Service boundary and native Agent Skill

Wires every side effect and native channel as an injectable interface behind a Provider that throws UnimplementedError until the…

Context cost
9.9k tokensestimated from the bundle, loaded when it triggers
Bundle
6 files · 43.8 kB2 scripts among them — read before you run
Licence
MITthe author’s own terms
Last change
—no release on file
Servers it uses
Noneruns standalone

What it does

Wires every side effect and native channel as an injectable interface behind a Provider that throws UnimplementedError until the composition root overrides it, with one live impl per flavor, value-typed signatures returning typed results, hand-written contract-honouring fakes over mocks, MethodChannel quarantined to one lib/native/ directory, and versioned cross-language contracts edited on both sides in one commit.

What this skill changes about the agent has not been set out here as a list. Its SKILL.md, shown below, says it in the author’s own words.

mobile

The skill itself

A skill is instructions the model reads, so you can see what you are getting: below is the start of its SKILL.md. The front matter loads with every session, the body loads when the skill triggers, and the whole file comes with the install.

SKILL.md18.7 kB · preview
--- name: service-boundary-and-native ---
6# Service boundary and native
7
8Every platform capability — reading "now", sharing, logging an event, fetching remote flags, calling a MethodChannel — crosses the app's layers as an **injected interface**. The interface names value types only; the concrete SDK lives in exactly one class; the composition root (main) wires one live impl; tests wire a deterministic fake. **Abstract exactly what cannot run in a test — nothing more.** This is the seam that keeps features pure, SDKs swappable, and the whole app testable headlessly.
9
10Read the reference for the task at hand:
11- references/service-interface.md — typed outcomes vs bool/throw, exhaustiveness, @useResult, hand-written env-fakes, the arrow-callback Future-drop hole, single write path.
12- references/native-channels.md — MethodChannel placement, the cross-language contract ownership table, versioned-file-vs-shared_preferences traps, native-fast-path independence, the round-trip integration_test.
13- references/multi-flavor.md — line-for-line composition roots, build flavors over runtime detection, per-flavor plugin pins, the banned-dependency CI graph gate.
14
15Run scripts/check-service-boundaries.sh and scripts/check-flavor-graph.sh before a PR.
16
17## Non-negotiable rules
18
191. **Every side effect is an injected interface, never a concrete SDK at the call site.** Features, Notifiers, and repositories depend on the *interface*; the app imports the real SDK in exactly one live-impl class. WHY: one seam to swap, one place to quarantine platform quirks, zero SDK reach into pure code.
202. **Abstract only what cannot run in flutter test.** One interface + one real impl + one fake = justified. One interface + one impl + no fake = delete the interface. WHY: interfaces "for symmetry" are dead weight that hide where the real risk (untestable I/O) is.
213. **Interface signatures reference value types only.** Name a DateTime, a domain value object, a sealed outcome — never a plugin type, never a dart:io symbol. WHY: keeps the interface nameable from any layer and the SDK confined to one class.
224. **Every method returns a typed result — failure is control flow, not a swallowed exception.** Model a cancel, an empty fetch, an unavailable target as a sealed outcome or Result<T, F extends Failure>, never a thrown-and-forgotten exception. WHY: Dart requires no throws declaration, so an unhandled failure silently vanishes. See error-handling-typed-results for the Result/Failure spine.
235. **The provider throws UnimplementedError until overridden — the provider IS the DI.** No get_it, no service locator, no second container, no business logic in the provider body. WHY: a forgotten wiring becomes a loud startup failure instead of silent null data or a live SDK opening at import.
246. **Wire exactly one live impl per interface at the composition root.** main (per flavor) is the *only* place a live SDK is constructed; app.dart and every widget below are wiring-blind. WHY: the composition root is the single seam where the real world is chosen.
This is a preview of SKILL.md. Download or install the skill to get the whole thing.
In the file
SKILL.md2,480 words
Files6
LicenceMIT
Why you can read it

Nothing in a skill executes. The client loads the text and the model follows it, so a skill can be audited the way a runbook is — by reading it.

What it costs in context

Skills are not billed by the call. They are paid for in context: every token the instructions occupy is a token your code, your diff and your conversation cannot use. Here is what this one takes and when it takes it.

≈240
always loaded
The name and description, so the model knows the skill exists and when to reach for it.
9,660
on trigger
The instruction body and 5 supporting files, read only when the skill fires.
5.0%
of a 200k window
Ten skills this size would take about 50% of the window before you open a file.
050k100k150k200k context window

9.9k tokens, estimated from the bundle at four bytes to the token, held for the rest of the session once it triggers. Heavy. Teams tend to install this one per project rather than globally, and load it only when the job comes up.

Servers bill, skills cost

A server charges by the month. A skill charges once per session, in context, and then keeps charging it for as long as the session lives.

Before and after

The same question, put to the same model twice: once as it comes, and once with these instructions loaded.

No worked example has been published for this skill yet.

The procedure it runs

The procedure has not been set out here step by step. It is in the body of its SKILL.md: the start of it is shown above, and the whole file comes with the install.

Prose, not code

These steps are written for a model to follow, not executed by a runtime. It can still be told to skip one, and it will say so when it does.

Servers it uses

None. This skill calls no MCP servers at all.

Everything it needs is in the instructions, so it works in a project with nothing connected — the model reads the file and changes how it works with what it can already reach.

Its SKILL.md does not declare which tools it asks the agent for, so what it touches is whatever the instructions themselves ask for — read them before you install it.

What it asks for
Writes filesnot declared
Network accessnot declared

This SKILL.md carries no allowed-tools line, so neither is declared. A skill grants no permissions of its own — it can only ask for tools your client already has.

What it will not do

Every skill is narrow, and the useful ones say where they stop. These are the jobs this one is the wrong tool for.

What this skill is not for has not been published here. Nothing is implied by that: it is a section the author has not filled in.

What is in the bundle

6 files, 43.8 kB on disk. Mostly text — the instructions the model reads — with 2 scripts in it that your client would run only if the instructions tell it to. 2 files of its source are not in the download — listed after these, with why.

  • SKILL.md18.7 kB
  • references/multi-flavor.md5.2 kB
  • references/native-channels.md7.9 kB
  • references/service-interface.md7.8 kB
  • scripts/check-flavor-graph.sh1.4 kB
  • scripts/check-service-boundaries.sh2.8 kB

Not in the download

Not carried is a type a skill download does not hold. Take them from its source.

  • examples/native_gateway.dartnot carried
  • examples/service_boundary.dartnot carried
What is not in it

A skill installs nothing and depends on nothing: it is a folder your client reads. This one carries 2 scripts beside the text, so the download is 6 files — not the whole of its source: 2 files are left out. They are listed under the files, with why. The MIT licence covers the templates and examples as well as the instructions.

Install

Installing copies the bundle into your project. Nothing runs at install time — the files sit on disk until the model reads them.

# Service boundary and native · 9.9k tokens when loaded · macOS, Linux mkdir -p .claude/skills && t=$(mktemp -d) && curl -fsSL -o "$t/bundle.tar.gz" https://mcprush.com/api/skills/service-boundary-and-native/bundle.tar.gz && tar -xzf "$t/bundle.tar.gz" -C "$t" && [ -d "$t/service-boundary-and-native" ] && { [ ! -e .claude/skills/service-boundary-and-native ] || { mv .claude/skills/service-boundary-and-native "$t/.previous" && echo "Previous copy moved to $t/.previous"; }; } && mv "$t/service-boundary-and-native" .claude/skills/ && echo "Installed .claude/skills/service-boundary-and-native"
On Windows, or any OS with Node 20+
npx mcprush@latest skill add zakariaf/service-boundary-and-native

Windows PowerShell says 'running scripts is disabled on this system'? Run Set-ExecutionPolicy -Scope CurrentUser RemoteSigned once, or type npx.cmd instead of npx (codex.cmd, gemini.cmd, grok.cmd likewise). PowerShell 7 needs neither. Writes to .claude/skills/service-boundary-and-native/ in the current project. For every project instead, run mkdir -p ~/.claude/skills && t=$(mktemp -d) && curl -fsSL -o "$t/bundle.tar.gz" https://mcprush.com/api/skills/service-boundary-and-native/bundle.tar.gz && tar -xzf "$t/bundle.tar.gz" -C "$t" && [ -d "$t/service-boundary-and-native" ] && { [ ! -e ~/.claude/skills/service-boundary-and-native ] || { mv ~/.claude/skills/service-boundary-and-native "$t/.previous" && echo "Previous copy moved to $t/.previous"; }; } && mv "$t/service-boundary-and-native" ~/.claude/skills/ && echo "Installed ~/.claude/skills/service-boundary-and-native" (macOS, Linux) — or add --global to the npx line. The curl line replaces .claude/skills/service-boundary-and-native/ only after the download succeeds, so files left there by another skill of the same name, or by an older release, do not mix in. A copy already there is moved aside, not deleted, and the line prints where. Not in this download: 2 files of its source (the full list, and why). Take them from its source.

Which clients pick it up on their own

A skill is a folder of text. Most of these read a skills folder and pick it up without being told; the rest take the same folder another way — an upload in the app, a parameter on an API call — and then choose it by its description just the same.

Claude Code.claude/skills/
Claude DesktopCustomize › Skills
ChatGPTSkills › Create › Upload
Cursor.cursor/skills/
VS Code.github/skills/
Codex CLI.agents/skills/
Gemini CLI.gemini/skills/
Grok Build.grok/skills/
Microsoft CopilotSkills › Upload a skill
PerplexityComputer › Skills › Upload a skill
DeepSeek Harness.dsh/skills/
Zed.agents/skills/
Devin Desktop.devin/skills/
Windsurf (legacy).windsurf/skills/
OpenAI Agents SDKInstructions or sandbox Skills
HTTP / APIa request parameter
This release
Versionnot versioned
Publishedno release date on file
PriceFree
Referencezakariaf/service-boundary-and-native

Versions

Its author publishes no version number, so there is nothing here to pin to: what you install is the folder as it stands today. Instructions change more often than APIs do — a skill can be rewritten entirely without anything it depends on moving.

Pinning

Nothing to pin to: this skill carries no version number of its own. What you install is what the folder holds on the day you install it.

Reviews

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