diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/AlternativeCombinator.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/AlternativeCombinator.kt
index cb10cec..bd17fa6 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/AlternativeCombinator.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/AlternativeCombinator.kt
@@ -38,9 +38,9 @@ open class AlternativeCombinator(vararg val tokens: Combinator) : Combinator {
     return match(value, offset, Combinator::SemanticMatch)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> =
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> =
     // Offer every alternative's matches, so the order of options no longer affects correctness.
-    tokens.asSequence().flatMap { it.parse(value, offset) }
+    tokens.asSequence().flatMap { it.parse(value, offset, frontier) }
 
   override fun toString(): String = toStringIndented(0)
 
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Combinator.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Combinator.kt
index b9140ad..b0f5411 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Combinator.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Combinator.kt
@@ -40,8 +40,12 @@ interface Combinator {
    * carries a `valid` flag for the strict (semantic) check. Because every alternative is offered
    * rather than the first greedy one committed to, matching is complete — e.g.
    * Seq(ZeroOrMore("a"), "a") on "aa" matches, because ZeroOrMore offers the shorter match too.
+   *
+   * [frontier] records the deepest offset reached and what was expected there, so that even when no
+   * path succeeds we can localize the error (and, later, drive completion). Combinators thread the
+   * same instance into their children; leaf matchers report themselves to it.
    */
-  fun parse(value: String, offset: Int): Sequence<Parse>
+  fun parse(value: String, offset: Int, frontier: Frontier = Frontier()): Sequence<Parse>
 
   fun toStringIndented(indent: Int): String
 }
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/EOF.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/EOF.kt
index aea72e8..d62c78e 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/EOF.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/EOF.kt
@@ -17,8 +17,10 @@ class EOF : Combinator {
     }
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> =
-    if (offset == value.length) sequenceOf(Parse(offset, emptyList())) else emptySequence()
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
+    frontier.reached(offset, this) // we expect end-of-input here
+    return if (offset == value.length) sequenceOf(Parse(offset, emptyList())) else emptySequence()
+  }
 
   override fun toStringIndented(indent: Int): String {
     return "EOF"
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/FlexibleLiteralChoiceTerminal.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/FlexibleLiteralChoiceTerminal.kt
index c24012a..601189f 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/FlexibleLiteralChoiceTerminal.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/FlexibleLiteralChoiceTerminal.kt
@@ -91,7 +91,8 @@ class FlexibleLiteralChoiceTerminal(vararg val choices: String) : TerminalCombin
     return NoMatch.copy(longestMatch = offset)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
+    frontier.reached(offset, this)
     // Lenient shape match (so a wrong token like AF_BOGUS still matches and can be highlighted),
     // valid only if the matched text is one of the exact choices.
     val m = syntaticMatch.matchAt(value, offset) ?: return emptySequence()
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Frontier.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Frontier.kt
new file mode 100644
index 0000000..d081b64
--- /dev/null
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Frontier.kt
@@ -0,0 +1,42 @@
+package net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.optionvalues.grammar
+
+/**
+ * The "frontier" — a high-water mark recorder threaded through parse() (#467 step 3).
+ *
+ * parse() only ever returns *successful* matches, so when a value is malformed the failing paths
+ * vanish and we lose all trace of how far we got. The frontier is a side-channel that survives
+ * failure: every leaf matcher (a terminal, or EOF) reports itself here when it is consulted at an
+ * offset, and the frontier keeps only the DEEPEST offset reached and the set of matchers wanted
+ * there.
+ *
+ * That gives two things from one mechanism:
+ *  - error localization: the deepest offset is where parsing got stuck, and `expected` is what
+ *    would have been valid there;
+ *  - the seed of completion (#343): "what could come next at this position?" is the same question.
+ *
+ * It is mutable and shared across the whole (lazy) exploration of a single value on purpose — it is
+ * the global deepest-reach across every path tried.
+ */
+class Frontier {
+  /** The deepest offset at which any leaf matcher was consulted. */
+  var position: Int = 0
+    private set
+
+  private val expectedAtPosition = linkedSetOf<Combinator>()
+
+  /** The matchers consulted at [position] — i.e. what the grammar was hoping to see there. */
+  val expected: Set<Combinator> get() = expectedAtPosition
+
+  /** Record that [matcher] was consulted at [offset]. Only the deepest offset's matchers are kept. */
+  fun reached(offset: Int, matcher: Combinator) {
+    when {
+      offset > position -> {
+        position = offset
+        expectedAtPosition.clear()
+        expectedAtPosition.add(matcher)
+      }
+      offset == position -> expectedAtPosition.add(matcher)
+      // offset < position: a shallower path, ignore.
+    }
+  }
+}
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/IntegerTerminal.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/IntegerTerminal.kt
index 744b5a3..45e4356 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/IntegerTerminal.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/IntegerTerminal.kt
@@ -29,7 +29,8 @@ class IntegerTerminal(private val minInclusive: Long,private val maxExclusive: L
     }
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
+    frontier.reached(offset, this)
     val m = intRegex.matchAt(value, offset) ?: return emptySequence()
     val text = m.value
     // Lenient: any integer matches (so we can locate it); valid only if it is within range.
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/LiteralChoiceTerminal.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/LiteralChoiceTerminal.kt
index 17d6fbf..e7ae2cb 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/LiteralChoiceTerminal.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/LiteralChoiceTerminal.kt
@@ -24,11 +24,13 @@ class  LiteralChoiceTerminal(vararg var choices: String) : TerminalCombinator {
     return match(value, offset)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> =
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
+    frontier.reached(offset, this)
     // Offer every choice that matches here (e.g. both ":" and "::"); each is always strictly valid.
-    choices.asSequence()
+    return choices.asSequence()
       .filter { value.startsWith(it, offset) }
       .map { Parse(offset + it.length, listOf(ParsedToken(offset, offset + it.length, it, this, valid = true))) }
+  }
 
   override fun toString(): String {
     return if (choices.size == 1) {
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/OneOrMore.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/OneOrMore.kt
index fc999cd..e9d7518 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/OneOrMore.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/OneOrMore.kt
@@ -40,15 +40,15 @@ class OneOrMore(val combinator : Combinator) : Combinator {
     return match(value, offset, combinator::SemanticMatch)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
     // Same as ZeroOrMore, but the first repetition is mandatory (and must make progress).
     fun extend(from: Parse): Sequence<Parse> = sequence {
       yield(from)
-      for (step in combinator.parse(value, from.end)) {
+      for (step in combinator.parse(value, from.end, frontier)) {
         if (step.end > from.end) yieldAll(extend(Parse(step.end, from.tokens + step.tokens)))
       }
     }
-    return combinator.parse(value, offset).filter { it.end > offset }.flatMap { extend(it) }
+    return combinator.parse(value, offset, frontier).filter { it.end > offset }.flatMap { extend(it) }
   }
 
   override fun toString(): String = toStringIndented(0)
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Parse.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Parse.kt
index aa3c389..8e63d2c 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Parse.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Parse.kt
@@ -34,8 +34,12 @@ sealed interface ParseOutcome {
   /** A path consumed the whole value, but a token is not strictly valid (well-formed but wrong). */
   data class SemanticError(val badToken: ParsedToken) : ParseOutcome
 
-  /** No path consumed the whole value. [furthest] is how far any path got (for error localization). */
-  data class SyntaxError(val furthest: Int) : ParseOutcome
+  /**
+   * No path consumed the whole value. [furthest] is the deepest offset any path reached, and
+   * [expected] is the set of matchers the grammar was hoping to see there (for error localization,
+   * and the seed of completion).
+   */
+  data class SyntaxError(val furthest: Int, val expected: Set<Combinator>) : ParseOutcome
 }
 
 /** Every way [this] grammar can consume the entire [value]. */
@@ -46,15 +50,19 @@ fun Combinator.fullParses(value: String): Sequence<Parse> =
  * One lenient parse answers both questions the old two passes did:
  *  - syntactic ("could be this, color it"): did any path consume the whole value?
  *  - semantic ("actually valid"):           did any such path use only valid tokens?
+ *
+ * A single shared [Frontier] rides along, so a SyntaxError can report where parsing got stuck.
  */
 fun Combinator.validate(value: String): ParseOutcome {
+  val frontier = Frontier()
   var firstBad: ParsedToken? = null
-  for (p in fullParses(value)) {
+  for (p in parse(value, 0, frontier)) {
+    if (p.end != value.length) continue
     val bad = p.tokens.firstOrNull { !it.valid }
     if (bad == null) return ParseOutcome.Valid // short-circuit on the first fully-valid full parse
     if (firstBad == null) firstBad = bad
   }
   if (firstBad != null) return ParseOutcome.SemanticError(firstBad)
-  val furthest = parse(value, 0).maxOfOrNull { it.end } ?: 0
-  return ParseOutcome.SyntaxError(furthest)
+  // No full parse: exhausting the loop above has populated the frontier with the deepest reach.
+  return ParseOutcome.SyntaxError(frontier.position, frontier.expected)
 }
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/RegexTerminal.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/RegexTerminal.kt
index f31b854..c338b7f 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/RegexTerminal.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/RegexTerminal.kt
@@ -18,7 +18,8 @@ class RegexTerminal(syntaticMatchStr : String, semanticMatchStr: String ) : Term
     return MatchResult(listOf(matchResult.value), offset + matchResult.value.length, listOf(this), offset + matchResult.value.length)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
+    frontier.reached(offset, this)
     // The syntactic regex gives the lenient span; valid iff the semantic regex matches that same span.
     val syn = syntaticMatch.matchAt(value, offset) ?: return emptySequence()
     val text = syn.value
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Repeat.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Repeat.kt
index 3cec7fa..c624e6f 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Repeat.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/Repeat.kt
@@ -62,13 +62,13 @@ class Repeat(val combinator : Combinator, val minInclusive: Int, val maxExclusiv
     return match(value, offset, combinator::SemanticMatch)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
     // Offer every repetition count in [minInclusive, maxExclusive] (maxExclusive is the cap on the
     // count, mirroring the existing match() loop). Yield only once enough repetitions have happened.
     fun extend(from: Parse, count: Int): Sequence<Parse> = sequence {
       if (count >= minInclusive) yield(from)
       if (count < maxExclusive) {
-        for (step in combinator.parse(value, from.end)) {
+        for (step in combinator.parse(value, from.end, frontier)) {
           if (step.end > from.end) yieldAll(extend(Parse(step.end, from.tokens + step.tokens), count + 1))
         }
       }
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/SequenceCombinator.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/SequenceCombinator.kt
index c959fd5..0692677 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/SequenceCombinator.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/SequenceCombinator.kt
@@ -55,12 +55,12 @@ open class SequenceCombinator(vararg val tokens: Combinator) : Combinator {
     return MatchResult(resultTokens, index, resultTerminals, maxLength)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
     // Thread each possibility of one part into the next: the cartesian product of the parts.
     var results = sequenceOf(Parse(offset, emptyList()))
     for (token in tokens) {
       results = results.flatMap { acc ->
-        token.parse(value, acc.end).map { next -> Parse(next.end, acc.tokens + next.tokens) }
+        token.parse(value, acc.end, frontier).map { next -> Parse(next.end, acc.tokens + next.tokens) }
       }
     }
     return results
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/WhitespaceTerminal.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/WhitespaceTerminal.kt
index cf9be72..541a4ba 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/WhitespaceTerminal.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/WhitespaceTerminal.kt
@@ -27,7 +27,8 @@ class WhitespaceTerminal : TerminalCombinator {
     return match(value, offset)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
+    frontier.reached(offset, this)
     var end = offset
     while (end < value.length && value[end].isWhitespace()) end++
     return if (end == offset) emptySequence()
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrMore.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrMore.kt
index 0b68640..c94bf12 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrMore.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrMore.kt
@@ -42,12 +42,12 @@ class ZeroOrMore(val combinator : Combinator) : Combinator {
     return match(value, offset, combinator::SemanticMatch)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> {
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> {
     // Offer EVERY repetition count (0, 1, 2, ...), not just the greedy maximum. The `> from.end`
     // guard keeps an inner matcher that can match empty from looping forever.
     fun extend(from: Parse): Sequence<Parse> = sequence {
       yield(from) // stop repeating here...
-      for (step in combinator.parse(value, from.end)) {
+      for (step in combinator.parse(value, from.end, frontier)) {
         if (step.end > from.end) yieldAll(extend(Parse(step.end, from.tokens + step.tokens))) // ...or take one more
       }
     }
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrOne.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrOne.kt
index 279026b..6305bf4 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrOne.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ZeroOrOne.kt
@@ -40,9 +40,9 @@ class ZeroOrOne(val combinator : Combinator) : Combinator {
     return match(value, offset, combinator::SemanticMatch)
   }
 
-  override fun parse(value: String, offset: Int): Sequence<Parse> =
+  override fun parse(value: String, offset: Int, frontier: Frontier): Sequence<Parse> =
     // Both the empty match and whatever the inner matcher offers.
-    sequenceOf(Parse(offset, emptyList())) + combinator.parse(value, offset)
+    sequenceOf(Parse(offset, emptyList())) + combinator.parse(value, offset, frontier)
 
   override fun toString(): String = toStringIndented(0)
 
diff --git a/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ParseTest.kt b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ParseTest.kt
index 8e91dab..9e6b8ce 100644
--- a/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ParseTest.kt
+++ b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/grammar/ParseTest.kt
@@ -2,6 +2,7 @@ package net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.optionvalues.gra
 
 import net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.optionvalues.ai.ConfigParseAddressFamiliesOptionValue
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotNull
 import org.junit.Assert.assertTrue
 import org.junit.Test
 
@@ -44,13 +45,26 @@ class ParseTest {
     assertEquals("AF_BOGUS", (semantic as ParseOutcome.SemanticError).badToken.text)
 
     // Comma breaks the shape after "AF_INET" -> syntax error (malformed, not just an unknown name).
-    val syntax = grammar.validate("AF_INET, AF_INET6")
-    assertTrue(syntax is ParseOutcome.SyntaxError)
-    // KNOWN LIMITATION: `furthest` is best-effort. This grammar ends in EOF(), so the outer
-    // Seq(..., EOF()) drops the partial "AF_INET" path when EOF fails, and furthest collapses to 0
-    // (we'd want 7). Precise localization needs the frontier/expected-set layer — the same machinery
-    // that powers completion (#343) — which is deliberately not in this step. Pinned to document it.
-    assertEquals(0, (syntax as ParseOutcome.SyntaxError).furthest)
+    // Thanks to the frontier layer we now report WHERE it got stuck (offset 7, the comma) and WHAT
+    // was expected there: another whitespace-separated family, or end-of-input.
+    val syntax = grammar.validate("AF_INET, AF_INET6") as? ParseOutcome.SyntaxError
+    assertNotNull(syntax)
+    assertEquals(7, syntax!!.furthest)
+    assertTrue(syntax.expected.any { it is WhitespaceTerminal })
+    assertTrue(syntax.expected.any { it is EOF })
+  }
+
+  @Test
+  fun testFrontierSeedsCompletionAtStart() {
+    // The frontier's "expected set" is exactly what completion (#343) needs: at the caret position,
+    // which tokens could legally come next? For the empty value at offset 0, the grammar expects
+    // "none", the "~" inversion prefix, or an address-family name.
+    val grammar = ConfigParseAddressFamiliesOptionValue().combinator
+    val outcome = grammar.validate("") as? ParseOutcome.SyntaxError
+    assertNotNull(outcome)
+    assertEquals(0, outcome!!.furthest)
+    assertTrue(outcome.expected.any { it is FlexibleLiteralChoiceTerminal }) // the AF_* names
+    assertTrue(outcome.expected.any { it is LiteralChoiceTerminal })         // "none" / "~"
   }
 
   @Test