cli-forge-lld

name	cli-forge-lld
metadata	{"author":"clement"}
description	Generate a Low-Level Design (LLD) document for a software component or service. Use when the user mentions 'LLD', 'low-level design', 'detailed design', 'class diagram', 'sequence diagram', 'API spec', 'database schema', 'component design', 'module design', 'STRIDE', 'threat model', or asks for detailed technical design of a specific service/component. Produces C4 L3-L4 diagrams, class/sequence/state diagrams, API contracts, DB schemas, error handling, and testability design. Do NOT use for system-level architecture — use cli-forge-hld instead.
argument-hint	[component-or-service-name]
context	fork
agent	general-purpose
allowed-tools	["Read","Write","Grep","Glob","Bash","WebSearch","WebFetch"]

Optimization: This skill uses on-demand loading. Heavy content lives in references/ and is loaded only when needed.

Language rule: Skill instructions are written in English. When generating user-facing output, detect the project's primary language (from README, comments, docs, commit messages) and produce the document in that language. If the project is bilingual, ask the user which language to use before proceeding.

Diacritics rule: When the chosen output language uses diacritics (French, Spanish, Portuguese, etc.), render them correctly — including on capitals (É, À, Ç, Ô…). Never strip accents to ASCII: write "sécurité" not "securite", "Rôle" not "Role". ASCII-flattened French is a defect, not a style.

Writing style: Favor clarity — short sentences, one idea per sentence, plain words. A metaphor only when it clarifies. Keep one consistent register (impersonal/infinitive for operational docs). Separate narrative prose, meant to make the reader understand, from telegraphic content (tables, commands, checklists) which stays terse for action.

No fabricated output: Never invent command output, console captures, API responses, metrics, or logs. When real output is needed, emit a clearly-marked placeholder (🚧 "replace with a real capture"), never plausible-but-fake text presented as real.

Forge LLD — Low-Level / Detailed Design

Translate an HLD's "what" into the "how" — detailed enough that a developer can implement without ambiguity. (Robert C. Martin, Clean Architecture)

Core Principles

1. Mitosis — the document grows only the sections the component actually needs. A stateless function doesn't need a state machine. A read-only service doesn't need a migration plan. Kill empty sections, don't fill them with boilerplate
2. Implementable precision — every included section must be specific enough to code from. No hand-waving
3. Contract-first — define API contracts and interfaces before internals
4. SOLID by default — design visible in class diagrams must respect SOLID; flag violations explicitly
5. Patterns justified — only document a pattern when its use is non-obvious from reading the code
6. KISS — one clear diagram beats two pages of prose. One code example beats three paragraphs of explanation. Readers are developers — they scan for interfaces, schemas, and contracts
7. Gotchas — read ../gotchas.md before producing output to avoid known mistakes

---

Workflow

Step 1 — Identify scope

Signal	Output
"LLD", "low-level design", "detailed design"	Full LLD for a component/service
"class diagram", "module design"	Class/module design section only
"API spec", "contract", "OpenAPI"	API contract section only
"DB schema", "database design", "ER diagram"	Database schema section only
"sequence diagram", "flow"	Sequence diagram for a specific flow
"STRIDE", "threat model"	Security design section only
"state machine", "lifecycle"	State machine section only

Step 2 — Locate the HLD anchor

The LLD must reference its parent HLD:

1. Check if an HLD exists in the project (docs/*hld*, docs/*architecture*, docs/*design*)
2. If yes: identify which HLD container(s) this LLD covers
3. If no: note that this LLD is standalone and may need an HLD later

Step 3 — Gather implementation context

Read existing code if available:

• Source files for the component (understand current structure)
• Cargo.toml / package.json (dependencies, features)
• Config files (understand runtime configuration)
• Existing tests (understand testing approach)
• CI pipeline (understand deployment constraints)

Step 4 — Size the document (Mitosis)

Determine the component's complexity tier. The tier decides which sections to include.

Detect the tier from signals:

Signal	Tier
Pure function, utility module, single-file component	S
Single service with DB, one domain concept	M
Service with external deps, multiple domain concepts, async flows	L
Complex service: multi-aggregate, saga, high concurrency, regulated	XL

Section inclusion matrix:

#	Section	S	M	L	XL
1	Overview & HLD Anchor	Yes	Yes	Yes	Yes
2	Component Architecture (C4 L3)	—	Yes	Yes	Yes
3	Tactical Domain Model	—	If DDD	Yes	Yes
4	Class/Module Design	Key interfaces	Yes	Yes	Yes
5	API Contracts	If API exists	Yes	Yes	Yes
6	Database Schema	If persistence	Yes	Yes	Yes
7	Sequence Diagrams	Main flow only	Happy + error	All critical	All
8	State Machines	—	If >2 states	Yes	Yes
9	Error Handling & Resilience	Error codes only	Yes	Yes	Yes
10	Concurrency Design	—	—	If shared state	Yes
11	Caching Strategy	—	If cache exists	Yes	Yes
12	Component-Level Security (STRIDE)	—	Input validation	Yes	Yes
13	Testability Design	—	DI points only	Yes	Yes
14	Configuration & Feature Flags	—	If non-trivial	Yes	Yes
15	Micro-ADRs	—	If non-obvious	Yes	Yes
16	Migration Plan	—	—	If brownfield	If brownfield

Target document size:

Tier	Pages	Sections	Diagrams
S	1-3	3-5	1-2
M	4-8	7-10	3-5
L	8-15	12-14	5-8
XL	12-20	All 16	8+

Rules:

• Never include an empty section. Omit entirely — don't write "N/A" or "Not applicable"
• Merge small sections for tier S. Class Design + API Contract + DB Schema can be one "Implementation" section
• State the tier at the top so readers calibrate their expectations
• Diagram > prose. If the class diagram already shows the relationships, don't re-explain them in text
• Ask: "Would a developer make a wrong implementation choice without this section?" If no, cut it

Step 5 — Write the LLD

Only include sections selected by the mitosis matrix in Step 4. For tier S, merge related sections freely.

Read references/sections.md for full section templates with Mermaid examples. Only include sections selected by the mitosis matrix.

---

LLD Document Structure

# [Component/Service Name] — Low-Level Design

## 1. Overview & HLD Anchor
## 2. Component Architecture (C4 Level 3)
## 3. Tactical Domain Model
## 4. Class/Module Design
## 5. API Contracts
## 6. Database Schema
## 7. Sequence Diagrams (Critical Flows)
## 8. State Machines
## 9. Error Handling & Resilience
## 10. Concurrency Design
## 11. Caching Strategy
## 12. Component-Level Security (STRIDE)
## 13. Testability Design
## 14. Configuration & Feature Flags
## 15. Micro-ADRs
## 16. Migration Plan

---

HLD vs LLD Boundary

In LLD	NOT in LLD (→ HLD)
C4 Level 3-4 diagrams	C4 Level 1-2 diagrams
Full API contracts (OpenAPI)	API surface list
Table schemas with indexes	Entity-relationship sketch
Class diagrams with methods, SOLID	Container diagram
Circuit breaker config values	"We use circuit breakers"
Component-level STRIDE (SQLi, IDOR, input validation)	System-level trust boundaries, security policies
Retry/backoff configuration	Failure mode categories
Migration scripts / phases	"We'll migrate data"
Tactical DDD (aggregates, entities, value objects)	Strategic DDD (bounded contexts, context maps)
Concurrency design (locks, channels, race conditions)	Scalability strategy (horizontal, sharding)
Testability design (DI points, mock boundaries)	Test strategy overview
Cache TTLs, invalidation patterns	"We need caching for hot data"

If you're drawing system context or estimating capacity, you've drifted into HLD territory. Stop and suggest the user invoke /cli-forge-hld.

---

Quality Scoring

Read references/scoring.md for the full DCI-LLD formula, 12 checklist items with weights, thresholds, and derived metrics.

Anti-Patterns

Read references/anti-patterns.md for the 9 anti-patterns table with descriptions and fixes.

---

Review Checklist

All tiers: Tier stated | HLD referenced | Interfaces defined | Mermaid syntax | No empty sections | No HLD-level content

M+: C4 L3 | API contracts (schemas + errors) | DB schema (ER + indexes) | Sequence diagrams (happy + error) | Error taxonomy

L+: Class diagram (SOLID) | State machines | STRIDE | Testability (DI + mocks) | Micro-ADRs

XL: Concurrency | Tactical DDD | Migration plan (if brownfield)

Size: S: 1-3p | M: 4-8p | L: 8-15p | XL: 12-20p — no section exists just because it's in the template

---

Dynamic Handoffs

Condition detected	Recommend	Why
LLD reveals security threats (STRIDE)	/cli-audit-code on affected components	C9 security check
API contracts defined	/cli-audit-drift	Bootstrap CONTRACTS.md from the LLD
Component has complex state machine	/cli-forge-schema	Generate state diagram

Rule: Recommend, don't auto-execute.

Integration with other cli-* skills

Skill	Relationship
cli-forge-hld	Provides the container architecture that LLD zooms into
cli-forge-schema	Can generate the Mermaid diagrams used in the LLD
cli-audit-code	Validates the implementation matches the LLD design
cli-audit-test	Validates test coverage matches the testability design
cli-cycle	Calls cli-forge-lld as part of full project review

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Reference Sources

Books: Clean Architecture (Martin), DDD (Evans), IDDD (Vernon), Learning DDD (Khononov), PEAA (Fowler), Design Patterns (GoF), Release It! (Nygard), Domain Modeling Made Functional (Wlaschin), Working with Legacy Code (Feathers)

Templates: C4 Model, arc42, MADR, French DAT, awesome-low-level-design, STRIDE