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Embeddable Wasm sandbox for running untrusted mruby code from Ruby applications.

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# Kobako

[![Ask DeepWiki](https://deepwiki.com/badge.svg)](https://deepwiki.com/elct9620/kobako)

Kobako is a Ruby gem that embeds a Wasm-isolated mruby interpreter inside your application, so you can execute untrusted Ruby scripts (LLM-generated code, user formulas, student submissions, third-party plugins) in-process without giving them access to host memory, files, network, or credentials. Its home is the Ruby ecosystem; a Rust SDK offers the same sandbox to hosts written in other languages.

The host (`wasmtime`) runs a precompiled `kobako.wasm` guest containing mruby and a Transport proxy. The only way a guest script can reach the outside world is through Host App-declared **Services** — named host objects you explicitly inject into the sandbox; the guest sees each one as a proxy that forwards calls back to the host over the Transport wire.

```
        Host process                       Wasm guest
   ┌──────────────────────┐         ┌──────────────────────┐
   │  Kobako::Sandbox     │ ─eval─▶ │  mruby interpreter   │
   │                      │ ─run──▶ │                      │
   │  Services            │ ◀─call─ │  KV::Lookup.call(k)  │
   │   KV::Lookup         │ ─resp─▶ │                      │
   │                      │         │                      │
   │  Execution           │ ◀─pipe─ │  puts / warn         │
   │   value              │ ◀─last─ │  last expression     │
   │   stdout / stderr    │         │                      │
   │   usage              │         │                      │
   └──────────────────────┘         └──────────────────────┘
            trusted                       untrusted
```

## Requirements

- **Ruby ≥ 3.3.0**
- **Rust / Cargo** at install time — the native extension compiles from source via `rb_sys`
- **Linux** or **macOS** — Windows is not supported

The precompiled `kobako.wasm` Guest Binary ships inside the gem, so end users do **not** need a WASI toolchain. (The toolchain is only required if you build the gem from a source checkout — see [Development](#development).)

## Installation

```bash
bundle add kobako
# or
gem install kobako
```

## Quick Start

The same untrusted script runs identically from either host frontend. It executes inside the Wasm guest and cannot read your filesystem, open sockets, or touch your environment.

```ruby
require "kobako"

sandbox = Kobako::Sandbox.new
sandbox.eval("1 + 2").value  # => 3
```

```rust
use kobako::{Options, Sandbox};

// Options::default() is secure by default: no caps, hermetic isolation.
let sandbox = Sandbox::new("kobako.wasm", Options::default())?;
sandbox.eval("1 + 2")?.value()?;  // => Value::Int(3)
```

Each invocation returns the record of that one run — `Kobako::Execution` in Ruby, `Execution` in Rust — carrying the guest value alongside the run's output and resource usage. Nothing a run observes is stored on the Sandbox, so the same Sandbox serves concurrent invocations without them seeing each other.

The gem bundles its Guest Binary; a Rust host loads one explicitly — see [Frontends & Guest Binaries](#frontends--guest-binaries) for the packages and the Guest Binary variants.

## Frontends & Guest Binaries

Embedding kobako is two independent choices: the **host frontend** you build against, and the **Guest Binary** it runs. They compose freely — any frontend loads any Guest Binary, so a Ruby host can run a JSON-enabled guest and a Rust host can run the pure default.

How far down those choices go — and which ones a given starting point quietly makes for you — is laid out in [`docs/architecture.md`](docs/architecture.md).

### Host frontends

The host embeds the sandbox and owns the SPEC wire codec. Choose by your host language and how much of the contract you want pre-assembled.

| Frontend | Package | Add it | Best for |
|----------|---------|--------|----------|
| Ruby gem | `kobako` (RubyGems) | `gem install kobako` | A Ruby host — Services, Handles, snippets, and pooling out of the box |
| Rust SDK | `kobako` (crates.io) | `kobako = "0.12"` | A Rust host — the same behavior contract behind an idiomatic Rust API |
| Low-level crates | `kobako-wasmtime` + `kobako-runtime` + `kobako-transport` + `kobako-codec` | Cargo deps | A custom host, or driving the wire directly in another language |

The Rust crates are documented on [crates.io](https://crates.io/crates/kobako); the Ruby gem is this README. Three runnable Rust hosts show the choice: [`plugin-rs`](examples/plugin-rs) builds on the SDK, [`wire-rs`](examples/wire-rs) assembles a host by hand on the low-level crates, and [`fixed-schema-rs`](examples/fixed-schema-rs) replaces what both of those keep — the payload schema — with a guest and a host that agree on protobuf.

### Pre-built Guest Binaries

The gem bundles the pure `kobako.wasm`. Regexp and JSON are opt-in capabilities compiled into separate variants, each attached to every [GitHub Release](https://github.com/elct9620/kobako/releases). Download a variant and point your host at it — `Sandbox.new(wasm_path:)` in Ruby, `Sandbox::new(path, ...)` in Rust.

| Variant | File | Adds | Distribution |
|---------|------|------|--------------|
| Pure (default) | `kobako.wasm` | mruby + IO | Bundled in the gem, and a Release asset |
| +regexp | `kobako+regexp.wasm` | Regexp / MatchData (ASCII) | Release asset |
| +regexp-unicode | `kobako+regexp-unicode.wasm` | Regexp / MatchData (Unicode) | Release asset |
| +json | `kobako+json.wasm` | JSON | Release asset |
| +full | `kobako+full.wasm` | JSON + Regexp (ASCII) | Release asset |

```ruby
sandbox = Kobako::Sandbox.new(wasm_path: "kobako+full.wasm")
sandbox.eval('JSON.generate({ n: "42".to_i })').value  # => "{\"n\":42}"
```

### Custom Guest Binaries

When no pre-built variant matches your capability set, assemble a guest in Rust. `kobako-mruby` is the harness; its `init_gems` hook is where you install exactly the capability gems you want — the shipped `kobako-io` / `kobako-regexp` / `kobako-json`, or your own `beni::Gem`. `kobako-core`'s `export_guest!` emits the wasm ABI. `wasm/kobako-wasm/src/guest.rs` is the worked example.

| Guest crate | Role |
|-------------|------|
| `kobako-mruby` | mruby guest harness — the `MrbGuest` trait and provided flows |
| `kobako-core` | Guest ABI contract — the `Guest` trait and the `export_guest!` macro |
| `kobako-io` | IO / Kernel capability gem |
| `kobako-regexp` | Regexp / MatchData capability gem |
| `kobako-json` | JSON capability gem |

```rust
use beni::{Error, Mrb};

struct MyGuest;

// Pick the capability gems the guest exposes.
impl kobako_mruby::MrbGuest for MyGuest {
    fn init_gems(mrb: &Mrb) -> Result<(), Error> {
        mrb.init_gem::<kobako_io::KobakoIo>()?;
        mrb.init_gem::<kobako_json::KobakoJson>()?;
        Ok(())
    }
}

// Forward the ABI contract to the harness flows.
impl kobako_core::Guest for MyGuest {
    fn eval() { <MyGuest as kobako_mruby::MrbGuest>::eval() }
    fn run(env: &[u8]) { <MyGuest as kobako_mruby::MrbGuest>::run(env) }
    fn yield_to_block(req: &[u8]) -> u64 { <MyGuest as kobako_mruby::MrbGuest>::yield_to_block(req) }
}

kobako_core::export_guest!(MyGuest);
```

Build the crate as a `cdylib` for `wasm32-wasip1`, then bake the canonical boot state into the artifact (see [`CLAUDE.md`](CLAUDE.md) § Build Pipeline).

## Glossary

| Term | Meaning |
|------|---------|
| Sandbox | The reusable unit (`Kobako::Sandbox`) that runs guest code and returns a result or raises a typed error. It holds configuration only — no state from any run. |
| Service | A host object bound at a constant-path name (`MyService::KV`) — the guest's only path to host resources. |
| Invocation | One `#eval` or `#run`; capability state is scoped to it and ends with it. |
| Execution | The frozen record one invocation returns (`Kobako::Execution`): its `#value`, output captures, and `#usage`. A failed run raises, carrying the same record on the error's `#execution`. |
| Context | The per-invocation object the optional `#eval` / `#run` block receives; its `ctx.bind` supplies a Service object for that one run. |
| Snippet | Named mruby code (source or bytecode) replayed into a fresh state before every invocation. |
| Handle | An opaque token the guest holds for a host object the wire cannot transmit directly. |
| Block | A guest mruby block passed to a Service; each `yield` is a synchronous round-trip into the guest. |

## Usage

### Services

`bind` any Ruby object as a Service at a constant-path name; the guest reaches it as a `MyService::KV` (or top-level `File`) proxy and invokes its public methods through the Transport wire. See [`docs/behavior/registration.md`](docs/behavior/registration.md) B-08..B-12.

```ruby
class User
  attr_reader :name

def initialize(name:)
    @name = name
  end
end

sandbox.bind("Project::User", User.new(name: "alice"))
sandbox.bind("KV::Lookup",     ->(key) { redis.get(key) })

sandbox.eval(<<~RUBY)
  Project::User.name         # => "alice"
  KV::Lookup.call("user_42") # => "..."
RUBY
```

Each `::`-separated path segment must match `/\A[A-Z]\w*\z/`. Symbol kwargs travel transparently to the host method's keyword arguments. The registry seals at the first invocation (see [Invocation Lifecycle](#invocation-lifecycle)); later `#bind` raises `ArgumentError`.

### Per-Invocation Bindings

A setup-time `bind` fixes one object for the Sandbox's life. When the object belongs to a single run instead — the current request, the acting user, a per-tenant store — declare the path at setup and fill it per invocation. `bind(path)` with no object reserves the name as a *fillable*: the guest sees the constant, while an unfilled dispatch fails closed as `Kobako::ServiceError`. The optional `#eval` / `#run` block fills it ([`docs/behavior/registration.md`](docs/behavior/registration.md) B-62..B-63).

```ruby
sandbox.bind("Req::Current")  # declared, unfilled — stands for Kobako::Unresolved

sandbox.eval("Req::Current.user_id") { |ctx| ctx.bind("Req::Current", request) }
```

`ctx.bind` also shadows an already-bound path for that one run. The Context is spent once the block returns, so a `ctx` captured out of it raises `ArgumentError`. Choosing the object per invocation instead of sharing one is what lets concurrent Threads invoke a single Sandbox and still keep their identities apart — the [multi-tenant example](examples/multi-tenant/) runs that shape end to end.

### Output Capture

Guest writes through `puts` / `print` / `p` / `$stdout` / `$stderr` are buffered per-channel and read off the run's Execution, independently of its `#value` ([`docs/behavior/lifecycle.md`](docs/behavior/lifecycle.md) B-04). Each invocation captures its own; overflow is clipped at the cap and flagged by `#stdout_truncated?` / `#stderr_truncated?`.

```ruby
execution = sandbox.eval(<<~RUBY)
  puts "hello"
  warn "be careful"
  42
RUBY

execution.value   # => 42
execution.stdout  # => "hello\n"
execution.stderr  # => "be careful\n"
```

### Error Handling

Every invocation either returns an Execution or raises exactly one of three classes, so you can route faults without inspecting messages. The full taxonomy lives in [`lib/kobako/errors.rb`](lib/kobako/errors.rb).

```ruby
begin
  sandbox.eval(script)
rescue Kobako::TrapError
  # Wasm engine fault or cap exhaustion. Discard the Sandbox.
rescue Kobako::ServiceError
  # A host Service call failed and the script did not rescue it.
rescue Kobako::SandboxError => e
  # The script raised, failed to compile, or returned an unrepresentable value.
  logger.warn(e.execution.stderr)  # the failed run's own captures and usage
end
```

Each of these carries the failed run's Execution on `#execution`, so a rescue reads that run's output and usage exactly as a successful caller reads them off the returned one. `#failed?` keeps the two apart when both `#value` are `nil` — a script whose last expression was `nil` versus one that never produced a value. A failure caught before the guest ran at all — a host pre-flight refusal — leaves `#execution` `nil`, since there is no run to report.

| Class                           | Parent         | Trigger                                              |
|---------------------------------|----------------|------------------------------------------------------|
| `Kobako::TimeoutError`          | `TrapError`    | Per-invocation `timeout` exhausted                   |
| `Kobako::MemoryLimitError`      | `TrapError`    | Per-invocation `memory_limit` exhausted              |
| `Kobako::HandleExhaustedError` | `SandboxError` | Handle counter reached its 2³¹ − 1 cap               |
| `Kobako::BytecodeError`         | `SandboxError` | `#preload(binary:)` failed RITE validation at replay |
| `Kobako::UndefinedEntrypointError` | `SandboxError` | `#run` named a constant no snippet defined; carries `#name` and `#available` |

`SandboxError` and `ServiceError` carry structured `origin` / `klass` / `backtrace_lines` fields when the guest produced a panic envelope.

### Resource Limits

Each invocation enforces a wall-clock `timeout` and a per-invocation linear-memory `memory_limit`; exhaustion raises a `TrapError` subclass. Pass `nil` to `timeout` / `memory_limit` to disable that cap. Read [`Execution#usage`](lib/kobako/execution.rb) for actual consumption — populated on every outcome, so a rescued trap reports it just as a completed run does ([`docs/behavior/lifecycle.md`](docs/behavior/lifecycle.md) B-35).

```ruby
sandbox = Kobako::Sandbox.new(
  timeout:      5.0,              # seconds, default 60.0
  memory_limit: 10 * 1024 * 1024, # bytes,   default 1 MiB
  stdout_limit: 64 * 1024,        # bytes,   default 1 MiB
  stderr_limit: 64 * 1024
)
```

| Cap            | Raises                     | Default |
|----------------|----------------------------|---------|
| `timeout`      | `Kobako::TimeoutError`     | 60.0 s  |
| `memory_limit` | `Kobako::MemoryLimitError` | 1 MiB   |
| `stdout_limit` | output clipped (no raise)  | 1 MiB   |
| `stderr_limit` | output clipped (no raise)  | 1 MiB   |

`memory_limit` covers the per-invocation `memory.grow` delta from the entry baseline, so a Sandbox reused across invocations does not silently accumulate against a global budget.

Beyond the four caps, `profile:` requests the Sandbox's isolation posture on the `:permissive` < `:hermetic` ladder (default `:hermetic`). `:hermetic` denies the guest ambient time and entropy; `:permissive` lets the guest's `wasi:clocks` / `wasi:random` read live host sources — an explicit trade of reproducibility, with filesystem, environment, and network still unreachable. The request is also a floor: construction fails with `Kobako::SetupError` on a runtime that declares a weaker posture than requested. See [`docs/security-model.md`](docs/security-model.md) § Isolation profiles.

### Concurrency

A Sandbox keeps no state from any run, so concurrent Threads may invoke distinct Sandboxes or share a single one; each invocation owns its Handles, captures, and usage either way ([`docs/behavior/runtime.md`](docs/behavior/runtime.md) B-22). One Thread still runs one invocation at a time. Sharing a Sandbox adds a single obligation: an object bound once at setup is reached by every Thread and must itself be thread-safe, while an object supplied per invocation — `ctx.bind`, or an Extension `provider:` — carries no such requirement.

By default an invocation holds Ruby's GVL for its whole span, so guest execution across Threads serializes. `gvl: :release` drops the GVL for the guest span and re-acquires it for each guest→host dispatch, running guest code in parallel across Threads (B-64).

```ruby
sandbox = Kobako::Sandbox.new(gvl: :release)
```

The mode is per-Sandbox and fixed at construction; it changes scheduling only, leaving isolation, Handle lifetimes, captures, and outcomes identical. `:hold` remains the default because releasing pays a handoff cost at every dispatch: compute-bound scripts scale with Thread count, while dispatch-heavy ones match or trail `:hold`. `rake bench:gvl_scheduling` measures both ends on your own hardware.

### Invocation Lifecycle

One Sandbox serves many invocations. Service bindings and preloaded snippets persist across calls; everything a run produces — Handles, captured output, memory delta — belongs to that run alone.

```
   ───────────── setup phase (mutable) ─────────────

sandbox = Kobako::Sandbox.new
     sandbox.bind("KV::Lookup", ...)
     sandbox.bind("Req::Current")          # fillable: object comes later
     sandbox.preload(code: ..., name: :Adder)
     sandbox.preload(code: ..., name: :Greeter)

│
                          ▼

═════════════════ seal point ═════════════════
   First #eval or #run freezes the Service registry
   and snippet table. Further bind / preload now
   raise ArgumentError.

│
                          ▼

──────────────── invocation N ───────────────────

1. resolve this run's bindings, before the guest starts:
          ctx.bind("Req::Current", ...)  fills the fillable
          provider: backends yield this run's object

2. start from the canonical boot state
        (mruby pre-initialized into the artifact at build time)

3. replay snippets (in insertion order):
          :Adder     → defines Adder
          :Greeter   → defines Greeter

4. dispatch:  eval(source)  or  run(:Target, *args, **kwargs)

5. return the run's Execution to host

6. discard the instance; per-invocation state ends with it:
          · Handles invalidated
          · captures frozen into the Execution
          · memory delta zeroed

Services + snippets persist; invocation N+1 repeats.
```

For workloads that must be isolated from each other (one Sandbox per tenant, per student submission, per agent session), construct a fresh `Kobako::Sandbox` per scope — wasmtime's Engine and the compiled Module are cached at process scope, so additional Sandboxes amortize cold-start cost automatically.

### Pooling

For hosts that serve many short invocations, `Kobako::Pool` keeps a bounded set of warm, identically set-up Sandboxes and hands each one to a single exclusive holder at a time ([`docs/behavior/runtime.md`](docs/behavior/runtime.md) B-46..B-48). Construction forwards every `Sandbox.new` keyword verbatim; the optional block is the per-Sandbox setup window and runs exactly once per constructed Sandbox.

`Kobako::Pool` is experimental today and is best treated as a convenience for warm, pre-configured reuse rather than a throughput optimisation. B-49 bakes the shared boot state into the artifact and every dynamic script still compiles and runs per invocation, so all a pool actually saves is the host-side `Sandbox.new` — now under 3 µs, an order of magnitude below the invocation that follows it. For the workload kobako is built for — many small, short-lived Sandboxes running dynamic scripts — that is not a gain worth the coupling. What a Pool buys is warm setup and exclusive checkout, not isolation: a Sandbox holds no state from any run, so Threads sharing one are equally safe (see [Concurrency](#concurrency)).

```ruby
pool = Kobako::Pool.new(slots: 4) do |sandbox|
  sandbox.bind("KV::Lookup", ->(key) { redis.get(key) })
end

pool.with { |sandbox| sandbox.eval(%(KV::Lookup.call("user_42"))).value }
```

| Option | Meaning | Default |
|--------|---------|---------|
| `slots:` | Upper bound on constructed Sandboxes | required |
| `checkout_timeout:` | Seconds `#with` waits for a free Sandbox; `nil` waits indefinitely | 5.0 |

Sandboxes construct lazily on first demand. `#with` yields a Sandbox and returns the block's value; at block exit the Sandbox returns to the pool, except a block that raises `Kobako::TrapError` discards its Sandbox and the slot refills by a fresh construction on next demand. A checkout that waits past `checkout_timeout` raises `Kobako::PoolTimeoutError`. There is no teardown verb — a Pool releases everything with its own reachability.

### Service Blocks

A Service method can accept a guest-supplied block via `&blk` and `yield` into it. The block body runs inside the Wasm guest; `break` / `next` / exceptions follow normal Ruby semantics, scoped to the single dispatch. See [`docs/behavior/yield.md`](docs/behavior/yield.md) B-23..B-30.

```ruby
sandbox.bind("Seq::Map", ->(items, &blk) { items.map(&blk) })

sandbox.eval('Seq::Map.call([1, 2, 3]) { |x| x * 2 }').value
# => [2, 4, 6]
```

### Handle Management

A non-wire-representable host object — returned from a Service (B-14), passed to `#run` (B-34), or handed back from the guest (B-37) — crosses the boundary as an opaque `Kobako::Handle` proxy and is restored to the original object before host code sees it; any other unrepresentable value raises `Kobako::SandboxError`. Handles are scoped to a single invocation ([`docs/behavior/dispatch.md`](docs/behavior/dispatch.md) B-13..B-21, B-34, B-37).

```ruby
class Greeter
  def initialize(name) = @name = name
  def greet            = "hi, #{@name}"
end

sandbox.bind("Factory::Make", ->(name) { Greeter.new(name) })

sandbox.eval('Factory::Make.call("Bob").greet').value  # => "hi, Bob"  (Handle round-trip inside guest)
sandbox.eval('Factory::Make.call("Bob")').value        # => #<Greeter @name="Bob">  (B-37 restoration)
```

A `break` value from a guest block is the one exception: it unwinds back to the guest Service call rather than to host code, so a Handle in it stays a Handle — restoring would just re-wrap the same object into a new id on the return trip.

Each dispatch that hands back a non-wire-representable object allocates a *new* Handle — kobako never deduplicates by object identity (B-15, B-17). This is most visible with fluent / builder APIs. An `ActiveRecord::Relation` chain `spawn`s a fresh relation at each step, so every hop is an independent dispatch that binds its own Handle:

```
   guest chain                        host  (Catalog::Handles, one invocation)
   ───────────                        ─────────────────────────────────────────
   User.where(active: true)  ─call──▶ Relation #1 (fresh clone)  bound ▶ Handle 1
                             ◀─Handle 1
       .order(:created_at)   ─call──▶ Relation #2 (fresh clone)  bound ▶ Handle 2
                             ◀─Handle 2
       .limit(10)            ─call──▶ Relation #3 (fresh clone)  bound ▶ Handle 3
                             ◀─Handle 3

3 hops ─▶ 3 dispatches ─▶ 3 distinct relations ─▶ 3 Handles
   all stay live until the invocation ends, then reset together
```

This is deliberate, not a leak. Handle IDs run to 2³¹ − 1 per invocation and reset between invocations, so even deep chains stay far inside the range. Two consequences are worth keeping in mind: the same host object handed back twice yields two *different* Handles — the guest cannot tell they alias — and every intermediate Handle stays live until the invocation ends, since there is no per-Handle release (B-19).

### Snippets & Entrypoints

`Sandbox#preload` registers named mruby snippets that replay into every invocation's canonical boot state; `Sandbox#run(:Target, *args, **kwargs)` dispatches into a top-level `Object` constant defined by those snippets ([`docs/behavior/invocation.md`](docs/behavior/invocation.md) B-31..B-33).

```ruby
sandbox = Kobako::Sandbox.new
sandbox.preload(code: "Adder   = ->(a, b) { a + b }",                    name: :Adder)
sandbox.preload(code: 'Greeter = ->(opts) { "hello, #{opts[:name]}" }', name: :Greeter)

sandbox.run(:Adder, 2, 3).value            # => 5
sandbox.run(:Greeter, name: "world").value # => "hello, world"
```

An entrypoint's `kwargs` arrive as a trailing positional Hash — mruby's C-side call path carries no keyword arguments — so declare a Hash parameter and unpack it yourself.

A target no snippet defined raises `Kobako::UndefinedEntrypointError`, whose `#available` lists the top-level constants the snippets did contribute — so the name is corrected from the error rather than by reading the guest source.

```
   per-invocation replay (every #eval / #run, snippets in insertion order):

canonical boot state
            │
            ├──▶ replay :Adder            (defines Adder)
            │
            ├──▶ replay :Greeter          (defines Greeter)
            │
            └──▶ eval(source)  -or-  run(:Target, *args, **kwargs)
                       │
                       ▼
              return the Execution, then discard the instance
```

`#preload` accepts two payload forms:

| Form     | Signature                            | Snippet name source                   | Validation timing                                                          |
|----------|--------------------------------------|---------------------------------------|----------------------------------------------------------------------------|
| Source   | `preload(code: "...", name: :Const)` | The `name:` keyword                   | First invocation's replay; compile errors raise `Kobako::SandboxError`     |
| Bytecode | `preload(binary: bytes)`             | Read from the bytecode's `debug_info` | First invocation's replay; structural failure raises `Kobako::BytecodeError` |

Use the source form for snippets authored in your repo; use the bytecode form when snippets ship as build artifacts from a separate `mrbc` pipeline. Both replay through the same per-invocation path, so no snippet content failure surfaces at `#preload` — force the first replay with a no-op invocation (e.g. `sandbox.eval("nil")`) when you want validation before real traffic.

### Extensions

An Extension teaches the guest a native-style constant by pairing a guest idiom (`source`) with an optional host `backend`. `Sandbox#install` composes the two through the existing `#preload` and `#bind` verbs, adding no wire or Guest Binary surface: pure operations run in-guest with no round-trip, while the rest dispatch to the backend under the same isolation and reflection guarantees as any bound Service ([`docs/extensions.md`](docs/extensions.md), [`docs/behavior/extension.md`](docs/behavior/extension.md) B-55..B-57).

```ruby
FILE = <<~'MRUBY'
  class File
    extend Kobako::Proxy
    def self.basename(path) = path.split("/").last || ""  # pure: runs in-guest
    # read / write are undefined here, so they dispatch to the backend
  end
MRUBY

sandbox.install(
  Kobako::Extension.new(
    name: :File,
    source: FILE,
    backend: Kobako::Extension::Backend.new(
      path: "File",
      provider: -> { OverlayFileSystem.new(root) }  # invoked once per invocation
    )
  )
)

sandbox.eval('File.read("sample.txt")')  # dispatches to the backend's #read
```

A backend declares the bound object's lifetime by keyword, never by inference — so a static object that happens to be callable stays unambiguous.

| Keyword     | Lifetime                                                                    |
|-------------|-----------------------------------------------------------------------------|
| `object:`   | One object, shared by every invocation                                       |
| `provider:` | A no-argument callable invoked per invocation — what a writable backend needs, so its state cannot leak across runs |
| neither     | A fillable, standing for `Kobako::Unresolved` until `ctx.bind` supplies the run's object |

kobako ships no concrete Extension; the idiom and backend are yours. The [overlay VFS example](examples/vfs/) is a worked `File` that reads through to disk while protecting it from guest writes.

## Security

kobako isolates the guest, but **what it may reach is whatever you `bind`** — and `bind`
exposes *every* public method of the object. So bind a purpose-built object scoped to the
task, not a capable one whose other methods leak more than you intend.

```ruby
class ThemeReader          # only #color is reachable; AppConfig.secret_key is not
  def color = AppConfig.theme.color
end

sandbox = Kobako::Sandbox.new
sandbox.bind("Cfg::Settings", ThemeReader.new)  # not: bind("Cfg::Settings", AppConfig)

sandbox.eval('Cfg::Settings.color').value  # => "#3366ff"  — every other method raises NoMethodError
```

When a purpose-built wrapper is more than you need, an object can gate its own surface in
place: a private `respond_to_guest?(name)` answers, per method, whether the guest may call
it. Returning `false` for every name makes the object opaque — a credential the guest
forwards to another Service but never reads — while a named subset becomes an allow-list.

Guest code can name any `MyService::KV` path, but a forged name only resolves to
something you bound — the real authorization gate is this host-side allowlist. Give each
trust context its own Sandbox, and see [`docs/security-model.md`](docs/security-model.md) for the rest
as security-design concerns: validating untrusted input, default-deny external effects,
and controlling the return surface.

## Performance

Order-of-magnitude figures on macOS arm64, Ruby 3.4.7, YJIT off. Absolute values vary by hardware but ratios are stable across machines. Full numbers, methodology, and the +10%-regression gate live in [`benchmark/README.md`](benchmark/README.md).

| Phase                                                        | Cost                  |
|--------------------------------------------------------------|-----------------------|
| First `Sandbox.new` ever for a Guest Binary (Module JIT, then disk-cached) | ~500 ms once per machine |
| First `Sandbox.new` in a fresh process (`.cwasm` cache warm) | ~1.3 ms one-time      |
| Subsequent `Sandbox.new` (caches warm)                       | ~3 µs                 |
| Warm `#eval` with one Service call on a reused Sandbox       | ~71 µs                |
| Warm `#run(:Entrypoint, ...)` dispatch                       | ~81 µs                |
| Service call amortized inside one invocation                 | ~5.7 µs               |
| Snippet replay per invocation                                | ~7.5 µs each          |
| Per additional idle Sandbox (RSS)                            | ~1 KB                 |

The Cranelift JIT runs once per machine and gem version — the compiled artifact persists in a `.cwasm` disk cache, so later processes deserialize in milliseconds. An idle Sandbox holds no wasm instance (the canonical boot state is baked into the artifact and instantiated per invocation), which is why a thousand idle tenants cost ~33 MB total. Under the default `gvl: :hold`, wasm work is GVL-serialized: aggregate throughput stays around 16k `#eval`/s regardless of Thread count, though Ruby-side `#eval` setup still overlaps. Opting a Sandbox into `gvl: :release` lifts that ceiling for compute-bound scripts (see [Concurrency](#concurrency)). A +10% regression on any SPEC-mandated benchmark blocks release.

Regexp is an opt-in capability gem, excluded from the default binary and the gated set; its throughput is tracked in a separate non-gated characterization (`#11` in [`benchmark/README.md`](benchmark/README.md)). There `=~` (~5 µs/match) costs about 4.5× `match?` (~1.1 µs), because `=~` eagerly builds the `MatchData` and match globals — prefer `match?` for boolean tests.

```bash
bundle exec rake bench  # every gated regression benchmark (~5-8 min)
```

## Development

After checking out the repo:

```bash
bin/setup         # install dependencies
bundle exec rake  # default: compile + test + rubocop + steep
```

Building from source requires a WASI-capable Rust toolchain in addition to the standard host toolchain; the first compile walks the full chain — the [beni](https://github.com/elct9620/beni) gem vendors wasi-sdk + mruby and builds `libmruby.a` (`rake beni:build`), then `rake wasm:build` produces the Guest Binary. See [`CLAUDE.md`](CLAUDE.md) for the rake task map and pipeline layout. `bin/console` opens an IRB session with the gem preloaded; `bundle exec rake install` installs the local checkout as a gem.

## Contributing

Bug reports and pull requests are welcome at <https://github.com/elct9620/kobako>. Please open an issue before starting on non-trivial changes so we can align on scope.

Releases are automated with release-please across two tracks — the gem and the linked guest-crate group. The version rules, commit conventions, and how to cut each release live in [`docs/releasing.md`](docs/releasing.md).

## License

Kobako is released under the [Apache License 2.0](https://opensource.org/licenses/Apache-2.0).

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