Claude Skills Arguments: Passing Inputs and Testing Them

A Claude skill argument is the text you provide after the slash command when you invoke a skill. The skill file defines the instructions; the argument is the specific input you want the skill to work on. Getting the argument format right is the difference between a skill that produces a reliable result and one that has to be rerun or corrected.

Tools like fenbs keep tested argument templates and lessons about failed invocations in one place, so you do not rediscover the right format for every new session.

How skill arguments work

When you type /skill-name some input text, Claude Code passes everything after the skill name to the session as the argument. The skill file may say how to use the argument — "apply these steps to the pasted text" — or it may leave the handling implicit. If the skill file does not describe how to use the argument, Claude infers what to do from context.

Explicit handling is more reliable than implicit. A skill file that says "The input will follow the skill name. Apply step 1 to the first paragraph, then step 2 to the whole document" leaves less room for misinterpretation than one that says nothing about how input arrives. For where skill files are structured, see claude skills files. For finding skills with good argument handling, see claude skills marketplace.

Ways to pass arguments

The simplest argument is a direct paste: you invoke the skill and paste the text you want processed in the same message. This works for self-contained inputs like a paragraph, a list or a short document.

• Inline paste: /skill-name \n\n[your text here]. Simple and direct. Works when the input is short.
• Label the argument: /skill-name INPUT: [your text]. Adding a label helps when the skill processes multiple named sections.
• Structured input: if the skill expects several fields, pass them with headers — TITLE:, CONTEXT:, TASK:. The skill file should describe the expected structure.
• File reference: in some Claude Code environments you can reference a file path. Check the skill file to see if it supports this.

What makes an argument fail

An argument fails when the skill cannot tell where the instructions end and the input begins, or when the input is in a format the skill was not designed for. The most common failure modes are mixing the argument with additional instructions, passing input that is much longer than the skill expects and passing input in a language or format the skill does not handle.

If a skill produces an unexpected result, reread the skill file first. The issue is often that the skill expects a specific structure the argument did not follow. Change the argument format before changing the skill file.

Testing argument formats

1. Choose a known input: a task you completed recently where you have a good and a poor output to compare against.
2. Try the simplest argument format first: a direct paste with the skill invocation.
3. If the result is not what you expected, add structure: label the argument sections and rerun.
4. Once the output matches your expectation on two or three test inputs, write down the exact format you used.
5. Save that format as a template so you do not have to rediscover it next time.

Validating that the skill processed the argument correctly

After a skill runs, check whether the output reflects the specific input you provided or a generic response that could have been written without it. A skill that produces the same output regardless of the argument has not actually processed it.

Ask Claude to confirm what it read as the argument before proceeding. On a complex input, a brief summary confirms that the right text was received and reduces the chance of acting on a misread.

When the argument format matters most

Argument format has the most impact on tasks where the input has multiple distinct parts: a draft and its feedback, a code snippet and its expected behavior, a question and its context. When the parts are clearly labeled, Claude can address each one specifically rather than treating the whole argument as a single block of text.

Format matters less for simple, self-contained inputs like a single paragraph of prose, a short list or a standalone code snippet. These work well with a direct paste because there is only one thing for the skill to process.

When in doubt, add a label to your argument. A labeled input can always be read as plain text if the label is unnecessary; an unlabeled input with multiple parts requires the skill to infer the structure, which introduces a potential failure point you can avoid with one extra word.

How to do this with fenbs

1. Create a free account and add a project for the work these skills support.
2. For each skill you use regularly, add the skill text to Skills and prompts in fenbs. Below the skill text, add an example of the argument format that works best.
3. When an argument format fails in a way that is worth remembering — a specific input type the skill misreads — add a Lesson in fenbs linked to the skill, describing what failed and the correct format.
4. Connect your AI app over MCP from Integrations. It reads your approved skills and lessons at the start of each session.