diff --git a/docs/chat-chain-changes/2026-10-22-thinking-tool-width.md b/docs/chat-chain-changes/2026-10-22-thinking-tool-width.md
new file mode 100644
index 0000000..33b842a
--- /dev/null
+++ b/docs/chat-chain-changes/2026-10-22-thinking-tool-width.md
@@ -0,0 +1,16 @@
+---
+date: 2026-10-22
+feature: Thinking tool call row width
+impact: Use the available chat width instead of a fixed 520px row
+---
+
+PR/commit: pending.
+
+MessageList tool-call rows now use 100% of their panel width. The existing
+flex shrinking, ellipsis and max-width rules keep long arguments within the
+viewport. This also applies to the compression and abort status rows sharing
+the same class. Assistant reply sizing and chat events are unchanged.
+
+Validation: Playwright reproduces the fixed-width failure and passes after the
+change at 1920px, 900px and 390px, including long tool arguments and overflow
+checks. Production build and harness checks pass.
diff --git a/packages/client/src/components/hermes/chat/MessageList.vue b/packages/client/src/components/hermes/chat/MessageList.vue
index 7f10d99..299d9f8 100644
--- a/packages/client/src/components/hermes/chat/MessageList.vue
+++ b/packages/client/src/components/hermes/chat/MessageList.vue
@@ -1502,7 +1502,7 @@ defineExpose({
   display: flex;
   align-items: center;
   gap: 6px;
-  width: 520px;
+  width: 100%;
   max-width: 100%;
   min-width: 0;
   box-sizing: border-box;
diff --git a/tests/e2e/thinking-panel-width.spec.ts b/tests/e2e/thinking-panel-width.spec.ts
new file mode 100644
index 0000000..7f5cb30
--- /dev/null
+++ b/tests/e2e/thinking-panel-width.spec.ts
@@ -0,0 +1,39 @@
+import { expect, test } from '@playwright/test'
+import { authenticate, mockChatSocket, mockHermesApi, TEST_ACCESS_KEY } from './fixtures'
+
+test('thinking tool rows fill the chat width and shrink without overflowing', async ({ page }) => {
+  await page.setViewportSize({ width: 1920, height: 1080 })
+  await authenticate(page, TEST_ACCESS_KEY, 'research')
+  await mockHermesApi(page)
+  await mockChatSocket(page)
+  await page.goto('/#/hermes/chat')
+  await page.locator('textarea.input-textarea').fill('Inspect the working directory')
+  await page.getByRole('button', { name: 'Send', exact: true }).click()
+  await page.waitForFunction(() => (window as any).__PW_CHAT_SOCKET__?.emitted.some((event: any) => event.event === 'run'))
+  await page.evaluate(() => {
+    const state = (window as any).__PW_CHAT_SOCKET__
+    const sid = state.emitted.find((event: any) => event.event === 'run').payload.session_id
+    const common = { session_id: sid, run_id: 'width-run' }
+    state.latest.__trigger('run.started', { ...common, event: 'run.started' })
+    state.latest.__trigger('tool.started', {
+      ...common, event: 'tool.started', tool_call_id: 'width-tool', tool: 'terminal',
+      preview: 'Read a long project path and inspect the current repository state '.repeat(30),
+      arguments: JSON.stringify({ command: 'git status --short' }),
+    })
+  })
+  const panel = page.locator('.tool-calls-panel')
+  const row = panel.locator('.tool-call-item').first()
+  await expect(row).toBeVisible()
+  for (const width of [1920, 900, 390]) {
+    await page.setViewportSize({ width, height: 1080 })
+    await expect.poll(async () => {
+      const panelBox = await panel.boundingBox()
+      const rowBox = await row.boundingBox()
+      return panelBox && rowBox ? Math.abs(rowBox.width - panelBox.width) : Infinity
+    }).toBeLessThan(2)
+    const fits = await panel.evaluate(element => element.scrollWidth <= element.clientWidth + 1)
+    expect(fits).toBe(true)
+    const box = await row.boundingBox()
+    expect(box!.x + box!.width).toBeLessThanOrEqual(width)
+  }
+})