# V2 Tools

Status: **Current semantic overview.** The Plugin package owns the public tool type; Core owns registration, execution, and generic output bounding.

## Tools

V2 has one structural tool value for locally executable tools. Typed tools declare schemas and execution together:

```ts
const read = Tool.make({
  description: "Read a file",
  input: Schema.Struct({ path: Schema.String }),
  output: Schema.Struct({ content: Schema.String }),
  execute: ({ path }, context) =>
    readFile(path, context).pipe(Effect.map((output) => ({ output, content: output.content }))),
})
```

One tool response may carry three values: the declared, schema-validated `output` is the ephemeral machine value Code Mode receives; `content` is the model-facing value stored durably; and optional `metadata` is compact JSON for tool-specific UI. A tool without `output` intentionally returns only model-visible `content` and optional `metadata`. Dynamic MCP and manifest tools use the same tool shape with runtime JSON Schema.

Built-ins and statically authored plugin tools use this same constructor and execution contract.

`Tool.Tool` is a transparent structural value with exactly one `execute` function. Effect schemas and schemas implementing both Standard Schema V1 and Standard JSON Schema V1 are accepted. The Tool module derives inert model-facing `LLM.ToolDefinition` values and executes tools for the registry; callers normally rely on `Tool.make` inference rather than naming the nested type.

Standard input schemas validate model input into the tool input. Standard output schemas validate the tool response's `output` into the Code Mode machine value. Effect codecs retain their native decode-input and encode-output directions.

Input and output codecs are self-contained. Schema conversion cannot require services. Tool dependencies are acquired during construction and captured by `execute`.

## Every Call Has Durable Identity

Every local tool receives the same concrete invocation context:

```ts
interface Tool.Context {
  readonly sessionID: Session.ID
  readonly agent: Agent.ID
  readonly messageID: SessionMessage.ID
  readonly callID: string
  readonly progress: (update: Progress) => Effect.Effect<void>
}
```

`messageID` is the durable ID of the assistant message containing the call. The Session runner owns this association and supplies the complete context to the registry; the registry does not infer it. `callID` carries the same invocation identifier durable events use.

Decoded tool input is passed separately to `execute`. Raw provider input and domain services do not belong in the invocation context.

Effect interruption is the cancellation mechanism. Tools may translate expected typed failures into `ToolFailure`, but must not translate interruption or defects into model-visible failures.

## Registrations Are Scoped

Tools are named when registered:

```ts
yield *
  tools.register({
    read,
    write,
    grep,
  })
```

The record key is the authored name. Registration normalizes it before deriving the effective model-facing name. A reusable tool value has no intrinsic name.

```ts
interface Tools {
  readonly register: (
    tools: Readonly<Record<string, Tool.Any>>,
    options?: Tool.RegisterOptions,
  ) => Effect.Effect<void, Tool.RegistrationError, Scope.Scope>
}
```

Registration replaces unsupported name characters with `_` and reserves `execute` for Code Mode.

A Location plugin receives only the narrow `Tools` registration capability, not the internal registry. Each activation acquires the Location's services, constructs its tools, and registers them in a fresh plugin-owned Scope.

Within one placement:

- The latest active registration for a name wins.
- Closing a registration removes only that registration.
- Closing the winner reveals the next-latest active registration.
- Mutating the caller's registration record later does not change the captured registration.

## Built-Ins Use The Same Contract

Built-ins use the same tool API while capturing trusted Location services:

```ts
const filesystem = yield * FileSystem.Service
const permission = yield * PermissionV2.Service
const tools = yield * Tools.Service

yield *
  tools.register({
    grep: Tool.make({
      description: "Search file contents",
      input: Input,
      output: Output,
      execute: (input, context) =>
        Effect.gen(function* () {
          const root = yield* filesystem.resolveRoot(input)

          yield* permission.assert({
            sessionID: context.sessionID,
            agent: context.agent,
            source: {
              type: "tool",
              messageID: context.messageID,
              callID: context.callID,
            },
            action: "grep",
            resources: [input.pattern],
            save: ["*"],
            metadata: { root: root.resource },
          })

          return yield* filesystem.grep(input, root)
        }).pipe(/* translate expected typed errors to ToolFailure */),
    }),
  })
```

Trusted tools formulate and sequence permission requests. `PermissionV2` evaluates policy and manages approval. The registry does not inject an `assertPermission` helper.

Sharing a tool type does not imply equal authority. Built-ins and trusted Location plugins may capture services that are not available to application tools.

## Requests Capture Tool Values

The Location-scoped registry owns effective lookup and execution through one request-scoped snapshot pairing advertised LLM definitions with captured tools. For each local call it:

1. Resolves one effective named registration.
2. Decodes provider input with the input codec.
3. Executes the tool with the runner-supplied context.
4. Encodes the returned output with the output codec; the encoded value is the ephemeral machine output for Code Mode.
5. Normalizes the tool response into canonical non-empty model content and optional JSON metadata.
6. Bounds the model content; validates metadata, dropping invalid or oversized values with a warning rather than failing the call.
7. Runs `execute.after` hooks with the canonical outcome and managed output paths.
8. Returns one `ToolOutcome` — completed with output, content, and optional metadata, or an error with an optional final partial snapshot — to the runner for durable publication.

Invalid input never executes the tool. Invalid output never produces a successful execution.

When an output-bearing tool omits `content`, an encoded string becomes one text item and any other encoded JSON is serialized once. A tool without `output` must provide non-empty model content.

Each model request captures the effective registration for every advertised name. Execution uses those captured tools; later registration changes affect later requests. Unknown, hook-removed, and final-Step calls fail individually through the same execution seam; the final Step retains tool definitions with `toolChoice: "none"` where the provider supports it so the cached prompt prefix survives.

Durable terminal events are self-contained: success stores exactly the non-empty model content plus optional metadata; failure stores one error plus the final bounded snapshot of partial progress. Provider replay derives its wire value from canonical content; provider-hosted payloads that a protocol requires verbatim live in provider-owned result state, never in a generic result field.

## Producers And The Registry Own Different Limits

Producers may cap capture or spool data before a complete tool result exists. For example, a process tool may retain output it cannot keep in memory. Producer limits must report their own loss accurately; they are separate from registry bounding and cannot claim to reconstruct bytes already discarded.

After tool execution, the registry bounds the model content sent to the provider: only textual parts are measured, native media remains unchanged under producer-owned limits, and the default cut keeps a head-plus-tail split with the omission marker in the middle. Oversized text is retained in managed storage and replaced with a bounded preview; if complete retention fails, execution fails operationally rather than publishing lossy success. Metadata is validated and measured independently and never becomes an unbounded side channel. Managed paths never appear in `Tool.make` or tool output schemas solely for retention bookkeeping.

`execute.after` hooks receive the canonical bounded outcome and its internal managed paths. Hooks may deliberately transform that outcome; changed content is normalized and bounded again before publication.

## Failures Preserve Interruptions

Outcomes remain distinct:

- `ToolFailure` is an expected model-visible failure.
- Interruption cancels the invocation and is not a tool result.
- Unexpected typed errors and defects follow the runner's operational failure policy.
- Unknown and invalid calls become explicit model-visible execution errors without executing a tool.

Tools translate only errors they deliberately classify as recoverable. Broad cause-catching around `execute` is invalid because it consumes interruption and defects.

## Laws

- **Single execution:** `Tool.make(config)` can execute only `config.execute`.
- **Codec boundary:** a tool observes decoded input; Code Mode observes the validated encoded output; model content and metadata come from the tool response.
- **Canonical representation:** a completed call has exactly one stored model representation; a failed call has exactly one stored error plus at most one final partial snapshot. Every other view is derived at a named boundary.
- **Metadata opt-in:** absent response metadata produces absent metadata, never a copied output.
- **Durable identity:** invocation-owned records use the exact Session, agent, assistant message, and call IDs supplied by the runner.
- **Scoped registration:** closing a Scope removes exactly its registration and reveals any prior active overlay.
- **Captured execution:** a call executes the registered tool advertised in its model request.
- **Per-call rejection:** rejecting one unavailable call cannot fail another call.
- **Storage encapsulation:** domain output does not change according to model-output bounding or retention policy.