diff --git a/packages/web/src/features/models/model-group-expansion.ts b/packages/web/src/features/models/model-group-expansion.ts
index aad6d02..5f282ba 100644
--- a/packages/web/src/features/models/model-group-expansion.ts
+++ b/packages/web/src/features/models/model-group-expansion.ts
@@ -5,15 +5,23 @@
  * are derived only after the model rows load (user-defined groups arrive with that
  * async response), so a collapsed-set default cannot express "everything collapsed
  * except DeepSeek" without knowing every group id up front. An expanded set survives
- * late-arriving groups — anything not in it simply renders collapsed. Not persisted,
- * matching the previous collapse behavior.
+ * late-arriving groups — anything not in it simply renders collapsed.
+ *
+ * The user's toggles persist per Project in localStorage (#224 follow-up: DeepSeek-only
+ * is the first-visit default, not a per-visit reset), mirroring the sidebar's persisted
+ * group-collapse sets — a `penguin.…` key namespaced by projectId holding a JSON array
+ * of ids. Unlike the sidebar sets, whose default IS the empty set, the default here is
+ * non-empty, so "nothing stored / invalid" (fall back to the default) is distinguished
+ * from a stored empty array (the user collapsed everything; honored as-is). Storage is
+ * injectable (draft-cache.ts convention: vitest runs in Node, no localStorage); search
+ * force-open below stays derived and never writes storage.
  */
 
 /**
- * Provider ids expanded on first paint: DeepSeek only — the default model's provider and
- * the first group in MODEL_PROVIDERS, so the page opens with exactly its top group
- * unfolded. Returns a fresh Set per call (React state must never share a module-level
- * mutable instance).
+ * Provider ids expanded on first visit (nothing persisted yet): DeepSeek only — the
+ * default model's provider and the first group in MODEL_PROVIDERS, so the page opens
+ * with exactly its top group unfolded. Returns a fresh Set per call (React state must
+ * never share a module-level mutable instance).
  */
 export function defaultExpandedProviders(): Set<string> {
   return new Set(["deepseek"]);
@@ -44,3 +52,75 @@ export function toggleExpandedProvider(
   else next.add(providerId);
   return next;
 }
+
+/** Minimal storage interface (the subset of localStorage used here); tests inject an in-memory implementation. */
+export interface ExpansionStorage {
+  getItem(key: string): string | null;
+  setItem(key: string, value: string): void;
+}
+
+/**
+ * Storage key of one Project's expanded-group set (sidebar key-naming convention,
+ * `penguin.sidebarCollapsedGroups.<projectId>` &c.). Provider ids — including
+ * user-defined group names — are Project-scoped, hence one key per Project.
+ */
+export const expandedGroupsKey = (projectId: string): string =>
+  `penguin.modelsExpandedGroups.${projectId}`;
+
+/** Serialized form of an expanded set: a JSON array of provider ids. */
+export function serializeExpandedProviders(expanded: ReadonlySet<string>): string {
+  return JSON.stringify([...expanded]);
+}
+
+/**
+ * Parses a stored raw value. Returns null — not the default — for "nothing usable"
+ * (absent, malformed JSON, non-array): the caller owns the fallback. A valid array
+ * yields the set of its string elements (junk elements dropped), so a stored `[]`
+ * round-trips to the empty set: "user collapsed everything" is a persisted choice,
+ * never replaced by the default.
+ */
+export function parseExpandedProviders(raw: string | null): Set<string> | null {
+  if (raw === null) return null;
+  try {
+    const parsed: unknown = JSON.parse(raw);
+    if (!Array.isArray(parsed)) return null;
+    return new Set(parsed.filter((x): x is string => typeof x === "string"));
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Reads a Project's persisted expanded set; no Project yet, nothing stored, or
+ * corrupted storage falls back to the DeepSeek-only default. Stored ids of
+ * since-deleted groups pass through unpruned — harmless, expansion is a pure
+ * membership test and a group no longer rendered is never asked about.
+ */
+export function loadExpandedProviders(
+  projectId: string | null,
+  storage: ExpansionStorage = localStorage,
+): Set<string> {
+  if (projectId === null) return defaultExpandedProviders();
+  try {
+    return (
+      parseExpandedProviders(storage.getItem(expandedGroupsKey(projectId))) ??
+      defaultExpandedProviders()
+    );
+  } catch {
+    return defaultExpandedProviders();
+  }
+}
+
+/** Writes a Project's expanded set on every toggle (best-effort: quota limits / private browsing fail silently). */
+export function saveExpandedProviders(
+  projectId: string | null,
+  expanded: ReadonlySet<string>,
+  storage: ExpansionStorage = localStorage,
+): void {
+  if (projectId === null) return;
+  try {
+    storage.setItem(expandedGroupsKey(projectId), serializeExpandedProviders(expanded));
+  } catch {
+    /* best-effort persistence (quota limits / private browsing) */
+  }
+}
diff --git a/packages/web/src/features/models/models-page.tsx b/packages/web/src/features/models/models-page.tsx
index d733a8f..eb6d53a 100644
--- a/packages/web/src/features/models/models-page.tsx
+++ b/packages/web/src/features/models/models-page.tsx
@@ -72,8 +72,9 @@ import {
 import type { ModelProviderInfo } from "@prismshadow/penguin-core/model-catalog";
 import { groupModelRows, isFreeModel, sameModelRef, userProviderInfo } from "./model-grouping";
 import {
-  defaultExpandedProviders,
   isGroupExpanded,
+  loadExpandedProviders,
+  saveExpandedProviders,
   toggleExpandedProvider,
 } from "./model-group-expansion";
 import { clearDraftModelRef } from "../chat/draft-cache";
@@ -358,11 +359,17 @@ export function ModelsPage() {
   const [addingTo, setAddingTo] = useState<string | null>(null);
   const [query, setQuery] = useState("");
   /**
-   * Expanded vendor groups — only DeepSeek on first paint; every other group (including
-   * user-defined ones, which arrive with the async row load) starts collapsed. Searching
-   * force-opens the rendered groups without touching this set (see model-group-expansion.ts).
+   * Expanded vendor groups — hydrated from this Project's persisted set (DeepSeek-only
+   * on a first visit; every other group, including user-defined ones arriving with the
+   * async row load, starts collapsed), written back on every toggle so the user's
+   * choices survive a refresh. Searching force-opens the rendered groups without
+   * touching this set (see model-group-expansion.ts).
    */
-  const [expanded, setExpanded] = useState<Set<string>>(defaultExpandedProviders);
+  const [expanded, setExpanded] = useState<Set<string>>(() => loadExpandedProviders(projectId));
+  // Project resolved on first load / switched: swap in that Project's persisted expansion set.
+  useEffect(() => {
+    setExpanded(loadExpandedProviders(projectId));
+  }, [projectId]);
   /** Vendor group (provider id) currently having its API key configured in bulk. */
   const [groupKeyFor, setGroupKeyFor] = useState<string | null>(null);
   /** "Add group" popup (user-defined group): a valid name proceeds to that group's add-model dialog. */
@@ -528,11 +535,15 @@ export function ModelsPage() {
   /**
    * Header toggles are inert while searching: every rendered group is force-opened (see
    * isGroupExpanded), so a flip would change nothing visibly and only silently mutate the
-   * state restored once the query clears.
+   * state restored once the query clears. Computed outside the state updater (sidebar
+   * toggleGroup convention): the persistence write is a side effect, and updaters must
+   * stay pure (double-invoked in StrictMode).
    */
   const toggleGroup = (id: string) => {
     if (searching) return;
-    setExpanded((prev) => toggleExpandedProvider(prev, id));
+    const next = toggleExpandedProvider(expanded, id);
+    setExpanded(next);
+    saveExpandedProviders(projectId, next);
   };
 
   return (
diff --git a/packages/web/test/model-group-expansion.test.ts b/packages/web/test/model-group-expansion.test.ts
index b432262..8473263 100644
--- a/packages/web/test/model-group-expansion.test.ts
+++ b/packages/web/test/model-group-expansion.test.ts
@@ -1,22 +1,39 @@
 /**
  * model-group-expansion.ts unit tests: the models page's default-collapsed vendor groups.
- * Only DeepSeek (the catalog's first provider) is expanded on first paint; the state is a
+ * Only DeepSeek (the catalog's first provider) is expanded on a first visit; the state is a
  * set of EXPANDED ids so groups arriving late (user-defined groups load with the rows)
  * default to collapsed without being known up front. While a search query is active every
  * rendered group is force-opened — groupModelRows only returns match-holding groups when
  * searching, and a match hidden inside a collapsed group would look like a missing result —
- * without touching the stored set.
+ * without touching the stored set. The user's toggles persist per Project (localStorage,
+ * injectable storage): nothing stored or corrupted storage falls back to the default, a
+ * stored empty array is honored as "all collapsed", and stale ids of since-deleted groups
+ * stay inert.
  */
 import { describe, expect, it } from "vitest";
 import { MODEL_PROVIDERS } from "@prismshadow/penguin-core/model-catalog";
 import {
   defaultExpandedProviders,
+  expandedGroupsKey,
   isGroupExpanded,
+  loadExpandedProviders,
+  saveExpandedProviders,
   toggleExpandedProvider,
 } from "../src/features/models/model-group-expansion";
+import type { ExpansionStorage } from "../src/features/models/model-group-expansion";
 import { groupModelRows } from "../src/features/models/model-grouping";
 import type { ModelRowLike } from "../src/features/models/model-grouping";
 
+/** In-memory storage (vitest runs in a Node environment, no localStorage; draft-cache.test.ts convention). */
+function memStorage(): ExpansionStorage & { map: Map<string, string> } {
+  const map = new Map<string, string>();
+  return {
+    map,
+    getItem: (k) => map.get(k) ?? null,
+    setItem: (k, v) => void map.set(k, v),
+  };
+}
+
 describe("defaultExpandedProviders", () => {
   it("contains exactly deepseek, the first provider of the catalog", () => {
     expect([...defaultExpandedProviders()]).toEqual(["deepseek"]);
@@ -87,3 +104,80 @@ describe("toggleExpandedProvider", () => {
     expect(withAnthropic.has("deepseek")).toBe(true);
   });
 });
+
+describe("persisted expansion (per-Project localStorage)", () => {
+  it("nothing stored — or no Project yet — falls back to the DeepSeek-only default", () => {
+    const s = memStorage();
+    expect([...loadExpandedProviders("p1", s)]).toEqual(["deepseek"]);
+    expect([...loadExpandedProviders(null, s)]).toEqual(["deepseek"]);
+    expect(s.map.size).toBe(0); // load never writes; save without a Project is a no-op
+    saveExpandedProviders(null, new Set(["openai"]), s);
+    expect(s.map.size).toBe(0);
+  });
+
+  it("toggle → save → load round-trips the user's set", () => {
+    const s = memStorage();
+    let set = loadExpandedProviders("p1", s);
+    set = toggleExpandedProvider(set, "anthropic"); // open anthropic
+    set = toggleExpandedProvider(set, "deepseek"); // close deepseek
+    saveExpandedProviders("p1", set, s);
+    const restored = loadExpandedProviders("p1", s);
+    expect(restored).toEqual(new Set(["anthropic"]));
+    expect(restored).not.toBe(set); // fresh instance per load (React state discipline)
+  });
+
+  it("a stored empty array IS a choice (all collapsed), not a fallback to the default", () => {
+    const s = memStorage();
+    saveExpandedProviders("p1", new Set(), s);
+    expect(loadExpandedProviders("p1", s).size).toBe(0);
+  });
+
+  it("invalid JSON / non-array shapes fall back to the default; junk array elements are dropped", () => {
+    const s = memStorage();
+    for (const raw of ["{not json", '"deepseek"', "42", "null", "{}", ""]) {
+      s.map.set(expandedGroupsKey("p1"), raw);
+      expect([...loadExpandedProviders("p1", s)]).toEqual(["deepseek"]);
+    }
+    // An array survives element-level junk: non-strings are filtered, strings kept.
+    s.map.set(expandedGroupsKey("p1"), '["anthropic", 7, null, {"x": 1}]');
+    expect([...loadExpandedProviders("p1", s)]).toEqual(["anthropic"]);
+  });
+
+  it("Projects are isolated: each key holds its own set", () => {
+    const s = memStorage();
+    saveExpandedProviders("p1", new Set(["openai"]), s);
+    saveExpandedProviders("p2", new Set(["moonshot", "my-proxy"]), s);
+    expect([...loadExpandedProviders("p1", s)]).toEqual(["openai"]);
+    expect(loadExpandedProviders("p2", s)).toEqual(new Set(["moonshot", "my-proxy"]));
+    // A third Project is untouched by the writes above and opens on the default.
+    expect([...loadExpandedProviders("p3", s)]).toEqual(["deepseek"]);
+  });
+
+  it("stale ids of since-deleted groups are harmless and survive round-trips", () => {
+    const s = memStorage();
+    saveExpandedProviders("p1", new Set(["deepseek", "deleted-group"]), s);
+    const restored = loadExpandedProviders("p1", s);
+    // Membership renders nothing by itself: current groups derive exactly as before …
+    expect(isGroupExpanded(restored, "deepseek", false)).toBe(true);
+    expect(isGroupExpanded(restored, "anthropic", false)).toBe(false);
+    // … and the stored id cannot resurrect the group: only groups the rows produce render.
+    const rows: ModelRowLike[] = [{ provider: "deepseek", modelId: "deepseek-chat" }];
+    expect(groupModelRows(rows, "").some((g) => g.provider.id === "deleted-group")).toBe(false);
+    // Toggling other groups keeps the stale id inert but persisted (pruning is not load's job).
+    saveExpandedProviders("p1", toggleExpandedProvider(restored, "anthropic"), s);
+    expect(loadExpandedProviders("p1", s).has("deleted-group")).toBe(true);
+  });
+
+  it("storage throwing (quota/private mode): save does not throw, load yields the default", () => {
+    const broken: ExpansionStorage = {
+      getItem: () => {
+        throw new Error("denied");
+      },
+      setItem: () => {
+        throw new Error("denied");
+      },
+    };
+    expect(() => saveExpandedProviders("p1", new Set(["openai"]), broken)).not.toThrow();
+    expect([...loadExpandedProviders("p1", broken)]).toEqual(["deepseek"]);
+  });
+});