diff --git a/Sources/SstData/EntryWarnings.swift b/Sources/SstData/EntryWarnings.swift
new file mode 100644
index 0000000..b8b559f
--- /dev/null
+++ b/Sources/SstData/EntryWarnings.swift
@@ -0,0 +1,67 @@
+import Foundation
+
+public struct EntryWarning: Equatable, Sendable {
+    public enum Level: Int, Comparable, Sendable {
+        case note = 0, warning, error
+        public static func < (a: Level, b: Level) -> Bool { a.rawValue < b.rawValue }
+    }
+
+    public let level: Level
+    public let message: String
+
+    public init(level: Level, message: String) {
+        self.level = level
+        self.message = message
+    }
+}
+
+/// Flags dictionary entries that are likely to corrupt normal text.
+public enum EntryWarnings {
+    /// - Parameter others: all entries except the one being checked.
+    public static func check(_ kind: DictionaryEntry.Kind, among others: [DictionaryEntry], knowledge: WordKnowledge) -> [EntryWarning] {
+        let source: String
+        let isCorrection: Bool
+        switch kind {
+        case .term(let term): (source, isCorrection) = (term, false)
+        case .correction(let heard, _): (source, isCorrection) = (heard, true)
+        }
+
+        let parts = EntryKey.parts(source)
+        guard !parts.isEmpty else { return [EntryWarning(level: .error, message: "Leeg: vul een woord in.")] }
+
+        var result: [EntryWarning] = []
+        if others.contains(where: { EntryKey.key($0.source) == EntryKey.key(source) }) {
+            result.append(EntryWarning(level: .error, message: "Dubbele invoer: deze bron staat er al, de eerste telt."))
+        }
+
+        if !isCorrection {
+            if parts.count == 1, knowledge.isWord(source) {
+                result.append(EntryWarning(level: .note, message: "Gewoon woord: krijgt alleen voorrang bij het model, geen automatische hoofdletterregel."))
+            }
+            return result.sorted { $0.level > $1.level }
+        }
+
+        if parts.count == 1, knowledge.isWord(parts[0]) {
+            result.append(EntryWarning(level: .warning, message: "'\(parts[0])' is een gewoon woord en wordt overal in je tekst vervangen."))
+        }
+        if source.filter(\.isLetter).count < 3 {
+            result.append(EntryWarning(level: .warning, message: "Erg kort (minder dan 3 letters): veel kans op ongewenste vervangingen."))
+        }
+        if parts.count > 1 {
+            var flagged = false
+            if let common = parts.first(where: { knowledge.isFunctionWord($0) }) {
+                result.append(EntryWarning(level: .warning, message: "Bevat het veelvoorkomende woord '\(common)'; dit kan gewone zinnen raken."))
+                flagged = true
+            }
+            let glued = parts.joined()
+            if knowledge.isWord(glued) {
+                result.append(EntryWarning(level: .warning, message: "Aaneengeschreven ('\(glued)') is een bestaand woord en wordt ook vervangen."))
+                flagged = true
+            }
+            if !flagged, parts.allSatisfy({ knowledge.isWord($0) }) {
+                result.append(EntryWarning(level: .note, message: "Alle delen zijn gewone woorden: alleen de hele combinatie wordt vervangen."))
+            }
+        }
+        return result.sorted { $0.level > $1.level }
+    }
+}
diff --git a/Sources/SstData/VocabularyBuilder.swift b/Sources/SstData/VocabularyBuilder.swift
new file mode 100644
index 0000000..43a2dc0
--- /dev/null
+++ b/Sources/SstData/VocabularyBuilder.swift
@@ -0,0 +1,19 @@
+import Foundation
+
+/// The short word list handed to the speech engine as context.
+public enum VocabularyBuilder {
+    /// Terms first (file order), then the unique right-hand sides of correction pairs; at most `maxTerms` words.
+    public static func build(from entries: [DictionaryEntry], maxTerms: Int = 24) -> [String] {
+        var out: [String] = []
+        var seen = Set<String>()
+        func add(_ word: String) {
+            let key = word.lowercased()
+            guard !word.isEmpty, !seen.contains(key) else { return }
+            seen.insert(key)
+            out.append(word)
+        }
+        for entry in entries { if case .term(let term) = entry.kind { add(term) } }
+        for entry in entries { if case .correction(_, let wrote) = entry.kind { add(wrote) } }
+        return Array(out.prefix(maxTerms))
+    }
+}
diff --git a/Tests/SstDataTests/EntryWarningsTests.swift b/Tests/SstDataTests/EntryWarningsTests.swift
new file mode 100644
index 0000000..d37338a
--- /dev/null
+++ b/Tests/SstDataTests/EntryWarningsTests.swift
@@ -0,0 +1,60 @@
+import Testing
+@testable import SstData
+
+struct EntryWarningsTests {
+    let knowledge = FakeKnowledge()
+
+    func check(_ kind: DictionaryEntry.Kind, others: [DictionaryEntry] = []) -> [EntryWarning] {
+        EntryWarnings.check(kind, among: others, knowledge: knowledge)
+    }
+
+    @Test func singleCommonWordIsAWarning() {
+        let w = check(.correction(heard: "cloud", wrote: "Claude"))
+        #expect(w.contains { $0.level == .warning && $0.message.contains("cloud") })
+    }
+
+    @Test func veryShortSourceIsAWarning() {
+        #expect(check(.correction(heard: "ab", wrote: "AB")).contains { $0.level == .warning })
+    }
+
+    @Test func functionWordInsideAPhraseIsAWarning() {
+        #expect(check(.correction(heard: "in de", wrote: "indeed")).contains { $0.level == .warning })
+    }
+
+    @Test func gluedFormThatIsARealWordIsAWarning() {
+        let w = check(.correction(heard: "some thing", wrote: "Something"))
+        #expect(w.contains { $0.level == .warning && $0.message.contains("something") })
+    }
+
+    @Test func phraseOfCommonWordsOnlyGetsANote() {
+        let w = check(.correction(heard: "cloud code", wrote: "Claude Code"))
+        #expect(w.count == 1)
+        #expect(w[0].level == .note)
+    }
+
+    @Test func distinctiveEntriesGetNoWarnings() {
+        #expect(check(.correction(heard: "versel", wrote: "Vercel")).isEmpty)
+        #expect(check(.term("Supabase")).isEmpty)
+    }
+
+    @Test func duplicateSourceIsAnErrorEvenWithDifferentSpelling() {
+        let others = [pair("cloud code", "Claude Code")]
+        let w = check(.correction(heard: "Cloud-Code", wrote: "X"), others: others)
+        #expect(w.first?.level == .error)
+    }
+
+    @Test func termThatIsARealWordGetsANoteAboutBiasOnly() {
+        let w = check(.term("Reef"))
+        #expect(w.count == 1)
+        #expect(w[0].level == .note)
+    }
+
+    @Test func emptySourceIsAnError() {
+        #expect(check(.term("  ")).first?.level == .error)
+    }
+
+    @Test func warningsAreSortedMostSevereFirst() {
+        let w = check(.correction(heard: "cloud", wrote: "Claude"), others: [pair("cloud", "x")])
+        #expect(w.map(\.level) == w.map(\.level).sorted(by: >))
+    }
+}
diff --git a/Tests/SstDataTests/VocabularyBuilderTests.swift b/Tests/SstDataTests/VocabularyBuilderTests.swift
new file mode 100644
index 0000000..f9cd1ca
--- /dev/null
+++ b/Tests/SstDataTests/VocabularyBuilderTests.swift
@@ -0,0 +1,19 @@
+import Testing
+@testable import SstData
+
+struct VocabularyBuilderTests {
+    @Test func termsComeFirstThenCorrectionTargetsWithoutDuplicates() {
+        let entries = [pair("cloud code", "Claude Code"), term("Anthropic"), term("anthropic"), pair("versel", "Vercel"), term("Supabase")]
+        #expect(VocabularyBuilder.build(from: entries) == ["Anthropic", "Supabase", "Claude Code", "Vercel"])
+    }
+
+    @Test func listIsCapped() {
+        let entries = (1...40).map { term("Woord\($0)") }
+        #expect(VocabularyBuilder.build(from: entries).count == 24)
+        #expect(VocabularyBuilder.build(from: entries, maxTerms: 5).count == 5)
+    }
+
+    @Test func emptyDictionaryGivesNoVocabulary() {
+        #expect(VocabularyBuilder.build(from: []).isEmpty)
+    }
+}