Fizz Convert
pashov/skills
Convert English-language properties in PROPERTIES.md (produced by the Fizz skill) into Solidity assertions inside the existing fuzz harness, then flip their checkboxes.
EVM contract error and calldata decoder. An agent skill from sd0xdev/sd0x-harness.
Install Contract Decode
$ npx skills add sd0xdev/sd0x-harness --skill contract-decode -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install sd0xdev/sd0x-harness contract-decode --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/sd0xdev/sd0x-harness.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/contract-decode .claude/skills/contract-decode && rm -rf skills-srcUse ~/.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/
Install the "contract-decode" agent skill from https://github.com/sd0xdev/sd0x-harness/tree/main/skills/contract-decode into .claude/skills/contract-decode/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "contract-decode", 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 install https://github.com/sd0xdev/sd0x-harness/tree/main/skills/contract-decodeType 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.
$ npx skills add sd0xdev/sd0x-harness --skill contract-decode -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install sd0xdev/sd0x-harness contract-decode --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/sd0xdev/sd0x-harness.git skills-src && mkdir -p .agents/skills && cp -r skills-src/skills/contract-decode .agents/skills/contract-decode && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "contract-decode" agent skill from https://github.com/sd0xdev/sd0x-harness/tree/main/skills/contract-decode into .agents/skills/contract-decode/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "contract-decode", 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.
$ npx skills add sd0xdev/sd0x-harness --skill contract-decode -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install sd0xdev/sd0x-harness contract-decode --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/sd0xdev/sd0x-harness.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/skills/contract-decode .cursor/skills/contract-decode && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "contract-decode" agent skill from https://github.com/sd0xdev/sd0x-harness/tree/main/skills/contract-decode into .cursor/skills/contract-decode/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "contract-decode", 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 https://github.com/sd0xdev/sd0x-harness.git --path skills/contract-decode--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add sd0xdev/sd0x-harness --skill contract-decode -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install sd0xdev/sd0x-harness contract-decode --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/sd0xdev/sd0x-harness.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/skills/contract-decode .gemini/skills/contract-decode && rm -rf skills-srcUse ~/.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/
Install the "contract-decode" agent skill from https://github.com/sd0xdev/sd0x-harness/tree/main/skills/contract-decode into .gemini/skills/contract-decode/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "contract-decode", 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.
$ gh skill install sd0xdev/sd0x-harness contract-decodeInstalls 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).
$ npx skills add sd0xdev/sd0x-harness --skill contract-decode -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/sd0xdev/sd0x-harness.git skills-src && mkdir -p .github/skills && cp -r skills-src/skills/contract-decode .github/skills/contract-decode && rm -rf skills-srcUse ~/.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/
Install the "contract-decode" agent skill from https://github.com/sd0xdev/sd0x-harness/tree/main/skills/contract-decode into .github/skills/contract-decode/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "contract-decode", 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.
$ npx skills add sd0xdev/sd0x-harness --skill contract-decode -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install sd0xdev/sd0x-harness contract-decode --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/sd0xdev/sd0x-harness.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/skills/contract-decode .opencode/skills/contract-decode && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "contract-decode" agent skill from https://github.com/sd0xdev/sd0x-harness/tree/main/skills/contract-decode into .opencode/skills/contract-decode/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "contract-decode", 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
contract-decode- GitHub stars
- 192
- Token cost
- ~1.3k tokens
- SKILL.md length
- 372 words
- Files
- 2 (incl. references)
- Skills in repo
- 91
- Repo updated
- First seen
- Licence
- MIT
At a glance
EVM contract error and calldata decoder. An agent skill from sd0xdev/sd0x-harness.
- Works in 4 steps: Classify Input → Local Fast Decode (no external… → External API Query → …
- : user pastes hex revert data
- SKILL.md covers Trigger, When NOT to Use, Input Parsing and Workflow, plus 4 more sections
- Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
What it does
Contract Decode is an agent skill from sd0xdev/sd0x-harness. EVM contract error and calldata decoder. Use when: user pastes hex revert data, calldata, function selector, or mentions custom error, execution reverted, 4byte, decode. Input contains 0x + 8+ hex chars.
Its SKILL.md is about 1.3k tokens, which your agent loads only when the skill is triggered. The skill folder holds 2 other files, including reference files (for example `references/apis.md`).
It sits in Backend & APIs, covering Smart contracts. The repository describes itself as: The harness layer for Claude Code — a reference implementation of harness engineering with hook-enforced dual review, state-machine gates that survive context compaction, and… The licence is MIT.
When your agent uses it
- : user pastes hex revert data
- Function selector
- Mentions custom error
- Execution reverted
Example prompts
- “/contract-decode”
Requirements
- Pre-approved tools (allowed-tools): Read, Grep, Glob, Bash, WebFetch
Workflow steps
4 steps, taken from the step headings in SKILL.md.
What it can do on your machine
Read from SKILL.md and the folder at commit c9a2036. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves these tools, so the agent can use them without asking each time:
ReadGrepGlobBashWebFetch
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 bash and markdown).
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
Contract Decode loads about 1.3k tokens when it runs, and up to ~2.9k if it reads all its reference files. Until then it costs about 55 tokens; SKILL.md has 372 words of instructions outside code blocks.
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: notes
The automated check noted patterns worth knowing about, such as sudo or a known installer.
- NotePre-approves every shell command (allowed-tools: Bash)SKILL.md
allowed-tools: Read, Grep, Glob, Bash, WebFetch
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 sd0xdev/sd0x-harness at commit c9a2036, republished under its MIT licence (© sd0xdev). 372 words, ~1,314 tokens.
Download SKILL.mdSave it as
.claude/skills/contract-decode/SKILL.md (or your agent's skills folder). This skill also uses 1 other file; get the full folder from GitHub.- name
- contract-decode
- description
- EVM contract error and calldata decoder. Use when: user pastes hex revert data, calldata, function selector, or mentions custom error, execution reverted, 4byte, decode. Input contains 0x + 8+ hex chars.
- allowed-tools
- Read, Grep, Glob, Bash, WebFetch
- context
- fork
Contract Decode — EVM Error & Calldata Decoder
Decode EVM contract function selectors, custom errors, calldata, and revert data.
Trigger
- Keywords: revert, selector, 4byte, calldata, decode, custom error, execution reverted, abi decode
- Message contains
0x+ 8+ hex chars (selector or revert data)
When NOT to Use
- Standard
Error(string)revert already handled by your codebase - Non-EVM chain errors
- Only need error code definitions (not hex decoding)
Input Parsing
Extract from user input:
| Field | Source | Example |
|---|---|---|
revertData | Error data or user-pasted hex | 0xaca553e4 |
calldata | Transaction data | 0x5e15c749000...1c05 |
contractAddr | Contract address | 0x5874...f064 |
chainId | Chain identifier | 1 (Ethereum mainnet) |
Workflow
Step 1: Classify input → Step 2: Local fast decode → Step 3: ABI query → Step 4: Precise decode → ReportStep 1: Classify Input
| Length | Classification | Next Step |
|---|---|---|
4 bytes (0x + 8 hex) | Pure selector | Step 2 |
| > 4 bytes, with selector prefix | Calldata or revert data | Step 2 + Step 3 |
| Contract address | Need ABI first | Step 3 |
Step 2: Local Fast Decode (no external dependencies)
Try in order:
2a. Standard error decode
| Selector | Type | Decode Method |
|---|---|---|
0x08c379a0 | Error(string) | cast abi-decode "Error(string)" <data> |
0x4e487b71 | Panic(uint256) | cast abi-decode "Panic(uint256)" <data> → lookup panic code |
Panic code reference:
| Code | Meaning |
|---|---|
| 0x00 | generic compiler panic |
| 0x01 | assert failure |
| 0x11 | arithmetic overflow |
| 0x12 | division by zero |
| 0x21 | enum conversion |
| 0x22 | storage encoding |
| 0x31 | pop empty array |
| 0x32 | array out of bounds |
| 0x41 | too much memory |
| 0x51 | zero function pointer |
2b. Selector lookup (cast) — optional, skip if cast not installed
timeout 5 cast 4byte <selector>If cast is unavailable or crashes, fall back to Step 3 API query.
Show full SKILL.md (142 more words)Show less
Step 3: External API Query
See references/apis.md for full endpoints and parameters.
Query strategy:
Has contract address? → 3a. ABI path (precise) → get ABI → Step 4
No contract address? → 3b. Selector DB path (candidates) → get signature candidates3a. ABI path (has contract address + chainId)
- Sourcify (free, no key) → get full ABI JSON
- Etherscan v2 (needs key, free tier 3 req/s) → get ABI JSON
- If proxy → get implementation address → re-query ABI
3b. Selector DB path (no contract address or ABI query failed)
- 4byte.directory API → may return multiple candidates
- Multiple candidates → mark
confidence: low, list all possibilities
Step 4: Precise Decode
With ABI, use cast (if available):
# Decode revert data (needs ABI file)
cast decode-error <revert_data> --abi <abi_file>
# Decode calldata
cast decode-calldata <calldata> --abi <abi_file>
# Or decode with signature directly
cast calldata-decode "functionName(type1,type2)" <calldata>Without ABI but with signature candidates:
cast abi-decode "functionName(type1,type2)" <data_without_selector>Output Format
## Contract Decode Report
| Field | Value |
|-------|-------|
| Type | function call / revert error / event |
| Selector | `0x5e15c749` |
| Signature | `finalizeWithdrawal(uint256)` |
| Decoded Args | `tokenId: 7173` |
| Confidence | High (verified ABI) / Medium (selector DB) / Low (multiple candidates) |
| Source | Sourcify / Etherscan / 4byte.directory / cast |
| Contract | `0x5874...f064` |
| Chain | Ethereum Mainnet (chainId: 1) |
### Raw Data
- Revert: `0xaca553e4`
- Calldata: `0x5e15c749000...1c05`Multiple candidates:
### Ambiguous Candidates
| # | Signature | Confidence |
|---|-----------|------------|
| 1 | `WithdrawalRequestDoesNotExist()` | Likely (context match) |
| 2 | `SomeOtherError()` | Unlikely |References
| File | Purpose | When to Read |
|---|---|---|
references/apis.md | API endpoints, parameters, rate limits | Before Step 3 |
Verification
- Input correctly classified (selector / calldata / revert data)
- Local fast decode attempted (Error/Panic/cast)
- External query returned results or marked as failed
- Confidence level indicated
- Multiple candidates listed (if any)
Examples
Input: Decode 0xaca553e4
Action: 4 bytes → selector → cast 4byte → 4byte.directory → return candidate signaturesInput: What is this revert data 0x08c379a0000...
Action: selector = Error(string) → local cast abi-decode → return error messageInput: Decode this with contract 0x5874..., chainId 1, revert data 0xaca553e4
Action: Has contract + chainId → Sourcify ABI → cast decode-error → precise decode© sd0xdev, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
SKILL.md and 1 other file (references) in skills/contract-decode of sd0xdev/sd0x-harness.
- SKILL.md
- references/apis.md
Open the folder on GitHubat commit c9a2036
Compare with similar skills
Contract Decode 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.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Contract Decode this skillsd0xdev/sd0x-harness | 192 | — | ~1.3k | Automated safety check: Notes | MIT | |
| Fizz Convertpashov/skills | 1.2k | 2 repos | ~3.7k | Automated safety check: Pass | MIT | |
| Solana Devsolana-foundation/solana-dev-skill | 571 | — | ~3.8k | Automated safety check: Pass | MIT | |
| Feynman Auditor0xiehnnkta/nemesis-auditor | 244 | 1 repos | ~11k | Automated safety check: Pass | MIT | |
| Smart Contract Auditgreatpie/smart-contract-audit-skill | 101 | — | ~1.1k | Automated safety check: Pass | None | |
| RadarAuditware/radar | 154 | — | ~2.1k | Automated safety check: Pass | GPL-3.0 |
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Questions about Contract Decode
What does Contract Decode do?
EVM contract error and calldata decoder. An agent skill from sd0xdev/sd0x-harness. Contract Decode is an agent skill from sd0xdev/sd0x-harness. EVM contract error and calldata decoder.
When should I use Contract Decode?
Contract Decode fits situations like: : user pastes hex revert data; function selector; mentions custom error; execution reverted.
How do I install Contract Decode in Claude Code?
Run `npx skills add sd0xdev/sd0x-harness --skill contract-decode -a claude-code`. Or copy the skill folder (skills/contract-decode in sd0xdev/sd0x-harness) into .claude/skills/contract-decode in your project. Claude Code loads it when a task matches its description.
How do I install Contract Decode in Codex?
Run `npx skills add sd0xdev/sd0x-harness --skill contract-decode -a codex`. Or copy the skill folder (skills/contract-decode in sd0xdev/sd0x-harness) into .agents/skills/contract-decode in your project. Codex loads it when a task matches its description.
Can I use Contract Decode 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 sd0xdev/sd0x-harness --skill contract-decode -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/contract-decode, .gemini/skills/contract-decode, .github/skills/contract-decode and .opencode/skills/contract-decode in your project.
What does Contract Decode need to run?
SKILL.md names no scripts, command-line tools or credentials: Contract Decode is instructions for the agent only. Its frontmatter pre-approves these tools: Read, Grep, Glob, Bash, WebFetch.
Does Contract Decode 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 Contract Decode safe to install?
Our automated static check of SKILL.md found notes only (pre-approves every shell command (allowed-tools: bash)), nothing it rates as a warning. It is not a guarantee. Review the folder before installing.
What licence does Contract Decode use?
Contract Decode is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Contract Decode use?
About 1.3k tokens (SKILL.md is roughly 5.3k 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 1.6k tokens, read only when the agent opens those files.
What are the alternatives to Contract Decode?
Skills that share tags, products or a category with Contract Decode: Fizz Convert (pashov/skills, 1.2k stars), Solana Dev (solana-foundation/solana-dev-skill, 571 stars), Feynman Auditor (0xiehnnkta/nemesis-auditor, 244 stars) and Smart Contract Audit (greatpie/smart-contract-audit-skill, 101 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Contract Decode?
sd0xdev (a GitHub user) maintains it in sd0xdev/sd0x-harness, which has 192 GitHub stars. The repository holds 91 skills in this directory. The repository was last updated on October 6, 2026.
Source: sd0xdev/sd0x-harness on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.