diff --git a/apps/desktop/src/main/__tests__/renderer-style-pruning-contract.test.ts b/apps/desktop/src/main/__tests__/renderer-style-pruning-contract.test.ts
index 651a1ad2a5..c0790ef46e 100644
--- a/apps/desktop/src/main/__tests__/renderer-style-pruning-contract.test.ts
+++ b/apps/desktop/src/main/__tests__/renderer-style-pruning-contract.test.ts
@@ -63,9 +63,6 @@ const DYNAMIC_STYLE_HOOKS = new Set([
   'maka-sidebar-row',
   'maka-sidebar-section',
   'maka-titlebar',
-  'maka-tool-error-body',
-  'maka-tool-error-icon',
-  'maka-tool-error-title',
   'scrollbar-hide',
   'scrollbar-hover',
   'shadow-medium',
diff --git a/apps/desktop/src/main/__tests__/visible-copy-hygiene-contract.test.ts b/apps/desktop/src/main/__tests__/visible-copy-hygiene-contract.test.ts
index dee4779c62..5033c21f25 100644
--- a/apps/desktop/src/main/__tests__/visible-copy-hygiene-contract.test.ts
+++ b/apps/desktop/src/main/__tests__/visible-copy-hygiene-contract.test.ts
@@ -553,22 +553,39 @@ describe('tool error copy feedback contract', () => {
   });
 
   it('styles tool-error copy pending and failure states', async () => {
-    const src = await readRendererContractCss();
+    // `.maka-tool-error-copy[…]` retired onto the `@maka/ui` Alert primitive
+    // (issue #332 PR3c): the pending / copy-feedback chrome — which lived UNLAYERED
+    // in tool-output.css so it out-ranked the ghost button — now lives as literal
+    // arbitrary utilities inlined on the copy button's `className`. We slice the
+    // `ToolErrorBanner` block and require the utilities to appear on an actual
+    // `className="maka-button …"` so the assertion proves BOTH that the state
+    // utilities exist AND that the banner's copy button wears them — a whole-file
+    // scan would false-pass if the string drifted to another component. These are
+    // arbitrary-value utilities (source == computed), so this source contract is the
+    // proof; the computed-style harness only re-diffs the non-trivial container box.
+    const componentsPath = resolve(process.cwd(), '..', '..', 'packages', 'ui', 'src', 'components.tsx');
+    const src = await readFile(componentsPath, 'utf8');
+    const block = src.match(/function ToolErrorBanner[\s\S]*?export function OverlayHost/)?.[0] ?? '';
 
     assert.match(
-      src,
-      /\.maka-tool-error-copy\[data-pending="true"\]\s*\{[\s\S]*cursor:\s*progress;/,
+      block,
+      /className="maka-button \[align-self:start\] data-\[pending=true\]:cursor-progress/,
+      'The tool-error copy button must wear the leaf state utilities inline on its className.',
+    );
+    assert.match(
+      block,
+      /data-\[pending=true\]:cursor-progress/,
       'Tool-error pending copy should visibly indicate in-progress work.',
     );
     assert.match(
-      src,
-      /\.maka-tool-error-copy\[data-copy-feedback="copied"\]/,
-      'Tool-error copied state should have a stable CSS selector.',
+      block,
+      /data-\[copy-feedback=copied\]:text-\[color:var\(--accent\)\] data-\[copy-feedback=copied\]:border-\[oklch\(from_var\(--accent\)_l_c_h_\/_0\.35\)\]/,
+      'Tool-error copied state should have a stable color + border styling hook.',
     );
     assert.match(
-      src,
-      /\.maka-tool-error-copy\[data-copy-feedback="failed"\]/,
-      'Tool-error failed copy state should have a stable CSS selector.',
+      block,
+      /data-\[copy-feedback=failed\]:text-\[color:var\(--destructive\)\] data-\[copy-feedback=failed\]:border-\[oklch\(from_var\(--destructive\)_l_c_h_\/_0\.35\)\]/,
+      'Tool-error failed copy state should have a stable color + border styling hook.',
     );
   });
 });
diff --git a/apps/desktop/src/renderer/maka-tokens.css b/apps/desktop/src/renderer/maka-tokens.css
index 769076ef6b..3f3abbfa46 100644
--- a/apps/desktop/src/renderer/maka-tokens.css
+++ b/apps/desktop/src/renderer/maka-tokens.css
@@ -1496,55 +1496,6 @@
     50%      { opacity: 1;    transform: scale(1.1); }
   }
 
-  /* Inline failure banner shown at the top of an errored tool body so the
-   * user sees what failed without expanding the result blob. The wrapper
-   * already paints a red border via [data-status="errored"]; this banner
-   * gives the failure a heading + copy-button affordance. */
-  .maka-tool-error {
-    display: grid;
-    grid-template-columns: 18px minmax(0, 1fr) auto;
-    align-items: start;
-    gap: 10px;
-    padding: 10px 12px;
-    margin: 0 0 10px;
-    border: 1px solid oklch(from var(--destructive) l c h / 0.30);
-    border-radius: 10px;
-    background: oklch(from var(--destructive) l c h / 0.08);
-  }
-  .maka-tool-error-icon {
-    display: grid;
-    place-items: center;
-    color: var(--destructive);
-    margin-top: 1px;
-  }
-  .maka-tool-error-body {
-    display: grid;
-    gap: 4px;
-    min-width: 0;
-  }
-  .maka-tool-error-title {
-    color: var(--destructive);
-    font-size: 12px;
-    font-weight: 700;
-    letter-spacing: 0.02em;
-  }
-  .maka-tool-error-text {
-    margin: 0;
-    color: var(--foreground-80);
-    font-family: var(--font-mono);
-    font-size: 12px;
-    line-height: 1.5;
-    white-space: pre-wrap;
-    word-break: break-word;
-  }
-  .maka-tool-error-copy {
-    align-self: start;
-    display: inline-flex;
-    align-items: center;
-    gap: 4px;
-    font-size: 11px;
-  }
-
   /* ---- Composer --------------------------------------------------- */
 
   .maka-composer {
diff --git a/apps/desktop/src/renderer/styles/tool-output.css b/apps/desktop/src/renderer/styles/tool-output.css
index bcbf448967..ec464a7f1b 100644
--- a/apps/desktop/src/renderer/styles/tool-output.css
+++ b/apps/desktop/src/renderer/styles/tool-output.css
@@ -334,20 +334,6 @@
   border-color: oklch(from var(--destructive) l c h / 0.35);
 }
 
-.maka-tool-error-copy[data-pending="true"] {
-  cursor: progress;
-}
-
-.maka-tool-error-copy[data-copy-feedback="copied"] {
-  color: var(--accent);
-  border-color: oklch(from var(--accent) l c h / 0.35);
-}
-
-.maka-tool-error-copy[data-copy-feedback="failed"] {
-  color: var(--destructive);
-  border-color: oklch(from var(--destructive) l c h / 0.35);
-}
-
 .maka-office-document-preview {
   display: grid;
   gap: 8px;
diff --git a/packages/ui/src/components.tsx b/packages/ui/src/components.tsx
index cdcadde8a6..ddb42fa5ac 100644
--- a/packages/ui/src/components.tsx
+++ b/packages/ui/src/components.tsx
@@ -6325,6 +6325,9 @@ function ToolOutputStream(props: {
   );
 }
 
+// Preserve the retired `.maka-tool-error*` leaf utilities onto Alert (#332 PR3c) —
+// Alert owns the shell; these are the few declarations it doesn't set, kept arbitrary
+// so they map 1:1 to the old CSS (`[align-self:start]`, not Tailwind's `flex-start`).
 function ToolErrorBanner(props: { result: ToolActivityItem['result'] }) {
   // Tool stderr / raw provider errors occasionally slip credential paths,
   // bearer tokens, or API keys through main-side redaction. Apply a
@@ -6348,11 +6351,11 @@ function ToolErrorBanner(props: { result: ToolActivityItem['result'] }) {
   }
 
   return (
-    <Alert variant="error" className="maka-tool-error">
+    <Alert variant="error" className="mb-[10px]">
       <AlertOctagon size={16} strokeWidth={2} aria-hidden="true" />
       <AlertTitle>工具调用失败</AlertTitle>
       {errorText && (
-        <AlertDescription className="maka-tool-error-text">
+        <AlertDescription className="[font-family:var(--font-mono)] text-[12px] leading-[1.5] whitespace-pre-wrap [word-break:break-word]">
           {errorText.length > 240 ? `${errorText.slice(0, 240)}…` : errorText}
         </AlertDescription>
       )}
@@ -6362,7 +6365,7 @@ function ToolErrorBanner(props: { result: ToolActivityItem['result'] }) {
             type="button"
             variant="ghost"
             size="sm"
-            className="maka-button maka-tool-error-copy"
+            className="maka-button [align-self:start] data-[pending=true]:cursor-progress data-[copy-feedback=copied]:text-[color:var(--accent)] data-[copy-feedback=copied]:border-[oklch(from_var(--accent)_l_c_h_/_0.35)] data-[copy-feedback=failed]:text-[color:var(--destructive)] data-[copy-feedback=failed]:border-[oklch(from_var(--destructive)_l_c_h_/_0.35)]"
             data-pending={copyPending ? 'true' : undefined}
             data-copy-feedback={copyPhase ?? undefined}
             aria-label={`${copyLabel}错误信息`}
diff --git a/scripts/check-chat-marker-computed-style.mjs b/scripts/check-chat-marker-computed-style.mjs
index 219a460e27..f0820672ca 100644
--- a/scripts/check-chat-marker-computed-style.mjs
+++ b/scripts/check-chat-marker-computed-style.mjs
@@ -85,6 +85,7 @@ import { fileURLToPath, pathToFileURL } from 'node:url';
 const REPO_ROOT = resolve(fileURLToPath(new URL('..', import.meta.url)));
 const { buttonVariants, cn } = await import(pathToFileURL(resolve(REPO_ROOT, 'packages/ui/dist/ui.js')).href);
 const { markerVariants, streamVariants, toolVariants } = await import(pathToFileURL(resolve(REPO_ROOT, 'packages/ui/dist/primitives/chat.js')).href);
+const { Alert, AlertTitle, AlertDescription, AlertAction } = await import(pathToFileURL(resolve(REPO_ROOT, 'packages/ui/dist/primitives/alert.js')).href);
 
 const mainCssPath = process.argv[2] && resolve(process.argv[2]);
 const headCssPath = process.argv[3] && resolve(process.argv[3]);
@@ -206,6 +207,36 @@ const toolCardSection = (el) => {
     + STAT.map(card).join('\n'));
 };
 
+// PR3c — the tool-error banner CONTAINER. The ONE thing this harness uniquely proves
+// is that the retired `.maka-tool-error` CONTAINER declarations were INERT:
+// `.maka-tool-error*` sat in `@layer components` while Alert's slot utilities sit in
+// `@layer utilities` (which win regardless of specificity), so its bespoke 18px grid /
+// 10px radius / padding / border / background never rendered. The main side (real Alert
+// error class + `.maka-tool-error`) and the head side (same Alert class + `mb-[10px]`)
+// must therefore compute identically; a wrongly-surviving `.maka-tool-error`
+// declaration would surface as a real DIFF here, not a false green. The Alert slot
+// classes come from CALLING the real primitive components — single source of truth, no
+// hand-copy, no production-API change (a function component is a plain function, so
+// `.props.className` is exactly what it renders). The description / copy-button LEAF
+// utilities are NOT re-diffed here: they are arbitrary-value Tailwind (source ==
+// computed by construction) pinned by visible-copy-hygiene-contract instead.
+const ALERT_ERR = Alert({ variant: 'error' }).props.className;
+const ALERT_DESC = AlertDescription({}).props.className;
+const ALERT_ACTION = AlertAction({}).props.className;
+const ALERT_TITLE = AlertTitle({}).props.className;
+const errorBanner = (el) => {
+  const cont = pair(cn(ALERT_ERR, 'maka-tool-error'), cn(ALERT_ERR, 'mb-[10px]'));
+  // The svg + title / description / action slot children carry their real Alert slot
+  // classes only so Alert's `has-[>svg]:has-data-[slot=alert-action]` 3-col grid
+  // resolves on the container exactly as in production; only `err-banner` is diffed.
+  return el('div', 'err-banner', cont, 'data-slot="alert" role="alert"',
+      '<svg width="16" height="16" aria-hidden="true"></svg>'
+      + el('div', 'err-title', pair(ALERT_TITLE, ALERT_TITLE), 'data-slot="alert-title"', '工具调用失败')
+      + el('div', 'err-text', pair(ALERT_DESC, ALERT_DESC), 'data-slot="alert-description"', 'boom: command not found')
+      + el('div', 'err-action', pair(ALERT_ACTION, ALERT_ACTION), 'data-slot="alert-action"',
+          el('button', 'err-copy', pair(bv('ghost', 'sm'), bv('ghost', 'sm')), 'type="button"', '<svg width="11" height="11"></svg><span>复制</span>')));
+};
+
 // DOM tree mirroring TurnView nesting.
 const TREE = (side) => {
   const C = (p) => p[side];
@@ -251,6 +282,8 @@ const TREE = (side) => {
     streamPanel(el, 'stream-live', 'data-live="true"'),
     // The PR3b tool-activity card shell.
     toolCardSection(el),
+    // The PR3c tool-error banner (Alert primitive).
+    errorBanner(el),
   ].join('\n');
 };
 
@@ -272,7 +305,12 @@ const IDS = ['summary', 'summary-chip-1', 'summary-chip-2', 'summary-chip-tools'
   'tool-summary-collapsed', 'tool-name-collapsed', 'tool-body-collapsed',
   // …and every static dot, EXCEPT running's (its `maka-tool-pulse` ring is
   // animated → phase-dependent `getComputedStyle`; pinned by the keyframe contract).
-  ...STAT.filter((s) => s !== 'running').map((s) => `tool-dot-${s}`)];
+  ...STAT.filter((s) => s !== 'running').map((s) => `tool-dot-${s}`),
+  // PR3c tool-error banner: the CONTAINER box only — proves `.maka-tool-error` was
+  // inert (shadowed by Alert's `@layer utilities`). The description / copy-button leaf
+  // utilities are arbitrary-value (source == computed) and pinned by
+  // visible-copy-hygiene-contract, so they are not re-diffed here.
+  'err-banner'];
 // `::before` middot separators are now diffed for real (they render once the
 // CSS is inlined — the old `<link>` build couldn't apply them, masking this).
 // summary-chip-2 is a non-first chip (`[&:not(:first-child)]:before:…`);