diff --git a/packages/vite-plugin/src/build-start.ts b/packages/vite-plugin/src/build-start.ts
index d424b4c0c..a30fa85c9 100644
--- a/packages/vite-plugin/src/build-start.ts
+++ b/packages/vite-plugin/src/build-start.ts
@@ -38,7 +38,7 @@ export async function runBuildStart(
   // The hot path classifies file events by the Session's own scan policy.
   ctx.excludeMatcher = session.scanConfig.excludeMatcher;
   ctx.extensionsSet = session.scanConfig.extensionsSet;
-  // An earlier environment's closeBundle closed it: it joins its build again.
+  // An earlier build's end retired it: it joins its build again.
   session.reattach();
   session.hostResolve = resolveSpecifier;
   // A build fails on an aborted parse. A dev server starts through it with
diff --git a/packages/vite-plugin/src/context.ts b/packages/vite-plugin/src/context.ts
index 76e3e9075..36e35a70c 100644
--- a/packages/vite-plugin/src/context.ts
+++ b/packages/vite-plugin/src/context.ts
@@ -90,6 +90,19 @@ const NO_SHEETS: AssetSheets = {
   componentCss: '',
 };
 
+/** One environment's build, as Vite hands it to a hook: the object is its
+ *  identity, the name its key. */
+export interface BuildingEnvironment {
+  readonly name: string;
+}
+
+/** An environment's open build, and the promise its end settles. */
+interface OpenBuild {
+  environment: BuildingEnvironment;
+  ended: Promise<void>;
+  end(): void;
+}
+
 export class PluginContext {
   readonly options: AnimusExtractOptions;
   readonly verbose: boolean;
@@ -439,6 +452,43 @@ export class PluginContext {
     this.applyAssetSubstitutions();
   }
 
+  /** The environment a host without environments builds as. */
+  readonly singleEnvironment: BuildingEnvironment = { name: 'build' };
+
+  /** Each environment build that started and has not ended, by name. A
+   *  custom `buildApp` builds a shared plugin's environments in parallel, so
+   *  the context's build retires with the last of them, not the first. */
+  private readonly openBuilds = new Map<string, OpenBuild>();
+
+  /** Opens `environment`'s build, after any open build under its name has
+   *  ended: a builder reused while an earlier build still runs, or a second
+   *  environment under one name, builds after it rather than beside it.
+   *  Each build then ends its own generation once, in `buildEnd`. */
+  async openBuild(environment: BuildingEnvironment): Promise<void> {
+    for (;;) {
+      const open = this.openBuilds.get(environment.name);
+      if (open === undefined) break;
+      await open.ended;
+    }
+    let end = () => {};
+    const ended = new Promise<void>((settle) => {
+      end = settle;
+    });
+    this.openBuilds.set(environment.name, { environment, ended, end });
+  }
+
+  /** Ends `environment`'s open build; whether that left none open. Vite
+   *  ends each build once, in `buildEnd`, and a dev server ends only the
+   *  environments that started, so only the build that opened an entry
+   *  ends it. */
+  endBuild(environment: BuildingEnvironment): boolean {
+    const open = this.openBuilds.get(environment.name);
+    if (open === undefined || open.environment !== environment) return false;
+    this.openBuilds.delete(environment.name);
+    open.end();
+    return this.openBuilds.size === 0;
+  }
+
   /** Retires this context's build when Vite closes it: the Session leaves
    *  the registry (with its manifest and watch state) and the engine handle
    *  is released, injected engine or not. The next build start rejoins. */
diff --git a/packages/vite-plugin/src/index.ts b/packages/vite-plugin/src/index.ts
index d97206c63..6525032e1 100644
--- a/packages/vite-plugin/src/index.ts
+++ b/packages/vite-plugin/src/index.ts
@@ -206,14 +206,18 @@ export function animusExtract(options: AnimusExtractOptions): Plugin {
       applyResolvedConfig(ctx, config);
     },
 
-    // The end of a build, and of a dev server, which Vite closes the same
-    // way: the context's Session and engine are retired with it.
-    closeBundle() {
+    // The end of a build, and of a dev server, which Vite ends the same way:
+    // the last open environment build retires the context's Session and
+    // engine. buildEnd runs once per build, failed ones included; closeBundle
+    // can run twice for a failed one.
+    buildEnd() {
+      if (!ctx.endBuild(this.environment ?? ctx.singleEnvironment)) return;
       cancelSystemReload(ctx);
       ctx.close();
     },
 
     async buildStart() {
+      await ctx.openBuild(this.environment ?? ctx.singleEnvironment);
       await runBuildStart(
         ctx,
         async (specifier, importer) => {
diff --git a/packages/vite-plugin/tests/dev-server/build-app.test.ts b/packages/vite-plugin/tests/dev-server/build-app.test.ts
new file mode 100644
index 000000000..7c4718481
--- /dev/null
+++ b/packages/vite-plugin/tests/dev-server/build-app.test.ts
@@ -0,0 +1,196 @@
+import { readdirSync, readFileSync } from 'fs';
+import { join } from 'path';
+import { BuildEnvironment, createBuilder } from 'vite-plus';
+import { describe, expect, it } from 'vitest';
+
+import { animusExtract } from '../../src/index';
+import { createDevFixture } from './fixture';
+
+import type { ViteBuilder } from 'vite-plus';
+
+/** Fails the client build while `failing.client` is set. */
+const failing = { client: false };
+const failClient = {
+  name: 'fail-client',
+  transform(this: { environment: { name: string } }) {
+    if (failing.client && this.environment.name === 'client') {
+      throw new Error('planned client failure');
+    }
+    return null;
+  },
+};
+
+/** Delays every SSR transform, so the client environment closes first. */
+const slowSsr = {
+  name: 'slow-ssr',
+  enforce: 'pre' as const,
+  async transform(this: { environment: { name: string } }) {
+    if (this.environment.name === 'ssr') {
+      await new Promise((settle) => setTimeout(settle, 300));
+    }
+    return null;
+  },
+};
+
+interface EnvironmentBuild {
+  build: {
+    outDir: string;
+    ssr?: true;
+    rollupOptions: { input: string };
+  };
+}
+
+interface ParallelBuildConfig {
+  root: string;
+  configFile: false;
+  logLevel: 'silent';
+  plugins: Array<
+    typeof slowSsr | typeof failClient | ReturnType<typeof animusExtract>
+  >;
+  environments: { client: EnvironmentBuild; ssr: EnvironmentBuild };
+  builder: { buildApp(app: ViteBuilder): Promise<void> };
+}
+
+type StartParallelBuild = (
+  config: ParallelBuildConfig
+) => Promise<{ buildApp(): Promise<void> }>;
+
+function filesUnder(dir: string): string[] {
+  return readdirSync(dir, { withFileTypes: true }).flatMap((entry) =>
+    entry.isDirectory()
+      ? filesUnder(join(dir, entry.name))
+      : [join(dir, entry.name)]
+  );
+}
+
+/**
+ * A custom `buildApp` builds a shared plugin's environments in parallel: the
+ * first to close must not retire the Session and engine the others still
+ * transform with.
+ */
+describe('environments built in parallel', () => {
+  it('the first environment to close leaves the others their Session', async () => {
+    const fixture = createDevFixture();
+    try {
+      const input = join(fixture.root, 'src', 'main.ts');
+      // SAFETY: vite-plus runs Vite's builder and consumes the same plugin
+      // hooks; only the nominal declarations differ, and tsc overflows on them.
+      const startParallelBuild = createBuilder as StartParallelBuild;
+      const builder = await startParallelBuild({
+        root: fixture.root,
+        configFile: false,
+        logLevel: 'silent',
+        plugins: [slowSsr, animusExtract({ system: './src/ds.ts' })],
+        environments: {
+          client: {
+            build: { outDir: 'dist/client', rollupOptions: { input } },
+          },
+          ssr: {
+            build: { outDir: 'dist/ssr', ssr: true, rollupOptions: { input } },
+          },
+        },
+        builder: {
+          async buildApp(app) {
+            await Promise.all([
+              app.build(app.environments.client),
+              app.build(app.environments.ssr),
+            ]);
+          },
+        },
+      });
+      await builder.buildApp();
+
+      const ssrCode = filesUnder(join(fixture.root, 'dist', 'ssr'))
+        .filter((file) => file.endsWith('.js'))
+        .map((file) => readFileSync(file, 'utf-8'))
+        .join('\n');
+      expect(ssrCode).toContain('createComponent');
+      expect(ssrCode).not.toContain('.styles(');
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  it('a builder reused while an earlier build still runs builds after it', async () => {
+    const fixture = createDevFixture();
+    try {
+      // Every environment build either app starts, in order.
+      const builds: Array<Promise<object | void>> = [];
+      const builder = await parallelBuilder(fixture.root, async (app) => {
+        const started = [
+          app.build(app.environments.client),
+          app.build(app.environments.ssr),
+        ];
+        builds.push(...started);
+        await Promise.all(started);
+      });
+      // The client fails at once; the slow SSR build is still running when
+      // the builder is reused.
+      failing.client = true;
+      await expect(builder.buildApp()).rejects.toThrow(
+        'planned client failure'
+      );
+      failing.client = false;
+      await builder.buildApp();
+
+      const outcomes = await Promise.allSettled(builds);
+      expect(outcomes.map((outcome) => outcome.status)).toEqual([
+        'rejected',
+        'fulfilled',
+        'fulfilled',
+        'fulfilled',
+      ]);
+    } finally {
+      failing.client = false;
+      fixture.dispose();
+    }
+  });
+
+  it('two environments under one name build one after the other', async () => {
+    const fixture = createDevFixture();
+    try {
+      // A second, distinct environment object under the client's name.
+      const builder = await parallelBuilder(fixture.root, async (app) => {
+        const twin = new BuildEnvironment('client', app.config);
+        await twin.init();
+        await Promise.all([
+          app.build(app.environments.client),
+          app.build(twin),
+        ]);
+      });
+
+      await expect(builder.buildApp()).resolves.toBeUndefined();
+    } finally {
+      fixture.dispose();
+    }
+  });
+});
+
+/** A builder whose app builds client and SSR in parallel, SSR slowly,
+ *  unless `buildApp` builds otherwise. */
+function parallelBuilder(
+  root: string,
+  buildApp: (app: ViteBuilder) => Promise<void> = async (app) => {
+    await Promise.all([
+      app.build(app.environments.client),
+      app.build(app.environments.ssr),
+    ]);
+  }
+) {
+  const input = join(root, 'src', 'main.ts');
+  // SAFETY: as above.
+  const startParallelBuild = createBuilder as StartParallelBuild;
+  return startParallelBuild({
+    root,
+    configFile: false,
+    logLevel: 'silent',
+    plugins: [slowSsr, failClient, animusExtract({ system: './src/ds.ts' })],
+    environments: {
+      client: { build: { outDir: 'dist/client', rollupOptions: { input } } },
+      ssr: {
+        build: { outDir: 'dist/ssr', ssr: true, rollupOptions: { input } },
+      },
+    },
+    builder: { buildApp },
+  });
+}