feat: auto-compress oversized images before API submission

[noah79]

Summary

Images pasted from clipboard or read via the file read tool can exceed provider API limits (Anthropic's 5MB base64 limit, dimension caps). This adds automatic image compression using sharp at the provider transform layer, covering all image paths uniformly.

This supersedes #6455 (auto-closed after 60 days) with significant improvements, and addresses the read.ts coverage gap noted in #12069.

Changes

• New packages/opencode/src/util/image.ts — Image compression utility:

  • Three-phase compression: quality reduction → progressive dimension reduction → final fallback
  • Smart format selection: preserves PNG/WebP for transparency, converts opaque images to JPEG
  • Correct 3.75MB raw-byte threshold (base64 inflates ~4/3x → stays under 5MB API limit)

• Provider transform integration (transform.ts):

  • New compressImages() step runs before message normalization
  • Handles both image and file parts with graceful error fallback
  • Corrupt/unsupported images surface as error text instead of silently failing
  • message() is now async to support compression

• Read tool validation (read.ts):

  • Images validated and optimized via sharp before base64 encoding
  • Clear error for corrupt or unsupported image formats

• 23 unit tests covering compression, resizing, format detection, transparency handling, and error cases

Key differences from #6455

	Original PR	This PR
Compression	Quality reduction only	Three-phase: quality → dimension → fallback
Threshold	4MB (incorrect for base64)	3.75MB raw bytes (correct for 5MB base64 limit)
Coverage	Clipboard only	All paths via provider transform layer
read.ts	Not covered	Validated + optimized before encoding
Error handling	Silent catch	Surfaced as error text to model

Testing

cd packages/opencode
bun test test/util/image.test.ts      # 23 tests pass
bun test test/provider/transform.test.ts  # 121 tests pass
bun run typecheck                     # Clean across all 13 workspace packages

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Related PRs Found

Related PR:

• fix: resize images before sending to Anthropic API #12069 - fix: resize images before sending to Anthropic API
  • This PR is explicitly mentioned in the current PR's description as addressing the "read.ts coverage gap noted in fix: resize images before sending to Anthropic API #12069"

Note: The original PR #6455 (which this PR supersedes) was auto-closed after 60 days and is not returned in search results, but it's a predecessor rather than a duplicate.

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