Self-referential predicate hashes as statement template args (#494)

* Support quoted predicate hashes, including self-referential predicates

* Clippy

* Review feedback

src/backends/plonky2/circuits/common.rs

@@ -771,7 +771,8 @@ impl CustomPredicateEntryTarget {
         pw.set_target_arr(&self.id.elements, &predicate.batch.id().0)?;
         pw.set_target(self.index, F::from_canonical_usize(predicate.index))?;
 
-        // Replace statement templates of batch-self with (id,index)
+        // Replace BatchSelf predicates with Custom(batch, i), and
+        // SelfPredicateHash args with Literal(hash(Custom(batch, i)))
         let batch = &predicate.batch;
         let predicate = predicate.predicate();
         let statements = predicate
@@ -788,10 +789,22 @@ impl CustomPredicateEntryTarget {
                     }
                     x => x.clone(),
                 };
-                StatementTmpl {
+                let args = st_tmpl
-                    pred_or_wc,
+                    .args
-                    args: st_tmpl.args,
+                    .into_iter()
-                }
+                    .map(|arg| match arg {
+                        StatementTmplArg::SelfPredicateHash(i) => {
+                            let pred_hash = Predicate::Custom(CustomPredicateRef {
+                                batch: batch.clone(),
+                                index: i,
+                            })
+                            .hash();
+                            StatementTmplArg::Literal(Value::from(pred_hash))
+                        }
+                        other => other,
+                    })
+                    .collect();
+                StatementTmpl { pred_or_wc, args }
             })
             .collect_vec();
         let predicate = CustomPredicate {
@@ -2012,7 +2025,7 @@ pub(crate) mod tests {
         // Empty case
         let mut cpb_builder = CustomPredicateBatchBuilder::new(params.clone(), "empty".into());
         _ = cpb_builder.predicate_and("empty", &[], &[], &[])?;
-        let custom_predicate_batch = cpb_builder.finish();
+        let custom_predicate_batch = cpb_builder.finish()?;
         helper_custom_predicate_in_batch_target(&custom_predicate_batch).unwrap();
 
         // Some cases from the examples

src/backends/plonky2/circuits/mainpod.rs

@@ -54,8 +54,8 @@ use crate::{
     measure_gates_begin, measure_gates_end,
     middleware::{
         CustomPredicate, CustomPredicateBatch, CustomPredicateRef, NativeOperation,
-        NativePredicate, Params, PredicatePrefix, RawValue, Statement, ToFields, Value, F,
+        NativePredicate, Params, PredicatePrefix, RawValue, Statement, StatementTmplArgPrefix,
-        HASH_SIZE,
+        ToFields, Value, F, HASH_SIZE,
     },
 };
 //
@@ -1534,8 +1534,8 @@ pub fn calculate_statements_hash_circuit(
     sts_hash
 }
 
-// Replace predicates of batch-self with the corresponding global custom predicate batch_id and
+// Replace BatchSelf predicates with the corresponding Custom(batch_id, index), and
-// index
+// SelfPredicateHash args with Literal(hash(Custom(batch_id, index))).
 fn normalize_st_tmpl_circuit(
     params: &Params,
     builder: &mut CircuitBuilder,
@@ -1564,7 +1564,41 @@ fn normalize_st_tmpl_circuit(
     );
     let pred_hash_or_wc =
         PredicateHashOrWildcardTarget::new(st_tmpl.pred_hash_or_wc().elements[0], data);
-    StatementTmplTarget::new(pred_hash_or_wc, st_tmpl.args.clone())
+
+    // Normalize SelfPredicateHash args: replace prefix 4 with Literal containing the resolved
+    // predicate hash. Same pattern as the predicate normalization above.
+    let prefix_sph = builder.constant(F::from(StatementTmplArgPrefix::SelfPredicateHash));
+    let prefix_literal = builder.constant(F::from(StatementTmplArgPrefix::Literal));
+    let zero = builder.zero();
+    let normalized_args = st_tmpl
+        .args
+        .iter()
+        .map(|arg| {
+            let is_sph = builder.is_equal(arg.elements[0], prefix_sph);
+
+            // The predicate index is in elements[1] (same slot as WildcardLiteral).
+            let pred_index = arg.elements[1];
+
+            // Compute hash(Custom(batch_id, pred_index))
+            let pred_target = PredicateTarget::new_custom(builder, id, pred_index);
+            let pred_hash = pred_target.hash(builder);
+
+            // Build a Literal-encoded arg: [1, hash[0..4], 0, 0, 0, 0]
+            let mut literal_elements = [zero; Params::statement_tmpl_arg_size()];
+            literal_elements[0] = prefix_literal;
+            literal_elements[1] = pred_hash.elements[0];
+            literal_elements[2] = pred_hash.elements[1];
+            literal_elements[3] = pred_hash.elements[2];
+            literal_elements[4] = pred_hash.elements[3];
+            let normalized = StatementTmplArgTarget {
+                elements: literal_elements,
+            };
+
+            builder.select_flattenable(params, is_sph, &normalized, arg)
+        })
+        .collect();
+
+    StatementTmplTarget::new(pred_hash_or_wc, normalized_args)
 }
 
 /// Build a table of [batch_id, custom_predicate_index, custom_predicate] with queryable part as
@@ -3262,7 +3296,7 @@ mod tests {
             &[stb0.clone(), stb1.clone()],
         )?;
         let _ = builder.predicate_or("pred_or", &["id"], &["secret"], &[stb0, stb1])?;
-        let batch = builder.finish();
+        let batch = builder.finish()?;
 
         let dict = Hash([F(6), F(7), F(8), F(9)]);
 
@@ -3352,7 +3386,7 @@ mod tests {
             &[stb0.clone(), stb1.clone()],
         )?;
         let _ = builder.predicate_or("pred_or", &["id"], &["secret_id"], &[stb0, stb1])?;
-        let batch = builder.finish();
+        let batch = builder.finish()?;
 
         let dict = Hash([F(1), F(2), F(3), F(4)]);
         let secret_dict = Hash([F(6), F(7), F(8), F(9)]);
@@ -3570,4 +3604,100 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_normalize_st_tmpl_self_predicate_hash() -> Result<()> {
+        let params = Params::default();
+
+        // Build a batch with two predicates:
+        // pred_A: Equal(x, y)
+        // pred_B: Equal(x, SelfPredicateHash(0)), references pred_A's hash
+        use NativePredicate as NP;
+        let mut cpb = CustomPredicateBatchBuilder::new(params.clone(), "batch".into());
+        let stb_a = StatementTmplBuilder::new_from_pred(NP::Equal)
+            .arg("x")
+            .arg("y");
+        cpb.predicate_and("pred_A", &["x", "y"], &[], &[stb_a])
+            .unwrap();
+
+        // Build pred_B's template manually with SelfPredicateHash(0)
+        let stb_b_tmpl = StatementTmpl {
+            pred_or_wc: PredicateOrWildcard::Predicate(Predicate::Native(NP::Equal)),
+            args: vec![
+                StatementTmplArg::Wildcard(Wildcard::new("x".to_string(), 0)),
+                StatementTmplArg::SelfPredicateHash(0),
+            ],
+        };
+        let pred_b = CustomPredicate::new(
+            &params,
+            "pred_B".into(),
+            true,
+            vec![stb_b_tmpl],
+            1,
+            vec!["x".to_string()],
+        )
+        .unwrap();
+        cpb.predicates.push(pred_b);
+        let batch = cpb.finish().unwrap();
+
+        // Compute the expected resolved hash of pred_A
+        let pred_a_ref = CustomPredicateRef::new(batch.clone(), 0);
+        let pred_a_hash = Predicate::Custom(pred_a_ref).hash();
+        let expected_pred_a_value = Value::from(pred_a_hash);
+
+        // Test: normalize_st_tmpl_circuit should convert SelfPredicateHash(0) to
+        // Literal(pred_a_hash). Then make_statement_from_template_circuit should produce
+        // a statement with that literal value.
+        let pred_b_ref = CustomPredicateRef::new(batch.clone(), 1);
+        let pred_b_tmpl = &pred_b_ref.predicate().statements[0];
+
+        let config = CircuitConfig::standard_recursion_config();
+        let mut builder = CircuitBuilder::new(config);
+
+        // Create the template target and batch id target
+        let st_tmpl_target = builder.add_virtual_statement_tmpl(true);
+        let batch_id = builder.add_virtual_hash();
+
+        // Normalize the template (this is what we're testing)
+        let normalized =
+            normalize_st_tmpl_circuit(&params, &mut builder, &st_tmpl_target, batch_id);
+
+        // Feed normalized template into statement generation
+        let args_target: Vec<_> = (0..params.max_custom_predicate_wildcards)
+            .map(|_| builder.add_virtual_value())
+            .collect();
+        let st_target =
+            make_statement_from_template_circuit(&params, &mut builder, &normalized, &args_target);
+
+        // Connect to expected output
+        let expected_st_target = builder.add_virtual_statement(false);
+        builder.connect_flattenable(&expected_st_target, &st_target);
+
+        // Set witness
+        let mut pw = PartialWitness::<F>::new();
+        st_tmpl_target.set_targets(&mut pw, pred_b_tmpl)?;
+        pw.set_target_arr(&batch_id.elements, &batch.id().0)?;
+
+        let some_value = Value::from(42);
+        // args: first wildcard is "x" = some_value, rest are padding
+        let mut args_values = vec![some_value.clone()];
+        for _ in 1..params.max_custom_predicate_wildcards {
+            args_values.push(Value::from(EMPTY_VALUE));
+        }
+        for (target, value) in args_target.iter().zip(args_values.iter()) {
+            target.set_targets(&mut pw, value)?;
+        }
+
+        // Expected statement: Equal(Literal(some_value), Literal(pred_a_hash))
+        let expected_st: crate::backends::plonky2::mainpod::Statement =
+            Statement::equal(some_value, expected_pred_a_value).into();
+        expected_st_target.set_targets(&mut pw, &expected_st)?;
+
+        // Build and verify
+        let data = builder.build::<C>();
+        let proof = data.prove(pw)?;
+        data.verify(proof)?;
+
+        Ok(())
+    }
 }

src/backends/plonky2/mainpod/mod.rs

@@ -104,8 +104,9 @@ pub(crate) fn extract_custom_predicate_verifications(
         if let middleware::Operation::Custom(cpr, sts) = op {
             if let middleware::Statement::Custom(st_cpr, st_args) = st {
                 assert_eq!(cpr, st_cpr);
+                let normalized_pred = cpr.normalized_predicate();
                 let wildcard_values =
-                    wildcard_values_from_op_st(params, cpr.predicate(), sts, st_args)
+                    wildcard_values_from_op_st(params, &normalized_pred, sts, st_args)
                         .expect("resolved wildcards");
                 let sts = sts.iter().map(|s| Statement::from(s.clone())).collect();
                 let custom_predicate_table_index = custom_predicates
@@ -1096,7 +1097,7 @@ pub mod tests {
             &[stb0.clone(), stb1.clone()],
         )?;
         let _ = cpb_builder.predicate_or("pred_or", &["dict"], &["secret_dict"], &[stb0, stb1])?;
-        let cpb = cpb_builder.finish();
+        let cpb = cpb_builder.finish()?;
 
         let cpb_and = CustomPredicateRef::new(cpb.clone(), 0);
         let _cpb_or = CustomPredicateRef::new(cpb.clone(), 1);
@@ -1130,6 +1131,63 @@ pub mod tests {
         Ok(pod.verify()?)
     }
 
+    #[test]
+    fn test_main_self_predicate_hash() -> frontend::Result<()> {
+        use frontend::BuilderArg;
+
+        let params = Params {
+            max_signed_by: 0,
+            max_input_pods: 0,
+            max_statements: 6,
+            max_public_statements: 2,
+            max_operation_args: 5,
+            max_custom_predicate_wildcards: 4,
+            max_custom_predicate_verifications: 2,
+            max_merkle_proofs_containers: 0,
+            max_merkle_tree_state_transition_proofs_containers: 0,
+            ..Default::default()
+        };
+        let mut vds = DEFAULT_VD_LIST.clone();
+        vds.push(rec_main_pod_circuit_data(&params).1.verifier_only.clone());
+        let vd_set = VDSet::new(&vds);
+
+        // Build a batch: pred_A references pred_B's hash, pred_B references pred_A's hash
+        let mut cpb = CustomPredicateBatchBuilder::new(params.clone(), "batch".into());
+        let stb_a = STB::new_from_pred(NP::Equal)
+            .arg("x")
+            .arg(BuilderArg::SelfPredicateHash("pred_B".into()));
+        cpb.predicate_and("pred_A", &["x"], &[], &[stb_a])?;
+
+        let stb_b = STB::new_from_pred(NP::Equal)
+            .arg("x")
+            .arg(BuilderArg::SelfPredicateHash("pred_A".into()));
+        cpb.predicate_and("pred_B", &["x"], &[], &[stb_b])?;
+
+        let batch = cpb.finish()?;
+
+        let pred_a_ref = CustomPredicateRef::new(batch.clone(), 0);
+        let pred_b_ref = CustomPredicateRef::new(batch.clone(), 1);
+        let pred_b_hash = middleware::Value::from(middleware::Predicate::Custom(pred_b_ref).hash());
+
+        // Build a POD using pred_A: Equal(pred_b_hash, pred_b_hash)
+        let mut pod_builder = MainPodBuilder::new(&params, &vd_set);
+        let eq_st =
+            pod_builder.priv_op(frontend::Operation::eq(pred_b_hash.clone(), pred_b_hash))?;
+        pod_builder.pub_op(frontend::Operation::custom(pred_a_ref, [eq_st]))?;
+
+        // Mock
+        let prover = MockProver {};
+        let pod = pod_builder.prove(&prover)?;
+        assert!(pod.pod.verify().is_ok());
+
+        // Real
+        let prover = Prover {};
+        let pod = pod_builder.prove(&prover)?;
+        let pod = (pod.pod as Box<dyn Any>).downcast::<MainPod>().unwrap();
+
+        Ok(pod.verify()?)
+    }
+
     #[test]
     fn test_set_contains() -> frontend::Result<()> {
         let params = Params::default();

src/frontend/custom.rs

@@ -18,6 +18,8 @@ pub enum BuilderArg {
     /// Key: (origin, key), where origin is Wildcard and key is Key
     Key(String, String),
     WildcardLiteral(String),
+    /// Reference to a same-batch predicate's identity hash (resolved by name in finish()).
+    SelfPredicateHash(String),
 }
 
 /// When defining a `BuilderArg`, it can be done from 3 different inputs:
@@ -130,6 +132,8 @@ pub struct CustomPredicateBatchBuilder {
     params: Params,
     pub name: String,
     pub predicates: Vec<CustomPredicate>,
+    /// Forward references to resolve in finish(): (predicate_idx, statement_idx, arg_idx, name)
+    pending_self_pred_hashes: Vec<(usize, usize, usize, String)>,
 }
 
 impl CustomPredicateBatchBuilder {
@@ -138,6 +142,7 @@ impl CustomPredicateBatchBuilder {
             params,
             name,
             predicates: Vec::new(),
+            pending_self_pred_hashes: Vec::new(),
         }
     }
 
@@ -194,14 +199,18 @@ impl CustomPredicateBatchBuilder {
             ));
         }
 
+        let pred_idx = self.predicates.len();
+        let mut pending = Vec::new();
         let statements = sts
             .iter()
-            .map(|sb| {
+            .enumerate()
+            .map(|(stmt_idx, sb)| {
                 let stb = sb.clone().desugar();
                 let st_tmpl_args = stb
                     .args
                     .iter()
-                    .map(|a| {
+                    .enumerate()
+                    .map(|(arg_idx, a)| {
                         Ok::<_, Error>(match a {
                             BuilderArg::Literal(v) => StatementTmplArg::Literal(v.clone()),
                             BuilderArg::Key(root_wc, key_str) => StatementTmplArg::AnchoredKey(
@@ -211,6 +220,22 @@ impl CustomPredicateBatchBuilder {
                             BuilderArg::WildcardLiteral(v) => {
                                 StatementTmplArg::Wildcard(resolve_wildcard(args, priv_args, v)?)
                             }
+                            BuilderArg::SelfPredicateHash(pred_name) => {
+                                // Try backward reference first
+                                match self.predicates.iter().position(|p| p.name == *pred_name) {
+                                    Some(index) => StatementTmplArg::SelfPredicateHash(index),
+                                    None => {
+                                        // Forward reference - placeholder, resolved in finish()
+                                        pending.push((
+                                            pred_idx,
+                                            stmt_idx,
+                                            arg_idx,
+                                            pred_name.clone(),
+                                        ));
+                                        StatementTmplArg::SelfPredicateHash(0)
+                                    }
+                                }
+                            }
                         })
                     })
                     .collect::<Result<_>>()?;
@@ -240,11 +265,27 @@ impl CustomPredicateBatchBuilder {
                 .collect(),
         )?;
         self.predicates.push(custom_predicate);
+        self.pending_self_pred_hashes.extend(pending);
         Ok(Predicate::BatchSelf(self.predicates.len() - 1))
     }
 
-    pub fn finish(self) -> Arc<CustomPredicateBatch> {
+    pub fn finish(mut self) -> Result<Arc<CustomPredicateBatch>> {
-        CustomPredicateBatch::new(self.name, self.predicates)
+        // Resolve forward references for SelfPredicateHash
+        for (pred_idx, stmt_idx, arg_idx, ref name) in &self.pending_self_pred_hashes {
+            let target_idx = self
+                .predicates
+                .iter()
+                .position(|p| p.name == *name)
+                .ok_or_else(|| {
+                    Error::custom(format!(
+                        "SelfPredicateHash references unknown predicate '{}'",
+                        name
+                    ))
+                })?;
+            self.predicates[*pred_idx].statements[*stmt_idx].args[*arg_idx] =
+                StatementTmplArg::SelfPredicateHash(target_idx);
+        }
+        Ok(CustomPredicateBatch::new(self.name, self.predicates))
     }
 }
 
@@ -306,7 +347,7 @@ mod tests {
             .arg("s2");
 
         builder.predicate_and("gt_custom_pred", &["s1", "s2"], &[], &[gt_stb])?;
-        let batch = builder.finish();
+        let batch = builder.finish()?;
         let batch_clone = batch.clone();
         let gt_custom_pred = CustomPredicateRef::new(batch, 0);
 
@@ -356,7 +397,7 @@ mod tests {
             &[],
             &[set_contains_stb],
         )?;
-        let batch = builder.finish();
+        let batch = builder.finish()?;
         let batch_clone = batch.clone();
 
         let mut mp_builder = MainPodBuilder::new(&params, vd_set);
@@ -386,4 +427,83 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_builder_self_predicate_hash_unknown_ref() {
+        let params = Params::default();
+        let mut builder = CustomPredicateBatchBuilder::new(params.clone(), "batch".into());
+
+        let stb = StatementTmplBuilder::new_from_pred(NativePredicate::Equal)
+            .arg("x")
+            .arg(BuilderArg::SelfPredicateHash("nonexistent".into()));
+        builder
+            .predicate_and("pred_A", &["x"], &[], &[stb])
+            .unwrap();
+
+        // finish() should fail because "nonexistent" was never defined
+        assert!(builder.finish().is_err());
+    }
+
+    /// Tests cyclic SelfPredicateHash references end-to-end:
+    /// pred_A references pred_B's hash (forward ref), pred_B references pred_A's hash (backward
+    /// ref). Exercises forward reference resolution in finish(), then builds and verifies a POD
+    /// using pred_A via MockProver.
+    #[test]
+    fn test_builder_self_predicate_hash_e2e() -> Result<()> {
+        let params = Params::default();
+        let vd_set = &*MOCK_VD_SET;
+
+        let mut builder = CustomPredicateBatchBuilder::new(params.clone(), "batch".into());
+
+        // pred_A references pred_B's hash (forward ref, pred_B not yet defined)
+        let stb_a = StatementTmplBuilder::new_from_pred(NativePredicate::Equal)
+            .arg("x")
+            .arg(BuilderArg::SelfPredicateHash("pred_B".into()));
+        builder.predicate_and("pred_A", &["x"], &[], &[stb_a])?;
+
+        // pred_B references pred_A's hash (backward ref, pred_A already defined)
+        let stb_b = StatementTmplBuilder::new_from_pred(NativePredicate::Equal)
+            .arg("x")
+            .arg(BuilderArg::SelfPredicateHash("pred_A".into()));
+        builder.predicate_and("pred_B", &["x"], &[], &[stb_b])?;
+
+        let batch = builder.finish()?;
+
+        // Verify resolution: pred_A references pred_B (index 1), pred_B references pred_A (index 0)
+        assert_eq!(
+            batch.predicates()[0].statements[0].args[1],
+            StatementTmplArg::SelfPredicateHash(1)
+        );
+        assert_eq!(
+            batch.predicates()[1].statements[0].args[1],
+            StatementTmplArg::SelfPredicateHash(0)
+        );
+
+        // Compute concrete hashes
+        let pred_a_ref = CustomPredicateRef::new(batch.clone(), 0);
+        let pred_b_ref = CustomPredicateRef::new(batch.clone(), 1);
+        let pred_b_hash = Value::from(Predicate::Custom(pred_b_ref.clone()).hash());
+
+        // Build a POD using pred_A: Equal(pred_b_hash, pred_b_hash)
+        let mut mp_builder = MainPodBuilder::new(&params, vd_set);
+        let eq_st = mp_builder.priv_op(Operation::eq(pred_b_hash.clone(), pred_b_hash.clone()))?;
+        mp_builder.pub_op(Operation::custom(pred_a_ref, [eq_st]))?;
+
+        // Prove and verify
+        let prover = MockProver {};
+        let proof = mp_builder.prove(&prover)?;
+        proof.pod.verify()?;
+
+        // Verify the public statement contains pred_b_hash as its argument
+        let pub_sts = proof.pod.pub_self_statements();
+        let custom_st = pub_sts
+            .iter()
+            .find(|s| matches!(s, middleware::Statement::Custom(_, _)))
+            .expect("should have a custom statement");
+        if let middleware::Statement::Custom(_, args) = custom_st {
+            assert_eq!(args[0], pred_b_hash);
+        }
+
+        Ok(())
+    }
 }

src/frontend/mod.rs

@@ -578,7 +578,7 @@ impl MainPodBuilder {
                 }
             }
             OperationType::Custom(cpr) => {
-                let pred = &cpr.batch.predicates()[cpr.index];
+                let pred = cpr.normalized_predicate();
                 if pred.statements.len() != op.1.len() {
                     return Err(Error::custom(format!(
                         "Custom predicate operation needs {} statements but has {}.",
@@ -606,7 +606,7 @@ impl MainPodBuilder {
                     }
                     wildcard_map[index] = Some(value);
                 }
-                fill_wildcard_values(pred, &args, &mut wildcard_map)?;
+                fill_wildcard_values(&pred, &args, &mut wildcard_map)?;
                 let v_default = Value::from(0);
                 let st_args: Vec<_> = wildcard_map
                     .into_iter()

src/lang/frontend_ast_lower.rs

@@ -346,6 +346,9 @@ impl<'a> Lowerer<'a> {
                     let key = Key::from(key_str.as_str());
                     MWStatementTmplArg::AnchoredKey(wildcard, key)
                 }
+                BuilderArg::SelfPredicateHash(_) => {
+                    unreachable!("SelfPredicateHash should not appear in request lowering")
+                }
             };
             mw_args.push(mw_arg);
         }

src/lang/module.rs

@@ -345,7 +345,9 @@ fn build_single_batch(
             })?;
     }
 
-    Ok(builder.finish())
+    builder.finish().map_err(|e| BatchingError::Internal {
+        message: format!("Failed to finalize batch '{}': {}", batch_name, e),
+    })
 }
 
 /// Build a statement template with properly resolved predicate references

src/middleware/custom.rs

@@ -49,6 +49,9 @@ pub enum StatementTmplArg {
     // AnchoredKey where the origin is a wildcard
     AnchoredKey(Wildcard, Key),
     Wildcard(Wildcard),
+    /// Reference to a same-batch predicate's identity hash, resolved at verification time.
+    /// The usize is the predicate index within the batch.
+    SelfPredicateHash(usize),
 }
 
 #[derive(Clone, Copy)]
@@ -57,6 +60,7 @@ pub enum StatementTmplArgPrefix {
     Literal = 1,
     AnchoredKey = 2,
     WildcardLiteral = 3,
+    SelfPredicateHash = 4,
 }
 
 impl From<StatementTmplArgPrefix> for F {
@@ -68,11 +72,12 @@ impl From<StatementTmplArgPrefix> for F {
 impl ToFields for StatementTmplArg {
     fn to_fields(&self) -> Vec<F> {
         // Encoding:
-        // None =>                      (0,          0, 0, 0, 0,  0, 0, 0, 0)
+        // None =>                         (0,          0, 0, 0, 0,  0, 0, 0, 0)
-        // Literal(v) =>                (1,        [v         ],  0, 0, 0, 0)
+        // Literal(v) =>                   (1,        [v         ],  0, 0, 0, 0)
-        // Key(wc_index, key_or_wc) =>  (2, [wc_index], 0, 0, 0, [key_or_wc])
+        // Key(wc_index, key_or_wc) =>     (2, [wc_index], 0, 0, 0, [key_or_wc])
-        // WildcardLiteral(wc_index) => (3, [wc_index], 0, 0, 0,  0, 0, 0, 0)
+        // WildcardLiteral(wc_index) =>    (3, [wc_index], 0, 0, 0,  0, 0, 0, 0)
-        // In all three cases, we pad to 2 * hash_size + 1 = 9 field elements
+        // SelfPredicateHash(pred_index) => (4, pred_index, 0, 0, 0,  0, 0, 0, 0)
+        // In all cases, we pad to 2 * hash_size + 1 = 9 field elements
         match self {
             StatementTmplArg::None => iter::once(F::from(StatementTmplArgPrefix::None))
                 .chain(iter::repeat(F::ZERO))
@@ -97,6 +102,13 @@ impl ToFields for StatementTmplArg {
                     .take(Params::statement_tmpl_arg_size())
                     .collect_vec()
             }
+            StatementTmplArg::SelfPredicateHash(index) => {
+                iter::once(F::from(StatementTmplArgPrefix::SelfPredicateHash))
+                    .chain(iter::once(F::from_canonical_usize(*index)))
+                    .chain(iter::repeat(F::ZERO))
+                    .take(Params::statement_tmpl_arg_size())
+                    .collect_vec()
+            }
         }
     }
 }
@@ -113,6 +125,7 @@ impl fmt::Display for StatementTmplArg {
                 write!(f, "]")
             }
             Self::Wildcard(v) => v.fmt(f),
+            Self::SelfPredicateHash(i) => write!(f, "::self.{}", i),
         }
     }
 }
@@ -569,6 +582,44 @@ impl CustomPredicateRef {
     pub fn predicate(&self) -> &CustomPredicate {
         &self.batch.predicates()[self.index]
     }
+
+    /// Returns a copy of this predicate with all `SelfPredicateHash(i)` args
+    /// resolved to `Literal(hash(Custom(batch, i)))`.
+    pub fn normalized_predicate(&self) -> CustomPredicate {
+        let pred = self.predicate();
+        let normalized_statements = pred
+            .statements
+            .iter()
+            .map(|st_tmpl| {
+                let args = st_tmpl
+                    .args
+                    .iter()
+                    .map(|arg| match arg {
+                        StatementTmplArg::SelfPredicateHash(i) => {
+                            let pred_hash = Predicate::Custom(CustomPredicateRef {
+                                batch: self.batch.clone(),
+                                index: *i,
+                            })
+                            .hash();
+                            StatementTmplArg::Literal(Value::from(pred_hash))
+                        }
+                        other => other.clone(),
+                    })
+                    .collect();
+                StatementTmpl {
+                    pred_or_wc: st_tmpl.pred_or_wc.clone(),
+                    args,
+                }
+            })
+            .collect();
+        CustomPredicate {
+            name: pred.name.clone(),
+            conjunction: pred.conjunction,
+            statements: normalized_statements,
+            args_len: pred.args_len,
+            wildcard_names: pred.wildcard_names.clone(),
+        }
+    }
 }
 
 #[cfg(test)]
@@ -823,4 +874,164 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_normalized_predicate() -> Result<()> {
+        let params = Params::default();
+
+        // Build a batch: pred_A = Equal(x, y), pred_B = Equal(x, SelfPredicateHash(0))
+        let pred_a = CustomPredicate::and(
+            &params,
+            "pred_A".into(),
+            vec![st(
+                P::Native(NP::Equal),
+                vec![STA::Wildcard(wc(0)), STA::Wildcard(wc(1))],
+            )],
+            2,
+            names(&["x", "y"]),
+        )?;
+        let pred_b = CustomPredicate::and(
+            &params,
+            "pred_B".into(),
+            vec![st(
+                P::Native(NP::Equal),
+                vec![STA::Wildcard(wc(0)), STA::SelfPredicateHash(0)],
+            )],
+            1,
+            names(&["x"]),
+        )?;
+        let batch = CustomPredicateBatch::new("batch".into(), vec![pred_a, pred_b]);
+
+        // Compute expected pred_A hash
+        let pred_a_ref = CustomPredicateRef::new(batch.clone(), 0);
+        let expected_hash = Value::from(Predicate::Custom(pred_a_ref).hash());
+
+        // Normalize pred_B
+        let pred_b_ref = CustomPredicateRef::new(batch.clone(), 1);
+        let normalized = pred_b_ref.normalized_predicate();
+
+        // The second arg should be resolved to Literal(pred_A_hash)
+        assert_eq!(
+            normalized.statements[0].args[1],
+            STA::Literal(expected_hash)
+        );
+
+        // First arg should be unchanged (still a wildcard)
+        assert_eq!(normalized.statements[0].args[0], STA::Wildcard(wc(0)));
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_self_predicate_hash_check() -> Result<()> {
+        let params = Params::default();
+
+        // Build a batch: pred_A = Equal(x, y), pred_B = Equal(x, SelfPredicateHash(0))
+        let pred_a = CustomPredicate::and(
+            &params,
+            "pred_A".into(),
+            vec![st(
+                P::Native(NP::Equal),
+                vec![STA::Wildcard(wc(0)), STA::Wildcard(wc(1))],
+            )],
+            2,
+            names(&["x", "y"]),
+        )?;
+        let pred_b = CustomPredicate::and(
+            &params,
+            "pred_B".into(),
+            vec![st(
+                P::Native(NP::Equal),
+                vec![STA::Wildcard(wc(0)), STA::SelfPredicateHash(0)],
+            )],
+            1,
+            names(&["x"]),
+        )?;
+        let batch = CustomPredicateBatch::new("batch".into(), vec![pred_a, pred_b]);
+
+        let pred_a_ref = CustomPredicateRef::new(batch.clone(), 0);
+        let pred_a_hash = Value::from(Predicate::Custom(pred_a_ref).hash());
+
+        let pred_b_ref = CustomPredicateRef::new(batch.clone(), 1);
+
+        // Construct a valid operation: Equal(some_value, pred_a_hash)
+        let some_value = Value::from(42);
+        let op_args = vec![Statement::equal(some_value.clone(), pred_a_hash.clone())];
+
+        // The output statement
+        let output_st = Statement::Custom(pred_b_ref.clone(), vec![some_value.clone()]);
+
+        // This should pass
+        assert!(Operation::Custom(pred_b_ref.clone(), op_args).check(&params, &output_st)?);
+
+        // Now try with wrong hash, should fail
+        let wrong_hash = Value::from(999);
+        let bad_op_args = vec![Statement::equal(some_value.clone(), wrong_hash)];
+        assert!(Operation::Custom(pred_b_ref, bad_op_args)
+            .check(&params, &output_st)
+            .is_err());
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_self_predicate_hash_cyclic() -> Result<()> {
+        let params = Params::default();
+
+        // Build a batch where pred_A references pred_B's hash and vice versa
+        // pred_A = Equal(x, SelfPredicateHash(1))
+        // pred_B = Equal(x, SelfPredicateHash(0))
+        let pred_a = CustomPredicate::and(
+            &params,
+            "pred_A".into(),
+            vec![st(
+                P::Native(NP::Equal),
+                vec![STA::Wildcard(wc(0)), STA::SelfPredicateHash(1)],
+            )],
+            1,
+            names(&["x"]),
+        )?;
+        let pred_b = CustomPredicate::and(
+            &params,
+            "pred_B".into(),
+            vec![st(
+                P::Native(NP::Equal),
+                vec![STA::Wildcard(wc(0)), STA::SelfPredicateHash(0)],
+            )],
+            1,
+            names(&["x"]),
+        )?;
+        let batch = CustomPredicateBatch::new("batch".into(), vec![pred_a, pred_b]);
+
+        let pred_a_ref = CustomPredicateRef::new(batch.clone(), 0);
+        let pred_b_ref = CustomPredicateRef::new(batch.clone(), 1);
+        let pred_a_hash = Value::from(Predicate::Custom(pred_a_ref.clone()).hash());
+        let pred_b_hash = Value::from(Predicate::Custom(pred_b_ref.clone()).hash());
+
+        // pred_A's normalized form should reference pred_B's hash
+        let norm_a = pred_a_ref.normalized_predicate();
+        assert_eq!(
+            norm_a.statements[0].args[1],
+            STA::Literal(pred_b_hash.clone())
+        );
+
+        // pred_B's normalized form should reference pred_A's hash
+        let norm_b = pred_b_ref.normalized_predicate();
+        assert_eq!(
+            norm_b.statements[0].args[1],
+            STA::Literal(pred_a_hash.clone())
+        );
+
+        // Verify pred_A: Equal(pred_b_hash, pred_b_hash) should pass
+        let op_a = vec![Statement::equal(pred_b_hash.clone(), pred_b_hash.clone())];
+        let st_a = Statement::Custom(pred_a_ref.clone(), vec![pred_b_hash.clone()]);
+        assert!(Operation::Custom(pred_a_ref, op_a).check(&params, &st_a)?);
+
+        // Verify pred_B: Equal(pred_a_hash, pred_a_hash) should pass
+        let op_b = vec![Statement::equal(pred_a_hash.clone(), pred_a_hash.clone())];
+        let st_b = Statement::Custom(pred_b_ref.clone(), vec![pred_a_hash.clone()]);
+        assert!(Operation::Custom(pred_b_ref, op_b).check(&params, &st_b)?);
+
+        Ok(())
+    }
 }

src/middleware/operation.rs

@@ -595,6 +595,11 @@ pub fn check_st_tmpl(
         (StatementTmplArg::Wildcard(wc), StatementArg::Literal(v)) => {
             wc_check_or_set(v.clone(), wc, wildcard_map)
         }
+        (StatementTmplArg::SelfPredicateHash(_), _) => {
+            unreachable!(
+                "SelfPredicateHash should be normalized to Literal before template matching"
+            )
+        }
         _ => Err(Error::mismatched_statement_tmpl_arg(
             st_tmpl_arg.clone(),
             st_arg.clone(),
@@ -712,7 +717,7 @@ pub(crate) fn check_custom_pred(
     args: &[Statement],
     s_args: &[Value],
 ) -> Result<()> {
-    let pred = custom_pred_ref.predicate();
+    let pred = custom_pred_ref.normalized_predicate();
     if pred.statements.len() != args.len() {
         return Err(Error::diff_amount(
             "custom predicate operation".to_string(),
@@ -731,7 +736,7 @@ pub(crate) fn check_custom_pred(
     }
 
     // Check that the resolved wildcards match the statement arguments.
-    let wc_values = match wildcard_values_from_op_st(params, pred, args, s_args) {
+    let wc_values = match wildcard_values_from_op_st(params, &pred, args, s_args) {
         Ok(wc_values) => wc_values,
         Err(Error::Inner { inner, backtrace }) => match *inner {
             MiddlewareInnerError::InvalidWildcardAssignment(wc, v, prev)