This skill should be used when the user asks to "build background agent", "create hosted coding agent", "set up sandboxed execution", "implement multiplayer agent", or mentions background agents, sandboxed VMs, agent infrastructure, Modal sandboxes, self-spawning agents, or remote coding environments.
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Hosted agents run in remote sandboxed environments rather than on local machines. When designed well, they provide unlimited concurrency, consistent execution environments, and multiplayer collaboration. The critical insight is that session speed should be limited only by model provider time-to-first-token, with all infrastructure setup completed before the user starts their session.
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harness-engineering.multi-agent-patterns.tool-design.filesystem-context.Move agent execution to remote sandboxed environments to eliminate the fundamental limits of local execution: resource contention, environment inconsistency, and single-user constraints. Remote sandboxes unlock unlimited concurrency, reproducible environments, and collaborative workflows because each session gets its own isolated compute with a known-good environment image.
Design the architecture in three layers because each layer scales independently. Build sandbox infrastructure for isolated execution, an API layer for state management and client coordination, and client interfaces for user interaction across platforms. Keep these layers cleanly separated so sandbox changes do not ripple into clients.
The Core Challenge Eliminate sandbox spin-up latency because users perceive anything over a few seconds as broken. Development environments require cloning repositories, installing dependencies, and running build steps -- do all of this before the user ever submits a prompt.
Image Registry Pattern Pre-build environment images on a regular cadence (every 30 minutes works well) because this makes synchronization with the latest code a fast delta rather than a full clone. Include in each image:
When starting a session, spin up a sandbox from the most recent image. The repository is at most 30 minutes out of date, making the remaining git sync fast.
Snapshot and Restore Take filesystem snapshots at key points to enable instant restoration for follow-up prompts without re-running setup:
Git Configuration for Background Agents Configure git identity explicitly in every sandbox because background agents are not tied to a specific user during image builds:
user.name and user.email when committing and pushing changesWarm Pool Strategy Maintain a pool of pre-warmed sandboxes for high-volume repositories because cold starts are the primary source of user frustration:
Server-First Architecture Structure the agent framework as a server first, with TUI and desktop apps as thin clients, because this prevents duplicating agent logic across surfaces:
Code as Source of Truth Select frameworks where the agent can read its own source code to understand behavior. Prioritize this because having code as source of truth prevents the agent from hallucinating about its own capabilities -- an underrated failure mode in AI development.
Plugin System Requirements Require a plugin system that supports runtime interception because this enables safety controls and observability without modifying core agent logic:
tool.execute.before)Predictive Warm-Up Start warming the sandbox as soon as a user begins typing their prompt, not when they submit it, because the typing interval (5-30 seconds) is enough to complete most setup:
Parallel File Reading Allow the agent to start reading files immediately even if sync from latest base branch is not complete, because in large repositories incoming prompts rarely touch recently-changed files:
Maximize Build-Time Work Move everything possible to the image build step because build-time duration is invisible to users:
Agent-Spawned Sessions Build tools that allow agents to spawn new sessions because frontier models are capable of decomposing work and coordinating sub-tasks:
Expose three primitives: start a new session with specified parameters, read status of any session (check-in capability), and continue main work while sub-sessions run in parallel.
Prompt Engineering for Self-Spawning Engineer prompts that guide when agents should spawn sub-sessions rather than doing work inline:
Per-Session State Isolation Isolate state per session (SQLite per session works well) because cross-session interference is a subtle and hard-to-debug failure mode:
Real-Time Streaming Stream all agent work in real-time because high-frequency feedback is critical for user trust:
Use WebSocket connections with hibernation APIs to reduce compute costs during idle periods while maintaining open connections.
Synchronization Across Clients Build a single state system that synchronizes across all clients (chat interfaces, Slack bots, Chrome extensions, web interfaces, VS Code instances) because users switch surfaces frequently and expect continuity. All changes sync to the session state, enabling seamless client switching.
Why Multiplayer Matters Design for multiplayer from day one because it is nearly free to add with proper synchronization architecture, and it unlocks high-value workflows:
Implementation Requirements Build the data model so sessions are not tied to single authors because multiplayer fails silently if authorship is hardcoded:
User-Based Commits Use GitHub authentication to open PRs on behalf of the user (not the app) because this preserves the audit trail and prevents users from approving their own AI-generated changes:
Sandbox-to-API Flow Follow this sequence because it keeps sandbox permissions minimal while letting the API handle sensitive operations:
Slack Integration Prioritize Slack as the first distribution channel for internal adoption because it creates a virality loop as team members see others using it:
Web Interface Build a web interface with these features because it serves as the primary power-user surface:
Chrome Extension Build a Chrome extension for non-engineering users because DOM and React internals extraction gives higher precision than raw screenshots at lower token cost:
Before building the system, decide these infrastructure properties explicitly:
Choose between queueing and inserting follow-up messages sent during execution. Prefer queueing because it is simpler to manage and lets users send thoughts on next steps while the agent works. Build a mechanism to stop the agent mid-execution when needed, because without it users feel trapped.
Track these metrics because they indicate real value rather than vanity usage:
Drive internal adoption through visibility rather than mandates because forced usage breeds resentment:
Example 1: Background coding session lifecycle
user prompt
-> API allocates warm sandbox from current image
-> sandbox syncs latest branch delta
-> reads allowed immediately, writes blocked until sync completes
-> agent edits and tests inside isolated workspace
-> sandbox snapshots final filesystem
-> branch is pushed
-> API creates PR using user token
-> session summary, logs, and PR URL are returned to clients
This sequence keeps setup work outside the user-visible path while preserving auditability and user ownership of code changes.
Example 2: Boundary decision
If the task is "make the agent loop run for days with locked rubrics and PR approval," use harness-engineering. If the task is "run that loop in remote sandboxes with warm pools, session snapshots, streaming clients, and user-authored PRs," use this skill.
user.name or user.email causes commit failures in background agents. Always set git identity explicitly during sandbox configuration, never assume it carries over from the image.This skill owns hosted runtime infrastructure. Adjacent skills own the control system, topology, and tool contracts:
harness-engineering: governance, locked evaluators, rollback, novelty gates, and human approval boundaries around autonomous work.multi-agent-patterns: self-spawning and supervisor patterns once hosted infrastructure exists.tool-design: spawn, status, teardown, and PR tools exposed to agents.context-optimization: managing context across distributed hosted sessions.filesystem-context: using the sandbox filesystem for durable session state and artifacts.Internal reference:
Related skills in this collection:
External resources:
Created: 2026-01-12 Last Updated: 2026-05-15 Author: Agent Skills for Context Engineering Contributors Version: 1.2.0
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Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).