fix: bootstrap user preferences without invalidating DWH cache

backend/src/app.ts

@@ -64,7 +64,13 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
     // The real runner persists preferences through the harness repository. The file fallback
     // only keeps older isolated route tests and externally injected runners compatible.
     if (typeof runner.preferencesGet === "function") {
-      return effectiveSettings(config, await runner.preferencesGet());
+      const stored = await runner.preferencesGet() as Settings;
+      if (Object.keys(stored).length === 0) {
+        const seeded = effectiveSettings(config, loadSettings(config));
+        await runner.preferencesSet(seeded);
+        return seeded;
+      }
+      return effectiveSettings(config, stored);
     }
     return effectiveSettings(config, loadSettings(config));
   };

backend/test/routes-settings.test.ts

@@ -1,5 +1,5 @@
 import { test, expect } from "vitest";
-import { mkdtempSync, rmSync } from "node:fs";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { buildApp } from "../src/app.js";
@@ -77,6 +77,65 @@ test("settings are isolated by the authenticated repository principal", async ()
   expect(bob.json()).not.toMatchObject({ workspace: "psd", thinking: "high" });
 });
 
+test("GET /settings seeds an empty private profile from complete legacy settings once", async () => {
+  let preferences: Record<string, unknown> = {};
+  const writes: Record<string, unknown>[] = [];
+  const runner = {
+    withPrincipal: () => ({
+      preferencesGet: async () => preferences,
+      preferencesSet: async (next: Record<string, unknown>) => {
+        writes.push(next);
+        preferences = next;
+      },
+    }),
+  };
+  const { app, dir } = appWithTmpSettings({}, { thtRunner: runner as any, listModels: async () => [] });
+  try {
+    writeFileSync(join(dir, "settings.json"), JSON.stringify({
+      workspace: "local", provider: "local-qwen", model: "qwen3.6-35b-a3b", thinking: "low",
+    }));
+
+    const first = await app.inject({ method: "GET", url: "/settings" });
+    const second = await app.inject({ method: "GET", url: "/settings" });
+
+    const expected = {
+      workspace: "local", provider: "local-qwen", model: "qwen3.6-35b-a3b", thinking: "low",
+    };
+    expect(first.statusCode).toBe(200);
+    expect(first.json()).toEqual(expected);
+    expect(second.json()).toEqual(expected);
+    expect(writes).toEqual([expected]);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test("GET /settings does not overwrite an existing private profile with legacy settings", async () => {
+  const privateSettings = {
+    workspace: "private", provider: "zai", model: "glm-5.2", thinking: "high",
+  };
+  const preferencesSet = async () => { throw new Error("must not seed an existing profile"); };
+  const runner = {
+    withPrincipal: () => ({
+      preferencesGet: async () => privateSettings,
+      preferencesSet,
+    }),
+  };
+  const { app, dir } = appWithTmpSettings({}, { thtRunner: runner as any, listModels: async () => [] });
+  try {
+    writeFileSync(join(dir, "settings.json"), JSON.stringify({
+      workspace: "local", provider: "local-qwen", model: "qwen3.6-35b-a3b", thinking: "low",
+    }));
+
+    const response = await app.inject({ method: "GET", url: "/settings" });
+
+    expect(response.statusCode).toBe(200);
+    expect(response.json()).toEqual(privateSettings);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
 test("PUT /settings rejects an unknown model when a model list is available", async () => {
   const { app, dir } = appWithTmpSettings({}, {
     listModels: async () => [{ provider: "zai", id: "glm-5.2", name: "GLM 5.2", reasoning: true }],