diff --git a/clients/web/src/components/elements/AppRenderer/hostContext.test.ts b/clients/web/src/components/elements/AppRenderer/hostContext.test.ts
new file mode 100644
index 00000000..23041332
--- /dev/null
+++ b/clients/web/src/components/elements/AppRenderer/hostContext.test.ts
@@ -0,0 +1,208 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import type { McpUiDisplayMode } from "@modelcontextprotocol/ext-apps/app-bridge";
+import {
+  currentStyles,
+  currentTheme,
+  measureContainerDimensions,
+  snapshotHostContext,
+} from "./hostContext";
+
+const COLOR_SCHEME_ATTR = "data-mantine-color-scheme";
+
+/**
+ * Build a fake CSSStyleDeclaration whose `getPropertyValue` reads from a
+ * lookup table, so tests control exactly which CSS variables resolve (and to
+ * what) without depending on happy-dom's custom-property resolution.
+ */
+function fakeComputedStyle(
+  values: Record<string, string>,
+): CSSStyleDeclaration {
+  return {
+    getPropertyValue: (name: string) => values[name] ?? "",
+  } as unknown as CSSStyleDeclaration;
+}
+
+function stubComputedStyle(values: Record<string, string>) {
+  vi.spyOn(window, "getComputedStyle").mockReturnValue(
+    fakeComputedStyle(values),
+  );
+}
+
+afterEach(() => {
+  vi.restoreAllMocks();
+  vi.unstubAllGlobals();
+  document.documentElement.removeAttribute(COLOR_SCHEME_ATTR);
+});
+
+describe("currentTheme", () => {
+  it("returns the resolved dark scheme written by Mantine", () => {
+    document.documentElement.setAttribute(COLOR_SCHEME_ATTR, "dark");
+    expect(currentTheme()).toBe("dark");
+  });
+
+  it("returns the resolved light scheme written by Mantine", () => {
+    document.documentElement.setAttribute(COLOR_SCHEME_ATTR, "light");
+    expect(currentTheme()).toBe("light");
+  });
+
+  it("ignores an unrecognized attribute value and falls back", () => {
+    document.documentElement.setAttribute(COLOR_SCHEME_ATTR, "sepia");
+    vi.stubGlobal("matchMedia", () => ({ matches: false }));
+    expect(currentTheme()).toBe("light");
+  });
+
+  it("falls back to matchMedia dark when the attribute is absent", () => {
+    document.documentElement.removeAttribute(COLOR_SCHEME_ATTR);
+    vi.stubGlobal("matchMedia", (query: string) => ({
+      matches: query.includes("dark"),
+    }));
+    expect(currentTheme()).toBe("dark");
+  });
+
+  it("falls back to light when matchMedia does not match dark", () => {
+    document.documentElement.removeAttribute(COLOR_SCHEME_ATTR);
+    vi.stubGlobal("matchMedia", () => ({ matches: false }));
+    expect(currentTheme()).toBe("light");
+  });
+
+  it("falls back to light when matchMedia is unavailable", () => {
+    document.documentElement.removeAttribute(COLOR_SCHEME_ATTR);
+    vi.stubGlobal("matchMedia", undefined);
+    expect(currentTheme()).toBe("light");
+  });
+
+  it("returns light in a non-DOM environment (no document, no window)", () => {
+    vi.stubGlobal("document", undefined);
+    vi.stubGlobal("window", undefined);
+    expect(currentTheme()).toBe("light");
+  });
+
+  it("consults matchMedia when document is absent but window is present", () => {
+    vi.stubGlobal("document", undefined);
+    vi.stubGlobal("matchMedia", (query: string) => ({
+      matches: query.includes("dark"),
+    }));
+    expect(currentTheme()).toBe("dark");
+  });
+});
+
+describe("currentStyles", () => {
+  it("maps resolved Mantine tokens to spec style variables", () => {
+    stubComputedStyle({
+      "--mantine-color-body": "#101113",
+      "--mantine-color-text": "#c9c9c9",
+      "--mantine-radius-md": "0.5rem",
+    });
+    const styles = currentStyles();
+    expect(styles).toEqual({
+      variables: {
+        "--color-background-primary": "#101113",
+        "--color-text-primary": "#c9c9c9",
+        "--border-radius-md": "0.5rem",
+      },
+    });
+  });
+
+  it("trims whitespace and omits variables that resolve to empty", () => {
+    stubComputedStyle({
+      "--mantine-color-body": "  #ffffff  ",
+      "--mantine-color-text": "   ",
+    });
+    const styles = currentStyles();
+    expect(styles?.variables?.["--color-background-primary"]).toBe("#ffffff");
+    expect(styles?.variables?.["--color-text-primary"]).toBeUndefined();
+  });
+
+  it("returns undefined when nothing resolves", () => {
+    stubComputedStyle({});
+    expect(currentStyles()).toBeUndefined();
+  });
+
+  it("returns undefined in a non-DOM environment (no document)", () => {
+    vi.stubGlobal("document", undefined);
+    expect(currentStyles()).toBeUndefined();
+  });
+
+  it("returns undefined in a non-DOM environment (no window)", () => {
+    vi.stubGlobal("window", undefined);
+    expect(currentStyles()).toBeUndefined();
+  });
+});
+
+describe("measureContainerDimensions", () => {
+  it("returns whole-pixel width and height for a laid-out element", () => {
+    const el = document.createElement("div");
+    vi.spyOn(el, "getBoundingClientRect").mockReturnValue({
+      width: 320.6,
+      height: 199.4,
+    } as DOMRect);
+    expect(measureContainerDimensions(el)).toEqual({ width: 321, height: 199 });
+  });
+
+  it("returns undefined for a 0x0 (unattached) element", () => {
+    const el = document.createElement("div");
+    vi.spyOn(el, "getBoundingClientRect").mockReturnValue({
+      width: 0,
+      height: 0,
+    } as DOMRect);
+    expect(measureContainerDimensions(el)).toBeUndefined();
+  });
+
+  it("returns undefined when only one dimension is zero", () => {
+    const el = document.createElement("div");
+    vi.spyOn(el, "getBoundingClientRect").mockReturnValue({
+      width: 100,
+      height: 0,
+    } as DOMRect);
+    expect(measureContainerDimensions(el)).toBeUndefined();
+  });
+
+  it("returns undefined when getBoundingClientRect is unavailable", () => {
+    const el = {} as unknown as HTMLElement;
+    expect(measureContainerDimensions(el)).toBeUndefined();
+  });
+});
+
+describe("snapshotHostContext", () => {
+  const MODES: readonly McpUiDisplayMode[] = ["inline", "fullscreen"];
+
+  it("seeds theme, inline display mode, and a copied available-modes list", () => {
+    document.documentElement.setAttribute(COLOR_SCHEME_ATTR, "dark");
+    stubComputedStyle({});
+    const context = snapshotHostContext(null, MODES);
+    expect(context.theme).toBe("dark");
+    expect(context.displayMode).toBe("inline");
+    expect(context.availableDisplayModes).toEqual(["inline", "fullscreen"]);
+    // A copy, not the caller's array.
+    expect(context.availableDisplayModes).not.toBe(MODES);
+    expect(context.styles).toBeUndefined();
+    expect(context.containerDimensions).toBeUndefined();
+  });
+
+  it("includes styles and container dimensions when both resolve", () => {
+    document.documentElement.setAttribute(COLOR_SCHEME_ATTR, "light");
+    stubComputedStyle({ "--mantine-color-body": "#ffffff" });
+    const container = document.createElement("div");
+    vi.spyOn(container, "getBoundingClientRect").mockReturnValue({
+      width: 640,
+      height: 480,
+    } as DOMRect);
+    const context = snapshotHostContext(container, MODES);
+    expect(context.theme).toBe("light");
+    expect(context.styles).toEqual({
+      variables: { "--color-background-primary": "#ffffff" },
+    });
+    expect(context.containerDimensions).toEqual({ width: 640, height: 480 });
+  });
+
+  it("omits container dimensions when the container has no layout box", () => {
+    stubComputedStyle({});
+    const container = document.createElement("div");
+    vi.spyOn(container, "getBoundingClientRect").mockReturnValue({
+      width: 0,
+      height: 0,
+    } as DOMRect);
+    const context = snapshotHostContext(container, MODES);
+    expect(context.containerDimensions).toBeUndefined();
+  });
+});
diff --git a/clients/web/src/components/elements/AppRenderer/hostContext.ts b/clients/web/src/components/elements/AppRenderer/hostContext.ts
new file mode 100644
index 00000000..9ea6f8f0
--- /dev/null
+++ b/clients/web/src/components/elements/AppRenderer/hostContext.ts
@@ -0,0 +1,156 @@
+import type {
+  McpUiDisplayMode,
+  McpUiHostContext,
+  McpUiHostStyles,
+  McpUiStyles,
+  McpUiStyleVariableKey,
+} from "@modelcontextprotocol/ext-apps/app-bridge";
+
+/**
+ * Resolve the host theme from the DOM. Mantine writes the resolved color
+ * scheme to `<html data-mantine-color-scheme>`. Reading it here (rather than
+ * capturing React state) keeps the bridge factory's identity stable across
+ * theme toggles — the renderer treats a new factory identity as "rebuild the
+ * bridge", which would reload a running app's iframe on every theme flip.
+ *
+ * The attribute is only ever `"light"` or `"dark"` — Mantine resolves
+ * `defaultColorScheme="auto"` to the system value before paint and never
+ * writes `"auto"` here, so no `auto` branch is needed. The matchMedia
+ * fallback only covers the attribute being absent (e.g. a hydration race).
+ */
+export function currentTheme(): "light" | "dark" {
+  if (typeof document !== "undefined") {
+    const attr = document.documentElement.getAttribute(
+      "data-mantine-color-scheme",
+    );
+    if (attr === "dark" || attr === "light") return attr;
+  }
+  if (
+    typeof window !== "undefined" &&
+    window.matchMedia?.("(prefers-color-scheme: dark)").matches
+  ) {
+    return "dark";
+  }
+  return "light";
+}
+
+/**
+ * Maps the spec's host-style variable keys ({@link McpUiStyleVariableKey}) to
+ * the inspector's underlying CSS custom properties. The inspector themes itself
+ * with Mantine, so each spec token resolves to the matching Mantine design-token
+ * variable (or an `--inspector-*` token layered on top of one). Only a curated
+ * subset of the ~90 spec keys is mapped — the ones the inspector has a
+ * meaningful equivalent for; the rest are omitted, which the spec allows (hosts
+ * may provide any subset).
+ */
+const STYLE_VARIABLE_SOURCES: Partial<Record<McpUiStyleVariableKey, string>> = {
+  "--color-background-primary": "--mantine-color-body",
+  "--color-background-secondary": "--inspector-surface-card",
+  "--color-background-tertiary": "--inspector-surface-subtle",
+  "--color-text-primary": "--mantine-color-text",
+  "--color-text-secondary": "--inspector-text-secondary",
+  "--color-text-inverse": "--inspector-text-inverse",
+  "--color-text-info": "--inspector-log-info",
+  "--color-text-danger": "--inspector-log-error",
+  "--color-text-success": "--inspector-status-connected",
+  "--color-text-warning": "--inspector-log-warning",
+  "--color-border-primary": "--inspector-border-default",
+  "--color-border-secondary": "--inspector-border-subtle",
+  "--font-sans": "--mantine-font-family",
+  "--font-mono": "--mantine-font-family-monospace",
+  "--font-text-xs-size": "--mantine-font-size-xs",
+  "--font-text-sm-size": "--mantine-font-size-sm",
+  "--font-text-md-size": "--mantine-font-size-md",
+  "--font-text-lg-size": "--mantine-font-size-lg",
+  "--border-radius-xs": "--mantine-radius-xs",
+  "--border-radius-sm": "--mantine-radius-sm",
+  "--border-radius-md": "--mantine-radius-md",
+  "--border-radius-lg": "--mantine-radius-lg",
+  "--border-radius-xl": "--mantine-radius-xl",
+  "--shadow-sm": "--mantine-shadow-sm",
+  "--shadow-md": "--mantine-shadow-md",
+  "--shadow-lg": "--mantine-shadow-lg",
+};
+
+const STYLE_VARIABLE_ENTRIES = Object.entries(STYLE_VARIABLE_SOURCES) as [
+  McpUiStyleVariableKey,
+  string,
+][];
+
+/**
+ * Resolve the inspector's design tokens into a {@link McpUiHostStyles} for
+ * hostContext, so style-aware apps can theme themselves from the host instead
+ * of falling back to their own defaults. Reads the computed value of each
+ * mapped CSS variable from the document root — which reflects the active
+ * Mantine color scheme — and keeps only the ones that resolve to a non-empty
+ * value. Returns undefined when nothing resolves (e.g. a non-DOM/test
+ * environment) so we never advertise an empty styles object.
+ */
+export function currentStyles(): McpUiHostStyles | undefined {
+  if (typeof document === "undefined" || typeof window === "undefined") {
+    return undefined;
+  }
+  const computed = window.getComputedStyle(document.documentElement);
+  const variables: McpUiStyles = {} as McpUiStyles;
+  let resolved = false;
+  for (const [specKey, sourceVar] of STYLE_VARIABLE_ENTRIES) {
+    const value = computed.getPropertyValue(sourceVar).trim();
+    if (value) {
+      variables[specKey] = value;
+      resolved = true;
+    }
+  }
+  return resolved ? { variables } : undefined;
+}
+
+/**
+ * Spec shape for `hostContext.containerDimensions`. Derived from
+ * {@link McpUiHostContext} so the seed and live-push paths share one source of
+ * truth and stay in lockstep with the spec types.
+ */
+export type ContainerDimensions = NonNullable<
+  McpUiHostContext["containerDimensions"]
+>;
+
+/**
+ * Measure the host container an app renders into and return its concrete
+ * `{ width, height }` (whole pixels). Returns undefined when the element has
+ * no layout box yet (0×0 — e.g. before the iframe is attached, or in a
+ * non-DOM/test environment) so a meaningless size is never seeded into
+ * hostContext. The return type is the concrete pair rather than the spec's
+ * {@link ContainerDimensions} union so callers can compare both fields.
+ */
+export function measureContainerDimensions(
+  element: HTMLElement,
+): { width: number; height: number } | undefined {
+  if (typeof element.getBoundingClientRect !== "function") return undefined;
+  const rect = element.getBoundingClientRect();
+  const width = Math.round(rect.width);
+  const height = Math.round(rect.height);
+  if (width <= 0 || height <= 0) return undefined;
+  return { width, height };
+}
+
+/**
+ * Read the live host UI state into a {@link McpUiHostContext} for the bridge
+ * handshake — the single place that decides which fields the inspector seeds.
+ * Optional fields are omitted (not set undefined) so the SDK's diff stays
+ * accurate; subsequent live changes are pushed by the renderer's observers as
+ * partial `host-context-changed` notifications.
+ */
+export function snapshotHostContext(
+  container: HTMLElement | null,
+  availableDisplayModes: readonly McpUiDisplayMode[],
+): McpUiHostContext {
+  const styles = currentStyles();
+  const containerDimensions = container
+    ? measureContainerDimensions(container)
+    : undefined;
+  return {
+    theme: currentTheme(),
+    displayMode: "inline",
+    availableDisplayModes: [...availableDisplayModes],
+    ...(styles ? { styles } : {}),
+    ...(containerDimensions ? { containerDimensions } : {}),
+  };
+}