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promptkit/docs/policy/architecture.md

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# Architecture Policy
## Purpose
This document defines Promptkit's current high-level architecture and the
durable boundaries that implementation changes must preserve. The
[internal component overview](../internal/overview.md) inventories concrete
implemented packages without redefining these rules.
## System Shape
Promptkit is an importable Go library. It does not ship a command, an HTTP
service, or another application process. Repository examples demonstrate
library use but are not Promptkit applications or release artifacts.
The module root contains package `promptkit`, which is the public facade. It
provides the supported engine, configuration and source options, requests,
results, public values, extension interfaces, profiles, and error sentinels.
The implemented internal components consist of:
- `internal/domain`, which owns framework data values and source-neutral
invariants shared by later internal components;
- `internal/backend`, which owns validated immutable OpenAI-compatible backend
definitions and the built-in OpenRouter definition;
- `internal/capacity`, which owns engine-local bounded run admission and
model-generation scheduling for limited backends;
- `internal/defaults`, which owns application-neutral framework defaults and
constructs the default execution target;
- `internal/filecatalog`, which discovers YAML files and provides source-path
helpers for filesystem and `fs.FS` consumers;
- `internal/jsonvalue`, which validates and defensively copies JSON-compatible
extra-parameter trees;
- `internal/promptdef`, which loads and validates prompt definitions from
filesystem and `fs.FS` sources;
- `internal/profile`, which loads, validates, and overlays execution profiles
from filesystem and `fs.FS` sources;
- `internal/profile/builtin`, which embeds the built-in execution profile
catalog;
- `internal/prompt`, which renders prompt messages from Go templates;
- `internal/artifact`, which resolves ordinary inline and unrestricted
caller-selected file references;
- `internal/validate`, which validates basic, JSON, and JSON Schema output
using filesystem and `fs.FS` schema sources;
- `internal/llm`, which defines the provider-neutral generation boundary and
implements outbound OpenAI-compatible chat requests; and
- `internal/usecase`, which coordinates preparation and execution across the
internal framework components.
The `examples/go-library/prepare` and `examples/go-library/run` packages are
maintained downstream consumers of the root facade. They do not expose library
packages or participate in internal assembly.
The root facade assembles one immutable backend registry, one capacity manager,
the internal repositories, renderer, validator, outbound client, and use-case
runner while translating public values and errors at the library boundary. The
registry contains built-ins plus validated engine-scoped consumer additions.
The facade constructs the capacity manager from the registry's immutable
policy snapshot, wraps the selected built-in or injected model client, and
supplies bounded admission to the runner. The defaults and renderer depend on
the domain model. Prompt-definition and profile repositories use the domain
model, file catalog, and YAML decoder. The built-in profile repository supplies
an embedded `fs.FS` to the profile package. Artifact reading uses the domain
model and application-neutral defaults. Validation uses the domain model, file
catalog, and JSON Schema implementation. The model client uses the domain
model, application-neutral defaults, and an injected or standard-library HTTP
client. The use-case runner depends on the narrow interfaces owned by each
internal component, including backend lookup and run admission.
The current implementation follows this dependency direction:
```text
downstream consumers, including Scriptorium
|
v
root promptkit public facade
|
v
internal framework components
|
v
narrow injected abstractions
```
The backend registry depends on the domain model and shared JSON-value
validation, has no mutation API after construction, and consumes the
OpenAI-compatible reserved request-field rule owned by the model client. The
capacity component depends on the domain model and the narrow internal
model-client boundary, not on provider transport implementation. The model
client does not depend on registry or capacity configuration. The facade
coordinates internal components and adapts the supported public extension
interfaces to narrow internal abstractions. Internal components must not depend
on consumers or on Scriptorium.
`internal/domain` owns source-neutral invariants for values shared across
multiple input and execution boundaries, including execution-setting bounds,
session identifiers, and output-contract legality. Callers retain source
parsing, required-field rules, source-specific normalization, defaulting,
error classification, and other policy specific to their own boundary.
## Repository And Consumer Boundary
Scriptorium is a downstream application that consumes Promptkit through
the supported public facade. It is not a Promptkit package and must not become
an internal dependency.
Promptkit owns reusable, application-neutral library behavior. It does not own:
- binaries or executable packaging;
- CLI commands, parsing, streams, or exit codes;
- HTTP routes, servers, request DTOs, status mapping, or deployment policy;
- application configuration discovery or precedence;
- process lifecycle, operational state, or application logging; or
- consumer-specific filesystem or security policy.
Those concerns remain with Scriptorium or another consuming application.
## Package Ownership
The module root is the supported public facade. Framework implementation
packages belong under Go's `internal/` boundary unless a demonstrated, stable
consumer contract requires a public package.
Each package must have one cohesive responsibility and a clear dependency
direction. Internal packages must not expose their types merely to simplify
wiring, and the public facade must not leak internal representations through
exported signatures. New public packages require a durable consumer need that
cannot be served cleanly by the root facade.
The [internal component overview](../internal/overview.md) must be updated as
packages are implemented or their responsibilities change.
## Exported API Discipline
Export the smallest contract required by real consumers. Exported declarations
must have accurate GoDoc, stable semantics, and tests proportionate to their
compatibility risk. Avoid speculative extension points, aliases for internal
types, and public constructors that expose assembly details.
Once an exported API exists, its Go declaration and GoDoc own its exact public
contract. Architecture documentation owns boundary rules, not a duplicate API
reference.
## Error Boundaries
Internal failures must cross the public facade as errors meaningful to a Go
consumer without exposing private package types or transport-specific policy.
Wrapping should add useful context while preserving any public error identity
needed with `errors.Is` or `errors.As`.
Promptkit must not assign CLI exit codes or HTTP status codes. Consumers map
public library outcomes into their own transport behavior.
## Dependency Injection
External effects and consumer-selected policy must enter through narrow
interfaces or functions at the boundary that uses them. Dependencies should be
explicitly supplied during construction or invocation rather than read from
consumer configuration or hidden process-global state.
Interfaces should be owned by the code that consumes the behavior and should
contain only the operations that code requires. Provide defaults only for
application-neutral behavior; consumer-specific restrictions and adapters
remain injected from the consuming project.
## Repository Independence
Promptkit must build, test, and validate independently of Scriptorium. Do not
commit `go.work`, `go.work.sum`, or a local filesystem `replace` directive.
Temporary workspace or replacement configuration may support coordinated local
development, but it is not part of either repository's architecture or release
state.
## Current-State Maintenance
Do not list planned packages as implemented components. When implementation
introduces a package, update the internal inventory and the owning contract or
subsystem document in the same change.