Write tool JSON parsing error with large files and special characters

[rizwankce]

Bug Description

The Write tool consistently fails with JSON parsing errors when writing files that contain:

1. Large content (>~1000 characters)
2. Special characters (quotes, newlines, backticks)
3. Code blocks with nested structures

Error Messages

Common errors encountered:

• JSON Parse error: Unterminated string
• Invalid input for tool write: JSON parsing failed
• JSON Parse error: Expected ']'

Reproduction Steps

1. Try to write a Rust file with ~200+ lines using the Write tool
2. Content includes code with quotes, backticks, or special characters
3. Error occurs consistently

Example that fails:

/// Fixed-point arithmetic module (16.16 format)
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Fixed(i32);

impl Fixed {
 const FRACTIONAL_BITS: i32 = 16;
 // ... more code with quotes and special chars
}

Workaround

Using bash with heredoc works reliably:

cat > /path/to/file.rs << 'EOF'
// Large file content here
// With any special characters
EOF

Environment

• Platform: macOS (darwin)
• Model: kimi-k2.5-free
• Working with: Rust code files, Markdown documentation

Impact

• Must use bash workarounds for any substantial file writes
• Breaks flow when editing multiple files
• Limits ability to use Write tool effectively

Suggested Fix

Either:

1. Increase JSON string size limits
2. Better escaping for special characters
3. Alternative write method for large files
4. Document the limitation and recommend bash for large files

[github-actions]

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

• JSON Parse Error - Unterminated String on Write Tool #11531: JSON Parse Error - Unterminated String on Write Tool (similar issue with large content and special characters)
• [BUG] openai/gpt-5.2-codex Error message: JSON Parse error: Unterminated string errors #10755: JSON Parse error: Unterminated string errors with Write tool
• Incomplete JSON when writing out files #5890: Incomplete JSON when writing out files (missing closing braces)
• [BUG] Write tool continously getting aborted #11079: Write tool continuously getting aborted on macOS
• always stuck at “Preparing write...” #11112: Write tool stuck at "Preparing write..." state
• Icons/Unicode characters cause file editing failures #2904: Unicode/special characters causing file handling failures
• Issue: Opencode Fails to Write Files When Switching from Plan to Build Mode #11282: Write tool fails when switching from Plan to Build mode
• [aws bedrock: anthropic opus 4.1] The arguments provided to the tool are invalid: Invalid input for tool write: JSON parsing failed: Text #1803: JSON parsing failed when writing files with AWS Bedrock
• 🐛 Bug Report: MCP Tool JSON Parsing Errors on OpenCode (GLM-4.7) #8102: MCP Tool JSON Parsing Errors with invalid JSON arguments
• Tool.edit should recieve json object #6838: Tool.edit and Write tool JSON parsing issues
• Ollama (qwen2.5-coder): tool calls (edit/write) show as executed but no files are created/modified #7030: Tool calls appear executed but files aren't created
• Write Tool includes metadata in output and created filecontent #5362: Write Tool including metadata in file content

Feel free to ignore if none of these address your specific case.