# ImaGen architecture

ImaGen is intentionally small. The framework owns plumbing; adapters own the
upstream API. Each adapter only ever sees its own slice of `imagen.yaml`.

## Layers

```
        ┌───────────────────────┐
        │   cmd/imagen          │   CLI dispatch
        │   (or HTTP server)    │
        └──────────┬────────────┘
                   │
        ┌──────────▼────────────┐
        │   internal/prompt     │   style preset → prompt suffix
        │   internal/output     │   filename templating, sidecar
        │   internal/config     │   YAML loader, validation
        │   internal/preview    │   tmux-img window spawner
        └──────────┬────────────┘
                   │
        ┌──────────▼────────────┐
        │   internal/backend    │   Backend interface + Registry
        └──────────┬────────────┘
                   │
        ┌──────────▼────────────┐
        │   adapters            │   ComfyUI · Replicate · OpenAI · …
        │   (each one register- │   each registers a `type` name on
        │    s in init())       │   `backend.Default` at init time.
        └───────────────────────┘
```

## The Backend contract

```go
type Request struct {
    Prompt         string
    NegativePrompt string
    Width, Height  int
    Steps          int
    Seed           int64
    Style          string
    BackendOpts    map[string]any
}

type Result struct {
    ImageReader io.ReadCloser
    MimeType    string
    Metadata    map[string]any
}

type Backend interface {
    Name() string
    Generate(ctx context.Context, req Request) (*Result, error)
}
```

Adapters translate `Request` into whatever the upstream expects. Fields they
can't honour (e.g. `NegativePrompt` on DALL-E) are silently ignored.

## Registry

`backend.Default` holds the process-wide name → constructor map. Each adapter
calls `backend.Register("<type>", NewX)` from its `init()`. The CLI imports
`internal/backend` (which transitively triggers the mock's init) and any
extra adapter packages.

## Config flow

```
imagen.yaml
  backends:
    flux-schnell-local:
      type: comfyui                  ──┐
      base_url: http://mrock:8188      │  framework keeps `type`,
      model: flux1-schnell.safetensors │  hands the rest to the
      default_steps: 4                 │  comfyui adapter as cfg map[string]any
                                     ──┘
```

The framework never inspects fields below `type`. That's the adapter's
contract with itself, expressed however the adapter wants (typed struct,
map lookups, JSON tags — its call).

## Output

```
output:
  directory: ~/Pictures/imagen
  naming: "{date}-{slug}-{seed}.png"
  write_metadata_json: true
```

Placeholders: `{date}`, `{time}`, `{slug}` (lowercased prompt, alnum-only,
truncated to 40 chars), `{seed}`, `{backend}`, `{ext}`. The sidecar JSON
contains the prompt, backend instance name, seed, ISO timestamp, and the
`Result.Metadata` map verbatim.

## Where adapters fail fast

- Missing required field in their config block — return an error from the
  constructor; the CLI surfaces it as `imagen: backend "X": <err>`.
- Unset env-var for credentials — same.
- Network errors during `Generate` — wrap and return; no retry policy yet
  (decide per-adapter, or move to a shared retry helper if a pattern emerges).

## Out of scope (today)

- Image post-processing (cropping, watermarking).
- Cost-tracking (lands with the Replicate adapter, since only API backends bill).
- Multi-image `n>1` per request — backends that support it can expose it via
  `BackendOpts`; the framework doesn't have a first-class field yet.