diff --git a/contracts/witness/811c78184eb2609244c59534f015f0be2f90923120a3027c176ad56723d0d850.json b/contracts/witness/811c78184eb2609244c59534f015f0be2f90923120a3027c176ad56723d0d850.json
index 13582518f0..a5088c39a3 100644
--- a/contracts/witness/811c78184eb2609244c59534f015f0be2f90923120a3027c176ad56723d0d850.json
+++ b/contracts/witness/811c78184eb2609244c59534f015f0be2f90923120a3027c176ad56723d0d850.json
@@ -1,7 +1,6 @@
 {
   "census_id_set_sha256": "063d371c514532fccc38497ed2f0d85711be3c35061de05df561446f16e93d99",
   "relations_sha256": "811c78184eb2609244c59534f015f0be2f90923120a3027c176ad56723d0d850",
-  "reasoner_git_sha": "56ae1edeaf9f0761d3e0f9bf0754fb735e7c3d05",
   "checkable_n": 87,
   "result": {
     "kind": "model",
@@ -9,6 +8,5 @@
       "false": []
     }
   },
-  "pc_reasoner": "fired",
-  "cpu_ms": 2
+  "pc_reasoner": "fired"
 }
diff --git a/crates/aprender-contracts/src/ontology/witness.rs b/crates/aprender-contracts/src/ontology/witness.rs
index 710eb478ef..2c1e62bf54 100644
--- a/crates/aprender-contracts/src/ontology/witness.rs
+++ b/crates/aprender-contracts/src/ontology/witness.rs
@@ -160,11 +160,13 @@ pub struct Witness {
     pub census_id_set_sha256: String,
     pub relations_sha256: String,
     /// `git rev-parse HEAD` where pv-sat ran, when it could ask.
+    #[serde(default, skip_serializing)]
     pub reasoner_git_sha: Option<String>,
     pub checkable_n: usize,
     pub result: WitnessResult,
     /// `fired` — pv-sat's plant `A⇒B, B⇒C, contradicts(A,C)` yielded the core `[A, B, C]` before it wrote this.
     pub pc_reasoner: String,
+    #[serde(default, skip_serializing)]
     pub cpu_ms: u64,
 }
 
diff --git a/crates/aprender-contracts/src/ontology/witness_tests.rs b/crates/aprender-contracts/src/ontology/witness_tests.rs
index 1c21ee1362..eda50ce344 100644
--- a/crates/aprender-contracts/src/ontology/witness_tests.rs
+++ b/crates/aprender-contracts/src/ontology/witness_tests.rs
@@ -263,3 +263,39 @@ fn implication_adjacency_groups_targets_by_source() {
     assert_eq!(adj["B"], vec!["C"]);
     assert!(implication_adjacency(&ClauseSet::default()).is_empty());
 }
+
+fn volatile_witness(sha: Option<&str>, cpu_ms: u64) -> Witness {
+    Witness {
+        census_id_set_sha256: "0".repeat(64),
+        relations_sha256: "1".repeat(64),
+        reasoner_git_sha: sha.map(str::to_string),
+        checkable_n: 3,
+        result: good_core(),
+        pc_reasoner: FIRED.into(),
+        cpu_ms,
+    }
+}
+
+/// P5: a regen at another HEAD or with another timing must leave the committed file byte-identical,
+/// or every PR that regenerates conflicts with every other on the shared witness.
+#[test]
+fn volatile_fields_are_never_written() {
+    let a = serde_json::to_string_pretty(&volatile_witness(Some("aaaa"), 1)).expect("serializes");
+    let b = serde_json::to_string_pretty(&volatile_witness(Some("bbbb"), 2)).expect("serializes");
+    assert_eq!(a, b);
+    assert!(
+        !a.contains("cpu_ms") && !a.contains("reasoner_git_sha"),
+        "{a}"
+    );
+}
+
+/// Witnesses already on disk carry the two fields; they must still be read.
+#[test]
+fn a_witness_with_the_old_volatile_fields_still_parses() {
+    let mut v = serde_json::to_value(volatile_witness(None, 0)).expect("serializes");
+    v["cpu_ms"] = serde_json::json!(7);
+    v["reasoner_git_sha"] = serde_json::json!("abc");
+    let w: Witness = serde_json::from_value(v).expect("old shape parses");
+    assert_eq!(w.cpu_ms, 7);
+    assert_eq!(w.reasoner_git_sha.as_deref(), Some("abc"));
+}