gstack/docs/BROWSER_INTERNALS.md

Browser / sidebar / server internals

Scope: this is the internals reference for gstack's own browser engine — the browse daemon, GStack Browser, the sidebar extension. That engine is the automatic fallback when the Aside browser is not installed or not running (Linux, Windows, a closed Aside app); Aside is what every skill drives first. The Aside contract lives in scripts/resolvers/aside.ts and BROWSER.md.

Moved verbatim from CLAUDE.md (token-load reduction). These are the load-bearing invariants for browse/src/server.ts, the Chrome extension, the sidebar PTY, SSE endpoints, CDP sessions, and the sidebar security stack. Every rule here is additionally pinned by a CI tripwire test named in its paragraph.

Sidebar architecture: Before modifying sidepanel.js, background.js, content.js, terminal-agent.ts, or sidebar-related server endpoints, read docs/designs/SIDEBAR_MESSAGE_FLOW.md. The sidebar has one primary surface — the Terminal pane (interactive claude PTY) — with Activity / Refs / Inspector as debug overlays behind the footer's debug toggle. The chat queue path was ripped once the PTY proved out; sidebar-agent.ts and the /sidebar-command / /sidebar-chat / /sidebar-agent/event endpoints are gone. The doc covers the WS auth flow, dual-token model, and threat-model boundary — silent failures here usually trace to not understanding the cross-component flow.

Embedder terminal-agent ownership (v1.42.1.0+, identity-based kill v1.44.0.0+). buildFetchHandler in browse/src/server.ts accepts ServerConfig.ownsTerminalAgent?: boolean (default true). When true, factory shutdown runs the full teardown: identity-based kill via killAgentByRecord(readAgentRecord(stateDir)) from browse/src/terminal-agent-control.ts plus safeUnlinkQuiet on <stateDir>/terminal-port, <stateDir>/terminal-internal-token, and <stateDir>/terminal-agent-pid (the per-boot agent record introduced in v1.44). Embedders (e.g. the gbrowser phoenix overlay) that pre-launch their own PTY server must pass false so their discovery files survive gstack teardown cycles. The flag is the third caller-owned teardown gate in ServerConfig (alongside xvfb? and proxyBridge?); polarity is inverted (explicit bool vs presence) and documented in the field's JSDoc. CLI start() always passes true explicitly — the static-grep test in browse/test/server-embedder-terminal-port.test.ts fails CI if a refactor drops it. Pre-v1.44 used pkill -f terminal-agent\.ts (regex match) which would kill sibling gstack sessions on the same host; the new browse/test/terminal-agent-pid-identity.test.ts static-grep tripwire fails CI if any source file re-introduces pkill ... terminal-agent or spawnSync('pkill', ...).

WebSocket auth uses Sec-WebSocket-Protocol, not cookies. Browsers can't set Authorization on a WebSocket upgrade, but they CAN set Sec-WebSocket-Protocol via new WebSocket(url, [token]). The agent reads it, validates against validTokens, and MUST echo the protocol back in the upgrade response — without the echo, Chromium closes the connection immediately. Set-Cookie: gstack_pty=... is kept as a fallback for non-browser callers (the cross-port SameSite=Strict cookie path doesn't survive from a chrome-extension origin).

Cross-pane PTY injection. The toolbar's Cleanup button and the Inspector's "Send to Code" action both pipe text into the live claude PTY via window.gstackInjectToTerminal(text), exposed by sidepanel-terminal.js. No /sidebar-command POST — the live REPL is the only execution surface in the sidebar now.

/health MUST NOT surface any token — and it no longer does (v1.63+). The historical headed-mode leak of AUTH_TOKEN is fixed: GET /health is liveness/status only in every mode. Token bootstrap is POST /extension-token, which validates the caller's Origin against the pinned extension identity (the key field in extension/manifest.json pins the extension ID — GSTACK_EXTENSION_ID in browse/src/server.ts, derivation reproducible via bun browse/scripts/extension-id.ts) plus a loopback Host. PTY auth still flows through POST /pty-session only. Don't add any token to /health.

Transport-layer security (v1.6.0.0+). When pair-agent starts an ngrok tunnel, the daemon binds two HTTP listeners: a local listener (127.0.0.1, full command surface, never forwarded) and a tunnel listener (locked allowlist: /connect, /command with a scoped token + 26-command browser-driving allowlist, /sidebar-chat). ngrok forwards only the tunnel port. Root tokens over the tunnel return 403. SSE endpoints use a 30-minute HttpOnly gstack_sse cookie minted via POST /sse-session (never valid against /command). Tunnel-surface rejections go to ~/.gstack/security/attempts.jsonl via tunnel-denial-log.ts. Before editing server.ts, sse-session-cookie.ts, or tunnel-denial-log.ts, read ARCHITECTURE.md — the module boundary (no imports from token-registry.ts into sse-session-cookie.ts) is load-bearing for scope isolation.

Unicode sanitization at server egress (v1.38.0.0+). Every server egress that ships page-content-derived strings MUST go through JSON.stringify(payload, sanitizeReplacer) for object payloads or sanitizeLoneSurrogates(body) for text bodies. Lone UTF-16 surrogate halves from CDP page content otherwise reach the Anthropic API as \uD800-style escapes and trigger a 400. Wired at four egress points today: handleCommandInternal (HTTP + batch via a sanitizing wrapper around handleCommandInternalImpl) and both SSE producers (/activity/stream, /inspector/events). Post-stringify regex is a no-op — JSON.stringify has already escaped the surrogate before regex could match, so the replacer must run inside the encoding pipeline. Before adding a new SSE/WebSocket writer or HTTP response in server.ts, read ARCHITECTURE.md. browse/test/server-sanitize-surrogates.test.ts pins the wiring with invariant tests, so bypasses fail CI.

SSE endpoint helper (v1.51.0.0+). New SSE endpoints in server.ts MUST route through createSseEndpoint(req, config) from browse/src/sse-helpers.ts. The helper owns the cleanup contract (abort + enqueue-throw + heartbeat-throw, all idempotent) and bakes in sanitizeLoneSurrogates on every JSON.stringify, so new subscribers can't accidentally regress either invariant. Inline ReadableStream wiring leaked subscribers when the TCP connection died without firing req.signal.abort (Chromium MV3 service-worker suspend, intermediate proxy half-close). /activity/stream, /inspector/events, and /memory (SSE-eligible) all route through it. browse/test/sse-helpers.test.ts pins the cleanup contract.

CDP session lifecycle (v1.51.0.0+). Direct page.context().newCDPSession(page) calls outside browse/src/cdp-bridge.ts fail CI via the static-grep tripwire in browse/test/cdp-session-cleanup.test.ts. Use withCdpSession(page, async (s) => {...}) for one-shot CDP work (try/finally detach) or getOrCreateCdpSession(page, cache) for cached sessions tied to a page's lifetime (close-detach via Map<page, session>). Three sites migrated: cdp-bridge frame events, write-commands archive capture, cdp-inspector. The helpers prevent the per-session leak class where successful-path detach happened but error-path detach was missed.

Setup symlink hardening (v1.38.0.0+). Every link site in setup MUST route through the _link_or_copy SRC DST helper near the IS_WINDOWS detection. On Windows without Developer Mode, plain ln -snf produces frozen file copies that don't refresh on git pull — silent staleness across every host adapter. The helper preserves ln -snf on Unix and switches to cp -R / cp -f on Windows. test/setup-windows-fallback.test.ts enforces a static invariant: a single raw ln call outside the helper body fails CI. Windows users get a one-line note from _print_windows_copy_note_once reminding them to re-run ./setup after every git pull.

Sidebar security stack (layered defense against prompt injection):

Layer	Module	Lives in
L1-L3	content-security.ts	server + read path — datamarking, hidden element strip, ARIA regex, URL blocklist, envelope wrapping
L4	security-classifier.ts (TestSavantAI ONNX)	security sidecar subprocess only (security-sidecar-entry.ts, driven by security-sidecar-client.ts from server.ts)
Canary	security.ts (generate/inject/detect)	pure utilities — no production injector today (the chat prompt-builder that injected them was ripped)
Combiner	security.ts (combineVerdict + THRESHOLDS)	pure, tested; retains transcript/deberta vote handling for LayerSignal inputs no live layer produces anymore

History note: an L4b Haiku transcript classifier and an opt-in DeBERTa ensemble (GSTACK_SECURITY_ENSEMBLE=deberta) existed until the chat-path agent that invoked them was ripped; both were deleted as dead code (zero production callers). Do not re-document them as live.

Critical constraint: security-classifier.ts CANNOT be imported from the compiled browse binary. @huggingface/transformers v4 requires onnxruntime-node which fails to dlopen from Bun compile's temp extract dir — hence the sidecar subprocess. Only security.ts (pure-string operations — canary utilities, verdict combiner, status) is safe for server.ts. See ~/.gstack/projects/garrytan-gstack/ceo-plans/2026-04-19-prompt-injection-guard.md §"Pre-Impl Gate 1 Outcome" for the original architectural decision.

Thresholds (in security.ts): BLOCK: 0.85, WARN: 0.75, LOG_ONLY: 0.40, SOLO_CONTENT_BLOCK: 0.92 (label-less content classifiers can't distinguish "injection" from "phishing aimed at the user", so their solo bar is higher). The live L4 path applies these in server.ts's sidecar-scan handling; canary leak always BLOCKs (deterministic).

Env knobs:

• GSTACK_SECURITY_OFF=1 — emergency kill switch. Classifier stays off even if warmed; the L1-L3 filters keep running.
• Classifier model cache: ~/.gstack/models/testsavant-small/ (112MB, first run only)
• Attack log: ~/.gstack/security/attempts.jsonl — written by tunnel-denial-log.ts (tunnel-surface rejections; rotates at 10MB, 5 generations)

History note (#2557): the cross-process session state (~/.gstack/security/session-state.json), getStatus(), the /health security field, and the sidepanel SEC shield were all removed — the state file lost its only writer when sidebar-agent.ts was ripped, so the shield reported a permanent 'inactive' or a stale false-green 'protected' from leftover disk state. The live defenses (L1-L3 filters, L4 sidecar on the inject-scan path) report through their own call sites, never through /health. browse/test/server-security-surface.test.ts pins both the removal and the live L4 wiring. Do not re-document these as live.