agent-tool-design

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The Agent Tool Contract — 5 principles for designing tools agents call reliably: predictable signature, rich errors, token-efficient output, idempotency, graceful degradation. Includes anti-pattern table with 8 common mistakes.

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npx skill4agent add oimiragieo/agent-studio agent-tool-design

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agent-tool-designai-agenttool-contractidempotencyerror-handling

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Agent Tool Design

The Agent Tool Contract — 5 principles for designing tools that agents call reliably.

The 5 Principles

Principle 1: Predictable Signature

Tools must have typed, named parameters with clear required/optional distinction. No positional ambiguity.
Good:
javascript
// Clear, named, typed
function searchCode({ query, limit = 20, type = 'semantic' }) { ... }
Bad:
javascript
// Positional, ambiguous
function searchCode(q, n, t) { ... }

Principle 2: Rich Errors

Errors must include: error code (machine-readable), message (human-readable), context (debugging data).
Good:
javascript
throw {
  code: 'FILE_NOT_FOUND',
  message: `File not found: ${path}`,
  context: { path, cwd: process.cwd() },
};
Bad:
javascript
throw new Error('not found'); // No context for agent to act on

Principle 3: Token-Efficient Output

Tools return structured minimal data. No prose explanations, no redundant wrapping, no verbose status messages. Agents format output themselves.
Good:
javascript
return { files: ['a.js', 'b.js'], total: 2 };
Bad:
javascript
return { status: 'success', message: 'Found 2 files successfully', data: { files: [...], metadata: {...} } };
Rule of thumb: If the output contains prose an agent would re-read to extract facts, it's too verbose.

Principle 4: Idempotency

Tools must be safe to retry. Running a tool twice should produce the same result as running it once.
Good:
javascript
// Upsert instead of insert
db.upsert({ id, ...data });
// mkdir -p instead of mkdir
fs.mkdirSync(path, { recursive: true });
Bad:
javascript
// Fails on retry
db.insert({ id, ...data }); // duplicate key error
fs.mkdirSync(path); // EEXIST error

Principle 5: Graceful Degradation

Partial success > hard failure. Return what succeeded with a clear indication of what didn't.
Good:
javascript
return {
  succeeded: ['file1.js', 'file2.js'],
  failed: [{ file: 'file3.js', reason: 'PERMISSION_DENIED' }],
  partial: true,
};
Bad:
javascript
// One file fails -> entire batch throws
throw new Error('Failed to process file3.js');

Anti-Pattern Table

Anti-PatternProblemFix
Verbose status wrappingWastes tokens; agent re-parses to extract dataReturn data directly
Positional argsAmbiguous; breaks on refactorNamed params with types
Swallowed exceptionsAgent thinks success; work is lostAlways surface errors explicitly
Non-idempotent mutationsRetry causes duplicate data or errorsUpsert semantics; check-then-set
Hard failures on partial inputOne bad item breaks entire batchReturn partial results
Side-effect-heavy readsRead tools that trigger writes confuse agentsSeparate reads from writes
String error messages onlyAgent can't programmatically handle errorsInclude machine-readable error codes
Untyped return shapeAgent can't reliably destructure outputDocument and enforce return schema

Review Checklist

Before shipping any tool:
[ ] Parameters are named (not positional)
[ ] Required vs optional params are explicit
[ ] All error paths return { code, message, context }
[ ] Output contains no prose — only structured data
[ ] Tool is idempotent (safe to retry)
[ ] Partial failure returns partial results, not throws
[ ] Return shape is documented in JSDoc or TypeScript types
[ ] Token budget for output estimated (< 500 tokens for standard tools)

Iron Laws

  1. ALWAYS use named parameters — never positional arguments in tool signatures; positional args break on refactor and create ambiguity for agents.
  2. ALWAYS include machine-readable error codes — never surface plain string errors only; agents need 
    { code, message, context }
    to handle errors programmatically.
  3. NEVER mix reads and writes in the same tool — read tools that trigger side effects confuse agents and prevent safe retries.
  4. ALWAYS design for idempotency — retry must produce the same result as the first call; use upsert semantics and 
    mkdir -p
    patterns.
  5. ALWAYS return partial results on partial failure — never let one failing item abort the entire batch; return 
    { succeeded, failed, partial: true }
    .

Integration

  • Used by: 
    tool-creator
    skill when designing new tools
  • Reviewed by: 
    code-reviewer
    agent during tool PRs
  • Pairs with: 
    dynamic-api-integration
    skill (consuming external tools)
  • Complements: 
    agent-evaluation
    skill (evaluating tool output quality)

Memory Protocol (MANDATORY)

Before starting: Read 
.claude/context/memory/learnings.md
After completing:
  • New pattern -> 
    .claude/context/memory/learnings.md
  • Issue found -> 
    .claude/context/memory/issues.md
  • Decision made -> 
    .claude/context/memory/decisions.md
ASSUME INTERRUPTION: If it's not in memory, it didn't happen.