diff --git a/apps/control-plane/app/layout.tsx b/apps/control-plane/app/layout.tsx
index c482c82..71a7fa2 100644
--- a/apps/control-plane/app/layout.tsx
+++ b/apps/control-plane/app/layout.tsx
@@ -1,18 +1,21 @@
 /**
- * / - App Router root layout (coreci-chat#10).
+ * / - App Router root layout (coreci-chat#10 minimum; coreci-chat#16 slice 1).
  *
  * Next.js 15 requires a root layout that renders <html> and <body> above
  * every route. Without one, `next build` fails with "login/page.tsx does not
  * have a root layout", so apps/control-plane has never been buildable - the
  * pipeline never built the app, so nothing caught it.
  *
- * This is the build-enabling minimum only. The shell content (theme,
- * providers, session wrapper) is an M3 scope call for the line head and is
- * deliberately NOT decided here - it rides the M3 kickoff verdict on
- * coreci-chat#1.
+ * Slice 1 (M3) adds the minimal shell content around {children}: the theme
+ * provider and the auth/session provider (env-based client config), both
+ * defined in app/shell/providers.tsx. The shell is additive: it never
+ * redirects or gates - /login and every SSR page keep their server-side
+ * session resolution - and no boot-time DDL rides it (deploy owns DDL, the
+ * app boots schema-check only, coreci-chat#1 comment 16428).
  */
 
 import type { ReactNode } from "react";
+import Providers from "./shell/providers";
 
 export const metadata = {
   title: "CoreCI Chat - control plane",
@@ -20,8 +23,10 @@ export const metadata = {
 
 export default function RootLayout({ children }: { children: ReactNode }) {
   return (
-    <html lang="en">
-      <body>{children}</body>
+    <html lang="en" suppressHydrationWarning>
+      <body>
+        <Providers>{children}</Providers>
+      </body>
     </html>
   );
-}
+}
\ No newline at end of file
diff --git a/apps/control-plane/app/shell/_lib.ts b/apps/control-plane/app/shell/_lib.ts
new file mode 100644
index 0000000..683b12f
--- /dev/null
+++ b/apps/control-plane/app/shell/_lib.ts
@@ -0,0 +1,108 @@
+/**
+ * Root-shell pure logic (coreci-chat#16, M3 slice 1).
+ *
+ * Extracted from the React providers for node-testability, one repo
+ * convention applied from Wave J (app/_lib.ts files tested via vitest):
+ * no DOM, no Next imports — only decisions. The provider components in
+ * app/shell/providers.tsx consume these; env names live here too:
+ *   NEXT_PUBLIC_API_BASE_URL — client session config (default same-origin)
+ *   NEXT_PUBLIC_DEFAULT_THEME — fallback theme before user preference
+ */
+
+export type ThemeName = "light" | "dark" | "system";
+export type ResolvedTheme = "light" | "dark";
+
+export const THEME_STORAGE_KEY = "coreci.theme";
+
+/** Env names the shell reads (build-time inlined, undefined-safe). */
+export const SHELL_ENV = {
+  apiBaseUrl: "NEXT_PUBLIC_API_BASE_URL",
+  defaultTheme: "NEXT_PUBLIC_DEFAULT_THEME",
+} as const;
+
+/** Coerce a stored/default candidate into a theme name. */
+export function normalizeTheme(
+  value: string | null | undefined,
+  fallback: ThemeName = "system",
+): ThemeName {
+  return value === "light" || value === "dark" || value === "system"
+    ? value
+    : fallback;
+}
+
+/**
+ * Resolve a concrete theme: "system" follows prefers-color-scheme,
+ * everything else is explicit.
+ */
+export function resolveTheme(
+  pref: ThemeName,
+  systemPrefersDark: boolean,
+): ResolvedTheme {
+  if (pref === "system") return systemPrefersDark ? "dark" : "light";
+  return pref;
+}
+
+/** CSS class applied to <html> for the resolved theme. */
+export function themeClass(theme: ResolvedTheme): string {
+  return `theme-${theme}`;
+}
+
+/**
+ * Session client config (the GO's env-decided shape): the client's API base
+ * comes from NEXT_PUBLIC_API_BASE_URL and defaults to same-origin ("") — no
+ * runner-image or deployment assumption is baked in. Only same-origin or
+ * absolute http(s) URLs are accepted; anything else falls back to same-origin.
+ */
+export function resolveApiBase(value: string | null | undefined): string {
+  if (typeof value !== "string") return "";
+  const v = value.trim().replace(/\/+$/, "");
+  if (v === "") return "";
+  if (v.startsWith("/") && !v.startsWith("//")) return v;
+  if (v.startsWith("http://") || v.startsWith("https://")) return v;
+  return "";
+}
+
+/** Client-visible session shape — mirrors GET /api/me. */
+export interface SessionUser {
+  id: string;
+  tenantId: string;
+  role: "admin" | "operator" | "viewer";
+}
+
+export type SessionStatus = "loading" | "guest" | "authenticated" | "error";
+
+export interface SessionSnapshot {
+  status: SessionStatus;
+  user: SessionUser | null;
+}
+
+/**
+ * Classify a /api/me response into a session snapshot. 200 → authenticated,
+ * 401/403 → guest (the login route itself renders without a session — never
+ * redirect from the root shell), anything else → error.
+ */
+export function classifyMe(
+  httpStatus: number,
+  payload: unknown,
+): SessionSnapshot {
+  if (httpStatus === 200) {
+    const rec = payload as { user?: Record<string, unknown> } | null;
+    const user = rec?.user ?? undefined;
+    if (
+      user &&
+      typeof user.id === "string" &&
+      typeof user.tenantId === "string" &&
+      (user.role === "admin" || user.role === "operator" || user.role === "viewer")
+    ) {
+      return {
+        status: "authenticated",
+        user: { id: user.id, tenantId: user.tenantId, role: user.role },
+      };
+    }
+    return { status: "error", user: null };
+  }
+  if (httpStatus === 401 || httpStatus === 403) {
+    return { status: "guest", user: null };
+  }
+  return { status: "error", user: null };
+}
\ No newline at end of file
diff --git a/apps/control-plane/app/shell/providers.tsx b/apps/control-plane/app/shell/providers.tsx
new file mode 100644
index 0000000..d3e5350
--- /dev/null
+++ b/apps/control-plane/app/shell/providers.tsx
@@ -0,0 +1,169 @@
+"use client";
+
+/**
+ * Root-shell providers (coreci-chat#16, M3 slice 1).
+ *
+ * Minimal shell content for the app router root layout:
+ *   1. ThemeProvider — light/dark/system, persisted, applied as a class on
+ *      <html>; no external dependency.
+ *   2. SessionProvider — the client-side session wrapper over @coreci/auth's
+ *      server session lane: it reads the coreci_session cookie's identity via
+ *      GET /api/me and exposes it as context. The client config comes from
+ *      NEXT_PUBLIC_API_BASE_URL (default same-origin) — env-based, no
+ *      runner-image assumption.
+ *
+ * Deliberate limits (slice 1, per the task scope):
+ *   - The root shell NEVER redirects or gates: /login and every SSR page
+ *     keep their existing server-side session resolution (lib/session via
+ *     getMe()); this context is additive for the chat UI.
+ *   - No boot-time DDL and no DB access here — deploy owns DDL, the app
+ *     boots schema-check only (coreci-chat#1 comment 16428).
+ */
+
+import {
+  createContext,
+  useCallback,
+  useContext,
+  useEffect,
+  useMemo,
+  useState,
+  type ReactNode,
+} from "react";
+import {
+  classifyMe,
+  normalizeTheme,
+  resolveApiBase,
+  resolveTheme,
+  themeClass,
+  THEME_STORAGE_KEY,
+  SHELL_ENV,
+  type SessionSnapshot,
+  type ThemeName,
+  type ResolvedTheme,
+} from "./_lib.js";
+
+/* ------------------------------ theme ------------------------------ */
+
+interface ThemeContextValue {
+  pref: ThemeName;
+  theme: ResolvedTheme;
+  setPref: (next: ThemeName) => void;
+}
+
+const ThemeContext = createContext<ThemeContextValue>({
+  pref: "system",
+  theme: "light",
+  setPref: () => undefined,
+});
+
+function ThemeProvider({ children }: { children: ReactNode }) {
+  const [pref, setPrefState] = useState<ThemeName>(() =>
+    normalizeTheme(process.env[SHELL_ENV.defaultTheme], "system"),
+  );
+
+  useEffect(() => {
+    const stored = window.localStorage.getItem(THEME_STORAGE_KEY);
+    setPrefState(normalizeTheme(stored, normalizeTheme(process.env[SHELL_ENV.defaultTheme], "system")));
+  }, []);
+
+  const setPref = useCallback((next: ThemeName) => {
+    setPrefState(next);
+    try {
+      window.localStorage.setItem(THEME_STORAGE_KEY, next);
+    } catch {
+      // Storage may be unavailable; the preference still applies this session.
+    }
+  }, []);
+
+  // SSR-safe: "system" reads prefers-color-scheme only in the browser, after
+  // hydration, and tracks changes (the server render never touches window).
+  const [systemDark, setSystemDark] = useState(false);
+  useEffect(() => {
+    const query = window.matchMedia("(prefers-color-scheme: dark)");
+    setSystemDark(query.matches);
+    const onChange = (e: MediaQueryListEvent) => {
+      setSystemDark(e.matches);
+    };
+    query.addEventListener("change", onChange);
+    return () => query.removeEventListener("change", onChange);
+  }, []);
+
+  const theme = resolveTheme(pref, systemDark);
+
+  useEffect(() => {
+    const el = document.documentElement;
+    el.classList.add(themeClass(theme));
+    return () => el.classList.remove(themeClass(theme));
+  }, [theme]);
+
+  return <ThemeContext.Provider value={{ pref, theme, setPref }}>{children}</ThemeContext.Provider>;
+}
+
+/* ----------------------------- session ---------------------------- */
+
+interface SessionContextValue extends SessionSnapshot {
+  refresh: () => void;
+}
+
+const SessionContext = createContext<SessionContextValue>({
+  status: "loading",
+  user: null,
+  refresh: () => undefined,
+});
+
+function SessionProvider({ children }: { children: ReactNode }) {
+  const apiBase = useMemo(
+    () => resolveApiBase(process.env[SHELL_ENV.apiBaseUrl]),
+    [],
+  );
+  const [snapshot, setSnapshot] = useState<SessionSnapshot>({
+    status: "loading",
+    user: null,
+  });
+
+  const fetchMe = useCallback(async () => {
+    try {
+      const res = await fetch(`${apiBase}/api/me`, {
+        credentials: "same-origin",
+        cache: "no-store",
+      });
+      let payload: unknown = null;
+      try {
+        payload = await res.json();
+      } catch {
+        payload = null;
+      }
+      setSnapshot(classifyMe(res.status, payload));
+    } catch {
+      setSnapshot({ status: "error", user: null });
+    }
+  }, [apiBase]);
+
+  useEffect(() => {
+    void fetchMe();
+  }, [fetchMe]);
+
+  const refresh = useCallback(() => {
+    void fetchMe();
+  }, [fetchMe]);
+
+  return <SessionContext.Provider value={{ ...snapshot, refresh }}>{children}</SessionContext.Provider>;
+}
+
+/* ----------------------------- exports ---------------------------- */
+
+export function useShellTheme(): ThemeContextValue {
+  return useContext(ThemeContext);
+}
+
+export function useShellSession(): SessionContextValue {
+  return useContext(SessionContext);
+}
+
+export default function Providers({ children }: { children: ReactNode }) {
+  return (
+    <ThemeProvider>
+      <SessionProvider>{children}</SessionProvider>
+    </ThemeProvider>
+  );
+}
\ No newline at end of file
diff --git a/apps/control-plane/tests/shell-lib.test.ts b/apps/control-plane/tests/shell-lib.test.ts
new file mode 100644
index 0000000..69f22a5
--- /dev/null
+++ b/apps/control-plane/tests/shell-lib.test.ts
@@ -0,0 +1,95 @@
+/**
+ * Root-shell pure logic (coreci-chat#16, M3 slice 1).
+ */
+
+import { describe, expect, it } from "vitest";
+import {
+  classifyMe,
+  normalizeTheme,
+  resolveApiBase,
+  resolveTheme,
+  themeClass,
+  THEME_STORAGE_KEY,
+} from "../app/shell/_lib.js";
+
+describe("normalizeTheme", () => {
+  it("accepts the three theme names", () => {
+    expect(normalizeTheme("light")).toBe("light");
+    expect(normalizeTheme("dark")).toBe("dark");
+    expect(normalizeTheme("system")).toBe("system");
+  });
+
+  it("falls back for stored garbage and absence", () => {
+    expect(normalizeTheme("blue")).toBe("system");
+    expect(normalizeTheme(null)).toBe("system");
+    expect(normalizeTheme(undefined, "dark")).toBe("dark");
+  });
+
+  it("uses the env default as fallback (the shipped env default)", () => {
+    expect(normalizeTheme(null, "light")).toBe("light");
+  });
+});
+
+describe("resolveTheme + themeClass", () => {
+  it("resolves system to the media query result", () => {
+    expect(resolveTheme("system", false)).toBe("light");
+    expect(resolveTheme("system", true)).toBe("dark");
+  });
+
+  it("resolves explicit preferences over the media query", () => {
+    expect(resolveTheme("dark", false)).toBe("dark");
+    expect(resolveTheme("light", true)).toBe("light");
+  });
+
+  it("produces the html class name", () => {
+    expect(themeClass("dark")).toBe("theme-dark");
+    expect(themeClass("light")).toBe("theme-light");
+  });
+
+  it("exposes the storage key shape", () => {
+    expect(THEME_STORAGE_KEY).toBe("coreci.theme");
+  });
+});
+
+describe("resolveApiBase (env-based session client config)", () => {
+  it("defaults to same-origin when the env var is absent", () => {
+    expect(resolveApiBase(undefined)).toBe("");
+    expect(resolveApiBase(null)).toBe("");
+    expect(resolveApiBase("")).toBe("");
+    expect(resolveApiBase("   ")).toBe("");
+  });
+
+  it("accepts same-origin-relative and absolute http(s) bases", () => {
+    expect(resolveApiBase("/gw")).toBe("/gw");
+    expect(resolveApiBase("https://chat.coreci.dev/")).toBe("https://chat.coreci.dev");
+    expect(resolveApiBase("  http://localhost:3000/  ")).toBe("http://localhost:3000");
+  });
+
+  it("falls back to same-origin for unsupported forms", () => {
+    expect(resolveApiBase("//evil.example")).toBe("");
+    expect(resolveApiBase("ftp://x")).toBe("");
+    expect(resolveApiBase("javascript:alert(1)")).toBe("");
+  });
+});
+
+describe("classifyMe (/api/me responses)", () => {
+  it("classifies a valid 200 payload as authenticated with the user", () => {
+    const out = classifyMe(200, { user: { id: "u1", tenantId: "t1", role: "operator" } });
+    expect(out.status).toBe("authenticated");
+    expect(out.user).toEqual({ id: "u1", tenantId: "t1", role: "operator" });
+  });
+
+  it("treats a malformed 200 payload as an error, not a crash", () => {
+    expect(classifyMe(200, { user: { id: 7 } }).status).toBe("error");
+    expect(classifyMe(200, null).status).toBe("error");
+  });
+
+  it("treats 401/403 as guest — the shell never redirects", () => {
+    expect(classifyMe(401, null)).toEqual({ status: "guest", user: null });
+    expect(classifyMe(403, null)).toEqual({ status: "guest", user: null });
+  });
+
+  it("treats other statuses as an error state", () => {
+    expect(classifyMe(500, null)).toEqual({ status: "error", user: null });
+  });
+});
\ No newline at end of file
diff --git a/apps/control-plane/vitest.config.ts b/apps/control-plane/vitest.config.ts
index 8ac1c28..252dfa9 100644
--- a/apps/control-plane/vitest.config.ts
+++ b/apps/control-plane/vitest.config.ts
@@ -15,6 +15,8 @@ export default defineConfig({
         "app/dashboard/settings/adapters/_lib.ts",
         "app/dashboard/test-call/_helpers.ts",
         "app/dashboard/test-call/_lib.ts",
+        // M3 slice 1 root-shell pure logic (coreci-chat#16).
+        "app/shell/_lib.ts",
       ],
       reporter: ["text", "json"],
     },