# Transmission Manager - Agent Guide

## First Rule
Always read `project-context.md` before making code or documentation changes. If a change alters project purpose, architecture, APIs, file roles, workflows, constraints, or implementation patterns, update `project-context.md` in the same change.

## Quick Summary
Transmission Manager is a lightweight single-binary Go web app for managing and viewing torrents in a Transmission RPC client. It serves a dark-mode vanilla HTML/CSS/JS SPA and proxies Transmission RPC calls through a minimal Go backend.

## Implementation Guidelines
- Keep the backend stdlib-only. Do not add Gin, Fiber, Chi, Gorilla, or other Go web frameworks.
- Keep the frontend dependency-free. Do not add React, Vue, Svelte, bundlers, package managers, or transpilers.
- Preserve the single-binary model: static frontend files should remain embedded via `embed`.
- Format display numbers in the frontend unless there is a clear backend reason.
- Preserve in-place frontend updates during polling. Do not replace the whole torrent list on each refresh.
- Keep polling lightweight and avoid overlapping refresh requests.
- Keep UI dark-only, modern, compact, responsive, and based on CSS variables.

## Transmission RPC Notes
- RPC URL read from `TRANSMISSION_URL` (must be set).
- Optional HTTP Basic Auth credentials come from `TRANSMISSION_RPC_USERNAME` and `TRANSMISSION_RPC_PASSWORD`; unset, empty, or literal `null` values mean no credential value.
- Handle the Transmission session-ID handshake:
  1. POST to the RPC URL.
  2. If the response is `409`, read `X-Transmission-Session-Id`.
  3. Retry the same request with that header.
  4. Cache the session ID behind a mutex.
- "Peers at 100%" means count `peers[]` entries where `progress === 1.0`.
- `doneDate` is a Unix timestamp; `0` means unfinished.

## Do Not
- Do not add authentication unless explicitly requested.
- Do not add CI/CD or GitHub Actions.
- Do not create a GitHub repository.
- Do not vendor dependencies.
- Do not add broad tests or infrastructure for small changes; keep verification proportional.