A trusted agent stack for MCP

Listed on the official MCP Registry as io.github.rudranaresh0201/verimcp and io.github.rudranaresh0201/devmcp.

Problem

MCP tool calls are trusted by default, in two different ways that both bite in production agent workflows:

1. Claims aren't checked. When a server says a commit succeeded, a file was written, or a CI check passed, the Host has no way to know if that's true — isError: false only means the tool didn't crash, not that it did what it claimed. An agent wired up to git, CI, or infrastructure through MCP is one confidently-wrong tool response away from acting on a lie.
2. Retries aren't safe. When a Host resends a tools/call after an ambiguous response (a timeout, a dropped connection), nothing in MCP stops the backend from just running it again — a second commit, a second CI run, a second charge. The protocol has no concept of "didn't I already do this?"

Solution

verimcp is a transparent proxy that sits between an MCP Host and a backend server and forwards every message unchanged, except for two independent checks around each call:

• Before forwarding (pre-check) — if this exact call was already sent once and independently verified true this session, answer the retry from that confirmed result instead of hitting the backend again. --idempotent-replay, see Proof below.
• After the backend responds (post-check) — for calls it knows how to check, independently re-derive the real outcome (re-read the file, re-check the git log, re-run the safe parts of a CI step) and compare that against what the backend claimed. A lie gets rewritten into a real error before the Host ever sees it as a success.

devmcp is the backend it's proven against — a real git/CI MCP server, deliberately built with the full protocol surface (tools, resources, prompts, roots, sampling), because a proxy is only as convincing as what it's shown catching.

Host  <--stdio-->  verimcp (pre-check: dedupe retries + post-check: verify claims)  <--stdio-->  devmcp (git/CI server)

Both are independently installable and have no import dependency on each other — verimcp works in front of any MCP backend (proven against the official mcp-server-git too, not just devmcp), and devmcp works with any Host directly, unproxied.

Proof, not just claims

Two runnable benchmarks, real subprocess pipes, no mocks — the numbers below are what they actually print, gaps included:

pip install verimcp devmcp-server
python scripts/benchmark_claim_acceptance.py    # does verimcp catch a lying backend?
python scripts/benchmark_retry_duplication.py   # does --idempotent-replay stop a retry from duplicating a side effect?

Claim-acceptance — a real MCP Host with no verification layer accepts 100% of fabricated results by construction; that's not a benchmark artifact, it's the actual gap in the protocol:

RAW claim-acceptance rate:     12/12  (100%)
verimcp claim-catch rate:       9/12  (75%)

The 75%, not 100%, is deliberate and stated in the script's own output: 3 of the 12 scenarios exploit documented, principled gaps in specific verifiers (e.g. a hash-exists check can't tell this call created the hash vs an old one) — named plainly rather than hidden, because a tool claiming a perfect score on its own benchmark is the real red flag.

Retry-duplication — same idea applied to --idempotent-replay, against 5 realistic non-idempotent side effects (a counter bump, a notification send, an invoice increment, an audit entry, a two-step pipeline):

RAW duplicate-action rate:      5/5  (100%)
verimcp duplicate-action rate:  0/5  (0%)

scripts/demo.py is the smaller, narrative version of the same proof — one fabricated commit hash caught, one real write confirmed and passed through unchanged. tests/test_adversarial_corpus.py and tests/test_proxy_integration.py run the same claims through the real proxy pipe in CI on every commit, not just on demand.

Technical summary

• Correctness verifiers — one per tool/resource with an independently-checkable postcondition: write_file (disk hash-compare), git_commit/git_branch (re-derived from real git state), run_ci_pipeline (self-consistency + re-execution of steps marked safe to re-run), plus resource reads (repo://status, repo://log, repo://file/{path}). New verifiers are a plugin system (importlib.metadata.entry_points, the same mechanism pytest/Black use) — see docs/writing-a-verifier.md.
• Verify-before-retry (--idempotent-replay) — a retried tools/call is only ever answered from cache if the previous identical call was independently verified true by the checks above; a key match with no prior verified success is a cache miss, not a false dedupe. Adapted from arXiv:2608.02645, moved to the proxy layer so it works for any Host/backend pair verimcp fronts, not just one agent framework's own retry wrapper.
• Policy gates — allow/deny/require-approval rules evaluated before a call reaches the backend, for the calls that have no objective truth to check (sampling rate limits, arbitrary tool-name/argument policy via YAML, human-in-the-loop approval over real MCP elicitation/create).
• Audit + replay — every call verimcp handles is logged and served back as a real verimcp://audit MCP resource; verimcp replay re-runs recorded traffic against a new policy to backtest "would this have changed anything."
• Observability — every call gets an OpenTelemetry span/metric using the GenAI semantic conventions, exportable to any OTLP collector.
• Proven against real clients, not just our own tests — the official MCP Inspector and VS Code's native MCP support (Copilot Chat) each caught a real bug our own test suite never triggered, now fixed with regression tests.

Deeper design reasoning (why prompts/roots get no verifier on principle, why policy is a separate concept from verification, etc.) lives in docs/adr/ for anyone who wants to go that deep — the summary above is everything needed to use or evaluate the project.

Getting started

As a user — install straight from PyPI:

pip install verimcp devmcp-server
verimcp -- devmcp --repo-path ./some-repo

Note on names: the PyPI distribution is devmcp-server (devmcp was blocked by PyPI's typosquat-similarity check against an unrelated existing package), but the Python import and CLI command are both still plain devmcp — nothing above changes if you're reading devmcp's own source.

In Docker — no local Python/git needed:

docker build -t verimcp-devmcp .
docker run -i -v /path/to/your/repo:/repo verimcp-devmcp

As a contributor — editable installs from this repo:

pip install -e ".[dev]"
pip install -e "./devmcp[dev]"

pytest tests devmcp/tests   # 138 tests, real subprocess + real git repo, nothing mocked
ruff check src tests devmcp/src devmcp/tests

python scripts/inspector_smoke_test.py   # verify against the real MCP Inspector client (needs node/npx)

Try the observability yourself

# spans/metrics print to stderr (verimcp's stdout is the live MCP protocol channel)
verimcp --otel-exporter console -- devmcp --repo-path ./some-repo

# or point a real collector, Jaeger, or Grafana Agent at it
pip install "verimcp[otel]"
verimcp --otel-exporter otlp --otel-endpoint localhost:4317 -- devmcp --repo-path ./some-repo

Omitting --otel-exporter entirely means zero telemetry overhead — the default, same as every other opt-in flag here.

Try verify-before-retry yourself

verimcp --idempotent-replay -- devmcp --repo-path ./some-repo

Send the same tools/call twice in a row (same tool, same arguments, new request id — exactly what a client resending after a timeout looks like on the wire). The first call runs for real; the second is answered from the confirmed result without touching the backend again. Omit the flag for the unchanged default: every retry is re-executed, same as before this existed.

Contributing

See CONTRIBUTING.md — dev setup, how to add a verifier, this project's ADR discipline, and commit/PR conventions. See CHANGELOG.md for release history.

License

MIT — see LICENSE.