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+# RFC: Bring the CAP visual language into Fluent UI v9
+
+---
+
+_Contributors: (author: Enrico Gianoglio)_
+
+## Summary
+
+Make the **CAP visual language** a first-class part of Fluent UI v9 so that:
+
+1. A consumer of Fluent v9 can opt in to CAP **without installing any separate package** (no `@fluentui-contrib/*` dependency).
+2. All future CAP work happens inside the Fluent v9 repo, on the Fluent release cadence, with the same CI guarantees (bundle size, conformance, VR) as the rest of v9.
+3. CAP is discoverable and try-able directly on https://react.fluentui.dev/ — as a **visual-language toggle** that overlays the currently-selected base theme (web / teams / high-contrast / light / dark), _not_ as additional entries in the theme dropdown.
+4. Consumers who already pass their own `customStyleHooks_unstable` (brand overrides, internal design systems built on Fluent) can adopt CAP **without losing their overrides**. See [Phase 3 — `composeStyleHooks` helper](#phase-3--composestylehooks-helper).
+
+The change is small because the consumer-facing primitive already exists: `FluentProvider` accepts a `customStyleHooks_unstable` prop today and CAP is just a value to pass to it. We propose shipping that value (`CAP_STYLE_HOOKS`) from `@fluentui/react-components`, wiring it into the docs site as a toggle, moving the source into the Fluent v9 monorepo, and adding a small public helper (`composeStyleHooks`) so consumers can layer CAP on top of their own overrides.
+
+## Background
+
+### What CAP is
+
+CAP is a **visual-language overlay** — a set of style decisions (border-radius, spacing, hover/focus treatment, etc.) that re-skin a subset of Fluent v9 components.
+
+CAP **is**:
+
+- A single value: `CAP_STYLE_HOOKS`, a map matching `FluentProviderCustomStyleHooks`.
+- Composed of per-component override hooks across Fluent component packages.
+- Activated by passing it to the existing `customStyleHooks_unstable` prop on `FluentProvider`:
+
+```tsx
+import { FluentProvider, webLightTheme } from '@fluentui/react-components';
+import { CAP_STYLE_HOOKS } from '@fluentui-contrib/react-cap-theme';
+
+<FluentProvider theme={webLightTheme} customStyleHooks_unstable={CAP_STYLE_HOOKS}>
+  <App />
+</FluentProvider>;
+```
+
+That is the entire opt-in surface today.
+
+A sibling map `TEAMS_STYLE_HOOKS` is shipped from the same package and follows the same pattern.
+
+### Where CAP lives today and why that hurts
+
+`CAP_STYLE_HOOKS` is published from `@fluentui-contrib/react-cap-theme`. Consequences:
+
+- CAP is perceived as a third-party theme rather than a peer of `webLightTheme` / `teamsLightTheme`.
+- It is not selectable from the Fluent v9 docs, so adoption requires extra discovery.
+- Product teams must add a non-Fluent package to `package.json` just to access one exported value.
+- Every API gap forces a fork (an override hook in contrib) instead of an upstream contribution that benefits all Fluent users.
+- CAP releases are decoupled from Fluent's release pipeline; bundle-size and VR signals don't gate CAP changes against the Fluent components they override.
+
+## Problem statement
+
+> "We should try to get off this island of working in FluentUI Contrib so that the CAP theme becomes synonymous with Fluent and not a separate entity."
+
+Three things must change:
+
+1. **Consumer install surface.** A product team opting in to CAP should not see `@fluentui-contrib/*` in their `package.json`. CAP should be reachable from the package they already have.
+2. **Development location.** New CAP override hooks (and any related Fluent component extensions) should be authored and reviewed inside the Fluent v9 repo, with the same CI as the rest of v9.
+
+The proposal must also avoid:
+
+- Regressing bundle size for consumers who don't opt in to CAP.
+- Forcing every CAP consumer to rewrite provider plumbing.
+- Coupling brand-specific knowledge into base primitives like `FluentProvider`.
+
+## Detailed Design or Proposal
+
+### Phased approach
+
+| Phase                                  | Goal                                                                                                                                                                                | Risk                            |
+| -------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------- |
+| **1. Re-export from Fluent v9**        | `@fluentui/react-components` internally depends on `@fluentui-contrib/react-cap-theme` and re-exports `CAP_STYLE_HOOKS`. Products import CAP from Fluent. Docs site shows a toggle. | Low — additive only.            |
+| **2. Move source into Fluent v9**      | New `packages/react-components/react-cap-theme/` package in the Fluent monorepo. Contrib package becomes a deprecated shim re-exporting from Fluent.                                | Medium — coordinated migration. |
+| **3. Ship `composeStyleHooks` helper** | Public utility for layering multiple style-hook maps (consumer brand + CAP).                                                                                                        | Low.                            |
+| **4. Deprecate contrib package**       | Mark `@fluentui-contrib/react-cap-theme` deprecated on npm after one Fluent minor cycle.                                                                                            | Low.                            |
+
+### Phase 1 — Re-export contrib from Fluent v9
+
+`@fluentui/react-components` adds `@fluentui-contrib/react-cap-theme` as an internal dependency and re-exports the hooks map:
+
+```ts
+// packages/react-components/react-components/src/index.ts
+export { CAP_STYLE_HOOKS } from '@fluentui-contrib/react-cap-theme';
+// (and TEAMS_STYLE_HOOKS, if Fluent also wants to surface it)
+```
+
+That's the entire Phase 1 change to the public API.
+
+Result for the product team:
+
+```bash
+# what they already have
+yarn add @fluentui/react-components
+```
+
+```tsx
+// what they write
+import { FluentProvider, webLightTheme, CAP_STYLE_HOOKS } from '@fluentui/react-components';
+
+<FluentProvider theme={webLightTheme} customStyleHooks_unstable={CAP_STYLE_HOOKS}>
+  <App />
+</FluentProvider>;
+```
+
+No `@fluentui-contrib/*` in `package.json`. The contrib dependency is an internal implementation detail.
+
+#### Why no `CapProvider` wrapper
+
+Earlier drafts proposed a `<CapProvider>` component. Rejected: `FluentProvider` already has a first-class `customStyleHooks_unstable` prop. Adding a second provider component would duplicate the prop, force existing `<FluentProvider>` users to swap to a different component just to opt in, and complicate library code that wraps `<FluentProvider>` internally. The simpler answer is "expose the value and let consumers pass it to the prop they already use."
+
+### Phase 2 — Move source into Fluent v9
+
+Create a new sibling package:
+
+```
+packages/react-components/
+  react-cap-theme/                 # new — source of truth post-Phase-2
+    library/src/
+      capStyleHooks.ts             # exports CAP_STYLE_HOOKS
+      components/                  # per-component override hooks
+        react-accordion/
+        react-avatar/
+        react-button/
+        react-card/
+        ...
+      index.ts
+    stories/src/                   # optional CAP-specific stories
+```
+
+The folder structure mirrors today's contrib source layout.
+
+`@fluentui/react-cap-theme` replaces `@fluentui-contrib/react-cap-theme` as the source. The contrib package becomes a thin re-export shim with a deprecation notice:
+
+```ts
+// fluentui-contrib/packages/react-cap-theme/src/index.ts (post-Phase-2)
+/** @deprecated Import from `@fluentui/react-components` instead. */
+export { CAP_STYLE_HOOKS } from '@fluentui/react-cap-theme';
+```
+
+### Phase 3 — `composeStyleHooks` helper
+
+**This is the answer to the most common product-team blocker.** Today, `customStyleHooks_unstable` is a single map and assigning it replaces whatever was there. A product team that already does this:
+
+```tsx
+const productHooks = {
+  useButtonStyles_unstable: useBrandedButtonStyles,
+  useInputStyles_unstable: useBrandedInputStyles,
+};
+
+<FluentProvider customStyleHooks_unstable={productHooks}>
+```
+
+…cannot opt in to CAP without **losing their own overrides**.
+
+Ship a tiny composition utility:
+
+```ts
+// packages/react-components/react-provider/library/src/utils/composeStyleHooks.ts
+import type { FluentProviderCustomStyleHooks } from '../components/FluentProvider/FluentProvider.types';
+
+/**
+ * Combine multiple custom-style-hook maps into one. For each component, every
+ * matching hook runs in order; later hooks see the state mutated by earlier
+ * hooks. Conflicting className writes resolve by Griffel's `mergeClasses`
+ * rule (rightmost class wins on the same CSS rule).
+ *
+ * Typical usage: layer CAP on top of a consumer's brand overrides.
+ */
+export function composeStyleHooks(
+  ...maps: ReadonlyArray<FluentProviderCustomStyleHooks | undefined>
+): FluentProviderCustomStyleHooks {
+  const defined = maps.filter((m): m is FluentProviderCustomStyleHooks => Boolean(m));
+  const keys = new Set(defined.flatMap(Object.keys));
+  const out: Record<string, (state: unknown) => void> = {};
+
+  for (const key of keys) {
+    const hooks = defined.map(m => (m as Record<string, unknown>)[key]).filter(Boolean) as Array<(s: unknown) => void>;
+    out[key] = state => {
+      for (const h of hooks) h(state);
+    };
+  }
+
+  return out as FluentProviderCustomStyleHooks;
+}
+```
+
+Re-exported from `@fluentui/react-components`. Usage:
+
+```tsx
+import { FluentProvider, webLightTheme, CAP_STYLE_HOOKS, composeStyleHooks } from '@fluentui/react-components';
+
+const productHooks = {
+  useButtonStyles_unstable: useBrandedButtonStyles,
+};
+
+<FluentProvider theme={webLightTheme} customStyleHooks_unstable={composeStyleHooks(productHooks, CAP_STYLE_HOOKS)}>
+  <App />
+</FluentProvider>;
+```