diff --git a/Sources/CAtomics/include/CAtomics.h b/Sources/CAtomics/include/CAtomics.h
index a5d273647..16b797a57 100644
--- a/Sources/CAtomics/include/CAtomics.h
+++ b/Sources/CAtomics/include/CAtomics.h
@@ -70,7 +70,7 @@ typedef struct {
 } AtomicUInt32;
 
 __attribute__((swift_name("AtomicUInt32.init(initialValue:)")))
-static inline AtomicUInt32 atomic_int_create(uint8_t initialValue) {
+static inline AtomicUInt32 atomic_int_create(uint32_t initialValue) {
   AtomicUInt32 atomic;
   atomic.value = initialValue;
   return atomic;
diff --git a/Sources/SKCore/BuildServerBuildSystem.swift b/Sources/SKCore/BuildServerBuildSystem.swift
index 5f19066ab..5fa36777c 100644
--- a/Sources/SKCore/BuildServerBuildSystem.swift
+++ b/Sources/SKCore/BuildServerBuildSystem.swift
@@ -293,7 +293,7 @@ extension BuildServerBuildSystem: BuildSystem {
 
   public func prepare(
     targets: [ConfiguredTarget],
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws {
     throw PrepareNotSupportedError()
   }
diff --git a/Sources/SKCore/BuildSystem.swift b/Sources/SKCore/BuildSystem.swift
index 204dbefc3..f409c1ab5 100644
--- a/Sources/SKCore/BuildSystem.swift
+++ b/Sources/SKCore/BuildSystem.swift
@@ -169,7 +169,7 @@ public protocol BuildSystem: AnyObject, Sendable {
   /// dependencies.
   func prepare(
     targets: [ConfiguredTarget],
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws
 
   /// If the build system has knowledge about the language that this document should be compiled in, return it.
diff --git a/Sources/SKCore/BuildSystemManager.swift b/Sources/SKCore/BuildSystemManager.swift
index 134453ed8..415e98fc6 100644
--- a/Sources/SKCore/BuildSystemManager.swift
+++ b/Sources/SKCore/BuildSystemManager.swift
@@ -237,9 +237,9 @@ extension BuildSystemManager {
 
   public func prepare(
     targets: [ConfiguredTarget],
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws {
-    try await buildSystem?.prepare(targets: targets, indexProcessDidProduceResult: indexProcessDidProduceResult)
+    try await buildSystem?.prepare(targets: targets, logMessageToIndexLog: logMessageToIndexLog)
   }
 
   public func registerForChangeNotifications(for uri: DocumentURI, language: Language) async {
diff --git a/Sources/SKCore/CMakeLists.txt b/Sources/SKCore/CMakeLists.txt
index 913e1e087..a0653fa64 100644
--- a/Sources/SKCore/CMakeLists.txt
+++ b/Sources/SKCore/CMakeLists.txt
@@ -10,7 +10,7 @@ add_library(SKCore STATIC
   Debouncer.swift
   FallbackBuildSystem.swift
   FileBuildSettings.swift
-  IndexProcessResult.swift
+  IndexTaskID.swift
   MainFilesProvider.swift
   PathPrefixMapping.swift
   SplitShellCommand.swift
diff --git a/Sources/SKCore/CompilationDatabaseBuildSystem.swift b/Sources/SKCore/CompilationDatabaseBuildSystem.swift
index d601bd099..c906336b0 100644
--- a/Sources/SKCore/CompilationDatabaseBuildSystem.swift
+++ b/Sources/SKCore/CompilationDatabaseBuildSystem.swift
@@ -129,7 +129,7 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
 
   public func prepare(
     targets: [ConfiguredTarget],
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws {
     throw PrepareNotSupportedError()
   }
diff --git a/Sources/SKCore/IndexProcessResult.swift b/Sources/SKCore/IndexProcessResult.swift
deleted file mode 100644
index 07b71c7c8..000000000
--- a/Sources/SKCore/IndexProcessResult.swift
+++ /dev/null
@@ -1,71 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
-// Licensed under Apache License v2.0 with Runtime Library Exception
-//
-// See https://swift.org/LICENSE.txt for license information
-// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-//
-//===----------------------------------------------------------------------===//
-
-import struct TSCBasic.ProcessResult
-
-/// Result of a process that prepares a target or updates the index store. To be shown in the build log.
-///
-/// Abstracted over a `ProcessResult` to facilitate build systems that don't spawn a new process to prepare a target but
-/// prepare it from a build graph they have loaded in-process.
-public struct IndexProcessResult {
-  /// A human-readable description of what the process was trying to achieve, like `Preparing MyTarget`
-  public let taskDescription: String
-
-  /// The command that was run to produce the result.
-  public let command: String
-
-  /// The output that the process produced.
-  public let output: String
-
-  /// Whether the process failed.
-  public let failed: Bool
-
-  /// The duration it took for the process to execute.
-  public let duration: Duration
-
-  public init(taskDescription: String, command: String, output: String, failed: Bool, duration: Duration) {
-    self.taskDescription = taskDescription
-    self.command = command
-    self.output = output
-    self.failed = failed
-    self.duration = duration
-  }
-
-  public init(taskDescription: String, processResult: ProcessResult, start: ContinuousClock.Instant) {
-    let stdout = (try? String(bytes: processResult.output.get(), encoding: .utf8)) ?? "<failed to decode stdout>"
-    let stderr = (try? String(bytes: processResult.stderrOutput.get(), encoding: .utf8)) ?? "<failed to decode stderr>"
-    var outputComponents: [String] = []
-    if !stdout.isEmpty {
-      outputComponents.append(
-        """
-        Stdout:
-        \(stdout)
-        """
-      )
-    }
-    if !stderr.isEmpty {
-      outputComponents.append(
-        """
-        Stderr:
-        \(stderr)
-        """
-      )
-    }
-    self.init(
-      taskDescription: taskDescription,
-      command: processResult.arguments.joined(separator: " "),
-      output: outputComponents.joined(separator: "\n\n"),
-      failed: processResult.exitStatus != .terminated(code: 0),
-      duration: start.duration(to: .now)
-    )
-  }
-}
diff --git a/Sources/SKCore/IndexTaskID.swift b/Sources/SKCore/IndexTaskID.swift
new file mode 100644
index 000000000..4ca4564b3
--- /dev/null
+++ b/Sources/SKCore/IndexTaskID.swift
@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import struct TSCBasic.ProcessResult
+
+/// The ID of a preparation or update indexstore task. This allows us to log messages from multiple concurrently running
+/// indexing tasks to the index log while still being able to differentiate them.
+public enum IndexTaskID: Sendable {
+  case preparation(id: UInt32)
+  case updateIndexStore(id: UInt32)
+
+  private static func numberToEmojis(_ number: Int, numEmojis: Int) -> String {
+    let emojis = ["🟥", "🟩", "🟦", "🟧", "⬜️", "🟪", "⬛️", "🟨", "🟫"]
+    var number = abs(number)
+    var result = ""
+    for _ in 0..<numEmojis {
+      let (quotient, remainder) = number.quotientAndRemainder(dividingBy: emojis.count)
+      result += emojis[remainder]
+      number = quotient
+    }
+    return result
+  }
+
+  /// Returns a two-character emoji string that allows easy differentiation between different task IDs.
+  ///
+  /// This marker is prepended to every line in the index log.
+  public var emojiRepresentation: String {
+    // Multiply by 2 and optionally add 1 to make sure preparation and update index store have distinct IDs.
+    // Run .hashValue to make sure we semi-randomly pick new emoji markers for new tasks
+    switch self {
+    case .preparation(id: let id):
+      return Self.numberToEmojis((id * 2).hashValue, numEmojis: 2)
+    case .updateIndexStore(id: let id):
+      return Self.numberToEmojis((id * 2 + 1).hashValue, numEmojis: 2)
+    }
+  }
+}
diff --git a/Sources/SKSupport/CMakeLists.txt b/Sources/SKSupport/CMakeLists.txt
index 000ca70f8..561d8d9db 100644
--- a/Sources/SKSupport/CMakeLists.txt
+++ b/Sources/SKSupport/CMakeLists.txt
@@ -11,6 +11,7 @@ add_library(SKSupport STATIC
   DocumentURI+CustomLogStringConvertible.swift
   FileSystem.swift
   LineTable.swift
+  PipeAsStringHandler.swift
   Process+LaunchWithWorkingDirectoryIfPossible.swift
   Process+WaitUntilExitWithCancellation.swift
   Random.swift
diff --git a/Sources/SKSupport/PipeAsStringHandler.swift b/Sources/SKSupport/PipeAsStringHandler.swift
new file mode 100644
index 000000000..08669e0f0
--- /dev/null
+++ b/Sources/SKSupport/PipeAsStringHandler.swift
@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+/// Gathers data from a stdout or stderr pipe. When it has accumulated a full line, calls the handler to handle the
+/// string.
+public actor PipeAsStringHandler {
+  /// Queue on which all data from the pipe will be handled. This allows us to have a
+  /// nonisolated `handle` function but ensure that data gets processed in order.
+  private let queue = AsyncQueue<Serial>()
+  private var buffer = Data()
+
+  /// The closure that actually handles
+  private let handler: @Sendable (String) -> Void
+
+  public init(handler: @escaping @Sendable (String) -> Void) {
+    self.handler = handler
+  }
+
+  private func handleDataFromPipeImpl(_ newData: Data) {
+    self.buffer += newData
+    while let newlineIndex = self.buffer.firstIndex(of: UInt8(ascii: "\n")) {
+      // Output a separate log message for every line in the pipe.
+      // The reason why we don't output multiple lines in a single log message is that
+      //  a) os_log truncates log messages at about 1000 bytes. The assumption is that a single line is usually less
+      //     than 1000 bytes long but if we merge multiple lines into one message, we might easily exceed this limit.
+      //  b) It might be confusing why sometimes a single log message contains one line while sometimes it contains
+      //     multiple.
+      handler(String(data: self.buffer[...newlineIndex], encoding: .utf8) ?? "<invalid UTF-8>")
+      buffer = buffer[buffer.index(after: newlineIndex)...]
+    }
+  }
+
+  public nonisolated func handleDataFromPipe(_ newData: Data) {
+    queue.async {
+      await self.handleDataFromPipeImpl(newData)
+    }
+  }
+}
diff --git a/Sources/SKSupport/Process+LaunchWithWorkingDirectoryIfPossible.swift b/Sources/SKSupport/Process+LaunchWithWorkingDirectoryIfPossible.swift
index f57f905a3..45e513058 100644
--- a/Sources/SKSupport/Process+LaunchWithWorkingDirectoryIfPossible.swift
+++ b/Sources/SKSupport/Process+LaunchWithWorkingDirectoryIfPossible.swift
@@ -26,6 +26,7 @@ extension Process {
     arguments: [String],
     environmentBlock: ProcessEnvironmentBlock = ProcessEnv.block,
     workingDirectory: AbsolutePath?,
+    outputRedirection: OutputRedirection = .collect,
     startNewProcessGroup: Bool = true,
     loggingHandler: LoggingHandler? = .none
   ) throws -> Process {
@@ -35,6 +36,7 @@ extension Process {
           arguments: arguments,
           environmentBlock: environmentBlock,
           workingDirectory: workingDirectory,
+          outputRedirection: outputRedirection,
           startNewProcessGroup: startNewProcessGroup,
           loggingHandler: loggingHandler
         )
@@ -42,6 +44,7 @@ extension Process {
         Process(
           arguments: arguments,
           environmentBlock: environmentBlock,
+          outputRedirection: outputRedirection,
           startNewProcessGroup: startNewProcessGroup,
           loggingHandler: loggingHandler
         )
@@ -57,6 +60,7 @@ extension Process {
         arguments: arguments,
         environmentBlock: environmentBlock,
         workingDirectory: nil,
+        outputRedirection: outputRedirection,
         startNewProcessGroup: startNewProcessGroup,
         loggingHandler: loggingHandler
       )
diff --git a/Sources/SKSwiftPMWorkspace/SwiftPMBuildSystem.swift b/Sources/SKSwiftPMWorkspace/SwiftPMBuildSystem.swift
index c3a24b451..6ddab50bf 100644
--- a/Sources/SKSwiftPMWorkspace/SwiftPMBuildSystem.swift
+++ b/Sources/SKSwiftPMWorkspace/SwiftPMBuildSystem.swift
@@ -13,7 +13,9 @@
 import Basics
 import Build
 import BuildServerProtocol
+import CAtomics
 import Dispatch
+import Foundation
 import LSPLogging
 import LanguageServerProtocol
 import PackageGraph
@@ -71,6 +73,9 @@ fileprivate extension ConfiguredTarget {
   static let forPackageManifest = ConfiguredTarget(targetID: "", runDestinationID: "")
 }
 
+/// `nonisolated(unsafe)` is fine because `preparationTaskID` is atomic.
+fileprivate nonisolated(unsafe) var preparationTaskID: AtomicUInt32 = AtomicUInt32(initialValue: 0)
+
 /// Swift Package Manager build system and workspace support.
 ///
 /// This class implements the `BuildSystem` interface to provide the build settings for a Swift
@@ -535,12 +540,12 @@ extension SwiftPMBuildSystem: SKCore.BuildSystem {
 
   public func prepare(
     targets: [ConfiguredTarget],
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws {
     // TODO (indexing): Support preparation of multiple targets at once.
     // https://github.com/apple/sourcekit-lsp/issues/1262
     for target in targets {
-      try await prepare(singleTarget: target, indexProcessDidProduceResult: indexProcessDidProduceResult)
+      try await prepare(singleTarget: target, logMessageToIndexLog: logMessageToIndexLog)
     }
     let filesInPreparedTargets = targets.flatMap { self.targets[$0]?.buildTarget.sources ?? [] }
     await fileDependenciesUpdatedDebouncer.scheduleCall(Set(filesInPreparedTargets.map(DocumentURI.init)))
@@ -548,7 +553,7 @@ extension SwiftPMBuildSystem: SKCore.BuildSystem {
 
   private func prepare(
     singleTarget target: ConfiguredTarget,
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws {
     if target == .forPackageManifest {
       // Nothing to prepare for package manifests.
@@ -581,15 +586,28 @@ extension SwiftPMBuildSystem: SKCore.BuildSystem {
       return
     }
     let start = ContinuousClock.now
-    let process = try Process.launch(arguments: arguments, workingDirectory: nil)
-    let result = try await process.waitUntilExitSendingSigIntOnTaskCancellation()
-    indexProcessDidProduceResult(
-      IndexProcessResult(
-        taskDescription: "Preparing \(target.targetID) for \(target.runDestinationID)",
-        processResult: result,
-        start: start
+
+    let logID = IndexTaskID.preparation(id: preparationTaskID.fetchAndIncrement())
+    logMessageToIndexLog(
+      logID,
+      """
+      Preparing \(target.targetID) for \(target.runDestinationID)
+      \(arguments.joined(separator: " "))
+      """
+    )
+    let stdoutHandler = PipeAsStringHandler { logMessageToIndexLog(logID, $0) }
+    let stderrHandler = PipeAsStringHandler { logMessageToIndexLog(logID, $0) }
+
+    let process = try Process.launch(
+      arguments: arguments,
+      workingDirectory: nil,
+      outputRedirection: .stream(
+        stdout: { stdoutHandler.handleDataFromPipe(Data($0)) },
+        stderr: { stderrHandler.handleDataFromPipe(Data($0)) }
       )
     )
+    let result = try await process.waitUntilExitSendingSigIntOnTaskCancellation()
+    logMessageToIndexLog(logID, "Finished in \(start.duration(to: .now))")
     switch result.exitStatus.exhaustivelySwitchable {
     case .terminated(code: 0):
       break
diff --git a/Sources/SKTestSupport/TestSourceKitLSPClient.swift b/Sources/SKTestSupport/TestSourceKitLSPClient.swift
index eedf51d85..03cacd18e 100644
--- a/Sources/SKTestSupport/TestSourceKitLSPClient.swift
+++ b/Sources/SKTestSupport/TestSourceKitLSPClient.swift
@@ -273,12 +273,12 @@ public final class TestSourceKitLSPClient: MessageHandler {
   /// Ignores any notifications that are of a different type or that don't satisfy the predicate.
   public func nextNotification<ExpectedNotificationType: NotificationType>(
     ofType: ExpectedNotificationType.Type,
-    satisfying predicate: (ExpectedNotificationType) -> Bool = { _ in true },
+    satisfying predicate: (ExpectedNotificationType) throws -> Bool = { _ in true },
     timeout: Duration = .seconds(defaultTimeout)
   ) async throws -> ExpectedNotificationType {
     while true {
       let nextNotification = try await nextNotification(timeout: timeout)
-      if let notification = nextNotification as? ExpectedNotificationType, predicate(notification) {
+      if let notification = nextNotification as? ExpectedNotificationType, try predicate(notification) {
         return notification
       }
     }
diff --git a/Sources/SemanticIndex/PreparationTaskDescription.swift b/Sources/SemanticIndex/PreparationTaskDescription.swift
index c0adfeb31..db854e923 100644
--- a/Sources/SemanticIndex/PreparationTaskDescription.swift
+++ b/Sources/SemanticIndex/PreparationTaskDescription.swift
@@ -37,8 +37,8 @@ public struct PreparationTaskDescription: IndexTaskDescription {
 
   private let preparationUpToDateTracker: UpToDateTracker<ConfiguredTarget>
 
-  /// See `SemanticIndexManager.indexProcessDidProduceResult`
-  private let indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+  /// See `SemanticIndexManager.logMessageToIndexLog`.
+  private let logMessageToIndexLog: @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
 
   /// Test hooks that should be called when the preparation task finishes.
   private let testHooks: IndexTestHooks
@@ -60,13 +60,13 @@ public struct PreparationTaskDescription: IndexTaskDescription {
     targetsToPrepare: [ConfiguredTarget],
     buildSystemManager: BuildSystemManager,
     preparationUpToDateTracker: UpToDateTracker<ConfiguredTarget>,
-    indexProcessDidProduceResult: @escaping @Sendable (IndexProcessResult) -> Void,
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void,
     testHooks: IndexTestHooks
   ) {
     self.targetsToPrepare = targetsToPrepare
     self.buildSystemManager = buildSystemManager
     self.preparationUpToDateTracker = preparationUpToDateTracker
-    self.indexProcessDidProduceResult = indexProcessDidProduceResult
+    self.logMessageToIndexLog = logMessageToIndexLog
     self.testHooks = testHooks
   }
 
@@ -105,7 +105,7 @@ public struct PreparationTaskDescription: IndexTaskDescription {
       do {
         try await buildSystemManager.prepare(
           targets: targetsToPrepare,
-          indexProcessDidProduceResult: indexProcessDidProduceResult
+          logMessageToIndexLog: logMessageToIndexLog
         )
       } catch {
         logger.error(
diff --git a/Sources/SemanticIndex/SemanticIndexManager.swift b/Sources/SemanticIndex/SemanticIndexManager.swift
index dc2b663c0..2cea6efd7 100644
--- a/Sources/SemanticIndex/SemanticIndexManager.swift
+++ b/Sources/SemanticIndex/SemanticIndexManager.swift
@@ -160,11 +160,8 @@ public final actor SemanticIndexManager {
   /// workspaces.
   private let indexTaskScheduler: TaskScheduler<AnyIndexTaskDescription>
 
-  /// Callback to be called when the process to prepare a target finishes.
-  ///
-  /// Allows an index log to be displayed to the user that includes the command line invocations of all index-related
-  /// process launches, as well as their output.
-  private let indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+  /// Callback that is called when an indexing task produces output it wants to log to the index log.
+  private let logMessageToIndexLog: @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
 
   /// Called when files are scheduled to be indexed.
   ///
@@ -206,7 +203,7 @@ public final actor SemanticIndexManager {
     buildSystemManager: BuildSystemManager,
     testHooks: IndexTestHooks,
     indexTaskScheduler: TaskScheduler<AnyIndexTaskDescription>,
-    indexProcessDidProduceResult: @escaping @Sendable (IndexProcessResult) -> Void,
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void,
     indexTasksWereScheduled: @escaping @Sendable (Int) -> Void,
     indexProgressStatusDidChange: @escaping @Sendable () -> Void
   ) {
@@ -214,7 +211,7 @@ public final actor SemanticIndexManager {
     self.buildSystemManager = buildSystemManager
     self.testHooks = testHooks
     self.indexTaskScheduler = indexTaskScheduler
-    self.indexProcessDidProduceResult = indexProcessDidProduceResult
+    self.logMessageToIndexLog = logMessageToIndexLog
     self.indexTasksWereScheduled = indexTasksWereScheduled
     self.indexProgressStatusDidChange = indexProgressStatusDidChange
   }
@@ -467,7 +464,7 @@ public final actor SemanticIndexManager {
         targetsToPrepare: targetsToPrepare,
         buildSystemManager: self.buildSystemManager,
         preparationUpToDateTracker: preparationUpToDateTracker,
-        indexProcessDidProduceResult: indexProcessDidProduceResult,
+        logMessageToIndexLog: logMessageToIndexLog,
         testHooks: testHooks
       )
     )
@@ -514,7 +511,7 @@ public final actor SemanticIndexManager {
         buildSystemManager: self.buildSystemManager,
         index: index,
         indexStoreUpToDateTracker: indexStoreUpToDateTracker,
-        indexProcessDidProduceResult: indexProcessDidProduceResult,
+        logMessageToIndexLog: logMessageToIndexLog,
         testHooks: testHooks
       )
     )
diff --git a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
index 54d7dbeaa..4ee76e59d 100644
--- a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
+++ b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
@@ -111,8 +111,8 @@ public struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
   /// case we don't need to index it again.
   private let index: UncheckedIndex
 
-  /// See `SemanticIndexManager.indexProcessDidProduceResult`
-  private let indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+  /// See `SemanticIndexManager.logMessageToIndexLog`.
+  private let logMessageToIndexLog: @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
 
   /// Test hooks that should be called when the index task finishes.
   private let testHooks: IndexTestHooks
@@ -139,14 +139,14 @@ public struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
     buildSystemManager: BuildSystemManager,
     index: UncheckedIndex,
     indexStoreUpToDateTracker: UpToDateTracker<DocumentURI>,
-    indexProcessDidProduceResult: @escaping @Sendable (IndexProcessResult) -> Void,
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void,
     testHooks: IndexTestHooks
   ) {
     self.filesToIndex = filesToIndex
     self.buildSystemManager = buildSystemManager
     self.index = index
     self.indexStoreUpToDateTracker = indexStoreUpToDateTracker
-    self.indexProcessDidProduceResult = indexProcessDidProduceResult
+    self.logMessageToIndexLog = logMessageToIndexLog
     self.testHooks = testHooks
   }
 
@@ -337,9 +337,25 @@ public struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
     defer {
       signposter.endInterval("Indexing", state)
     }
+    let logID = IndexTaskID.updateIndexStore(id: id)
+    logMessageToIndexLog(
+      logID,
+      """
+      Indexing \(indexFile.pseudoPath)
+      \(processArguments.joined(separator: " "))
+      """
+    )
+
+    let stdoutHandler = PipeAsStringHandler { logMessageToIndexLog(logID, $0) }
+    let stderrHandler = PipeAsStringHandler { logMessageToIndexLog(logID, $0) }
+
     let process = try Process.launch(
       arguments: processArguments,
-      workingDirectory: workingDirectory
+      workingDirectory: workingDirectory,
+      outputRedirection: .stream(
+        stdout: { stdoutHandler.handleDataFromPipe(Data($0)) },
+        stderr: { stderrHandler.handleDataFromPipe(Data($0)) }
+      )
     )
     // Time out updating of the index store after 2 minutes. We don't expect any single file compilation to take longer
     // than 2 minutes in practice, so this indicates that the compiler has entered a loop and we probably won't make any
@@ -349,13 +365,7 @@ public struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
       try await process.waitUntilExitSendingSigIntOnTaskCancellation()
     }
 
-    indexProcessDidProduceResult(
-      IndexProcessResult(
-        taskDescription: "Indexing \(indexFile.pseudoPath)",
-        processResult: result,
-        start: start
-      )
-    )
+    logMessageToIndexLog(logID, "Finished in \(start.duration(to: .now))")
 
     switch result.exitStatus.exhaustivelySwitchable {
     case .terminated(code: 0):
diff --git a/Sources/SourceKitLSP/Clang/ClangLanguageService.swift b/Sources/SourceKitLSP/Clang/ClangLanguageService.swift
index 965d7b01e..b5b8efd08 100644
--- a/Sources/SourceKitLSP/Clang/ClangLanguageService.swift
+++ b/Sources/SourceKitLSP/Clang/ClangLanguageService.swift
@@ -32,35 +32,6 @@ extension NSLock {
   }
 }
 
-/// Gathers data from clangd's stderr pipe. When it has accumulated a full line, writes the the line to the logger.
-fileprivate actor ClangdStderrLogForwarder {
-  /// Queue on which all data from `clangd`’s stderr will be forwarded to `stderr`. This allows us to have a
-  /// nonisolated `handle` function but ensure that data gets processed in order.
-  private let queue = AsyncQueue<Serial>()
-  private var buffer = Data()
-
-  private func handleImpl(_ newData: Data) {
-    self.buffer += newData
-    while let newlineIndex = self.buffer.firstIndex(of: UInt8(ascii: "\n")) {
-      // Output a separate log message for every line in clangd's stderr.
-      // The reason why we don't output multiple lines in a single log message is that
-      //  a) os_log truncates log messages at about 1000 bytes. The assumption is that a single line is usually less
-      //     than 1000 bytes long but if we merge multiple lines into one message, we might easily exceed this limit.
-      //  b) It might be confusing why sometimes a single log message contains one line while sometimes it contains
-      //     multiple.
-      let logger = Logger(subsystem: LoggingScope.subsystem, category: "clangd-stderr")
-      logger.info("\(String(data: self.buffer[...newlineIndex], encoding: .utf8) ?? "<invalid UTF-8>")")
-      buffer = buffer[buffer.index(after: newlineIndex)...]
-    }
-  }
-
-  nonisolated func handle(_ newData: Data) {
-    queue.async {
-      await self.handleImpl(newData)
-    }
-  }
-}
-
 /// A thin wrapper over a connection to a clangd server providing build setting handling.
 ///
 /// In addition, it also intercepts notifications and replies from clangd in order to do things
@@ -233,14 +204,16 @@ actor ClangLanguageService: LanguageService, MessageHandler {
 
     process.standardOutput = clangdToUs
     process.standardInput = usToClangd
-    let logForwarder = ClangdStderrLogForwarder()
+    let logForwarder = PipeAsStringHandler {
+      Logger(subsystem: LoggingScope.subsystem, category: "clangd-stderr").info("\($0)")
+    }
     let stderrHandler = Pipe()
     stderrHandler.fileHandleForReading.readabilityHandler = { fileHandle in
       let newData = fileHandle.availableData
       if newData.count == 0 {
         stderrHandler.fileHandleForReading.readabilityHandler = nil
       } else {
-        logForwarder.handle(newData)
+        logForwarder.handleDataFromPipe(newData)
       }
     }
     process.standardError = stderrHandler
diff --git a/Sources/SourceKitLSP/SourceKitLSPServer.swift b/Sources/SourceKitLSP/SourceKitLSPServer.swift
index 8c11b4074..e96623e39 100644
--- a/Sources/SourceKitLSP/SourceKitLSPServer.swift
+++ b/Sources/SourceKitLSP/SourceKitLSPServer.swift
@@ -837,15 +837,18 @@ private extension LanguageServerProtocol.WorkspaceType {
 }
 
 extension SourceKitLSPServer {
-  nonisolated func indexTaskDidProduceResult(_ result: IndexProcessResult) {
+  nonisolated func logMessageToIndexLog(taskID: IndexTaskID, message: String) {
+    var message: Substring = message[...]
+    while message.last?.isNewline ?? false {
+      message = message.dropLast(1)
+    }
+    let messageWithEmojiLinePrefixes = message.split(separator: "\n", omittingEmptySubsequences: false).map {
+      "\(taskID.emojiRepresentation) \($0)"
+    }.joined(separator: "\n")
     self.sendNotificationToClient(
       LogMessageNotification(
-        type: result.failed ? .warning : .info,
-        message: """
-          \(result.taskDescription) finished in \(result.duration)
-          \(result.command)
-          \(result.output)
-          """,
+        type: .info,
+        message: messageWithEmojiLinePrefixes,
         logName: "SourceKit-LSP: Indexing"
       )
     )
@@ -932,8 +935,8 @@ extension SourceKitLSPServer {
       options: options,
       indexOptions: self.options.indexOptions,
       indexTaskScheduler: indexTaskScheduler,
-      indexProcessDidProduceResult: { [weak self] in
-        self?.indexTaskDidProduceResult($0)
+      logMessageToIndexLog: { [weak self] taskID, message in
+        self?.logMessageToIndexLog(taskID: taskID, message: message)
       },
       indexTasksWereScheduled: { [weak self] count in
         self?.indexProgressManager.indexTasksWereScheduled(count: count)
@@ -1013,8 +1016,8 @@ extension SourceKitLSPServer {
           index: nil,
           indexDelegate: nil,
           indexTaskScheduler: self.indexTaskScheduler,
-          indexProcessDidProduceResult: { [weak self] in
-            self?.indexTaskDidProduceResult($0)
+          logMessageToIndexLog: { [weak self] taskID, message in
+            self?.logMessageToIndexLog(taskID: taskID, message: message)
           },
           indexTasksWereScheduled: { [weak self] count in
             self?.indexProgressManager.indexTasksWereScheduled(count: count)
diff --git a/Sources/SourceKitLSP/Workspace.swift b/Sources/SourceKitLSP/Workspace.swift
index 43672de55..848324a24 100644
--- a/Sources/SourceKitLSP/Workspace.swift
+++ b/Sources/SourceKitLSP/Workspace.swift
@@ -93,7 +93,7 @@ public final class Workspace: Sendable {
     index uncheckedIndex: UncheckedIndex?,
     indexDelegate: SourceKitIndexDelegate?,
     indexTaskScheduler: TaskScheduler<AnyIndexTaskDescription>,
-    indexProcessDidProduceResult: @escaping @Sendable (IndexProcessResult) -> Void,
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void,
     indexTasksWereScheduled: @escaping @Sendable (Int) -> Void,
     indexProgressStatusDidChange: @escaping @Sendable () -> Void
   ) async {
@@ -117,7 +117,7 @@ public final class Workspace: Sendable {
         buildSystemManager: buildSystemManager,
         testHooks: options.indexTestHooks,
         indexTaskScheduler: indexTaskScheduler,
-        indexProcessDidProduceResult: indexProcessDidProduceResult,
+        logMessageToIndexLog: logMessageToIndexLog,
         indexTasksWereScheduled: indexTasksWereScheduled,
         indexProgressStatusDidChange: indexProgressStatusDidChange
       )
@@ -155,7 +155,7 @@ public final class Workspace: Sendable {
     options: SourceKitLSPServer.Options,
     indexOptions: IndexOptions = IndexOptions(),
     indexTaskScheduler: TaskScheduler<AnyIndexTaskDescription>,
-    indexProcessDidProduceResult: @escaping @Sendable (IndexProcessResult) -> Void,
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void,
     indexTasksWereScheduled: @Sendable @escaping (Int) -> Void,
     indexProgressStatusDidChange: @Sendable @escaping () -> Void
   ) async throws {
@@ -196,7 +196,7 @@ public final class Workspace: Sendable {
       index: UncheckedIndex(index),
       indexDelegate: indexDelegate,
       indexTaskScheduler: indexTaskScheduler,
-      indexProcessDidProduceResult: indexProcessDidProduceResult,
+      logMessageToIndexLog: logMessageToIndexLog,
       indexTasksWereScheduled: indexTasksWereScheduled,
       indexProgressStatusDidChange: indexProgressStatusDidChange
     )
diff --git a/Tests/SKCoreTests/BuildSystemManagerTests.swift b/Tests/SKCoreTests/BuildSystemManagerTests.swift
index 10583e945..ab894d5ea 100644
--- a/Tests/SKCoreTests/BuildSystemManagerTests.swift
+++ b/Tests/SKCoreTests/BuildSystemManagerTests.swift
@@ -471,7 +471,7 @@ class ManualBuildSystem: BuildSystem {
 
   public func prepare(
     targets: [ConfiguredTarget],
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws {
     throw PrepareNotSupportedError()
   }
diff --git a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
index 6e5ed554e..81c419017 100644
--- a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
+++ b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
@@ -709,22 +709,44 @@ final class BackgroundIndexingTests: XCTestCase {
   }
 
   public func testProduceIndexLog() async throws {
+    let didReceivePreparationIndexLogMessage = self.expectation(description: "Did receive preparation log message")
+    let didReceiveIndexingLogMessage = self.expectation(description: "Did receive indexing log message")
+    let updateIndexStoreTaskDidFinish = self.expectation(description: "Update index store task did finish")
+
+    // Block the index tasks until we have received a log notification to make sure we stream out results as they come
+    // in and not only when the indexing task has finished
+    var serverOptions = SourceKitLSPServer.Options.testDefault
+    serverOptions.indexTestHooks.preparationTaskDidFinish = { _ in
+      await self.fulfillment(of: [didReceivePreparationIndexLogMessage], timeout: defaultTimeout)
+    }
+    serverOptions.indexTestHooks.updateIndexStoreTaskDidFinish = { _ in
+      await self.fulfillment(of: [didReceiveIndexingLogMessage], timeout: defaultTimeout)
+      updateIndexStoreTaskDidFinish.fulfill()
+    }
+
     let project = try await SwiftPMTestProject(
       files: [
         "MyFile.swift": ""
       ],
-      enableBackgroundIndexing: true
+      serverOptions: serverOptions,
+      enableBackgroundIndexing: true,
+      pollIndex: false
     )
-    let targetPrepareNotification = try await project.testClient.nextNotification(ofType: LogMessageNotification.self)
-    XCTAssert(
-      targetPrepareNotification.message.hasPrefix("Preparing MyLibrary"),
-      "\(targetPrepareNotification.message) does not have the expected prefix"
+    _ = try await project.testClient.nextNotification(
+      ofType: LogMessageNotification.self,
+      satisfying: { notification in
+        return notification.message.contains("Preparing MyLibrary")
+      }
     )
-    let indexFileNotification = try await project.testClient.nextNotification(ofType: LogMessageNotification.self)
-    XCTAssert(
-      indexFileNotification.message.hasPrefix("Indexing \(try project.uri(for: "MyFile.swift").pseudoPath)"),
-      "\(indexFileNotification.message) does not have the expected prefix"
+    didReceivePreparationIndexLogMessage.fulfill()
+    _ = try await project.testClient.nextNotification(
+      ofType: LogMessageNotification.self,
+      satisfying: { notification in
+        notification.message.contains("Indexing \(try project.uri(for: "MyFile.swift").pseudoPath)")
+      }
     )
+    didReceiveIndexingLogMessage.fulfill()
+    try await fulfillmentOfOrThrow([updateIndexStoreTaskDidFinish])
   }
 
   func testIndexingHappensInParallel() async throws {
diff --git a/Tests/SourceKitLSPTests/BuildSystemTests.swift b/Tests/SourceKitLSPTests/BuildSystemTests.swift
index 44c4031e9..c5439ca67 100644
--- a/Tests/SourceKitLSPTests/BuildSystemTests.swift
+++ b/Tests/SourceKitLSPTests/BuildSystemTests.swift
@@ -63,7 +63,7 @@ actor TestBuildSystem: BuildSystem {
 
   public func prepare(
     targets: [ConfiguredTarget],
-    indexProcessDidProduceResult: @Sendable (IndexProcessResult) -> Void
+    logMessageToIndexLog: @escaping @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
   ) async throws {
     throw PrepareNotSupportedError()
   }
@@ -141,7 +141,7 @@ final class BuildSystemTests: XCTestCase {
       index: nil,
       indexDelegate: nil,
       indexTaskScheduler: .forTesting,
-      indexProcessDidProduceResult: { _ in },
+      logMessageToIndexLog: { _, _ in },
       indexTasksWereScheduled: { _ in },
       indexProgressStatusDidChange: {}
     )