8.3 — Agentic Customisation

CCDV-F Domain 8 · Task 8.3

What You Need to Know

Four mechanisms: built-in tools, custom tools, Skills, MCP servers. This sub-skill is choosing between them.

Built-In Tools

No code, no handler, no deployment. Server-executed ones leave no execution to manage. When a built-in fits, it is the cheapest correct answer. Cost is reach: a server-executed tool runs in the platform’s environment — nothing connects it to a database on your private subnet.

Built-in ≠ Anthropic-executed. Client tools have an Anthropic-defined schema but your application handles execution.

Skill or Tool: Ask What Is Missing

Missing	Mechanism
Information it cannot reach, or an action it cannot perform	Tool
Knowledge — does the task inconsistently, wrong format, wrong conventions	Skill

Instructions are not access. On the Claude API, Skills run in a sandboxed container with no network access. A Skill that documents a private system still does not perform the lookup.

Progressive Disclosure

Level	When loaded	Token cost
Metadata (name + description)	Always	~100 tokens per Skill
Instructions (SKILL.md body)	When triggered	Under 5k
Resources (bundled files/scripts)	As needed	None until accessed

Description is the trigger — must say both what the Skill does and when to use it.

Custom Tool or MCP Server: Reuse and Ownership

Three facts decide:

1. How many applications/surfaces need this capability?
2. Should it be maintained independently of any one application’s release cycle?
3. Who owns the underlying system, and should they own the integration?

One application, one team → custom tool. Several surfaces, or system-owning team should own the integration → MCP server.

The Four Postures That Recur (Wrong)

1. Pasting data into context on every request (stale snapshot; wastes context)
2. Hard-coding the procedure into every system prompt (copies drift)
3. Assuming a built-in tool can reach an internal service (platform environment)
4. Reaching for fine-tuning (wrong class of problem for knowledge packaging or tooling)

Running the Decision

1. Does the platform already provide this? → built-in
2. Does the model need to DO something, or KNOW how? → tool vs Skill
3. How many consumers? Independent ownership? → custom tool vs MCP
4. Mix of requirements → assign each the right mechanism; uniformity is almost always wrong