Prefill and Placeholders

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Normative language (MUST/SHOULD/MAY) follows the conventions defined in Conformance Language.

Overview

Placeholders are template tokens embedded in Skill content that are resolved at runtime. Prefill is a mechanism to pre-execute a tool and embed its result into the Skill before delivery.

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)

Skill is requested (with input parameters)
prefill[] entries are executed in order
Results are stored temporarily
{{input:key}} tokens are replaced with user parameters
{{prefill:ns/tool/name}} tokens are replaced with pre-executed results
{{tool:...}}, {{resource:...}}, {{prompt:...}} tokens are resolved from catalog
{{listName:alias}} tokens are resolved from Shared Lists
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.

Depends on: 00-overview.md, 16-id-schema.md, 14-skills.md
Related: 12-prompt-architecture.md, 17-selections.md, 02-parameters.md