diff --git a/Core/Sources/Core/InputUtils/RomanCaseCandidates.swift b/Core/Sources/Core/InputUtils/RomanCaseCandidates.swift
new file mode 100644
index 00000000..ef4c51d6
--- /dev/null
+++ b/Core/Sources/Core/InputUtils/RomanCaseCandidates.swift
@@ -0,0 +1,15 @@
+import Foundation
+
+/// Case variants of the original keyboard input, before kana conversion.
+enum RomanCaseCandidates {
+    static func variants(for input: String) -> [String] {
+        guard input.unicodeScalars.allSatisfy({ (0x21...0x7e).contains($0.value) }),
+              input.unicodeScalars.contains(where: { (0x41...0x5a).contains($0.value) || (0x61...0x7a).contains($0.value) }) else {
+            return []
+        }
+        let lower = input.lowercased()
+        let title = lower.prefix(1).uppercased() + lower.dropFirst()
+        var seen = Set<String>()
+        return [lower, title, input.uppercased()].filter { seen.insert($0).inserted }
+    }
+}
diff --git a/Core/Sources/Core/InputUtils/SegmentsManager.swift b/Core/Sources/Core/InputUtils/SegmentsManager.swift
index 3188ca36..b51b8f51 100644
--- a/Core/Sources/Core/InputUtils/SegmentsManager.swift
+++ b/Core/Sources/Core/InputUtils/SegmentsManager.swift
@@ -564,7 +564,7 @@ public final class SegmentsManager {
 
         let leftSideContext = forcedLeftSideContext ?? self.getCleanLeftSideContext(maxCount: ContextLength.conversion)
         let rightSideContext = forcedRightSideContext ?? self.getCleanRightSideContext(maxCount: ContextLength.conversion)
-        let result = self.kanaKanjiConverter.requestCandidates(
+        var result = self.kanaKanjiConverter.requestCandidates(
             self.composingText,
             options: options(
                 leftSideContext: leftSideContext,
@@ -574,6 +574,22 @@ public final class SegmentsManager {
                 requireEnglishPrediction: Config.DebugPredictiveTyping().value ? .manualMix : .disabled
             )
         )
+        let romanComposingText = self.composingText.prefixToCursorPosition()
+        let romanInput = romanComposingText.input.map(\.piece).inputString(preferIntention: false)
+        let romanVariants = RomanCaseCandidates.variants(for: romanInput)
+        if !romanVariants.isEmpty {
+            let romanTexts = Set(result.mainResults.map(\.text))
+            let romanCandidates = romanVariants.filter { !romanTexts.contains($0) }.map { text in
+                Candidate(
+                    text: text, value: -18,
+                    composingCount: .inputCount(romanComposingText.input.count),
+                    lastMid: MIDData.一般.mid,
+                    data: [.init(word: text, ruby: romanInput, cid: CIDData.固有名詞.cid, mid: MIDData.一般.mid, value: -18)],
+                    isLearningTarget: false
+                )
+            }
+            result.mainResults.insert(contentsOf: romanCandidates, at: min(5, result.mainResults.count))
+        }
         self.rawCandidates = result
     }
 
diff --git a/Core/Tests/CoreTests/InputUtilsTests/EditedVariantRangeTests.swift b/Core/Tests/CoreTests/InputUtilsTests/EditedVariantRangeTests.swift
new file mode 100644
index 00000000..6d525266
--- /dev/null
+++ b/Core/Tests/CoreTests/InputUtilsTests/EditedVariantRangeTests.swift
@@ -0,0 +1,23 @@
+import Core
+import Foundation
+import KanaKanjiConverterModuleWithDefaultDictionary
+import Testing
+
+@MainActor
+@Test func editedVariantConsumesOnlySelectedPrefix() throws {
+    let directory = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+    try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
+    defer { try? FileManager.default.removeItem(at: directory) }
+    let manager = SegmentsManager(kanaKanjiConverter: .withDefaultDictionary(), applicationDirectoryURL: directory,
+                                  containerURL: nil, context: .init(useZenzai: false))
+    manager.insertAtCursorPosition("sushi", inputStyle: .roman2kana)
+    manager.editSegment(count: -1)
+    manager.requestSetCandidateWindowState(visible: true)
+    guard case .selecting(let choices, _) = manager.getCurrentCandidateWindow(inputState: .selecting) else {
+        Issue.record("Expected candidates for edited segment")
+        return
+    }
+    let candidate = try #require(choices.first { $0.text == "SU" })
+    manager.prefixCandidateCommited(candidate, leftSideContext: "")
+    #expect(manager.convertTarget == "し")
+}
diff --git a/Core/Tests/CoreTests/InputUtilsTests/RomanCaseCandidatesTests.swift b/Core/Tests/CoreTests/InputUtilsTests/RomanCaseCandidatesTests.swift
new file mode 100644
index 00000000..5a98ece1
--- /dev/null
+++ b/Core/Tests/CoreTests/InputUtilsTests/RomanCaseCandidatesTests.swift
@@ -0,0 +1,47 @@
+@testable import Core
+import Foundation
+import KanaKanjiConverterModuleWithDefaultDictionary
+import Testing
+
+@MainActor
+struct RomanCaseCandidatesTests {
+    @Test func variantsAndExclusions() {
+        #expect(RomanCaseCandidates.variants(for: "fable") == ["fable", "Fable", "FABLE"])
+        #expect(RomanCaseCandidates.variants(for: "GitHub") == ["github", "Github", "GITHUB"])
+        #expect(RomanCaseCandidates.variants(for: "a") == ["a", "A"])
+        for input in ["", "1111", "ふぁbぇ", "コーヒー", "abc\ndef"] {
+            #expect(RomanCaseCandidates.variants(for: input).isEmpty)
+        }
+    }
+
+    @Test(arguments: ["fable", "github", "FABLE"], ["lower", "title", "upper"])
+    func originalKeysCanBeConvertedAndFullyCommitted(input: String, form: String) throws {
+        let directory = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+        try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: directory) }
+        let manager = SegmentsManager(kanaKanjiConverter: .withDefaultDictionary(), applicationDirectoryURL: directory,
+                                      containerURL: nil, context: .init(useZenzai: false))
+        for character in input {
+            manager.insertAtCursorPosition(String(character), inputStyle: .roman2kana)
+        }
+        if input == "fable" { #expect(manager.convertTarget == "ふぁbぇ") }
+        let lower = input.lowercased()
+        let expected = form == "lower" ? lower : form == "title" ? lower.prefix(1).uppercased() + lower.dropFirst() : input.uppercased()
+        for rich in [false, true] {
+            manager.update(requestRichCandidates: rich)
+            manager.requestSetCandidateWindowState(visible: true)
+            guard case .selecting(let choices, _) = manager.getCurrentCandidateWindow(inputState: .selecting) else {
+                Issue.record("Expected conversion candidates")
+                return
+            }
+            #expect(choices.filter { $0.text == expected }.count == 1)
+            if rich {
+                let row = try #require(choices.firstIndex { $0.text == expected })
+                manager.requestSelectingRow(row)
+                let candidate = try #require(manager.selectedCandidate)
+                manager.prefixCandidateCommited(candidate, leftSideContext: "")
+                #expect(manager.isEmpty)
+            }
+        }
+    }
+}