A skill for designing the tool interface of an agent system: tool descriptions, input schemas, response formats, naming, error messages, and MCP servers. MCP is a standard interface that lets agents call tools.

Use it to create or review tool contracts, consolidate overlapping tools, design MCP interfaces, and write errors that help agents recover.

It helps make tools clear enough for an AI agent to select and use correctly, reducing ambiguous requests and malformed calls.

tool-design — Skill for Claude Code and Codex by navendubrajesh

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add navendubrajesh/context-management-for-agents --skill tool-design

Clone the repo

git clone --depth 1 https://github.com/navendubrajesh/context-management-for-agents

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/navendubrajesh/context-management-for-agents/tool-design/github.svg)](https://agentmods.dev/skills/navendubrajesh/context-management-for-agents/tool-design)

Your own site

<a href="https://agentmods.dev/skills/navendubrajesh/context-management-for-agents/tool-design"><img src="https://agentmods.dev/badge/skills/navendubrajesh/context-management-for-agents/tool-design/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/navendubrajesh/context-management-for-agents/tool-design"><img src="https://agentmods.dev/badge/skills/navendubrajesh/context-management-for-agents/tool-design.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 86 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,270 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00086	$0.01270
Opus 5.5	$0.00034	$0.00508
Sonnet 5.5	$0.00017	$0.00254
Haiku 4.5	$0.00009	$0.00127

Measured 2mo ago against content hash 11894c344b92, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

tool-design scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/tool-design/SKILL.md · 102 lines

How it starts

The opening of the file, as written. The whole thing — 102 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Tool Design for Agents

Design every tool as a contract between a deterministic system and a non-deterministic agent. Unlike human-facing APIs, agent-facing tools must make the contract unambiguous through the description alone: agents infer intent from descriptions and generate calls that must match expected formats. Every ambiguity becomes a potential failure mode that no amount of prompt engineering can fix.

The unit of work for this skill is a single tool or a tool catalog. Project-shape, pipeline architecture, task-model-fit, and cost-at-the-project-level decisions belong to project-development. Deciding whether to introduce sub-agents belongs to multi-agent-patterns.

When to Activate

Activate this skill when the unit of work is a tool:

Writing a new tool description, schema, or response format.
Debugging cases where the agent picked the wrong tool or generated malformed calls.
Consolidating an overlapping tool catalog.
Designing actionable error messages so the agent can self-correct.
Naming tools and parameters consistently across a catalog (MCP namespacing, verb-noun naming).
Evaluating a third-party tool against the consolidation principle before adding it.

Do not activate this skill for adjacent work owned by other skills:

Deciding whether the project should use LLMs at all, or what the pipeline stages should be: project-development.
Deciding whether to split work across sub-agents or run a single agent with more tools: multi-agent-patterns.
Reducing the token weight of tool outputs at the trajectory level: context-optimization.
Headless browser automation for QA or scraping: GStack /browse — centralize Chromium there; do not embed puppeteer per tool (see GStack ETHOS).

Core Concepts

Design tools around the consolidation principle: if a human engineer cannot definitively say which tool should be used in a given situation, an agent cannot be expected to do better. Reduce the tool set until each tool has one unambiguous purpose.

Read the full file on GitHub · 102 lines

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

references/tool-contracts.md 995 B

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

2mo ago First seen · 102 lines · 86 tokens per session scan A 11894c344b92

Subscribe to this mod's changes

tool-design is a skill published in the GitHub repository navendubrajesh/context-management-for-agents (2 stars, last pushed 1mo ago), licensed MIT. It adds 86 tokens to every session and 1,270 once invoked, about $0.0003 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

Building agents

tool-design next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
tool-designnavendubrajesh/context-management-for-agents	Skill	2	1mo ago	MIT	A	86	Claude Code, Codex
imagegenopenai/codex	Skill	133k	yesterday	Apache-2.0	A	113	Codex
playwright-climicrosoft/playwright	Skill	99k	yesterday	Apache-2.0	A	19	Claude Code
local-ai-agentsmicrosoft/ai-agents-for-beginners	Skill	79k	4d ago	MIT	A	200	Claude Code, Codex
systematic-debuggingobra/superpowers	Skill	313k	3d ago	MIT	A	21	Claude Code
next-cache-components-adoptionvercel/next.js	Skill	143k	yesterday	MIT	A	95	Claude Code
next-partial-prefetching-adoptionvercel/next.js	Skill	143k	yesterday	MIT	A	103	Claude Code