Agent harness design

Agent Harness Design

Use when designing or improving how an agent invokes tools, handles errors, and decides when to stop.

When to Use

• Designing tool definitions for a new agent or subagent
• Observing high retry rates or ambiguous tool invocations
• Agent is failing silently or completing without verifying outcomes
• Reviewing an existing agent harness for quality issues

Tool Design Rules

Naming

• Use explicit, stable names: read_file, run_tests, apply_patch
• No generic names: do_action, execute, handle
• One tool per distinct operation; do not overload parameters to compensate

Schema

Keep tool inputs narrow:

• Required fields only; no optional fields that change behavior
• Use enums for mode/type values — never free-text strings that require parsing
• Validate at the boundary; reject malformed input with a clear error, not a fallback

{
  "name": "run_tests",
  "parameters": {
    "path": { "type": "string", "description": "Path to test file or directory" },
    "filter": { "type": "string", "description": "Optional test name filter" }
  }
}

Output Shape

Every tool response must include:

• status: "success" | "warning" | "error"
• summary: one-line result (human-readable)
• next_actions: list of follow-up steps the agent should consider
• artifacts: file paths or IDs produced (empty list if none)

Catch-All Tools

Avoid run_bash / shell_exec style catch-all tools unless:

• The task is genuinely open-ended and the toolset cannot be pre-defined
• You explicitly document the risk and add an allowlist or preflight check

If you must use a catch-all, add a PreToolUse validation hook for dangerous patterns.

Error Path Rules

Every tool must define what happens on failure:

Case	Required response
Invalid input	Reject immediately with status: "error" and exact field name
Transient failure	Include retry_after hint and idempotency note
Non-recoverable	State stop: true and describe the manual resolution step

Do not return partial success with no indication that something failed.

Retry and Stop Conditions

Define retry limits in the harness, not inside tool implementations:

max_retries: 2
stop_conditions:
  - tool returns status: "error" with stop: true
  - same tool called with identical inputs twice in a row
  - completion signal received

Never retry indefinitely. Declare a hard ceiling.

Context Budget

• Keep system prompt fixed and minimal — it is loaded on every turn
• Put large reference material (schemas, docs) in skills loaded on demand
• Compact at phase boundaries (after research, after planning, after debugging)
• Do not pass growing tool-call history to subagents; summarize into a context bundle

Granularity Guide

Risk level	Tool granularity
High (deploy, migrate, permissions)	Micro — one action, one confirmation
Medium (edit, read, search)	Standard — composite is fine
Low (format, report, list)	Macro — batch operations acceptable

Benchmarks to Track

• Completion rate (task finished without escalation)
• Retries per task
• Pass@1 rate (completed on first attempt)
• Cost per successful task

Anti-Patterns

• Overlapping tool semantics (agent cannot choose between them)
• Tool returns only on error — no output on success
• No explicit stop condition — agent loops indefinitely
• Context overload — every tool call inlines full file contents

Verification

• Every tool has an explicit, stable name and exactly one distinct operation
• Schemas use required fields and enums — no free-text mode strings, no behavior-changing optionals
• Every tool response carries status, summary, next_actions, and artifacts
• Every tool defines its invalid-input, transient-failure, and non-recoverable paths
• Retry ceiling and stop conditions are declared in the harness, not inside tools
• Any catch-all tool is documented with its risk and guarded by an allowlist or preflight check