Modular Prompts & Agent Skills

Modular Prompts

Rather than duplicating text across many prompts, compose prompts from reusable, independently versioned fragments: a role definition, a policy block, a format spec. Each fragment can be updated once and propagated everywhere it's used, which is both a maintenance win and a way to keep the cacheable prefix stable -- a change to one fragment doesn't require rewriting the whole prompt.

Agent Skills

Skills package reusable instructions and procedures -- know-how for a specific capability -- so that capability is authored once and reused across applications and teams, without standing up a running service for it. Where modular prompts are about composing a single prompt from fragments, Skills are about packaging a capability so multiple different agents/apps can invoke it without re-implementing the instructions each time.

Reuse as an Affordability Lever

Caching, modular prompts, and Skills are three angles on the same goal: making a well-designed prompt affordable and maintainable at production scale, rather than something that has to be rebuilt or repriced for every new use.

Common exam traps

• Treating modular prompts purely as a code-organization nicety and missing that they also help keep the cacheable prefix stable across revisions.
• Confusing Skills (packaged, reusable instructions/procedures without a running service) with a deployed tool or microservice -- Skills are a prompt/instruction-reuse mechanism, not an infrastructure component.