diff --git a/.changeset/add-acknowledgement-lexicon.md b/.changeset/add-acknowledgement-lexicon.md
new file mode 100644
index 00000000..802401cf
--- /dev/null
+++ b/.changeset/add-acknowledgement-lexicon.md
@@ -0,0 +1,5 @@
+---
+"@hypercerts-org/lexicon": minor
+---
+
+Add org.hypercerts.acknowledgement lexicon for bidirectional inclusion links between records across PDS repos
diff --git a/ERD.puml b/ERD.puml
index 7d06514e..574e502f 100644
--- a/ERD.puml
+++ b/ERD.puml
@@ -200,6 +200,17 @@ dataclass collection {
     !endif
 }
 
+' org.hypercerts.acknowledgement
+dataclass acknowledgement {
+    !if (SHOW_FIELDS == "true")
+    subject
+    context
+    acknowledged
+    comment?
+    createdAt
+    !endif
+}
+
 'together {
 
 ' Funders are represented by DIDs or human-readable strings
@@ -296,6 +307,11 @@ collection::items --> activity
 collection::items --> collection : "recursive\nnesting"
 collection::location --> location
 
+acknowledgement::subject --> activity
+acknowledgement::subject --> contributorInformation
+acknowledgement::context --> collection
+acknowledgement::context --> activity
+
 activity::contributors -l--> contributorInformation
 activity::contributors --> contributionDetails
 activity::rights --> rights
diff --git a/README.md b/README.md
index bf469e78..a4f9a033 100644
--- a/README.md
+++ b/README.md
@@ -373,6 +373,73 @@ const collectionRecord = {
 are optional and support either embedded image blobs or URI references to
 external images.
 
+### Acknowledging Inclusion
+
+The `org.hypercerts.acknowledgement` record enables bidirectional
+linking between records that live in different PDS repositories. When
+one user includes another user's record (e.g. adding an activity to a
+collection), the owner of the included record can create an
+acknowledgement to confirm or reject the inclusion. This forms a
+two-way link that an AppView can verify.
+
+Each acknowledgement uses `com.atproto.repo.strongRef` fields to
+reference both the **subject** (the record being included) and the
+**context** (the record it's being included in).
+
+See [SCHEMAS.md](SCHEMAS.md) for the full property reference.
+
+#### Use Case: Activity Included in a Collection
+
+A project organizer (Alice) creates a collection and adds Bob's
+activity to it via a `strongRef` in the collection's `items[]` array.
+Bob then creates an acknowledgement in his own repo to confirm:
+
+```typescript
+import { ACKNOWLEDGEMENT_NSID } from "@hypercerts-org/lexicon";
+
+// Bob acknowledges that his activity is included in Alice's collection
+const ack = {
+  $type: ACKNOWLEDGEMENT_NSID,
+  subject: {
+    uri: "at://did:plc:bob/org.hypercerts.claim.activity/3k2abc",
+    cid: "bafy...",
+  },
+  context: {
+    uri: "at://did:plc:alice/org.hypercerts.claim.collection/7x9def",
+    cid: "bafy...",
+  },
+  acknowledged: true,
+  createdAt: new Date().toISOString(),
+};
+```
+
+#### Use Case: Contributor Included in an Activity
+
+Alice creates an activity that lists Bob as a contributor. Bob creates
+an acknowledgement in his own repo to confirm his participation:
+
+```typescript
+const ack = {
+  $type: ACKNOWLEDGEMENT_NSID,
+  subject: {
+    // Bob's contributor information record
+    uri: "at://did:plc:bob/org.hypercerts.claim.contributorInformation/abc123",
+    cid: "bafy...",
+  },
+  context: {
+    // Alice's activity that lists Bob as contributor
+    uri: "at://did:plc:alice/org.hypercerts.claim.activity/3k2abc",
+    cid: "bafy...",
+  },
+  acknowledged: true,
+  comment: "Confirming my contribution to this reforestation project",
+  createdAt: new Date().toISOString(),
+};
+```
+
+Setting `acknowledged: false` explicitly rejects inclusion, which an
+AppView can use to flag disputed associations.
+
 ### Adding Locations to Activities
 
 The `locations` field in activity records is an array of strong references
diff --git a/SCHEMAS.md b/SCHEMAS.md
index 8591248c..a638d21b 100644
--- a/SCHEMAS.md
+++ b/SCHEMAS.md
@@ -7,6 +7,24 @@
 
 Hypercerts-specific lexicons for tracking impact work and claims.
 
+### `org.hypercerts.acknowledgement`
+
+**Description:** Acknowledges the inclusion of one record (subject) within another (context). Typically created in the subject owner's repo to form a bidirectional link. For example, a contributor acknowledging inclusion in an activity, or an activity owner acknowledging inclusion in a collection.
+
+**Key:** `tid`
+
+#### Properties
+
+| Property       | Type      | Required | Description                                                                                                                             | Comments        |
+| -------------- | --------- | -------- | --------------------------------------------------------------------------------------------------------------------------------------- | --------------- |
+| `subject`      | `ref`     | ✅       | The record whose inclusion is being acknowledged (e.g. an activity, a contributor information record).                                  |                 |
+| `context`      | `ref`     | ✅       | The record that includes the subject (e.g. a collection/project that includes an activity, or an activity that includes a contributor). |                 |
+| `acknowledged` | `boolean` | ✅       | Whether inclusion is acknowledged (true) or rejected (false).                                                                           |                 |
+| `comment`      | `string`  | ❌       | Optional comment providing additional context or reasoning.                                                                             | maxLength: 1000 |
+| `createdAt`    | `string`  | ✅       | Client-declared timestamp when this record was originally created.                                                                      |                 |
+
+---
+
 ### `org.hypercerts.claim.activity`
 
 **Description:** A hypercert record tracking impact work.
diff --git a/lexicons/org/hypercerts/acknowledgement.json b/lexicons/org/hypercerts/acknowledgement.json
new file mode 100644
index 00000000..21137bf0
--- /dev/null
+++ b/lexicons/org/hypercerts/acknowledgement.json
@@ -0,0 +1,41 @@
+{
+  "lexicon": 1,
+  "id": "org.hypercerts.acknowledgement",
+  "defs": {
+    "main": {
+      "type": "record",
+      "description": "Acknowledges the inclusion of one record (subject) within another (context). Typically created in the subject owner's repo to form a bidirectional link. For example, a contributor acknowledging inclusion in an activity, or an activity owner acknowledging inclusion in a collection.",
+      "key": "tid",
+      "record": {
+        "type": "object",
+        "required": ["subject", "context", "acknowledged", "createdAt"],
+        "properties": {
+          "subject": {
+            "type": "ref",
+            "ref": "com.atproto.repo.strongRef",
+            "description": "The record whose inclusion is being acknowledged (e.g. an activity, a contributor information record)."
+          },
+          "context": {
+            "type": "ref",
+            "ref": "com.atproto.repo.strongRef",
+            "description": "The record that includes the subject (e.g. a collection/project that includes an activity, or an activity that includes a contributor)."
+          },
+          "acknowledged": {
+            "type": "boolean",
+            "description": "Whether inclusion is acknowledged (true) or rejected (false)."
+          },
+          "comment": {
+            "type": "string",
+            "description": "Optional comment providing additional context or reasoning.",
+            "maxLength": 1000
+          },
+          "createdAt": {
+            "type": "string",
+            "format": "datetime",
+            "description": "Client-declared timestamp when this record was originally created."
+          }
+        }
+      }
+    }
+  }
+}