Fix ApplyTextEdits replacement index calculation and port probe handshake

[dsarno]

Summary

• Avoid invalid substring lengths when auto-upgrading ApplyTextEdits to replace_method
• Compute replacement snippet relative to the method span with clamped ranges
• Honor FRAMING=1 handshake when probing Unity MCP ports so framed ping/pong works

Testing

• pytest -q

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https://chatgpt.com/codex/tasks/task_e_68a296f62bcc8327addf3431c744a0da

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[greptile-apps]

Greptile Summary

This PR addresses two critical fixes in the Unity MCP Bridge system. The first fix resolves a dangerous bug in the ApplyTextEdits method's auto-upgrade logic within ManageScript.cs. The original implementation had a flawed substring calculation that could cause IndexOutOfRangeException or produce incorrect text replacements when converting text edits to structured replace_method operations. The new implementation properly extracts the method text first, converts edit positions to relative coordinates within that method, clamps ranges to prevent out-of-bounds access, and then performs safe replacement operations.

The second fix updates the port discovery mechanism in port_discovery.py to support the new framing protocol (FRAMING=1). The Unity MCP Bridge now sends greeting messages advertising framing capabilities, and the probe function has been enhanced to detect these greetings and handle both framed and unframed ping/pong exchanges. When FRAMING=1 is detected, messages are sent/received with 8-byte big-endian length prefixes, ensuring proper communication with modern Unity MCP servers while maintaining backward compatibility.

These changes integrate well with the existing codebase architecture - the ManageScript.cs fix preserves the auto-upgrade functionality for text editing operations while making them crash-safe, and the port_discovery.py enhancement extends the existing port probing mechanism to work with the new protocol framing system that appears to be part of a broader protocol evolution in this MCP implementation.

Important Files Changed

Click to expand file changes

Filename	Score	Overview
UnityMcpBridge/Editor/Tools/ManageScript.cs	4/5	Fixed critical substring calculation bug in ApplyTextEdits auto-upgrade logic
UnityMcpBridge/UnityMcpServer~/src/port_discovery.py	4/5	Added FRAMING=1 protocol support to port discovery probe mechanism

Confidence score: 4/5

• This PR is safe to merge with minimal risk as both fixes address specific, well-defined issues
• Score reflects solid bug fixes with clear improvements to system reliability and protocol compatibility
• Pay close attention to UnityMcpBridge/Editor/Tools/ManageScript.cs to ensure the new substring logic handles all edge cases correctly

Sequence Diagram

sequenceDiagram
    participant User
    participant MCP_Server as MCP Server
    participant Port_Discovery as Port Discovery
    participant Unity_Bridge as Unity Bridge
    participant Unity_Editor as Unity Editor
    participant Script_Manager as Script Manager
    participant Validation as Validation System

    User->>MCP_Server: "apply_text_edits request"
    MCP_Server->>Port_Discovery: "discover_unity_port()"
    Port_Discovery->>Port_Discovery: "list_candidate_files()"
    Port_Discovery->>Unity_Bridge: "TCP connect + probe with FRAMING=1"
    Unity_Bridge-->>Port_Discovery: "greeting with FRAMING=1"
    Port_Discovery->>Unity_Bridge: "framed ping (8-byte header + payload)"
    Unity_Bridge-->>Port_Discovery: "framed pong response"
    Port_Discovery-->>MCP_Server: "validated port number"
    
    MCP_Server->>Unity_Bridge: "forward apply_text_edits request"
    Unity_Bridge->>Script_Manager: "HandleCommand(apply_text_edits)"
    Script_Manager->>Script_Manager: "validate precondition_sha256"
    Script_Manager->>Script_Manager: "convert edits to absolute indices"
    
    alt single edit targeting method
        Script_Manager->>Script_Manager: "detect method span for auto-upgrade"
        Script_Manager->>Script_Manager: "compute replacement relative to method span"
        Script_Manager->>Script_Manager: "upgrade to replace_method operation"
    else multiple/other edits
        Script_Manager->>Script_Manager: "apply text edits in reverse order"
    end
    
    Script_Manager->>Validation: "CheckBalancedDelimiters()"
    Validation-->>Script_Manager: "validation result"
    
    opt USE_ROSLYN defined
        Script_Manager->>Validation: "Roslyn syntax validation"
        Validation-->>Script_Manager: "syntax diagnostics"
    end
    
    Script_Manager->>Unity_Editor: "atomic file write with backup"
    Script_Manager->>Unity_Editor: "ScheduleScriptRefresh()"
    Unity_Editor->>Unity_Editor: "debounced asset import and compilation"
    
    Script_Manager-->>Unity_Bridge: "success response with new SHA256"
    Unity_Bridge-->>MCP_Server: "operation result"
    MCP_Server-->>User: "edits applied successfully"

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