diff --git a/crates/vex-chora/src/bridge.rs b/crates/vex-chora/src/bridge.rs
index c89cfa1..56a16bd 100644
--- a/crates/vex-chora/src/bridge.rs
+++ b/crates/vex-chora/src/bridge.rs
@@ -57,11 +57,12 @@ impl AuthorityBridge {
         };
 
         // 4. WitnessData — populated from the live CHORA response.
-        let now = chrono::Utc::now().to_rfc3339();
+        let now = chrono::Utc::now().timestamp() as u64;
         let witness = WitnessData {
             chora_node_id: response.authority.capsule_id.clone(),
             receipt_hash: response.signature.clone(),
             timestamp: now,
+            metadata: serde_json::Value::Null,
         };
 
         // 5. Build Pillar Hashes
@@ -78,7 +79,7 @@ impl AuthorityBridge {
 
         let authority_hash = hash_seg(&response.authority)?;
         let identity_hash = hash_seg(&identity)?;
-        let witness_hash = hash_seg(&witness)?;
+        let witness_hash = witness.to_commitment_hash()?;
 
         // 6. Build Composite Capsule
         let mut capsule = Capsule {
diff --git a/crates/vex-core/src/segment.rs b/crates/vex-core/src/segment.rs
index dd38627..8dd0df2 100644
--- a/crates/vex-core/src/segment.rs
+++ b/crates/vex-core/src/segment.rs
@@ -54,7 +54,48 @@ fn default_sensor_value() -> serde_json::Value {
 pub struct WitnessData {
     pub chora_node_id: String,
     pub receipt_hash: String,
-    pub timestamp: String,
+    pub timestamp: u64,
+    /// Diagnostic or display-only fields that are NOT part of the commitment surface.
+    #[serde(flatten, default)]
+    pub metadata: serde_json::Value,
+}
+
+/// The "Commitment Surface" for the Witness segment.
+/// Lexicographical order and field presence are strictly enforced here for interop parity.
+#[derive(Serialize)]
+struct MinimalWitness<'a> {
+    chora_node_id: &'a str,
+    receipt_hash: &'a str,
+    timestamp: u64,
+}
+
+impl WitnessData {
+    /// Compute the SHA-256 hash of the JCS-canonicalized MINIMAL witness structure.
+    /// This ensures that adding extra display fields to the JSON does not break the commitment.
+    pub fn to_commitment_hash(&self) -> Result<String, String> {
+        let minimal = MinimalWitness {
+            chora_node_id: &self.chora_node_id,
+            receipt_hash: &self.receipt_hash,
+            timestamp: self.timestamp,
+        };
+
+        let jcs_bytes = serde_jcs::to_vec(&minimal)
+            .map_err(|e| format!("JCS serialization of minimal witness failed: {}", e))?;
+
+        let mut hasher = Sha256::new();
+        hasher.update(&jcs_bytes);
+        Ok(hex::encode(hasher.finalize()))
+    }
+
+    pub fn to_jcs_hash(&self) -> Result<Hash, String> {
+        let hex = self.to_commitment_hash()?;
+        Ok(Hash::from_bytes(
+            hex::decode(hex)
+                .map_err(|e| e.to_string())?
+                .try_into()
+                .map_err(|_| "Invalid hash length")?,
+        ))
+    }
 }
 
 /// Identity Data (Attest Pillar)
@@ -132,7 +173,7 @@ impl Capsule {
 
         let authority_hash_hex = hash_seg(&self.authority)?;
         let identity_hash_hex = hash_seg(&self.identity)?;
-        let witness_hash_hex = hash_seg(&self.witness)?;
+        let witness_hash_hex = self.witness.to_commitment_hash()?;
 
         // Build the Canonical Composite Object
         let composite_root = serde_json::json!({
@@ -191,4 +232,31 @@ mod tests {
 
         assert_ne!(hash1, hash2, "Hashes must change when content changes");
     }
+
+    #[test]
+    fn test_witness_metadata_exclusion() {
+        let base_witness = WitnessData {
+            chora_node_id: "node-1".to_string(),
+            receipt_hash: "hash-1".to_string(),
+            timestamp: 1710396000,
+            metadata: serde_json::Value::Null,
+        };
+
+        let hash_base = base_witness.to_commitment_hash().unwrap();
+
+        let mut metadata_witness = base_witness.clone();
+        metadata_witness.metadata = serde_json::json!({
+            "witness_mode": "sentinel",
+            "diagnostics": {
+                "latency_ms": 42
+            }
+        });
+
+        let hash_with_metadata = metadata_witness.to_commitment_hash().unwrap();
+
+        assert_eq!(
+            hash_base, hash_with_metadata,
+            "Witness hash must be invariant to extra metadata fields"
+        );
+    }
 }
diff --git a/crates/vex-core/src/vep.rs b/crates/vex-core/src/vep.rs
index 07ef61e..0a48c69 100644
--- a/crates/vex-core/src/vep.rs
+++ b/crates/vex-core/src/vep.rs
@@ -1,4 +1,4 @@
-//! VEP (Viking Enveloped Packet) Binary Format
+//! VEP (Verifiable Evidence Packet) Binary Format
 //!
 //! A zero-copy, high-performance binary envelope for segmented VEX audit data.
 
@@ -174,7 +174,7 @@ impl<'a> VepPacket<'a> {
 
         let authority_hash = hash_seg(&authority)?;
         let identity_hash = hash_seg(&identity)?;
-        let witness_hash = hash_seg(&witness)?;
+        let witness_hash = witness.to_commitment_hash()?;
 
         let mut capsule = Capsule {
             capsule_id: authority.capsule_id.clone(),
diff --git a/crates/vex-core/tests/chora_parity.rs b/crates/vex-core/tests/chora_parity.rs
index 5c678a8..c81dba1 100644
--- a/crates/vex-core/tests/chora_parity.rs
+++ b/crates/vex-core/tests/chora_parity.rs
@@ -30,7 +30,8 @@ fn test_capsule_jcs_parity() {
     let witness = WitnessData {
         chora_node_id: "test-chora-node".into(),
         receipt_hash: "deadbeef".into(),
-        timestamp: "2024-03-09T10:00:00Z".into(),
+        timestamp: 1710396000,
+        metadata: serde_json::Value::Null,
     };
 
     let capsule = Capsule {
@@ -66,7 +67,7 @@ fn test_capsule_jcs_parity() {
 
     let auth_hash = hash_seg(&authority);
     let id_hash = hash_seg(&identity);
-    let wit_hash = hash_seg(&witness);
+    let wit_hash = witness.to_commitment_hash().unwrap();
 
     println!("--- RUST NATIVE HASHES ---");
     println!("Intent Hash:    {}", intent_hash.to_hex());
diff --git a/crates/vex-runtime/src/audit/vep.rs b/crates/vex-runtime/src/audit/vep.rs
index 3c28039..ae4891b 100644
--- a/crates/vex-runtime/src/audit/vep.rs
+++ b/crates/vex-runtime/src/audit/vep.rs
@@ -52,7 +52,28 @@ pub struct IdentitySegment {
 pub struct WitnessSegment {
     pub chora_node_id: String,
     pub receipt_hash: String,
-    pub timestamp: String,
+    pub timestamp: u64,
+    /// Diagnostic or display-only fields that are NOT part of the commitment surface.
+    #[serde(flatten, default)]
+    pub metadata: serde_json::Value,
+}
+
+impl WitnessSegment {
+    /// Compute the SHA-256 hash of the JCS-canonicalized MINIMAL witness structure.
+    pub fn to_commitment_hash(&self) -> Result<String, VepError> {
+        // Strict commitment to only the core fields (Unix timestamp handles JCS parity)
+        let minimal = serde_json::json!({
+            "chora_node_id": self.chora_node_id,
+            "receipt_hash": self.receipt_hash,
+            "timestamp": self.timestamp,
+        });
+
+        let jcs_bytes = serde_jcs::to_vec(&minimal).map_err(|e| VepError::Jcs(e.to_string()))?;
+
+        let mut hasher = Sha256::new();
+        hasher.update(&jcs_bytes);
+        Ok(hex::encode(hasher.finalize()))
+    }
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -101,7 +122,7 @@ impl EvidenceCapsuleV0 {
         let intent_hash = hash_segment(&intent)?;
         let authority_hash = hash_segment(&authority)?;
         let identity_hash = hash_segment(&identity)?;
-        let witness_hash = hash_segment(&witness)?;
+        let witness_hash = witness.to_commitment_hash()?;
 
         // Root commitment: JCS lexicographical order is handled by serde_jcs/serde_json
         let root_map = serde_json::json!({
diff --git a/crates/vex-runtime/src/audit/verify.rs b/crates/vex-runtime/src/audit/verify.rs
index 61d2df3..787be23 100644
--- a/crates/vex-runtime/src/audit/verify.rs
+++ b/crates/vex-runtime/src/audit/verify.rs
@@ -73,6 +73,7 @@ impl VepVerifier {
             chora_node_id: core_capsule.witness.chora_node_id,
             receipt_hash: core_capsule.witness.receipt_hash,
             timestamp: core_capsule.witness.timestamp,
+            metadata: core_capsule.witness.metadata,
         };
 
         let request_commitment = core_capsule.request_commitment.map(|rc| RequestCommitment {
@@ -168,7 +169,8 @@ mod tests {
         let witness = WitnessSegment {
             chora_node_id: "node1".to_string(),
             receipt_hash: "rh".to_string(),
-            timestamp: "now".to_string(),
+            timestamp: 1710403200,
+            metadata: serde_json::Value::Null,
         };
 
         let mut capsule =
@@ -230,7 +232,8 @@ mod tests {
         let witness = WitnessSegment {
             chora_node_id: "nodeB".to_string(),
             receipt_hash: "receiptB".to_string(),
-            timestamp: "2024-03-12T00:00:00Z".to_string(),
+            timestamp: 1710403200,
+            metadata: serde_json::Value::Null,
         };
 
         let mut capsule =
diff --git a/crates/vex-runtime/src/gate/titan.rs b/crates/vex-runtime/src/gate/titan.rs
index bf6153d..f848ceb 100644
--- a/crates/vex-runtime/src/gate/titan.rs
+++ b/crates/vex-runtime/src/gate/titan.rs
@@ -349,7 +349,8 @@ impl Gate for TitanGate {
                         let witness = WitnessSegment {
                             chora_node_id: "chora-primary-v1".to_string(),
                             receipt_hash: chora_resp.signature.clone(),
-                            timestamp: chrono::Utc::now().to_rfc3339(),
+                            timestamp: chrono::Utc::now().timestamp() as u64,
+                            metadata: serde_json::Value::Null,
                         };
 
                         let mut v0_capsule = match EvidenceCapsuleV0::new(
diff --git a/crates/vex-runtime/tests/vep_verification.rs b/crates/vex-runtime/tests/vep_verification.rs
index 386962f..7b3ecfc 100644
--- a/crates/vex-runtime/tests/vep_verification.rs
+++ b/crates/vex-runtime/tests/vep_verification.rs
@@ -30,7 +30,8 @@ fn test_vep_binary_serialization() {
     let witness = WitnessSegment {
         chora_node_id: "node-1".to_string(),
         receipt_hash: "2".repeat(64),
-        timestamp: "2026-03-11T22:00:00Z".to_string(),
+        timestamp: 1710403200,
+        metadata: serde_json::json!({}),
     };
 
     let capsule = EvidenceCapsuleV0::new(intent, authority, identity, witness, None).unwrap();
@@ -111,7 +112,8 @@ fn test_vep_signature_verification() {
         WitnessSegment {
             chora_node_id: "n".into(),
             receipt_hash: "2".repeat(64),
-            timestamp: "2026".into(),
+            timestamp: 1710403200,
+            metadata: serde_json::json!({}),
         },
         None,
     )
diff --git a/crates/vex-sidecar/src/main.rs b/crates/vex-sidecar/src/main.rs
index c4926e0..cc1efde 100644
--- a/crates/vex-sidecar/src/main.rs
+++ b/crates/vex-sidecar/src/main.rs
@@ -105,7 +105,8 @@ async fn proxy_handler(
     let witness = WitnessData {
         chora_node_id: "sidecar-local".to_string(),
         receipt_hash: payload_hash.clone(), // In proxy mode, we link to payload
-        timestamp: chrono::Utc::now().to_rfc3339(),
+        timestamp: chrono::Utc::now().timestamp() as u64,
+        metadata: serde_json::Value::Null,
     };
 
     // 5. Build Pillar Hashes
@@ -118,7 +119,7 @@ async fn proxy_handler(
 
     let authority_hash = hash_seg(&authority);
     let identity_hash = hash_seg(&identity);
-    let witness_hash = hash_seg(&witness);
+    let witness_hash = witness.to_commitment_hash().unwrap();
 
     // 6. Assemble Hardened Capsule
     let mut capsule = Capsule {