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feat(imagegen): optional per-request generation settings

Add Steps, CFGScale, NegativePrompt, Sampler, Seed to imagegen.Request
(pointer/empty = leave the backend's per-model default), with mirror
options, and forward them in the llamaswap wire payload as the
stable-diffusion.cpp fields (steps/cfg_scale/negative_prompt/
sample_method/seed). Unset fields are omitted so sd-server keeps its
baked defaults.

Lets callers (e.g. mort drawbots) override only what they explicitly set.

2026-06-28 19:05:49 -04:00

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ADR-0016: imagegen — a canonical text-to-image interface

Status: Accepted — 2026-06-27

Context

mort needs to generate images (via llama-swap's stable-diffusion.cpp backend), and majordomo had no image-generation surface. Image generation does not fit the chat contract: there are no conversation messages, tools, streaming, or failover-chain semantics — forcing it through llm.Request/llm.Response/ llm.Model would overload that contract with mostly-unused fields. The user asked for "a new ai image interface as opposed to llm".

Decision

A new canonical leaf package imagegen, parallel to llm, re-exported from the root (ImageModel, ImageProvider, ImageRequest, ImageResult, ImageOption, plus WithImageCount/WithImageSize). Providers import imagegen; mort codes to the interface, not to llama-swap.
Minimal v1 surface (text-to-image only):
- Request{ Prompt string; N int; Size string } — zero values mean provider default (N=0 → backend default count; "" Size → backend default).
- Result{ Images []Image; Raw any }.
- Model.Generate(ctx, Request, ...Option) (*Result, error) and Provider.ImageModel(id, ...ModelOption) (Model, error).
- Functional options + Request.Apply, mirroring llm.
type Image = llm.ImagePart (bytes + MIME). Reusing the chat content type means a generated image drops straight back into a chat turn (llm.UserParts(res.Images[0])) with no conversion — the key interop win.
Out of scope for v1 (designed-for, deferred): image edits / img2img, the raw A1111 SDAPI, masks/seeds/steps, streaming, and registry-level image-model DSN resolution (construct the provider directly for now).
First implementation: provider/llamaswap, targeting OpenAI /v1/images/generations with response_format: "b64_json" (bytes inline; we never fetch remote URLs — mirrors ImagePart's bytes-only contract).

Consequences

Image generation is provider-agnostic from day one; a future OpenAI DALL·E or Gemini image backend implements the same interface.
The narrow interface keeps the door open for richer requests without breaking callers (additive fields/options).
No health/failover for image models yet; if needed it can be added as a separate chain type rather than retrofitting the chat chain.

Update — optional per-request settings

Request gained additive optional overrides — Steps *int, CFGScale *float64, NegativePrompt string, Sampler string, Seed *int64 — with mirror options (WithSteps, …). nil/"" means "leave the backend's per-model default", so the v1 contract is unchanged for callers that don't set them. provider/llamaswap forwards them to sd-server as steps/cfg_scale/negative_prompt/sample_method/ seed (omitempty). This realizes the "seeds/steps … additive fields" note above; img2img/masks/streaming remain deferred.

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