diff --git a/Examples/math/Math.swift b/Examples/math/Math.swift
index b4160dda..eec24fee 100644
--- a/Examples/math/Math.swift
+++ b/Examples/math/Math.swift
@@ -259,7 +259,7 @@ extension Math.Statistics {
   }
 }
 
-func customCompletion(_ s: [String], _: Int, _: Int) -> [String] {
+func customCompletion(_ s: [String], _: Int, _: String) -> [String] {
   (s.last ?? "").starts(with: "a")
     ? ["aardvark", "aaaaalbert"]
     : ["hello", "helicopter", "heliotrope"]
diff --git a/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift b/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift
index 2d229ba2..e6ca3803 100644
--- a/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift	
+++ b/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift	
@@ -40,7 +40,7 @@ public struct CompletionKind {
     case file(extensions: [String])
     case directory
     case shellCommand(String)
-    case custom(@Sendable ([String], Int, Int) -> [String])
+    case custom(@Sendable ([String], Int, String) -> [String])
     case customDeprecated(@Sendable ([String]) -> [String])
   }
 
@@ -126,11 +126,11 @@ public struct CompletionKind {
   /// passed to Swift as `"abc\\""def"` (i.e. the Swift String's contents would
   /// include all 4 of the double quotes and the 2 consecutive backslashes).
   ///
-  /// The first of the two `Int` arguments is the 0-based index of the word
-  /// for which completions are being requested within the given `[String]`.
+  /// The second argument (an `Int`) is the 0-based index of the word for which
+  /// completions are being requested within the given `[String]`.
   ///
-  /// The second of the two `Int` arguments is the 0-based index of the shell
-  /// cursor within the word for which completions are being requested.
+  /// The third argument (a `String`) is the prefix of the word for which
+  /// completions are being requested that precedes the cursor.
   ///
   /// ### bash
   ///
@@ -171,21 +171,21 @@ public struct CompletionKind {
   /// character, not as after the backslash.
   @preconcurrency
   public static func custom(
-    _ completion: @Sendable @escaping ([String], Int, Int) -> [String]
+    _ completion: @Sendable @escaping ([String], Int, String) -> [String]
   ) -> CompletionKind {
     CompletionKind(kind: .custom(completion))
   }
 
   /// Deprecated; only kept for backwards compatibility.
   ///
-  /// The same as `custom(@Sendable @escaping ([String], Int, Int) -> [String])`,
-  /// except that index arguments are not supplied.
+  /// The same as `custom(@Sendable @escaping ([String], Int, String) -> [String])`,
+  /// except that the last two closure arguments are not supplied.
   @preconcurrency
   @available(
     *,
     deprecated,
     message:
-      "Provide a three-parameter closure instead. See custom(@Sendable @escaping ([String], Int, Int) -> [String])."
+      "Provide a three-parameter closure instead. See custom(@Sendable @escaping ([String], Int, String) -> [String])."
   )
   public static func custom(
     _ completion: @Sendable @escaping ([String]) -> [String]
diff --git a/Sources/ArgumentParser/Parsing/CommandParser.swift b/Sources/ArgumentParser/Parsing/CommandParser.swift
index 5c24586a..f45939c6 100644
--- a/Sources/ArgumentParser/Parsing/CommandParser.swift
+++ b/Sources/ArgumentParser/Parsing/CommandParser.swift
@@ -456,16 +456,27 @@ extension CommandParser {
       }
 
       guard
-        let s = args.popFirst(),
-        let cursorIndexWithinCompletingArgument = Int(s)
+        let arg = args.popFirst(),
+        let cursorIndexWithinCompletingArgument = Int(arg)
       else {
         throw ParserError.invalidState
       }
 
+      let completingPrefix: String
+      if let completingArgument = args.last {
+        completingPrefix = String(
+          completingArgument.prefix(cursorIndexWithinCompletingArgument)
+        )
+      } else if cursorIndexWithinCompletingArgument == 0 {
+        completingPrefix = ""
+      } else {
+        throw ParserError.invalidState
+      }
+
       completions = complete(
         Array(args),
         completingArgumentIndex,
-        cursorIndexWithinCompletingArgument
+        completingPrefix
       )
     case .customDeprecated(let complete):
       completions = complete(args)
diff --git a/Sources/ArgumentParserToolInfo/ToolInfo.swift b/Sources/ArgumentParserToolInfo/ToolInfo.swift
index df35f15f..808106a8 100644
--- a/Sources/ArgumentParserToolInfo/ToolInfo.swift
+++ b/Sources/ArgumentParserToolInfo/ToolInfo.swift
@@ -149,9 +149,9 @@ public struct ArgumentInfoV0: Codable, Hashable {
     case directory
     /// Call the given shell command to generate completions.
     case shellCommand(command: String)
-    /// Generate completions using the given closure including index arguments.
+    /// Generate completions using the given three-parameter closure.
     case custom
-    /// Generate completions using the given closure without index arguments.
+    /// Generate completions using the given one-parameter closure.
     @available(*, deprecated, message: "Use custom instead.")
     case customDeprecated
   }