TUI hangs on 'Preparing to write...' with large file content (unbounded diff rendering)

[caramaschiHG]

Summary

The TUI freezes indefinitely on "Preparing to write..." when the write or bash tool receives large content (150+ lines). Small content (1-2 lines) works fine.

Environment

• OS: Windows 11
• OpenCode Version: 1.17.3 (npm global install)
• Terminal: TUI mode
• Plugin: oh-my-openagent@latest

Reproduction Steps

1. Start OpenCode TUI session
2. Request AI to write a file with 150+ lines of content
3. Observe TUI freezing at "Preparing to write..."
4. The operation never completes (requires manual termination)

Works: write with 1-10 lines of content
Hangs: write with 150+ lines of content
Same behavior: bash with large heredoc/content in parameters

Root Cause Analysis

The hang occurs in the permission rendering layer, not in tool execution:

1. Permission metadata includes unbounded diff

In packages/core/src/permission.ts, the metadata field has no size constraint:

metadata: Schema.Record(Schema.String, Schema.Unknown).pipe(Schema.optional)

2. TUI renders entire diff synchronously

In packages/tui/src/routes/session/permission.tsx (EditBody component):

<scrollbox height="100%">
  <diff
    diff={diff()}  // ← ENTIRE DIFF, NO SIZE LIMIT
    view={view()}
    ...
  />
</scrollbox>

The <diff> component from @opentui/core parses the entire diff synchronously, blocking the event loop when content is large.

3. Existing limits don't protect against this

Component	Limit	Protects Permission Rendering?
tool-output-store.ts	50 KB / 2000 lines	❌ No
bash.ts	1 MB per stream	❌ No
Permission diff	None	❌ This is the gap

Log Evidence

From %TEMP%\oh-my-opencode.log:

[tool-execute-after] Unable to recover stored metadata and no native session linkage was present {"tool":"write",...}

This pattern repeats during hangs, suggesting metadata handling issues with large content.

Proposed Solutions

Option A: Truncate diff in permission metadata

Add a size limit (e.g., 100 KB) when creating permission requests. Show truncation notice in UI.

Option B: Add max_diff_size config option

Similar to existing tool_output.max_bytes, allow users to configure:

{
  "permission": {
    "max_diff_size": 102400
  }
}

Option C: Async/lazy diff rendering

Render diff asynchronously or implement lazy-loading for large diffs in the permission dialog.

Related Issues

• fix(opencode): skip O(N^2) diff computation on large files #20471 - fix(opencode): skip O(N²) diff computation on large files (closed for revision)
• always stuck at “Preparing write...” #11112 - always stuck at "Preparing write..."
• TUI hangs indefinitely when resuming sessions with large file diffs (formatPatch/structuredPatch blocks event loop) #23362 - TUI hangs indefinitely when resuming sessions with large file diffs

Workarounds (Current)

1. Use edit tool instead of write for incremental changes
2. Write files in smaller chunks
3. Set "scroll_acceleration": { "enabled": false } in config (marginal improvement)

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• always stuck at “Preparing write...” #11112: always stuck at "Preparing write..." (same symptom)
• write/edit freezes when writing long content ("Preparing write..." hangs) #20477: write/edit freezes when writing long content ("Preparing write..." hangs) (same symptom with large files)
• [bug] 使用opencode经常出现preparing patch....问题 #31843: similar "preparing patch" hang issue

Please review those issues and consider whether your report adds new information. If it does, it would be helpful to comment there rather than open a new issue.