heso 0.1.0

Minimal web runtime for agents: fetch, JS, DOM, forms, clicks → JSON. No rendering pipeline, no Chromium, no Node.

heso — The agent half of the web. One Rust binary.

Site: heso.ca · Docs: heso.ca/docs · npm · PyPI · Releases

The minimal web runtime for agents: fetch, JS, DOM, forms, clicks → JSON. No rendering pipeline, no Chromium, no Node. Failures come back as structured data (partial: true, bot_challenge, cassette miss) — not opaque browser crashes. Every run can be stamped into a signed plat with an embedded network cassette, then byte-identically re-executed off-network with heso run. Anyone, any machine, same output.

binary       9.73 MB
cold start   ~77 ms   (open https://example.com, network included)
engine only  ~28 ms   (no network)
batch        ~1.1 s   for 8 URLs in parallel

A 50-second real recording — an LLM agent (Gemini) drives heso to find and compare two GitHub repositories by star count and README description, then stamps the run into a verifiable plat (tamper one byte → the hash flags it). No Chromium, no rendering pipeline, no driver. ▶ Watch the full demo on heso.ca

Contents

• Install
• What it can do
• What it can't do
• Why not just use X?
• Use as a library
• Examples
• Signed receipts
• Error handling
• Plug into agent harnesses
• Verbs are open
• Use as an agent skill
• Stats
• Building from source
• Status
• License

Install

# Python (uv, pipx, or pip — any of them)
uv tool install heso          # or: pipx install heso  /  pip install heso

# Node
npm install -g @ixla/heso     # or one-shot: npx @ixla/heso open https://example.com

# Direct binary installers
# macOS / Linux:
curl --proto '=https' --tlsv1.2 -LsSf https://github.com/blank3rs/heso/releases/latest/download/heso-cli-installer.sh | sh

# Windows:
powershell -ExecutionPolicy Bypass -c "irm https://github.com/blank3rs/heso/releases/latest/download/heso-cli-installer.ps1 | iex"

Shipping v0.1.0 for Windows-x64, Linux x64 + arm64, macOS x64 + arm64. cargo-dist builds every target on tag; npm/PyPI publish through the same workflow.

After install, heso is on $PATH:

heso open https://example.com
# → { url, title, description, tree, actions, plat_hash, ... }

You get JSON: title, description, a heading tree, and a list of clickable elements numbered @e0, @e1, and so on.

What it can do

Find and read things.

• heso search "<query>" — searches the web (DuckDuckGo + Wikipedia, optional SearXNG). No API key.
• heso open <url> — fetches and returns a page summary: title, headings, actionable elements.
• heso read <url> — fetches, runs JS, returns the full picture: title, visible text, actions, forms, cookies, console output, framework detection. One call.
• heso read <url> --complete — same, but heso loops "fire pending observers + click load-more + wait for DOM to settle" until the page stops changing. For lazy-loaded sites.
• heso batch [open|read] <urls...> — runs many URLs in parallel. Shared cookie jar, JSON-Lines out.
• heso wait <url> --selector-exists ".foo" (also --text-contains, --url-matches, --network-idle, --time) — blocks until a condition is true. No polling loop.

Interact with sites.

• heso click <url> @e7 — click by element ref.
• heso click <url> --text "Sign in" — or by visible text, CSS selector, or aria-label.
• heso fill <url> @e3 "hello" — type into an input.
• heso submit <url> @e9 — submit a form.
• heso serve exposes a JSON-RPC navigate method for changing URL inside a stateful session.
• heso eval-dom <url> "<js>" — fetch, run scripts, then run your JS against the resulting DOM.

Bundle, edit, replay, and re-execute action sequences.

A plan is a JSON array of canonical actions (open, click, fill, submit). A plat is an observation, plus an embedded network cassette — every (method, URL, request-body) → (status, headers, response-body) tuple the engine touched during the run. Four verbs close the loop:

• heso stamp <plan.json> — executes the plan against the live web and mints a fresh plat that embeds the plan, the recorded cassette, and a per-step log. Accepts a bare Action[] array, a plat with a "plan" field, or a TraceFingerprint. Exit 0 on a clean run; 1 if any step failed (still prints the partial plat with error + steps).
• heso run <plat.plat> — re-executes the plan against the embedded cassette. No network. For an unchanged cassette the output plat_hash equals the input's — byte-identical replay (ADR 0008). If the cassette has drifted (page changed since stamping), the failing step carries a structured cassette miss: METHOD URL not recorded error and run exits 1 — graceful, never silent.
• heso replay <plat.plat> — pure observation. Reads the recorded step log from the plat and prints it. No engine, no JS, no cassette lookup, no network. Use run if you want to re-execute.
• heso unpack <plat.plat> — extracts just the plan field. Edit it standalone and pipe back into stamp to re-mint a fresh plat (with a fresh cassette since the requests changed).

cat > plan.json <<EOF
[
  {"verb": "open",   "url": "https://news.ycombinator.com/"},
  {"verb": "click",  "ref": "@e3"},
  {"verb": "fill",   "ref": "@e7", "value": "claude"},
  {"verb": "submit", "ref": "@form1"}
]
EOF
heso stamp plan.json > out.plat           # plan → plat (records cassette)
heso run out.plat > replay.plat           # plat → plat (off-network, byte-identical)
heso replay out.plat                      # plat → step log (pure read, no execution)
heso unpack out.plat > plan-again.json    # plat → plan (edit, restamp)

The plat's plat_hash (BLAKE3 over canonical JSON via RFC 8785) commits to the plan, the observed content, AND the embedded cassette. Tamper with any of them and the hash no longer matches; heso plat-verify will say so. Two different <url> inputs always produce different plat_hash values — the URL is part of the hashed canonical bytes, and a regression test in crates/heso-engine-fetch/src/plat.rs::tests pins that invariant against future drift.

Inspect a plat. Text dev tools, all baked into the main binary:

heso plat-info   out.plat                       # human summary: hash, plan / cassette / steps counts, sealed status
heso plat-diff   before.plat after.plat         # what changed (plan, cassette URLs, fields, url / title / description)
heso plat-redact cookies   my.plat > clean.plat # strip a top-level field; recomputes plat_hash
heso plat-seal   my.plat > sealed.plat          # wrap in Ed25519 envelope (default key: heso-local-data/identity.key)
heso plat-unseal sealed.plat                    # verify; exit 0 valid / 1 invalid / 2 wrong-alg or malformed
heso plat-unseal sealed.plat --extract          # verify, then print the inner plat body for piping

plat-redact refuses sealed envelopes (would break the signature). Removing any present content field recomputes plat_hash; if the field contributed to the plat body, the hash changes and any prior signature is invalidated. The top-level plat_hash field itself is bookkeeping and is excluded from its own digest.

plat-seal produces a SealedPlat JSON envelope ({alg, content, signature}) that any holder of the envelope + the heso binary can verify offline — no key material, no network, no clock. Mint a key once with heso identity init; from then on the same key signs every plat. plat-unseal checks the algorithm tag, the embedded plat_hash, and the Ed25519 signature in order, and refuses to silently treat an unknown alg as Ed25519.

Replay a published plat in one command. Install heso (uv tool install heso / pipx install heso / npm install -g @ixla/heso), then:

curl -sL https://github.com/blank3rs/heso/releases/download/v0.0.10/replay-demo-1-goldfinger.plat.json \
  | heso run - \
  | jq -r .plat_hash
# → d93c08ba32b762dd6e47091a1d4bd4aa4d8308dbdbf44869f81146a3f5b8033a

That hash is BLAKE3 over the canonical bytes of the resulting plat. Anyone, any machine, any time — same hash. The cassette inside the plat carries every HTTP response the engine touched when it was stamped against the live Wikipedia article. No network is involved in heso run itself.

Three sample plats live as release assets on v0.0.10:

• replay-demo-1-goldfinger.plat.json — Wikipedia Goldfinger_(film) (1 MB plat, hash d93c08ba…)
• replay-demo-2-torvalds-bio.plat.json — Wikipedia Linus_Torvalds (1 MB plat, hash 27e66b0d…)
• replay-demo-3-rust-lang-rust.plat.json — github.com/rust-lang/rust (640 KB plat, hash 201e9410…)

Recover from broken sites.

• --best-effort on open / read / wait — exit 0 even when scripts crash. Output includes partial: true, partial_reason: "script_crash" | "wait_timeout" | "fetch_failed" | "parse_error", and failed_scripts: [...]. The agent sees what broke and decides what to try next.
• --inject-script "<inline-js>" or --inject-script @file.js — run JS before the page's own scripts. Use it to shim a missing global (the canonical window.lunr cascade kind of thing).

Detect cross-call state changes.

• heso read always returns a content_hash. Pass --since <prev_hash> to get a delta describing what changed (actions_added, actions_removed, forms_changed, text_changed, title_changed).

Honest about failure.

• Every open / read / fetch response carries http_status (200, 403, 503, ...) — captured pre-body-consumption so 4xx/5xx pages never come back wearing a 200 mask. Cloudflare-style "Just a moment..." interstitials are detected via __cf_chl_opt / challenge-token markers and surfaced as partial_reason: "bot_challenge". No more silent "I got something" when the server returned an error page.
• heso click @e7 on an <a href="..."> actually follows the link — the response carries the destination page's title, tree, actions, and http_status, not the source page.

Web platform coverage.

• XMLHttpRequest (sync + async, backed by the same reqwest client as fetch), performance.mark / performance.measure, document.getElementsByClassName / getElementsByName / getElementsByTagName, 60+ HTMLElement subclass constructors (new HTMLDivElement() works, instanceof HTMLScriptElement works), element.style = "color: red" string-coercion setter, data: URL fast path in <script src>.
• MutationObserver + IntersectionObserver fire on real DOM mutations and viewport intersections; setTimeout / setInterval accept the 1-arg form per WHATWG HTML; classic <script> runs sloppy-mode per spec (so sites like Apple and Wikipedia that use var = ... at the top level work); ES modules (<script type="module">) stay strict per ECMA-262.

Stateful sessions.

• heso serve — JSON-RPC over stdin/stdout. Cookies, DOM mutations, listeners, and history persist across calls. Useful for login → navigate → scrape flows.

What it can't do

• No rendering. No canvas, WebGL, CSS layout, or video. If the meaning is in pixels, use a real browser.
• CAPTCHAs and hard bot-detect. Hits one, stops. The default user-agent is heso/<version> so anything fingerprinting will see us coming. We detect Cloudflare interstitials and surface them as partial_reason: "bot_challenge" rather than pretending the page loaded.
• Service Workers, WebRTC, WebUSB, WebBluetooth. Not implemented. The JS engine itself runs modern Next.js / React / Vue / Svelte / SSR sites cleanly; the gaps are in browser features above ECMAScript.
• Sibling-script cascades we haven't shimmed. When script A sets window.X and script B reads it, and X doesn't exist on first load, heso surfaces the crash and the agent can --inject-script a stub.

Why not just use X?

Partial overlap everywhere; no exact shelf neighbor. The win is not "smaller browser" — it is smaller failure surface when the task is structured data, not pixels.

Layer	Examples	What they ship	Gap vs heso
Full Chromium stack	Playwright, Puppeteer, Browser Use, Stagehand, Skyvern	V8 + full browser; often an AI planner on top	Heavy deps, opaque failures, no native JSON verb surface, no plat replay
Smaller browser engine	Lightpanda	Zig engine, V8, CDP — drop-in for Playwright/Puppeteer	Still a browser mental model; agents drive it through CDP/wrappers, not verbs; no plat/cassette/receipt story
Scraper APIs	Firecrawl, Jina Reader, Crawl4AI	Fetch + extract markdown/JSON	Weak or no real click/fill/submit; often no honest partial-failure envelope
DOM simulators (Node)	jsdom, happy-dom	Minimal DOM + JS in JS	Proven lane for the agent-relevant half; test harnesses, not shipped agent products
Built-in fetch tools	WebFetch (Claude), curl	Static HTML / no JS hydration	No DOM events, no forms, no session
heso	this repo	QuickJS + agent-shaped DOM + verbs + plats	QuickJS ≠ V8 (honest limit); CAPTCHAs/hard bot-detect still stop you

What heso adds on top of the capability list: explicit in/out scope, verb-native JSON (no Playwright/CDP/Node required), structured partial failures, and byte-identical off-network replay via stamp/run.

Use as a library

The Python (heso) and Node (@ixla/heso) packages each ship two faces of the same bundled binary: a CLI on $PATH and a programmatic API that spawns that binary under the hood and gives you back parsed JSON as native objects. No FFI, no Python extension module, no N-API addon — subprocess + JSON is the contract.

# Python
import heso

page    = heso.open("https://example.com")              # -> dict
results = heso.search("rust web scraping", limit=5)     # -> dict
content = heso.read("https://example.com", complete=True)

# Stateful flow over one long-lived `heso serve` process:
with heso.session() as s:
    s.open("https://example.com")
    s.click(text="More information...")
    page = s.read()

// Node
import { open, search, read, session } from "@ixla/heso";

const page    = await open("https://example.com");
const results = await search("rust web scraping", { limit: 5 });
const content = await read("https://example.com", { complete: true });

await session(async (s) => {
  await s.open("https://example.com");
  await s.click({ text: "More information..." });
  const page = await s.read();
});

Per-language idioms: Python is snake_case + sync, Node is camelCase + Promises. Full API at heso.ca/docs.

Examples

Search the web, then read the top hits in parallel:

heso search "rust web scraping" --limit 5
heso batch read url1 url2 url3 --parallel 2

Read everything from one page in one call:

heso read https://nextjs.org/
# → { title, text, actions, forms, cookies, console, framework,
#     content_hash, lazy_hints, partial: false, ... }

Find by visible text, click, follow:

heso click https://news.ycombinator.com --text "More"

Wait for an SPA condition:

heso wait https://app.example.com/ --selector-exists ".dashboard" --timeout 5s

Rescue a broken site with a polyfill:

heso open https://shoelace.style --best-effort \
  --inject-script "window.lunr = (() => ({ Index: { load: () => ({}) } }))()"

Multi-step session over stdio:

heso serve
# → JSON-RPC. Page state, cookies, DOM all persist across requests.

Reproducibility (same seed → same output across machines):

heso eval-js --seed 42 'Math.random()'   # 0.5140492957650241
heso eval-js --seed 42 'Math.random()'   # 0.5140492957650241

Signed receipts

Every heso open / heso read call can emit a signed receipt alongside its JSON output — an Ed25519-signed envelope describing what was run, what came back, and the BLAKE3 trace hash. The recipient verifies the signature against an allowlist of trusted public keys (or rejects the receipt). Per ADR 0005 + ADR 0008.

One-time setup — generate a local Ed25519 identity:

heso identity init
# → {"path": "heso-local-data/identity.key", "public_key": "fdibx2...IE=", "algorithm": "Ed25519"}

Sign a receipt on every call by passing --receipt PATH:

heso open https://example.com/ --receipt receipt.json
# stdout: the normal page JSON
# receipt.json (sibling file):
# {
#   "trace": [{"op": "cd", "target": {"kind": "url", "url": "https://example.com/"}}],
#   "results": [{"op": "cd", "url": "https://example.com/"}],
#   "trace_hash": "7e501fac...",
#   "seed": 0, "mode": "deterministic", "cost": {...},
#   "signature": {"algorithm": "Ed25519", "public_key": "fdibx2...IE=", "signature": "bNBb...Cg=="}
# }

Verify the receipt — bind it to a trusted signer with --trusted-keys:

# trusted.json is a JSON array of base64 pubkeys you accept signatures from.
echo '["fdibx2rLqGfrIf+duGbRKlM1iPwVSynHUq+nEisjwIE="]' > trusted.json

heso receipt-verify --trusted-keys trusted.json receipt.json
# → OK fdibx2rLqGfrIf+duGbRKlM1iPwVSynHUq+nEisjwIE=
# exit 0

Or via the HESO_TRUSTED_KEYS=<path> env var if you'd rather not pass the flag every call.

Verify enforces three rejections:

# 1. Tampered receipt — any byte change invalidates the signature
sed -i 's/"seed": 0/"seed": 999/' receipt.json
heso receipt-verify --trusted-keys trusted.json receipt.json
# → INVALID: signature verification failed       (exit 1)

# 2. Wrong signer — receipt is well-formed but the pubkey isn't allowlisted
heso receipt-verify --trusted-keys other_keys.json receipt.json
# → INVALID: signing pubkey `...` is not in the trusted-keys allowlist   (exit 1)

# 3. `mode: live` — live runs use real time + real network and aren't
#    replay-safe, so the signature has no replay value (ADR 0008)
heso open https://example.com/ --receipt live.json --mode live
heso receipt-verify --trusted-keys trusted.json live.json
# → INVALID: receipt `mode: live` is not replay-safe — per ADR 0008 ...   (exit 1)

Verify without an allowlist still works for backwards compatibility, but emits a stderr warning so the missing trust anchor isn't silent:

heso receipt-verify receipt.json
# stderr: warning: no pubkey allowlist configured (pass --trusted-keys PATH or set HESO_TRUSTED_KEYS ...)
# stdout: OK fdibx2...IE=
# exit 0

Exit codes: 0 valid + (allowlist empty OR pubkey allowlisted), 1 invalid (tampered, wrong signer, or mode: live), 2 missing/malformed receipt or --trusted-keys load failure.

Error handling

Both libraries throw a structured error (HesoError in Python, HesoError extends Error in Node) when the binary exits non-zero. Fields on the error tell you what to retry:

import heso
try:
    page = heso.read("https://shoelace.style")
except heso.HesoError as e:
    print(e.returncode, e.stderr[:200])  # exit code + first 200 chars of stderr

import { read, HesoError } from "@ixla/heso";
try {
  const page = await read("https://shoelace.style");
} catch (e) {
  if (e instanceof HesoError) {
    console.error(e.code, e.stderr.slice(0, 200));
  }
}

For sites that crash some scripts, use best_effort / bestEffort instead — heso exits 0 with a partial: true envelope so you handle the failure as data, not an exception:

page = heso.read("https://shoelace.style", best_effort=True)
if page["partial"]:
    print("got partial:", page["partial_reason"], page["failed_scripts"])

Plug into agent harnesses

heso is harness-agnostic. The same package serves five integration patterns:

Harness style	How heso fits
Python frameworks (LangChain, Pydantic AI, LangGraph, smolagents, AgentScope)	import heso. Each function returns a dict. Wrap with @tool / Tool(...) / a function schema.
Node / TS frameworks (Mastra, Vercel AI SDK, LangGraph.js, Stagehand, Browser Use TS)	import { open, search } from "@ixla/heso". All async; TypeScript types ship in index.d.ts.
Skill-markdown harnesses (Claude Code, Cursor, Aider, Cline, Continue, Windsurf)	Drop the manifest in the "Use as an agent skill" block below into ~/.claude/skills/heso/SKILL.md (or the harness's skills dir). The harness auto-discovers; heso on PATH does the rest.
CLI-spawning harnesses (Aider, shell-script agents, homegrown loops)	Same heso <verb> ... CLI used by both libraries. JSON on stdout. No special integration.
Long-running JSON-RPC harnesses	heso serve is a JSON-RPC 2.0 server over stdin/stdout. Cookies + DOM state persist across calls.

The verbs are the contract (see ADR 0017) — no heso-specific framework dependency, no adapter layer.

Verbs are open

HESO/1.0 is an open protocol; the heso binary is one implementation of it. The spec defines a closed core set of bare verb names (open, read, click, fill, submit, wait, stamp, run, replay, unpack, identity-init, receipt-verify, plat-hash, plat-verify, plat-seal, plat-unseal) — every conformant implementation must dispatch these. Beyond the core, anyone can define a verb under a domain they control, reverse-DNS style:

{"verb": "com.example.scrape-pricing", "url": "https://example.com/products"}
{"verb": "org.archive.warc-import",    "path": "./snapshot.warc"}

No registration server, no central authority. DNS ownership is the squat-defence — the same model as Java packages, Android application IDs, Maven groups, and OCI image labels. Publish a doc under your domain describing the input/output shape; any HESO/1.0 implementation that knows that verb can dispatch it.

Today, the reference implementation (this binary, v0.1.0) ships only the core verbs — typing heso com.example.foo ... exits with unknown subcommand. Third-party verb dispatch is in the spec, not yet in the reference impl. To use an extension verb today you implement HESO/1.0 yourself, in any language; the protocol does not lock vocabulary to one binary.

Use as an agent skill

heso is built to be a tool an agent calls, not a library a human drives. The cleanest integration is the skill markdown pattern that Claude Code, Cursor, Aider, Cline, and similar harnesses use:

---
name: heso
description: Use heso, the agent-native page reader (one Rust binary, no Chromium, no Node) for fetch, JS, DOM, forms, clicks, and JSON — minimal failure surface for agent web tasks. Prefer this over WebFetch when you need a DOM, stateful clicks, or framework-rendered content.
---

## Verbs

- `heso search "<query>" [--limit N]` — web search via DDG + Wikipedia
- `heso open <url>` — page summary
- `heso read <url> [--complete]` — full content + actions + forms (use --complete for lazy-loaded sites)
- `heso wait <url> --selector-exists ".x"` — block until a condition is true
- `heso batch [open|read] <urls...> [--parallel N]` — parallel scrape
- `heso click <url> --text "..." | --selector "..." | @eN` — click
- `heso fill <url> @eN "value"` — type into input
- `heso submit <url> @eN` — submit form
- `heso eval-dom <url> "<js>"` — run JS against the page
- `heso serve` — multi-step JSON-RPC session
- `--best-effort` on open/read/wait — exit 0 on partial failures, surface what broke
- `--inject-script "<js>" | @file` — inject a polyfill before page scripts run

The verbs are the contract. Same shape works in any harness that does tool or skill markdown.

Stats

Measured on Windows 11, AMD x86_64, with the release binary:

Thing	Number
Binary size	9.73 MB
Cold start (open https://example.com, network included)	~77 ms
Engine-only (no network, local fixture)	~28 ms
Batch (8 URLs, --parallel 8)	~1.1 s total
Search (DDG, 5 results)	~1 s

Building from source

If you want to hack on heso itself (prebuilt binaries for Windows x64, Linux x64+arm64, macOS x64+arm64 ship from each release tag — see Install above):

git clone https://github.com/blank3rs/heso
cd heso
cargo build --release -p heso-cli
./target/release/heso search "rust web scraping" --limit 5

Requires Rust 1.90 (rustup from https://rustup.rs).

Status

Pre-alpha. v0.1.0 is on every registry. Worth trying if the use case fits; not worth depending on in production yet — APIs may still shift.

License

MIT or Apache-2.0, your choice.

---

Full docs: heso.ca/docs · Site: heso.ca · npm: @ixla/heso · PyPI: heso