diff --git a/CHANGELOG.md b/CHANGELOG.md
index ea5e4525b..549e1f0ab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,17 @@
 Notable API additions and breaking changes. For the full commit log, see
 [GitHub Releases](https://github.com/mirrorstack-ai/web-ui-kit/releases).
 
+## 0.5.9
+
+- Agent sidebar reload **clobber fix**: a mount-time mutation (the host bridge's
+  `setOpen`, a route-driven tab change) could `put` the empty, un-hydrated strip
+  before the mount GET settled, permanently overwriting the server's saved
+  state with `{tabs:[], placeholder width}`. Reloads then "reverted to default /
+  new chat" and stayed. `schedulePut` now refuses to persist until the strip has
+  hydrated (the flag is set even on GET failure, so saves never stall). This is
+  the live-reproduced root cause that 0.5.7/0.5.8 did not resolve. Regression
+  test included (a pre-hydration mutation must not `put`).
+
 ## 0.5.8
 
 - Agent sidebar reload reliability: the persisted drag-width and the restored
diff --git a/package.json b/package.json
index 24ee47f50..d2aeba8cf 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@mirrorstack-ai/web-ui-kit",
   "packageManager": "pnpm@10.29.3",
-  "version": "0.5.8",
+  "version": "0.5.9",
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",
   "exports": {
diff --git a/src/hooks/agent-chat/useAgentTabs.test.ts b/src/hooks/agent-chat/useAgentTabs.test.ts
index df2a46f3d..6cb231f2f 100644
--- a/src/hooks/agent-chat/useAgentTabs.test.ts
+++ b/src/hooks/agent-chat/useAgentTabs.test.ts
@@ -95,6 +95,45 @@ describe("useAgentTabs", () => {
     expect(put).not.toHaveBeenCalled();
   });
 
+  it("does NOT persist before hydration settles — a mount-time mutation must not clobber the server", async () => {
+    // Deferred GET: hydration is still in flight when a mount-time mutation
+    // fires (the host bridge's setOpen, a route-driven tab change).
+    let resolveGet!: (s: AgentSidebarState) => void;
+    const get = vi.fn().mockImplementation(
+      () => new Promise<AgentSidebarState>((r) => { resolveGet = r; }),
+    );
+    const put = vi.fn().mockResolvedValue(undefined);
+    const persistence: AgentTabsPersistence = { get, put };
+    const { result } = renderHook(() => useAgentTabs(persistence));
+
+    // Pre-fix this scheduled a PUT of the empty, un-hydrated strip (tabs:[],
+    // placeholder width), permanently clobbering the server's saved state —
+    // the "reload reverts to default / new chat" bug. The gate suppresses it.
+    act(() => {
+      result.current.openConversation("c-9");
+    });
+    await advance(5000); // well past the debounce window
+    expect(result.current.hydrated).toBe(false);
+    expect(put).not.toHaveBeenCalled();
+
+    // Hydration settles with the real saved state (tabs + a real width).
+    await act(async () => {
+      resolveGet(state({ width: 700 }));
+    });
+    expect(result.current.hydrated).toBe(true);
+    expect(result.current.tabs).toEqual(["c-1", "c-2"]); // restored, not wiped
+    expect(put).not.toHaveBeenCalled(); // hydration itself never writes
+
+    // A genuine post-hydrate mutation DOES persist, carrying the hydrated
+    // width — never a placeholder.
+    act(() => {
+      result.current.openConversation("c-3");
+    });
+    await advance(5000);
+    expect(put).toHaveBeenCalledTimes(1);
+    expect(put.mock.calls[0][0].width).toBe(700);
+  });
+
   it("debounces mutations into ONE trailing PUT carrying the final state", async () => {
     const { persistence, put } = fakePersistence();
     const { result } = renderHook(() => useAgentTabs(persistence));
diff --git a/src/hooks/agent-chat/useAgentTabs.ts b/src/hooks/agent-chat/useAgentTabs.ts
index cd0b4718f..09f92981c 100644
--- a/src/hooks/agent-chat/useAgentTabs.ts
+++ b/src/hooks/agent-chat/useAgentTabs.ts
@@ -383,7 +383,19 @@ export function useAgentTabs(
   const hasHydratedRef = useRef(false);
 
   const schedulePut = useCallback(() => {
-    if (!persistenceRef.current || !enabledRef.current) return;
+    // NEVER persist before the mount GET has settled. A mount-time mutation
+    // (the host bridge's setOpen, a route-driven newTab/selectTab) fires
+    // `mutate -> schedulePut` while the strip is still the empty, un-hydrated
+    // placeholder; without this gate that empty strip (tabs:[], placeholder
+    // width) gets PUT and permanently clobbers the server's real saved state
+    // — the reload "reverts to default / new chat" bug. hasHydratedRef flips
+    // true in reconcile's finally even on GET failure, so saves never stall.
+    if (
+      !persistenceRef.current ||
+      !enabledRef.current ||
+      !hasHydratedRef.current
+    )
+      return;
     pendingRef.current = true;
     if (timerRef.current) clearTimeout(timerRef.current);
     timerRef.current = setTimeout(() => {