Agent skill

Formal Spec Generator

Generate formal specifications (definitions, predicates, invariants, pre/post-conditions) in Isabelle/HOL or Coq from informal requirements, source code, pseudocode, or mathematical descriptions.

Apache-2.0Auto-check passed

Install Formal Spec Generator

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE formal-spec-generator --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/formal-spec-generator .claude/skills/formal-spec-generator && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Paste into Claude Code

Install the "formal-spec-generator" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/formal-spec-generator into .claude/skills/formal-spec-generator/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "formal-spec-generator", then confirm the skill loads.

Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

$skill-installer

$skill-installer install https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/formal-spec-generator

Type this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a codex

Project install goes to .agents/skills/; add -g for ~/.codex/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE formal-spec-generator --agent codex

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .agents/skills && cp -r skills-src/skills/formal-spec-generator .agents/skills/formal-spec-generator && rm -rf skills-src

Use ~/.agents/skills/ instead of .agents/skills for a personal install.

Codex skills documentation · loads skills from .agents/skills/

Paste into Codex

Install the "formal-spec-generator" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/formal-spec-generator into .agents/skills/formal-spec-generator/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "formal-spec-generator", then confirm the skill loads.

Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a cursor

Project install goes to .agents/skills/; add -g for ~/.cursor/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE formal-spec-generator --agent cursor

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/skills/formal-spec-generator .cursor/skills/formal-spec-generator && rm -rf skills-src

Use ~/.cursor/skills/ instead of .cursor/skills for a personal install.

Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/

Paste into Cursor

Install the "formal-spec-generator" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/formal-spec-generator into .cursor/skills/formal-spec-generator/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "formal-spec-generator", then confirm the skill loads.

Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

gemini skills install

$ gemini skills install https://github.com/ArabelaTso/Skills-4-SE.git --path skills/formal-spec-generator

--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a gemini-cli

Project install goes to .agents/skills/; add -g for ~/.gemini/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE formal-spec-generator --agent gemini-cli

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/skills/formal-spec-generator .gemini/skills/formal-spec-generator && rm -rf skills-src

Use ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.

Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/

Paste into Gemini CLI

Install the "formal-spec-generator" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/formal-spec-generator into .gemini/skills/formal-spec-generator/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "formal-spec-generator", then confirm the skill loads.

Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE formal-spec-generator

Installs for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a github-copilot

Project install goes to .agents/skills/; add -g for ~/.copilot/skills/.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .github/skills && cp -r skills-src/skills/formal-spec-generator .github/skills/formal-spec-generator && rm -rf skills-src

Use ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.

GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/

Paste into GitHub Copilot

Install the "formal-spec-generator" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/formal-spec-generator into .github/skills/formal-spec-generator/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "formal-spec-generator", then confirm the skill loads.

GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a opencode

OpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE formal-spec-generator --agent opencode

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/skills/formal-spec-generator .opencode/skills/formal-spec-generator && rm -rf skills-src

Use ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.

OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/

Paste into OpenCode

Install the "formal-spec-generator" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/formal-spec-generator into .opencode/skills/formal-spec-generator/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "formal-spec-generator", then confirm the skill loads.

OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

Facts

Skill name: formal-spec-generator
GitHub stars: 253
Token cost: ~1.5k tokens
SKILL.md length: 561 words
Files: 4 (incl. references)
Skills in repo: 170
Repo updated: Aug 21, 2026
First seen: Oct 7, 2026
Licence: Apache-2.0

At a glance

Generate formal specifications (definitions, predicates, invariants, pre/post-conditions) in Isabelle/HOL or Coq from informal requirements, source code, pseudocode, or mathematical descriptions.

Works in 5 steps: Understand the Input → Choose Target System → Identify Specification Components → …
Formalize algorithms
SKILL.md covers Workflow, Key Principles, Common Patterns and Examples, plus 1 more section
Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Formal Spec Generator is an agent skill from ArabelaTso/Skills-4-SE. Generate formal specifications (definitions, predicates, invariants, pre/post-conditions) in Isabelle/HOL or Coq from informal requirements, source code, pseudocode, or mathematical descriptions. Use when users need to: (1) Formalize algorithms or data structures, (2) Create function specifications with contracts, (3) Generate predicates and properties for verification, (4) Translate informal requirements into formal logic, (5) Specify invariants for loops or data structures, or (6) Create formal definitions for…

Its SKILL.md is about 1.5k tokens, which your agent loads only when the skill is triggered. The skill folder holds 4 other files, including reference files (for example `references/coq_patterns.md`, `references/examples.md` and `references/isabelle_patterns.md`).

The repository describes itself as: A curated list of 180+ useful Claude Skills for Software Engineering and resources for customizing AI for SE workflows. The licence is Apache-2.0.

When your agent uses it

Formalize algorithms
Data structures
Create function specifications with contracts
Generate predicates and properties for verification

Example prompts

“/formal-spec-generator”

Workflow steps

5 steps, taken from the step headings in SKILL.md.

Understand the Input
Choose Target System
Identify Specification Components
Generate Formal Specifications
Structure the Output

What it can do on your machine

Read from SKILL.md and the folder at commit 4f38503. It shows what the files ask for, not the result of running them.

Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md (its code samples are isabelle and coq).
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Formal Spec Generator loads about 1.5k tokens when it runs, and up to ~4.7k if it reads all its reference files. Until then it costs about 152 tokens; SKILL.md has 561 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it: ~152
When it runs · the whole SKILL.md, loaded when a task matches: ~1.5k
With references · SKILL.md plus every file in references/, read only if the agent opens them: ~4.7k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from ArabelaTso/Skills-4-SE at commit 4f38503, republished under its Apache-2.0 licence (© ArabelaTso). 561 words, ~1,508 tokens.

Download SKILL.mdSave it as .claude/skills/formal-spec-generator/SKILL.md (or your agent's skills folder). This skill also uses 3 other files; get the full folder from GitHub.

name: formal-spec-generator
description: Generate formal specifications (definitions, predicates, invariants, pre/post-conditions) in Isabelle/HOL or Coq from informal requirements, source code, pseudocode, or mathematical descriptions. Use when users need to: (1) Formalize algorithms or data structures, (2) Create function specifications with contracts, (3) Generate predicates and properties for verification, (4) Translate informal requirements into formal logic, (5) Specify invariants for loops or data structures, or (6) Create formal definitions for mathematical concepts. Supports both Isabelle/HOL and Coq equally.

Formal Specification Generator

Generate formal specifications in Isabelle/HOL or Coq from informal descriptions, source code, or mathematical statements.

Workflow

1. Understand the Input

Identify what type of input is provided:

Informal requirements: Natural language descriptions (e.g., "a function that sorts a list")
Source code: Existing implementations in Python, C, Java, or other languages
Pseudocode: Algorithmic descriptions with semi-formal structure
Mathematical definitions: Properties or theorems to formalize

2. Choose Target System

Ask the user which formal proof assistant to target:

Isabelle/HOL: Preferred for higher-order logic, functional programming style
Coq: Preferred for constructive logic, dependent types, proof automation
Both: Generate specifications in both systems when requested

If not specified, default to generating both versions.

3. Identify Specification Components

Determine what needs to be formalized:

Function definitions: Type signatures and implementations
Data types: Algebraic data types, records, or inductive types
Predicates: Properties and logical relationships
Pre/post-conditions: Function contracts and correctness specifications
Invariants: Loop invariants or data structure invariants

4. Generate Formal Specifications

Use the reference files for syntax and patterns:

Isabelle patterns: See isabelle_patterns.md
Coq patterns: See coq_patterns.md
Complete examples: See examples.md

Generate specifications that include:

Type definitions for data structures
Function definitions with proper types
Predicates describing properties
Correctness specifications relating inputs to outputs
Theorem statements (proofs can be left as sorry in Isabelle or Admitted in Coq)

5. Structure the Output

Organize the generated specifications clearly:

For Isabelle/HOL:

isabelle

theory TheoryName
  imports Main
begin

(* Data type definitions *)
datatype ...

(* Function definitions *)
fun function_name :: "types" where
  ...

(* Predicates and properties *)
definition property_name :: "type" where
  ...

(* Correctness specifications *)
theorem theorem_name:
  "specification"
  sorry

end

For Coq:

coq

Require Import List Arith.
Import ListNotations.

(* Data type definitions *)
Inductive ...

(* Function definitions *)
Fixpoint function_name ... :=
  ...

(* Predicates and properties *)
Definition property_name ... : Prop :=
  ...

(* Correctness theorems *)
Theorem theorem_name :
  specification.
Proof.
  Admitted.

Key Principles

Completeness

Include all necessary type definitions
Specify both preconditions and postconditions
Define helper predicates when needed
State correctness theorems even if proofs are omitted

Clarity

Use descriptive names for functions and predicates
Add comments explaining non-obvious specifications
Structure code logically (types, then functions, then properties)
Keep specifications close to the informal description

Correctness

Ensure type signatures are accurate
Match the semantics of the informal specification
Use appropriate logical operators (∀, ∃, ⟶, ∧, ∨)
Verify that pre/post-conditions capture the intended behavior

Idiomatic Style

Follow standard conventions for each system
Use built-in libraries (List, Arith, etc.) when available
Prefer simple definitions over complex ones
Use pattern matching for recursive structures

Show full SKILL.md (221 more words)Show less

Common Patterns

From Informal Requirements

When given natural language descriptions:

Extract the function signature (inputs, outputs, types)
Identify preconditions (assumptions about inputs)
Identify postconditions (guarantees about outputs)
Define helper predicates for complex properties
State the correctness theorem

Example: "A function that finds the maximum element in a non-empty list"

Input: list nat (precondition: non-empty)
Output: nat
Postcondition: result is in the list AND result ≥ all elements

From Source Code

When given existing implementations:

Translate the data structures to formal types
Translate the function logic to formal definitions
Infer the implicit preconditions and postconditions
Formalize the expected behavior as predicates
State correctness theorems

From Mathematical Definitions

When given mathematical statements:

Choose appropriate formal types (nat, int, real, etc.)
Translate mathematical notation to formal syntax
Define predicates for mathematical properties
State theorems for mathematical facts

Examples

For complete worked examples including insertion sort, binary search, and stack data structures, see examples.md.

Tips

Start simple: Begin with basic definitions, then add complexity
Use libraries: Import standard libraries (List, Arith, etc.) for common operations
Leave proofs: Focus on specifications; proofs can be sorry/Admitted
Test syntax: Ensure generated code is syntactically valid
Explain choices: Comment on design decisions in the specifications
Both systems: When generating both Isabelle and Coq, ensure semantic equivalence

Files

SKILL.md and 3 other files (references) in skills/formal-spec-generator of ArabelaTso/Skills-4-SE.

SKILL.md
references/coq_patterns.md
references/examples.md
references/isabelle_patterns.md

Open the folder on GitHubat commit 4f38503

Compare with similar skills

Formal Spec Generator next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Formal Spec Generator compared with similar skills
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Questions about Formal Spec Generator

What does Formal Spec Generator do?

Generate formal specifications (definitions, predicates, invariants, pre/post-conditions) in Isabelle/HOL or Coq from informal requirements, source code, pseudocode, or mathematical descriptions. Formal Spec Generator is an agent skill from ArabelaTso/Skills-4-SE. Generate formal specifications (definitions, predicates, invariants, pre/post-conditions) in Isabelle/HOL or Coq from informal requirements, source code, pseudocode, or mathematical descriptions.

When should I use Formal Spec Generator?

Formal Spec Generator fits situations like: formalize algorithms; data structures; create function specifications with contracts; generate predicates and properties for verification.

How do I install Formal Spec Generator in Claude Code?

Run `npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a claude-code`. Or copy the skill folder (skills/formal-spec-generator in ArabelaTso/Skills-4-SE) into .claude/skills/formal-spec-generator in your project. Claude Code loads it when a task matches its description.

How do I install Formal Spec Generator in Codex?

Run `npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a codex`. Or copy the skill folder (skills/formal-spec-generator in ArabelaTso/Skills-4-SE) into .agents/skills/formal-spec-generator in your project. Codex loads it when a task matches its description.

Can I use Formal Spec Generator in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add ArabelaTso/Skills-4-SE --skill formal-spec-generator -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/formal-spec-generator, .gemini/skills/formal-spec-generator, .github/skills/formal-spec-generator and .opencode/skills/formal-spec-generator in your project.

What does Formal Spec Generator need to run?

SKILL.md names no scripts, command-line tools or credentials: Formal Spec Generator is instructions for the agent only.

Does Formal Spec Generator access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Formal Spec Generator safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Formal Spec Generator use?

Formal Spec Generator is published under the Apache-2.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Formal Spec Generator use?

About 1.5k tokens (SKILL.md is roughly 6k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 3.2k tokens, read only when the agent opens those files.

What are the alternatives to Formal Spec Generator?

Skills that share tags, products or a category with Formal Spec Generator: Generate (alirezarezvani/claude-skills, 28k stars), Definition Of Done Generator (jeremylongshore/tons-of-skills-marketplace, 2.8k stars), Fal Generate (nexu-io/open-design, 100k stars) and Video Generation (bytedance/deer-flow, 84k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Formal Spec Generator?

ArabelaTso (a GitHub user) maintains it in ArabelaTso/Skills-4-SE, which has 253 GitHub stars. The repository holds 170 skills in this directory. The repository was last updated on August 21, 2026.

Source: ArabelaTso/Skills-4-SE on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.

Install Formal Spec Generator

Facts

At a glance

What it does

When your agent uses it

Example prompts

Workflow steps

What it can do on your machine

Tool permissions

Runs code

Network

Credentials

Context cost

Safety

SKILL.md

Formal Specification Generator

Workflow

1. Understand the Input

2. Choose Target System

3. Identify Specification Components

4. Generate Formal Specifications

5. Structure the Output

Key Principles

Completeness

Clarity

Correctness

Idiomatic Style

Common Patterns

From Informal Requirements

From Source Code

From Mathematical Definitions

Examples

Tips

Files

Compare with similar skills

Similar skills

More from ArabelaTso/Skills-4-SE

Questions about Formal Spec Generator