feat(filesystem): add append_file

[altrao]

Description

Added append_file capability to filesystem server:

• Appends content to existing files or creates new files

Server Details

• Server: filesystem
• Changes to: tools

Motivation and Context

Need to provide log-style file operations for LLM clients:

• append_file enables incremental writes without full file rewrites

How Has This Been Tested?

Testing performed for append_file:

1. Basic functionality:
   • Append to existing files
   • Append to new files (creates file)
2. Security boundaries:
   • Works in allowed dir
   • Fails in not specified dir
3. Edge cases:
   • Empty files
   • Large content appends
   • Empty append to new file (creates empty file)
   • Empty append to existing file (maintains empty state)
4. Error handling:
   • Invalid paths
   • Permission issues

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

[olaservo]

Edit: original review below didn't account for conflicts.

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Review: APPROVE

Clean implementation that follows existing codebase patterns.

Strengths

• Proper Zod schema validation (AppendFileArgsSchema)
• Uses validatePath() for security boundary enforcement
• Correct use of fs.appendFile() with UTF-8 encoding
• Documentation updated in README

Note for Maintainers

This PR is nearly identical to #1233 (by gloridea), which also adds append_file with the same implementation approach. Please choose one to merge and close the other to avoid duplication.

Both PRs are solid - the main difference is that #1233 includes a prompt enhancement for edit_file to clarify when to use append vs edit.

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Reviewed with Claude Opus 4.5

[olaservo]

Closing this PR — it has merge conflicts due to the major architecture refactoring (Nov 2025) and would need a full rewrite to use the current registerTool() API with outputSchema, structuredContent, and annotations.

The append_file feature is still wanted (tracked in issue #1208). PR #2816 covers the same functionality and is currently mergeable against main, so we'll focus review efforts there.

Thank you for the clean implementation — it validated the approach that #2816 builds on.

This comment was posted by Claude Code on behalf of @olaservo.