A tab can't see an HTTP response the way a desktop app can: CORS hides most headers (2 of 8 in a typical response), redirects are followed silently, and there is no timeline. So the tab renders the request and posts it here, and this process puts it on the network with the desktop's own engine (yaak-http) and streams back everything that happened — every header, every redirect hop, DNS timing, the body — for the tab to store.

It is a stateless executor. It keeps nothing: no database, no files, no sessions, no cookies between calls. Every byte it sees comes from the tab in the request, and every byte it returns is stored by the tab. Restart it any time.

Self-hosting it

One container, no configuration, nothing behind it:

docker run -p 8080:8080 ghcr.io/mountain-loop/yaak-web

Open http://localhost:8080. The image carries the built web client and this binary, which serves it — so the app and its sends are on one origin, and the tab's send URL is a path (/v1/http/send) rather than an address anyone has to configure. The image is linux/amd64 and linux/arm64, built from Dockerfile.web at the repo root.

Your data lives in your browser (SQLite compiled to wasm, in IndexedDB), not in the container. The container is stateless: nothing is written to disk, so upgrading is docker pull and nothing else.

Two settings are worth knowing about:

docker run -p 8080:8080 \
  -e YAAK_WEB_ALLOW_PRIVATE_NETWORKS=true \
  -e YAAK_WEB_RATE_LIMIT_PER_MINUTE=0 \
  ghcr.io/mountain-loop/yaak-web

YAAK_WEB_ALLOW_PRIVATE_NETWORKS=true lets sends reach loopback, private and link-local addresses. Off by default, and it should stay off on anything strangers can reach — see What it refuses. Turn it on for an instance on your own network, where calling the API on the next machine is the whole point. Note that "private" is relative to the container: 127.0.0.1 is the container itself, and reaching the Docker host means host.docker.internal (or --network host).
YAAK_WEB_RATE_LIMIT_PER_MINUTE defaults to 120 sends per client IP, which suits a public instance and not a team of your own; 0 disables it.

Behind a reverse proxy, add YAAK_WEB_TRUST_FORWARDED_FOR=true so the rate limit sees real client addresses instead of its own — and only then, since otherwise anyone can spoof the header. If the reverse proxy buffers responses, tell it not to: sends are streamed, and the X-Accel-Buffering: no header this binary sets is honoured by nginx-shaped ones.

Running it from source

cargo run -p yaak-web -- --serve dist/apps/yaak-client

after a YAAK_TARGET=web SKIP_WASM_BUILD=1 npx vp -C apps/yaak-client build. Without --serve it is the send executor alone, which is what the frontend dev server wants:

cargo run -p yaak-web
YAAK_TARGET=web npm run dev --workspace @yaakapp/yaak-client

A dev build looks for the server at http://127.0.0.1:9227 (the Vite server is a different origin and serves no /v1); a production build sends to its own origin unless VITE_YAAK_WEB_URL was set when it was built.

Configuration

Every flag has a YAAK_WEB_* environment variable, so a container needs no arguments; --help lists them all.

Flag	Default	What
`--serve`	off	Also serve a built web client from this directory, on the same origin.
`--bind`	`127.0.0.1:9227`	Listen address. The image sets `0.0.0.0:8080`.
`--allow-private-networks`	off	Allow sends to loopback, private and link-local addresses.
`--allowed-origins`	`*`	CORS origins, comma-separated. Unused when the app is served from here: same origin, no CORS.
`--max-request-bytes`	16 MiB	Largest rendered request accepted from the tab.
`--max-response-bytes`	64 MiB	Largest upstream body relayed before the send is cut off.
`--max-timeout-secs`	60	Ceiling on a send's timeout; a request asking for more (or none) gets this.
`--rate-limit-per-minute`	120	Sends per client IP per minute; 0 disables.
`--max-concurrent`	256	Sends in flight at once.
`--trust-forwarded-for`	off	Take the client IP from `X-Forwarded-For`. Only behind a load balancer that sets it.

Serving the app

--serve DIR puts a file server behind the API routes: /v1/* is matched first, everything else comes from DIR, and a path with no file behind it gets index.html so the app's own routes survive a refresh. Responses are compressed (gzip or zstd) on the fly. /assets/* is cached forever — Vite content-hashes those names — and everything else is no-cache, so a new deploy arrives on the next reload.

Serving files changes nothing about sending: the same rendered request, the same destination policy, the same stateless executor. It exists so that a self-hosted Yaak is one thing to run rather than two.

Split deployments

The app and the sender can still be separate services — one CDN-hosted bundle and one server elsewhere, or one server shared by several fronts. Then the bundle has to be told where to send, at build time:

docker build -f Dockerfile.web \
  --build-arg VITE_YAAK_WEB_URL=https://send.example.com .

and the server needs the CORS origins its callers use, since the requests are no longer same-origin:

docker run -p 8080:8080 \
  -e YAAK_WEB_ALLOWED_ORIGINS=https://yaak.example.com \
  ghcr.io/mountain-loop/yaak-web \
  yaak-web

The trailing yaak-web is a command override: the same image run without --serve, so it executes sends and serves no app.

What it refuses, and why

A hosted sender is, by construction, a machine that makes HTTP requests on behalf of strangers. Left alone that is an open relay into whatever network it sits on. So by default it refuses to connect to:

loopback (127/8, ::1), private (10/8, 172.16/12, 192.168/16, fc00::/7), link-local (169.254/16 — where cloud metadata lives — and fe80::/10), carrier-grade NAT, multicast, reserved and unspecified ranges, IPv4 addresses carried inside IPv6 forms (::ffff:a.b.c.d, the well-known NAT64 prefix, 6to4), and the whole NAT64 local-use range;
anything not http:// or https://.

The check runs on the resolved addresses, after DNS, for every hop of a redirect chain, so a public hostname that points at an internal address is caught, and so is a Location: header that points at one. It also refuses body types that would read files on its own disk (binary, multipart file fields), since no browser tab could legitimately mean those.

Refusals are logged with the reason. On a public instance (web.yaak.app, or anything else strangers can reach) this must stay on: the machine's private network is the host's, not the user's, so a localhost or LAN API is not the user's to reach through it — the desktop app is what reaches those. On an instance you run for yourself, that reasoning is inverted, and --allow-private-networks inverts the policy with it. It allows every range above, including 169.254.169.254, so use it only where the network on the other side is one the users are entitled to.

There is no authentication either way: an instance is anonymous, protected by the per-client rate limit and the destination policy. Anything more (a shared token, per-user quotas) is a later slice and would sit in front of send_http in main.rs. Put TLS in front of a public instance.

The wire

POST /v1/http/send with a JSON body:

{
  "request":  { "url": "https://…", "method": "GET", "headers": […], "body": {…}, "bodyType": null, "urlParameters": […] },
  "settings": { "validateCertificates": true, "followRedirects": true, "timeoutMs": 0, "sendCookies": true, "storeCookies": true },
  "cookies":  [ … ] 
}

request is a Yaak HttpRequest in the desktop's own model shape with every template already rendered by the tab; the server builds the URL, headers and body from it exactly the way the desktop does after rendering. cookies is the jar's contents (or null for no jar).

The reply is application/x-ndjson, one JSON frame per line, in the order things happened:

`type`	When	Carries
`event`	as the engine produces them	one timeline event, in the desktop's `http_response_event.event` shape
`response`	once, when the final hop's headers arrive	status, all headers, request headers as sent, remote address, HTTP version, timing
`body`	as the body is read	a decompressed chunk, base64
`done`	last, on success	elapsed, byte counts, and the cookie jar as the send left it
`error`	last, on failure	the reason, and any cookies collected before the failure

Refusals that happen before anything is sent (a blocked destination, a bad body, rate limit, capacity) are plain HTTP errors (403, 400, 429, 503) with {"error": "…"}, not streams.

Why a streamed HTTP response and not a WebSocket: one POST is stateless by construction, cancellable by closing the connection, readable with curl, and needs no upgrade handling on either side. A WebSocket only earns its keep when traffic is bidirectional, which a single send is not.

The TypeScript side of this contract is generated from src/wire.rs by ts-rs into bindings/ (run cargo test -p yaak-web after changing a frame) and published to the tab as @yaakapp-internal/web, so a change to the wire on one side is a type error on the other.

GET /v1/health reports the version and the effective limits.

What comes later

Not built, by design, but the router is shaped for it: a WebSocket relay (/v1/ws/relay) and a gRPC relay (/v1/grpc/relay) would be long-lived, bidirectional endpoints on the same binary, behind the same destination policy and limits. They differ from this endpoint in holding per-connection in-memory state while a connection is open (never persisted), which brings connection limits and a larger abuse surface — the reason they are separate work.