diff --git a/.changeset/10021-legend-swatch-custom-property.md b/.changeset/10021-legend-swatch-custom-property.md
new file mode 100644
index 0000000000..37e1c97a1c
--- /dev/null
+++ b/.changeset/10021-legend-swatch-custom-property.md
@@ -0,0 +1,12 @@
+---
+'@object-ui/plugin-charts': patch
+---
+
+fix(plugin-charts): the legend swatch carries its series colour as a custom property
+
+`ChartLegendContent` painted each swatch with an inline `backgroundColor`, the
+colour-bearing property AGENTS.md's styling carve-out names as forbidden. The
+swatch now publishes the author-declared colour as `--color-bg` and a static
+`bg-(--color-bg)` utility consumes it, the same shape the tooltip indicator in
+the same file already uses, so the theme keeps control of the rule. The
+rendered colour is unchanged.
diff --git a/packages/plugin-charts/src/ChartContainerImpl.legendSwatchColour.test.tsx b/packages/plugin-charts/src/ChartContainerImpl.legendSwatchColour.test.tsx
new file mode 100644
index 0000000000..9f1263c1d1
--- /dev/null
+++ b/packages/plugin-charts/src/ChartContainerImpl.legendSwatchColour.test.tsx
@@ -0,0 +1,67 @@
+/**
+ * ObjectUI
+ * Copyright (c) 2024-present ObjectStack Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * The legend swatch carries the series colour the way AGENTS.md's styling
+ * carve-out prescribes (objectui#10021).
+ *
+ * A series colour is author-declared, so it may reach the DOM through
+ * `style={{}}` — but only as a CSS custom property that a STATIC Tailwind
+ * utility consumes, never as a colour-bearing property (`backgroundColor`)
+ * written inline. The swatch used to write `backgroundColor: item.color`; the
+ * tooltip indicator in the same file already used the compliant shape
+ * (`bg-(--color-bg)` + `"--color-bg"`), and the swatch now copies it.
+ */
+
+import React from 'react';
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { render, cleanup } from '@testing-library/react';
+
+// ResponsiveContainer measures 0x0 under happy-dom; a passthrough keeps the
+// legend content (rendered as a plain child) in the tree.
+vi.mock('recharts', async () => {
+  const actual = await vi.importActual<Record<string, unknown>>('recharts');
+  return {
+    ...actual,
+    ResponsiveContainer: ({ children }: { children: React.ReactElement }) => children,
+  };
+});
+
+import { ChartContainer, ChartLegendContent } from './ChartContainerImpl';
+
+afterEach(() => {
+  cleanup();
+});
+
+function renderSwatch(color: string): HTMLElement {
+  const { container } = render(
+    <ChartContainer config={{}}>
+      <ChartLegendContent
+        payload={[{ value: 'Revenue', dataKey: 'revenue', type: 'square', color }]}
+      />
+    </ChartContainer>,
+  );
+  const swatch = container.querySelector<HTMLElement>('.rounded-\\[2px\\]');
+  expect(swatch).not.toBeNull();
+  return swatch!;
+}
+
+describe('ChartLegendContent swatch colour (objectui#10021)', () => {
+  it('writes no colour-bearing property inline', () => {
+    const swatch = renderSwatch('#ff0000');
+    expect(swatch.style.backgroundColor).toBe('');
+    expect(swatch.style.color).toBe('');
+    expect(swatch.getAttribute('style') ?? '').not.toMatch(/background|(^|;)\s*color\s*:/i);
+  });
+
+  it('publishes the author colour as a custom property a static utility consumes', () => {
+    const swatch = renderSwatch('#ff0000');
+    expect(swatch.style.getPropertyValue('--color-bg')).toBe('#ff0000');
+    expect(swatch.classList.contains('bg-(--color-bg)')).toBe(true);
+  });
+});
diff --git a/packages/plugin-charts/src/ChartContainerImpl.tsx b/packages/plugin-charts/src/ChartContainerImpl.tsx
index e423bc6bd3..9fbd59c616 100644
--- a/packages/plugin-charts/src/ChartContainerImpl.tsx
+++ b/packages/plugin-charts/src/ChartContainerImpl.tsx
@@ -482,11 +482,17 @@ function ChartLegendContent({
               {itemConfig?.icon && !hideIcon ? (
                 <itemConfig.icon />
               ) : (
+                // The series colour is author-declared, so it reaches the DOM
+                // only as a custom property a static utility consumes — the
+                // same shape as the tooltip indicator above (AGENTS.md styling
+                // carve-out; objectui#10021).
                 <div
-                  className="h-2 w-2 shrink-0 rounded-[2px]"
-                  style={{
-                    backgroundColor: item.color,
-                  }}
+                  className="h-2 w-2 shrink-0 rounded-[2px] bg-(--color-bg)"
+                  style={
+                    {
+                      "--color-bg": item.color,
+                    } as React.CSSProperties
+                  }
                 />
               )}
               {/* Fall back to the series name recharts itself put on the