SolidStart 1.0 Server Function Boundary & Reactive Signal Invariants

--- description: SolidStart 1.0 Server Function Boundary & Reactive Signal Invariants - Enforces strict parameter serialization and reactivity separation across SolidStart 1.0 server$ RPC boundaries. Prevents runtime serialization failures, dead-code elimination leaks, and broken signal subscription graphs across client-server execution contexts. globs: src/**/*.{ts,tsx} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/solidstart-server-function-boundary-reactivity" --- # SolidStart 1.0 Server Function Boundary & Reactive Signal Invariants > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Implicit Client Signal Closures Inside server$ RPCs Causing Stale Serialization and Broken Fine-Grained Reactive Tracking ## System Constraints You are a Principal Software Architect enforcing SolidStart 1.0 server function and async boundary safety. Mandate explicit data passing across server$ boundaries, correct createResource dependency declaration, and strict payload serializability. SolidStart server functions compile to separate server and client bundles via Vinxi; closing over client signals inside server$ closures will silently capture undefined values or fail AST dead-code elimination. Ensure every async boundary uses Suspense or ErrorBoundary gracefully. ## Strict Guidelines - Engineers MUST ALWAYS pass client-side signals into `server$` functions as explicit function arguments rather than closing over signal accessors inside the `server$` body. - Engineers MUST use the accessor source argument in `createResource(source, fetcher)` to ensure fine-grained reactivity tracks signal changes and triggers re-fetching automatically. - Engineers MUST NOT rely on client state, browser globals (`window`, `document`), or reactive contexts within `server$` execution scopes. - Code MUST strictly validate and type all input parameters passed to `server$` functions using runtime schema validation (e.g., Valibot, Zod) to guard RPC endpoints. - Services MUST return only JSON-serializable primitives, plain objects, arrays, or standard HTTP `Response` objects from `server$` functions. - Components NEVER render asynchronous resource values without wrapping their evaluation in a `<Suspense>` or `<ErrorBoundary>` boundary. ## Bad vs Good Implementations ### Closing Over Signals in server$ vs Explicit Source Passing in createResource #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Accessing userId() inside the server$ closure fails because server$ is compiled to an external HTTP RPC endpoint where client lexical scope does not exist. Additionally, omitting the source accessor in createResource breaks Solid's fine-grained dependency tracking. ```typescript import { createSignal, createResource } from "solid-js"; import { server$ } from "@solidjs/start"; import { db } from "~/server/db"; export default function UserProfile() { const [userId, setUserId] = createSignal("usr_100"); // ANTI-PATTERN: Closure over client signal 'userId()' inside server$ // 1. Vinxi compiler splits this function; client closure is lost at runtime. // 2. createResource has no reactive source, so signal mutations will not re-trigger fetch. const [user] = createResource(async () => { return await server$(async () => { const id = userId(); // Stale or undefined on server execution! return await db.user.findUnique({ where: { id } }); })(); }); return <div>{user()?.name}</div>; } ``` #### Production Standard (Correct) > **Standard**: The server function `fetchUser` is defined at the module level accepting a strictly validated primitive argument. `createResource` uses `userId` as its reactive source, correctly orchestrating Suspense transitions whenever the signal updates. ```typescript import { createSignal, createResource, Suspense, Show } from "solid-js"; import { server$ } from "@solidjs/start"; import { db } from "~/server/db"; // Module-level RPC declaration with explicit serializable input const fetchUser = server$(async (id: string) => { "use server"; if (!id || typeof id !== "string") { throw new Error("Invalid user ID"); } return await db.user.findUnique({ where: { id }, select: { id: true, name: true, email: true } }); }); export default function UserProfile() { const [userId, setUserId] = createSignal<string>("usr_100"); // INVARIANT: Pass signal accessor as the first argument (source) // createResource automatically tracks userId and calls fetchUser when it changes const [user] = createResource(userId, fetchUser); return ( <Suspense fallback={<p>Loading profile...</p>}> <Show when={user()} fallback={<p>User not found</p>}> {(u) => <div><h1>{u().name}</h1><p>{u().email}</p></div>} </Show> </Suspense> ); } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: SolidStart 1.0 Server Function Boundary & Reactive Signal Invariants

SolidStart 1.0 Server Function Boundary & Reactive Signal Invariants defines an industrial-grade behavioral standard for frontend engineering. Enforces strict parameter serialization and reactivity separation across SolidStart 1.0 server$ RPC boundaries. Prevents runtime serialization failures, dead-code elimination leaks, and broken signal subscription graphs across client-server execution contexts. 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: Implicit Client Signal Closures Inside server$ RPCs Causing Stale Serialization and Broken Fine-Grained Reactive Tracking 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 Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Implicit Client Signal Closures Inside server$ RPCs Causing Stale Serialization and Broken Fine-Grained Reactive Tracking	Critical	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 Inconsistency	AI 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:

Engineers MUST ALWAYS pass client-side signals into `server$` functions as explicit function arguments rather than closing over signal accessors inside the `server$` body.

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.