diff --git a/src/web-ui/src/flow_chat/components/ModelSelector.scss b/src/web-ui/src/flow_chat/components/ModelSelector.scss
index bc0117510b..4c9583127c 100644
--- a/src/web-ui/src/flow_chat/components/ModelSelector.scss
+++ b/src/web-ui/src/flow_chat/components/ModelSelector.scss
@@ -138,8 +138,9 @@
     position: absolute;
     bottom: calc(100% + 6px);
     left: 0;
-    min-width: 220px;
-    max-width: 280px;
+    width: 220px;
+    min-width: 0;
+    max-width: calc(100vw - 16px);
     background: var(--color-bg-elevated);
     border: 1px solid var(--color-overlay-white-12);
     border-radius: 6px;
@@ -158,6 +159,8 @@
     display: flex;
     align-items: center;
     justify-content: space-between;
+    gap: 8px;
+    min-width: 0;
     padding: 8px 12px;
     border-bottom: 1px solid var(--color-overlay-white-08);
     font-size: var(--flowchat-font-size-xxs);
@@ -168,6 +171,10 @@
   }
   
   &__dropdown-hint {
+    min-width: 0;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
     font-size: var(--flowchat-font-size-2xs);
     color: var(--color-text-muted);
     font-weight: 400;
@@ -212,6 +219,7 @@
     align-items: center;
     justify-content: space-between;
     gap: 8px;
+    min-width: 0;
     padding: 7px 12px;
     cursor: pointer;
     transition: all 0.15s ease;
diff --git a/src/web-ui/src/flow_chat/components/ModelSelector.tsx b/src/web-ui/src/flow_chat/components/ModelSelector.tsx
index 949c62f42e..41ff90321b 100644
--- a/src/web-ui/src/flow_chat/components/ModelSelector.tsx
+++ b/src/web-ui/src/flow_chat/components/ModelSelector.tsx
@@ -29,6 +29,7 @@ import {
   type ContextUsageSource,
 } from '../utils/tokenUsageDisplay';
 import { createLogger } from '@/shared/utils/logger';
+import { getModelSelectorDropdownStyle } from './modelSelectorDropdownPosition';
 import './ModelSelector.scss';
 
 const log = createLogger('ModelSelector');
@@ -340,18 +341,14 @@ export const ModelSelector: React.FC<ModelSelectorProps> = ({
 
     const updatePosition = () => {
       if (!dropdownRef.current) return;
-      const rect = dropdownRef.current.getBoundingClientRect();
-      const placementStyle = dropdownPlacement === 'bottom'
-        ? { top: `${rect.bottom + 6}px`, bottom: 'auto' }
-        : { top: 'auto', bottom: `${window.innerHeight - rect.top + 6}px` };
-      setDropdownStyle({
-        position: 'fixed',
-        visibility: 'visible',
-        left: `${rect.left}px`,
-        minWidth: '220px',
-        maxWidth: '280px',
-        ...placementStyle,
-      });
+      setDropdownStyle(getModelSelectorDropdownStyle(
+        dropdownRef.current.getBoundingClientRect(),
+        dropdownPlacement,
+        {
+          width: window.innerWidth,
+          height: window.innerHeight,
+        },
+      ));
     };
 
     updatePosition();
diff --git a/src/web-ui/src/flow_chat/components/modelSelectorDropdownPosition.test.ts b/src/web-ui/src/flow_chat/components/modelSelectorDropdownPosition.test.ts
new file mode 100644
index 0000000000..850278dcae
--- /dev/null
+++ b/src/web-ui/src/flow_chat/components/modelSelectorDropdownPosition.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, it } from 'vitest';
+import { getModelSelectorDropdownStyle } from './modelSelectorDropdownPosition';
+
+describe('getModelSelectorDropdownStyle', () => {
+  it('keeps the dropdown inside the right edge of a narrow viewport', () => {
+    const style = getModelSelectorDropdownStyle(
+      { left: 250, top: 700, bottom: 724, width: 90 },
+      'top',
+      { width: 320, height: 800 },
+    );
+
+    expect(style.left).toBe('92px');
+    expect(style.width).toBe('220px');
+    expect(style.maxWidth).toBe('304px');
+    expect(style.bottom).toBe('106px');
+  });
+
+  it('shrinks the dropdown when the viewport cannot fit the preferred minimum width', () => {
+    const style = getModelSelectorDropdownStyle(
+      { left: 80, top: 600, bottom: 624, width: 80 },
+      'bottom',
+      { width: 180, height: 700 },
+    );
+
+    expect(style.left).toBe('8px');
+    expect(style.width).toBe('164px');
+    expect(style.maxWidth).toBe('164px');
+    expect(style.top).toBe('630px');
+  });
+
+  it('caps wide triggers at the dropdown maximum width', () => {
+    const style = getModelSelectorDropdownStyle(
+      { left: 32, top: 400, bottom: 424, width: 360 },
+      'bottom',
+      { width: 900, height: 700 },
+    );
+
+    expect(style.left).toBe('32px');
+    expect(style.width).toBe('280px');
+  });
+});
diff --git a/src/web-ui/src/flow_chat/components/modelSelectorDropdownPosition.ts b/src/web-ui/src/flow_chat/components/modelSelectorDropdownPosition.ts
new file mode 100644
index 0000000000..c659dcb426
--- /dev/null
+++ b/src/web-ui/src/flow_chat/components/modelSelectorDropdownPosition.ts
@@ -0,0 +1,62 @@
+export type ModelSelectorDropdownPlacement = 'top' | 'bottom';
+
+export interface ModelSelectorDropdownAnchorRect {
+  left: number;
+  top: number;
+  bottom: number;
+  width: number;
+}
+
+export interface ModelSelectorDropdownViewport {
+  width: number;
+  height: number;
+}
+
+export interface ModelSelectorDropdownStyle {
+  position: 'fixed';
+  visibility: 'visible';
+  left: string;
+  top: string;
+  bottom: string;
+  width: string;
+  minWidth: string;
+  maxWidth: string;
+}
+
+const DROPDOWN_GAP_PX = 6;
+const DROPDOWN_MAX_WIDTH_PX = 280;
+const DROPDOWN_MIN_WIDTH_PX = 220;
+const VIEWPORT_PADDING_PX = 8;
+
+const clamp = (value: number, min: number, max: number): number => {
+  return Math.min(Math.max(value, min), Math.max(min, max));
+};
+
+export function getModelSelectorDropdownStyle(
+  rect: ModelSelectorDropdownAnchorRect,
+  placement: ModelSelectorDropdownPlacement,
+  viewport: ModelSelectorDropdownViewport,
+): ModelSelectorDropdownStyle {
+  const availableWidth = Math.max(viewport.width - VIEWPORT_PADDING_PX * 2, 1);
+  const preferredWidth = Math.min(
+    DROPDOWN_MAX_WIDTH_PX,
+    Math.max(DROPDOWN_MIN_WIDTH_PX, rect.width),
+  );
+  const width = Math.min(preferredWidth, availableWidth);
+  const maxLeft = viewport.width - VIEWPORT_PADDING_PX - width;
+  const left = clamp(rect.left, VIEWPORT_PADDING_PX, maxLeft);
+
+  const placementStyle = placement === 'bottom'
+    ? { top: `${rect.bottom + DROPDOWN_GAP_PX}px`, bottom: 'auto' }
+    : { top: 'auto', bottom: `${viewport.height - rect.top + DROPDOWN_GAP_PX}px` };
+
+  return {
+    position: 'fixed',
+    visibility: 'visible',
+    left: `${left}px`,
+    width: `${width}px`,
+    minWidth: '0',
+    maxWidth: `${availableWidth}px`,
+    ...placementStyle,
+  };
+}