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frontendcritical PriorityID: vite-6-environment-api-heterogeneous-ssr-isolation

Vite 6 Heterogeneous SSR Runtime Isolation via Environment API

Enforces strict runtime-agnostic module separation and module graph isolation across Node.js, WinterCG (Edge), and Browser environments utilizing Vite 6's Environment API. Prevents cross-environment global pollution, host-specific built-in leakage, and shared runtime singleton corruption during multi-target SSR compilation and hydration.

Prohibited Anti-Pattern: Monolithic SSR Module Graph and Cross-Contaminating Host Primitives

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---
description: Vite 6 Heterogeneous SSR Runtime Isolation via Environment API - Enforces strict runtime-agnostic module separation and module graph isolation across Node.js, WinterCG (Edge), and Browser environments utilizing Vite 6's Environment API. Prevents cross-environment global pollution, host-specific built-in leakage, and shared runtime singleton corruption during multi-target SSR compilation and hydration.
globs: vite.config.{ts,js,mjs}, src/**/*.{ts,tsx,js,jsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/vite-6-environment-api-heterogeneous-ssr-isolation"
---

# Vite 6 Heterogeneous SSR Runtime Isolation via Environment API

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Monolithic SSR Module Graph and Cross-Contaminating Host Primitives

## System Constraints
You are a Principal Software Architect enforcing Vite 6 Environment API Heterogeneous SSR Runtime Isolation. Vite 6 decouples the dev server from assuming a binary client/ssr model by introducing multi-environment compilation and execution graphs. You MUST mandate strict isolation across runtime targets (Node.js, WinterCG/Edge, Deno, Browser), preventing runtime primitive leaks, conflicting module resolution conditions, and cross-polluted ModuleRunner instances.

## Strict Guidelines
- Engineers MUST ALWAYS declare explicit isolated environment profiles under `environments` in `vite.config.ts` (e.g., `client`, `ssrNode`, `ssrEdge`) rather than relying on legacy top-level `ssr` configurations.
- Engineers MUST NOT leak Node.js core built-ins (e.g., `node:fs`, `node:crypto`, `node:path`) into modules resolved within WinterCG, Edge, or Browser environment targets.
- Code targeting heterogeneous runtimes MUST decouple environment-specific side-effects behind explicit dependency injection interfaces or runtime capabilities checks, never top-level un-guarded global references (`process`, `Deno`, or `WebSocketPair`).
- Module runners MUST be instantiated uniquely per environment using Vite 6's `ModuleRunner` or specialized `DevEnvironment` runners to eliminate shared module cache pollution between Edge and Node targets during local development.
- Vite configuration MUST explicitly declare discrete `resolve.conditions` per environment (e.g., `['node', 'import']` vs `['workerd', 'worker', 'import']` vs `['browser', 'import']`).
- Shared route modules and components NEVER evaluate runtime-specific singletons at module scope; any platform-specific persistence or state MUST be bound to request-scoped context.

## Bad vs Good Implementations
### Vite 6 Multi-Environment Configuration and Runtime Adapter Isolation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy monolithic Vite SSR configurations treat server-side execution as a homogenous target. Importing Node built-ins into shared routes crashes Edge/WinterCG workers, and baking `process.env` globals breaks runtime environment isolation.
```typescript
import { defineConfig } from 'vite';
import fs from 'node:fs';

// ANTI-PATTERN: Monolithic SSR configuration using legacy Vite patterns.
// Mixes Node.js dependencies and Edge constraints in a single SSR pipeline.
export default defineConfig({
  ssr: {
    target: 'node',
    noExternal: true
  },
  define: {
    // Leaks host machine process env into all bundles regardless of runtime
    'process.env.API_SECRET': JSON.stringify(process.env.API_SECRET)
  }
});

// In application code (shared route):
export async function handleRequest(req: Request) {
  // FAILS on Cloudflare Workers / Fastly / Deno: node:fs invoked directly in shared SSR code
  const template = fs.readFileSync('./index.html', 'utf-8');
  return new Response(template, { headers: { 'content-type': 'text/html' } });
}
```

#### Production Standard (Correct)
> **Standard**: Declares explicit environment trees with specialized `conditions` and externalization boundaries under Vite 6 Environment API. Application code accesses platform capabilities through injected adapters rather than direct host globals.
```typescript
import { defineConfig } from 'vite';

// PRODUCTION INVARIANT: Vite 6 Environment API with discrete runtime environments
export default defineConfig({
  environments: {
    client: {
      build: {
        outDir: 'dist/client'
      },
      resolve: {
        conditions: ['browser', 'import']
      }
    },
    ssrNode: {
      build: {
        outDir: 'dist/server/node',
        ssr: true
      },
      resolve: {
        conditions: ['node', 'import'],
        external: ['node:fs', 'node:path', 'node:crypto']
      }
    },
    ssrEdge: {
      build: {
        outDir: 'dist/server/edge',
        ssr: true
      },
      resolve: {
        conditions: ['workerd', 'worker', 'web', 'import'],
        noExternal: true
      }
    }
  }
});

// Application Code: Port-and-Adapter runtime decoupling
export interface ServerPlatformContext {
  readStaticAsset: (path: string) => Promise<string>;
}

export async function renderApp(req: Request, platform: ServerPlatformContext): Promise<Response> {
  const template = await platform.readStaticAsset('index.html');
  return new Response(template, { headers: { 'content-type': 'text/html; charset=utf-8' } });
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Vite Ecosystem MCP Inspector
Validates module resolution graphs and runtime boundary leaks across Vite 6 development environments.
WinterCG Runtime Validator MCP
Verifies that Edge bundle outputs conform strictly to the WinterCG Minimum Common Web Platform API specification.
Recommended Agent Skills (askills.dev)

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

Vite Environment Graph Auditor
CLI tool to inspect Vite 6 EnvironmentModuleGraph instances and ensure Node.js APIs are excluded from edge bundles.
Heterogeneous SSR Matrix Tester
Concurrently executes SSR test suites across Node.js (v20+), Deno (v2+), and workerd to enforce hydration parity.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Vite 6 Heterogeneous SSR Runtime Isolation via Environment API

Vite 6 Heterogeneous SSR Runtime Isolation via Environment API defines an industrial-grade behavioral standard for frontend engineering. Enforces strict runtime-agnostic module separation and module graph isolation across Node.js, WinterCG (Edge), and Browser environments utilizing Vite 6's Environment API. Prevents cross-environment global pollution, host-specific built-in leakage, and shared runtime singleton corruption during multi-target SSR compilation and hydration. 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: Monolithic SSR Module Graph and Cross-Contaminating Host Primitives 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 DriftMonolithic SSR Module Graph and Cross-Contaminating Host PrimitivesCritical
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 declare explicit isolated environment profiles under `environments` in `vite.config.ts` (e.g., `client`, `ssrNode`, `ssrEdge`) rather than relying on legacy top-level `ssr` configurations.

Engineers MUST ALWAYS declare explicit isolated environment profiles under `environments` in `vite.config.ts` (e.g., `client`, `ssrNode`, `ssrEdge`) rather than relying on legacy top-level `ssr` configurations.

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 NOT leak Node.js core built-ins (e.g., `node

Engineers MUST NOT leak Node.js core built-ins (e.g., `node:fs`, `node:crypto`, `node:path`) into modules resolved within WinterCG, Edge, or Browser environment targets.

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

Code targeting heterogeneous runtimes MUST decouple environment-specific side-effects behind explicit dependency injection interfaces or runtime capabilities checks, never top-level un-guarded global references (`process`, `Deno`, or `WebSocketPair`).

Code targeting heterogeneous runtimes MUST decouple environment-specific side-effects behind explicit dependency injection interfaces or runtime capabilities checks, never top-level un-guarded global references (`process`, `Deno`, or `WebSocketPair`).

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

Module runners MUST be instantiated uniquely per environment using Vite 6's `ModuleRunner` or specialized `DevEnvironment` runners to eliminate shared module cache pollution between Edge and Node targets during local development.

Module runners MUST be instantiated uniquely per environment using Vite 6's `ModuleRunner` or specialized `DevEnvironment` runners to eliminate shared module cache pollution between Edge and Node targets during local development.

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

Vite configuration MUST explicitly declare discrete `resolve.conditions` per environment (e.g., `['node', 'import']` vs `['workerd', 'worker', 'import']` vs `['browser', 'import']`).

Vite configuration MUST explicitly declare discrete `resolve.conditions` per environment (e.g., `['node', 'import']` vs `['workerd', 'worker', 'import']` vs `['browser', 'import']`).

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.
6

Shared route modules and components NEVER evaluate runtime-specific singletons at module scope

Shared route modules and components NEVER evaluate runtime-specific singletons at module scope; any platform-specific persistence or state MUST be bound to request-scoped context.

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 Heterogeneous SSR Runtime Isolation via Environment API critical for AI coding assistants?

Enforces strict runtime-agnostic module separation and module graph isolation across Node.js, WinterCG (Edge), and Browser environments utilizing Vite 6's Environment API. Prevents cross-environment global pollution, host-specific built-in leakage, and shared runtime singleton corruption during multi-target SSR compilation and hydration. 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 (Monolithic SSR Module Graph and Cross-Contaminating Host Primitives) 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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