diff --git a/packages/rstack/src/config.ts b/packages/rstack/src/config.ts
index 9f1ec1e4..07216548 100644
--- a/packages/rstack/src/config.ts
+++ b/packages/rstack/src/config.ts
@@ -13,6 +13,7 @@ import type { UserConfig, UserConfigAsyncFn } from '@rspress/core';
 import type { RstestConfigExport } from '@rstest/core';
 import type { FmtConfigDefinition } from './fmt/types.ts';
 import type { StagedConfig } from './staged.ts';
+import { composeConfigLayers } from './configLayers.ts';
 
 export type RslintConfigDefinition =
   RslintConfig | (() => Promise<RslintConfig>);
@@ -58,6 +59,7 @@ export const normalizeRstackConfig = ({
     : { ...configs, lint: normalizeLintConfig(lint) };
 
 export type LoadedRstackConfig = {
+  /** Effective tool definitions. Configuration factories remain unevaluated. */
   configs: Configs;
   filePath: string | null;
   dependencies: string[];
@@ -251,7 +253,7 @@ export const loadRstackConfig = async ({
       });
 
       return {
-        configs: session.configs,
+        configs: await composeConfigLayers([session.configs]),
         filePath,
         dependencies,
       };
diff --git a/packages/rstack/src/configLayers.ts b/packages/rstack/src/configLayers.ts
index 8795a918..bc46628e 100644
--- a/packages/rstack/src/configLayers.ts
+++ b/packages/rstack/src/configLayers.ts
@@ -112,3 +112,79 @@ export const resolveStagedConfig = async (
       : { ...merged, ...config },
   );
 };
+
+export const resolveFmtConfigLayers = async (
+  layers: readonly Configs[],
+): Promise<FmtConfig | undefined> => {
+  const configs = await resolveConfigLayers(layers, 'fmt');
+  return configs.length > 1
+    ? (Object.assign({}, ...configs) as FmtConfig)
+    : configs[0];
+};
+
+/** Compose effective definitions without executing any tool or task factories. */
+export const composeConfigLayers = async (
+  layers: readonly Configs[],
+): Promise<Configs> => {
+  if (layers.length <= 1) {
+    return layers[0] ?? {};
+  }
+
+  const configs: Configs = {};
+  for (const kind of [
+    'app',
+    'lib',
+    'doc',
+    'test',
+    'lint',
+    'fmt',
+    'staged',
+  ] as const) {
+    const matchingLayers = layers.filter((layer) => layer[kind] !== undefined);
+    if (matchingLayers.length === 0) {
+      continue;
+    }
+    if (matchingLayers.length === 1) {
+      Object.assign(configs, { [kind]: matchingLayers[0][kind] });
+      continue;
+    }
+
+    switch (kind) {
+      case 'app':
+        configs.app = async (params) => {
+          const { resolveRsbuildConfig } = await import('./rsbuildConfig.ts');
+          return resolveRsbuildConfig(matchingLayers, params);
+        };
+        break;
+      case 'lib':
+        configs.lib = async (params) => {
+          const { resolveRslibConfig } = await import('./rslibConfig.ts');
+          return resolveRslibConfig(matchingLayers, params);
+        };
+        break;
+      case 'doc':
+        configs.doc = async () => {
+          const { resolveRspressConfig } = await import('./rspressConfig.ts');
+          return resolveRspressConfig(matchingLayers);
+        };
+        break;
+      case 'test':
+        configs.test = async () => {
+          const { mergeRstestConfigLayers } = await import('./rstestConfig.ts');
+          return mergeRstestConfigLayers(matchingLayers);
+        };
+        break;
+      case 'lint':
+        configs.lint = async () => (await resolveRslintConfig(matchingLayers))!;
+        break;
+      case 'fmt':
+        configs.fmt = async () =>
+          (await resolveFmtConfigLayers(matchingLayers))!;
+        break;
+      case 'staged':
+        configs.staged = await resolveStagedConfig(matchingLayers);
+        break;
+    }
+  }
+  return configs;
+};
diff --git a/packages/rstack/src/fmt/config.ts b/packages/rstack/src/fmt/config.ts
index 54d735b7..1d9ba7b0 100644
--- a/packages/rstack/src/fmt/config.ts
+++ b/packages/rstack/src/fmt/config.ts
@@ -1,7 +1,7 @@
 import { dirname } from 'node:path';
 import micromatch from 'micromatch';
 import type { Configs } from '../config.ts';
-import { resolveConfigLayers } from '../configLayers.ts';
+import { resolveFmtConfigLayers } from '../configLayers.ts';
 import { createRelativePathResolver } from './pathHelpers.ts';
 import type {
   FmtConfig,
@@ -157,11 +157,7 @@ const resolveFmtConfig = async ({
   configFilePath,
   cwd,
 }: ResolveFmtConfigOptions): Promise<ResolvedFmtConfig> => {
-  const configs = await resolveConfigLayers(layers, 'fmt');
-  const config =
-    configs.length > 1
-      ? (Object.assign({}, ...configs) as FmtConfig)
-      : configs[0];
+  const config = await resolveFmtConfigLayers(layers);
   const rootPath = configFilePath ? dirname(configFilePath) : cwd;
 
   return normalizeFmtConfig(config, rootPath);
diff --git a/packages/rstack/src/rstestConfig.ts b/packages/rstack/src/rstestConfig.ts
index c69bdacb..d02868ba 100644
--- a/packages/rstack/src/rstestConfig.ts
+++ b/packages/rstack/src/rstestConfig.ts
@@ -87,13 +87,20 @@ const extendsConfig = async (
   };
 };
 
+export const mergeRstestConfigLayers = async (
+  layers: readonly Configs[],
+): Promise<RstestConfig> => {
+  const configs = await resolveConfigLayers(layers, 'test');
+  return configs.length > 1
+    ? mergeRstestConfig(...configs)
+    : (configs[0] ?? {});
+};
+
 export const resolveRstestConfig = async (
   layers: readonly Configs[],
   params: ConfigParams,
 ): Promise<RstestConfig> => {
-  const configs = await resolveConfigLayers(layers, 'test');
-  const testConfig =
-    configs.length > 1 ? mergeRstestConfig(...configs) : (configs[0] ?? {});
+  const testConfig = await mergeRstestConfigLayers(layers);
   return extendsConfig(layers, testConfig, params);
 };
 
diff --git a/packages/rstack/tests/config/compose.test.ts b/packages/rstack/tests/config/compose.test.ts
new file mode 100644
index 00000000..ed52afc0
--- /dev/null
+++ b/packages/rstack/tests/config/compose.test.ts
@@ -0,0 +1,89 @@
+import assert from 'node:assert/strict';
+import { createRequire } from 'node:module';
+import { expect, rs, test } from 'rstack/test';
+import {
+  composeConfigLayers,
+  resolveConfigLayers,
+} from '../../src/configLayers.ts';
+import { resolveFmtConfig } from '../../src/fmt/config.ts';
+import { resolveRstestConfig } from '../../src/rstestConfig.ts';
+
+// TODO: Import the source directly after upgrading Rstest to fix
+// https://github.com/web-infra-dev/rstest/issues/1891.
+rs.mock(
+  '../../src/rstestConfig.ts',
+  () =>
+    createRequire(import.meta.url)(
+      '../../dist/rstestConfig.js',
+    ) as typeof import('../../src/rstestConfig.ts'),
+);
+
+test('preserves single definitions and leaves missing tools absent', async () => {
+  const app = rs.fn(() => ({}));
+  const base = { app };
+  const fmt = {};
+
+  expect(await composeConfigLayers([])).toEqual({});
+  expect(await composeConfigLayers([base])).toBe(base);
+  const configs = await composeConfigLayers([base, { fmt }]);
+  expect(configs).toEqual({ app, fmt });
+  expect(configs.fmt).toBe(fmt);
+  expect(app).not.toHaveBeenCalled();
+});
+
+test('resolves only the requested tool and preserves staged task generators', async () => {
+  const app = rs.fn(() => ({}));
+  const fmt = rs.fn(() => ({ semi: false }));
+  const staged = rs.fn(() => 'rs check');
+  const configs = await composeConfigLayers([
+    { app, fmt, staged: { '*.ts': 'rs lint' } },
+    { app: {}, fmt: { singleQuote: true }, staged },
+  ]);
+
+  expect(fmt).not.toHaveBeenCalled();
+  expect(
+    await resolveFmtConfig({
+      layers: [configs],
+      configFilePath: '/project/rstack.config.ts',
+      cwd: '/other',
+    }),
+  ).toEqual({
+    rootPath: '/project',
+    baseOptions: { semi: false, singleQuote: true },
+    overrides: [],
+    ignorePatterns: [],
+  });
+  expect(fmt).toHaveBeenCalledExactlyOnceWith();
+  expect(app).not.toHaveBeenCalled();
+  expect(configs.staged).toBe(staged);
+  expect(staged).not.toHaveBeenCalled();
+});
+
+test('applies automatic inheritance after merging test definitions', async () => {
+  const app = rs.fn(() => ({ source: { define: { SHARED: true } } }));
+  const configs = await composeConfigLayers([
+    { app, test: { retry: 1 } },
+    {
+      app: { source: { define: { PROJECT: true } } },
+      test: { testTimeout: 5000 },
+    },
+  ]);
+
+  expect(await resolveConfigLayers([configs], 'test')).toEqual([
+    { retry: 1, testTimeout: 5000 },
+  ]);
+  expect(app).not.toHaveBeenCalled();
+
+  const params = { command: 'build', env: 'production' } as const;
+  const config = await resolveRstestConfig([configs], params);
+  assert(typeof config.extends === 'function');
+  expect(config).toEqual({
+    retry: 1,
+    testTimeout: 5000,
+    extends: config.extends,
+  });
+  expect(await config.extends(config)).toMatchObject({
+    source: { define: { SHARED: true, PROJECT: true } },
+  });
+  expect(app).toHaveBeenCalledExactlyOnceWith(params);
+});