diff --git a/docs/solutions/integration-issues/vite-api-source-modules-proxied-to-backend.md b/docs/solutions/integration-issues/vite-api-source-modules-proxied-to-backend.md
new file mode 100644
index 0000000000..b47346f552
--- /dev/null
+++ b/docs/solutions/integration-issues/vite-api-source-modules-proxied-to-backend.md
@@ -0,0 +1,106 @@
+---
+title: Vite API source modules proxied to the backend in dashboard dev mode
+date: 2026-06-08
+category: integration-issues
+module: dashboard-dev-server
+problem_type: integration_issue
+component: tooling
+symptoms:
+  - "Agent-browser snapshots show a blank or broken dashboard even though the dashboard source compiles"
+  - "Vite module requests such as /api.ts, /api-node.ts, or /api/legacy.ts are forwarded to the backend instead of served by Vite"
+  - "Hot-module or cache-busted source requests with query strings can re-enter the backend proxy"
+root_cause: config_error
+resolution_type: config_change
+severity: medium
+related_components:
+  - development_workflow
+tags: [vite, proxy, agent-browser, hmr, dashboard, worktree, source-modules]
+---
+
+# Vite API source modules proxied to the backend in dashboard dev mode
+
+## Problem
+
+The dashboard dev server used a broad `/api` proxy rule that caught the app's own Vite source-module requests. In linked-worktree browser verification, that made `agent-browser` see a blank or broken dashboard because requests for files like `/api.ts` and `/api/legacy.ts` were sent to the backend instead of Vite's module pipeline.
+
+## Symptoms
+
+- `agent-browser` could open the page but could not reliably inspect the Settings UI because the dashboard failed during module loading.
+- Direct requests for `/api.ts`, `/api-node.ts`, and `/api/legacy.ts` returned backend/proxy behavior instead of JavaScript from Vite.
+- `/api/health` still needed to proxy to the dashboard backend, so simply disabling the proxy was not a valid fix.
+- The first regex fix excluded only paths ending in `.ts`; review caught that Vite can append query strings, so `/api/foo.ts?x=1` would still proxy.
+
+## What Didn't Work
+
+- Treating this as an `agent-browser` problem. The browser automation was working; the page it loaded was failing because module requests were misrouted.
+- Debugging linked-worktree HMR as a stale bundle issue. Stale bundles are a real worktree trap, but here direct URL checks showed Vite source module paths were being routed to the backend.
+- Switching the proxy from `"/api"` to a regex that only excluded `\.ts$`. That fixed plain `/api/foo.ts` but missed query-string requests that Vite may use for module import and HMR cache busting.
+- Removing or bypassing the API proxy. Real backend endpoints such as `/api/health` still need to reach the API server during dev.
+
+## Solution
+
+Keep the backend proxy for real API endpoints, but make the proxy key exclude Vite source-module paths under `/api*`.
+
+```ts
+// Before: prefix proxy caught both backend endpoints and source modules.
+proxy: {
+  "/api": {
+    target: `http://localhost:${process.env.FUSION_API_PORT ?? "4040"}`,
+    changeOrigin: true,
+    ws: true,
+  },
+}
+```
+
+The final proxy rule:
+
+```ts
+// packages/dashboard/vite.config.ts
+server: {
+  proxy: {
+    // Keep Vite source modules under app/api* on the dev server while proxying real API endpoints.
+    "^/api(?!/.*\\.[jt]sx?(?:\\?|$))(/|$)": {
+      target: `http://localhost:${process.env.FUSION_API_PORT ?? "4040"}`,
+      changeOrigin: true,
+      ws: true,
+    },
+  },
+}
+```
+
+Verify both sides of the boundary:
+
+```bash
+# Should be served by Vite as JavaScript modules.
+curl -i "$DASHBOARD_URL/api.ts"
+curl -i "$DASHBOARD_URL/api-node.ts"
+curl -i "$DASHBOARD_URL/api/legacy.ts?import"
+
+# Should still proxy to the backend API server.
+curl -i "$DASHBOARD_URL/api/health"
+```
+
+After the fix, `agent-browser` could snapshot the dashboard and navigate Settings -> Project Models, including the Plan/Triage, Executor, and Reviewer controls.
+
+## Why This Works
+
+Vite treats proxy keys beginning with `^` as regular expressions matched against the request URL. The dashboard has source modules whose served URLs begin with `/api`, while the backend API namespace also begins with `/api`; a prefix rule cannot distinguish them.
+
+The negative lookahead rejects source-module extensions before the proxy claims the request:
+
+- `/api/health` matches the proxy and reaches the backend.
+- `/api/legacy.ts` does not match the proxy and stays on Vite.
+- `/api/legacy.ts?import` also stays on Vite because the source-extension check allows an optional query string.
+- `.tsx`, `.js`, and `.jsx` are excluded too so future source modules do not rediscover the same failure.
+
+## Prevention
+
+- When adding a Vite dev-server proxy, test both a real backend endpoint and any source modules that share the same URL prefix.
+- Include query-string cases in proxy-regex checks; Vite module and HMR requests are often not bare pathnames.
+- Prefer a short inline comment for subtle proxy regexes so future maintainers know which requests must stay on Vite.
+- If browser automation sees a blank dashboard, confirm module routing with direct `curl` checks before debugging the browser tool.
+
+## Related Issues
+
+- [Browser-testing the Fusion dashboard from a worktree safely](../developer-experience/browser-testing-dashboard-from-worktree-safely.md) — neighboring worktree browser-testing traps: live engine hazards, reserved ports, and stale bundles.
+- [CSS animations silently frozen by transition tokens used as durations](../ui-bugs/css-animation-frozen-by-transition-token-shape-mismatch.md) — mentions the older dev-server blank-page trap from broad `/api` proxying.