Building a Skill Once, Shipping It to 20 Different Agents

There's a specific kind of arithmetic that makes builders' eyes light up in this category: a skill written once, in a single afternoon, that reaches every developer using any of more than 20 compatible agents and tools — Claude Code, Cursor, Codex CLI, Gemini CLI, GitHub Copilot, OpenClaw among them — rather than the fraction of the market locked into whichever single tool the skill happened to be written for. That's not marketing spin. It's a real structural advantage that didn't exist a year earlier, when a skill built for one agent's proprietary format was, functionally, a skill for that agent alone.

The honest version of this story has two halves, though, and most of the enthusiasm stops at the first one. The leverage is real. The friction that determines whether the leverage actually pays off is more specific, and less discussed, than the "write once, run everywhere" framing suggests.

Why the Leverage Argument Is Genuinely Strong

Compare this to the economics of building software for a single platform's proprietary plugin system, which is the default alternative in most software categories — a Shopify app, a VS Code extension, a Salesforce integration. Each of those markets is real, but building for one means accepting that market's ceiling as your ceiling. A skill built to the SKILL.md convention doesn't make that trade, because the format's simplicity — plain markdown plus optional scripts, no required schema, described earlier in this series — means most compatible agents can read and act on it with little to no adapter work.

That changes the math on whether a given skill idea is worth building at all. A tool that would only be useful to, say, 5% of developers on any single agent might not clear the bar to justify a weekend of work. The same tool, reachable across the combined user base of 20-plus agents, clears that bar easily. Portability doesn't just distribute an existing skill more widely — it makes categories of skill viable to build in the first place that wouldn't have been worth building for a single platform. This is the same dynamic that made cross-platform mobile frameworks valuable a decade earlier: the leverage isn't in the code being reused, it's in projects becoming worth starting that otherwise wouldn't have cleared the bar.

Where "Compatible" Starts Doing a Lot of Work

The friction shows up the moment a skill needs to do more than describe a procedure in prose. The markdown portion of a skill — the description, the step-by-step guidance — genuinely does travel cleanly across agents, because reading and following natural-language instructions is close to a universal capability across every serious agent in the category. The scripts attached to a skill are a different story, and this is where "compatible across 20+ agents" quietly becomes "compatible across 20+ agents, assuming their tool-calling and permission models line up closely enough."

Different agents expose different levels of access to the filesystem, to shell execution, to network calls, and — critically — gate that access through different permission and confirmation flows. A script that assumes it can shell out freely on one agent might hit a permission wall on another that requires explicit user confirmation for the same action, or might simply not have an equivalent capability exposed at all. The markdown layer of a skill is close to perfectly portable. The scripted layer is portable in proportion to how conservatively it was written, and skills that lean hard on scripts to do the actual work — rather than using scripts sparingly, for the narrow slice of a task that genuinely needs determinism, as discussed in the earlier piece on choosing between skills, subagents, hooks, and MCP — hit this friction hardest.

The Testing Burden Nobody Budgets For

Writing a skill once and assuming it works everywhere is a trap that costs creators real credibility when it turns out not to be true. A skill that was only ever tested against one agent, then listed as broadly compatible because the format technically permits it, is exactly the kind of gap between claim and behavior that damages trust — the same failure mode covered in the piece on what makes a skill get reused, just at the portability layer instead of the functional layer.

The honest cost of cross-agent portability isn't in the writing — it's in the testing. A skill creator serious about the 20-plus-agent claim needs to actually verify behavior across a meaningful sample of those agents, not just the one they personally use daily, which is a real and often underestimated time cost relative to the effort of writing the skill in the first place. This is a place where the category still lacks good tooling: there's no standard cross-agent test harness yet that lets a creator run a skill against a dozen different agent runtimes and get a clean pass/fail report, the way cross-browser testing tools solved an analogous problem for the web two decades earlier. Someone building that tooling well has a real business in front of them.

Portability Compounds With Everything Else in This Series

The leverage argument connects directly to the pricing discussion earlier in this series: a creator with real outside options, because their skill is portable across many agents rather than locked to one, has genuine bargaining power in marketplace revenue-split negotiations, which is part of why 70/30 splits have held rather than eroded toward the platform. It connects to discovery too — a skill's addressable market is the combined user base of every compatible agent, which is a stronger position to be discovered from, but only if the marketplace itself is agent-agnostic rather than tied to promoting one vendor's ecosystem over the others.

And it connects to the closing piece in this series, on what the app store era got right and wrong: app stores never had this portability problem in the same way, because an iOS app and an Android app were, for most of the category's history, genuinely different codebases requiring genuinely separate builds. The skill economy's portability story is closer to what the web promised and mostly delivered — write once, run in any standards-compliant environment — which is a stronger analogy than the app store comparison in this one specific respect, even though the app store comparison holds up better elsewhere.

What "Build Once" Actually Requires in Practice

The realistic guidance for a creator chasing this leverage: keep the scripted portion of a skill as thin and conservative as the task allows, lean on the markdown layer to carry as much of the actual instruction as possible since that's the layer with the strongest portability guarantee, and actually test against more than one agent before making a broad compatibility claim rather than assuming the format's theoretical portability translates automatically into practical portability. The 20-plus-agent number is real and it's the single strongest structural argument for building in this category right now — but it's a ceiling the format makes possible, not a guarantee any individual skill gets for free just by being written in markdown.

Part of the "The Skill Economy" series on aiskill.market.