diff --git a/apps/web/src/components/chat/ModelPickerContent.tsx b/apps/web/src/components/chat/ModelPickerContent.tsx
index 211bbd59dbd98..ca6e34141cdf3 100644
--- a/apps/web/src/components/chat/ModelPickerContent.tsx
+++ b/apps/web/src/components/chat/ModelPickerContent.tsx
@@ -834,6 +834,7 @@ export const ModelPickerContent = memo(function ModelPickerContent(props: {
         {showSidebar && (
           <ModelPickerSidebar
             selectedInstanceId={selectedInstanceId}
+            activeInstanceId={props.activeInstanceId}
             onSelectInstance={handleSelectInstance}
             onFocusSearch={focusSearchInput}
             instanceEntries={sidebarInstanceEntries}
diff --git a/apps/web/src/components/chat/ModelPickerSidebar.tsx b/apps/web/src/components/chat/ModelPickerSidebar.tsx
index c07b1f33c5fa9..4685ff5358372 100644
--- a/apps/web/src/components/chat/ModelPickerSidebar.tsx
+++ b/apps/web/src/components/chat/ModelPickerSidebar.tsx
@@ -31,6 +31,8 @@ const PICKER_TOOLTIP_SIDE_OFFSET = 8;
 
 export const ModelPickerSidebar = memo(function ModelPickerSidebar(props: {
   selectedInstanceId: ProviderInstanceId | "favorites";
+  /** The instance the thread runs on; it stays marked while another one is browsed. */
+  activeInstanceId?: ProviderInstanceId;
   onSelectInstance: (instanceId: ProviderInstanceId | "favorites") => void;
   onFocusSearch: () => void;
   instanceEntries: ReadonlyArray<ProviderInstanceEntry>;
@@ -131,6 +133,7 @@ export const ModelPickerSidebar = memo(function ModelPickerSidebar(props: {
             const isDisabled =
               (isUnavailable && !unavailableSelectionIsReachable) || isContextDisabled;
             const isSelected = props.selectedInstanceId === entry.instanceId;
+            const isActive = props.activeInstanceId === entry.instanceId;
             const isHovered = hoveredInstanceId === entry.instanceId;
             const showInstanceBadge = shouldShowInstanceBadge(entry, props.instanceEntries);
 
@@ -138,7 +141,9 @@ export const ModelPickerSidebar = memo(function ModelPickerSidebar(props: {
               ? describeUnavailableInstance(entry)
               : isContextDisabled
                 ? (props.getDisabledInstanceTooltip?.(entry) ?? entry.displayName)
-                : entry.displayName;
+                : isActive
+                  ? `${entry.displayName} · Active`
+                  : entry.displayName;
 
             const button = (
               <Toolbar.Button
@@ -159,6 +164,7 @@ export const ModelPickerSidebar = memo(function ModelPickerSidebar(props: {
                 disabled={isDisabled}
                 focusableWhenDisabled={!isDisabled}
                 aria-pressed={isSelected}
+                aria-current={isActive ? "true" : undefined}
                 type="button"
                 aria-label={isUnavailable || isContextDisabled ? tooltip : entry.displayName}
               >
@@ -180,6 +186,13 @@ export const ModelPickerSidebar = memo(function ModelPickerSidebar(props: {
                   }
                   {...(entry.accentColor ? { badgeClassName: "h-3 min-w-3 px-0.5 text-5xs" } : {})}
                 />
+                {isActive ? (
+                  <span
+                    aria-hidden
+                    data-model-picker-active-instance
+                    className="absolute top-1 right-1 z-40 size-2 rounded-full bg-success ring-2 ring-popover"
+                  />
+                ) : null}
               </Toolbar.Button>
             );
 
diff --git a/apps/web/src/components/chat/ProviderModelPicker.browser.test.tsx b/apps/web/src/components/chat/ProviderModelPicker.browser.test.tsx
index b6dd1a38e86cf..252164bcb60cd 100644
--- a/apps/web/src/components/chat/ProviderModelPicker.browser.test.tsx
+++ b/apps/web/src/components/chat/ProviderModelPicker.browser.test.tsx
@@ -79,6 +79,12 @@ it("keeps each registry image when adding a model from another ACP agent", async
   await vi.waitFor(() => expect(imageWidths()).toEqual([11]));
   await screen.getByRole("button", { name: "First model", exact: true }).click();
   await page.getByRole("button", { name: "Second agent", exact: true }).click();
+  await expect
+    .element(page.getByRole("button", { name: "First agent", exact: true }))
+    .toHaveAttribute("aria-current", "true");
+  await expect
+    .element(page.getByRole("button", { name: "Second agent", exact: true }))
+    .not.toHaveAttribute("aria-current");
   await userEvent.keyboard("{Shift>}");
   try {
     await page.getByRole("option", { name: /Second model/ }).click();
diff --git a/apps/web/src/components/chat/ProviderModelPicker.tsx b/apps/web/src/components/chat/ProviderModelPicker.tsx
index 2e47b4c2fe3b4..de528feabdf37 100644
--- a/apps/web/src/components/chat/ProviderModelPicker.tsx
+++ b/apps/web/src/components/chat/ProviderModelPicker.tsx
@@ -7,9 +7,11 @@ import {
 } from "@t3tools/contracts";
 import { createContext, memo, type ReactNode, useRef, useState } from "react";
 import { Badge } from "../ui/badge";
+import { Kbd } from "../ui/kbd";
 import { Popover, PopoverPopup, PopoverTrigger } from "../ui/popover";
 import { Tooltip, TooltipPopup, TooltipTrigger } from "../ui/tooltip";
 import { cn } from "~/lib/utils";
+import { ThreadHoverCard, ThreadHoverCardPopup } from "../ThreadHoverCard";
 import { ModelPickerContent, resolveModelPickerSelectedModel } from "./ModelPickerContent";
 import { ChatGptSharingControl } from "./ChatGptSharingControl";
 import { ProviderInstanceIcon } from "./ProviderInstanceIcon";
@@ -228,6 +230,9 @@ export const ProviderModelPicker = memo(function ProviderModelPicker(props: {
   const allModelNames = selectedEntries
     ? selectedEntries.map((selection) => selection.fullLabel).join(", ") || "Choose models"
     : undefined;
+  // Only when same-driver instances could be confused, e.g. one Claude instance per account.
+  const activeInstanceLabel =
+    showInstanceBadge && !selectedEntries ? activeEntry?.displayName : undefined;
   const triggerTooltipContent = [
     props.triggerLabel ?? allModelNames ?? triggerLabel,
     props.triggerDetail,
@@ -309,6 +314,7 @@ export const ProviderModelPicker = memo(function ProviderModelPicker(props: {
             aria-label={[
               props.triggerAriaLabel ?? props.triggerLabel ?? allModelNames ?? triggerLabel,
               props.triggerDetail,
+              activeInstanceLabel,
             ]
               .filter(Boolean)
               .join(", ")}
@@ -387,12 +393,42 @@ export const ProviderModelPicker = memo(function ProviderModelPicker(props: {
               />
             </span>
           ) : null}
-          {props.isComposerOwned ? (
+          {props.isComposerOwned && !activeInstanceLabel ? (
             <span {...triggerTextProps}>{triggerText}</span>
           ) : (
             <Tooltip>
               <TooltipTrigger render={<span {...triggerTextProps} />}>{triggerText}</TooltipTrigger>
-              <TooltipPopup side="top">{triggerTooltipContent}</TooltipPopup>
+              {activeInstanceLabel && activeEntry ? (
+                // Same card as the agents panel, so the thread's instance reads the same way.
+                <ThreadHoverCardPopup side="top">
+                  <ThreadHoverCard title={props.triggerLabel ?? triggerLabel}>
+                    <div className="flex min-w-0 items-center gap-2">
+                      <ProviderInstanceIcon
+                        driverKind={activeEntry.driverKind}
+                        displayName={activeEntry.displayName}
+                        accentColor={activeEntry.accentColor}
+                        acpRegistryAgentId={activeEntry.acpRegistryAgentId}
+                        acpRegistryIconUrl={activeEntry.acpRegistryIconUrl}
+                        showBadge
+                        badgeClassName="right-[-0.125rem] bottom-[-0.125rem] h-3 min-w-3 px-0.5 text-5xs"
+                        className="size-4"
+                        iconClassName="size-4"
+                      />
+                      <span className="min-w-0 truncate text-foreground/75">
+                        {activeInstanceLabel}
+                      </span>
+                    </div>
+                    {props.triggerDetail || shortcutLabel ? (
+                      <div className="flex min-w-0 items-center justify-between gap-4">
+                        <span className="min-w-0 truncate">{props.triggerDetail}</span>
+                        {shortcutLabel ? <Kbd>{shortcutLabel}</Kbd> : null}
+                      </div>
+                    ) : null}
+                  </ThreadHoverCard>
+                </ThreadHoverCardPopup>
+              ) : (
+                <TooltipPopup side="top">{triggerTooltipContent}</TooltipPopup>
+              )}
             </Tooltip>
           )}
           {props.triggerSuffix}