diff --git a/src/uix/eidos/active-eidos-config.test.ts b/src/uix/eidos/active-eidos-config.test.ts
index f7fed711d..fcaa94b5e 100644
--- a/src/uix/eidos/active-eidos-config.test.ts
+++ b/src/uix/eidos/active-eidos-config.test.ts
@@ -561,6 +561,14 @@ describe('ActiveEidos config', () => {
 		expect(css).toContain('--shape-smoothing: 3;');
 	});
 
+	it('applyShape retunes the channel at runtime (jaula-abierta shape builder)', () => {
+		const eidos = createThemeBaseEidos();
+		const result = eidos.applyShape({ smoothing: 1.4, families: { continuous: 'squircle' } });
+		expect(result.variables).toContain('--shape-smoothing: 1.4;');
+		expect(result.families).toContain("[data-shape='continuous'] { corner-shape: squircle; }");
+		eidos.clearShape(); // reverts cleanly, no throw
+	});
+
 	it('emits the Phase 3 typography scales + optical tracking', () => {
 		const css = createThemeBaseEidos().renderStaticCss()
 		expect(css).toContain('--tracking-tight: -0.02em;')
diff --git a/src/uix/eidos/active-eidos.svelte.ts b/src/uix/eidos/active-eidos.svelte.ts
index d4d296b8c..9b6bbb69c 100644
--- a/src/uix/eidos/active-eidos.svelte.ts
+++ b/src/uix/eidos/active-eidos.svelte.ts
@@ -54,6 +54,7 @@ import {
 	type BuildTypeScaleResult
 } from './lib/build-type-scale';
 import { buildDepth, type DepthOverrides, type BuildDepthResult } from './lib/build-depth';
+import { buildShape, type ShapeSeed, type BuildShapeResult } from './lib/build-shape';
 import { collectFontPreloads, type FontPreload } from './lib/font-preload';
 import type { Oklch } from '$color';
 import type { EngineMotion } from '$motion';
@@ -158,6 +159,17 @@ interface ActiveEidosDepthSpec {
 	readonly options: ApplyDepthOptions;
 }
 
+/** Options for {@link ActiveEidos.applyShape}. */
+export interface ApplyShapeOptions {
+	/** CSS selector the variable override targets. @default ':root' */
+	readonly selector?: string;
+}
+
+interface ActiveEidosShapeSpec {
+	readonly seed: ShapeSeed;
+	readonly options: ApplyShapeOptions;
+}
+
 export interface ActiveEidosOptions {
 	readonly config?: EidosConfig | EidosConfigDocument;
 	readonly themeBase?: EidosConfigPatch;
@@ -219,6 +231,7 @@ export class ActiveEidos {
 	#schemeSpec: ActiveEidosSchemeSpec | undefined;
 	#typeScaleSpec: ActiveEidosTypeScaleSpec | undefined;
 	#depthSpec: ActiveEidosDepthSpec | undefined;
+	#shapeSpec: ActiveEidosShapeSpec | undefined;
 	#unsubscribe: (() => void) | undefined;
 	#lastAttrs: ActiveEidosLastAttrs | undefined;
 	#disposed = false;
@@ -528,6 +541,26 @@ export class ActiveEidos {
 		this.apply();
 	}
 
+	/**
+	 * Retune the shape channel at runtime — the shape analogue of {@link applyColorScheme} /
+	 * {@link applyTypeScale} / {@link applyDepth}. Dial `smoothing` (corner continuity / squircle
+	 * intensity) + `nestGap`, or override/add `families`, written as a managed block that wins over
+	 * the foundation. The jaula-abierta runtime of the shape channel.
+	 */
+	applyShape(seed: ShapeSeed, options: ApplyShapeOptions = {}): BuildShapeResult {
+		this.#shapeSpec = { seed, options };
+		const result = buildShape(seed);
+		this.apply();
+		return result;
+	}
+
+	/** Remove the applied shape retune, reverting to the foundation's shape system. */
+	clearShape(): void {
+		if (!this.#shapeSpec) return;
+		this.#shapeSpec = undefined;
+		this.apply();
+	}
+
 	#buildSchemeResult(): BuildSchemeResult {
 		const spec = this.#schemeSpec;
 		if (!spec) throw new ActiveEidosConfigError('no color scheme applied');
@@ -623,6 +656,16 @@ export class ActiveEidos {
 		} else {
 			this.#removeStyle(host, depthStyleId);
 		}
+
+		// The runtime shape retune (applyShape) is written last so its `--shape-*` + family rules
+		// win over the static foundation's shape system at equal specificity.
+		const shapeStyleId = `${this.#styleId}-shape`;
+		const shapeCss = this.#renderShapeCss();
+		if (shapeCss) {
+			this.#writeStyle(host, shapeStyleId, shapeCss);
+		} else {
+			this.#removeStyle(host, shapeStyleId);
+		}
 	}
 
 	#renderSchemeCss(): string {
@@ -657,6 +700,15 @@ export class ActiveEidos {
 		return body ? `${selector} {\n${body}\n}` : '';
 	}
 
+	#renderShapeCss(): string {
+		if (!this.#shapeSpec) return '';
+		const result = buildShape(this.#shapeSpec.seed);
+		const selector = this.#shapeSpec.options.selector ?? ':root';
+		const body = result.variables.map((line) => `\t${line}`).join('\n');
+		const varBlock = body ? `${selector} {\n${body}\n}` : '';
+		return [varBlock, ...result.families].filter(Boolean).join('\n');
+	}
+
 	dispose(): void {
 		if (this.#disposed) return;
 		this.#disposed = true;
diff --git a/src/uix/eidos/index.ts b/src/uix/eidos/index.ts
index b9c410a01..f13013a7d 100644
--- a/src/uix/eidos/index.ts
+++ b/src/uix/eidos/index.ts
@@ -28,6 +28,8 @@ export type {
 } from './lib/build-type-scale';
 export { buildDepth, depthDeclarations } from './lib/build-depth';
 export type { DepthOverrides, BuildDepthResult } from './lib/build-depth';
+export { buildShape } from './lib/build-shape';
+export type { ShapeSeed, BuildShapeResult } from './lib/build-shape';
 export { collectFontPreloads } from './lib/font-preload';
 export type { FontPreload } from './lib/font-preload';
 export {
@@ -102,7 +104,8 @@ export type {
 	ActiveEidosUserOptions,
 	ApplyColorSchemeOptions,
 	ApplyTypeScaleOptions,
-	ApplyDepthOptions
+	ApplyDepthOptions,
+	ApplyShapeOptions
 } from './active-eidos.svelte';
 export type {
 	EidosConfigDocument,
diff --git a/src/uix/eidos/lib/build-shape.ts b/src/uix/eidos/lib/build-shape.ts
new file mode 100644
index 000000000..54ef864f0
--- /dev/null
+++ b/src/uix/eidos/lib/build-shape.ts
@@ -0,0 +1,42 @@
+/**
+ * Runtime shape retune — the shape analogue of `build-scheme.ts` (color),
+ * `build-type-scale.ts` (typography) and `build-depth.ts` (depth). Pure + isomorphic, DOM-free.
+ *
+ * Given a seed it emits the shape overrides for a managed block: `--shape-smoothing` (live
+ * continuity / squircle intensity) + `--shape-nest-gap` as `:root` variables, and any
+ * `families` as `[data-shape='{family}']` rules that win over the foundation. The jaula-abierta
+ * runtime of the shape channel — dial the corner continuity from a theme editor, no rebuild.
+ *
+ * `ActiveEidos.applyShape(seed)` writes the result as a managed block; the composition is here,
+ * the DOM application lives on ActiveEidos, matching the other three builders.
+ */
+
+export interface ShapeSeed {
+	/** Superellipse exponent for the `continuous` family (1 = arc, 2 = squircle). */
+	readonly smoothing?: string | number;
+	/** Concentric nesting gap (`--shape-nest-gap`). */
+	readonly nestGap?: string;
+	/** `data-shape='{family}'` → `corner-shape` value — override or add families at runtime. */
+	readonly families?: Record<string, string>;
+}
+
+export interface BuildShapeResult {
+	/** `:root` variable declarations (`--shape-smoothing` / `--shape-nest-gap`). */
+	readonly variables: readonly string[];
+	/** Full `[data-shape='{family}'] { corner-shape: … }` rule strings. */
+	readonly families: readonly string[];
+}
+
+/** Build the shape override declarations + family rules from a runtime seed. */
+export function buildShape(seed: ShapeSeed): BuildShapeResult {
+	const variables: string[] = [];
+	if (seed.smoothing !== undefined) variables.push(`--shape-smoothing: ${seed.smoothing};`);
+	if (seed.nestGap !== undefined) variables.push(`--shape-nest-gap: ${seed.nestGap};`);
+
+	const families: string[] = [];
+	for (const [family, value] of Object.entries(seed.families ?? {})) {
+		families.push(`[data-shape='${family}'] { corner-shape: ${value}; }`);
+	}
+
+	return { variables, families };
+}