From eec8a14ab34ee106ccf28dc80b4c062efab1188e Mon Sep 17 00:00:00 2001
From: Akhil <akhil_c@apple.com>
Date: Wed, 21 Oct 2026 22:46:22 -0700
Subject: [PATCH 1/2] fix: detect base64 from the data URL header, not the
 payload

Data.parseDataURL decided base64 with string.contains(";base64,"), which
also searched the payload, so "data:text/plain,hello;base64,world" was
treated as base64 and returned nil even though isDataURL accepted it.
The regex now captures the optional ;base64 marker and that capture
drives the decision.
---
 Sources/MCP/Extensions/Data+Extensions.swift | 10 +++++----
 Tests/MCPTests/Base64DataTests.swift         | 23 ++++++++++++++++++++
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/Sources/MCP/Extensions/Data+Extensions.swift b/Sources/MCP/Extensions/Data+Extensions.swift
index 745b4a30..be35f91d 100644
--- a/Sources/MCP/Extensions/Data+Extensions.swift
+++ b/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)
diff --git a/Tests/MCPTests/Base64DataTests.swift b/Tests/MCPTests/Base64DataTests.swift
index 96025935..bced6f73 100644
--- a/Tests/MCPTests/Base64DataTests.swift
+++ b/Tests/MCPTests/Base64DataTests.swift
@@ -105,6 +105,29 @@ 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("Test data URL encoding")
     func testDataURLEncoding() {
         let testData = "Hello, world!".data(using: .utf8)!

From 34c8cfbcc29ea8ba368defb1cb6ea0f6ac2488e3 Mon Sep 17 00:00:00 2001
From: Akhil <akhil_c@apple.com>
Date: Wed, 21 Oct 2026 23:03:24 -0700
Subject: [PATCH 2/2] test: cover ;base64 after a charset parameter

---
 Tests/MCPTests/Base64DataTests.swift | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Tests/MCPTests/Base64DataTests.swift b/Tests/MCPTests/Base64DataTests.swift
index bced6f73..a5e04347 100644
--- a/Tests/MCPTests/Base64DataTests.swift
+++ b/Tests/MCPTests/Base64DataTests.swift
@@ -128,6 +128,16 @@ struct Base64DataTests {
         }
     }
 
+    @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)!