fix(cli): reject unsupported webfetch images

[chrarnoldus]

Summary

Exclude ICO MIME variants from images returned as webfetch attachments so they cannot reach vision providers that reject icon containers. Other image formats remain attachments, while SVG and FastBidSheet responses retain their existing text behavior.

This prevents provider-level invalid request errors for fetched icons without introducing a restrictive image-format allowlist.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Other Observations (not in diff)

Issues found in unchanged code that cannot receive inline comments:

File	Line	Issue
packages/opencode/src/tool/read.ts	28	SUPPORTED_IMAGE_MIMES hardcodes four MIME types (jpeg, png, gif, webp) that are now a strict subset of what isImageAttachment accepts. Now that isImageAttachment uses a broad allowlist (all image/* except svg, fastbidsheet, and icons), read.ts could import and use isImageAttachment instead of maintaining a divergent set — adding a new supported type to media.ts would silently leave read.ts unaffected.

Incremental Review Notes

The new commit (714f920b) moves the icon rejection earlier and changes it from returning a soft { output: "Unsupported image format..." } result to throwing new Error(...). This is consistent with how other error conditions (oversized responses, invalid URLs) are handled in the same Effect generator — thrown errors surface as tool-level failures rather than LLM-visible output strings. The test is correctly updated to rejects.toThrow().

Files Reviewed (4 files)

• .changeset/calm-icons-fetch.md
• packages/opencode/src/tool/webfetch.ts
• packages/opencode/src/util/media.ts
• packages/opencode/test/tool/webfetch.test.ts

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_{Reviewed by claude-4.6-sonnet-20260217 · 414,562 tokens}

_{Review guidance: REVIEW.md from base branch main}

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