Edit tool diff preview renders entire file as all-green additions (no removals, no context)

Symptom

When Claude Code proposes an Edit tool change, the diff preview shows the entire new file content as one big green block — no red removed lines, no -/+ interleaving, no context lines. Just the full new content rendered as all-additions. Happens for tiny single-line edits to a normal-sized Python file (~200 lines).

Reproduction

1. In a Claude Code session, ask Claude to make a one-character or one-line change to an existing Python file.
2. Observe the Edit tool's diff preview before approving.

Expected: unified diff with red - removed lines and green + added lines, with surrounding context.

Actual: full new file content shown in green, old lines absent entirely.

Environment

• Claude Code version: 2.1.141
• OS: WSL2 Ubuntu on Windows 11
• Node: v24.15.0 (bundled binary)
• Settings: {"model": "opus", "theme": "auto"} — no other overrides
• TERM: xterm-256color
• COLORTERM: unset
• CLAUDE_EFFORT: xhigh

Ruled out

• Not line endings — file is LF, file confirms no CRLF.
• Not the JetBrains plugin's IDE diff handoff — persists with ENABLE_IDE_INTEGRATION unset and CLAUDE_CODE_SSE_PORT unset.
• Not the PyCharm "Reworked Terminal" — switched to Classic Terminal, reproduces. Also reproduces in standalone Ubuntu/WSL terminal outside PyCharm entirely.
• Not a per-project setting — project has no .claude/settings.json or CLAUDE.md.

Notes

Binary string search shows the renderer has DiffTooLarge, Binary file - cannot display diff, diffAdded/diffAddedDimmed/diffRemoved/diffRemovedDimmed constants, so a fallback code path appears to exist — possibly being triggered incorrectly here.

Happy to provide more diagnostic info (debug logs, etc.) if useful.

Update: likely related plugin-side stack trace

Caught this exception thrown by the Claude Code JetBrains plugin (com.anthropic.code.plugin) version 0.1.14-beta on PyCharm 2026.1 + WSL2 Ubuntu. The plugin's DiagnosticTools MCP handler is calling the IntelliJ EEL (WSL bridge) blocking API on the EDT, which the platform forbids:

java.lang.IllegalStateException: This method is forbidden on EDT because it does not pump the event queue. Switch to a BGT, or use com.intellij.openapi.progress.TasksKt.runWithModalProgressBlocking.
    at com.intellij.openapi.progress.CoroutinesKt.assertBackgroundThreadAndNoWriteAction(coroutines.kt:613)
    at com.intellij.openapi.progress.CoroutinesKt.runBlockingCancellable(coroutines.kt:150)
    at com.intellij.openapi.progress.CoroutinesKt.runBlockingMaybeCancellable(coroutines.kt:214)
    at com.intellij.platform.eel.provider.EelProviderUtil.toEelApiBlocking(EelProvider.kt:195)
    at com.intellij.openapi.vfs.impl.local.LocalFileSystemEelUtil.readWholeFileIfNotTooLargeWithEel(util.kt:81)
    at com.intellij.openapi.vfs.impl.local.LocalFileSystemBase.readIfNotTooLarge(LocalFileSystemBase.java:447)
    at com.intellij.openapi.vfs.impl.local.LocalFileSystemImpl.readContent(LocalFileSystemImpl.java:484)
    ...
    at com.intellij.codeInsight.daemon.impl.DaemonCodeAnalyzerImpl.restart(DaemonCodeAnalyzerImpl.java:637)
    at com.intellij.codeInsight.daemon.DaemonCodeAnalyzer.restart(DaemonCodeAnalyzer.java:75)
    at com.anthropic.code.plugin.mcp.tools.DiagnosticTools$addTools$1.invokeSuspend$lambda$1(DiagnosticTools.kt:164)

This is plausibly the root cause of the diff-rendering bug above: if the MCP handler the plugin exposes to Claude Code for fetching file state crashes when reading WSL-side files, Claude Code never gets the "before" state, and the Edit-tool preview falls back to showing only the "after" content (rendered as all-additions).

Fix is on the plugin side: wrap the EEL read in withContext(Dispatchers.IO) / runWithModalProgressBlocking, or otherwise move it off the EDT.

[github-actions]

Found 2 possible duplicate issues:

1. [BUG] Edit preview/diff not showing in VSCode extension UI when confirming changes #8660
2. [BUG] VSCode extension diff preview fails with CRLF line endings - "String not found in file" #30290

This issue will be automatically closed as a duplicate in 3 days.

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🤖 Generated with Claude Code

[github-actions]

This issue has been automatically closed as a duplicate of #8660.

If this is incorrect, please re-open this issue or create a new one.

🤖 Generated with Claude Code

[github-actions]

This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.