define agent profiles

Create an agents.yaml file listing the agents to compare:

# agents.yaml
agents:
  - name: claude-sonnet
    command: python agents/claude_agent.py
    env:
      MODEL: claude-3-5-sonnet-20241022
    description: Anthropic Claude Sonnet 3.5

  - name: gpt-4o
    command: python agents/openai_agent.py
    env:
      MODEL: gpt-4o
    description: OpenAI GPT-4o

Or embed profiles directly in mcptest.yaml so no extra file is needed:

# mcptest.yaml
agents:
  - name: claude-sonnet
    command: python agents/claude_agent.py
    env: { MODEL: claude-3-5-sonnet-20241022 }
  - name: gpt-4o
    command: python agents/openai_agent.py
    env: { MODEL: gpt-4o }

Run the benchmark

# Explicit profiles file
mcptest bench tests/ --agents agents.yaml

# Profiles from mcptest.yaml
mcptest bench tests/

# Machine-readable JSON (pipe to jq, store in CI artifacts, etc.)
mcptest bench tests/ --agents agents.yaml --json | jq .best_agent

The output includes three Rich tables:

• Leaderboard — agents ranked by composite score with pass rate, duration, and a BEST badge for the winner.
• Metric Comparison — pivot table of per-agent average scores for each quality metric, colour-coded green/yellow/red.
• Per-Test Breakdown — pass/fail grid across agents and test cases, highlighting where agents diverge.

CI integration

# Exit 1 if the best agent's composite score is below 0.75
mcptest bench tests/ --agents agents.yaml --ci --fail-under 0.75

Add to your pipeline:

# .github/workflows/bench.yml (example)
- name: Benchmark agents
  run: mcptest bench tests/ --agents agents.yaml --ci --fail-under 0.75

Options

Flag	Description
--agents <file>	Load profiles from this YAML file
--json	Emit JSON instead of Rich tables
--ci	Exit non-zero when best score < --fail-under
--fail-under <float>	CI composite-score threshold (default 0.0)
--retry <n>	Override retry count for every case
--tolerance <float>	Override pass-rate tolerance (0.0–1.0)
-j/--parallel <n>	Parallel workers per agent

Semantic evaluation

mcptest eval scores agent text output against named criteria — no LLM API calls required. Grading is deterministic: keyword coverage, regex patterns, and text similarity (levenshtein / Jaccard / cosine). It is ideal for CI pipelines where speed and cost predictability matter.

Define a rubric

Create a rubric YAML that describes what a good answer looks like:

# rubrics/booking.yaml
rubric:
  name: booking-quality
  criteria:
    - name: correctness
      weight: 0.5
      method: keywords
      expected: [confirmed, booking_id, receipt]
      threshold: 0.6        # ≥60 % of keywords must be present
    - name: format
      weight: 0.3
      method: pattern
      expected: "Booking \\w+ confirmed"
      threshold: 1.0        # regex must match
    - name: completeness
      weight: 0.2
      method: similarity
      expected: "Your booking ABC123 is confirmed. You will receive a receipt."
      threshold: 0.7        # text similarity must be ≥ 0.7

Grading methods

Method	Description
keywords	Fraction of expected keywords found in the text (sub-string)
pattern	Binary regex match anywhere in the text (1.0 or 0.0)
similarity	Best of levenshtein / Jaccard / cosine similarity against a reference
contains	Binary sub-string check (1.0 or 0.0)
custom	Reserved for plug-in graders (returns 0.0 by default)

Run evaluations

# Grade every test case against a rubric file
mcptest eval tests/ --rubric rubrics/booking.yaml

# Machine-readable JSON
mcptest eval tests/ --rubric rubrics/booking.yaml --json

# CI gate: exit 1 if mean composite score < 0.75 or any criterion fails
mcptest eval tests/ --rubric rubrics/booking.yaml --ci --fail-under 0.75

Inline rubric in test spec

Embed an eval: section directly in a test case to avoid a separate file:

# tests/booking.yaml
name: Booking agent
fixtures: [fixtures/booking.yaml]
agent:
  command: python agent.py
cases:
  - name: confirm booking
    input: "Book a table for 2 at 7pm"
    assertions:
      - tool_called: create_booking
    eval:
      name: booking-quality
      criteria:
        - name: correctness
          method: keywords
          expected: [confirmed, booking_id]
          weight: 1.0
          threshold: 0.5

Output

The Rich table shows per-criterion average scores, pass rates, and verdicts:

Evaluation Report — rubric: booking-quality

 Criterion      Avg Score  Pass Rate  Verdict
 ──────────────────────────────────────────────
 correctness    0.833      100.0%     PASS
 format         1.000      100.0%     PASS
 completeness   0.721       80.0%     PARTIAL

Overall: 4/5 passed (80.0%) — composite score 0.851

Options

Flag	Description
--rubric <file>	Load rubric from this YAML file (overrides inline eval:)
--json	Emit JSON instead of Rich tables
--ci	Exit non-zero when any criterion fails or score < --fail-under
--fail-under <float>	CI composite-score threshold (default 0.0)
--retry <n>	Override retry count for every case
--tolerance <float>	Override pass-rate tolerance (0.0–1.0)
-j/--parallel <n>	Parallel workers

Programmatic usage

from pathlib import Path
from mcptest.eval import Grader, load_rubric, aggregate_results

rubric = load_rubric(Path("rubrics/booking.yaml"))
grader = Grader(rubric)

texts = [
    "Your booking ABC123 is confirmed. Receipt sent.",
    "Booking confirmed.",
]
results = [grader.grade(t) for t in texts]
summary = aggregate_results(results)
print(summary.pass_rate, summary.mean_composite)

Plugins

Plugins let you add custom assertions, metrics, and exporters without forking mcptest. Any module that calls the registration decorators at import time is a valid plugin.

Load via config file (dotted module name or file path):

# mcptest.yaml
plugins:
  - my_company.mcptest_extensions  # installed package
  - ./custom_assertions.py         # local file

Load via confmcptest.py (auto-discovered, like pytest's conftest.py):

# tests/confmcptest.py
from mcptest.assertions.base import register_assertion, TraceAssertion
from mcptest.runner.trace import Trace
from mcptest.assertions.base import AssertionResult

@register_assertion
class response_is_json(TraceAssertion):
    yaml_key = "response_is_json"

    def check(self, trace: Trace) -> AssertionResult:
        ok = all(
            "json" in (call.result or "").lower()
            for call in trace.tool_calls
        )
        return AssertionResult(
            passed=ok,
            name=self.yaml_key,
            message="all tool responses are JSON" if ok else "non-JSON response found",
        )

Load via entry points (for distributable packages):

# pyproject.toml of your plugin package
[project.entry-points."mcptest.assertions"]
my_assertions = "my_package.assertions"

[project.entry-points."mcptest.metrics"]
my_metrics = "my_package.metrics"

[project.entry-points."mcptest.exporters"]
my_exporter = "my_package.exporters"

Once installed, your assertions are available by yaml_key in any test YAML:

assertions:
  - response_is_json: true

Cloud dashboard

mcptest ships a lightweight web UI for the cloud backend that runs with zero build tooling — Tailwind CSS, htmx, and Chart.js are all loaded from CDN.

Launch

# Install cloud extras
pip install 'mcptest[cloud]'

# Start the dashboard (opens browser automatically)
mcptest dashboard

# Custom host / port / database
mcptest dashboard --host 0.0.0.0 --port 8200 --db ./prod.db --no-browser

The server starts at http://127.0.0.1:8100/dashboard/ by default.

Pages

Page	URL	Description
Overview	/dashboard/	Stats cards (total runs, pass rate, avg duration, tool calls, baselines), recent-runs table, per-suite pass/fail bars
Runs	/dashboard/runs	Filterable, paginated run list. Dropdowns for suite, branch, status, and environment update the table live via htmx without a full page reload.
Run detail	/dashboard/runs/{id}	Full run info: metric scores (horizontal bar chart), collapsible tool-call timeline with arguments and results, input/output panels, promote-as-baseline button.
Trends	/dashboard/trends	Chart.js line chart of any metric over time. Baseline runs are marked with stars. Controls for metric, suite, branch, and data limit update the chart instantly.
Baselines	/dashboard/baselines	Active baseline table with one-click demote (htmx). Compare any two runs by ID and see a metric delta table with regression indicators.
Webhooks	/dashboard/webhooks	Register and manage HTTP webhook endpoints. Create webhooks with event subscriptions, view delivery history, and send test pings.

Configuration

Flag	Default	Description
--host	127.0.0.1	Bind address
--port	8100	Listen port
--db	./mcptest_cloud.db	SQLite database path (or set MCPTEST_DATABASE_URL)
--no-browser	off	Skip auto-opening the browser

The dashboard is backed by the same FastAPI app as mcptest cloud-push; all API endpoints remain available at their existing paths alongside the dashboard routes.

Webhook system

mcptest cloud can POST signed JSON notifications to external HTTP endpoints when key events occur — completing the CI loop from cloud-push through auto-regression check to team alert.

Events

Event	Fires when
run.created	A new test run is pushed via POST /runs
regression.detected	POST /runs/{id}/check finds metric regressions vs baseline
baseline.promoted	A run is promoted as the suite baseline
baseline.demoted	A baseline is demoted/removed

API

# Register a webhook
curl -X POST http://localhost:8100/webhooks \
  -H "X-API-Key: $KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://hooks.slack.com/services/...",
    "secret": "my-signing-secret",
    "events": ["regression.detected", "baseline.promoted"],
    "suite_filter": "smoke"
  }'

# List webhooks
curl http://localhost:8100/webhooks -H "X-API-Key: $KEY"

# Send a test ping
curl -X POST http://localhost:8100/webhooks/{id}/test -H "X-API-Key: $KEY"

# View delivery history
curl http://localhost:8100/webhooks/{id}/deliveries -H "X-API-Key: $KEY"

Payload format

Every delivery POSTs the same envelope:

{
  "event": "regression.detected",
  "timestamp": "2026-01-15T12:00:00+00:00",
  "data": {
    "head_id": 42,
    "base_id": 37,
    "suite": "smoke",
    "branch": "feat/new-prompt",
    "regression_count": 2,
    "deltas": [
      {"name": "tool_efficiency", "base_score": 0.9, "head_score": 0.6, "delta": -0.3}
    ]
  }
}

Signature verification

When a secret is set, each request includes an X-MCPTest-Signature: sha256=<hex> header. Verify it in your receiver:

import hashlib, hmac

def verify(secret: str, body: bytes, header: str) -> bool:
    expected = hmac.new(secret.encode(), body, hashlib.sha256).hexdigest()
    provided = header.removeprefix("sha256=")
    return hmac.compare_digest(expected, provided)

Delivery retries up to 3 times with exponential back-off (1 s, 4 s, 16 s) on connection errors or 5xx responses. Every attempt is logged in the webhook_deliveries table and visible in the dashboard.

Status

Alpha. The core loop (mock server → runner → assertions → CLI) is functional; SSE transport and test packs are under active development.

License

MIT — see LICENSE.