Prefill and Placeholders

Skill content carries two complementary template mechanisms that the runtime processes before handing the Skill to an agent. Placeholders are tokens embedded directly in the content — {{type:reference}} — that are substituted with tool descriptions, resource references, user input, or shared-list values at resolution time. Prefill goes one step further: a Skill can declare tool calls in a prefill array that the runtime executes up front, then embeds the live results into the content via {{prefill:...}} tokens, so the agent receives a Skill already populated with current data.

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Placeholder Syntax

All placeholders use double-brace syntax: {{type:reference}} or {{type:reference:field}}.

Complete Placeholder Table

Syntax	Resolves To
{{tool:ns/name}}	Complete tool block (description + parameters)
{{tool:ns/name:description}}	Tool description only
{{tool:ns/name:parameters}}	Parameter table only
{{tool:ns/name:test}}	First test case as example call
{{tool:ns/name:meta}}	Meta flags (isReadOnly, isDestructive, etc.)
{{tool:ns/name:call}}	Ready-to-use call command
{{resource:ns/name}}	Resource reference
{{prompt:ns/name}}	Prompt reference
{{skill:name}}	Skill reference (1 level, no nesting)
{{input:key}}	User input parameter
{{prefill:ns/tool/name}}	Pre-executed tool result
{{listName:alias}}	Shared List value via alias

Prefill Declaration

In a Skill, prefill is declared in the prefill array:

export const skill = {
    name: 'contract-analysis',
    prefill: [
        {
            tool: 'etherscan-io/tool/getNetworkStatus',
            params: { network: '{{input:chain}}' }
        }
    ],
    content: `
Network status: {{prefill:etherscan-io/tool/getNetworkStatus}}

Now analyze contract at {{input:address}}...
`
}

Resolution Flow (8 Steps)

1. Skill is requested (with input parameters)
2. prefill[] entries are executed in order
3. Results are stored temporarily
4. {{input:key}} tokens are replaced with user parameters
5. {{prefill:ns/tool/name}} tokens are replaced with pre-executed results
6. {{tool:...}}, {{resource:...}}, {{prompt:...}} tokens are resolved from catalog
7. {{listName:alias}} tokens are resolved from Shared Lists
8. Fully resolved Skill content is delivered to the agent

Error Handling

Principle: Always deliver the Skill. Errors are visible in the placeholder, not silent.

If a placeholder cannot be resolved, the error message replaces the placeholder:

{{tool:unknown/ns/missingTool}} → [ERROR: Tool 'unknown/ns/missingTool' not found]
{{prefill:ns/tool/name}} → [ERROR: Prefill execution failed: HTTP 503]

This ensures the Skill is always delivered — even if some references are broken.

Related

• ./00-overview.md — see chapter 00.
• ./02-parameters.md — see chapter 02.
• ./12-prompt-architecture.md — see chapter 12.
• ./14-skills.md — see chapter 14.
• ./16-id-schema.md — see chapter 16.
• ./17-selections.md — see chapter 17.