diff --git a/packages/app/e2e/tool-call-shimmer.spec.ts b/packages/app/e2e/tool-call-shimmer.spec.ts
new file mode 100644
index 000000000..c0d1ad514
--- /dev/null
+++ b/packages/app/e2e/tool-call-shimmer.spec.ts
@@ -0,0 +1,238 @@
+import type { Locator, Page } from "@playwright/test";
+import { test, expect } from "./fixtures";
+import { daemonWsRoutePattern } from "./helpers/daemon-port";
+import { openAgentRoute, seedMockAgentWorkspace } from "./helpers/mock-agent";
+
+type WebSocketMessage = string | Buffer;
+
+interface ShimmerEvidence {
+  animationDuration: string;
+  endPx: number;
+  label: string;
+  renderedWidth: number;
+  startPx: number;
+}
+
+function parseSessionMessage(message: WebSocketMessage): Record<string, unknown> | null {
+  const raw = typeof message === "string" ? message : message.toString("utf8");
+  try {
+    const envelope = JSON.parse(raw) as { type?: unknown; message?: unknown };
+    return envelope.type === "session" && envelope.message && typeof envelope.message === "object"
+      ? (envelope.message as Record<string, unknown>)
+      : null;
+  } catch {
+    return null;
+  }
+}
+
+function getToolCallStatus(
+  message: WebSocketMessage,
+  agentId: string,
+): { callId: string; status: string } | null {
+  const sessionMessage = parseSessionMessage(message);
+  if (sessionMessage?.type !== "agent_stream") {
+    return null;
+  }
+  const payload = sessionMessage.payload as Record<string, unknown> | undefined;
+  if (payload?.agentId !== agentId) {
+    return null;
+  }
+  const event = payload.event as Record<string, unknown> | undefined;
+  const item = event?.item as Record<string, unknown> | undefined;
+  return event?.type === "timeline" &&
+    item?.type === "tool_call" &&
+    typeof item.callId === "string" &&
+    typeof item.status === "string"
+    ? { callId: item.callId, status: item.status }
+    : null;
+}
+
+function replaceToolCallStatus(message: WebSocketMessage, status: string): string {
+  const raw = typeof message === "string" ? message : message.toString("utf8");
+  const envelope = JSON.parse(raw) as {
+    message?: { payload?: { event?: { item?: { status?: string } } } };
+  };
+  const item = envelope.message?.payload?.event?.item;
+  if (!item) {
+    throw new Error("Expected a tool-call session message");
+  }
+  item.status = status;
+  return JSON.stringify(envelope);
+}
+
+async function gateSecondToolCall(page: Page, agentId: string) {
+  let firstCallId: string | null = null;
+  let secondCallId: string | null = null;
+  let secondRunningMessage: WebSocketMessage | null = null;
+  let releaseSecondRequested = false;
+  let pauseServerMessages = false;
+  let secondRunningForwarded = false;
+  let forwardSecondRunning: (() => void) | null = null;
+  let resolveFirstCompleted!: () => void;
+  let resolveSecondRunning!: () => void;
+  const firstCompleted = new Promise<void>((resolve) => {
+    resolveFirstCompleted = resolve;
+  });
+  const secondRunning = new Promise<void>((resolve) => {
+    resolveSecondRunning = resolve;
+  });
+
+  await page.routeWebSocket(daemonWsRoutePattern(), (ws) => {
+    const server = ws.connectToServer();
+    forwardSecondRunning = () => {
+      if (!secondRunningMessage || secondRunningForwarded) {
+        return;
+      }
+      ws.send(secondRunningMessage);
+      secondRunningForwarded = true;
+    };
+    ws.onMessage((message) => server.send(message));
+    server.onMessage((message) => {
+      if (pauseServerMessages) {
+        return;
+      }
+
+      const toolCall = getToolCallStatus(message, agentId);
+      if (!toolCall) {
+        ws.send(message);
+        return;
+      }
+      if (!firstCallId) {
+        firstCallId = toolCall.callId;
+      }
+      if (toolCall.callId === firstCallId) {
+        if (toolCall.status === "running" || toolCall.status === "executing") {
+          return;
+        }
+        if (toolCall.status === "completed") {
+          // The mock tool completes inside the daemon's coalescing window, so the
+          // browser naturally receives its authoritative completed state first.
+          ws.send(message);
+          resolveFirstCompleted();
+          return;
+        }
+      }
+
+      secondCallId ??= toolCall.callId;
+      if (
+        toolCall.callId === secondCallId &&
+        (toolCall.status === "running" || toolCall.status === "executing")
+      ) {
+        secondRunningMessage = message;
+        pauseServerMessages = true;
+        resolveSecondRunning();
+        if (releaseSecondRequested) {
+          forwardSecondRunning?.();
+        }
+        return;
+      }
+
+      if (toolCall.callId === secondCallId && toolCall.status === "completed") {
+        // Keep the second real tool call inspectably active. Production providers
+        // send this same status shape while their work remains in flight.
+        secondRunningMessage = replaceToolCallStatus(message, "running");
+        pauseServerMessages = true;
+        resolveSecondRunning();
+        if (releaseSecondRequested) {
+          forwardSecondRunning?.();
+        }
+        return;
+      }
+
+      ws.send(message);
+    });
+  });
+
+  return {
+    waitForFirstCompleted: () => firstCompleted,
+    waitForSecondRunning: () => secondRunning,
+    releaseSecondRunning() {
+      releaseSecondRequested = true;
+      if (secondRunningMessage) {
+        forwardSecondRunning?.();
+      }
+    },
+  };
+}
+
+async function readShimmerEvidence(locator: Locator): Promise<ShimmerEvidence> {
+  return locator.evaluate((root) => {
+    const shimmer = Array.from(root.querySelectorAll<HTMLElement>("*")).find((element) =>
+      getComputedStyle(element).animationName.includes("paseo-toolcall-shimmer"),
+    );
+    if (!shimmer) {
+      throw new Error("Expected a running shimmer inside the badge");
+    }
+    const style = getComputedStyle(shimmer);
+    return {
+      animationDuration: style.animationDuration,
+      endPx: Number.parseFloat(style.getPropertyValue("--paseo-shimmer-end")),
+      label: shimmer.textContent ?? "",
+      renderedWidth: shimmer.getBoundingClientRect().width,
+      startPx: Number.parseFloat(style.getPropertyValue("--paseo-shimmer-start")),
+    };
+  });
+}
+
+test("measures an overview heading that becomes loading after its idle mount", async ({
+  page,
+}, testInfo) => {
+  test.setTimeout(120_000);
+  await page.addInitScript(() => {
+    localStorage.setItem(
+      "@paseo:app-settings",
+      JSON.stringify({ toolCallDetailLevel: "overview" }),
+    );
+  });
+  const agent = await seedMockAgentWorkspace({
+    repoPrefix: "tool-call-shimmer-",
+    title: "Tool-call shimmer",
+    model: "ten-second-stream",
+  });
+
+  try {
+    const gate = await gateSecondToolCall(page, agent.agentId);
+    await openAgentRoute(page, {
+      workspaceId: agent.workspaceId,
+      agentId: agent.agentId,
+    });
+    await agent.client.sendAgentMessage(agent.agentId, "Prove the overview shimmer lifecycle.");
+
+    await gate.waitForFirstCompleted();
+    const group = page.getByTestId("tool-call-group");
+    await expect(group).toBeVisible();
+    const idleGroupHandle = await group.elementHandle();
+    if (!idleGroupHandle) {
+      throw new Error("Expected the idle tool-call group to be mounted");
+    }
+    await expect(group.locator('[style*="paseo-toolcall-shimmer"]')).toHaveCount(0);
+
+    await gate.waitForSecondRunning();
+    gate.releaseSecondRunning();
+    await expect(group.locator('[style*="paseo-toolcall-shimmer"]')).not.toHaveCount(0);
+    const sameGroupNode = await group.evaluate(
+      (node, previous) => node === previous,
+      idleGroupHandle,
+    );
+
+    await group.click();
+    const runningChild = page.getByTestId("tool-call-badge").last();
+    await expect(runningChild).toBeVisible();
+    const header = await readShimmerEvidence(group);
+    const child = await readShimmerEvidence(runningChild);
+    const evidence = { sameGroupNode, header, child };
+    await testInfo.attach("tool-call-shimmer-evidence", {
+      body: JSON.stringify(evidence, null, 2),
+      contentType: "application/json",
+    });
+
+    expect(sameGroupNode).toBe(true);
+    expect(child.endPx).toBeGreaterThan(0);
+    expect(
+      header.endPx,
+      `The retained header rendered ${header.renderedWidth}px wide but its shimmer still ends at ${header.endPx}px`,
+    ).toBeGreaterThan(0);
+  } finally {
+    await agent.cleanup();
+  }
+});
diff --git a/packages/app/src/components/message.tsx b/packages/app/src/components/message.tsx
index b17982a6e..87fd88d4a 100644
--- a/packages/app/src/components/message.tsx
+++ b/packages/app/src/components/message.tsx
@@ -2657,7 +2657,9 @@ function computeShimmerMetrics(input: {
     Math.min(120, input.labelRowWidth > 0 ? input.labelRowWidth * 0.28 : 0),
   );
   const isWebShimmer = input.isLoading && isWeb;
-  const shouldMeasureWebShimmer = isWebShimmer;
+  // React Native Web only observes a node when onLayout exists at mount. Keep
+  // measuring while idle so a retained badge has dimensions when it starts loading.
+  const shouldMeasureWebShimmer = isWeb;
   const shouldMeasureNativeShimmer = input.isLoading && isNative;
   const isNativeShimmer =
     shouldMeasureNativeShimmer && input.labelRowWidth > 0 && input.labelRowHeight > 0;