diff --git a/crates/asap_types/src/sds.rs b/crates/asap_types/src/sds.rs
index dc08cee6..b54c5029 100644
--- a/crates/asap_types/src/sds.rs
+++ b/crates/asap_types/src/sds.rs
@@ -64,6 +64,14 @@ impl SummaryInstanceId {
         }
         Ok(Self(value))
     }
+
+    pub fn validate(&self) -> Result<(), SdsError> {
+        if self.0.trim().is_empty() {
+            Err(SdsError("summary instance ID must not be empty".into()))
+        } else {
+            Ok(())
+        }
+    }
 }
 
 /// Immutable identity of the desired catalog generation used to create state.
@@ -77,13 +85,118 @@ pub struct CatalogGeneration {
     pub snapshot_sha256: String,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
 #[serde(deny_unknown_fields)]
 pub struct HalfOpenTimeRange {
     pub start_ms: i64,
     pub end_ms: i64,
 }
 
+/// One ordered producer partition. Completion and watermark claims are scoped
+/// to this identity; a maximum timestamp observed by an unrelated worker is
+/// never a source-completeness signal.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SummarySourcePartition {
+    pub producer_id: String,
+    pub partition_id: String,
+    /// Changes whenever a producer restarts or loses its sequence state.
+    pub producer_epoch: u64,
+}
+
+/// A closed materialization bucket, including named group values so a
+/// downstream maintenance DAG can project or shuffle groups without parsing a
+/// routing key.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SummaryInstanceCoordinates {
+    pub summary_definition_id: SummaryDefinitionId,
+    pub time_range: HalfOpenTimeRange,
+    pub group_values: BTreeMap<String, String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SummaryWindowCompletion {
+    pub catalog_generation: CatalogGeneration,
+    pub source: SummarySourcePartition,
+    /// Concrete SDS instance identity allocated by the storage engine.
+    pub instance_id: SummaryInstanceId,
+    pub coordinates: SummaryInstanceCoordinates,
+    /// Collision-free opaque producer lineage. Retries repeat these bytes.
+    pub input_lineage: Vec<u8>,
+}
+
+impl HalfOpenTimeRange {
+    pub fn validate(self) -> Result<(), SdsError> {
+        if self.start_ms >= self.end_ms {
+            Err(SdsError(
+                "time range must be non-empty and half-open".into(),
+            ))
+        } else {
+            Ok(())
+        }
+    }
+}
+
+impl SummarySourcePartition {
+    pub fn validate(&self) -> Result<(), SdsError> {
+        if self.producer_id.trim().is_empty() || self.partition_id.trim().is_empty() {
+            return Err(SdsError(
+                "source producer and partition must be non-empty".into(),
+            ));
+        }
+        if self.producer_epoch == 0 {
+            return Err(SdsError("source producer epoch must be positive".into()));
+        }
+        Ok(())
+    }
+}
+
+impl SummaryWindowCompletion {
+    pub fn validate(&self) -> Result<(), SdsError> {
+        self.source.validate()?;
+        self.instance_id.validate()?;
+        self.coordinates.time_range.validate()?;
+        if self.input_lineage.is_empty() {
+            return Err(SdsError(
+                "summary completion lineage must not be empty".into(),
+            ));
+        }
+        validate_catalog_generation(&self.catalog_generation)
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SummaryWatermarkBarrier {
+    pub catalog_generation: CatalogGeneration,
+    pub source: SummarySourcePartition,
+    /// Monotonic sequence within the producer partition.
+    pub sequence: u64,
+    /// Every event in this partition at or before this event-time watermark
+    /// was published before the barrier.
+    pub watermark_ms: i64,
+}
+
+impl SummaryWatermarkBarrier {
+    pub fn validate(&self) -> Result<(), SdsError> {
+        self.source.validate()?;
+        if self.sequence == 0 {
+            return Err(SdsError("watermark sequence must be positive".into()));
+        }
+        validate_catalog_generation(&self.catalog_generation)
+    }
+}
+
+fn validate_catalog_generation(generation: &CatalogGeneration) -> Result<(), SdsError> {
+    if generation.schema_version == 0 || generation.snapshot_sha256.trim().is_empty() {
+        Err(SdsError("catalog generation identity is invalid".into()))
+    } else {
+        Ok(())
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum InstanceCompleteness {
@@ -712,6 +825,83 @@ fn data_descriptor_id(
 mod tests {
     use super::*;
 
+    fn completion(epoch: u64) -> SummaryWindowCompletion {
+        SummaryWindowCompletion {
+            catalog_generation: CatalogGeneration {
+                schema_version: 1,
+                plan_id: 4,
+                plan_version: 2,
+                snapshot_sha256: "snapshot".into(),
+            },
+            source: SummarySourcePartition {
+                producer_id: "producer-a".into(),
+                partition_id: "partition-0".into(),
+                producer_epoch: epoch,
+            },
+            instance_id: SummaryInstanceId::new("instance-1").unwrap(),
+            coordinates: SummaryInstanceCoordinates {
+                summary_definition_id: SummaryDefinitionId(crate::PolicyFingerprint(7)),
+                time_range: HalfOpenTimeRange {
+                    start_ms: 1_000,
+                    end_ms: 2_000,
+                },
+                group_values: BTreeMap::from([("job".into(), "api".into())]),
+            },
+            input_lineage: vec![1, 2, 3],
+        }
+    }
+
+    #[test]
+    fn completion_contract_validates_identity_epoch_range_and_lineage() {
+        completion(1).validate().unwrap();
+
+        let mut invalid = completion(1);
+        invalid.coordinates.time_range.end_ms = invalid.coordinates.time_range.start_ms;
+        assert!(invalid.validate().is_err());
+        invalid = completion(1);
+        invalid.source.producer_id.clear();
+        assert!(invalid.validate().is_err());
+        invalid = completion(0);
+        assert!(invalid.validate().is_err());
+        invalid = completion(1);
+        invalid.input_lineage.clear();
+        assert!(invalid.validate().is_err());
+
+        let mut wire = serde_json::to_value(completion(1)).unwrap();
+        wire.as_object_mut()
+            .unwrap()
+            .insert("future_field".into(), json!(true));
+        assert!(serde_json::from_value::<SummaryWindowCompletion>(wire).is_err());
+    }
+
+    #[test]
+    fn producer_epoch_separates_restart_incarnations() {
+        let first = completion(1).source;
+        let second = completion(2).source;
+        assert_ne!(first, second);
+        assert_eq!(BTreeSet::from([first, second]).len(), 2);
+    }
+
+    #[test]
+    fn watermark_contract_rejects_zero_sequence_and_unknown_fields() {
+        let completion = completion(3);
+        let mut barrier = SummaryWatermarkBarrier {
+            catalog_generation: completion.catalog_generation,
+            source: completion.source,
+            sequence: 1,
+            watermark_ms: 2_000,
+        };
+        barrier.validate().unwrap();
+        barrier.sequence = 0;
+        assert!(barrier.validate().is_err());
+
+        let mut wire = serde_json::to_value(&barrier).unwrap();
+        wire.as_object_mut()
+            .unwrap()
+            .insert("ignored".into(), json!(1));
+        assert!(serde_json::from_value::<SummaryWatermarkBarrier>(wire).is_err());
+    }
+
     fn observed_instance(lifecycle: InstanceLifecycle) -> SummaryInstance {
         SummaryInstance {
             instance_id: SummaryInstanceId::new("instance-1").unwrap(),
diff --git a/data_plane/src/precompute_engine/output_sink.rs b/data_plane/src/precompute_engine/output_sink.rs
index 47d5cb31..bbc112c0 100644
--- a/data_plane/src/precompute_engine/output_sink.rs
+++ b/data_plane/src/precompute_engine/output_sink.rs
@@ -28,6 +28,18 @@ pub trait OutputSink: Send + Sync {
         &self,
         outputs: Vec<(PrecomputedOutput, Box<dyn AggregateCore>)>,
     ) -> Result<(), Box<dyn std::error::Error + Send + Sync>>;
+
+    /// Publish an explicit source-partition event-time barrier after every
+    /// preceding window from that partition has reached this sink.
+    fn advance_summary_watermark(
+        &self,
+        _barrier: &asap_types::sds::SummaryWatermarkBarrier,
+    ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+        Err(Box::new(std::io::Error::new(
+            std::io::ErrorKind::Unsupported,
+            "output sink does not support summary watermark barriers",
+        )))
+    }
 }
 
 fn consume_in_order<T>(items: Vec<T>, mut persist: impl FnMut(&T) -> bool) -> usize {
@@ -268,6 +280,45 @@ mod tests {
     use std::collections::HashMap;
     use std::sync::atomic::{AtomicUsize, Ordering};
 
+    struct EmitOnlySink;
+
+    impl OutputSink for EmitOnlySink {
+        fn emit_batch(
+            &self,
+            _outputs: Vec<(PrecomputedOutput, Box<dyn AggregateCore>)>,
+        ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn watermark_support_must_be_explicit() {
+        let completion_source = asap_types::sds::SummarySourcePartition {
+            producer_id: "producer".into(),
+            partition_id: "partition".into(),
+            producer_epoch: 1,
+        };
+        let barrier = asap_types::sds::SummaryWatermarkBarrier {
+            catalog_generation: asap_types::sds::CatalogGeneration {
+                schema_version: 1,
+                plan_id: 1,
+                plan_version: 1,
+                snapshot_sha256: "snapshot".into(),
+            },
+            source: completion_source,
+            sequence: 1,
+            watermark_ms: 10,
+        };
+        let err = EmitOnlySink
+            .advance_summary_watermark(&barrier)
+            .expect_err("sink must reject an unsupported barrier");
+        assert_eq!(
+            err.downcast_ref::<std::io::Error>()
+                .map(std::io::Error::kind),
+            Some(std::io::ErrorKind::Unsupported)
+        );
+    }
+
     fn sum_agg_config(_id: u64, metric: &str, grouping_keys: &[&str]) -> AggregationConfig {
         // `_id` is unused after PR 5 — identity is content-addressed
         // via `PolicyFingerprint::from_config`. Callers obtain the id