diff --git a/crates/aura-composites/src/lib.rs b/crates/aura-composites/src/lib.rs
index a9f95b1..c0edf08 100644
--- a/crates/aura-composites/src/lib.rs
+++ b/crates/aura-composites/src/lib.rs
@@ -52,8 +52,8 @@ fn vol_stop_inner(knobs: Option<(i64, f64)>) -> Composite {
     let sqrt = g.add(Sqrt::builder()); // → σ (price units)
     // k·σ: weights[0] bound (Some) or open and named `stop_k` (None).
     let scale = g.add(match knobs {
-        Some((_, k)) => LinComb::builder(1).bind("weights[0]", Scalar::f64(k)),
-        None => LinComb::builder(1).named("stop_k"),
+        Some((_, k)) => LinComb::configured(1).bind("weights[0]", Scalar::f64(k)),
+        None => LinComb::configured(1).named("stop_k"),
     });
     g.feed(price, [delay.input("series"), sub.input("lhs")]);
     g.connect(delay.output("value"), sub.input("rhs"));
@@ -174,7 +174,7 @@ pub fn cost_graph(cost_nodes: Vec<PrimitiveBuilder>) -> Composite {
     let entry = g.input_role("entry_price");
     let stop = g.input_role("stop_price");
 
-    let agg = g.add(CostSum::builder(n));
+    let agg = g.add(CostSum::configured(n));
 
     // Per-role geometry fan targets, collected across all nodes, fed once per role.
     let mut closed_t = Vec::with_capacity(n);
diff --git a/crates/aura-engine/src/blueprint.rs b/crates/aura-engine/src/blueprint.rs
index c6a5b72..d4513ec 100644
--- a/crates/aura-engine/src/blueprint.rs
+++ b/crates/aura-engine/src/blueprint.rs
@@ -1821,7 +1821,7 @@ mod tests {
         Composite::new(
             "sig3",
             vec![
-                LinComb::builder(2).into(),
+                LinComb::configured(2).into(),
                 Bias::builder().into(),
                 Sma::builder().named("c").into(),
             ],
@@ -1860,7 +1860,7 @@ mod tests {
         Composite::new(
             "sig3",
             vec![
-                LinComb::builder(2).into(),
+                LinComb::configured(2).into(),
                 Bias::builder().into(),
                 Sma::builder().named("c").into(),
             ],
@@ -3233,7 +3233,7 @@ mod tests {
         // outer composite "strategy": the inner composite + a LinComb([1,-1])
         let strategy = Composite::new(
             "strategy",
-            vec![BlueprintNode::Composite(fast_slow), LinComb::builder(2).into()],
+            vec![BlueprintNode::Composite(fast_slow), LinComb::configured(2).into()],
             vec![],
             vec![Role { name: "price".into(), targets: vec![Target { node: 0, slot: 0 }], source: None }],
             vec![OutField { node: 0, field: 0, name: "out".into() }],
@@ -3441,7 +3441,7 @@ mod tests {
         // outer composite "strategy": the inner composite + a LinComb([1,-1])
         let strategy = Composite::new(
             "strategy",
-            vec![BlueprintNode::Composite(fast_slow), LinComb::builder(2).into()],
+            vec![BlueprintNode::Composite(fast_slow), LinComb::configured(2).into()],
             // fan fast_slow's output into both LinComb terms so every interior slot
             // is wired (the totality check, cycle 0040); param order is unaffected.
             vec![
@@ -3501,7 +3501,7 @@ mod tests {
         use aura_std::{LinComb, Sma};
         let bp = Composite::new(
             "root",
-            vec![Sma::builder().into(), LinComb::builder(2).into()],
+            vec![Sma::builder().into(), LinComb::configured(2).into()],
             vec![],
             vec![],
             vec![], // output
diff --git a/crates/aura-engine/tests/construction_e2e.rs b/crates/aura-engine/tests/construction_e2e.rs
index 73d7208..968412a 100644
--- a/crates/aura-engine/tests/construction_e2e.rs
+++ b/crates/aura-engine/tests/construction_e2e.rs
@@ -34,10 +34,10 @@ fn params() -> [Scalar; 3] {
 fn signal_ops() -> Vec<Op> {
     vec![
         Op::Source { role: "price".into(), kind: ScalarKind::F64 },
-        Op::Add { type_id: "SMA".into(), as_name: Some("fast".into()), bind: vec![] },
-        Op::Add { type_id: "SMA".into(), as_name: Some("slow".into()), bind: vec![] },
-        Op::Add { type_id: "Sub".into(), as_name: None, bind: vec![] },
-        Op::Add { type_id: "Bias".into(), as_name: None, bind: vec![] },
+        Op::Add { type_id: "SMA".into(), as_name: Some("fast".into()), args: vec![], bind: vec![] },
+        Op::Add { type_id: "SMA".into(), as_name: Some("slow".into()), args: vec![], bind: vec![] },
+        Op::Add { type_id: "Sub".into(), as_name: None, args: vec![], bind: vec![] },
+        Op::Add { type_id: "Bias".into(), as_name: None, args: vec![], bind: vec![] },
         Op::Feed { role: "price".into(), into: vec!["fast.series".into(), "slow.series".into()] },
         Op::Connect { from: "fast.value".into(), to: "sub.lhs".into() },
         Op::Connect { from: "slow.value".into(), to: "sub.rhs".into() },
@@ -96,10 +96,10 @@ fn replayed_construction_compiles_identically_to_graphbuilder() {
 #[test]
 fn replay_attributes_eager_fault_to_its_op_index() {
     let ops = vec![
-        Op::Add { type_id: "SMA".into(), as_name: Some("fast".into()), bind: vec![] }, // 0 ok
-        Op::Add { type_id: "Sub".into(), as_name: None, bind: vec![] },                // 1 ok
-        Op::Add { type_id: "Nope".into(), as_name: None, bind: vec![] },               // 2 fails
-        Op::Add { type_id: "Bias".into(), as_name: None, bind: vec![] },               // 3 never runs
+        Op::Add { type_id: "SMA".into(), as_name: Some("fast".into()), args: vec![], bind: vec![] }, // 0 ok
+        Op::Add { type_id: "Sub".into(), as_name: None, args: vec![], bind: vec![] },                // 1 ok
+        Op::Add { type_id: "Nope".into(), as_name: None, args: vec![], bind: vec![] },               // 2 fails
+        Op::Add { type_id: "Bias".into(), as_name: None, args: vec![], bind: vec![] },               // 3 never runs
     ];
     // `.err().unwrap()` (not `unwrap_err`): the Ok arm `Composite` holds build
     // closures and is not `Debug`, which `unwrap_err`'s bound would require.
@@ -120,9 +120,9 @@ fn replay_attributes_eager_fault_to_its_op_index() {
 fn replay_attributes_holistic_fault_to_finalize_step() {
     let ops = vec![
         Op::Source { role: "price".into(), kind: ScalarKind::F64 },                    // 0
-        Op::Add { type_id: "SMA".into(), as_name: Some("fast".into()), bind: vec![] }, // 1
-        Op::Add { type_id: "SMA".into(), as_name: Some("slow".into()), bind: vec![] }, // 2
-        Op::Add { type_id: "Sub".into(), as_name: None, bind: vec![] },                // 3
+        Op::Add { type_id: "SMA".into(), as_name: Some("fast".into()), args: vec![], bind: vec![] }, // 1
+        Op::Add { type_id: "SMA".into(), as_name: Some("slow".into()), args: vec![], bind: vec![] }, // 2
+        Op::Add { type_id: "Sub".into(), as_name: None, args: vec![], bind: vec![] },                // 3
         Op::Feed { role: "price".into(), into: vec!["fast.series".into(), "slow.series".into()] }, // 4
         Op::Connect { from: "fast.value".into(), to: "sub.lhs".into() },               // 5
         // sub.rhs deliberately left unwired -> only finalize can decide totality.
diff --git a/crates/aura-engine/tests/ger40_breakout.rs b/crates/aura-engine/tests/ger40_breakout.rs
index 6a0bf5d..478d823 100644
--- a/crates/aura-engine/tests/ger40_breakout.rs
+++ b/crates/aura-engine/tests/ger40_breakout.rs
@@ -110,7 +110,7 @@ fn build_harness(
         Resample::builder().schema().clone(),                                  // 0
         Delay::builder().schema().clone(),                                     // 1
         Gt::builder().schema().clone(),                                        // 2
-        Session::builder(9, 0, Berlin, 15).schema().clone(),                   // 3
+        Session::configured(9, 0, Berlin, 15).schema().clone(),                   // 3
         EqConst::builder().schema().clone(),                                   // 4
         EqConst::builder().schema().clone(),                                   // 5
         And::builder().schema().clone(),                                       // 6
diff --git a/crates/aura-engine/tests/r_meanrev_e2e.rs b/crates/aura-engine/tests/r_meanrev_e2e.rs
index 046d8cc..113a19c 100644
--- a/crates/aura-engine/tests/r_meanrev_e2e.rs
+++ b/crates/aura-engine/tests/r_meanrev_e2e.rs
@@ -21,7 +21,7 @@ fn run_meanrev_bias(closes: &[f64], n: i64, k: f64) -> Vec<f64> {
     let sq = g.add(Mul::builder()); // dev * dev
     let var = g.add(Ema::builder().bind("length", Scalar::i64(n))); // EWMA variance
     let sigma = g.add(Sqrt::builder());
-    let band = g.add(LinComb::builder(1).bind("weights[0]", Scalar::f64(k))); // k*sigma
+    let band = g.add(LinComb::configured(1).bind("weights[0]", Scalar::f64(k))); // k*sigma
     let upper = g.add(Add::builder()); // mean + k*sigma
     let lower = g.add(Sub::builder()); // mean - k*sigma
     let gt_hi = g.add(Gt::builder()); // price > upper
diff --git a/crates/aura-ingest/examples/shared/breakout_real.rs b/crates/aura-ingest/examples/shared/breakout_real.rs
index 373b710..065e75a 100644
--- a/crates/aura-ingest/examples/shared/breakout_real.rs
+++ b/crates/aura-ingest/examples/shared/breakout_real.rs
@@ -112,7 +112,7 @@ pub fn build_harness() -> (Harness, Taps) {
         Resample::builder().schema().clone(), // 0
         Delay::builder().schema().clone(),     // 1
         Gt::builder().schema().clone(),        // 2
-        Session::builder(SESSION_HOUR, SESSION_MINUTE, Berlin, BAR_MINUTES)
+        Session::configured(SESSION_HOUR, SESSION_MINUTE, Berlin, BAR_MINUTES)
             .schema()
             .clone(), // 3
         EqConst::builder().schema().clone(), // 4
@@ -248,7 +248,7 @@ pub fn ger40_breakout_blueprint(
     );
     let delay = g.add(Delay::builder().bind("lag", Scalar::i64(1)));
     let gt = g.add(Gt::builder());
-    let session = g.add(Session::builder(open_hour, open_minute, tz, bar_period_minutes));
+    let session = g.add(Session::configured(open_hour, open_minute, tz, bar_period_minutes));
     // The ONLY two params: the EqConst targets, named so the space reads
     // `entry_bar.target` / `exit_bar.target`. Their `target` is left UNBOUND.
     let entry_bar = g.add(EqConst::builder().named("entry_bar"));
diff --git a/crates/aura-market/src/session.rs b/crates/aura-market/src/session.rs
index 6941d83..3c71aa3 100644
--- a/crates/aura-market/src/session.rs
+++ b/crates/aura-market/src/session.rs
@@ -33,10 +33,17 @@
 //! **Cold guard.** If the trigger column is empty (not yet fired) → `None`.
 
 use aura_core::{
-    Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, ParamSpec, PortSpec, PrimitiveBuilder, ScalarKind,
+    ArgKind, ArgSpec, ArgValue, Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, ParamSpec, PortSpec,
+    PrimitiveBuilder, Scalar, ScalarKind,
 };
 use chrono::{TimeZone, Timelike};
 
+/// The declared construction args of a `Session` recipe (spec §Concrete code
+/// shapes): the timezone and local open time — structural, non-scalar
+/// configuration, never a swept param.
+const SESSION_ARGS: &[ArgSpec] =
+    &[ArgSpec { name: "tz", kind: ArgKind::Tz }, ArgSpec { name: "open", kind: ArgKind::TimeOfDay }];
+
 /// Frankfurt-session bar indexer. Holds the open offset (minutes past local
 /// midnight), the IANA timezone, and the bar period — baked at construction,
 /// never streamed and never a swept param (a timezone is not a scalar).
@@ -60,23 +67,57 @@ impl Session {
         }
     }
 
-    /// The param-generic recipe for a blueprint primitive. The open time,
-    /// timezone, and period are **structural** config baked into the closure
-    /// (like `SimBroker::builder` bakes `pip_size`), NOT scalar params — the
-    /// declared param list is empty and the `build_fn` ignores its slice (like
-    /// `Gt` / `Latch`).
-    pub fn builder(open_hour: u32, open_minute: u32, tz: chrono_tz::Tz, period_minutes: i64) -> PrimitiveBuilder {
+    /// Roster factory: zero-arg, arg-bearing (spec §Concrete code shapes) — the
+    /// timezone and open time come through the `args` channel (`try_args`),
+    /// declared by [`SESSION_ARGS`]. `period_minutes` becomes an ordinary
+    /// `ParamSpec` once `make` runs (a real scalar knob, unlike tz/open).
+    pub fn builder() -> PrimitiveBuilder {
+        PrimitiveBuilder::pending(
+            "Session",
+            "bars elapsed since the session open, from the configured open time and timezone",
+            SESSION_ARGS,
+            Self::make,
+        )
+    }
+
+    /// Turn a validated `(tz, open)` value pair into the real signature: one
+    /// `trigger` input, one `bars_since_open` output, and `period_minutes` as
+    /// the sole (now scalar) `ParamSpec` — read from the param slice at build.
+    fn make(values: &[(String, ArgValue)]) -> PrimitiveBuilder {
+        let tz = values
+            .iter()
+            .find_map(|(n, v)| match (n.as_str(), v) {
+                ("tz", ArgValue::Tz(tz)) => Some(*tz),
+                _ => None,
+            })
+            .expect("try_args validated `tz` as ArgKind::Tz before calling make");
+        let (open_hour, open_minute) = values
+            .iter()
+            .find_map(|(n, v)| match (n.as_str(), v) {
+                ("open", ArgValue::TimeOfDay { hour, minute }) => Some((*hour, *minute)),
+                _ => None,
+            })
+            .expect("try_args validated `open` as ArgKind::TimeOfDay before calling make");
         PrimitiveBuilder::new(
             "Session",
             NodeSchema {
                 inputs: vec![PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: "trigger".into() }],
                 output: vec![FieldSpec { name: "bars_since_open".into(), kind: ScalarKind::I64 }],
-                params: vec![],
+                params: vec![ParamSpec { name: "period_minutes".into(), kind: ScalarKind::I64 }],
                 doc: "bars elapsed since the session open, from the configured open time and timezone",
             },
-            move |_| Box::new(Session::new(open_hour, open_minute, tz, period_minutes)),
+            move |p| Box::new(Session::new(open_hour, open_minute, tz, p[0].i64())),
         )
     }
+
+    /// Rust-path convenience — the same recipe as the data path (twin
+    /// identity): `builder().try_args([tz, open]) + .bind("period_minutes", …)`.
+    pub fn configured(open_hour: u32, open_minute: u32, tz: chrono_tz::Tz, period_minutes: i64) -> PrimitiveBuilder {
+        Self::builder()
+            .try_args(&[("tz".to_string(), tz.name().to_string()), ("open".to_string(), format!("{open_hour:02}:{open_minute:02}"))])
+            .expect("configured: a valid chrono_tz::Tz and in-range hour/minute always parse")
+            .bind("period_minutes", Scalar::i64(period_minutes))
+    }
 }
 
 /// The zero-arg `SessionFrankfurt` roster preset (#261): the Frankfurt open
@@ -244,4 +285,36 @@ mod tests {
         let inputs = trigger_input();
         assert_eq!(node.eval(Ctx::new(&inputs, Timestamp(0))), None);
     }
+
+    /// Twin identity (spec §aura-market): `configured` is exactly
+    /// `builder().try_args([tz, open]).bind("period_minutes", …)` — same
+    /// accepted args, same declared signature, whichever path authored it.
+    #[test]
+    fn session_configured_twin_equals_builder_try_args() {
+        let via_configured = Session::configured(9, 30, Berlin, 15);
+        let via_try_args = Session::builder()
+            .try_args(&[("tz".into(), "Europe/Berlin".into()), ("open".into(), "09:30".into())])
+            .expect("valid tz/open configure")
+            .bind("period_minutes", Scalar::i64(15));
+        assert_eq!(via_configured.construction_args(), via_try_args.construction_args());
+        assert_eq!(via_configured.schema(), via_try_args.schema());
+    }
+
+    /// Behaviour re-anchor (#271 Task 2): the DST-aware headline property
+    /// (local 09:45 reads bar 3 in both CEST summer and CET winter) holds
+    /// identically when the node is built through `configured` instead of
+    /// `Session::new` directly.
+    #[test]
+    fn session_configured_behaviour_unchanged() {
+        let mut node = Session::configured(9, 0, Berlin, 15).build(&[]);
+        let mut inputs = trigger_input();
+
+        let summer = Berlin.with_ymd_and_hms(2024, 7, 1, 9, 45, 0).unwrap().timestamp_nanos_opt().unwrap();
+        inputs[0].push(Scalar::f64(0.0)).unwrap();
+        assert_eq!(node.eval(Ctx::new(&inputs, Timestamp(summer))).map(|r| r[0].i64()), Some(3));
+
+        let winter = Berlin.with_ymd_and_hms(2024, 1, 2, 9, 45, 0).unwrap().timestamp_nanos_opt().unwrap();
+        inputs[0].push(Scalar::f64(0.0)).unwrap();
+        assert_eq!(node.eval(Ctx::new(&inputs, Timestamp(winter))).map(|r| r[0].i64()), Some(3));
+    }
 }
diff --git a/crates/aura-runner/src/member.rs b/crates/aura-runner/src/member.rs
index 2c77f44..cd7eee5 100644
--- a/crates/aura-runner/src/member.rs
+++ b/crates/aura-runner/src/member.rs
@@ -327,7 +327,7 @@ pub fn wrap_r(
     if !reduce {
         // r_equity = cum_realized_r + unrealized_r — one tapped series for charting.
         let r_equity = g.add(
-            LinComb::builder(2)
+            LinComb::configured(2)
                 .bind("weights[0]", Scalar::f64(1.0))
                 .bind("weights[1]", Scalar::f64(1.0)),
         );
@@ -411,7 +411,7 @@ pub fn wrap_r(
             // net_r_equity = cum_realized_r + unrealized_r − Σcum_cost_in_r
             // − Σopen_cost_in_r (the #221-deleted LinComb(4), weights 1,1,-1,-1).
             let net_eq = g.add(
-                LinComb::builder(4)
+                LinComb::configured(4)
                     .bind("weights[0]", Scalar::f64(1.0))
                     .bind("weights[1]", Scalar::f64(1.0))
                     .bind("weights[2]", Scalar::f64(-1.0))
diff --git a/crates/aura-std/Cargo.toml b/crates/aura-std/Cargo.toml
index a0c8658..c5345b5 100644
--- a/crates/aura-std/Cargo.toml
+++ b/crates/aura-std/Cargo.toml
@@ -7,13 +7,6 @@ publish.workspace = true
 
 [dependencies]
 aura-core = { path = "../aura-core" }
-# DST-correct wall-clock math for the `Session` node. A vetted
-# standard crate for timezone/DST — never hand-rolled (C16 per-case policy).
-# `chrono` is already a transitive workspace dep (0.4 via aura-ingest's
-# data-server); aligned to that major. `chrono-tz` brings the IANA `Europe/Berlin`
-# zone + DST transitions.
-chrono = { version = "0.4", default-features = false, features = ["clock"] }
-chrono-tz = { version = "0.10", default-features = false }
 
 [dev-dependencies]
 aura-strategy = { path = "../aura-strategy" }
diff --git a/crates/aura-std/src/lincomb.rs b/crates/aura-std/src/lincomb.rs
index b02c899..beb7c3f 100644
--- a/crates/aura-std/src/lincomb.rs
+++ b/crates/aura-std/src/lincomb.rs
@@ -7,10 +7,15 @@
 //! indexed `F64` knobs that `Composite::param_space` aggregates (C8/C12/C19).
 
 use aura_core::{
-    Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, ParamSpec, PortSpec, PrimitiveBuilder,
-    ScalarKind,
+    ArgKind, ArgSpec, ArgValue, Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, ParamSpec, PortSpec,
+    PrimitiveBuilder, ScalarKind,
 };
 
+/// The declared construction args of a `LinComb` recipe: `arity` fixes the
+/// node's input/param count (topology, C19), taken through the `args`
+/// channel instead of a Rust-side builder parameter.
+const LINCOMB_ARGS: &[ArgSpec] = &[ArgSpec { name: "arity", kind: ArgKind::Count }];
+
 /// Weighted sum of `N` f64 inputs: `Σ weights[i] · input[i]`. The `weights` are
 /// construction parameters that configure the node and fix its arity
 /// (`weights.len()` inputs, in slot order). Emits `None` until *all* inputs
@@ -40,10 +45,29 @@ impl LinComb {
         Self { weights, out: [Cell::from_f64(0.0)] }
     }
 
-    /// The param-generic recipe for a blueprint primitive. The `arity` is topology
-    /// (fixed per blueprint, C19), taken as a builder arg; only the weight *values*
-    /// are injected, slot by slot, through `LinComb::new` (the single sizing gate).
-    pub fn builder(arity: usize) -> PrimitiveBuilder {
+    /// Roster factory: zero-arg, arg-bearing. `arity` is topology (fixed per
+    /// blueprint, C19), now taken through the `args` channel instead of a
+    /// Rust-side parameter.
+    pub fn builder() -> PrimitiveBuilder {
+        PrimitiveBuilder::pending(
+            "LinComb",
+            "linear combination of its inputs with constant weights",
+            LINCOMB_ARGS,
+            Self::make,
+        )
+    }
+
+    /// Turn a validated `arity` into the real, arity-sized signature; only
+    /// the weight *values* are injected, slot by slot, through `LinComb::new`
+    /// (the single sizing gate) once params are bound/built.
+    fn make(values: &[(String, ArgValue)]) -> PrimitiveBuilder {
+        let arity = values
+            .iter()
+            .find_map(|(n, v)| match (n.as_str(), v) {
+                ("arity", ArgValue::Count(n)) => Some(*n),
+                _ => None,
+            })
+            .expect("try_args validated `arity` as ArgKind::Count before calling make");
         let inputs = (0..arity)
             .map(|i| PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: format!("term[{i}]") })
             .collect();
@@ -63,6 +87,13 @@ impl LinComb {
             )),
         )
     }
+
+    /// Rust-path convenience — same recipe as the data path (twin identity).
+    pub fn configured(arity: usize) -> PrimitiveBuilder {
+        Self::builder()
+            .try_args(&[("arity".to_string(), arity.to_string())])
+            .expect("configured: a positive arity is always the canonical strict-form Count string")
+    }
 }
 
 impl Node for LinComb {
@@ -149,7 +180,7 @@ mod tests {
 
     #[test]
     fn input_slots_are_named_term_index() {
-        let lc = LinComb::builder(3);
+        let lc = LinComb::configured(3);
         let names: Vec<String> = lc.schema().inputs.iter().map(|p| p.name.clone()).collect();
         assert_eq!(names, ["term[0]", "term[1]", "term[2]"]);
     }
@@ -157,7 +188,7 @@ mod tests {
     #[test]
     fn chained_bind_reconstructs_positional_vector() {
         // bind BOTH weights, in reverse slot order, to DISTINCT values; build empty.
-        let builder = LinComb::builder(2)
+        let builder = LinComb::configured(2)
             .bind("weights[1]", Scalar::f64(2.0))
             .bind("weights[0]", Scalar::f64(0.5));
         assert!(builder.params().is_empty());
@@ -173,7 +204,7 @@ mod tests {
         assert_eq!(lc.eval(Ctx::new(&inputs, Timestamp(0))), Some([Cell::from_f64(11.0)].as_slice()));
 
         // partial: bind weights[0], leave weights[1] open → inject it at build
-        let partial = LinComb::builder(2).bind("weights[0]", Scalar::f64(0.5));
+        let partial = LinComb::configured(2).bind("weights[0]", Scalar::f64(0.5));
         assert_eq!(
             partial.params().iter().map(|p| p.name.as_str()).collect::<Vec<_>>(),
             ["weights[1]"],
@@ -187,4 +218,14 @@ mod tests {
         inputs2[1].push(Scalar::f64(3.0)).unwrap();
         assert_eq!(lc2.eval(Ctx::new(&inputs2, Timestamp(0))), Some([Cell::from_f64(11.0)].as_slice()));
     }
+
+    /// Twin identity (spec §aura-std): `configured(arity)` is exactly
+    /// `builder().try_args([("arity", arity)])`.
+    #[test]
+    fn lincomb_configured_twin_equals_builder_try_args() {
+        let via_configured = LinComb::configured(3);
+        let via_try_args = LinComb::builder().try_args(&[("arity".into(), "3".into())]).expect("valid arity configures");
+        assert_eq!(via_configured.construction_args(), via_try_args.construction_args());
+        assert_eq!(via_configured.schema(), via_try_args.schema());
+    }
 }
diff --git a/crates/aura-std/src/sma.rs b/crates/aura-std/src/sma.rs
index d30c743..2a430a9 100644
--- a/crates/aura-std/src/sma.rs
+++ b/crates/aura-std/src/sma.rs
@@ -238,7 +238,7 @@ mod tests {
             vec![ParamSpec { name: "scale".into(), kind: ScalarKind::F64 }],
         );
         // vector knob expands flat to N indexed F64 entries
-        let lc = LinComb::builder(2).schema().params.clone();
+        let lc = LinComb::configured(2).schema().params.clone();
         assert_eq!(lc.len(), 2);
         assert_eq!(lc[0].name, "weights[0]");
         assert_eq!(lc[1].name, "weights[1]");
diff --git a/crates/aura-strategy/src/cost.rs b/crates/aura-strategy/src/cost.rs
index afabe62..115b69b 100644
--- a/crates/aura-strategy/src/cost.rs
+++ b/crates/aura-strategy/src/cost.rs
@@ -555,10 +555,10 @@ mod tests {
             ConstantCost::builder().schema().output.iter().map(|f| f.name.clone()).collect();
         assert_eq!(prod, COST_FIELD_NAMES.to_vec());
         let agg_out: Vec<String> =
-            CostSum::builder(1).schema().output.iter().map(|f| f.name.clone()).collect();
+            CostSum::configured(1).schema().output.iter().map(|f| f.name.clone()).collect();
         assert_eq!(agg_out, COST_FIELD_NAMES.to_vec());
         let agg_in: Vec<String> =
-            CostSum::builder(1).schema().inputs.iter().map(|p| p.name.clone()).collect();
+            CostSum::configured(1).schema().inputs.iter().map(|p| p.name.clone()).collect();
         let expected: Vec<String> =
             COST_FIELD_NAMES.iter().map(|f| format!("cost[0].{f}")).collect();
         assert_eq!(agg_in, expected);
diff --git a/crates/aura-strategy/src/cost_sum.rs b/crates/aura-strategy/src/cost_sum.rs
index 12fa5a4..427ada7 100644
--- a/crates/aura-strategy/src/cost_sum.rs
+++ b/crates/aura-strategy/src/cost_sum.rs
@@ -7,9 +7,15 @@
 //! the cost path is uniform whether one or several cost nodes are wired.
 
 use aura_core::{
-    Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, PortSpec, PrimitiveBuilder, ScalarKind,
+    ArgKind, ArgSpec, ArgValue, Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, PortSpec,
+    PrimitiveBuilder, ScalarKind,
 };
 
+/// The declared construction args of a `CostSum` recipe: `n_costs` fixes the
+/// node's input count (topology, C19), taken through the `args` channel
+/// instead of a Rust-side builder parameter.
+const COST_SUM_ARGS: &[ArgSpec] = &[ArgSpec { name: "n_costs", kind: ArgKind::Count }];
+
 /// The cost-record field triple and its width come from the shared cost contract
 /// (`crate::cost::{COST_FIELD_NAMES, COST_WIDTH}`) — one source of truth, read by
 /// both the producer side (`cost_node_builder`) and this aggregator. The input-name
@@ -32,10 +38,29 @@ impl CostSum {
         Self { n_costs, out: [Cell::from_f64(0.0); COST_WIDTH] }
     }
 
-    /// The param-generic recipe. `n_costs` is topology (fixed per blueprint, C19),
-    /// captured by the build closure (no per-build params). The input names are a
-    /// lockstep contract with the connect side (`cost[k].<field>`).
-    pub fn builder(n_costs: usize) -> PrimitiveBuilder {
+    /// Roster factory: zero-arg, arg-bearing. `n_costs` is topology (fixed per
+    /// blueprint, C19), now taken through the `args` channel instead of a
+    /// Rust-side parameter.
+    pub fn builder() -> PrimitiveBuilder {
+        PrimitiveBuilder::pending(
+            "CostSum",
+            "sums cost-model contributions into one cost-in-R stream",
+            COST_SUM_ARGS,
+            Self::make,
+        )
+    }
+
+    /// Turn a validated `n_costs` into the real, arity-sized signature. The
+    /// input names are a lockstep contract with the connect side
+    /// (`cost[k].<field>`).
+    fn make(values: &[(String, ArgValue)]) -> PrimitiveBuilder {
+        let n_costs = values
+            .iter()
+            .find_map(|(n, v)| match (n.as_str(), v) {
+                ("n_costs", ArgValue::Count(n)) => Some(*n),
+                _ => None,
+            })
+            .expect("try_args validated `n_costs` as ArgKind::Count before calling make");
         let mut inputs = Vec::with_capacity(n_costs * COST_WIDTH);
         for k in 0..n_costs {
             for field in COST_FIELD_NAMES {
@@ -63,6 +88,13 @@ impl CostSum {
             move |_| Box::new(CostSum::new(n_costs)),
         )
     }
+
+    /// Rust-path convenience — same recipe as the data path (twin identity).
+    pub fn configured(n_costs: usize) -> PrimitiveBuilder {
+        Self::builder()
+            .try_args(&[("n_costs".to_string(), n_costs.to_string())])
+            .expect("configured: a positive n_costs is always the canonical strict-form Count string")
+    }
 }
 
 impl Node for CostSum {
@@ -140,7 +172,7 @@ mod tests {
 
     #[test]
     fn input_slots_are_named_cost_index_field() {
-        let s = CostSum::builder(2);
+        let s = CostSum::configured(2);
         let names: Vec<String> = s.schema().inputs.iter().map(|p| p.name.clone()).collect();
         assert_eq!(
             names,
@@ -161,4 +193,15 @@ mod tests {
     fn new_panics_on_zero() {
         let _ = CostSum::new(0);
     }
+
+    /// Twin identity (spec §aura-strategy): `configured(n_costs)` is exactly
+    /// `builder().try_args([("n_costs", n_costs)])`.
+    #[test]
+    fn cost_sum_configured_twin_equals_builder_try_args() {
+        let via_configured = CostSum::configured(2);
+        let via_try_args =
+            CostSum::builder().try_args(&[("n_costs".into(), "2".into())]).expect("valid n_costs configures");
+        assert_eq!(via_configured.construction_args(), via_try_args.construction_args());
+        assert_eq!(via_configured.schema(), via_try_args.schema());
+    }
 }