diff --git a/flake.nix b/flake.nix
index 54ad388..0477270 100644
--- a/flake.nix
+++ b/flake.nix
@@ -52,6 +52,7 @@
               --bundle --platform=node --target=node24 --format=esm \
               --outfile=quixos-lock-check.mjs
             esbuild dist/src/bindings/cli.js --bundle --platform=node --target=node24 --format=esm --outfile=quixos-codegen-ts.mjs
+            esbuild dist/src/bindings/react-platform.js --bundle --platform=node --target=node24 --format=esm --outfile=react-platform.mjs
             esbuild dist/src/capability-language/tool-cli.js --bundle --platform=node --target=node24 --format=esm --outfile=quixos-qx.mjs
             runHook postBuild
           '';
@@ -105,6 +106,7 @@ EOF
             chmod +x "$out/bin/quixos-lock-check"
             mkdir -p "$out/libexec/quixos-protocol"
             install -m644 client-codegen.mjs "$out/libexec/quixos-protocol/client-codegen.mjs"
+            install -m644 react-platform.mjs "$out/libexec/quixos-protocol/react-platform.mjs"
             install -m644 quixos-codegen-ts.mjs quixos-qx.mjs "$out/libexec/quixos-protocol/"
             install -m644 quixos-descriptor-check.mjs "$out/libexec/quixos-protocol/quixos-descriptor-check.mjs"
             install -m644 quixos-capability-compile.mjs "$out/libexec/quixos-protocol/quixos-capability-compile.mjs"
diff --git a/src/bindings/cli.ts b/src/bindings/cli.ts
index f3a0c01..21f34fc 100644
--- a/src/bindings/cli.ts
+++ b/src/bindings/cli.ts
@@ -1,13 +1,18 @@
 #!/usr/bin/env node
 import { readFile, writeFile } from "node:fs/promises";
 import { generateTypeScriptBindings } from "./index.js";
+import path from "node:path";
+import {reactPlatformTypes} from "./react-platform.js";
 
 const main = async () => {
   const [schema, revision, output, options, ...rest] = process.argv.slice(2);
   if (!schema || !revision || !output || rest.length) throw new Error(
     "usage: quixos-codegen-ts SCHEMA.json PACKAGE_REVISION OUTPUT.ts [OPTIONS.json]");
-  const generated = generateTypeScriptBindings(JSON.parse(await readFile(schema, "utf8")), revision,
-    options ? JSON.parse(await readFile(options, "utf8")) : {});
+  const config = options ? JSON.parse(await readFile(options, "utf8")) : {};
+  const generated = generateTypeScriptBindings(JSON.parse(await readFile(schema, "utf8")), revision, config);
   await writeFile(output, generated);
+  if (config.messages?.["org.quixos.web-studio.ReactProps"]) {
+    await writeFile(path.join(path.dirname(output), "web-studio-react-runtime.d.ts"), reactPlatformTypes);
+  }
 };
 main().catch((error: unknown) => { console.error(error instanceof Error ? error.message : error); process.exitCode = 1; });
diff --git a/src/bindings/react-platform.ts b/src/bindings/react-platform.ts
new file mode 100644
index 0000000..0d3a997
--- /dev/null
+++ b/src/bindings/react-platform.ts
@@ -0,0 +1,62 @@
+/** One authoring contract, distributed by scaffolding and immutable codegen.
+ * Web Studio checks its concrete exports against this contract during its build. */
+export const reactPlatformTypes = `// Generated by the Quixos React platform contract. Do not edit.
+declare module "@quixos/web-studio-react-runtime" {
+  import type * as React from "react";
+  export function useComponentOverlayContainer(): HTMLElement;
+  export function useComponentStyleRoot(): ShadowRoot;
+  export type ObjectRef<AtomId extends string> = string & {
+    readonly $quixosAtom: AtomId;
+  };
+  export type LiveFieldProp<T> = {
+    value: T;
+    source: {
+      objectId: string;
+      slotId: string;
+      valueType?: string;
+      storagePolicy?: string;
+      revision?: string | number | bigint;
+      crdtSnapshot?: {
+        type: string;
+        encoding: string;
+        payload: string;
+      };
+    };
+  };
+  export type ReactComponentHostProps<Action> = {
+    onAction?: (action: Action) => void;
+    fallback?: React.ReactNode;
+    className?: string;
+    style?: React.CSSProperties;
+    onError?: (error: Error) => void;
+  };
+  export type ReactComponentImplementationProps<CaminoProps, RenderProps, Action> = {
+    camino: CaminoProps;
+    render: RenderProps;
+    dispatch: (action: Action) => void;
+  };
+  export const createWebStudioComponent: <
+    ForObject extends ObjectRef<string>, RenderProps extends object, Action,
+  >(config: { expectedAtomId: string }) => (props: {
+    forObject: ForObject;
+  } & RenderProps & ReactComponentHostProps<Action>) => any;
+  export const invokeCapability: <Result = unknown>(
+    objectId: string,
+    interfaceRevisionId: string,
+    operationId: string,
+    value?: unknown,
+    options?: {clientMutationId?: string},
+  ) => Promise<Result>;
+  export const h: typeof React.createElement;
+  export const useLiveField: <T>(
+    field: LiveFieldProp<T>,
+    options?: {
+      reconcileRegister?: (state: {
+        confirmed: T;
+        optimistic: T;
+        pending: boolean;
+      }) => T;
+    },
+  ) => readonly [T, (value: T) => Promise<void>];
+}
+`;
diff --git a/src/capability-language/authoring-check.ts b/src/capability-language/authoring-check.ts
index 656bab8..3dae757 100644
--- a/src/capability-language/authoring-check.ts
+++ b/src/capability-language/authoring-check.ts
@@ -22,6 +22,7 @@ export async function checkAuthoring(start: string, output: string, options: { b
     directory, checker: checkerIdentity(), candidateOnly: true, activationEvidence: false, blockers: [], phase: "convergence", output,
   };
   try {
+    console.error(`[${new Date().toISOString()}] Check: capture source and converge dependencies`);
     // Serialize only source capture, not the potentially slow Nix build.
     // Repository-scoped agents can check separate immutable candidates in parallel.
     const captured = await promisify(callback)("quixos-qx", ["converge", context.workbench, directory], {
@@ -32,6 +33,7 @@ export async function checkAuthoring(start: string, output: string, options: { b
     });
     const converged = JSON.parse(captured.stdout) as Awaited<ReturnType<typeof convergeAuthoring>>;
     timings.captureMs = Math.round(performance.now() - started);
+    console.error(`[${new Date().toISOString()}] Check: source captured in ${(timings.captureMs / 1000).toFixed(1)}s`);
     if (!converged.candidate) {
       report.phase = converged.worklist.find(entry => entry.phase !== "dependency")?.phase ?? "convergence";
       throw new Error(converged.worklist.map(entry => `${entry.directory} [${entry.phase}]: ${entry.message}`).join("\n"));
@@ -39,8 +41,13 @@ export async function checkAuthoring(start: string, output: string, options: { b
     report.commit = converged.candidate.commit;
     report.phase = "verification";
     const buildStarted = performance.now();
-    report.artifactPath = await buildImmutableCandidate(converged.candidate, resource.kind, path.join(output, "nix.log"), options.contractOnly);
-    timings.immutableCheckMs = Math.round(performance.now() - buildStarted);
+    console.error(`[${new Date().toISOString()}] Check: immutable Nix ${options.contractOnly ? "contract" : "verification"}; build output: ${path.join(output, "nix.log")}`);
+    try {
+      report.artifactPath = await buildImmutableCandidate(converged.candidate, resource.kind, path.join(output, "nix.log"), options.contractOnly);
+    } finally {
+      timings.immutableCheckMs = Math.round(performance.now() - buildStarted);
+      console.error(`[${new Date().toISOString()}] Check: immutable phase ended after ${(timings.immutableCheckMs / 1000).toFixed(1)}s`);
+    }
     const candidateText = await fs.readFile(path.join(report.artifactPath, "candidate.json"), "utf8");
     await fs.writeFile(path.join(output, "candidate.json"), candidateText);
     report.compilation = "passed";
diff --git a/src/capability-language/scaffold-recipes.ts b/src/capability-language/scaffold-recipes.ts
index 057f3d3..8f8e0e9 100644
--- a/src/capability-language/scaffold-recipes.ts
+++ b/src/capability-language/scaffold-recipes.ts
@@ -6,6 +6,7 @@ import {formatQuixosLock, type GitSource} from "../resource-lock/index.js";
 import {parseQx, walkSyntax} from "./source.js";
 import type {StructuralRequest} from "./structural-plan.js";
 import {addImplementation} from "./implementation-edit.js";
+import {reactPlatformTypes} from "../bindings/react-platform.js";
 
 type Source = {repository: string; commit: string};
 type PackageEditModel = {generatedBy: "qx-scaffold-v1"; name: string; id: string; revision: string; exports: {name: string; id: string; file: string; migration?: boolean}[]};
@@ -69,6 +70,7 @@ export const scaffoldRecipe = async (root: string, command: "package" | "functio
       create("src/component.tsx", `// Props are opaque at the platform boundary until capability generics exist.\nexport default function Component(_props: {camino: unknown; render: unknown; dispatch: (action: unknown) => void}) {\n  return <section><h1>${name}</h1><p>Edit this component, then run qx-workspace check.</p></section>;\n}\n`);
       create("src/impl/sourceGet.ts", `import componentSource from "../component.js?browser-source";\nimport type {Implementation} from "../gen/qx.js";\nexport const handler: Implementation["sourceGet"] = () => componentSource;\n`);
       create("src/browser-assets.d.ts", `declare module "*?browser-source" { const source: string; export default source; }\n`);
+      create("src/gen/web-studio-react-runtime.d.ts", reactPlatformTypes);
     }
     create(".gitignore", "node_modules/\ndist/\n.quixos/\nresult\n.yarn/install-state.gz\n");
     create(".yarnrc.yml", `nodeLinker: node-modules\nenableScripts: true\nnpmMinimalAgeGate: 0\napprovedGitRepositories:\n  - ${JSON.stringify(spec.tools.sdk.repository)}\nsupportedArchitectures:\n  os: [current, linux]\n  cpu: [current, x64, arm64]\n  libc: [current, glibc]\n`);
diff --git a/test/scaffold-recipes.test.ts b/test/scaffold-recipes.test.ts
index 9ffc174..6b9605a 100644
--- a/test/scaffold-recipes.test.ts
+++ b/test/scaffold-recipes.test.ts
@@ -9,6 +9,7 @@ import {scaffoldRecipe} from "../src/capability-language/scaffold-recipes.js";
 import {planStructure, applyStructure} from "../src/capability-language/structural-plan.js";
 import {contentDigest} from "../src/capability-model/evolution.js";
 import {sealMigrations} from "../src/capability-language/migration-seal.js";
+import {reactPlatformTypes} from "../src/bindings/react-platform.js";
 const execFile = promisify(callback);
 
 test("React preset applies its browser build script and shared-platform imports", async (context) => {
@@ -20,6 +21,7 @@ test("React preset applies its browser build script and shared-platform imports"
   await applyStructure(await planStructure(root, request));
   assert.match(await fs.readFile(path.join(root, "src/impl/sourceGet.ts"), "utf8"), /component.js\?browser-source/);
   assert.match(await fs.readFile(path.join(root, "flake.nix"), "utf8"), /browserSources = true/);
+  assert.equal(await fs.readFile(path.join(root, "src/gen/web-studio-react-runtime.d.ts"), "utf8"), reactPlatformTypes);
   assert.equal(JSON.parse(await fs.readFile(path.join(root, "quixos.check.json"), "utf8")).options.messages["org.quixos.web-studio.ReactProps"].export, "opaqueReactPropsBinding");
   await assert.rejects(fs.access(path.join(root, "quixos.scaffold.json")));
   assert.equal(JSON.parse(await fs.readFile(path.join(root, "tsconfig.json"), "utf8")).compilerOptions.jsx, "react-jsx");