HermesHandler

HermesHandler is a lightweight, framework-agnostic message router for browser extensions and event-driven systems. It provides structured request dispatching with a strict response envelope { ok:true, result?, info? } | { ok:false, error, info? }, built-in timeout handling, cooperative cancellation, and safe normalization.

Designed for reliability and clarity, HermesHandler is especially well-suited for LLM-driven agents, modular browser architectures, automation layers, and distributed runtime systems.

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✨ Features

• 🔁 Deterministic message routing via type
• 📦 Strict response envelope: { ok:true, result?, info? } | { ok:false, error, info? }
• ⏱ Built-in timeout handling
• 🛑 Cooperative cancellation via AbortSignal
• 🧊 Immutable (shallow-frozen) responses
• 🧠 LLM-friendly deterministic contract
• 🧩 Framework-agnostic (no runtime dependencies)
• 📘 Type-safe via generated .d.ts

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Installation

npm install hermes-handler

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Quick Start

import { HermesHandler } from "hermes-handler";

const handlers = {
  ping: () => ({ ok: true, result: "pong" }),

  greet: (msg) => {
    return { ok: true, result: `Hello ${msg.payload.name}` };
  }
};

const hermes = new HermesHandler(handlers);

const res = await hermes.dispatch({ type: "ping" });

if (res.ok) {
  console.log(res.result); // "pong"
}

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Browser Extension Usage

Attach HermesHandler to a runtime listener:

browser.runtime.onMessage.addListener(
  hermes.getListener()
);

HermesHandler supports both:

• Promise-returning listeners (MV3 / Firefox / polyfill)
• Callback-style sendResponse + return true

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Response Contract

All responses follow a strict envelope.

Hermes never mutates result; any unexpected or conflicting fields are preserved under info.

If a handler returns inconsistent envelopes (e.g. { ok:false, error, result }), Hermes warns and preserves extras under info.

If a handler returns an envelope that already includes info, Hermes preserves it under info.handlerInfo when additional fields must also be recorded.

Success

{ ok: true, result: any, info?: any }

Error

{ ok: false, error: string, info?: any }

Primitive return values are automatically normalized:

return "hello";

Becomes:

{ ok: true, result: "hello" }

Malformed envelopes are coerced into valid error responses.

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Timeouts

Handlers can be time-limited:

const hermes = new HermesHandler(handlers, {
  timeoutMs: 7000
});

If exceeded, HermesHandler returns:

{ ok: false, error: "Handler <type> timed out (7000 ms)" }

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Cooperative Cancellation

Each handler receives an AbortSignal:

async function longTask(msg, ctx) {
  if (ctx.signal?.aborted) {
    return { ok: false, error: "Cancelled" };
  }

  ctx.signal?.addEventListener("abort", () => {
    console.log("Cancelled externally");
  });
}

HermesHandler aborts the signal once a request lifecycle completes.

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API

new HermesHandler(initialHandlers?, options?)

initialHandlers Record<string, HermesHandlerFn>

options

• timeoutMs?: number
• onUnknown?: (msg, ctx) => HermesResponse
• onError?: (err, msg, ctx) => HermesResponse
• logger?: HermesLogger | null

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.register(type, fn)

Register or overwrite a handler.

.registerMany(map)

Register multiple handlers at once.

.unregister(type)

Remove a handler.

.has(type)

Check if a handler exists.

.getListener()

Returns a runtime-compatible message listener.

.dispatch(msg, sender?)

Dispatch a message manually (useful for testing or non-extension environments).

.types()

List registered message types (registration order).

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Logging

HermesHandler emits warnings and errors through a configurable logger.

By default, it uses the global console. You can disable logging entirely or provide a custom logger implementation.

Disable Logging

const hermes = new HermesHandler(handlers, {
  logger: null
});

Custom Logger

const hermes = new HermesHandler(handlers, {
  logger: {
    warn: (...args) => myLogger.warn(...args),
    error: (...args) => myLogger.error(...args)
  }
});

HermesLogger shape

interface HermesLogger {
  debug?(message?: any, ...optionalParams: any[]): void;
  info?(message?: any, ...optionalParams: any[]): void;
  warn?(message?: any, ...optionalParams: any[]): void;
  error?(message?: any, ...optionalParams: any[]): void;
}

If logger is null, HermesHandler will not emit any console output.

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Design Goals

HermesHandler enforces a predictable and deterministic runtime contract. By standardizing request/response handling and isolating message dispatch logic, it simplifies reasoning about complex systems—particularly those involving automation, background scripts, or LLM-driven tool execution.

The core remains intentionally minimal, dependency-free, and portable.

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License

MIT