The SKILL.md format is the detail that makes Ask Mio’s Agent Skills feature worth paying attention to beyond its immediate usefulness: it’s not a proprietary file format locked to one product. As the features page puts it, “Skills written in the open SKILL.md format (the same one Claude, Gemini and Cursor use) are loaded and can be picked like any other skill.” One of the 9 “How Mio works” features, Agent Skills is built on a format that already has adoption beyond Ask Mio itself.
That’s worth pausing on before getting into the mechanics, because it changes the calculation around building something custom on top of an AI assistant — the usual worry is that time invested in customising one platform is time you’d lose the moment you switch to another. An open, shared format changes that math.
What a skill file actually is
A SKILL.md file is a structured Markdown document that describes an area of expertise: what it’s for, when it should be used, and instructions the model should follow while operating under it. Think of it as a written brief you’d hand a knowledgeable colleague before asking them to help with a specific kind of task — except it’s persistent, reusable, and the same file can be loaded by more than one AI assistant, since the format itself isn’t tied to any single vendor’s implementation. A skill for, say, reviewing legal contracts under a specific jurisdiction’s conventions, or following a company’s exact style guide for customer replies, can be written once as a SKILL.md file and used consistently rather than re-explained in every new conversation.
How a skill gets loaded
Skills sit in your Skills list and get activated either by explicitly selecting one before starting a conversation — turning Mio into that specific expert for the whole chat — or, if you have Automatic skill turned on, by Mio recognising which skill best fits your question and loading it itself, telling you which one it picked rather than answering as a generalist by default. Once loaded, the skill’s instructions shape how Mio approaches the conversation for as long as it’s active — the same underlying model, but operating under a specific, written brief instead of general-purpose defaults.
Adding your own skill
You can add a skill from the Skills page in your account, either by writing one directly (name it, describe when it should be used and what it must do — which is also exactly what the separate skill creator job automates from a conversation) or by importing an existing SKILL.md file. Because the format is shared, a skill written for another assistant that already supports SKILL.md can often be imported directly rather than rebuilt from scratch — the practical value of “open format” made concrete rather than abstract.
Why an open format matters if you ever leave
This is the part that’s easy to undervalue until it actually matters. Most AI product features — a custom GPT, a proprietary “persona” system, a vendor-specific automation — live entirely inside that vendor’s platform. The moment you want to switch products, or just use a second one alongside the first, that customisation doesn’t travel with you; you rebuild it, often imperfectly, in whatever format the new platform uses instead. A skill built as a SKILL.md file doesn’t have that problem in the same way, because the format itself — not just the idea behind it — is shared across at least Claude, Gemini, Cursor and Ask Mio. Time spent writing a good skill file is portable in a way a locked-in equivalent isn’t.
This matters most for anything with real effort behind it: a skill encoding a company’s specific compliance requirements, a detailed domain expertise built up over many revisions, or a coding convention a whole team has agreed to follow. Building that once, in a format more than one tool can read, is a materially different investment than building it inside a single vendor’s walled garden.
What’s actually inside a SKILL.md file
At a structural level, a SKILL.md file typically includes a short description of the skill’s purpose (used both for humans browsing a skill list and for automatic matching, since Mio’s Automatic skill feature reads this to decide relevance), the operating instructions the model should follow while the skill is active, and any specific constraints, formats or conventions the skill should enforce — things like “always cite a source,” “respond in formal register,” “structure output as the following sections.” It’s plain-text Markdown, which is precisely why it’s practical as a shared standard: no proprietary schema, no special editor required, readable and editable by a person or another tool without translation.
Comparison: skill formats and portability
| Approach | Works across multiple AI assistants | Editable as plain text | Requires vendor lock-in |
|---|---|---|---|
| SKILL.md (Ask Mio, Claude, Gemini, Cursor) | Yes | Yes | No |
| Proprietary “custom persona/GPT” feature | No | Sometimes, within that platform’s editor | Yes |
| A saved prompt template you re-paste manually | Yes, trivially | Yes | No, but no automatic loading or matching |
| A fine-tuned or custom-trained model | No | No | Yes, and far more costly to build |
A worked example
Say your team has a specific way it wants customer support replies written — a fixed tone, a required structure (acknowledge, explain, resolve, offer follow-up), and a rule to never promise a refund without escalation. Written as a skill, that becomes a SKILL.md file with the tone and structure spelled out as instructions and the refund rule as an explicit constraint. Load that skill for support conversations and every reply follows the same standard, without re-explaining the rules each time — and because it’s a standard file, the same skill definition could, in principle, also be loaded into another SKILL.md-compatible assistant your team happens to use for a different purpose, without rewriting the brief from scratch.
Automatic skill and how it uses the same files
The related Automatic skill feature reads your available SKILL.md files’ descriptions to decide, per question, whether one fits better than answering generically — and tells you which one it picked, so the choice is never silent. This is covered in more depth in the next guide in this series, but it’s worth noting here because it’s the mechanism that makes having several skills genuinely useful rather than something you have to remember to switch manually.
External context: SKILL.md as an emerging shared standard
When more than one independent product — built by different companies, targeting different users — agrees to read the same file format for the same purpose, that’s the early shape of a de facto standard, the way plain-text formats like Markdown itself or JSON became defaults not because a single company mandated them but because enough tools agreed to support them. SKILL.md sits in that lineage: a specification for describing an AI assistant’s task-specific expertise as a Markdown document, readable and editable without special tooling, and increasingly recognised by more than one assistant. It’s worth being precise that this is a young, evolving area rather than a decades-old fixed standard — but the direction, more tools reading the same format rather than fewer, is the one that benefits anyone actually investing time in writing skills.
For anyone evaluating AI tools for a team, this is a genuinely practical consideration, not just a technical detail: a feature that stores its customisation in an open, portable format is a smaller commitment than one that locks the same customisation inside a single vendor’s proprietary system, because switching or adding a second tool doesn’t mean starting over.
Common mistakes when writing a skill
The most frequent one is writing a skill that’s too broad to be useful — “be a helpful business expert” doesn’t give Mio anything more specific to work from than its own defaults already provide. A good skill narrows scope deliberately: a specific domain, a specific format, specific rules to follow or avoid. The second is forgetting to write a clear description of when the skill should apply, which matters most if you’re using Automatic skill — a vague description makes it harder for Mio to correctly match a question to the right skill versus another one you’ve written, or versus no skill at all. The third is treating a skill as a one-time setup rather than something to refine — the first version rarely captures every edge case, and revising a skill after seeing how it actually performs in real conversations is normal, not a sign it was written wrong the first time.
Where this fits
Agent Skills is one of 9 features under “How Mio works” — the switchable, account-level parts of the product, alongside Headroom, Setup assistant and Document memory. It’s distinct from the pre-built 39 experts covered in the first run of this series, which are ready-made rather than something you author yourself. Every guide in this batch is collected under the “What Mio can do” tag.
A brief note on where the idea comes from
Structured briefs that shape how an AI model approaches a task aren’t a new idea in themselves — system prompts and instruction files have existed in various forms since the earliest general-purpose assistants. What SKILL.md represents is that idea becoming a named, shared, file-based convention rather than something each product implements its own incompatible way. That shift — from “every tool has its own private way of doing this” to “several tools read the same file” — is the specific thing worth understanding, more than the file format’s syntax itself. It’s still early days for this kind of standardisation across AI assistants generally, and which conventions end up widely adopted will keep evolving, but the direction favours anyone who’s already invested time writing a skill rather than anyone trying to lock them into one product.
Frequently Asked Questions
Is SKILL.md an Ask Mio invention?
No — it’s an open format also used by Claude, Gemini and Cursor, per the features page. Ask Mio supports it rather than defining a competing proprietary one.
Can I import a skill file written for another assistant?
Because the format is shared, a SKILL.md file written for another compatible tool can generally be imported directly through the Skills page rather than rewritten from scratch — the specifics depend on how closely that file follows the standard structure.
Do I need to know Markdown to write a skill?
Basic Markdown helps if you’re writing the file by hand, but you don’t need to — the skill creator job builds a SKILL.md file from a plain-language description of the expert you want, without you writing the file yourself.
How is a skill different from a system prompt I write myself in a chat?
A skill is persistent and reusable across conversations and loadable by name (or automatically), while a system prompt typed into one chat applies only there and has to be re-entered every time.
Can a skill be shared with teammates?
On plans with team seats, a skill can be part of a shared team setup rather than staying private to one person — check your specific plan’s team features on the pricing page.
What happens if two skills could both apply to a question?
With Automatic skill enabled, Mio picks the closest match and tells you which one it chose; you can always override by explicitly selecting a different skill yourself.
Is Agent Skills available on the Free plan?
Bigger account-level features under “How Mio works” vary by plan — check the pricing page for what’s included at each tier.
The Bottom Line
Agent Skills matters twice over: it makes Mio genuinely specialised for a repeated task, and it does so through a format — SKILL.md — that isn’t locked to Ask Mio alone, which means the work of writing a good skill isn’t wasted if your tooling changes later. Add a skill from the Skills page, or import one already written elsewhere, and treat the first version as a draft you’ll sharpen once you see how it performs in real conversations. See the full list of “How Mio works” features or start on the Chat plan.