Customizing Agents

Fullsend agents work out of the box, but most teams want to tailor them — add coding conventions, domain knowledge, extra skills, or entirely new agent roles. This page lists every customization approach from lightest to heaviest so you can pick the right one.

Quick decision guide

Goal	Approach	Effort
Teach agents your coding style, test commands, or architecture rules	AGENTS.md	Low
Give an agent domain-specific knowledge or a new capability	Skills	Low
Change model, timeout, image, or add env vars to an existing agent	Harness configuration	Medium
Build a completely new agent with its own trigger, scripts, and schema	Bring Your Own Agent	High

Start at the top and move down only when a lighter option doesn't cover your needs. Most teams only need AGENTS.md and perhaps a skill or two.

AGENTS.md

The simplest way to influence agent behavior. Add an AGENTS.md file to your repository with conventions that apply to all contributors — human and agent alike:

# AGENTS.md

## Testing
- Always run `make test` before committing.

## Code style
- Use structured logging via `slog`. Do not use `log.Printf`.

Every agent reads AGENTS.md automatically. No fullsend configuration changes needed.

Best for: coding conventions, test commands, architecture rules, domain context.

Guide: Configuring with AGENTS.md

Skills

Skills are self-contained markdown documents that teach an agent how to perform a specific task. Place them in .agents/skills/ in your repo and symlink .claude/skills to make them discoverable:

your-repo/
  .agents/skills/
    deployment-checks/
      SKILL.md
  .claude/skills -> ../.agents/skills

Skills extend what agents know — linting rules, deployment checklists, customer data sources — without changing any fullsend configuration.

Best for: agent-specific domain knowledge, helper scripts, extension points, replacing built-in skills.

Guide: Configuring with Skills

Harness configuration

To change how an existing agent runs — its model, timeout, sandbox image, environment variables, or skills — use base: harness composition. Create a thin harness that inherits from the upstream agent and overrides only what differs:

# .fullsend/harness/code.yaml
base: https://raw.githubusercontent.com/fullsend-ai/agents/<sha>/harness/code.yaml#sha256=abc...

model: sonnet
timeout_minutes: 45
skills:
  - skills/my-custom-linting
env:
  sandbox:
    JIRA_PROJECT: "MYPROJ"

Register the agent in .fullsend/config.yaml:

agents:
  - name: code
    source: harness/code.yaml

Best for: changing model or timeout, adding environment variables, adding skills via harness, extending the sandbox image, disabling agents.

Guides:

• Configuring Agent Behavior — harness composition, status notifications, disabling agents
• Harness Field Reference — complete field reference, merge rules, and advanced configuration

Bring Your Own Agent

When you need a completely new agent — with its own trigger, scripts, output schema, and possibly its own GitHub App identity — build one from scratch:

.fullsend/
  harness/my-agent.yaml    # Execution config
  agents/my-agent.md       # Agent prompt
  policies/base.yaml       # Sandbox policy
  scripts/pre-my-agent.sh  # Data fetching (before sandbox)
  scripts/post-my-agent.sh # Action execution (after sandbox)

Register it in config.yaml and it runs automatically when matching events arrive.

Best for: entirely new agent roles, custom triggers, specialized output schemas.

Guide: Bring Your Own Agent

See also

• Default, derived, and custom agents — when does configuration cross into custom agent territory?
• Escalation ladder — prove-it path before deriving or replacing a core agent
• Bugfix Workflow — how agents work together end to end