Fix data-URL strings decoding as binary values

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
           # Current leg: catches breakage from new Xcode and SDK releases.
           - os: macos-latest
             swift-version: Xcode default
-          - os: ubuntu-latest
+          - os: ubuntu-24.04
             swift-version: 6.1.0
 
     runs-on: ${{ matrix.os }}
@@ -35,13 +35,13 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Prepare Linux apt dependencies
-        if: matrix.os == 'ubuntu-latest'
+        if: matrix.os == 'ubuntu-24.04'
         run: |
           sudo -n apt-get update
           sudo -n apt-get install -y --no-install-recommends libcurl4-openssl-dev
 
       - name: Setup Swift on Linux
-        if: matrix.os == 'ubuntu-latest'
+        if: matrix.os == 'ubuntu-24.04'
         uses: vapor/swiftly-action@v0.2
         with:
           toolchain: ${{ matrix.swift-version }}

Sources/MCP/Base/Value.swift

@@ -2,13 +2,16 @@ import struct Foundation.Data
 import class Foundation.JSONDecoder
 import class Foundation.JSONEncoder
 
-/// A codable value.
+/// A JSON value. Encoding `.data` produces a data-URL string; decoding that JSON
+/// produces `.string`, since an ordinary JSON string does not carry a binary type.
 public enum Value: Hashable, Sendable {
     case null
     case bool(Bool)
     case int(Int)
     case double(Double)
     case string(String)
+    /// Encodes as a data-URL string. Decoding that JSON produces `.string`.
+    /// Use `Data.parseDataURL(_:)` to recover the bytes when needed.
     case data(mimeType: String? = nil, Data)
     case array([Value])
     case object([String: Value])
@@ -120,13 +123,7 @@ extension Value: Codable {
         } else if let value = try? container.decode(Double.self) {
             self = .double(value)
         } else if let value = try? container.decode(String.self) {
-            if Data.isDataURL(string: value),
-                case let (mimeType, data)? = Data.parseDataURL(value)
-            {
-                self = .data(mimeType: mimeType, data)
-            } else {
-                self = .string(value)
-            }
+            self = .string(value)
         } else if let value = try? container.decode([Value].self) {
             self = .array(value)
         } else if let value = try? container.decode([String: Value].self) {

Tests/MCPTests/ClientTests.swift

@@ -14,10 +14,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -124,10 +122,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -174,10 +170,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -230,10 +224,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -321,10 +313,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -401,10 +391,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -481,10 +469,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -521,10 +507,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -656,10 +640,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,
@@ -717,10 +699,8 @@ struct ClientTests {
 
         // Set up a task to handle the initialize response
         let initTask = Task {
-            try await Task.sleep(for: .milliseconds(10))
-            if let lastMessage = await transport.sentMessages.last,
-                let data = lastMessage.data(using: .utf8),
-                let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
+            let data = await transport.waitForFirstSentData()
+            if let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
             {
                 let response = Initialize.response(
                     id: request.id,

Tests/MCPTests/Helpers/MockTransport.swift

@@ -17,6 +17,7 @@ actor MockTransport: Transport {
     var isConnected = false
 
     private(set) var sentData: [Data] = []
+    private var firstSendWaiters: [CheckedContinuation<Data, Never>] = []
     var sentMessages: [String] {
         return sentData.compactMap { data in
             guard let string = String(data: data, encoding: .utf8) else {
@@ -57,6 +58,20 @@ actor MockTransport: Transport {
             throw MCPError.transportError(POSIXError(.EIO))
         }
         sentData.append(message)
+        let waiters = firstSendWaiters
+        firstSendWaiters.removeAll()
+        for waiter in waiters {
+            waiter.resume(returning: message)
+        }
+    }
+
+    func waitForFirstSentData() async -> Data {
+        if let data = sentData.first {
+            return data
+        }
+        return await withCheckedContinuation { continuation in
+            firstSendWaiters.append(continuation)
+        }
     }
 
     public func receive() -> AsyncThrowingStream<Data, Swift.Error> {

Tests/MCPTests/ValueDecodingTests.swift

@@ -0,0 +1,49 @@
+import Foundation
+import Testing
+
+@testable import MCP
+
+@Suite("Value JSON String Tests")
+struct ValueDecodingTests {
+    @Test("Data-URL-looking strings stay strings in nested JSON")
+    func dataURLStringsStayStrings() throws {
+        let strings = [
+            "data:text/plain,Hello%20World",
+            "data:text/plain;base64,SGVsbG8=",
+            "data:application/octet-stream;base64,AAEC/w==",
+        ]
+        let expected = Value.object(["values": .array(strings.map(Value.string))])
+        let json = try JSONEncoder().encode(expected)
+        #expect(try JSONDecoder().decode(Value.self, from: json) == expected)
+    }
+
+    @Test("Explicit data encoding does not change how generic JSON strings decode")
+    func explicitDataNeedsAnExplicitType() throws {
+        let data = Value.data(mimeType: "application/octet-stream", Data([0, 1, 2]))
+        let json = try JSONEncoder().encode(data)
+        let string = try JSONDecoder().decode(String.self, from: json)
+        #expect(string == "data:application/octet-stream;base64,AAEC")
+        #expect(try JSONDecoder().decode(Value.self, from: json) == .string(string))
+        let parsed = try #require(Data.parseDataURL(string))
+        #expect(parsed.mimeType == "application/octet-stream")
+        #expect(parsed.data == Data([0, 1, 2]))
+    }
+
+    @Test("Tool text content survives Value erasure")
+    func toolTextContentSurvivesValueErasure() throws {
+        let text = "data:text/plain,Hello%20World"
+        let result = CallTool.Result(content: [
+            .text(text: text, annotations: nil, _meta: nil)
+        ])
+        let erased = try Value(result)
+        let recovered = try JSONDecoder().decode(
+            CallTool.Result.self,
+            from: JSONEncoder().encode(erased)
+        )
+        guard case .text(let recoveredText, _, _) = recovered.content.first else {
+            Issue.record("Expected text content")
+            return
+        }
+        #expect(recoveredText == text)
+    }
+}