/fp-brief

name: fp-brief
description: "First-principles briefing from technical documents. Use when: understanding why decisions were made, onboarding to feature reasoning, reviewing decision chains, explaining doc from first principles. Not for: PM/CTO summary (use project-brief), pre-doc analysis (use feasibility-study), code explanation (use codex-explain). Output: structured reasoning chain with sensitivity analysis."
allowed-tools: Read, Grep, Glob, Write, Bash(node:*)

First-Principles Briefing Skill

Trigger

• Keywords: first principles, fp brief, why was this decided, reasoning chain, decision sensitivity, explain decisions, assumption analysis, onboarding brief

When NOT to Use

| Scenario | Alternative | |----------|------------| | PM/CTO executive summary (strip technical details) | `/project-brief` | | Pre-doc feasibility analysis (before writing spec) | `/feasibility-study` | | Code explanation at function/file level | `/codex-explain` | | Code architecture overview | `/code-explore` | | Simple document summary | Ask Claude directly |

Command Signature

/fp-brief <doc-path> [--depth brief|normal|deep] [--verify off|codex] [--output <path>] [--no-save]

| Flag | Default | Description | |------|---------|-------------| | `<doc-path>` | Required | Source markdown document path | | `--depth` | `normal` | Output detail level | | `--verify` | `off` | Independent Codex reasoning verification | | `--output` | Same dir, `-fp-brief.md` suffix | Custom output path | | `--no-save` | false | Print to stdout instead of file |

Workflow

sequenceDiagram
    participant U as User
    participant S as /fp-brief
    participant D as Source Doc
    participant O as Output File
    participant X as Codex (optional)

    U->>S: /fp-brief <doc-path> [--depth] [--verify]
    Note over S: Phase 1: Input Resolution
    S->>S: Validate path (repo boundary)
    S->>D: Read source document
    S->>S: Redaction scan (fail-safe)
    S->>S: Auto-detect format (hybrid)
    Note over S: Phase 2: First-Principles Extraction
    S->>S: Extract Root Problem (5-Why)
    S->>S: Build Assumptions Register
    S->>S: Build Reasoning Chain
    S->>S: Build Alternative Rejection Log
    S->>S: Build Decision Sensitivity
    S->>S: Identify Open Unknowns
    Note over S: Phase 3: Output Assembly
    S->>O: Write *-fp-brief.md
    alt --verify codex
        S->>X: Independent reasoning verification
        X-->>S: Verification Delta
        S->>O: Append Verification Delta
    end
    S-->>U: Report complete

Phase 1: Input Resolution

1. **Path validation**: Normalize, reject `..` traversal, enforce repo boundary 2. **Read source document** 3. **Redaction scan**: High-confidence secret patterns → abort; medium → mask `[REDACTED]` 4. **Format auto-detection**: See `references/detection-rules.md` 5. **Select extraction template** based on detected format

Phase 2: First-Principles Extraction

See `references/extraction-guide.md` for section-by-section heuristics.

| Section | Core Question | |---------|--------------| | Root Problem | What fundamental truth makes this problem unavoidable? | | Assumptions Register | What are we taking for granted, and why? | | Reasoning Chain | How does each decision trace back to a principle? | | Alternative Rejection Log | Why do other approaches violate our principles? | | Decision Sensitivity | If assumption X breaks, which decisions collapse? | | Open Unknowns | What don't we know, and what should we find out? |

For long documents (>500 lines): split by `##` headings, extract per-section, merge + dedup.

Phase 3: Output Assembly

1. Apply depth filter (section inclusion matrix) 2. Apply source citations (reference source doc section headings) 3. Apply Evidence Insufficient Rule — never fabricate content for thin sections 4. Write output file (or stdout if `--no-save` — which `--verify codex` rejects, see § Save Behavior) 5. If `--verify codex`: dispatch verification per `references/codex-verify-prompt.md`

Depth Levels

| Level | Description | Sections Included | |-------|-------------|-------------------| | brief | Core reasoning only (~500 words max) | Root Problem (full), Assumptions (top 3), Reasoning Chain (key decisions), Sensitivity (top 3) | | normal | Full reasoning chain (~1500 words max) | All 6 sections with citations | | deep | Full chain + analysis (~2500 words max) | All 6 sections + challenge questions, evidence ratings, counterfactual analysis, risk-weighted unknowns |

Verification Delta (section 7) appears only when `--verify codex` is used, at any depth level.

**Length policy**: These are upper bounds, not targets. If source doc is thin, output will be shorter. The Evidence Insufficient Rule applies: `[Evidence insufficient — source doc lacks data for this section]`.

Output

See `references/output-template.md` for full template.

# First-Principles Briefing: <title>

> Source: <path> | Depth: <level> | Format: <type> | Generated: <timestamp>

## 1. Root Problem
## 2. Assumptions Register
## 3. Reasoning Chain
## 4. Alternative Rejection Log
## 5. Decision Sensitivity
## 6. Open Unknowns
## 7. Verification Delta (optional)

Save Behavior

| Condition | Output Path | |-----------|------------| | Default | Same directory as source, `-fp-brief.md` suffix | | `--output <path>` | Specified path | | `--no-save` | stdout only, no file written. **Incompatible with `--verify codex`** — refuse the combination and say why: the verification prompt is built around `${OUTPUT_PATH}` and instructs Codex to `cat` that file, so with nothing on disk there is no subject to verify. Run them separately, or drop `--no-save` for the verified run |

Example: `docs/features/auth/2-tech-spec.md` → `docs/features/auth/2-tech-spec-fp-brief.md`

Verification

• [ ] Input path validated (repo boundary enforced)
• [ ] Secret redaction scan executed
• [ ] Format auto-detection result shown in output header
• [ ] Eac