ADR 0101: Model-aware image attachment transport

• Status: Accepted
• Date: 2026-08-18
• Deciders: Nib runtime and desktop UI maintainers
• Amends: D197, ADR 0059, ADR 0070
• Amended by: D361 (inline bound is 10 MB, matching MiniMax's OpenAI-compatible cap), D392 / ADR 0218 (binding image-input overrides)

Context

Clipboard images were materialized successfully, but the composer reduced every file to @<path> text. A model that supports image input therefore received a filesystem reference instead of an image block. The capability signal must be owned by the same model catalog that pi-ai uses to serialize provider requests; renderer discovery and user-entered model ids are not sufficient evidence.

Decision

1. Keep the composer draft compact and textual. Each file reference retains its kind, name, MIME type, and source path as structured metadata; the visible textarea never contains pasted binary data.
2. Resolve vision capability from the exact pi-ai model record. Only model.input.includes("image") enables image transport. Unknown and custom model ids stay on the conservative non-vision path, even when discovery metadata claims vision.
3. Electron main remains the attachment boundary. It validates every source path against the session scratch root, the session-bound project root, or the content-addressed attachment root. Images are stored as attachments/<sha256> and the durable UiMessage stores only the reference and metadata.
4. For a vision-capable model, an image within the 10 MB inline limit crosses the sidecar only as transient base64 data and becomes a pi-ai image content block. The base64 value is never persisted in SQLite, JSONL, or renderer transcript state.
5. For a non-vision model, an unknown model, or an image above the per-image inline limit, the prompt receives a safe @path fallback. Replayed content-store images are copied into the session scratch replayed/ directory before that fallback is exposed to the model.
6. Restored vision history has a 30 MB aggregate raw-image-byte budget in addition to the 10 MB per-image limit. The sidecar considers newest attachments first, preserving all images when the history fits; older images beyond the budget use the safe path fallback. The current prompt row is excluded before history hydration so its transient payload does not consume the history budget.
7. The Composer shows a compact accessible status row when an image is attached: it states whether the selected model can receive visual input. The model picker and session/provider summaries expose the same authoritative vision capability.

Consequences

• GPT/vision-capable models can inspect pasted images without requiring a model-specific renderer implementation.
• Text-only and non-vision models retain the existing file-tool workflow.
• Image bytes are deduplicated and can survive retry, fork, and runtime recreation without putting binary data in the transcript.
• Restored sessions preserve historical vision blocks regardless of age while the images fit within one fixed aggregate budget; over-budget history has a deterministic memory ceiling and a safe path fallback for older images.
• Attachment garbage collection remains a later storage task; references are content-addressed so it can be added without changing the message contract.
• Full visual previews, drag-and-drop, image transforms, and provider-specific image limits remain out of scope. The 10 MB inline threshold is an app-side safety bound; providers may impose stricter limits and return their normal provider error.

Rejected alternatives

• Always send @path text: preserves non-vision behavior but cannot satisfy vision models.
• Trust renderer/provider discovery flags: can advertise a capability that the runtime adapter cannot serialize and would make unknown models unsafe to classify.
• Persist base64 in messages: makes reloads and SQLite/JSONL growth unbounded and crosses the persistence boundary unnecessarily.