diff --git a/CHANGELOG.md b/CHANGELOG.md
index 86bf61d..5b4233c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
 - Sentence segmentation fallback: when a corpus has no `s` spans in `structure.xml` (common for TEI conversions of custom corpora), sentence boundaries now fall back to other segment-like TEI elements, in order of preference: `posting` (chat/CMC), `l` (verse line), `seg` (segment), `u` (utterance). This makes integrated taggers/parsers and KorAP sentence-based queries work on such corpora.
 - New `dck_sample.zip` test resource: two-text excerpt from the CC BY licensed Dortmunder Chat-Korpus with custom `cmc` tokenization/annotations (in one text the `s` spans are removed to exercise the sentence fallback)
 - Krill output now indexes extra inline `<w>`-level annotations from TEI-derived corpora as their own token layers, alongside POS (`p`) and lemma (`l`): `norm`, `orig`, `phon` and `trans` (read from the correspondingly named `<f>` features in `morpho.xml`). Each becomes a `foundry/<name>=tokens` layer with `foundry/<name>:<value>` terms (original casing preserved), grouped under the `morpho` foundry. The set is intentionally restricted to these four names to keep the Krill hot path fast. These layers are Krill-only and do not affect CoNLL-U output.
+- Krill output now always emits an `availability` field, which is legally required for a KorAP instance to serve a text. It is inherited from the corpus or document header when not set on the text (a lower level overrides a higher one); if no value is found at any level it defaults to `unknown`, logged per text at `FINE` and summarised in a single closing warning with the affected text count.
 
 ### Fixed
 
diff --git a/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt b/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt
index 69b655b..d3e3b39 100644
--- a/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt
+++ b/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt
@@ -1319,6 +1319,9 @@
     var krillOutputCount = java.util.concurrent.atomic.AtomicInteger(0)
     // Texts dropped from Krill output because they contain no tokens (see enqueueKrillCompression).
     val krillEmptyTextCount = java.util.concurrent.atomic.AtomicInteger(0)
+    // Texts for which no availability metadata could be resolved and that were
+    // defaulted to "unknown" (see applyInheritedKrillMetadata).
+    val krillMissingAvailabilityCount = java.util.concurrent.atomic.AtomicInteger(0)
     private val krillPeakRawPending = AtomicInteger(0)
     private val krillPeakCompressedPending = AtomicInteger(0)
     private val krillPeakCompressionInFlight = AtomicInteger(0)
@@ -1956,6 +1959,10 @@
                 if (emptyDropped > 0) {
                     LOGGER.warning("Dropped $emptyDropped empty text(s) with no tokens from Krill output")
                 }
+                val missingAvailability = krillMissingAvailabilityCount.get()
+                if (missingAvailability > 0) {
+                    LOGGER.warning("Set availability to \"unknown\" for $missingAvailability text(s) with no availability metadata in their text, doc or corpus header")
+                }
                 LOGGER.info("Closed krill TAR file: $krillOutputFileName (total texts output: $krillOutputCount)")
             } catch (e: Exception) {
                 LOGGER.severe("ERROR generating krill output: ${e.message}")
@@ -6189,6 +6196,20 @@
             corpusMetadata = corpusMetadata,
             docMetadata = docMetadata
         )
+
+        // availability is legally crucial for a KorAP instance to serve a text.
+        // resolveHeaderMetadata already inherits it corpus -> doc -> text (a lower
+        // level overrides a higher one); if it is still missing, default to
+        // "unknown" so the field is always present, and record it for a summary
+        // warning. The per-text detail is logged at FINE to avoid flooding the
+        // log when a whole corpus lacks the field.
+        val availability = resolvedMetadata["availability"]
+        if (availability == null || (availability is String && availability.isBlank())) {
+            resolvedMetadata["availability"] = "unknown"
+            krillMissingAvailabilityCount.incrementAndGet()
+            LOGGER.fine("No availability metadata for $textId (absent from text, doc and corpus header); set to \"unknown\"")
+        }
+
         textData.headerMetadata.clear()
         textData.headerMetadata.putAll(resolvedMetadata)
 
diff --git a/app/src/test/kotlin/de/ids_mannheim/korapxmltools/KrillJsonGeneratorTest.kt b/app/src/test/kotlin/de/ids_mannheim/korapxmltools/KrillJsonGeneratorTest.kt
index 9cbc88b..89f229b 100644
--- a/app/src/test/kotlin/de/ids_mannheim/korapxmltools/KrillJsonGeneratorTest.kt
+++ b/app/src/test/kotlin/de/ids_mannheim/korapxmltools/KrillJsonGeneratorTest.kt
@@ -134,6 +134,14 @@
         return tool.corpusMetadata.getValue(corpusSigle)
     }
 
+    private fun applyInheritedKrillMetadata(tool: KorapXmlTool, textId: String, textData: KrillJsonGenerator.KrillTextData) {
+        val method = KorapXmlTool::class.java.getDeclaredMethod(
+            "applyInheritedKrillMetadata", String::class.java, KrillJsonGenerator.KrillTextData::class.java
+        )
+        method.isAccessible = true
+        method.invoke(tool, textId, textData)
+    }
+
     private fun krillFieldValue(json: String, fieldName: String): String? =
         Regex(
             """"key"\s*:\s*"$fieldName".*?"value"\s*:\s*"([^"]*)"""",
@@ -591,6 +599,29 @@
     }
 
     @Test
+    fun availabilityIsInheritedFromCorpusWhenAbsentOnText() {
+        val tool = KorapXmlTool()
+        tool.corpusMetadata["TEST"] = mutableMapOf<String, Any>("availability" to "CC-BY-SA")
+        val textData = KrillJsonGenerator.KrillTextData(textId = "TEST_DOC.1")
+
+        applyInheritedKrillMetadata(tool, "TEST_DOC.1", textData)
+
+        assertEquals("CC-BY-SA", textData.headerMetadata["availability"])
+        assertEquals(0, tool.krillMissingAvailabilityCount.get(), "Inherited availability must not count as missing")
+    }
+
+    @Test
+    fun availabilityDefaultsToUnknownAndIsCountedWhenMissingEverywhere() {
+        val tool = KorapXmlTool()
+        val textData = KrillJsonGenerator.KrillTextData(textId = "TEST_DOC.1")
+
+        applyInheritedKrillMetadata(tool, "TEST_DOC.1", textData)
+
+        assertEquals("unknown", textData.headerMetadata["availability"])
+        assertEquals(1, tool.krillMissingAvailabilityCount.get(), "Missing availability must be counted for the summary warning")
+    }
+
+    @Test
     fun docAndCorpusMetadataCollectorsExposeCommonInheritedFields() {
         val tool = KorapXmlTool()