`replace_string_in_file` reports success but doesn't persist changes

Start by reproducing replace_string_in_file on the affected Swift files in an Xcode workspace, then compare its on-disk result with insert_edit_into_file. Trace the replace_string_in_file entry point and verify that successful edits persist and that failed writes or unmatched replacements report an error rather than success.

Summary

The replace_string_in_file editing tool consistently reports successful edits but fails to write changes to disk, requiring escalation to insert_edit_into_file. This silent failure wastes significant time and breaks trust in the tool.

Core Problem

What happens:

1. Tool reports: "✅ The following files were successfully edited"
2. File on disk remains unchanged (verified via grep, sed)
3. Compilation errors persist for missing code
4. No error message or warning given

Scale: 15-20 failed operations across multiple files in a single session

Reproduction Steps

Attempt 1: Basic Replace

// Trying to add property to struct
OLD: public var existingProperty: Type
NEW: public var existingProperty: Type
     public var newProperty: NewType  // ← Should be added

Result: Tool reports success, property not added

Attempt 2: More Context

Added 5+ lines of surrounding context for uniqueness
Result: Same false success, no file modification

Attempt 3: Smaller Edits

Broke into multiple atomic replace_string_in_file calls
Result: All reported success, none persisted

Attempt 4: Different Tool (Workaround)

Switched to insert_edit_into_file with full code block
Result: ✅ SUCCESS - File actually modified

Examples of Silent Failures

Example 1: Adding Property

// Attempting to add to line 21:
public var radius: Double
public var newConfig: Config  // ← Not added despite success report
public var model: ModelType

Verified via: grep "newConfig" File.swift (no results)

Example 2: Renaming Class

OLD: public class OldName {
NEW: public class NewName {

Verified via: grep "class NewName" (empty - still OldName)

Example 3: Updating Parameters

Changed type in 4 method signatures
All 4 reported success, none actually changed
Verified via: grep "OldType" (still 4 occurrences)

File Context

Common factors:

• Xcode workspace (.xcworkspace)
• Swift files (.swift)
• Some had extended attributes (@ flag)
• Files actively loaded in Xcode
• 100-30,000 lines
• Complex Swift (generics, protocols, async/await)

Impact

• Time lost: 60-80 minutes debugging
• Trust: Tool reliability undermined
• Workflow: Required terminal verification for every edit
• Escalation: Eventually abandoned replace_string_in_file entirely

Expected vs Actual Behavior

Expected	Actual
File modified on disk	File unchanged
Compilation reflects changes	Compilation errors persist
Subsequent reads show new content	Old content remains
OR: Error reported if write fails	False success reported

Successful Workaround

# 1. Detect failure with terminal:
grep "expected_symbol" file.swift  # (no output)
sed -n 'line,linep' file.swift     # Shows old code

# 2. Use insert_edit_into_file instead:
# - Provide complete code block
# - Use // ...existing code... comments
# - Tool writes successfully

# 3. Verify after edit:
grep "expected_symbol" file.swift  # Now found

Potential Causes

1. File caching - Editor cache not flushed
2. Xcode locks - Files locked by IDE
3. Extended attributes - @ flag preventing writes
4. Path resolution - Writing to wrong location
5. Silent match failure - Can't find string, reports success anyway

Recommended Fixes

1. Verify writes - Read file back after write to confirm
2. Report actual errors - Show permission/lock failures
3. Flush buffers - Explicitly sync to disk before success
4. Detect locks - Check if Xcode has file locked
5. Auto-fallback - Suggest insert_edit_into_file on failure

Evidence

# Tool says success:
✅ The following files were successfully edited: /path/to/File.swift

# Reality check:
$ grep "newProperty" File.swift
# (no output - property was NOT added)

$ sed -n '19,23p' File.swift
    public var oldProperty: Type
    public var anotherOld: Type
# (missing newProperty that should be between them)

Reproducibility

• Frequency: 100% for affected files
• Pattern: Consistent across multiple file types/sizes
• Workaround: insert_edit_into_file always worked

Key Observation

The fact that insert_edit_into_file works reliably suggests this is a replace_string_in_file-specific issue, not a general file writing problem.

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Priority: High - Core functionality broken
Request: Investigate silent failures in Xcode workspace context with Swift files