diff --git a/Package.swift b/Package.swift
index 99e0630ef..68d0d41e9 100644
--- a/Package.swift
+++ b/Package.swift
@@ -143,7 +143,6 @@ let package = Package(
         .product(name: "ArgumentParser", package: "swift-argument-parser"),
         .product(name: "SwiftSyntax", package: "swift-syntax"),
         .product(name: "SwiftParser", package: "swift-syntax"),
-        .product(name: "TSCBasic", package: "swift-tools-support-core"),
       ]
     ),
 
@@ -223,15 +222,10 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
       url: "https://github.com/apple/swift-syntax.git",
       branch: "main"
     ),
-    .package(
-      url: "https://github.com/apple/swift-tools-support-core.git",
-      exact: Version("0.4.0")
-    ),
   ]
 } else {
   package.dependencies += [
     .package(path: "../swift-argument-parser"),
     .package(path: "../swift-syntax"),
-    .package(path: "../swift-tools-support-core"),
   ]
 }
diff --git a/Sources/swift-format/Frontend/FormatFrontend.swift b/Sources/swift-format/Frontend/FormatFrontend.swift
index 996b1a924..aac36856b 100644
--- a/Sources/swift-format/Frontend/FormatFrontend.swift
+++ b/Sources/swift-format/Frontend/FormatFrontend.swift
@@ -40,7 +40,8 @@ class FormatFrontend: Frontend {
       return
     }
 
-    let diagnosticHandler: (Diagnostic, SourceLocation) -> () =  { (diagnostic, location) in
+    let diagnosticHandler: (SwiftDiagnostics.Diagnostic, SourceLocation) -> () = {
+      (diagnostic, location) in
       guard !self.lintFormatOptions.ignoreUnparsableFiles else {
         // No diagnostics should be emitted in this mode.
         return
diff --git a/Sources/swift-format/Frontend/Frontend.swift b/Sources/swift-format/Frontend/Frontend.swift
index fa9611fac..0c8c4a50f 100644
--- a/Sources/swift-format/Frontend/Frontend.swift
+++ b/Sources/swift-format/Frontend/Frontend.swift
@@ -57,7 +57,7 @@ class Frontend {
   final let diagnosticPrinter: StderrDiagnosticPrinter
 
   /// The diagnostic engine to which warnings and errors will be emitted.
-  final let diagnosticsEngine: UnifiedDiagnosticsEngine
+  final let diagnosticsEngine: DiagnosticsEngine
 
   /// Options that apply during formatting or linting.
   final let lintFormatOptions: LintFormatOptions
@@ -83,7 +83,7 @@ class Frontend {
     self.diagnosticPrinter = StderrDiagnosticPrinter(
       colorMode: lintFormatOptions.colorDiagnostics.map { $0 ? .on : .off } ?? .auto)
     self.diagnosticsEngine =
-      UnifiedDiagnosticsEngine(diagnosticsHandlers: [diagnosticPrinter.printDiagnostic])
+      DiagnosticsEngine(diagnosticsHandlers: [diagnosticPrinter.printDiagnostic])
   }
 
   /// Runs the linter or formatter over the inputs.
diff --git a/Sources/swift-format/Utilities/Diagnostic.swift b/Sources/swift-format/Utilities/Diagnostic.swift
new file mode 100644
index 000000000..3a80333a9
--- /dev/null
+++ b/Sources/swift-format/Utilities/Diagnostic.swift
@@ -0,0 +1,80 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 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 SwiftFormatCore
+import SwiftSyntax
+
+/// Diagnostic data that retains the separation of a finding category (if present) from the rest of
+/// the message, allowing diagnostic printers that want to print those values separately to do so.
+struct Diagnostic {
+  /// The severity of the diagnostic.
+  enum Severity {
+    case note
+    case warning
+    case error
+  }
+
+  /// Represents the location of a diagnostic.
+  struct Location {
+    /// The file path associated with the diagnostic.
+    var file: String
+
+    /// The 1-based line number where the diagnostic occurred.
+    var line: Int
+
+    /// The 1-based column number where the diagnostic occurred.
+    var column: Int
+
+    /// Creates a new diagnostic location from the given source location.
+    init(_ sourceLocation: SourceLocation) {
+      self.file = sourceLocation.file!
+      self.line = sourceLocation.line!
+      self.column = sourceLocation.column!
+    }
+
+    /// Creates a new diagnostic location with the given finding location.
+    init(_ findingLocation: Finding.Location) {
+      self.file = findingLocation.file
+      self.line = findingLocation.line
+      self.column = findingLocation.column
+    }
+  }
+
+  /// The severity of the diagnostic.
+  var severity: Severity
+
+  /// The location where the diagnostic occurred, if known.
+  var location: Location?
+
+  /// The category of the diagnostic, if any.
+  var category: String?
+
+  /// The message text associated with the diagnostic.
+  var message: String
+
+  var description: String {
+    if let category = category {
+      return "[\(category)] \(message)"
+    } else {
+      return message
+    }
+  }
+
+  /// Creates a new diagnostic with the given severity, location, optional category, and
+  /// message.
+  init(severity: Severity, location: Location?, category: String? = nil, message: String) {
+    self.severity = severity
+    self.location = location
+    self.category = category
+    self.message = message
+  }
+}
diff --git a/Sources/swift-format/Utilities/DiagnosticsEngine.swift b/Sources/swift-format/Utilities/DiagnosticsEngine.swift
new file mode 100644
index 000000000..52e0b8909
--- /dev/null
+++ b/Sources/swift-format/Utilities/DiagnosticsEngine.swift
@@ -0,0 +1,126 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 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 SwiftFormatCore
+import SwiftSyntax
+import SwiftDiagnostics
+
+/// Unifies the handling of findings from the linter, parsing errors from the syntax parser, and
+/// generic errors from the frontend so that they are emitted in a uniform fashion.
+final class DiagnosticsEngine {
+  /// The handler functions that will be called to process diagnostics that are emitted.
+  private let handlers: [(Diagnostic) -> Void]
+
+  /// A Boolean value indicating whether any errors were emitted by the diagnostics engine.
+  private(set) var hasErrors: Bool
+
+  /// A Boolean value indicating whether any warnings were emitted by the diagnostics engine.
+  private(set) var hasWarnings: Bool
+
+  /// Creates a new diagnostics engine with the given diagnostic handlers.
+  ///
+  /// - Parameter diagnosticsHandlers: An array of functions, each of which takes a `Diagnostic` as
+  ///   its sole argument and returns `Void`. The functions are called whenever a diagnostic is
+  ///   received by the engine.
+  init(diagnosticsHandlers: [(Diagnostic) -> Void]) {
+    self.handlers = diagnosticsHandlers
+    self.hasErrors = false
+    self.hasWarnings = false
+  }
+
+  /// Emits the diagnostic by passing it to the registered handlers, and tracks whether it was an
+  /// error or warning diagnostic.
+  private func emit(_ diagnostic: Diagnostic) {
+    switch diagnostic.severity {
+    case .error: self.hasErrors = true
+    case .warning: self.hasWarnings = true
+    default: break
+    }
+
+    for handler in handlers {
+      handler(diagnostic)
+    }
+  }
+
+  /// Emits a generic error message.
+  ///
+  /// - Parameters:
+  ///   - message: The message associated with the error.
+  ///   - location: The location in the source code associated with the error, or nil if there is no
+  ///     location associated with the error.
+  func emitError(_ message: String, location: SourceLocation? = nil) {
+    emit(
+      Diagnostic(
+        severity: .error,
+        location: location.map(Diagnostic.Location.init),
+        message: message))
+  }
+
+  /// Emits a finding from the linter and any of its associated notes as diagnostics.
+  ///
+  /// - Parameter finding: The finding that should be emitted.
+  func consumeFinding(_ finding: Finding) {
+    emit(diagnosticMessage(for: finding))
+
+    for note in finding.notes {
+      emit(
+        Diagnostic(
+          severity: .note,
+          location: note.location.map(Diagnostic.Location.init),
+          message: "\(note.message)"))
+    }
+  }
+
+  /// Emits a diagnostic from the syntax parser and any of its associated notes.
+  ///
+  /// - Parameter diagnostic: The syntax parser diagnostic that should be emitted.
+  func consumeParserDiagnostic(
+    _ diagnostic: SwiftDiagnostics.Diagnostic,
+    _ location: SourceLocation
+  ) {
+    emit(diagnosticMessage(for: diagnostic.diagMessage, at: location))
+  }
+
+  /// Converts a diagnostic message from the syntax parser into a diagnostic message that can be
+  /// used by the `TSCBasic` diagnostics engine and returns it.
+  private func diagnosticMessage(
+    for message: SwiftDiagnostics.DiagnosticMessage,
+    at location: SourceLocation
+  ) -> Diagnostic {
+    let severity: Diagnostic.Severity
+    switch message.severity {
+    case .error: severity = .error
+    case .warning: severity = .warning
+    case .note: severity = .note
+    }
+    return Diagnostic(
+      severity: severity,
+      location: Diagnostic.Location(location),
+      category: nil,
+      message: message.message)
+  }
+
+  /// Converts a lint finding into a diagnostic message that can be used by the `TSCBasic`
+  /// diagnostics engine and returns it.
+  private func diagnosticMessage(for finding: Finding) -> Diagnostic {
+    let severity: Diagnostic.Severity
+    switch finding.severity {
+    case .error: severity = .error
+    case .warning: severity = .warning
+    }
+    return Diagnostic(
+      severity: severity,
+      location: finding.location.map(Diagnostic.Location.init),
+      category: "\(finding.category)",
+      message: "\(finding.message.text)")
+  }
+}
diff --git a/Sources/swift-format/Utilities/StderrDiagnosticPrinter.swift b/Sources/swift-format/Utilities/StderrDiagnosticPrinter.swift
index f6452be82..f7730f00c 100644
--- a/Sources/swift-format/Utilities/StderrDiagnosticPrinter.swift
+++ b/Sources/swift-format/Utilities/StderrDiagnosticPrinter.swift
@@ -12,7 +12,6 @@
 
 import Dispatch
 import Foundation
-import TSCBasic
 
 /// Manages printing of diagnostics to standard error.
 final class StderrDiagnosticPrinter {
@@ -49,11 +48,7 @@ final class StderrDiagnosticPrinter {
   init(colorMode: ColorMode) {
     switch colorMode {
     case .auto:
-      if let stream = stderrStream.stream as? LocalFileOutputByteStream {
-        useColors = TerminalController.isTTY(stream)
-      } else {
-        useColors = false
-      }
+      useColors = isTTY(FileHandle.standardError)
     case .off:
       useColors = false
     case .on:
@@ -62,25 +57,32 @@ final class StderrDiagnosticPrinter {
   }
 
   /// Prints a diagnostic to standard error.
-  func printDiagnostic(_ diagnostic: TSCBasic.Diagnostic) {
+  func printDiagnostic(_ diagnostic: Diagnostic) {
     printQueue.sync {
       let stderr = FileHandleTextOutputStream(FileHandle.standardError)
 
-      stderr.write("\(ansiSGR(.boldWhite))\(diagnostic.location): ")
+      stderr.write("\(ansiSGR(.boldWhite))\(description(of: diagnostic.location)): ")
 
-      switch diagnostic.behavior {
+      switch diagnostic.severity {
       case .error: stderr.write("\(ansiSGR(.boldRed))error: ")
       case .warning: stderr.write("\(ansiSGR(.boldMagenta))warning: ")
       case .note: stderr.write("\(ansiSGR(.boldGray))note: ")
-      case .remark, .ignored: break
       }
 
-      let data = diagnostic.data as! UnifiedDiagnosticData
-      if let category = data.category {
+      if let category = diagnostic.category {
         stderr.write("\(ansiSGR(.boldYellow))[\(category)] ")
       }
-      stderr.write("\(ansiSGR(.boldWhite))\(data.message)\(ansiSGR(.reset))\n")
+      stderr.write("\(ansiSGR(.boldWhite))\(diagnostic.message)\(ansiSGR(.reset))\n")
+    }
+  }
+
+  /// Returns a string representation of the given diagnostic location, or a fallback string if the
+  /// location was not known.
+  private func description(of location: Diagnostic.Location?) -> String {
+    if let location = location {
+      return "\(location.file):\(location.line):\(location.column)"
     }
+    return "<unknown>"
   }
 
   /// Returns the complete ANSI sequence used to enable the given SGR if colors are enabled in the
diff --git a/Sources/swift-format/Utilities/TTY.swift b/Sources/swift-format/Utilities/TTY.swift
new file mode 100644
index 000000000..35fc35841
--- /dev/null
+++ b/Sources/swift-format/Utilities/TTY.swift
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 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
+
+/// Returns a value indicating whether or not the stream is a TTY.
+func isTTY(_ fileHandle: FileHandle) -> Bool {
+  // The implementation of this function is adapted from `TerminalController.swift` in
+  // swift-tools-support-core.
+  #if os(Windows)
+    // The TSC implementation of this function only returns `.file` or `.dumb` for Windows,
+    // neither of which is a TTY.
+    return false
+  #else
+    if ProcessInfo.processInfo.environment["TERM"] == "dumb" {
+      return false
+    }
+    return isatty(fileHandle.fileDescriptor) != 0
+  #endif
+}
diff --git a/Sources/swift-format/Utilities/UnifiedDiagnosticsEngine.swift b/Sources/swift-format/Utilities/UnifiedDiagnosticsEngine.swift
deleted file mode 100644
index de6963f58..000000000
--- a/Sources/swift-format/Utilities/UnifiedDiagnosticsEngine.swift
+++ /dev/null
@@ -1,151 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2014 - 2021 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 SwiftFormatCore
-import SwiftSyntax
-import SwiftDiagnostics
-import TSCBasic
-
-/// Diagnostic data that retains the separation of a finding category (if present) from the rest of
-/// the message, allowing diagnostic printers that want to print those values separately to do so.
-struct UnifiedDiagnosticData: DiagnosticData {
-  /// The category of the diagnostic, if any.
-  var category: String?
-
-  /// The message text associated with the diagnostic.
-  var message: String
-
-  var description: String {
-    if let category = category {
-      return "[\(category)] \(message)"
-    } else {
-      return message
-    }
-  }
-
-  /// Creates a new unified diagnostic with the given optional category and message.
-  init(category: String? = nil, message: String) {
-    self.category = category
-    self.message = message
-  }
-}
-
-/// Unifies the handling of findings from the linter, parsing errors from the syntax parser, and
-/// generic errors from the frontend so that they are treated uniformly by the underlying
-/// diagnostics engine from the `swift-tools-support-core` package.
-final class UnifiedDiagnosticsEngine {
-  /// Represents a location from either the linter or the syntax parser and supports converting it
-  /// to a string representation for printing.
-  private enum UnifiedLocation: DiagnosticLocation {
-    /// A location received from the swift parser.
-    case parserLocation(SourceLocation)
-
-    /// A location received from the linter.
-    case findingLocation(Finding.Location)
-
-    var description: String {
-      switch self {
-      case .parserLocation(let location):
-        // SwiftSyntax's old diagnostic printer also force-unwrapped these, so we assume that they
-        // will always be present if the location itself is non-nil.
-        return "\(location.file!):\(location.line!):\(location.column!)"
-      case .findingLocation(let location):
-        return "\(location.file):\(location.line):\(location.column)"
-      }
-    }
-  }
-
-  /// The underlying diagnostics engine.
-  private let diagnosticsEngine: DiagnosticsEngine
-
-  /// A Boolean value indicating whether any errors were emitted by the diagnostics engine.
-  var hasErrors: Bool { diagnosticsEngine.hasErrors }
-
-  /// A Boolean value indicating whether any warnings were emitted by the diagnostics engine.
-  var hasWarnings: Bool {
-    diagnosticsEngine.diagnostics.contains { $0.behavior == .warning }
-  }
-
-  /// Creates a new unified diagnostics engine with the given diagnostic handlers.
-  ///
-  /// - Parameter diagnosticsHandlers: An array of functions, each of which takes a `Diagnostic` as
-  ///   its sole argument and returns `Void`. The functions are called whenever a diagnostic is
-  ///   received by the engine.
-  init(diagnosticsHandlers: [DiagnosticsEngine.DiagnosticsHandler]) {
-    self.diagnosticsEngine = DiagnosticsEngine(handlers: diagnosticsHandlers)
-  }
-
-  /// Emits a generic error message.
-  ///
-  /// - Parameters:
-  ///   - message: The message associated with the error.
-  ///   - location: The location in the source code associated with the error, or nil if there is no
-  ///     location associated with the error.
-  func emitError(_ message: String, location: SourceLocation? = nil) {
-    diagnosticsEngine.emit(
-      .error(UnifiedDiagnosticData(message: message)),
-      location: location.map(UnifiedLocation.parserLocation))
-  }
-
-  /// Emits a finding from the linter and any of its associated notes as diagnostics.
-  ///
-  /// - Parameter finding: The finding that should be emitted.
-  func consumeFinding(_ finding: Finding) {
-    diagnosticsEngine.emit(
-      diagnosticMessage(for: finding),
-      location: finding.location.map(UnifiedLocation.findingLocation))
-
-    for note in finding.notes {
-      diagnosticsEngine.emit(
-        .note(UnifiedDiagnosticData(message: "\(note.message)")),
-        location: note.location.map(UnifiedLocation.findingLocation))
-    }
-  }
-
-  /// Emits a diagnostic from the syntax parser and any of its associated notes.
-  ///
-  /// - Parameter diagnostic: The syntax parser diagnostic that should be emitted.
-  func consumeParserDiagnostic(
-    _ diagnostic: SwiftDiagnostics.Diagnostic,
-    _ location: SourceLocation
-  ) {
-    diagnosticsEngine.emit(
-      diagnosticMessage(for: diagnostic.diagMessage),
-      location: UnifiedLocation.parserLocation(location))
-  }
-
-  /// Converts a diagnostic message from the syntax parser into a diagnostic message that can be
-  /// used by the `TSCBasic` diagnostics engine and returns it.
-  private func diagnosticMessage(for message: SwiftDiagnostics.DiagnosticMessage)
-    -> TSCBasic.Diagnostic.Message
-  {
-    let data = UnifiedDiagnosticData(category: nil, message: message.message)
-
-    switch message.severity {
-    case .error: return .error(data)
-    case .warning: return .warning(data)
-    case .note: return .note(data)
-    }
-  }
-
-  /// Converts a lint finding into a diagnostic message that can be used by the `TSCBasic`
-  /// diagnostics engine and returns it.
-  private func diagnosticMessage(for finding: Finding) -> TSCBasic.Diagnostic.Message {
-    let data =
-      UnifiedDiagnosticData(category: "\(finding.category)", message: "\(finding.message.text)")
-
-    switch finding.severity {
-    case .error: return .error(data)
-    case .warning: return .warning(data)
-    }
-  }
-}