[Bug] Tool Execution Issues: Parallel execution, Grep regex error handling, and Read negative offset validation with Claude Cowork

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [ ] This is a single bug report (Note: 3 related tool bugs consolidated for context)
• [x] I am using the latest version of Claude Code

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What's Wrong?

Three tool execution issues discovered during Cowork mode testing:

Bug 1: Parallel Tool Calls Execute Sequentially

When multiple tool calls are made simultaneously in a single message, they execute sequentially rather than in parallel.

Bug 2: Grep Tool Returns Misleading Message for Invalid Regex

The Grep tool returns "No files found" instead of indicating a regex syntax error when given an invalid pattern.

Bug 3: Read Tool Accepts Negative Offset

The Read tool accepts negative offset values and produces output with negative line numbers (e.g., -5→, -4→).

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What Should Happen?

Bug 1: Parallel Execution

• Multiple independent tool calls should execute concurrently
• 3 × 2-second sleep commands should complete in ~2s, not ~10s

Bug 2: Grep Error Handling

• Invalid regex patterns should return clear error messages
• Example: "Error: Invalid regular expression - Unmatched '(' at position 16"

Bug 3: Read Validation

• Negative offset values should be rejected with a validation error
• Example: "Error: offset must be a non-negative integer"

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Steps to Reproduce

Bug 1: Parallel Execution

1. Open Claude for Mac → Cowork mode
2. Request: "Run three independent sleep 2 commands in parallel"
3. Observe timestamps show sequential execution (~10s total, not ~2s)

Evidence:

Task 1 開始: 13:34:47.831843934
Task 1 結束: 13:34:49.833217566

Task 2 開始: 13:34:51.706488544  ← Started ~2s AFTER Task 1 ended
Task 2 結束: 13:34:53.707604785

Task 3 開始: 13:34:55.399567678  ← Started ~2s AFTER Task 2 ended
Task 3 結束: 13:34:57.400674391

Bug 2: Grep Invalid Regex

1. Use Grep tool with pattern: [invalid regex (((
2. Observe response: "No files found" (misleading)

Comparison with ripgrep directly:

$ rg "[invalid regex (((" /path
error: regex parse error:
    [invalid regex (((
                    ^
error: unclosed group

Bug 3: Read Negative Offset

1. Use Read tool with offset=-5 on any file
2. Observe output shows negative line numbers:

    -5→Test file for Read tool
    -4→

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Claude Code Version

Claude for Mac Version 1.0.3218 (8679c9)

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Is this a regression?

I don't know

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Platform

Other (Claude for Mac - Cowork/Sandbox mode)

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Operating System

macOS

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Terminal/Shell

Claude for Mac (native app)

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Error Messages/Logs

Bug 1 - No error, just unexpected sequential timing

Bug 2 - Misleading "No files found" instead of regex parse error

Bug 3 - No error, invalid output with negative line numbers

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Impact Summary

| Bug | Severity | Impact |
|-----|----------|--------|
| Parallel Execution | Medium | 5x performance degradation |
| Grep Error Handling | Low | User confusion, wasted debugging time |
| Read Negative Offset | Low | Invalid output, poor input validation |

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Suggested Fixes

1. Parallel Execution: Enable concurrent tool execution or clarify documentation
2. Grep Error Handling: Surface ripgrep's error messages to users
3. Read Validation: Add "minimum": 0 constraint to offset parameter schema

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Reporter: Claude Code Community Testing
Date: 2026-01-13