Seamcheck

A small tool that looks for the bugs that sit between things — your frontend and your backend, your code and your database schema, one file and another. It reads your source. It never runs your code.

_{A small Express shop, scanned. Read it downwards: the browser at the top, the seam where requests cross the network, the server underneath.}

Why I made it

I was building a game — a fairly large Django app with a lot of hand-written JavaScript — and I kept losing afternoons to the same kind of bug. The Python was fine. The JavaScript was fine. The route one asked for and the route the other served were one character apart, and nothing I already had read both sides of that.

So I wrote something to find them, for myself, on that project. It kept catching things I would not have found on my own, and after a while it seemed like other people might have the same afternoons to lose. So here it is.

It does not catch everything, and I am sure there are things it gets wrong. When it cannot tell, it tries to say uncertain rather than guess. If you find it being confidently wrong somewhere, please open an issue — that is the most useful thing anyone can send me.

Install

Seamcheck reads a Django project by importing it, so install it into the same virtualenv your project runs in.

source .venv/bin/activate        # your project's virtualenv
pip install seamcheck
seamcheck map

That is the whole setup. It opens a map of your project and prints a link you can open on your phone. Have a look around before reading any further — it explains itself better than this page does.

Needs Python 3.10 or newer. Django is not a dependency — only a Django project needs it, and such a project already has it.

For the agent server: pip install 'seamcheck[mcp]'.

python3.12 -m venv .venv
source .venv/bin/activate
pip install seamcheck

brew install python@3.12
cd your-project
python3.12 -m venv .venv
source .venv/bin/activate
pip install seamcheck

sudo apt install python3-venv        # if `python3 -m venv` is missing
python3 -m venv .venv
source .venv/bin/activate
pip install seamcheck

py -3.12 -m venv .venv
.venv\Scripts\activate
pip install seamcheck

pipx install seamcheck
uv tool install seamcheck        # uv fetches its own Python, so no Homebrew needed

pip install --no-cache-dir --upgrade seamcheck
seamcheck --version

What it looks like

Click a red one. The chain that reaches it lights up and everything else recedes, so you can see where the request came from and where it stopped — with the file and line for every hop, and a sentence saying what it means and what to check.

_{checkout.js asks for /api/shipping/quotes. The server serves /api/shipping/quote. One character, valid on both sides, and nothing else would have told you.}

Everything it is willing to claim, each one explained in a sentence, worst first.

It reads on a phone, because that is where you end up looking at it.

Five looks, if you care. Aurora is the default.

Does it work with my stack?

No configuration. It works out which one you are using from what is in the repo.

Each one is checked against a small app written for the purpose, with a deliberately mistyped endpoint in it, and each one finds it:

flask     /api/orders            → the route is /api/order
fastapi   /api/sign-up           → the route is /api/signup
express   /api/does-not-exist    → there is no such route
fastify   /api/comments          → the route is /api/comment
nestjs    /api/orders/check-out  → the route is /api/orders/checkout
nextjs    /api/account/profil    → the file is pages/api/account/profile.js

Only Django is imported; the rest are read from source, so nothing has to run and none of their dependencies have to be installed.

Frontends — plain JavaScript · TypeScript · Django templates · CSS and design tokens Also reads — Stripe webhooks · Celery tasks and beat schedules · GraphQL schemas

Also tried against 20 open-source projects: 53,361 files, 9.5M lines, 2,435 routes. If yours does not work, I would genuinely like to know.

It reads your data layer too

The seam is a name crossing a boundary nothing checks. A route string is one instance. Where you keep your data is another, and usually nobody is checking that at all.

Supabase

Your client names a table, a column, a function and an edge function as strings, and they are checked against supabase/migrations/*.sql.

UNRESOLVED  db_table_use       order              the migrations declare `orders`
UNRESOLVED  db_column_use      order.total        PostgREST returns the row WITHOUT it
UNRESOLVED  db_function_use    get_statistics     the function is `get_stats`
UNRESOLVED  edge_function_use  send-mail          the directory is `send-email`
UNUSED      db_table           audit_log          no client code touches it
UNRESOLVED  db_policy          orders             read by the client, RLS off

A mistyped column is the quiet one. PostgREST returns the rows without it, the client reads undefined, and a blank field ships. Nothing raises. supabase gen types catches it only if you regenerate after every migration — and that drift is the bug.

The last line is a security check: the anon key ships in your browser bundle, so a table the client reads with row level security off is readable by anyone who opens devtools.

The schema reader is not Supabase-specific — it also reads migrations/, db/migrate/, database/migrations/ and sql/, so Alembic, dbmate, Sqitch and hand-rolled folders work.

Firebase

httpsCallable('sendEmail') is checked against what your functions directory exports — same shape as a fetch against a route.

Firestore is schemaless, so "does this collection exist" has no answer in your files and is never claimed. But firestore.rules is a declaration: a collection with no match block is denied by default and fails silently in production, and a match block for a collection nobody touches is usually a rename left behind.

Redis

No schema either, so a key is checked against its counterpart.

UNRESOLVED  redis_key  user:*:stat     read here, written nowhere — can only ever miss
UNUSED      redis_key  user:*:legacy   written here, read nowhere
UNRESOLVED  redis_ttl  cache:board:*   names itself a cache, written with no expiry

Key patterns are normalised before they are compared, so user:{uid}:stats, user:${id}:stats and user:%s:stats are one key — a Python writer meets a JavaScript reader.

Four words, and it never says more than it can prove

uncertain is a real answer rather than a cop-out. A route assembled at runtime genuinely cannot be known by reading the source, and I would rather it admitted that than pretended otherwise. It is also why the other three can be trusted.

The commands

seamcheck map        # scan, then open the canvas. Start here.
seamcheck check      # the CI gate. Exit 1 on new findings, 2 with no baseline, 0 clean.
seamcheck report     # the findings digest, as text or markdown
seamcheck explain    # why one symbol is classified the way it is
seamcheck triage     # record "this one is fine, and here is why"
seamcheck backfill   # scan the last N commits so the map has history
seamcheck observe    # drive your pages in a real browser and record what it saw
seamcheck config     # what was detected, and how it was worked out

seamcheck help <command> explains any of them with examples.

Useful flags: --format terminal|markdown|html|map|json · --out FILE · --serve / --no-serve · --tunnel (a temporary public HTTPS link, for your phone) · --local-only · --since REF · --open.

If you build with an AI agent

This is where I have found it most useful, honestly. Asked who writes this element?, an agent tends to grep, read a handful of files, and make a reasonable guess. On a big repo that costs a lot of context and is still a guess.

And there is a failure I have run into more than once: asked to fix something on screen, an agent will sometimes add a second place that writes the same element rather than finding the one already there. The symptom moves, the next session adds another, and it slowly gets worse.

So there is an MCP server. The agent asks, and gets the answer with the exact lines.

pip install 'seamcheck[mcp]'
claude mcp add seamcheck -- seamcheck-mcp

The server talks over stdin/stdout — no port, no daemon. Run it with the agent's working directory set to the project root. For a Django project it has to run inside that project's virtualenv, for the same reason the CLI does: it reads the project by importing it. Every other backend is read from source, so anywhere works.

The tools are thin wrappers over the same functions the CLI runs. If the agent and your terminal ever disagreed, neither would be worth trusting.

In CI, if you want it there

seamcheck check --since $BASE_SHA

Exit 1 on new findings, 2 if there is no baseline yet, 0 when clean. It also writes a markdown digest you can post as a PR comment with --format markdown.

What it cannot see

Routes built at runtime from variables. Elements a framework renders from components rather than templates — React and Vue handlers are props, not listeners, and the trail stops at the module boundary. Anything reached only by a string it cannot resolve.

In the data layer: Firestore collections, because Firestore has no schema and there is nothing in your files that declares one — only the rules are checked. Storage buckets, which are usually created in a dashboard rather than in the repo. And a Redis key whose name is assembled from variables end to end.

It says uncertain in all of those cases instead of guessing. That is the whole discipline: uncertain is a real answer, and it is why the other three can be trusted.

Contributing

Issues and pull requests welcome, especially "it got this wrong and here is the file".

License

MIT. Take it, fork it, improve it.

_{A note on the copy: the wording on this page was drafted with an LLM, and partly for them — realistically a coding agent is going to read this README before a person does, and decide whether to install it. So it is written to be easy to parse as well as to read. The tool itself does the opposite: it will not tell you anything without showing you the line it came from.}