Closes audit gap #2 (FilePart \u2192 MediaPart not implemented).
The bridge now lowers `MessageV2.FilePart` on user messages into
`LLM.MediaPart`, unblocking image and document inputs. The first
pass supports `data:` URLs only — the inline base64 form most
commonly produced by the OpenCode UI for pasted screenshots and
attached files. `http(s):` and `file:` URLs are explicitly
rejected with a clear error so a future fetch / filesystem-read
path can plug in cleanly without regressing safety.
Implementation:
- New `lowerFilePart` helper extracts the base64 payload from a
data URL via a single regex; failure yields a typed
`UnsupportedContentError` carrying both the partType and a
`reason` that includes the offending URL for debuggability.
- New `lowerUserPart` dispatches user-side parts: text \u2192
`LLM.text`, file \u2192 `MediaPart`. Returns identity-empty
for any unsupported part type the static gate would have caught.
- `userMessage` is now `Effect.fnUntraced` so file conversion can
yield typed errors. `lowerMessage` (the per-message dispatcher,
renamed from `messages` to free the local name) cascades the
Effect through the request flow via `Effect.forEach`.
- `supportsPart` static gate now allows `file` parts on user
messages. Assistant messages still reject file parts (the LLM
IR's MediaPart isn't valid in assistant content for any
adapter we ship today).
- `UnsupportedContentError` gains an optional `reason` field that
appends to the canonical message as `<base>: <reason>`. Existing
static-gate failures keep the same shape (no reason).
Tests (3 new, 1 rewritten):
- Image data URL with filename round-trips to MediaPart with
base64-stripped data.
- PDF data URL preserves filename and base64 payload.
- `https:` URL rejected with an error mentioning both the file
partType, the message ID, and the offending URL.
- The pre-existing "fails instead of dropping unsupported native
parts" test now uses a reasoning part on a user message
(reasoning is valid for assistants only) since file parts with
data URLs are no longer rejected by the static gate.
Out of scope, intentional follow-ups:
- HTTP/HTTPS URL fetching (would need HttpClient.HttpClient and a
decision on caching, retries, size limits).
- File path / file:// URL reading (would need FileSystem.FileSystem
and a permission check against the session's working directory).
- File parts on assistant messages (LLM IR doesn't model
assistant-side media; defer until we hit a provider that needs it).
- text/plain and application/x-directory file parts that the
AI-SDK path converts to text inline at message-v2.ts:791 — for
the bridge, those should be converted upstream before reaching
LLMNative.request rather than handled here.
Verified: bun typecheck clean, 28/0/0 across native + bridge
tests (was 21; +7 from the FilePart additions plus the rewritten
unsupported-parts test).
Kit Langton
b653261772