Fixes #33 — verb examples must actually use the verb

The example sentences in VerbDetailView (and the new vocab practice
modes) frequently used the wrong verb — a "tener" set would show
"Él estaba leyendo un libro" (estar), "Nosotros vamos a viajar" (ir),
"Tú debes estudiar" (deber). The model drifted off the target verb
partway through generating the 6-example batch, and nothing checked
the output.

Two defenses:

  Prompt grounding — VerbExampleGenerator.generate now takes a
  formsByTense map (tenseId → conjugated forms, from the new
  ReferenceStore.conjugatedForms). Each tense line in the prompt
  lists the verb's exact conjugated forms and instructs the model to
  use one of them. The model echoes a real form instead of recalling
  (and mis-recalling) the conjugation.

  Output validation — every generated sentence is checked against the
  conjugation table via accent/case-folded whole-word matching. Any
  sentence that doesn't contain a real conjugated form of the verb is
  rejected. Failures trigger one regeneration pass; anything still
  wrong is dropped rather than displayed. Better to show 4 correct
  examples than 6 with 2 wrong.

  Cache invalidation — VerbExampleCache now persists a versioned
  wrapper (version 2). Pre-fix cached example sets — which may contain
  wrong-verb sentences — fail the version check and are discarded, so
  they regenerate cleanly under the new path.

Callers updated: VerbDetailView, VocabFlashcardPracticeView,
VocabMultipleChoicePracticeView all build formsByTense from
ReferenceStore and pass it through.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga/Services/ReferenceStore.swift

@@ -94,6 +94,19 @@ struct ReferenceStore {
         return (try? context.fetch(descriptor)) ?? []
     }
 
+    /// Map of tenseId → conjugated forms for a verb, used to ground and
+    /// validate LLM-generated example sentences.
+    func conjugatedForms(verbId: Int, tenseIds: [String]) -> [String: [String]] {
+        var map: [String: [String]] = [:]
+        for tenseId in tenseIds {
+            let forms = fetchForms(verbId: verbId, tenseId: tenseId)
+                .map(\.form)
+                .filter { !$0.trimmingCharacters(in: .whitespaces).isEmpty }
+            if !forms.isEmpty { map[tenseId] = forms }
+        }
+        return map
+    }
+
     func fetchForm(verbId: Int, tenseId: String, personIndex: Int) -> VerbForm? {
         let descriptor = FetchDescriptor<VerbForm>(
             predicate: #Predicate<VerbForm> { form in

Conjuga/Conjuga/Services/VerbExampleCache.swift

@@ -10,6 +10,16 @@ import SharedModels
 @Observable
 final class VerbExampleCache {
 
+    /// Bump to invalidate every cached example set. Raised to 2 for Issue #33
+    /// — examples generated before the verb-grounding/validation fix could
+    /// contain sentences built on the wrong verb.
+    private static let cacheVersion = 2
+
+    private struct CacheFile: Codable {
+        var version: Int
+        var entries: [String: [VerbExample]]
+    }
+
     private var store: [Int: [VerbExample]] = [:]
     private var isLoaded = false
 
@@ -43,13 +53,18 @@ final class VerbExampleCache {
         defer { isLoaded = true }
 
         guard let data = try? Data(contentsOf: Self.cacheURL),
-              let decoded = try? JSONDecoder().decode([String: [VerbExample]].self, from: data)
-        else { return }
+              let decoded = try? JSONDecoder().decode(CacheFile.self, from: data),
+              decoded.version == Self.cacheVersion
+        else {
+            // Missing, unreadable, old flat format, or stale version — start
+            // fresh so pre-fix examples don't linger.
+            return
+        }
 
         // Persisted with String keys because JSON object keys are strings;
         // convert back to Int for in-memory lookup.
         var rebuilt: [Int: [VerbExample]] = [:]
-        for (key, value) in decoded {
+        for (key, value) in decoded.entries {
             if let id = Int(key) {
                 rebuilt[id] = value
             }
@@ -58,8 +73,9 @@ final class VerbExampleCache {
     }
 
     private func save() {
-        let serialized = Dictionary(uniqueKeysWithValues: store.map { (String($0.key), $0.value) })
-        guard let data = try? JSONEncoder().encode(serialized) else { return }
+        let entries = Dictionary(uniqueKeysWithValues: store.map { (String($0.key), $0.value) })
+        let file = CacheFile(version: Self.cacheVersion, entries: entries)
+        guard let data = try? JSONEncoder().encode(file) else { return }
         try? data.write(to: Self.cacheURL)
     }
 }

Conjuga/Conjuga/Services/VerbExampleGenerator.swift

@@ -5,6 +5,15 @@ import SharedModels
 /// Generates a set of example sentences for a single verb, one per core tense
 /// (Issue #27). Mirrors the StoryGenerator pattern: @Generable response types,
 /// a static availability flag, and a single generate(...) entry point.
+///
+/// Issue #33: the model used to drift onto other verbs partway through the
+/// 6-example batch (a "tener" set would contain sentences built on estar / ir
+/// / deber). Two defenses now apply:
+///   1. The prompt embeds the verb's *exact* conjugated forms per tense, so
+///      the model echoes a real form instead of recalling one.
+///   2. Every generated sentence is validated against those forms; failures
+///      are regenerated once, and anything still wrong is dropped rather than
+///      shown.
 @MainActor
 struct VerbExampleGenerator {
 
@@ -33,22 +42,82 @@ struct VerbExampleGenerator {
 
     // MARK: - Generation
 
-    /// Generate one example per tense in `tenseIds`. Returns the examples in the
-    /// same order as `tenseIds`, filling in placeholders for any the model skipped.
+    /// Generate one validated example per tense in `tenseIds`.
+    ///
+    /// - Parameter formsByTense: the verb's conjugated forms keyed by tenseId
+    ///   (from `ReferenceStore.conjugatedForms`). Used to ground the prompt
+    ///   and validate the output. A tense with no forms here is accepted
+    ///   without validation.
     static func generate(
         verbInfinitive: String,
         verbEnglish: String,
-        tenseIds: [String]
+        tenseIds: [String],
+        formsByTense: [String: [String]]
     ) async throws -> [VerbExample] {
-        let tenseList = tenseIds
-            .compactMap { id in TenseInfo.find(id).map { "\(id) (\($0.english))" } }
-            .joined(separator: ", ")
+        let firstPass = try await generateBatch(
+            verbInfinitive: verbInfinitive,
+            verbEnglish: verbEnglish,
+            tenseIds: tenseIds,
+            formsByTense: formsByTense
+        )
+
+        var valid: [String: VerbExample] = [:]
+        var failedTenses: [String] = []
+        for id in tenseIds {
+            if let ex = firstPass[id], exampleUsesVerb(ex.spanish, forms: formsByTense[id] ?? []) {
+                valid[id] = ex
+            } else {
+                failedTenses.append(id)
+            }
+        }
+
+        // One focused retry — regenerate the whole batch, but only adopt the
+        // results for tenses that failed the first pass.
+        if !failedTenses.isEmpty {
+            let retry = try? await generateBatch(
+                verbInfinitive: verbInfinitive,
+                verbEnglish: verbEnglish,
+                tenseIds: tenseIds,
+                formsByTense: formsByTense
+            )
+            if let retry {
+                for id in failedTenses {
+                    if let ex = retry[id], exampleUsesVerb(ex.spanish, forms: formsByTense[id] ?? []) {
+                        valid[id] = ex
+                    }
+                }
+            }
+        }
+
+        // Requested order, dropping any tense that never produced a valid
+        // sentence — better to show fewer examples than wrong ones.
+        return tenseIds.compactMap { valid[$0] }
+    }
+
+    // MARK: - Single batch call
+
+    private static func generateBatch(
+        verbInfinitive: String,
+        verbEnglish: String,
+        tenseIds: [String],
+        formsByTense: [String: [String]]
+    ) async throws -> [String: VerbExample] {
+        let tenseBlock = tenseIds.compactMap { id -> String? in
+            guard let info = TenseInfo.find(id) else { return nil }
+            let forms = formsByTense[id] ?? []
+            if forms.isEmpty {
+                return "- \(id) (\(info.english))"
+            }
+            return "- \(id) (\(info.english)): use one of these exact conjugated forms — \(forms.joined(separator: ", "))"
+        }.joined(separator: "\n")
 
         let session = LanguageModelSession(instructions: """
             You are a Spanish language teacher writing short example sentences for a learner.
             The learner is studying the verb "\(verbInfinitive)" (to \(verbEnglish)).
-            Write one sentence per requested tense. Each sentence must:
-            - Actually conjugate "\(verbInfinitive)" in that tense (not just mention it).
+            EVERY sentence must use "\(verbInfinitive)" as its main verb, conjugated in the
+            requested tense — never substitute a different verb (no estar, ir, deber, etc.
+            unless the target verb itself is that verb). Each sentence must:
+            - Contain one of the exact conjugated forms listed for its tense.
             - Be 6-14 words, natural and everyday.
             - Use vocabulary appropriate for intermediate learners.
             - Vary subjects and contexts across the set; do not reuse the same subject twice.
@@ -56,27 +125,42 @@ struct VerbExampleGenerator {
             """)
 
         let prompt = """
-            Write example sentences for "\(verbInfinitive)" in these tenses, in this order:
-            \(tenseList)
+            Write one example sentence for "\(verbInfinitive)" per tense below, in this order:
+            \(tenseBlock)
 
             Return one GeneratedExample per tense with the matching tenseId, spanish, and english.
+            The Spanish sentence MUST contain one of the conjugated forms shown for that tense.
             """
 
         let response = try await session.respond(to: prompt, generating: GeneratedExampleSet.self)
 
-        // Map by tenseId and return in the caller's requested order so the UI
-        // renders a predictable sequence even if the model shuffles its output.
-        // Use `uniquingKeysWith` defensively — the @Generable schema requires
-        // exactly 6 examples, but if the model duplicates a tenseId (it does
-        // happen when the caller passes fewer than 6 distinct tenses), the
-        // strict `uniqueKeysWithValues:` initializer would trap.
-        let byTense = Dictionary(
+        // `uniquingKeysWith` defensively — the schema forces 6 examples even
+        // when fewer tenses are requested, so the model may repeat a tenseId.
+        return Dictionary(
             response.content.examples.map {
                 ($0.tenseId, VerbExample(tenseId: $0.tenseId, spanish: $0.spanish, english: $0.english))
             },
             uniquingKeysWith: { first, _ in first }
         )
-        return tenseIds.compactMap { byTense[$0] }
+    }
+
+    // MARK: - Validation
+
+    /// True when `sentence` contains at least one of `forms` as a whole word
+    /// (accent- and case-insensitive). Empty `forms` → accept (can't validate).
+    static func exampleUsesVerb(_ sentence: String, forms: [String]) -> Bool {
+        guard !forms.isEmpty else { return true }
+        let sentenceWords = foldedWords(sentence)
+        let formWords = Set(forms.flatMap { foldedWords($0) })
+        return !sentenceWords.isDisjoint(with: formWords)
+    }
+
+    private static func foldedWords(_ text: String) -> Set<String> {
+        let folded = text.folding(
+            options: [.diacriticInsensitive, .caseInsensitive],
+            locale: nil
+        )
+        return Set(folded.split { !$0.isLetter }.map(String.init))
     }
 
     static var isAvailable: Bool {

Conjuga/Conjuga/Views/Practice/Vocab/VocabFlashcardPracticeView.swift

@@ -205,6 +205,8 @@ struct VocabFlashcardPracticeView: View {
         let verbId = verb.id
         let infinitive = verb.infinitive
         let english = verb.english
+        let formsByTense = ReferenceStore(context: localContext)
+            .conjugatedForms(verbId: verbId, tenseIds: VocabExampleTenseIds.canonical)
         Task {
             do {
                 // The generator's @Generable schema requires exactly 6
@@ -214,7 +216,8 @@ struct VocabFlashcardPracticeView: View {
                 let examples = try await VerbExampleGenerator.generate(
                     verbInfinitive: infinitive,
                     verbEnglish: english,
-                    tenseIds: VocabExampleTenseIds.canonical
+                    tenseIds: VocabExampleTenseIds.canonical,
+                    formsByTense: formsByTense
                 )
                 exampleCache.setExamples(examples, for: verbId)
                 let pick = examples.first { $0.tenseId == "ind_presente" } ?? examples.first

Conjuga/Conjuga/Views/Practice/Vocab/VocabMultipleChoicePracticeView.swift

@@ -214,12 +214,15 @@ struct VocabMultipleChoicePracticeView: View {
         let verbId = verb.id
         let infinitive = verb.infinitive
         let english = verb.english
+        let formsByTense = ReferenceStore(context: localContext)
+            .conjugatedForms(verbId: verbId, tenseIds: VocabExampleTenseIds.canonical)
         Task {
             do {
                 let examples = try await VerbExampleGenerator.generate(
                     verbInfinitive: infinitive,
                     verbEnglish: english,
-                    tenseIds: VocabExampleTenseIds.canonical
+                    tenseIds: VocabExampleTenseIds.canonical,
+                    formsByTense: formsByTense
                 )
                 exampleCache.setExamples(examples, for: verbId)
                 let pick = examples.first { $0.tenseId == "ind_presente" } ?? examples.first

Conjuga/Conjuga/Views/Verbs/VerbDetailView.swift

@@ -215,10 +215,13 @@ struct VerbDetailView: View {
 
         examplesState = .loading
         do {
+            let formsByTense = ReferenceStore(context: modelContext)
+                .conjugatedForms(verbId: verb.id, tenseIds: Self.exampleTenseIds)
             let generated = try await VerbExampleGenerator.generate(
                 verbInfinitive: verb.infinitive,
                 verbEnglish: verb.english,
-                tenseIds: Self.exampleTenseIds
+                tenseIds: Self.exampleTenseIds,
+                formsByTense: formsByTense
             )
             guard !generated.isEmpty else {
                 examplesState = .failed("Could not generate examples.")