Detect base64 from the data URL header, not the payload

Sources/MCP/Extensions/Data+Extensions.swift

@@ -4,7 +4,7 @@ import RegexBuilder
 extension Data {
     /// Regex pattern for data URLs
     @inline(__always) private static var dataURLRegex:
-        Regex<(Substring, Substring, Substring?, Substring)>
+        Regex<(Substring, Substring, Substring?, Substring?, Substring)>
     {
         Regex {
             "data:"
@@ -21,7 +21,9 @@ extension Data {
                     }
                 }
             }
-            Optionally { ";base64" }
+            Optionally {
+                Capture { ";base64" }
+            }
             ","
             Capture {
                 ZeroOrMore { .any }
@@ -49,9 +51,9 @@ extension Data {
         }
 
         // Extract components using strongly typed captures
-        let (_, mediatype, charset, encodedData) = match.output
+        let (_, mediatype, charset, base64Marker, encodedData) = match.output
 
-        let isBase64 = string.contains(";base64,")
+        let isBase64 = base64Marker != nil
 
         // Process MIME type
         var mimeType = mediatype.isEmpty ? "text/plain" : String(mediatype)

Tests/MCPTests/Base64DataTests.swift

@@ -105,6 +105,39 @@ struct Base64DataTests {
         }
     }
 
+    @Test("Parse plain-text data URL whose payload contains ;base64,")
+    func testParseDataURLBase64MarkerInPayload() {
+        let dataURL = "data:text/plain,hello;base64,world"
+        #expect(Data.isDataURL(string: dataURL))
+
+        let result = Data.parseDataURL(dataURL)
+        #expect(result != nil)
+        if let (mimeType, data) = result {
+            #expect(mimeType == "text/plain")
+            #expect(String(data: data, encoding: .utf8) == "hello;base64,world")
+        }
+    }
+
+    @Test("Parse data URL with ;base64, header still decodes base64")
+    func testParseDataURLBase64HeaderRegression() {
+        let result = Data.parseDataURL("data:application/octet-stream;base64,/wA=")
+        #expect(result != nil)
+        if let (mimeType, data) = result {
+            #expect(mimeType == "application/octet-stream")
+            #expect(data == Data([0xFF, 0x00]))
+        }
+    }
+
+    @Test("Parse data URL with charset and ;base64,")
+    func testParseDataURLCharsetAndBase64() {
+        let result = Data.parseDataURL("data:text/plain;charset=utf-8;base64,SGk=")
+        #expect(result != nil)
+        if let (mimeType, data) = result {
+            #expect(mimeType == "text/plain;charset=utf-8")
+            #expect(String(data: data, encoding: .utf8) == "Hi")
+        }
+    }
+
     @Test("Test data URL encoding")
     func testDataURLEncoding() {
         let testData = "Hello, world!".data(using: .utf8)!