Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add ychampion/cskill-agents --skill fallback-source-presence-sufficiencygit clone --depth 1 https://github.com/ychampion/cskill-agentsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/ychampion/cskill-agents/fallback-source-presence-sufficiency)<a href="https://agentmods.dev/skills/ychampion/cskill-agents/fallback-source-presence-sufficiency"><img src="https://agentmods.dev/badge/skills/ychampion/cskill-agents/fallback-source-presence-sufficiency/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/ychampion/cskill-agents/fallback-source-presence-sufficiency"><img src="https://agentmods.dev/badge/skills/ychampion/cskill-agents/fallback-source-presence-sufficiency.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00034 | $0.00398 |
| Opus 5.5 | $0.00014 | $0.00159 |
| Sonnet 5.5 | $0.00007 | $0.00080 |
| Haiku 4.5 | $0.00003 | $0.00040 |
Grade A, and why
fallback-source-presence-sufficiency scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
What it actually says
SKILL: Fallback Source Presence Sufficiency
Domain: extensions-mcp
Trigger: Apply when a configuration entry represents a fallback source and existing materialized state from another source should be preserved if it already satisfies presence.
Source Pattern: Distilled from reviewed extension lifecycle and source-reconciliation patterns.
Core Method
Distinguish fallback intent from authoritative source declarations. If the declared source exists only to satisfy the missing case, then the presence of any already-materialized source is enough and should be treated as up to date. This prevents the reconciler from overwriting valid state merely because the default source differs from the source that actually materialized the resource.
Key Rules
- Carry a flag that distinguishes fallback/default declarations from authoritative source directives.
- For fallback entries, compare on presence first and skip source-equality enforcement once materialized state exists.
- Preserve seeded, mirrored, or previously installed sources when they already satisfy the resource contract.
- Restrict this relaxation to fallback intent only; authoritative declarations should still drive updates when the source changes.
- Document why fallback entries are special so future maintainers do not "simplify" the logic back into destructive churn.
Example Application
If a package manager uses an official registry as a fallback only when no mirror is installed, leave an already-seeded internal mirror alone. The fallback declaration should not force a re-download from the official registry just because the source URLs differ.
Anti-Patterns (What NOT to do)
- Do not compare fallback declarations against materialized sources as if they were hard directives; you will overwrite healthy state unnecessarily.
- Do not erase the distinction between "default if missing" and "must come from here"; they drive very different reconcile behavior.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 29 lines · 34 tokens per session scan A 00e07994ab48
fallback-source-presence-sufficiency is a skill published in the GitHub repository ychampion/cskill-agents (36 stars, last pushed 6mo ago), licensed MIT. It adds 34 tokens to every session and 398 once invoked, about $0.0001 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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