[FEATURE]: Rust-backed async diff engine via claw-code (fixes write/edit freeze + unlocks claw-code Rust primitives) #20479

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Note: The original aggregated RFC I posted has been broken up. This issue focuses on one specific feature: a Rust-backed (or Worker-offloaded) async diff engine, inspired by the independently-derived claw-code Rust implementation. The other areas from the original RFC that already have open issues are linked below for cross-reference.

Describe the enhancement you want to request

Problem

The createTwoFilesPatch call in the write and patch tools runs synchronously on the main thread using a pure-JS Myers diff (O(N²) in line count). On files larger than ~500 lines this visibly blocks the Node.js/Bun event loop — users see Preparing write... or Preparing patch... hang for several seconds before the permission prompt appears. This is already documented in #20477.

The root fix is to move diff computation off the main thread. The opportunity here is to go further and adopt a Rust-backed diff engine, which is what claw-code implements for exactly this reason — the Rust similar / imara-diff crates are 10–50x faster than JS Myers implementations for large files and run in a separate OS thread.

Proposed solution

Two-tier implementation:

1. Immediate (pure JS): Move createTwoFilesPatch into a Worker thread so it never blocks the event loop. This is a small, low-risk change that fixes write/edit freezes when writing long content ("Preparing write..." hangs) #20477 regardless of anything else.

2. Follow-on (optional native): Add an optional NAPI native addon (packages/opencode/src/tool/diff-native/) wrapping claw-code's Rust diff crate. The JS Worker path becomes the fallback when the native addon isn't compiled/available (e.g., CI, container, unsupported arch).

// packages/opencode/src/tool/diff.ts

// Resolves to native NAPI binding if available, falls back to Worker thread
export async function createPatchAsync(
  originalPath: string,
  original: string,
  updated: string
): Promise {
  if (NativeDiff.isAvailable()) {
    return NativeDiff.createPatch(originalPath, original, updated)
  }
  return WorkerDiff.createPatch(originalPath, original, updated)
}

Why claw-code specifically

claw-code (instructkr/claw-code/rust) is an open-source, independently-derived Rust reimplementation of agentic coding tool primitives — it is not derived from any proprietary source. Its diff module uses imara-diff which produces unified diffs compatible with the existing patch format OpenCode already consumes. License compatibility with OpenCode's MIT license should be verified before bundling.

Acceptance criteria

• Diff operations on files up to 10k lines complete in under 100ms without blocking the event loop
• Worker thread fallback works in all existing environments (Linux, macOS, Windows, containers)
• Native NAPI addon is opt-in at build time (OPENCODE_NATIVE_DIFF=1), not required
• Existing diff output format (unified patch) is byte-for-byte compatible with current behavior
• License compatibility of claw-code Rust crates confirmed before any native bundling

Related / cross-reference

Previously linked:

• Async diff / write freeze: write/edit freezes when writing long content ("Preparing write..." hangs) #20477 (this issue directly addresses that)
• Background task result batching: [FEATURE]: Silent result collection for background tasks to avoid consuming LLM turns #20460
• Tool result cache / session loop perf: [FEATURE]: Session loop performance — message cache, tool cache, doom loop optimization, summary debounce, parallel plugin events #20285, [FEATURE]: Session loop performance — doom loop in-memory, summary debounce, parallel plugin events #20302
• Context summarization / auto-memory: [FEATURE]: Native auto-memory for cross-session learning #20322
• Orchestrator-executor topology / coordinator shell: [FEATURE]: Coordinator shell + fork mode #20368, [FEATURE]: Add hidden field to skill frontmatter to exclude skills from agent selection #20474
• Learnings from claude-code source: Lessons from claude-code-main? #20475

New claw-code derived feature requests (filed 2026-04-01):

• AI-callable Config tool: [FEATURE]: AI-callable Config tool for runtime self-configuration by agents #20481
• SendUserMessage / proactive sub-agent notifications: [FEATURE]: SendUserMessage / Brief tool for proactive async notifications from sub-agents #20482
• Sub-agent tool-set isolation by role: [FEATURE]: Sub-agent tool-set isolation by role (Explore / Plan / Verification scopes) #20483
• TodoWrite parallel workflow + verificationNudgeNeeded: [FEATURE]: TodoWrite parallel workflow support + verificationNudgeNeeded signal #20484
• StructuredOutput tool for typed agent data exchange: [FEATURE]: StructuredOutput tool for reliable typed data extraction from sub-agents #20485
• Sleep tool for non-blocking agent waits: [FEATURE]: Sleep tool for agents to wait without holding a shell process #20486
• NotebookEdit tool for Jupyter cell-level editing: [FEATURE]: NotebookEdit tool for Jupyter notebook cell-level editing #20487
• REPL tool for isolated language-specific execution: [FEATURE]: REPL tool for isolated language-specific code execution separate from bash #20488
• ToolSearch for lazy/deferred tool discovery: [FEATURE]: ToolSearch for lazy/deferred tool discovery to reduce per-turn context size #20489