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frontendcritical PriorityID: vite-6-environment-api-ssr-target-isolation-guardrail

Vite 6 Environment API SSR Runtime Target Isolation Guardrail

Enforces strict module graph separation, dedicated DevEnvironment instances, and isolated global/hot scopes across client, edge, and node runtimes in Vite 6 SSR architectures. Prevents cross-environment state leakage, dual-module graph contamination, and cumulative memory leaks caused by sharing server-side state or module execution runners across distinct request boundaries.

Prohibited Anti-Pattern: Global Module Contamination and Shared Environment State in Vite SSR

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---
description: Vite 6 Environment API SSR Runtime Target Isolation Guardrail - Enforces strict module graph separation, dedicated DevEnvironment instances, and isolated global/hot scopes across client, edge, and node runtimes in Vite 6 SSR architectures. Prevents cross-environment state leakage, dual-module graph contamination, and cumulative memory leaks caused by sharing server-side state or module execution runners across distinct request boundaries.
globs: vite.config.ts, vite.config.*.ts, src/entry-server.ts, src/entry-client.ts, src/server/**/*.ts, src/plugins/**/*.ts
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/vite-6-environment-api-ssr-target-isolation-guardrail"
---

# Vite 6 Environment API SSR Runtime Target Isolation Guardrail

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Global Module Contamination and Shared Environment State in Vite SSR

## System Constraints
You are a Principal Software Architect enforcing Vite 6 Environment API SSR runtime target isolation. Vite 6 decouples dev servers into distinct DevEnvironment instances per target (e.g., client, ssr, edge, workerd). You must strictly mandate separate module graphs, target-specific module runners (ModuleRunner / RunnableDevEnvironment), and explicit conditional exports per environment. Disallow legacy SSR APIs (viteServer.ssrLoadModule), global state leaks across request contexts, and cross-target imports that poison client bundles with node internals or edge runtimes with ambient Node.js globals.

## Strict Guidelines
- Engineers MUST ALWAYS configure distinct environments (client, ssr, and optionally edge) using the Vite 6 Environment API (`environments` config key) instead of relying on legacy flat `ssr: true` flags.
- Engineers MUST ALWAYS instantiate target-specific `ModuleRunner` or access `server.environments[name]` rather than calling deprecated `server.ssrLoadModule` or `server.ssrFixStacktrace`.
- Engineers MUST NOT import Node.js native modules (`fs`, `path`, `crypto`) or write to node-specific globals (`process.env`, `global`) inside shared runtime code without environment-specific conditional guards or entry points.
- Plugins MUST explicitly declare supported environments or hook filters via `applyToEnvironment(environment)` to prevent node/server transforms from executing over client graph modules.
- SSR context state (request stores, user tokens, caches) MUST be scoped to AsyncLocalStorage or explicit request parameters, NEVER module-scoped top-level variables subject to HMR leaks.

## Bad vs Good Implementations
### Vite 6 Environment Configuration Isolation vs. Legacy SSR Setup

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy Vite SSR configuration bundles server and client settings into a single target definition, creating shared plugin execution states and preventing isolated module graphs for different runtimes (Node.js vs. WebWorker vs. Edge).
```typescript
import { defineConfig } from 'vite';

// ANTI-PATTERN: Vite 5 legacy config mixing SSR and Client targets in a single graph
export default defineConfig({
  ssr: {
    noExternal: ['shared-ui-lib'],
    target: 'node'
  },
  plugins: [
    {
      name: 'leaky-plugin',
      transform(code, id, options) {
        // Pollutes both client and server graphs uniformly
        if (options?.ssr) {
          return code.replace('__TARGET__', JSON.stringify('ssr'));
        }
        return code.replace('__TARGET__', JSON.stringify('client'));
      }
    }
  ]
});
```

#### Production Standard (Correct)
> **Standard**: Explicitly declares independent environments (`client`, `ssr`, `edge`) using Vite 6 Environment API. Plugins selectively attach to targeted environments via `applyToEnvironment`, preserving complete module graph isolation.
```typescript
import { defineConfig } from 'vite';
import type { DevEnvironment } from 'vite';

// PRODUCTION INVARIANT: Vite 6 isolated environments with dedicated module graphs
export default defineConfig({
  environments: {
    client: {
      build: {
        outDir: 'dist/client'
      }
    },
    ssr: {
      build: {
        outDir: 'dist/server',
        target: 'node22'
      },
      resolve: {
        conditions: ['node', 'import']
      }
    },
    edge: {
      build: {
        outDir: 'dist/edge',
        target: 'esnext'
      },
      resolve: {
        noExternal: true,
        conditions: ['worker', 'edge-routine']
      }
    }
  },
  plugins: [
    {
      name: 'isolated-target-plugin',
      applyToEnvironment(env: DevEnvironment) {
        // Enforces plugin execution strictly within the target environment module graph
        return env.name === 'ssr' || env.name === 'edge';
      },
      transform(code) {
        return code.replace('__RUNTIME_ENV__', JSON.stringify('isolated-ssr'));
      }
    }
  ]
});
```

### Server Request Handling via ModuleRunner vs. Deprecated ssrLoadModule

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Using `server.ssrLoadModule` bypasses Vite 6 DevEnvironment isolation, caches a static module instance across requests causing cross-tenant state bleeding, and prevents proper environment-scoped HMR invalidation.
```typescript
import type { ViteDevServer } from 'vite';
import type { IncomingMessage, ServerResponse } from 'node:http';

// ANTI-PATTERN: Shared module state and legacy ssrLoadModule execution
let cachedRenderer: unknown = null;

export async function handleRequest(server: ViteDevServer, req: IncomingMessage, res: ServerResponse): Promise<void> {
  // Leaks memory across requests; bypasses isolated Environment API runtime runner
  if (!cachedRenderer) {
    cachedRenderer = await server.ssrLoadModule('/src/entry-server.ts');
  }
  
  const { render } = cachedRenderer as { render: (url: string) => Promise<string> };
  const html = await render(req.url ?? '/');
  res.end(html);
}
```

#### Production Standard (Correct)
> **Standard**: Leverages Vite 6 `createServerModuleRunner` scoped specifically to `server.environments.ssr`. Ensures HMR invalidations and module executions stay inside the isolated SSR environment without polluting other targets or module scopes.
```typescript
import type { ViteDevServer } from 'vite';
import type { IncomingMessage, ServerResponse } from 'node:http';
import { createServerModuleRunner } from 'vite/module-runner';

interface ServerEntry {
  render: (url: string, context: Record<string, unknown>) => Promise<string>;
}

export async function handleRequest(server: ViteDevServer, req: IncomingMessage, res: ServerResponse): Promise<void> {
  const ssrEnvironment = server.environments.ssr;
  if (!ssrEnvironment) {
    throw new Error('SSR Environment is not initialized on Vite Server');
  }

  // PRODUCTION INVARIANT: Environment-scoped ModuleRunner with per-request clean dispatch
  const runner = createServerModuleRunner(ssrEnvironment);
  const entry = (await runner.import('/src/entry-server.ts')) as ServerEntry;

  // Request context passed explicitly to avoid module-level global pollution
  const html = await entry.render(req.url ?? '/', {
    requestId: crypto.randomUUID(),
    timestamp: Date.now()
  });

  res.setHeader('Content-Type', 'text/html; charset=utf-8');
  res.end(html);
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Vite AST Inspector MCP
Analyzes Vite module graph transforms and dependency trees to detect cross-environment module leaks.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Vite Environment Verifier
Validates that target configurations, DevEnvironment instances, and runner instances conform to Vite 6 standards.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Vite 6 Environment API SSR Runtime Target Isolation Guardrail

Vite 6 Environment API SSR Runtime Target Isolation Guardrail defines an industrial-grade behavioral standard for frontend engineering. Enforces strict module graph separation, dedicated DevEnvironment instances, and isolated global/hot scopes across client, edge, and node runtimes in Vite 6 SSR architectures. Prevents cross-environment state leakage, dual-module graph contamination, and cumulative memory leaks caused by sharing server-side state or module execution runners across distinct request boundaries. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Global Module Contamination and Shared Environment State in Vite SSR Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftGlobal Module Contamination and Shared Environment State in Vite SSRCritical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS configure distinct environments (client, ssr, and optionally edge) using the Vite 6 Environment API (`environments` config key) instead of relying on legacy flat `ssr

Engineers MUST ALWAYS configure distinct environments (client, ssr, and optionally edge) using the Vite 6 Environment API (`environments` config key) instead of relying on legacy flat `ssr: true` flags.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST ALWAYS instantiate target-specific `ModuleRunner` or access `server.environments[name]` rather than calling deprecated `server.ssrLoadModule` or `server.ssrFixStacktrace`.

Engineers MUST ALWAYS instantiate target-specific `ModuleRunner` or access `server.environments[name]` rather than calling deprecated `server.ssrLoadModule` or `server.ssrFixStacktrace`.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Engineers MUST NOT import Node.js native modules (`fs`, `path`, `crypto`) or write to node-specific globals (`process.env`, `global`) inside shared runtime code without environment-specific conditional guards or entry points.

Engineers MUST NOT import Node.js native modules (`fs`, `path`, `crypto`) or write to node-specific globals (`process.env`, `global`) inside shared runtime code without environment-specific conditional guards or entry points.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Plugins MUST explicitly declare supported environments or hook filters via `applyToEnvironment(environment)` to prevent node/server transforms from executing over client graph modules.

Plugins MUST explicitly declare supported environments or hook filters via `applyToEnvironment(environment)` to prevent node/server transforms from executing over client graph modules.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

SSR context state (request stores, user tokens, caches) MUST be scoped to AsyncLocalStorage or explicit request parameters, NEVER module-scoped top-level variables subject to HMR leaks.

SSR context state (request stores, user tokens, caches) MUST be scoped to AsyncLocalStorage or explicit request parameters, NEVER module-scoped top-level variables subject to HMR leaks.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Vite 6 Environment API SSR Runtime Target Isolation Guardrail critical for AI coding assistants?

Enforces strict module graph separation, dedicated DevEnvironment instances, and isolated global/hot scopes across client, edge, and node runtimes in Vite 6 SSR architectures. Prevents cross-environment state leakage, dual-module graph contamination, and cumulative memory leaks caused by sharing server-side state or module execution runners across distinct request boundaries. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Global Module Contamination and Shared Environment State in Vite SSR) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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