Allow reserved expansion to include empty values

Author: halter73

src/ModelContextProtocol.Core/UriTemplate.cs

@@ -84,12 +84,12 @@ public static Regex CreateParser(string uriTemplate)
 
             switch (m.Groups["operator"].Value)
             {
-                case "+": AppendExpression(ref pattern, paramNames, null, "[^?&#]+"); break;
-                case "#": AppendExpression(ref pattern, paramNames, '#', "[^,]+"); break;
-                case ".": AppendExpression(ref pattern, paramNames, '.', "[^./?#]+"); break;
-                case "/": AppendExpression(ref pattern, paramNames, '/', "[^/?#]+"); break;
+                case "+": AppendExpression(ref pattern, paramNames, null, "[^?&#]*"); break;
+                case "#": AppendExpression(ref pattern, paramNames, '#', "[^,]*"); break;
+                case ".": AppendExpression(ref pattern, paramNames, '.', "[^./?#]*"); break;
+                case "/": AppendExpression(ref pattern, paramNames, '/', "[^/?#]*"); break;
                 case ";": AppendPathParameterExpression(ref pattern, paramNames); break;
-                default:  AppendExpression(ref pattern, paramNames, null, "[^/?&#]+"); break;
+                default:  AppendExpression(ref pattern, paramNames, null, "[^/?&#]*"); break;
 
                 case "?": AppendQueryExpression(ref pattern, paramNames, '?'); break;
                 case "&": AppendQueryExpression(ref pattern, paramNames, '&'); break;
@@ -135,7 +135,7 @@ static void AppendParameter(ref DefaultInterpolatedStringHandler pattern, string
                     pattern.AppendFormatted(paramName);
                     pattern.AppendFormatted("=(?<");
                     pattern.AppendFormatted(paramName);
-                    pattern.AppendFormatted(">[^/?&]+))?");
+                    pattern.AppendFormatted(">[^/?&]*))?");
                 }
             }
 
@@ -210,7 +210,7 @@ static void AppendParameter(ref DefaultInterpolatedStringHandler pattern, string
                     pattern.AppendLiteral(paramName);
                     pattern.AppendLiteral("(?:=(?<");
                     pattern.AppendLiteral(paramName);
-                    pattern.AppendLiteral(">[^;/?&]+))?)?");
+                    pattern.AppendLiteral(">[^;/?&]*))?)?");
                 }
             }
         }

tests/ModelContextProtocol.Tests/Configuration/McpServerResourceRoutingTests.cs

@@ -271,6 +271,49 @@ await AssertNoMatchAsync(
             uri: "wrongscheme://example.com/foo");
     }
 
+    /// <summary>
+    /// RFC 6570 specifies that empty values should expand to empty strings.
+    /// See https://datatracker.ietf.org/doc/html/rfc6570#page-22 test cases: O{+empty}X matches OX.
+    /// </summary>
+    [Fact]
+    public async Task ReservedExpansion_MatchesEmptyValue()
+    {
+        // Per RFC 6570: O{+empty}X should match OX when empty is ""
+        await AssertMatchAsync(
+            uriTemplate: "test://O{+empty}X",
+            method: (string empty) => $"empty:[{empty}]",
+            uri: "test://OX",
+            expectedResult: "empty:[]");
+    }
+
+    /// <summary>
+    /// RFC 6570 empty expansion test - reserved expansion at end of template.
+    /// </summary>
+    [Fact]
+    public async Task ReservedExpansion_MatchesEmptyValueAtEnd()
+    {
+        // {+var} at the end should match empty string
+        await AssertMatchAsync(
+            uriTemplate: "test://prefix{+suffix}",
+            method: (string suffix) => $"suffix:[{suffix}]",
+            uri: "test://prefix",
+            expectedResult: "suffix:[]");
+    }
+
+    /// <summary>
+    /// RFC 6570 empty expansion test - reserved expansion at start of template.
+    /// </summary>
+    [Fact]
+    public async Task ReservedExpansion_MatchesEmptyValueAtStart()
+    {
+        // {+var} at the start should match empty string
+        await AssertMatchAsync(
+            uriTemplate: "test://{+prefix}suffix",
+            method: (string prefix) => $"prefix:[{prefix}]",
+            uri: "test://suffix",
+            expectedResult: "prefix:[]");
+    }
+
     #endregion
 
     #region Level 2: Fragment Expansion {#var}