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Agent skill

Assertions

by onsi in onsi/gomega

Write correct synchronous Gomega assertions — Expect/Ω notation, the To/NotTo/ToNot/Should/ShouldNot equivalences, the multi-return error idiom, Succeed vs HaveOccurred, the .Error() chaining form…

MITAuto-check passedDevelopment

Install Assertions

skills CLI
$ npx skills add onsi/gomega --skill assertions -a claude-code

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

GitHub CLI
$ gh skill install onsi/gomega assertions --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/onsi/gomega.git skills-src && mkdir -p .claude/skills && cp -r skills-src/plugins/gomega/skills/assertions .claude/skills/assertions && 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/

Facts

Skill name
assertions
GitHub stars
2.4k
Token cost
~2.1k tokens
SKILL.md length
660 words
Files
1
Skills in repo
12
Repo updated
First seen
Licence
MIT

At a glance

Write correct synchronous Gomega assertions — Expect/Ω notation, the To/NotTo/ToNot/Should/ShouldNot equivalences, the multi-return error idiom, Succeed vs HaveOccurred, the .Error() chaining form…

  • Reviewing synchronous (non-polling) Gomega assertions
  • SKILL.md covers The two notations are identical, The multi-return error idiom, Succeed and HaveOccurred and The .Error() chaining form, plus 3 more sections
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Assertions is an agent skill from onsi/gomega. Write correct synchronous Gomega assertions — Expect/Ω notation, the To/NotTo/ToNot/Should/ShouldNot equivalences, the multi-return error idiom, Succeed vs HaveOccurred, the .Error() chaining form, annotating assertions (format-string and func()string), tuning failure output via the format subpackage (MaxLength/MaxDepth/UseStringerRepresentation/GomegaStringer/TruncatedDiff/RegisterCustomFormatter/format.Object), and asserting inside helper functions with GinkgoHelper/WithOffset/ExpectWithOffset, NewWithT(t) for…

Its SKILL.md is about 2.1k tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.

It sits in Development. The repository describes itself as: Ginkgo's Preferred Matcher Library. The licence is MIT.

When your agent uses it

  • Reviewing synchronous (non-polling) Gomega assertions

Example prompts

  • “/assertions”

What it can do on your machine

Read from SKILL.md and the folder at commit 37fa900. 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 go).

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    Links to these hosts (documentation or services it may open):

    • onsi.github.io

    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

Assertions loads about 2.1k tokens when it runs. Until then it costs about 161 tokens; SKILL.md has 660 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~161
When it runs · the whole SKILL.md, loaded when a task matches
~2.1k

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 onsi/gomega at commit 37fa900, republished under its MIT licence (© onsi). 660 words, ~2,051 tokens.

Download SKILL.mdSave it as .claude/skills/assertions/SKILL.md (or your agent's skills folder).
name
assertions
description
Write correct synchronous Gomega assertions — Expect/Ω notation, the To/NotTo/ToNot/Should/ShouldNot equivalences, the multi-return error idiom, Succeed vs HaveOccurred, the .Error() chaining form, annotating assertions (format-string and func()string), tuning failure output via the format subpackage (MaxLength/MaxDepth/UseStringerRepresentation/GomegaStringer/TruncatedDiff/RegisterCustomFormatter/format.Object), and asserting inside helper functions with GinkgoHelper/WithOffset/ExpectWithOffset, NewWithT(t) for plain testing, and the g Gomega callback. Use when writing or reviewing synchronous (non-polling) Gomega assertions.

Synchronous assertions

The core mechanics of asserting now (vs. polling — see gomega:async). Assumes a dot-import (. "github.com/onsi/gomega"). Narrative docs: https://onsi.github.io/gomega/#making-assertions. For the big picture and skill map see gomega:overview.

The two notations are identical

go
Expect(ACTUAL).To(Equal(EXPECTED))      // Expect notation
Expect(ACTUAL).NotTo(Equal(EXPECTED))
Expect(ACTUAL).ToNot(Equal(EXPECTED))   // ToNot == NotTo

Ω(ACTUAL).Should(Equal(EXPECTED))       // Ω notation (⌥z on macOS)
Ω(ACTUAL).ShouldNot(Equal(EXPECTED))

To == Should, and NotTo == ToNot == ShouldNot — pure syntactic sugar. ACTUAL (left) is anything; the matcher (right) must satisfy GomegaMatcher. Pick one notation per codebase and stay consistent. Matchers are values — see gomega:matchers.

The multi-return error idiom

Expect/Ω accept multiple arguments. The matcher runs against the first; the assertion fails unless every trailing argument is nil or zero-valued. This collapses Go's (value, error) pattern:

go
// instead of:
result, err := DoSomethingHard()
Expect(err).NotTo(HaveOccurred())
Expect(result).To(Equal("foo"))

// write:
Expect(DoSomethingHard()).To(Equal("foo"))   // passes only if return is ("foo", nil)

Succeed and HaveOccurred

For a function returning only error, use Succeed:

go
Expect(DoSomethingSimple()).To(Succeed())     // func() error
Expect(DoSomethingSimple()).NotTo(Succeed())

For an error value you already hold, use HaveOccurred:

go
err := DoSomethingSimple()
Expect(err).NotTo(HaveOccurred())
Expect(err).To(HaveOccurred())

Use Succeed when calling the function inline; use HaveOccurred when you have an err variable. To assert on the error's content (not just its presence), reach for MatchError and friends in gomega:matchers.

Don't use Succeed with a multi-return function. The matcher only sees the first return value; the rest are consumed by the multi-return idiom above. Expect(DoSomethingHard()).To(Succeed()) matches against the string, not the error, and Expect(DoSomethingHard()).NotTo(Succeed()) can never pass.

The .Error() chaining form

To assert on the trailing error of a multi-return function while ignoring the other returns, chain .Error():

go
Expect(MultipleReturnValuesFunc()).Error().To(HaveOccurred())
Expect(MultipleReturnValuesFunc()).Error().NotTo(HaveOccurred())

.Error() retargets the assertion at the last return value (the error). The To(HaveOccurred()) form additionally asserts that all the other returns are zero-valued; the NotTo(HaveOccurred()) form lets the other returns be anything. The plain alternative is to capture and assert manually:

go
_, _, _, err := MultipleReturnValuesFunc()
Expect(err).To(HaveOccurred())

Annotating assertions

Pass a format string (with fmt.Sprintf args) or a func() string after the matcher. It's printed alongside the failure message to add context:

go
Expect(ACTUAL).To(Equal(EXPECTED), "my annotation %d", foo)
Expect(ACTUAL).To(Equal(EXPECTED), func() string { return expensive() })

The func() string form is lazily evaluated — only called if the assertion fails — so use it for any annotation that's expensive to build.

Adjusting failure output

On failure Gomega prints a recursive rendering of the objects involved, produced by the format subpackage. Tune it via package-level globals (set once, e.g. in a test helper or TestMain):

go
import "github.com/onsi/gomega/format"

format.MaxLength = 4000   // truncate rendered output to N chars; 0 disables truncation
format.MaxDepth = 10      // max recursion depth into nested structures
format.TruncatedDiff = true   // for long strings, show only where they differ (false = full strings)
format.UseStringerRepresentation = false  // true => call String()/GoString() on Stringer/GoStringer types
format.PrintContextObjects = false        // true => print contents of context.Context objects

UseStringerRepresentation defaults to false on purpose: a String() method often hides fields you need to diagnose a failure. Leave it off unless you know the stringer is complete.

GomegaStringer interface. Implement GomegaString() string on a type and Gomega always uses it for that type's rendering, regardless of UseStringerRepresentation. Best practice: define it in a _test.go helper file so you don't leak it into your package's exported API.

go
func (w Widget) GomegaString() string { return fmt.Sprintf("Widget<%s>", w.id) }

Custom formatters. Register a format.CustomFormatter (func(value any) (string, bool)) — return (rendered, true) to handle a value, or ("", false) to pass. Custom formatters take precedence over GomegaStringer and UseStringerRepresentation, and their output is not truncated:

go
key := format.RegisterCustomFormatter(myFormatter)
defer format.UnregisterCustomFormatter(key)

format.Object. Reuse Gomega's renderer directly (the int is indentation depth):

go
fmt.Println(format.Object(theThing, 1))
Show full SKILL.md (221 more words)Show less

Making assertions inside helper functions

Factoring assertions into a helper reduces boilerplate, but a naive helper reports failures at the line inside the helper, not the caller. Fix the reported line number with one of these.

With Ginkgo — GinkgoHelper() (recommended). Call it first thing in the helper; Ginkgo then attributes failures to the call site. It nests cleanly across multiple helper layers:

go
func expectComponents(te TurboEncabulator, components ...string) {
    GinkgoHelper()
    got, err := te.GetComponents()
    Expect(err).NotTo(HaveOccurred())
    Expect(got).To(ConsistOf(components))
}

Without Ginkgo — WithOffset / ExpectWithOffset. The offset is how many stack frames to skip; 1 points at the helper's caller. These are equivalent:

go
Expect(err).WithOffset(1).NotTo(HaveOccurred())
ExpectWithOffset(1, err).NotTo(HaveOccurred())

(Eventually/Consistently have the same WithOffset / …WithOffset forms — see gomega:async.)

Plain testing — NewWithT(t). Wrap a *testing.T to get a *WithT carrying Expect, Eventually, and Consistently. Create a fresh one per test (no global fail handler needed):

go
func TestFarmHasCow(t *testing.T) {
    g := NewWithT(t)
    f := farm.New([]string{"Cow", "Horse"})
    g.Expect(f.HasCow()).To(BeTrue(), "farm should have a cow")
}

The g Gomega callback pattern. A helper can accept a Gomega and assert through it, decoupling the helper from how failures are reported:

go
func expectValidWidget(g Gomega, w Widget) {
    g.Expect(w.ID).NotTo(BeEmpty())
    g.Expect(w.Size).To(BeNumerically(">", 0))
}

g := NewWithT(t)        // or the g passed into an Eventually callback
expectValidWidget(g, w)

This is the same g Gomega that Eventually(func(g Gomega) {...}) passes into polled callbacks (gomega:async) — so a g Gomega helper works in both synchronous and polled contexts. To build a Gomega wired to your own fail handler, use NewGomega(failHandler).

Don't call the global Expect/Ω inside an Eventually/Consistently callback — those failures won't be intercepted by the poller. Always assert through the passed-in g Gomega there. See gomega:async.

For richer, reusable assertions consider a real matcher instead of a helper — see gomega:custom-matchers.

© onsi, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

Just SKILL.md in plugins/gomega/skills/assertions of onsi/gomega.

Open the folder on GitHubat commit 37fa900

Compare with similar skills

Assertions 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.

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PR Babysitteropeninterpreter/openinterpreter69k3 repos~4.2kAutomated safety check: PassApache-2.0
Code Review ChecklistshareAI-lab/learn-claude-code78k5 repos~1.1kAutomated safety check: PassMIT

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Categories

Questions about Assertions

What does Assertions do?

Write correct synchronous Gomega assertions — Expect/Ω notation, the To/NotTo/ToNot/Should/ShouldNot equivalences, the multi-return error idiom, Succeed vs HaveOccurred, the .Error() chaining form…. Assertions is an agent skill from onsi/gomega.

When should I use Assertions?

Assertions fits situations like: reviewing synchronous (non-polling) Gomega assertions.

How do I install Assertions in Claude Code?

Run `npx skills add onsi/gomega --skill assertions -a claude-code`. Or copy the skill folder (plugins/gomega/skills/assertions in onsi/gomega) into .claude/skills/assertions in your project. Claude Code loads it when a task matches its description.

How do I install Assertions in Codex?

Run `npx skills add onsi/gomega --skill assertions -a codex`. Or copy the skill folder (plugins/gomega/skills/assertions in onsi/gomega) into .agents/skills/assertions in your project. Codex loads it when a task matches its description.

Can I use Assertions 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 onsi/gomega --skill assertions -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/assertions, .gemini/skills/assertions, .github/skills/assertions and .opencode/skills/assertions in your project.

What does Assertions need to run?

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

Does Assertions access the network?

SKILL.md names 1 domain. As links in the text: onsi.github.io. This is read from the text; nothing was executed.

Is Assertions 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 Assertions use?

Assertions 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 Assertions use?

About 2.1k tokens (SKILL.md is roughly 8.2k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Assertions?

Skills that share tags, products or a category with Assertions: Vercel Composition Patterns (supabase/supabase, 111k stars), Finishing a Development Branch (obra/superpowers, 296k stars), Typescript Advanced Types (rolling-scopes/rsschool-app, 10k stars) and PR Babysitter (openinterpreter/openinterpreter, 69k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Assertions?

onsi (a GitHub user) maintains it in onsi/gomega, which has 2,357 GitHub stars. The repository holds 12 skills in this directory. The repository was last updated on September 24, 2026.

Source: onsi/gomega on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.