# Migration Step 4: Public-Facade Adapter Boundary

## Status

Complete as of 2026-07-28. The public-facade adapter boundary is established;
the [implementation plan](implementation.md) records the completed work, and
the [main migration roadmap](migration.md) identifies repository creation as
the next gate.

## Purpose

Make Scriptorium's CLI and HTTP adapters genuine consumers of the public
framework facade before that facade moves to Promptkit. This establishes and
tests the dependency boundary inside the current repository, where it can be
changed atomically, before the framework and application are separated across
repositories.

The [accepted split decision](../adr/0002-split-promptkit-from-scriptorium.md)
owns the long-term project boundary. The
[main migration roadmap](migration.md) owns the overall sequence. This feature
roadmap defines the desired Step 4 state, not an implementation sequence.

## Achieved Boundary

The CLI, HTTP handler, and prepared-run formatter now consume public engine
values and errors. `serve` injects Scriptorium's HTTP-owned restricted artifact
reader through the public extension point, while the root facade continues to
compose the framework implementation inside this repository. A repository-level
dependency test protects the direct-import boundary.

## Target State

Scriptorium's executable path is an ordinary consumer of the same public
framework boundary used by other Go applications:

```text
cmd/scriptorium
       |
       v
Scriptorium CLI and HTTP adapters
       |
       v
public Engine, requests, results, errors, and extension interfaces
       |
       v
framework implementation packages
```

The CLI and HTTP production packages do not import framework-owned internal
packages. They translate application configuration, flags, and HTTP DTOs into
public engine configuration and request values; call `Prepare` or `Run`; and
translate public results and errors back into their owned interfaces.

The root facade may continue to use the existing internal implementation during
Step 4. That implementation moves in later migration steps. The important
result here is that no Scriptorium-owned adapter or presentation component
depends on it directly.

## Public Facade Requirements

### Engine Consumption

The current public `Engine`, `Config`, `RunRequest`, `PreparedRun`,
`RunResult`, request helpers, result values, and existing broad error sentinels
remain the primary boundary. Step 4 must not introduce a second facade,
adapter-specific engine, exported internal runner, or public repository
constructor.

Scriptorium-local interfaces may abstract the methods an adapter needs for test
substitution. They must:

- be declared on the consuming side;
- use only public facade types;
- contain only `Prepare`, `Run`, or the narrower subset required by that
  consumer; and
- be satisfied directly by `*Engine`.

Promptkit must not acquire CLI, HTTP, status-code, or response-format concepts
to satisfy these interfaces.

### Artifact Reader Extension

The public facade will expose the demonstrated artifact-loading extension point:

```go
type ArtifactReader interface {
    Read(context.Context, ArtifactRef) (*Artifact, error)
}

func WithArtifactReader(ArtifactReader) Option
```

This option replaces the engine's ordinary composite artifact reader for all
input references. The default remains the framework's ordinary inline and
caller-selected file behavior.

The boundary must have the same defensive behavior as other public injection
points:

- a nil reader is rejected as `ErrInvalidConfig`;
- a reader response of `(nil, nil)` becomes `ErrArtifactLoad` rather than
  causing a panic;
- reader failures are wrapped as `ErrArtifactLoad` while retaining the
  original error identity for `errors.Is`;
- conversion between public and internal values does not expose internal domain
  types; and
- mutable artifact content is copied across the boundary where needed to avoid
  unintended aliasing.

This is the only new framework extension interface required by Step 4. No
public prompt repository, profile repository, renderer, validator, or internal
runner interface is needed for the current adapters.

### Public Error Detail

The facade's existing broad errors remain authoritative for general consumers.
To preserve Scriptorium's current HTTP contract without inspecting Promptkit
internals, the public error chain must additionally retain stable identities
for:

- profile selection being required because neither the request nor the prompt
  supplies a profile; and
- a selected credential environment variable being unset or empty.

These identities will be exposed as `ErrProfileRequired` and
`ErrAPIKeyEnvMissing`. Each remains nested under `ErrInvalidRequest`, so callers
that need only the broad category continue to match it. Scriptorium uses the
more specific identities only to preserve its existing HTTP error codes.

No error-string parsing is permitted. Prompt, profile, artifact, render,
generation, and validation failures continue to use the existing public
sentinels. Scriptorium-owned HTTP artifact-policy errors remain in Scriptorium
and survive wrapping through the public artifact-reader boundary.

## CLI Boundary

The `run` and `render` commands will construct the public engine from resolved
application settings:

- `prompt_dir`, `profile_dir`, and `schema_dir` map to public engine
  configuration;
- the public engine supplies the built-in profile registry, renderer,
  validator, ordinary artifact reader, and default OpenAI-compatible client;
- CLI input paths map to public file artifact references;
- CLI runtime flags map to the public request override while preserving
  numeric presence, including explicit zero; and
- omitted application values remain omitted so framework defaults are not
  duplicated in Scriptorium.

`run` consumes the public result for artifact output, validation exit status,
and the stderr summary. `render` consumes the public prepared value. The
Scriptorium-owned prepared-run formatter will accept public prepared values
instead of internal domain values without changing its text or JSON contract.

The `serve` command will construct the same public engine with Scriptorium's
restricted artifact reader injected through `WithArtifactReader`. It will pass
that engine to the HTTP handler through a Scriptorium-local run interface.

CLI construction must handle public engine-construction errors explicitly.
The executable must not construct an internal LLM client, repository, renderer,
validator, artifact reader, or runner.

## HTTP Boundary

The HTTP handler's consumer interface will accept public `RunRequest` values
and return public `RunResult` values. DTO mapping will preserve the existing
HTTP contract:

- strict JSON decoding and request-size enforcement remain in Scriptorium;
- prompt, profile, input, variable, and execution-override fields map to their
  corresponding public values;
- pointer-valued numeric overrides retain omitted-versus-explicit-zero
  semantics;
- raw API keys remain absent from the HTTP request shape;
- public run results map to the current response DTOs;
- raw model output remains opt-in; and
- response-size enforcement remains an HTTP concern.

HTTP error mapping will inspect only:

- public framework errors;
- Scriptorium's HTTP artifact-policy errors; and
- standard-library transport errors owned by the handler.

It will not inspect internal prompt, profile, artifact, domain, or use-case
errors. Existing HTTP statuses, error codes, and sanitized messages remain
unchanged.

## HTTP Artifact Policy

The restricted artifact reader remains Scriptorium-owned and becomes an
implementation of the public `ArtifactReader` interface. It belongs with the
HTTP adapter rather than the Promptkit-destined general artifact package.

The reader will continue to:

- resolve inline references without permitting empty inline bodies;
- deny file references when no artifact root is configured;
- enforce the configured maximum artifact size;
- enforce the documented lexical root-containment rule without resolving
  symlinks;
- return complete public artifact metadata; and
- preserve distinct Scriptorium errors for a denied or out-of-root file and an
  oversized file.

The public engine treats those errors as artifact-load failures while preserving
their identities. The HTTP mapper checks the Scriptorium-specific identity
before the broad public `ErrArtifactLoad` identity, retaining the current
`artifact_not_allowed`, `artifact_too_large`, and general artifact-read
outcomes.

General inline and unrestricted file reading remains framework-owned. Step 4
separates the HTTP policy from that implementation far enough that each side
can later move to its target repository without redesigning the interface.

## Package And Dependency Boundaries

The completed dependency state is:

| Component | Permitted framework dependency |
| --- | --- |
| `internal/adapter/cli` | Public facade types, constructors, options, errors, and methods only |
| `internal/adapter/http` | Public facade types and errors only; Scriptorium-owned HTTP artifact policy remains local |
| `internal/format` | Public prepared-run and rendered-message values only |
| `internal/config` | Scriptorium application settings and Scriptorium-owned defaults; no framework orchestration |
| `cmd/scriptorium` | CLI adapter only |
| Root facade | Existing internal framework implementation until extraction |

In particular, Scriptorium-owned adapter, formatter, and HTTP artifact-policy
production files must not import:

- `internal/domain`;
- `internal/usecase`;
- `internal/promptdef` or `internal/prompt`;
- `internal/profile` or `internal/profile/builtin`;
- `internal/validate`;
- `internal/llm`; or
- the Promptkit-destined general artifact implementation.

Tests for Scriptorium-owned components should follow the same public boundary
except when directly testing a Scriptorium-owned package.

## Observable Behavior

Step 4 is an architectural refactor plus the minimum additive public extension
surface required to support it. It is not a redesign of the executable
interfaces.

The following behavior must remain unchanged:

- CLI commands, flags, aliases, precedence, output routing, summaries, and exit
  codes;
- application configuration discovery, strict decoding, fields, and defaults;
- HTTP routes, strict decoding, DTOs, statuses, codes, messages, and limits;
- HTTP artifact containment and size enforcement;
- prompt/profile selection and override precedence;
- explicit numeric-zero behavior;
- built-in profile fallback and custom-profile overlays;
- structured-output and validation behavior;
- timeout layering;
- secret handling and redaction; and
- maintained executable examples.

The intended public additions are limited to artifact-reader injection and the
specific error identities required by the HTTP mapper. No compatibility shim is
needed because this repository still owns the facade during Step 4 and the
overall migration is intentionally breaking.

## Test Ownership And Verification

Tests will protect the boundary at the layer that owns each risk:

- public engine tests own artifact-reader option validation, conversion,
  invocation, nil-response handling, error wrapping, and error identity;
- CLI tests own flag and configuration mapping into public requests, public
  engine wiring, presentation, output, and exit behavior;
- HTTP tests own strict DTO mapping to public requests, public result mapping,
  error/status mapping, limits, and raw-output opt-in;
- HTTP artifact-reader tests own denied, contained, escaped, oversized, inline,
  cancellation, and metadata behavior;
- formatter tests own stable text and JSON presentation of public prepared
  values; and
- existing framework tests continue to own orchestration, source, validation,
  provider, and broad public error behavior.

Adapter tests that currently construct internal runners or assert internal
sentinels will be rewritten against the public engine or small public-typed
fakes. Duplicate framework-semantic assertions should be removed when the
public contract suite already owns the risk.

The final suite must include an enforceable dependency check showing that
Scriptorium-owned adapter and presentation production packages do not import
Promptkit-destined internal packages. This may be a focused architecture test
or an equivalent deterministic repository check; it must diagnose the
forbidden import clearly.

## Documentation Outcome

When the boundary is implemented, current-behavior documentation will be
reconciled in the same change:

- the public Go package contract will define `ArtifactReader`,
  `WithArtifactReader`, and the new error identities;
- adapter internals will describe public-engine composition and public-value
  mapping;
- source internals will distinguish the public reader extension, general
  framework readers, and Scriptorium's HTTP reader;
- the internal overview and architecture policy will reflect that executable
  adapters consume the public facade; and
- CLI, configuration, HTTP, integration, and operations contracts will change
  only if verification finds an observable correction is necessary.

Documents will keep exact external contracts in their existing canonical
owners and link rather than duplicate them.

## Required Validation Outcome

The completed boundary must pass:

- the full Go test suite;
- `go vet` across all packages;
- a temporary-output executable build;
- repeated public artifact-reader and adapter boundary tests;
- both maintained application configurations;
- maintained render, HTTP-request, and Go-package examples;
- CLI and HTTP smoke checks that exercise the public engine path;
- the forbidden-import dependency check;
- local Markdown-link validation; and
- whitespace validation.

All default validation remains offline, deterministic, and independent of real
credentials.

## Out Of Scope

Step 4 does not:

- create the Promptkit repository or module;
- change the module or root package name;
- move framework implementation packages or built-in profiles out of this
  repository;
- remove the current root facade;
- add compatibility aliases or forwarding packages;
- make Scriptorium depend on an external Promptkit revision;
- broadly export framework repositories, domain values, validators, renderers,
  or runner constructors;
- redesign prompt, profile, schema, request, response, CLI, or configuration
  formats;
- change HTTP containment from lexical path checking to symlink resolution;
- add new execution or repair behavior; or
- perform unrelated facade cleanup.

Those changes belong to later migration steps or a separately accepted feature.

## Completion Criteria

Step 4 is complete when:

- CLI `run` and `render` execute through the public engine;
- CLI `serve` injects the Scriptorium-owned restricted reader into the public
  engine and passes that engine to the HTTP handler;
- HTTP and CLI map only public framework request, result, and error values;
- prepared-run formatting consumes the public prepared value;
- the public artifact-reader extension has the specified validation,
  conversion, nil-response, and error-preservation behavior;
- public error identities preserve every distinction required by the current
  HTTP contract;
- Scriptorium-owned adapters, formatter, and HTTP artifact reader have no
  Promptkit-destined internal imports;
- HTTP containment, limits, error mapping, and all existing executable
  behavior remain protected by passing tests;
- current-behavior documentation reflects the implemented boundary;
- every required validation check passes;
- no out-of-scope extraction or compatibility work is included; and
- the main migration roadmap records Step 4 as complete and identifies
  repository creation in Step 5 as the next gate.

Migration Step 5 must not begin until these criteria are satisfied.

## Lifecycle

This feature roadmap is a temporary migration artifact. It may be removed after
Step 4 is complete and no longer needs to guide active work; repository history
will retain the decision and completion record.