devils-advocate

devils-advocate is a skill for Claude Code, Codex from haabe/mycelium. It costs 27 tokens per session (2,573 once invoked), scanned A, original, MIT.

A structured way to question the assumptions behind an important decision before acting on it. It uses failure planning, opposite-case questions, and deliberate criticism from viewpoints such as users, competitors, security reviewers, and accessibility advocates.

In plain words
What is it for?
Use it before major product, architecture, or planning decisions to look for failure causes, test assumptions, and identify concerns your team may have missed.
Why use it?
It helps expose confirmation bias, groupthink, overconfidence, ignored evidence, and weak assumptions before they cause costly problems.

Skill for Claude CodeCodex

Installs and runs on its own, but its text points at files inside its plugin — anything it tells you to read at a ${CLAUDE_PLUGIN_ROOT} path is only there once the plugin is installed. Installing the plugin gets both.

Part of the mycelium plugin — 54 skills, 6 hooks shipped together

Install

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.

agentmods
npx agentmods add skills/haabe/mycelium/devils-advocate
Any agent
npx skills add haabe/mycelium --skill devils-advocate
Clone the repo
git clone --depth 1 https://github.com/haabe/mycelium

Made for: Claude Code, Codex.

Or install mycelium, the plugin that ships this one along with the rest of its 54 skills, 6 hooks.

Wrote 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.

agentmods badge for devils-advocate

README.md
[![agentmods](https://agentmods.dev/badge/skills/haabe/mycelium/devils-advocate.svg)](https://agentmods.dev/skills/haabe/mycelium/devils-advocate)
Your own site
<a href="https://agentmods.dev/skills/haabe/mycelium/devils-advocate"><img src="https://agentmods.dev/badge/skills/haabe/mycelium/devils-advocate.svg" alt="Measured on agentmods" height="20"></a>
Per session 27 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 2,573 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5 $0.00027 $0.02573
Opus 5 $0.00014 $0.01287
Sonnet 5 $0.00005 $0.00515
Haiku 4.5 $0.00003 $0.00257

Measured today against content hash 651da22cec45, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

devils-advocate 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 today.

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.

plugins/mycelium/skills/devils-advocate/SKILL.md · 124 lines

How it starts

The opening of the file, as written. The whole thing — 124 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Devil's Advocate

Run before every major diamond transition and architecture decision. Source: Kahneman, Shotton.

Technique 1: Pre-Mortem

Imagine it's 6 months from now and this decision FAILED spectacularly.

  1. What went wrong?
  2. What assumption was the weakest link?
  3. What signal did we ignore?
  4. Who was affected and how?

Technique 2: Assumption Reversal

For each key assumption:

  • State the assumption explicitly
  • Ask: "What if the OPPOSITE is true?"
  • What evidence would support the opposite?
  • Is there any evidence we've dismissed?

Technique 3: Red Team

Attack your own position:

  • What would a competitor say about this approach?
  • What would a skeptical user say?
  • What would a security auditor find?
  • What would an accessibility advocate flag?

10 Challenge Questions

  1. What are we most confident about? (That's where overconfidence hides)
  2. What evidence have we dismissed or downweighted?
  3. Are we anchored on our first idea? (Shotton - anchoring bias)
  4. Have we tested with users who DON'T match our ideal profile?
  5. What would make us abandon this direction entirely?
  6. Are we building for ourselves or for actual users?
  7. What's the simplest version that could validate/invalidate this?
  8. What have we NOT measured that we should?
  9. If we had to start over, would we make the same choice?
  10. Who disagrees with us and what's their strongest argument?

Technique 4: Attribution-vs-Consistency Check

Per anti-pattern Consistency-as-Evidence (#7) — graduated 2026-05-09 from a recurring failure where causal chains were built from observational consistency rather than verified attribution.

For each piece of evidence supporting the current claim, label it:

  • Cleanly-attributed: the cause was demonstrably driving the effect (the action was Mycelium-specific, the variable was isolated, the alternative explanations were ruled out).
  • Consistency-only: the data is compatible with the hypothesis but doesn't isolate the cause (the user reported X in a context where Y was also true; the trend matches the prediction but matches three other predictions equally well).
  • Unrelated: the evidence is a different question entirely; don't include it in the chain.

Read the full file on GitHub · 124 lines

Changes

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.

  1. today Changed · +2 lines 651da22cec45
  2. 4d ago First seen · 122 lines · 27 tokens per session scan A 97c404fce8d6

Subscribe to this mod's changes

devils-advocate is a skill published in the GitHub repository haabe/mycelium (45 stars, last pushed today), licensed MIT. It adds 27 tokens to every session and 2,573 once invoked, about $0.0001 per session on Opus 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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