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Plugin Install Flow — Learnings

Why This Exists

The Directory → "Anthropic & Partners" tab has a non-obvious install flow that caused a structural bug (#396) on older versions. Key insight: the renderer that populates pluginContext.mode and pluginContext.pluginSource is served remotely from claude.ai in a BrowserView, not bundled locally. Static source inspection only sees the main-process gate; its inputs originate in server-rendered JS outside the asar.

Architecture

The main window is https://claude.ai/task/new loaded in a BrowserView. Only ~288 KB of JS lives locally under .vite/renderer/main_window/assets/; neither installPlugin nor pluginContext appears there.

When the user clicks install on a plugin:

  1. Remote web UI calls CustomPlugins.installPlugin(pluginId, egressAllowedDomains, pluginContext) via IPC (preload bridge → main process).
  2. Main-process IPC handler validates pluginContext via Qg() (runtime type check): { mode: string, workspacePath?, settingsLevel?, pluginSource?, marketplaceScope?, telemetryAttempt? }.
  3. Main-process installPlugin applies the gate, optionally calls the Anthropic API, and falls back to the claude CLI if the remote path is skipped or fails.

The values of mode and pluginSource are decided remotely by claude.ai based on which UI surface called install. The desktop app has no control over them; it only enforces the gate.

Install Gate (current, 1.3109.0)

Location: index.js:490853 inside the minified app.asar.

const a = s?.pluginSource === "local";   // user-uploaded .zip
const c = s?.pluginSource === "remote";  // remote marketplace install
if (!a && (c || s?.mode === "cowork") && (await A0())) {
  // remote API: /api/organizations/{orgId}/plugins/...
} else {
  // skip, log reason: "local-sourced" |
  //                   "not-cowork-not-remote" |
  //                   "sparkplug-disabled"
}
// always falls through to CLI install on failure
  • A0() (index.js:489947) = GrowthBook flag "2340532315" via isFeatureEnabled(), cached locally. Server-controlled.
  • On CLI fallback for a non-local marketplace like knowledge-work-plugins, install fails with Plugin "X" not found in marketplace "knowledge-work-plugins".

Plugin Listing Filter

Four places in 1.3109.0 gate on A0():

Line Function If flag off
490342 syncRemotePlugins {newlyInstalled: []}
490355 getDownloadedRemotePlugins []
491026 listAvailablePlugins local plugins only
491060 listRemotePluginsPage {plugins: [], hasMore: false}

If A0() is false, the Anthropic & Partners tab is empty. Users whose account doesn't have the flag enabled server-side never see these plugins at all.

Backend Endpoints

All served from https://claude.ai (base URL from Jr() = main-window URL). Main-process net.fetch adds identity headers via an onBeforeSendHeaders interceptor at index.js:504876:

Header Value
anthropic-client-platform "desktop_app" (constant)
anthropic-client-app "com.anthropic.claudefordesktop"
anthropic-client-version app.getVersion()
anthropic-client-os-platform process.platform — "linux" / "darwin" / "win32"
anthropic-client-os-version process.getSystemVersion()
anthropic-desktop-topbar "1"

Key endpoints:

Purpose URL Source line
GrowthBook flags GET /api/desktop/features 190336
Default marketplaces (Directory source) GET /api/organizations/{orgId}/marketplaces/list-default-marketplaces —
Account-attached marketplaces (user-added) GET /api/organizations/{orgId}/marketplaces/list-account-marketplaces —
Directory feed GET /api/organizations/{orgId}/plugins/list-plugins?installation_preference=... 246164
Plugin by-id GET /api/organizations/{orgId}/plugins/{id} 246212
Plugin by-name GET /api/organizations/{orgId}/plugins/by-name/{name}?marketplace_name=... 246221
Plugin download GET /api/organizations/{orgId}/plugins/{id}/download 246229

Auth is via the sessionKey cookie. orgId is read from the lastActiveOrg cookie by an() at index.js:191235. No orgId → fetchers return null → install falls back to CLI.

Issue #396 Post-Mortem

Filed on Claude Desktop 1.1.7714. That version had:

Install gate (index.js:230901 in 1.1.7714):

if (!c && (a?.mode) === "cowork" && (await Tg())) {
  // remote API
}
// reasons: "local-sourced" | "not-cowork" | "sparkplug-disabled"

Listing filter (index.js:231032):

if ((s?.mode) !== "cowork" || !(await Tg())) return o;  // local only
// else merge remote

listRemotePluginsPage (index.js:231066):

if (!(await Tg())) return { plugins: [], hasMore: !1 };
// else fetch and return

listRemotePluginsPage gated only on Tg(), not on cowork mode, so the Directory showed remote plugins whenever the sparkplug flag was on. But the install gate required mode === "cowork" specifically. Users browsing the Directory outside a cowork session received pluginContext without mode: "cowork" from the renderer → install gate failed → reason=not-cowork → CLI fallback → "marketplace not found."

Structural bug: plugins visible but uninstallable unless the user was actively inside a cowork session.

Fixed upstream in 1.3109.0 via two coordinated Anthropic-side changes:

  1. Install gate relaxed to accept pluginSource === "remote" as equivalent to mode === "cowork".
  2. claude.ai renderer updated to send pluginSource: "remote" for installs from the Anthropic & Partners Directory regardless of cowork session state.

PR #435 proposed a client-side Linux-specific short-circuit (process.platform === "linux" || ...). Correct strategy for the bug as it existed; obsolete after upstream fix. Closed as obsolete.

Live Investigation Recipe

To debug plugin-flow bugs on a running client:

1. Enable main-process DevTools

echo '{"allowDevTools": true}' > ~/.config/Claude/developer_settings.json

Then fully quit and relaunch the app. Open the (now visible) Enable Main Process Debugger menu item (under Help when dev tools are enabled) — this starts a Node inspector on 127.0.0.1:9229. Connect via chrome://inspect in any Chromium browser and click inspect on the Node target.

Source refs:

  • allowDevTools schema: index.js:299085
  • developer_settings.json path: index.js:299089
  • Debugger menu: index.js:494282

2. List webContents

require('electron').webContents.getAllWebContents()
  .map(w => ({ id: w.id, type: w.getType(), url: w.getURL() }))

Typically three: the find-in-page overlay, the claude.ai BrowserView (id 2), and the main window shell (id 1). The claude.ai one is where the plugin directory UI lives; open its DevTools separately via webContents.fromId(n).openDevTools() to inspect the renderer-side code.

3. Check the cached GrowthBook flag state

(async () => {
  const res = await require('electron').net.fetch(
    'https://claude.ai/api/desktop/features');
  const body = await res.json();
  console.log(body.features['2340532315']);
})();

Expected for users with the force rule: {value: true, source: "force", ruleId: "fr_..."}. If it's {value: false, source: "defaultValue", ruleId: null}, the user won't see any remote plugins — listAvailablePlugins and listRemotePluginsPage filter them out.

4. Header-spoofing harness

Electron only allows one onBeforeSendHeaders listener at a time. Registering a test listener replaces the app's injector (index.js:504876), so the harness re-implements the baseline injection and adds a per-test override layer:

const { app, session, net } = require('electron');

const APP_HEADERS = {
  'anthropic-client-platform': 'desktop_app',
  'anthropic-client-app': 'com.anthropic.claudefordesktop',
  'anthropic-client-version': app.getVersion(),
  'anthropic-client-os-platform': process.platform,
  'anthropic-client-os-version': process.getSystemVersion(),
  'anthropic-desktop-topbar': '1',
};

globalThis.__testOverrides = {};
globalThis.__testRemove = new Set();

session.defaultSession.webRequest.onBeforeSendHeaders(
  { urls: ['https://claude.ai/*', 'https://claude.com/*'] },
  (d, cb) => {
    const h = { ...d.requestHeaders, ...APP_HEADERS,
                ...globalThis.__testOverrides };
    for (const k of globalThis.__testRemove) delete h[k];
    cb({ requestHeaders: h });
  }
);

async function runTest(label, { set = {}, remove = [] } = {},
                      url = 'https://claude.ai/api/desktop/features') {
  globalThis.__testOverrides = set;
  globalThis.__testRemove = new Set(remove);
  const res = await net.fetch(url);
  const ct = res.headers.get('content-type') || '';
  const body = ct.includes('json') ? await res.json()
                                   : await res.text();
  globalThis.__testOverrides = {};
  globalThis.__testRemove = new Set();
  return { label, status: res.status, body };
}

Example: test whether flag depends on OS claim:

(async () => {
  const r = await runTest('darwin', {
    set: { 'anthropic-client-os-platform': 'darwin',
           'anthropic-client-os-version': '15.0' } });
  console.log(r.body.features['2340532315']);
})();

If the flag value changes when you spoof OS, the server is platform-gating; if not, the gate lives at a different layer (account-scoped rule, tier, cohort, or the remote renderer's local JS gating).

5. Breakpoint on the install gate

In main-process DevTools Sources: Ctrl+P → index.js → Ctrl+F → search installPlugin: attempting remote API install. Click the line number to set a breakpoint. Trigger an install in the app. When it breaks, inspect s (the pluginContext) and evaluate await A0() in a watch expression.

The companion breakpoint on installPlugin: skipping remote API path tells you which reason the gate chose if it failed.

Getting the Minified Source for Any Shipped Version

The repo's releases include reference-source.tar.gz (~6.5 MB) — beautified asar contents of the exact Claude Desktop build that was packaged. Much smaller than the AppImage (~133 MB) and sufficient for code diffing between versions.

gh release download "v1.3.23+claude1.1.7714" \
  -R aaddrick/claude-desktop-debian \
  -p 'reference-source.tar.gz' \
  -D /tmp/old-version --clobber
tar -xzf /tmp/old-version/reference-source.tar.gz -C /tmp/old-version
# Compare with current: /tmp/old-version/app-extracted/.vite/build/index.js

This is how #396's post-mortem was done — side-by-side comparison of installPlugin (230901 old vs 490853 current) and listAvailablePlugins (231032 old vs 491026 current) revealed both the structural bug and the upstream fix.

Key Files

  • scripts/patches/cowork.sh — patch_cowork_linux() applies the cowork patches to the asar. Patches 1–10 handle cowork mode infrastructure on Linux.
  • scripts/cowork-vm-service.js — Linux cowork VM daemon (separate subsystem, see cowork-vm-daemon.md).
  • Minified install flow in the running app: app.asar.contents/.vite/build/index.js around line 490853 on 1.3109.0 (subject to minifier drift — anchor on the log string [CustomPlugins] installPlugin: attempting remote API install when writing patches).