diff --git a/JavaToCSharp.Tests/ConcurrencyTests.cs b/JavaToCSharp.Tests/ConcurrencyTests.cs
new file mode 100644
index 0000000..bc2beef
--- /dev/null
+++ b/JavaToCSharp.Tests/ConcurrencyTests.cs
@@ -0,0 +1,82 @@
+using JavaToCSharp;
+
+namespace JavaToCSharp.Tests;
+
+/// <summary>
+/// Regression tests for shared mutable state that previously corrupted conversions
+/// when they ran concurrently. See the <c>voidvoid</c> return-type corruption caused by
+/// <see cref="TypeNameParser"/> holding its parse state in static fields.
+/// </summary>
+public class ConcurrencyTests
+{
+    /// <summary>
+    /// Hammers the type-name parser from many threads at once. Before the fix, the parser's
+    /// shared static StringBuilder caused threads to splice each other's output, producing
+    /// results like "voidvoid" or another thread's type name entirely.
+    /// </summary>
+    [Fact]
+    public void ConvertType_IsThreadSafe()
+    {
+        (string Java, string Expected)[] cases =
+        [
+            ("void", "void"),
+            ("String", "string"),
+            ("Integer", "int"),
+            ("List<String>", "IList<string>"),
+            ("Map<String, Integer>", "Dictionary<string, int>"),
+            ("int[]", "int[]"),
+            ("List<Map<String, Object>>", "IList<Dictionary<string, object>>"),
+        ];
+
+        var failures = new System.Collections.Concurrent.ConcurrentBag<string>();
+
+        Parallel.For(0, 2000, new ParallelOptions { MaxDegreeOfParallelism = 16 }, i =>
+        {
+            var (java, expected) = cases[i % cases.Length];
+            var actual = TypeHelper.ConvertType(java);
+
+            if (actual != expected)
+            {
+                failures.Add($"ConvertType(\"{java}\") returned \"{actual}\", expected \"{expected}\"");
+            }
+        });
+
+        Assert.Empty(failures);
+    }
+
+    /// <summary>
+    /// Runs full conversions in parallel. Before the fix these corrupted each other's output,
+    /// which surfaced as intermittent failures in unrelated test classes.
+    /// </summary>
+    [Fact]
+    public void Convert_IsThreadSafe()
+    {
+        const string java = """
+            package com.example;
+
+            public class Program {
+                public static void main(String[] args) {
+                    System.out.println("Hello world!");
+                }
+
+                public List<String> getNames(Map<String, Integer> input) {
+                    return null;
+                }
+            }
+            """;
+
+        var expected = JavaToCSharpConverter.ConvertText(java, new JavaConversionOptions());
+        Assert.NotNull(expected);
+        Assert.Contains("void Main", expected);
+
+        var results = new System.Collections.Concurrent.ConcurrentBag<string?>();
+
+        Parallel.For(0, 200, new ParallelOptions { MaxDegreeOfParallelism = 16 }, _ =>
+        {
+            results.Add(JavaToCSharpConverter.ConvertText(java, new JavaConversionOptions()));
+        });
+
+        // Every concurrent conversion must match the single-threaded result exactly.
+        Assert.All(results, r => Assert.Equal(expected, r));
+    }
+}
diff --git a/JavaToCSharp/TypeHelper.cs b/JavaToCSharp/TypeHelper.cs
index 7dcaafa..9b648a3 100644
--- a/JavaToCSharp/TypeHelper.cs
+++ b/JavaToCSharp/TypeHelper.cs
@@ -1,4 +1,5 @@
-using com.github.javaparser.ast.expr;
+using System.Collections.Concurrent;
+using com.github.javaparser.ast.expr;
 using com.github.javaparser.ast.type;
 using JavaToCSharp.Expressions;
 using Microsoft.CodeAnalysis;
@@ -11,7 +12,10 @@ namespace JavaToCSharp;
 
 public static class TypeHelper
 {
-    private static readonly Dictionary<string, string> _typeNameConversions = new()
+    // Mutated at conversion time via AddOrUpdateTypeNameConversions (e.g. to register interface
+    // renames when StartInterfaceNamesWithI is set) while other threads read it in ConvertType,
+    // so this must be a concurrent collection to keep parallel conversions safe.
+    private static readonly ConcurrentDictionary<string, string> _typeNameConversions = new()
     {
         // Simple types
         ["boolean"] = "bool",
diff --git a/JavaToCSharp/TypeNameParser.cs b/JavaToCSharp/TypeNameParser.cs
index 6b297f6..a8c0d14 100644
--- a/JavaToCSharp/TypeNameParser.cs
+++ b/JavaToCSharp/TypeNameParser.cs
@@ -1,9 +1,16 @@
-using System.Text;
+using System.Text;
 using System.Text.RegularExpressions;
 
 namespace JavaToCSharp;
 
-public static partial class TypeNameParser
+/// <summary>
+/// Parses and translates Java type names into their C# equivalents.
+/// </summary>
+/// <remarks>
+/// All parse state is held in instance fields, and a fresh instance is created per
+/// <see cref="ParseTypeName"/> call, so concurrent conversions cannot corrupt each other.
+/// </remarks>
+public sealed partial class TypeNameParser
 {
     private enum TokenType
     {
@@ -22,11 +29,19 @@ private enum TokenType
     [GeneratedRegex(@"\w+|\[|\]|<|>|,|\?", RegexOptions.Compiled)]
     private static partial Regex TokenizePattern { get; }
 
-    private static (string, TokenType)[]? _tokens;
-    private static (string text, TokenType type) _token;
-    private static int _currentIndex;
-    private static readonly StringBuilder _sb = new();
-    private static Func<string, string>? _translate;
+    private readonly (string, TokenType)[] _tokens;
+    private readonly StringBuilder _sb = new();
+    private readonly Func<string, string> _translate;
+    private (string text, TokenType type) _token;
+    private int _currentIndex;
+
+    private TypeNameParser(string typename, Func<string, string> translateIdentifier)
+    {
+        _translate = translateIdentifier;
+        _tokens = Tokenize(typename);
+        _currentIndex = -1;
+        NextToken();
+    }
 
     internal static string ParseTypeName(string typename, Func<string, string> translateIdentifier)
     {
@@ -36,16 +51,12 @@ internal static string ParseTypeName(string typename, Func<string, string> trans
         // TypeName = identifier [ "<" TypeArgument { "," TypeArgument } ">" ] { "[" "]" }.
         // TypeArgument = [ "?" [ "extends" | "super" ] ] TypeName.
 
-        _translate = translateIdentifier;
-        _tokens = Tokenize(typename);
-        _currentIndex = -1;
-        NextToken();
-        _sb.Clear();
+        var parser = new TypeNameParser(typename, translateIdentifier);
 
-        if (TypeName() && _token.type is TokenType.EndOfString)
+        if (parser.TypeName() && parser._token.type is TokenType.EndOfString)
         {
             // Otherwise we have extra tokens.
-            return _sb.ToString();
+            return parser._sb.ToString();
         }
 
         return typename;
@@ -75,13 +86,13 @@ private static (string, TokenType)[] Tokenize(string typeName)
         return tokens;
     }
 
-    private static void NextToken()
+    private void NextToken()
     {
         _currentIndex++;
-        _token = _tokens is not null && _currentIndex < _tokens.Length ? _tokens[_currentIndex] : default;
+        _token = _currentIndex < _tokens.Length ? _tokens[_currentIndex] : default;
     }
 
-    private static bool TypeName()
+    private bool TypeName()
     {
         // TypeName = identifier [ "<" TypeArgument { "," TypeArgument } ">" ] { "[" "]" }.
         if (_token.type is not TokenType.Identifier)
@@ -89,7 +100,7 @@ private static bool TypeName()
             return false;
         }
 
-        _sb.Append(_translate?.Invoke(_token.text));
+        _sb.Append(_translate(_token.text));
         NextToken();
 
         if (_token.type is TokenType.LeftAngleBracket)
@@ -114,7 +125,7 @@ private static bool TypeName()
         return ArraySuffix();
     }
 
-    private static bool ArraySuffix()
+    private bool ArraySuffix()
     {
         while (_token.type is TokenType.LeftSquareBracket)
         {
@@ -128,7 +139,7 @@ private static bool ArraySuffix()
         return true;
     }
 
-    private static bool TypeArgument()
+    private bool TypeArgument()
     {
         // TypeArgument = [ "?" [ "extends" | "super" ] ] TypeName.
         if (_token.type is TokenType.QuestionMark)