🤖 fix: normalize mux-gateway file parts so image attachments are accepted

[Neppkun]

Summary

Attaching an image to a chat message on a mux-gateway model fails with "invalid request", while text-only turns work. The AI SDK v7 upgrade changed how @ai-sdk/gateway serializes file parts on the wire, and the gateway server still validates the old shape. This PR adds a wire-level fetch shim that rewrites file parts back to the encoding the server accepts.

Background

#3707 bumped @ai-sdk/gateway from 3.0.x (spec v3) to 4.0.15 (spec v4). The two versions encode inline file data differently:

SDK	file part data on the wire
3.x (spec v3)	"data:image/png;base64,iVBOR…" — plain string
4.x (spec v4)	{ "type": "data", "data": "iVBOR…" } — tagged object (or { "type": "url", "url": … })

The request captured from a failing turn shows the v4 object shape, and the gateway server rejects it. (The server also still emits v2-style flat usage that gatewayStreamNormalization.ts patches around, so it lagging the SDK's wire protocol is consistent.)

Implementation

• src/node/utils/gatewayFilePartNormalization.ts (new): wrapFetchWithGatewayFilePartNormalization inspects gateway POST bodies and rewrites {type:"data"} → data:<mediaType>;base64,… (falling back to application/octet-stream, matching the 3.x SDK) and {type:"url"} → the URL string. Covers user/assistant file and reasoning-file parts and files nested in tool-result content output — the same set the SDK's own maybeEncodeFileParts touches.
• Requests without file parts are forwarded with the original init untouched (cheap substring pre-check, no JSON round-trip); content-length is dropped when the body is rewritten.
• providerModelFactory.ts: wraps the mux-gateway baseFetch with it as the innermost layer, so the existing Anthropic cache-control and auto-logout wrappers still apply on top.

The module doc marks the shim as removable once the gateway server accepts spec-v4 file parts.

Validation

• Confirmed the encoding difference by diffing maybeEncodeFileParts between @ai-sdk/gateway@3.0.110 and @4.0.15.
• Unit tests cover the data/url/string/unknown conversions, in-place prompt rewriting (including reasoning-file and tool-result nesting), and the fetch wrapper's pass-through cases (text-only, non-POST, non-string body, unparseable JSON).

Risks

Low and scoped to mux-gateway requests that contain file parts; text-only requests are byte-for-byte unchanged. If the gateway server is later upgraded to accept only the v4 shape, this shim would need to be removed — the module comment calls that out.

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Generated with xum • Model: anthropic:claude-fable-5-1 • Thinking: medium • Cost: $4.15

[Neppkun]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: 064fee5da8

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