Part of duplicate code analysis: #1661

Summary

Three functions in internal/server/unified.go contain a near-identical block for executing a backend MCP tool call over a session-aware connection. The shared logic — launching/getting a connection, sending a tools/call request, checking the error response, and unmarshalling the JSON result — is ~18 lines long and diverges only in return types and the logging context.

Duplication Details

Pattern: launch connection → SendRequestWithServerID → check error → unmarshal result

• Severity: High
• Occurrences: 3
• Locations:
  • internal/server/unified.go lines 569–604 (guardBackendCaller.CallTool)
  • internal/server/unified.go lines 703–730 (callBackendTool, Phase 3)
  • internal/server/unified.go lines 269–295 (registerToolsFromBackend, partial – uses tools/list)

Code in guardBackendCaller.CallTool (lines 569–604):

conn, err := launcher.GetOrLaunchForSession(g.server.launcher, g.serverID, sessionID.(string))
if err != nil {
    return nil, fmt.Errorf("failed to connect: %w", err)
}
response, err := conn.SendRequestWithServerID(g.ctx, "tools/call", map[string]interface{}{
    "name":      toolName,
    "arguments": args,
}, g.serverID)
if err != nil {
    return nil, err
}
if response.Error != nil {
    return nil, fmt.Errorf("backend error: code=%d, message=%s", response.Error.Code, response.Error.Message)
}
var result interface{}
if err := json.Unmarshal(response.Result, &result); err != nil {
    return nil, fmt.Errorf("failed to parse result: %w", err)
}
return result, nil

Nearly identical block in callBackendTool Phase 3 (lines 703–730):

conn, err := launcher.GetOrLaunchForSession(us.launcher, serverID, sessionID)
if err != nil {
    return newErrorCallToolResult(fmt.Errorf("failed to connect: %w", err))
}
response, err := conn.SendRequestWithServerID(ctx, "tools/call", map[string]interface{}{
    "name":      toolName,
    "arguments": args,
}, serverID)
if err != nil {
    return newErrorCallToolResult(err)
}
if response.Error != nil {
    return newErrorCallToolResult(fmt.Errorf("backend error: code=%d, message=%s", response.Error.Code, response.Error.Message))
}
var backendResult interface{}
if err := json.Unmarshal(response.Result, &backendResult); err != nil {
    return newErrorCallToolResult(fmt.Errorf("failed to parse result: %w", err))
}

The only structural differences are:

1. Return type: (interface{}, error) vs (*sdk.CallToolResult, interface{}, error)
2. Error wrapper: return nil, err vs return newErrorCallToolResult(err)
3. Session resolution: g.ctx.Value(...) vs us.getSessionID(ctx) (already resolved before the block)

Impact Analysis

• Maintainability: Any change to error message format, response error handling, or JSON unmarshal logic must be applied in both places. The current identical error string "backend error: code=%d, message=%s" is already evidence of copy-paste.
• Bug Risk: Medium — a future fix to one branch (e.g., adding retry logic or telemetry) is likely to miss the other.
• Code Bloat: ~18 lines duplicated per instance.

Refactoring Recommendations

1. Extract executeBackendToolCall helper

   • Location: internal/server/unified.go (or new internal/server/backend_call.go)
   • Signature: func executeBackendToolCall(ctx context.Context, l *launcher.Launcher, serverID, toolName string, args interface{}) (interface{}, error)
   • Returns raw (interface{}, error) — the callers adapt the result to their specific return types.
   • guardBackendCaller.CallTool can call this directly.
   • callBackendTool calls this, then applies DIFC Phase 4–5 on top.
   • Estimated effort: ~1 hour, low risk.

2. Align session ID resolution before the shared block

   • guardBackendCaller.CallTool extracts the session ID inline via g.ctx.Value(...).
   • Extracting this to before the helper call aligns with how callBackendTool already calls us.getSessionID(ctx) first.

Implementation Checklist

• Review duplication findings
• Extract executeBackendToolCall(ctx, launcher, serverID, toolName, args) helper
• Update guardBackendCaller.CallTool to use helper
• Update callBackendTool Phase 3 to use helper
• Verify registerToolsFromBackend pattern (uses tools/list — only first half overlaps; can share connect+error pattern)
• Update tests
• Verify no functionality broken

Parent Issue

See parent analysis report: #1661
Related to #1661

Generated by Duplicate Code Detector

• expires on Mar 15, 2026, 3:01 AM UTC