diff --git a/packages/static/src/plugin/clientPackages.test.ts b/packages/static/src/plugin/clientPackages.test.ts
new file mode 100644
index 0000000..79f8684
--- /dev/null
+++ b/packages/static/src/plugin/clientPackages.test.ts
@@ -0,0 +1,107 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { findClientPackages } from "./clientPackages";
+
+/**
+ * Build a throwaway project on disk: a root `package.json` plus a
+ * `node_modules/<name>/package.json` for each dependency, so `findClientPackages`
+ * can resolve and inspect them like a real install.
+ */
+function makeProject(
+  root: string,
+  rootPkg: Record<string, unknown>,
+  deps: Record<string, Record<string, unknown>>,
+): void {
+  fs.mkdirSync(root, { recursive: true });
+  fs.writeFileSync(
+    path.join(root, "package.json"),
+    JSON.stringify({ name: "test-project", ...rootPkg }),
+  );
+  for (const [name, pkgJson] of Object.entries(deps)) {
+    const dir = path.join(root, "node_modules", name);
+    fs.mkdirSync(dir, { recursive: true });
+    fs.writeFileSync(
+      path.join(dir, "package.json"),
+      JSON.stringify({ name, version: "1.0.0", ...pkgJson }),
+    );
+  }
+}
+
+const reactPeer = { peerDependencies: { react: "^19.0.0" } };
+
+describe("findClientPackages", () => {
+  let root: string;
+
+  beforeEach(() => {
+    root = fs.mkdtempSync(path.join(os.tmpdir(), "funstack-clientpkgs-"));
+  });
+
+  afterEach(() => {
+    fs.rmSync(root, { recursive: true, force: true });
+  });
+
+  it("detects dependencies that declare react as a peer dependency", () => {
+    makeProject(
+      root,
+      { dependencies: { "ui-kit": "^1.0.0", lodash: "^4.0.0" } },
+      { "ui-kit": reactPeer, lodash: {} },
+    );
+    expect(findClientPackages(root)).toEqual(["ui-kit"]);
+  });
+
+  it("considers devDependencies and optionalDependencies", () => {
+    makeProject(
+      root,
+      {
+        devDependencies: { "dev-widget": "^1.0.0" },
+        optionalDependencies: { "opt-widget": "^1.0.0" },
+      },
+      { "dev-widget": reactPeer, "opt-widget": reactPeer },
+    );
+    expect(findClientPackages(root).sort()).toEqual([
+      "dev-widget",
+      "opt-widget",
+    ]);
+  });
+
+  it("skips @funstack/static and react-dom even with a react peer dependency", () => {
+    makeProject(
+      root,
+      {
+        dependencies: {
+          "@funstack/static": "^1.0.0",
+          "react-dom": "^19.0.0",
+          "@funstack/router": "^1.0.0",
+        },
+      },
+      {
+        "@funstack/static": reactPeer,
+        "react-dom": reactPeer,
+        "@funstack/router": reactPeer,
+      },
+    );
+    expect(findClientPackages(root)).toEqual(["@funstack/router"]);
+  });
+
+  it("ignores transitive dependencies (only direct ones are scanned)", () => {
+    // `ui-kit` is a direct dep; `nested-widget` is only present in
+    // node_modules (a transitive dep) and must not be returned.
+    makeProject(
+      root,
+      { dependencies: { "ui-kit": "^1.0.0" } },
+      { "ui-kit": reactPeer, "nested-widget": reactPeer },
+    );
+    expect(findClientPackages(root)).toEqual(["ui-kit"]);
+  });
+
+  it("skips declared dependencies that are not installed", () => {
+    makeProject(root, { dependencies: { "missing-pkg": "^1.0.0" } }, {});
+    expect(findClientPackages(root)).toEqual([]);
+  });
+
+  it("returns an empty list when there is no package.json", () => {
+    expect(findClientPackages(path.join(root, "does-not-exist"))).toEqual([]);
+  });
+});
diff --git a/packages/static/src/plugin/clientPackages.ts b/packages/static/src/plugin/clientPackages.ts
new file mode 100644
index 0000000..f343640
--- /dev/null
+++ b/packages/static/src/plugin/clientPackages.ts
@@ -0,0 +1,88 @@
+import fs from "node:fs";
+import { createRequire } from "node:module";
+import path from "node:path";
+
+/**
+ * Discover the project's "client packages": directly-declared dependencies that
+ * list `react` as a peer dependency.
+ *
+ * `@vitejs/plugin-rsc` treats exactly these (a `react` peer dependency) as
+ * framework packages. When a server component imports one, the package is
+ * reached on the browser through a runtime-generated `client-package-proxy`
+ * virtual module that the dependency optimizer's initial scan cannot follow, so
+ * the package is otherwise only discovered at request time. That late discovery
+ * triggers a re-optimization pass which — mid-flight — corrupts CJS-interop
+ * module references (duplicate React on cold start, #128; broken dev-JSX, #124).
+ * Pre-bundling these packages in the browser environment keeps the optimizer to
+ * a single pass, so no re-optimization happens on the first render.
+ *
+ * Only directly-declared dependencies are considered: those are the bare
+ * specifiers a server component can import (and that Vite can resolve from the
+ * project root), whereas a framework package nested inside another dependency
+ * is pre-bundled together with its parent. `@funstack/static` and `react-dom`
+ * are skipped — `@funstack/static` is excluded from optimizeDeps (it imports
+ * virtual modules), and `react-dom` is handled separately.
+ */
+export function findClientPackages(root: string): string[] {
+  let rootPkg: Record<string, unknown>;
+  try {
+    rootPkg = JSON.parse(
+      fs.readFileSync(path.join(root, "package.json"), "utf-8"),
+    );
+  } catch {
+    return [];
+  }
+  const declared = new Set<string>();
+  for (const field of [
+    "dependencies",
+    "devDependencies",
+    "optionalDependencies",
+  ]) {
+    const deps = rootPkg[field];
+    if (deps && typeof deps === "object") {
+      for (const name of Object.keys(deps)) {
+        declared.add(name);
+      }
+    }
+  }
+  const require = createRequire(path.join(root, "package.json"));
+  const clientPackages: string[] = [];
+  for (const name of declared) {
+    if (name === "@funstack/static" || name === "react-dom") {
+      continue;
+    }
+    const pkgJson = readDependencyPackageJson(require, root, name);
+    const peerDependencies = pkgJson?.["peerDependencies"];
+    if (
+      peerDependencies &&
+      typeof peerDependencies === "object" &&
+      "react" in peerDependencies
+    ) {
+      clientPackages.push(name);
+    }
+  }
+  return clientPackages;
+}
+
+/**
+ * Read a dependency's `package.json`. Prefers Node resolution (handles pnpm and
+ * other layouts), falling back to a direct `node_modules` read for packages
+ * that do not expose `./package.json` via their `exports` map.
+ */
+function readDependencyPackageJson(
+  require: NodeJS.Require,
+  root: string,
+  name: string,
+): Record<string, unknown> | undefined {
+  for (const candidate of [
+    () => require.resolve(`${name}/package.json`),
+    () => path.join(root, "node_modules", name, "package.json"),
+  ]) {
+    try {
+      return JSON.parse(fs.readFileSync(candidate(), "utf-8"));
+    } catch {
+      // Try the next resolution strategy.
+    }
+  }
+  return undefined;
+}
diff --git a/packages/static/src/plugin/index.test.ts b/packages/static/src/plugin/index.test.ts
new file mode 100644
index 0000000..8772dc5
--- /dev/null
+++ b/packages/static/src/plugin/index.test.ts
@@ -0,0 +1,83 @@
+import { describe, it, expect } from "vitest";
+import type { Plugin } from "vite";
+import funstackStatic, { type FunstackStaticOptions } from "./index";
+
+/**
+ * Extract the `@funstack/static:config` plugin, which owns the
+ * `configEnvironment` hook that tweaks `optimizeDeps`.
+ */
+function getConfigPlugin(options: FunstackStaticOptions): Plugin {
+  const plugin = funstackStatic(options)
+    .flat()
+    .find((p): p is Plugin => p.name === "@funstack/static:config");
+  if (!plugin) {
+    throw new Error("config plugin not found");
+  }
+  return plugin;
+}
+
+/**
+ * Run the `configEnvironment` hook for the given environment and return the
+ * mutated `optimizeDeps` config.
+ */
+function runConfigEnvironment(
+  options: FunstackStaticOptions,
+  name: string,
+): { include: string[]; exclude: string[] } {
+  const plugin = getConfigPlugin(options);
+  const hook = plugin.configEnvironment;
+  const handler = typeof hook === "function" ? hook : hook?.handler;
+  if (!handler) {
+    throw new Error("configEnvironment hook not found");
+  }
+  const config: { optimizeDeps?: { include?: string[]; exclude?: string[] } } =
+    {};
+  // The hook only reads `name` and mutates `config.optimizeDeps`; the third
+  // `ConfigEnv` argument is unused, so an empty object is sufficient.
+  handler.call({} as never, name, config as never, {} as never);
+  return {
+    include: config.optimizeDeps?.include ?? [],
+    exclude: config.optimizeDeps?.exclude ?? [],
+  };
+}
+
+const fsRoutesOptions: FunstackStaticOptions = {
+  fsRoutes: {
+    dir: "./src/pages",
+    root: "./src/root.tsx",
+    adapter: "@funstack/static/fs-routes/next-adapter",
+  },
+};
+
+describe("configEnvironment optimizeDeps", () => {
+  it("includes React and ReactDOM as a single (bare) optimized chunk", () => {
+    const { include, exclude } = runConfigEnvironment(
+      fsRoutesOptions,
+      "client",
+    );
+    // Bare specifiers (not the nested `@funstack/static > react` form) so React
+    // merges with the copy the optimizer scanner already discovers — one chunk,
+    // no duplicate-React flip on a cold-started dev server (#128).
+    expect(include).toContain("react");
+    expect(include).toContain("react-dom");
+    expect(include).not.toContain("@funstack/static > react");
+    expect(include).not.toContain("@funstack/static > react-dom");
+    expect(exclude).toContain("@funstack/static");
+  });
+
+  it("pre-bundles client packages (react peer dependency) in the client environment", () => {
+    // The detection runs against this package, whose own devDependencies
+    // include @funstack/router (a `react` peer-dependency client package).
+    // Pre-bundling such packages avoids a cold-start re-optimization that would
+    // corrupt CJS-interop module references (#124, #128).
+    const { include } = runConfigEnvironment(fsRoutesOptions, "client");
+    expect(include).toContain("@funstack/router");
+  });
+
+  it("only pre-bundles client packages in the client environment", () => {
+    for (const name of ["rsc", "ssr"]) {
+      const { include } = runConfigEnvironment(fsRoutesOptions, name);
+      expect(include).not.toContain("@funstack/router");
+    }
+  });
+});
diff --git a/packages/static/src/plugin/index.ts b/packages/static/src/plugin/index.ts
index 8168e2d..2b82057 100644
--- a/packages/static/src/plugin/index.ts
+++ b/packages/static/src/plugin/index.ts
@@ -3,6 +3,7 @@ import { normalizePath, type Plugin } from "vite";
 import rsc from "@vitejs/plugin-rsc";
 import { buildApp } from "../build/buildApp";
 import { serverPlugin } from "./server";
+import { findClientPackages } from "./clientPackages";
 import { defaultRscPayloadDir } from "../rsc/rscModule";
 
 interface FunstackStaticBaseOptions {
@@ -259,7 +260,7 @@ export default function funstackStatic(
           );
         }
       },
-      configEnvironment(_name, config) {
+      configEnvironment(name, config) {
         if (!config.optimizeDeps) {
           config.optimizeDeps = {};
         }
@@ -278,12 +279,32 @@ export default function funstackStatic(
         // Since code includes imports to virtual modules, we need to exclude
         // us from Optimize Deps.
         config.optimizeDeps.exclude.push("@funstack/static");
-        // However, since Vite prohibits excluding a CommonJS package,
-        // we need to include React and ReactDOM so they are bundled properly.
-        config.optimizeDeps.include.push(
-          "@funstack/static > react",
-          "@funstack/static > react-dom",
-        );
+        // Vite prohibits excluding a CommonJS package, so include React and
+        // ReactDOM as plain entries. Using the bare specifiers (rather than the
+        // nested `@funstack/static > react` form) merges them with the copy the
+        // optimizer's scanner already discovers, so React is bundled into a
+        // SINGLE optimized chunk. The nested form produced a second, redundant
+        // React chunk from the same source file; whenever a re-optimization
+        // changed which chunk was canonical, two live React instances ended up
+        // loaded on a cold start (#128). One chunk makes that impossible.
+        config.optimizeDeps.include.push("react", "react-dom");
+        // Pre-bundle the project's client packages in the browser environment
+        // so they are discovered in the optimizer's INITIAL pass. Otherwise
+        // @vitejs/plugin-rsc reaches them at request time through a
+        // `client-package-proxy` virtual module that the scanner cannot follow,
+        // forcing a re-optimization on the first render. That mid-flight
+        // re-optimization corrupts CJS-interop module references — duplicate
+        // React (#128) and broken dev-JSX (`react/jsx-runtime` "export named
+        // 't'", #124). See findClientPackages for the full explanation.
+        if (name === "client") {
+          // process.cwd() matches the root @vitejs/plugin-rsc uses to detect
+          // these same framework packages, so the two stay consistent.
+          for (const pkg of findClientPackages(process.cwd())) {
+            if (!config.optimizeDeps.include.includes(pkg)) {
+              config.optimizeDeps.include.push(pkg);
+            }
+          }
+        }
       },
     },
     {