diff --git a/.github/instructions/Razor.instructions.md b/.github/instructions/Razor.instructions.md
index 39d949aa8ce67..c5302c5680581 100644
--- a/.github/instructions/Razor.instructions.md
+++ b/.github/instructions/Razor.instructions.md
@@ -36,6 +36,9 @@ their original sub-tree layout
   fields through the constructor. When adding a new field, thread it through every existing
   `With*` method. Prefer computing derived data via extension methods (e.g.,
   `GetUnusedDirectives()`) rather than storing computed results as fields.
+- **Razor engine concurrency**: Hosts can process multiple documents concurrently through one
+  `RazorProjectEngine`. Phase and pass instances are shared, so keep per-document state in locals
+  or an execution context rather than mutable instance fields.
 - **Razor documents in Roslyn**: Stored as additional documents. Resolve via
   `solution.GetDocumentIdsWithFilePath(filePath)` then `solution.GetAdditionalDocument(documentId)`.
 - **Razor documents with virtual URIs**: Remote Razor document classification preserves the full
diff --git a/src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/DefaultTagHelperResolutionPhaseTest.cs b/src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/DefaultTagHelperResolutionPhaseTest.cs
index 2e80286a53770..6783ec3da1272 100644
--- a/src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/DefaultTagHelperResolutionPhaseTest.cs
+++ b/src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/DefaultTagHelperResolutionPhaseTest.cs
@@ -1,19 +1,59 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Immutable;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Language;
 
 public class DefaultTagHelperResolutionPhaseTest
 {
+    [Fact]
+    public void Process_ConcurrentMixedFileKinds_UsesCorrectResolver()
+    {
+        // Arrange
+        var configuration = new RazorConfiguration(
+            RazorLanguageVersion.Version_5_0,
+            "MVC-3.0",
+            Extensions: []);
+
+        var projectEngine = RazorProjectEngine.Create(
+            configuration,
+            RazorProjectFileSystem.Empty,
+            builder => builder.RegisterExtensions());
+
+        var inputs = new[]
+        {
+            (Source: RazorSourceDocument.Create("<h1>View</h1>", "/Views/Index.cshtml"), FileKind: RazorFileKind.Legacy),
+            (Source: RazorSourceDocument.Create("<p>Component</p>", "/Components/App.razor"), FileKind: RazorFileKind.Component),
+        };
+
+        // Act and assert: match classic rzc's concurrency while repeating enough to exercise the timing-sensitive overlap.
+        var parallelOptions = new ParallelOptions { MaxDegreeOfParallelism = 4 };
+        Parallel.For(0, 2_000, parallelOptions, iteration =>
+        {
+            var (source, fileKind) = inputs[iteration % inputs.Length];
+            var codeDocument = projectEngine.Process(
+                source,
+                fileKind,
+                ImmutableArray<RazorSourceDocument>.Empty,
+                tagHelpers: null);
+
+            var generatedCode = codeDocument.GetCSharpDocument().GeneratedCode.ToString();
+            Assert.Contains(
+                fileKind == RazorFileKind.Component ? "BuildRenderTree" : "ExecuteAsync",
+                generatedCode);
+        });
+    }
+
     [Fact]
     public void MergeSourceSpans_SameLine_ReturnsCorrectSpan()
     {
         // Arrange
         var filePath = "test.razor";
         var first = new SourceSpan(filePath, absoluteIndex: 10, lineIndex: 2, characterIndex: 5, length: 3, lineCount: 0, endCharacterIndex: 8);
-        var last  = new SourceSpan(filePath, absoluteIndex: 15, lineIndex: 2, characterIndex: 10, length: 4, lineCount: 0, endCharacterIndex: 14);
+        var last = new SourceSpan(filePath, absoluteIndex: 15, lineIndex: 2, characterIndex: 10, length: 4, lineCount: 0, endCharacterIndex: 14);
 
         // Act
         var result = DefaultTagHelperResolutionPhase.MergeSourceSpans(first, last);
@@ -36,7 +76,7 @@ public void MergeSourceSpans_MultiLine_ReturnsCorrectSpan()
         // first spans lines 1-2 (lineCount = 1 means it crosses into the next line)
         var first = new SourceSpan(filePath, absoluteIndex: 0, lineIndex: 1, characterIndex: 0, length: 10, lineCount: 1, endCharacterIndex: 5);
         // last is on line 3 (lineIndex = 3)
-        var last  = new SourceSpan(filePath, absoluteIndex: 20, lineIndex: 3, characterIndex: 2, length: 5, lineCount: 0, endCharacterIndex: 7);
+        var last = new SourceSpan(filePath, absoluteIndex: 20, lineIndex: 3, characterIndex: 2, length: 5, lineCount: 0, endCharacterIndex: 7);
 
         // Act
         var result = DefaultTagHelperResolutionPhase.MergeSourceSpans(first, last);
@@ -58,7 +98,7 @@ public void MergeSourceSpans_AdjacentSpans_ReturnsCorrectSpan()
         var filePath = "test.razor";
         var first = new SourceSpan(filePath, absoluteIndex: 5, lineIndex: 0, characterIndex: 5, length: 3, lineCount: 0, endCharacterIndex: 8);
         // last starts right where first ends
-        var last  = new SourceSpan(filePath, absoluteIndex: 8, lineIndex: 0, characterIndex: 8, length: 4, lineCount: 0, endCharacterIndex: 12);
+        var last = new SourceSpan(filePath, absoluteIndex: 8, lineIndex: 0, characterIndex: 8, length: 4, lineCount: 0, endCharacterIndex: 12);
 
         // Act
         var result = DefaultTagHelperResolutionPhase.MergeSourceSpans(first, last);
@@ -94,7 +134,7 @@ public void MergeSourceSpans_NullFilePath_PreservesNullFilePath()
     {
         // Arrange — file path is null (e.g. for in-memory content)
         var first = new SourceSpan(filePath: null, absoluteIndex: 0, lineIndex: 0, characterIndex: 0, length: 3, lineCount: 0, endCharacterIndex: 3);
-        var last  = new SourceSpan(filePath: null, absoluteIndex: 5, lineIndex: 0, characterIndex: 5, length: 2, lineCount: 0, endCharacterIndex: 7);
+        var last = new SourceSpan(filePath: null, absoluteIndex: 5, lineIndex: 0, characterIndex: 5, length: 2, lineCount: 0, endCharacterIndex: 7);
 
         // Act
         var result = DefaultTagHelperResolutionPhase.MergeSourceSpans(first, last);
diff --git a/src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/DefaultTagHelperResolutionPhase.cs b/src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/DefaultTagHelperResolutionPhase.cs
index 5932ffe6a7f06..574c93e7ad6c5 100644
--- a/src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/DefaultTagHelperResolutionPhase.cs
+++ b/src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/DefaultTagHelperResolutionPhase.cs
@@ -24,8 +24,6 @@ namespace Microsoft.AspNetCore.Razor.Language;
 /// </summary>
 internal partial class DefaultTagHelperResolutionPhase : RazorEnginePhaseBase
 {
-    private TagHelperResolver _resolver;
-
     /// <summary>
     /// Entry point: resolves all unresolved <see cref="UnresolvedElementIntermediateNode"/> nodes
     /// in the IR tree. For each, matches against tag helper bindings and either converts to a
@@ -50,7 +48,7 @@ protected override RazorCodeDocument ExecuteCore(RazorCodeDocument codeDocument,
         // Choose resolver based on file kind and language version. Component features
         // (MarkupElementIntermediateNode, RZ10012 diagnostics) require Version_3_0+ because
         // the ComponentDocumentClassifierPass is only registered at that version.
-        _resolver = (codeDocument.FileKind.IsComponent() || codeDocument.FileKind.IsComponentImport())
+        TagHelperResolver resolver = (codeDocument.FileKind.IsComponent() || codeDocument.FileKind.IsComponentImport())
             && parserOptions.LanguageVersion >= RazorLanguageVersion.Version_3_0
             ? new ComponentTagHelperResolver()
             : new LegacyTagHelperResolver();
@@ -58,7 +56,7 @@ protected override RazorCodeDocument ExecuteCore(RazorCodeDocument codeDocument,
         if (tagHelperContext == null || tagHelperContext.TagHelpers is [])
         {
             // No tag helpers discovered - unwrap all UnresolvedElement nodes to their fallback.
-            UnwrapAllElements(documentNode, documentNode);
+            UnwrapAllElements(documentNode, resolver, documentNode);
 
             // Still need to set referenced tag helpers for downstream phases.
             return codeDocument.WithReferencedTagHelpers([]);
@@ -69,7 +67,7 @@ protected override RazorCodeDocument ExecuteCore(RazorCodeDocument codeDocument,
 
         using var usedHelpers = new TagHelperCollection.Builder();
         var sourceDocument = codeDocument.Source;
-        var context = new ResolutionContext(sourceDocument, documentNode);
+        var context = new ResolutionContext(sourceDocument, documentNode, resolver);
         ResolveElements(documentNode, binder, prefix, usedHelpers, in context);
 
         // Add tag helper descriptor validation diagnostics (e.g. RZ3003).
@@ -95,11 +93,16 @@ private readonly struct ResolutionContext
     {
         public readonly RazorSourceDocument SourceDocument;
         public readonly DocumentIntermediateNode DocumentNode;
+        public readonly TagHelperResolver Resolver;
 
-        public ResolutionContext(RazorSourceDocument sourceDocument, DocumentIntermediateNode documentNode)
+        public ResolutionContext(
+            RazorSourceDocument sourceDocument,
+            DocumentIntermediateNode documentNode,
+            TagHelperResolver resolver)
         {
             SourceDocument = sourceDocument;
             DocumentNode = documentNode;
+            Resolver = resolver;
         }
     }
 
@@ -160,7 +163,7 @@ private TagHelperIntermediateNode ResolveElement(
         {
             TryAddMalformedEndTagDiagnostic(elementNode, tagName, binder, attributes, parent, tagHelperParent, prefix);
 
-            _resolver.ConvertToPlainElement(parent, index, elementNode);
+            context.Resolver.ConvertToPlainElement(parent, index, elementNode);
             return null;
         }
 
@@ -264,7 +267,7 @@ private TagHelperIntermediateNode ResolveElement(
 
         // Add resolver-specific diagnostics (e.g. RZ10012 for component-like elements,
         // case mismatch between start/end tags).
-        _resolver.AddMatchedElementDiagnostics(tagHelperNode, elementNode, binding, in context);
+        context.Resolver.AddMatchedElementDiagnostics(tagHelperNode, elementNode, binding, in context);
 
         // Check if resolved tag name is a void element (handles prefixed elements like th:input).
         var isResolvedVoidElement = elementNode.IsVoidElement || Legacy.ParserHelpers.VoidElements.Contains(resolvedTagName);
@@ -281,7 +284,7 @@ private TagHelperIntermediateNode ResolveElement(
 
         // Build body and attributes.
         var bodyNode = new TagHelperBodyIntermediateNode();
-        _resolver.BuildTagHelper(tagHelperNode, bodyNode, elementNode, binding, context.SourceDocument, in context);
+        context.Resolver.BuildTagHelper(tagHelperNode, bodyNode, elementNode, binding, context.SourceDocument, in context);
 
         return (tagHelperNode, bodyNode);
     }
@@ -445,12 +448,12 @@ private void ConvertToPlainElementAndResolve(
         bool emitDiagnostics = true)
     {
         var childCountBefore = parent.Children.Count;
-        _resolver.ConvertToPlainElement(parent, index, elementNode);
+        context.Resolver.ConvertToPlainElement(parent, index, elementNode);
         var resultCount = parent.Children.Count - childCountBefore + 1; // +1 because the original was removed
 
         if (emitDiagnostics && resultCount > 0)
         {
-            _resolver.AddUnmatchedElementDiagnostic(parent.Children[index], elementNode, context.DocumentNode);
+            context.Resolver.AddUnmatchedElementDiagnostic(parent.Children[index], elementNode, context.DocumentNode);
         }
 
         for (var j = index + resultCount - 1; j >= index; j--)
@@ -785,7 +788,10 @@ private static void FlattenToDirectCSharpTokens(IntermediateNode source, Interme
     /// resolved by tag helper matching. Converts each to a plain element using the resolver.
     /// Recursively processes the tree to handle nested elements.
     /// </summary>
-    private void UnwrapAllElements(IntermediateNode node, DocumentIntermediateNode documentNode = null)
+    private static void UnwrapAllElements(
+        IntermediateNode node,
+        TagHelperResolver resolver,
+        DocumentIntermediateNode documentNode = null)
     {
         if (node is DocumentIntermediateNode doc)
         {
@@ -795,17 +801,17 @@ private void UnwrapAllElements(IntermediateNode node, DocumentIntermediateNode d
         for (var i = node.Children.Count - 1; i >= 0; i--)
         {
             var child = node.Children[i];
-            UnwrapAllElements(child, documentNode);
+            UnwrapAllElements(child, resolver, documentNode);
 
             if (child is UnresolvedElementIntermediateNode elementNode)
             {
                 var countBefore = node.Children.Count;
-                _resolver.ConvertToPlainElement(node, i, elementNode);
+                resolver.ConvertToPlainElement(node, i, elementNode);
                 var resultCount = node.Children.Count - countBefore + 1;
 
                 if (resultCount > 0)
                 {
-                    _resolver.AddUnmatchedElementDiagnostic(node.Children[i], elementNode, documentNode);
+                    resolver.AddUnmatchedElementDiagnostic(node.Children[i], elementNode, documentNode);
                 }
             }
         }