| 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, mcp__codex__codex, mcp__codex__codex-reply |
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
- Path validation: Normalize, reject
.. traversal, enforce repo boundary
- Read source document
- Redaction scan: High-confidence secret patterns → abort; medium → mask
[REDACTED]
- Format auto-detection: See
references/detection-rules.md
- 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
- Apply depth filter (section inclusion matrix)
- Apply source citations (reference source doc section headings)
- Apply Evidence Insufficient Rule — never fabricate content for thin sections
- Write output file (or stdout if
--no-save)
- 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 |
Example: docs/features/auth/2-tech-spec.md → docs/features/auth/2-tech-spec-fp-brief.md
Verification
References
- Output template:
references/output-template.md
- Detection rules:
references/detection-rules.md
- Extraction guide:
references/extraction-guide.md
- Codex verification:
references/codex-verify-prompt.md
Examples
Input: /fp-brief docs/features/seek-verdict/2-tech-spec.md
Action: Read spec → detect tech-spec → extract 6 sections → write 2-tech-spec-fp-brief.md
Input: /fp-brief docs/features/auth/2-tech-spec.md --depth brief
Action: Read spec → extract Root Problem + top assumptions + key decisions + top sensitivity → brief output
Input: /fp-brief docs/features/auth/2-tech-spec.md --depth deep --verify codex
Action: Read spec → full extraction → Codex independent verification → write with Verification Delta
Input: /fp-brief notes/design-decisions.md --no-save
Action: Read doc → detect unknown format → generic extraction → print to stdout
