feat(research,campaign): optional selection group on the sweep stage (#210 T1-T2)

The std::sweep selection triple (metric/select/deflate) becomes ONE
optional group, Option<SweepSelection>, serde-flattened so the wire
form keeps the flat slots: every existing document parses to Some and
its canonical bytes — and content id — are unchanged (golden pin
byte-untouched). A half-populated group is refused by the schema-strict
parser (all-or-nothing); a bare {"block":"std::sweep"} is the
selection-free sweep.

Campaign side: preflight permits a selection-free sweep only as the
pipeline's terminal stage (new ExecFault::SelectionFreeSweepNotTerminal,
prose in aura-cli's exhaustive exec_fault_prose); the executor's
selection-free arm appends the family exactly as before but records no
StageSelection and produces no nominee (recording one would fabricate
intent — #210 fork decision 1).

Introspection follows: the sweep block's schema table marks the group
optional; CLI introspection pins updated. New tests: group-or-nothing
refusals, selection-free round-trip + distinct content id, terminal-only
preflight accept/refuse, selection-free execute realization.

refs #210

crates/aura-campaign/src/exec.rs

@@ -243,7 +243,7 @@ fn run_cell(
 
     for (stage_ordinal, stage) in process.pipeline.iter().enumerate() {
         match stage {
-            StageBlock::Sweep { metric, select, deflate } => {
+            StageBlock::Sweep { selection } => {
                 // Deterministic, self-describing family name (the "-{run}"
                 // suffix is appended by the registry).
                 let family_name = format!(
@@ -254,32 +254,55 @@ fn run_cell(
                 let family_id = registry
                     .append_family(&family_name, FamilyKind::Sweep, &sweep_member_reports(&family))
                     .map_err(ExecFault::Registry)?;
-                let (winner, selection) =
-                    select_sweep_winner(&family, &grid.axis_lens, metric, *select, *deflate, seed)?;
-                let winner_ordinal = family
-                    .points
-                    .iter()
-                    .position(|p| p == &winner)
-                    .expect("the winner is a member of its own family");
-                let params = zip_params(&family.space, &winner.params);
-                selections.push(StageSelectionOut {
-                    stage: stage_ordinal,
-                    block: "std::sweep",
-                    family_id: family_id.clone(),
-                    winner_ordinal,
-                    params: params.clone(),
-                    selection: selection.clone(),
-                });
-                // Nominate the sweep winner (superseded by a later
-                // walk_forward stage; cleared by an empty gate).
-                nominee = Some((params.clone(), winner.report.clone()));
-                stages.push(StageRealization {
-                    block: "std::sweep".to_string(),
-                    family_id: Some(family_id.clone()),
-                    survivor_ordinals: None,
-                    selection: Some(StageSelection { winner_ordinal, params, selection }),
-                    bootstrap: None,
-                });
+                match selection {
+                    Some(sel) => {
+                        let (winner, selection) = select_sweep_winner(
+                            &family,
+                            &grid.axis_lens,
+                            &sel.metric,
+                            sel.select,
+                            sel.deflate,
+                            seed,
+                        )?;
+                        let winner_ordinal = family
+                            .points
+                            .iter()
+                            .position(|p| p == &winner)
+                            .expect("the winner is a member of its own family");
+                        let params = zip_params(&family.space, &winner.params);
+                        selections.push(StageSelectionOut {
+                            stage: stage_ordinal,
+                            block: "std::sweep",
+                            family_id: family_id.clone(),
+                            winner_ordinal,
+                            params: params.clone(),
+                            selection: selection.clone(),
+                        });
+                        // Nominate the sweep winner (superseded by a later
+                        // walk_forward stage; cleared by an empty gate).
+                        nominee = Some((params.clone(), winner.report.clone()));
+                        stages.push(StageRealization {
+                            block: "std::sweep".to_string(),
+                            family_id: Some(family_id.clone()),
+                            survivor_ordinals: None,
+                            selection: Some(StageSelection { winner_ordinal, params, selection }),
+                            bootstrap: None,
+                        });
+                    }
+                    None => {
+                        // Selection-free sweep (terminal, preflight-checked):
+                        // the family is the whole result — no StageSelection
+                        // is recorded (recording one would fabricate intent),
+                        // no nominee is produced.
+                        stages.push(StageRealization {
+                            block: "std::sweep".to_string(),
+                            family_id: Some(family_id.clone()),
+                            survivor_ordinals: None,
+                            selection: None,
+                            bootstrap: None,
+                        });
+                    }
+                }
                 // The whole family flows on (decision 3: `select` names the
                 // recorded selection, never a filter).
                 survivors = family

crates/aura-campaign/src/lib.rs

@@ -77,6 +77,13 @@ pub enum ExecFault {
     PlateauInWalkForward { stage: usize },
     /// sweep `deflate: true` with a non-argmax select rule.
     DeflatePlateauConflict { stage: usize },
+    /// A selection-free sweep (no metric/select group) anywhere but the
+    /// pipeline's terminal stage: gate/walk_forward/monte_carlo run fine off
+    /// `survivors` (a selection-free sweep still produces the whole family),
+    /// but no winner/nominee is recorded — a stage that needs the cell's
+    /// final candidate (`generalize`, campaign-scope) would have nothing to
+    /// grade.
+    SelectionFreeSweepNotTerminal { stage: usize },
     /// generalize needs >= 2 instruments in the campaign (static).
     GeneralizeNeedsInstruments { available: usize },
     /// generalize's metric must be an R metric (static — the intrinsic tier
@@ -235,14 +242,24 @@ pub fn preflight(process: &ProcessDoc, campaign: &CampaignDoc) -> Result<(), Exe
     // per-stage slot rules (shape already established above).
     for (i, stage) in process.pipeline.iter().enumerate() {
         match stage {
-            StageBlock::Sweep { metric, select, deflate } => {
-                if !RANKABLE_METRICS.contains(&metric.as_str()) {
-                    return Err(ExecFault::UnrankableMetric { stage: i, metric: metric.clone() });
+            StageBlock::Sweep { selection } => match selection {
+                Some(sel) => {
+                    if !RANKABLE_METRICS.contains(&sel.metric.as_str()) {
+                        return Err(ExecFault::UnrankableMetric {
+                            stage: i,
+                            metric: sel.metric.clone(),
+                        });
+                    }
+                    if sel.deflate && sel.select != SelectRule::Argmax {
+                        return Err(ExecFault::DeflatePlateauConflict { stage: i });
+                    }
                 }
-                if *deflate && *select != SelectRule::Argmax {
-                    return Err(ExecFault::DeflatePlateauConflict { stage: i });
+                None => {
+                    if i + 1 != process.pipeline.len() {
+                        return Err(ExecFault::SelectionFreeSweepNotTerminal { stage: i });
+                    }
                 }
-            }
+            },
             StageBlock::Gate { all } => {
                 for p in all {
                     if !PER_MEMBER_METRICS.contains(&p.metric.as_str()) {
@@ -305,6 +322,7 @@ pub fn preflight(process: &ProcessDoc, campaign: &CampaignDoc) -> Result<(), Exe
 mod tests {
     use super::*;
     use aura_engine::{RMetrics, RunManifest, RunMetrics, Timestamp};
+    use aura_research::SweepSelection;
 
     /// A report with every per-member scalar planted to a distinct value
     /// (1..=14 in `PER_MEMBER_METRICS` order), r block present.
@@ -467,7 +485,9 @@ mod tests {
     }
 
     fn sweep_stage(metric: &str, select: SelectRule, deflate: bool) -> StageBlock {
-        StageBlock::Sweep { metric: metric.to_string(), select, deflate }
+        StageBlock::Sweep {
+            selection: Some(SweepSelection { metric: metric.to_string(), select, deflate }),
+        }
     }
 
     fn gate_stage(metric: &str) -> StageBlock {
@@ -690,6 +710,30 @@ mod tests {
         ));
     }
 
+    /// A selection-free sweep is permitted as the terminal (and here, only)
+    /// stage of a process: the family itself is the result, no winner needed.
+    #[test]
+    fn preflight_accepts_a_terminal_selection_free_sweep() {
+        let c = campaign();
+        let process = process_of(vec![StageBlock::Sweep { selection: None }]);
+        assert!(preflight(&process, &c).is_ok());
+    }
+
+    /// A selection-free sweep followed by any other stage is refused: a
+    /// downstream stage would have no selection/nominee to consume.
+    #[test]
+    fn preflight_refuses_a_non_terminal_selection_free_sweep() {
+        let c = campaign();
+        let process = process_of(vec![
+            StageBlock::Sweep { selection: None },
+            gate_stage("sqn"),
+        ]);
+        match preflight(&process, &c) {
+            Err(ExecFault::SelectionFreeSweepNotTerminal { stage: 0 }) => {}
+            other => panic!("expected SelectionFreeSweepNotTerminal, got {other:?}"),
+        }
+    }
+
     #[test]
     fn preflight_refuses_non_sweep_first_and_double_sweep() {
         let c = campaign();
@@ -804,7 +848,8 @@ mod wf_tests {
     use aura_registry::{FamilyKind, Registry};
     use aura_research::{
         Axis, CampaignDoc, Cmp, DataSection, DocKind, DocRef, Predicate, Presentation,
-        ProcessDoc, ProcessRef, SelectRule, StageBlock, StrategyEntry, WfMode, Window,
+        ProcessDoc, ProcessRef, SelectRule, StageBlock, StrategyEntry, SweepSelection, WfMode,
+        Window,
     };
 
     use super::{execute, CampaignOutcome, CellSpec, ExecFault, MemberFault, MemberRunner};
@@ -857,9 +902,11 @@ mod wf_tests {
     /// stages on total_pips / argmax, rolling mode.
     fn wf_process(gate_gt: Option<f64>, is_ms: u64, oos_ms: u64, step_ms: u64) -> ProcessDoc {
         let mut pipeline = vec![StageBlock::Sweep {
-            metric: "total_pips".to_string(),
-            select: SelectRule::Argmax,
-            deflate: false,
+            selection: Some(SweepSelection {
+                metric: "total_pips".to_string(),
+                select: SelectRule::Argmax,
+                deflate: false,
+            }),
         }];
         if let Some(threshold) = gate_gt {
             pipeline.push(StageBlock::Gate {
@@ -1175,9 +1222,11 @@ mod wf_tests {
                 name: "wf-plateau-proc".to_string(),
                 description: None,
                 pipeline: vec![StageBlock::Sweep {
-                    metric: "total_pips".to_string(),
-                    select,
-                    deflate: false,
+                    selection: Some(SweepSelection {
+                        metric: "total_pips".to_string(),
+                        select,
+                        deflate: false,
+                    }),
                 }],
             };
             let runner = WfFakeRunner::new();
@@ -1205,9 +1254,11 @@ mod wf_tests {
             description: None,
             pipeline: vec![
                 StageBlock::Sweep {
-                    metric: "total_pips".to_string(),
-                    select: SelectRule::Argmax,
-                    deflate: false,
+                    selection: Some(SweepSelection {
+                        metric: "total_pips".to_string(),
+                        select: SelectRule::Argmax,
+                        deflate: false,
+                    }),
                 },
                 StageBlock::WalkForward {
                     in_sample_ms: 40,

crates/aura-campaign/tests/execute.rs

@@ -10,7 +10,7 @@ use aura_engine::{r_bootstrap, RMetrics, RunManifest, RunMetrics, RunReport, Sel
 use aura_registry::{generalization, FamilyKind, Registry, StageBootstrap};
 use aura_research::{
     Axis, CampaignDoc, Cmp, DataSection, DocKind, DocRef, Predicate, Presentation, ProcessDoc,
-    ProcessRef, SelectRule, StageBlock, StrategyEntry, WfMode, Window,
+    ProcessRef, SelectRule, StageBlock, StrategyEntry, SweepSelection, WfMode, Window,
 };
 
 const CAMPAIGN_ID: &str = "aaaabbbbccccddddeeeeffff0000111122223333444455556666777788889999";
@@ -162,12 +162,18 @@ fn campaign(instruments: &[&str]) -> CampaignDoc {
 
 fn sweep_stage(deflate: bool) -> StageBlock {
     StageBlock::Sweep {
-        metric: "net_expectancy_r".to_string(),
-        select: SelectRule::Argmax,
-        deflate,
+        selection: Some(SweepSelection {
+            metric: "net_expectancy_r".to_string(),
+            select: SelectRule::Argmax,
+            deflate,
+        }),
     }
 }
 
+fn selection_free_sweep_stage() -> StageBlock {
+    StageBlock::Sweep { selection: None }
+}
+
 fn gate_stage(value: f64) -> StageBlock {
     StageBlock::Gate {
         all: vec![Predicate { metric: "net_expectancy_r".to_string(), cmp: Cmp::Gt, value }],
@@ -276,6 +282,46 @@ fn execute_sweep_only_records_family_and_selection() {
     assert_eq!(stored.selection.seed, None, "no deflation annotation without deflate");
 }
 
+/// A selection-free sweep records the family (the whole grid, exactly as a
+/// selected sweep does) but never a selection or a nominee: there is no
+/// winner to name, and nothing downstream to nominate for.
+#[test]
+fn execute_selection_free_sweep_records_family_without_selection() {
+    let reg = temp_registry("selection_free_sweep");
+    let doc = campaign(&["EURUSD"]);
+    let proc_doc = process(vec![selection_free_sweep_stage()]);
+    let out = execute(CAMPAIGN_ID, &doc, &proc_doc, &strategies(), &FakeRunner::clean(), &reg)
+        .expect("selection-free sweep campaign executes");
+
+    // outcome payloads: one cell, one family of 4 members, NO selection
+    assert_eq!(out.cells.len(), 1);
+    assert_eq!(out.cells[0].families.len(), 1);
+    assert_eq!(out.cells[0].families[0].reports.len(), 4);
+    assert_eq!(out.cells[0].families[0].block, "std::sweep");
+    let expected_family_id = format!("{}-0-EURUSD-w0-s0-0", &CAMPAIGN_ID[..8]);
+    assert_eq!(out.cells[0].families[0].family_id, expected_family_id);
+    assert!(out.cells[0].selections.is_empty(), "no winner is named without a selection group");
+
+    // family persisted: 4 sweep members under the derived id, same as a
+    // selected sweep — the selection-free path differs only in what it
+    // records ABOUT the family, never in the family itself.
+    let members = reg.load_family_members().expect("load members");
+    assert_eq!(members.len(), 4);
+    assert!(members.iter().all(|m| m.kind == FamilyKind::Sweep));
+
+    // realization: the stage carries a family_id but no selection; no
+    // nominee-dependent stage ran, so no generalization is recorded either.
+    let runs = reg.load_campaign_runs().expect("load campaign runs");
+    assert_eq!(runs.len(), 1);
+    assert_eq!(runs[0], out.record);
+    assert_eq!(runs[0].cells[0].stages.len(), 1);
+    let stage = &runs[0].cells[0].stages[0];
+    assert_eq!(stage.block, "std::sweep");
+    assert_eq!(stage.family_id.as_deref(), Some(expected_family_id.as_str()));
+    assert!(stage.selection.is_none(), "a selection-free sweep records no selection");
+    assert!(runs[0].generalizations.is_empty());
+}
+
 #[test]
 fn execute_gate_filters_per_member() {
     let reg = temp_registry("gate_filters");

crates/aura-cli/src/campaign_run.rs

@@ -67,6 +67,10 @@ pub(crate) fn exec_fault_prose(f: &ExecFault) -> String {
         ExecFault::DeflatePlateauConflict { stage } => format!(
             "process stage {stage}: sweep deflate: true composes only with select \"argmax\""
         ),
+        ExecFault::SelectionFreeSweepNotTerminal { stage } => format!(
+            "process stage {stage}: a sweep without a selection group (metric + \
+             select) must be the last stage of its process"
+        ),
         ExecFault::GeneralizeNeedsInstruments { available } => format!(
             "campaign has {available} instrument(s); std::generalize needs at least 2"
         ),

crates/aura-cli/tests/research_docs.rs

@@ -138,7 +138,9 @@ fn process_introspect_vocabulary_block_and_content_id() {
     let (out, code) = run_code(&["process", "introspect", "--block", "std::sweep"]);
     assert_eq!(code, Some(0));
     assert!(out.contains("metric"));
-    assert!(out.contains("required"));
+    // The selection group (metric/select/deflate) is optional — a sweep with
+    // no selection is a terminal, selection-free stage (#210).
+    assert!(out.contains("optional"));
 
     let (out, code) = run_code(&["process", "introspect", "--block", "std::walk_forward"]);
     assert_eq!(code, Some(0));
@@ -170,13 +172,16 @@ fn process_introspect_unknown_block_is_prose_exit_1() {
 #[test]
 fn process_introspect_unwired_lists_open_slots() {
     let dir = temp_cwd("process-introspect-unwired");
+    // std::gate's "all" slot is required (unlike std::sweep's now-optional
+    // selection group, #210), so it still exercises the pipeline-level open
+    // slot listing alongside the top-level "name" slot.
     let partial = r#"{ "format_version": 1, "kind": "process",
-        "pipeline": [ { "block": "std::sweep", "metric": "sqn" } ] }"#;
+        "pipeline": [ { "block": "std::gate" } ] }"#;
     write_doc(&dir, "partial.process.json", partial);
     let (out, code) = run_code_in(&dir, &["process", "introspect", "--unwired", "partial.process.json"]);
     assert_eq!(code, Some(0));
     assert!(out.contains("open slot: name"));
-    assert!(out.contains("open slot: pipeline[0].select"));
+    assert!(out.contains("open slot: pipeline[0].all"));
 }
 
 #[test]
@@ -1033,6 +1038,98 @@ fn campaign_run_refuses_mc_before_walk_forward() {
     assert!(!out.contains("PipelineShape"), "Debug leak: {out}");
 }
 
+/// A v2 process doc whose `std::sweep` carries no selection group (no
+/// `metric`/`select`) followed by a `std::gate` stage: intrinsically valid
+/// (`process register` accepts a selection-free sweep as any other stage
+/// shape) but not an executable v2 shape, since a selection-free sweep is
+/// permitted ONLY as the terminal stage of its process (#210's terminal
+/// rule — the executor's preflight rule, not the intrinsic validator's).
+const SELECTION_FREE_SWEEP_THEN_GATE_PROCESS_DOC: &str = r#"{
+  "format_version": 1,
+  "kind": "process",
+  "name": "sweep-then-gate-no-selection",
+  "pipeline": [
+    { "block": "std::sweep" },
+    { "block": "std::gate", "all": [ { "metric": "n_trades", "cmp": "ge", "value": 0.0 } ] }
+  ]
+}"#;
+
+/// Property (#210, sweep-dissolution-a): a selection-free `std::sweep` (no
+/// metric/select group) anywhere but the LAST stage of its process is
+/// refused by `campaign run`'s data-free preflight — the same seam that
+/// already refuses `[sweep, monte_carlo, walk_forward]`'s ordering fault
+/// above — before any member runs (the [1, 2] 1970-epoch-ms window is never
+/// reached), naming the offending stage index, Debug-free. A regression
+/// that let a non-terminal selection-free sweep through would silently
+/// drop a stage's nominee downstream (nothing to select a winner from,
+/// nothing for a following stage to consume).
+#[test]
+fn campaign_run_refuses_a_non_terminal_selection_free_sweep() {
+    let _fixture = project_lock();
+    let dir = built_project();
+    let runs_dir = dir.join("runs");
+    std::fs::remove_dir_all(&runs_dir).ok();
+    let _cleanup = ScratchGuard(vec![
+        ScratchPath::Dir(runs_dir.clone()),
+        ScratchPath::File(dir.join("selfree.process.json")),
+        ScratchPath::File(dir.join("selfree.campaign.json")),
+    ]);
+    let bp_id = seed_blueprint(dir, "campaign-run-selfree-seed");
+    let proc_id =
+        register_process_doc(dir, "selfree.process.json", SELECTION_FREE_SWEEP_THEN_GATE_PROCESS_DOC);
+    write_doc(dir, "selfree.campaign.json", &campaign_doc_json(&bp_id, &proc_id, (1, 2), "", ""));
+    let (out, code) = run_code_in(dir, &["campaign", "run", "selfree.campaign.json"]);
+    assert_eq!(code, Some(1), "stdout/stderr: {out}");
+    assert!(
+        out.contains(
+            "process stage 0: a sweep without a selection group (metric + select) must be \
+             the last stage of its process"
+        ),
+        "the detail names the terminal-only rule: {out}"
+    );
+    assert!(!out.contains("SelectionFreeSweepNotTerminal"), "Debug leak: {out}");
+}
+
+/// The same process shape, but the selection-free sweep IS the (only, hence
+/// terminal) stage: `campaign validate`'s third (executor-preflight) tier
+/// accepts it, data-free, exit 0 — the selection-free arm is a legitimate
+/// executable shape, not merely a document-tier-only accept. Pairs with the
+/// refusal above: together they pin the exact placement boundary (terminal:
+/// yes / anywhere else: no) the executor preflight enforces.
+const SELECTION_FREE_SWEEP_ONLY_PROCESS_DOC: &str = r#"{
+  "format_version": 1,
+  "kind": "process",
+  "name": "sweep-only-no-selection",
+  "pipeline": [ { "block": "std::sweep" } ]
+}"#;
+
+#[test]
+fn campaign_validate_accepts_a_terminal_selection_free_sweep() {
+    let _fixture = project_lock();
+    let dir = built_project();
+    let runs_dir = dir.join("runs");
+    std::fs::remove_dir_all(&runs_dir).ok();
+    let _cleanup = ScratchGuard(vec![
+        ScratchPath::Dir(runs_dir.clone()),
+        ScratchPath::File(dir.join("selfreeonly.process.json")),
+        ScratchPath::File(dir.join("selfreeonly.campaign.json")),
+    ]);
+    let bp_id = seed_blueprint(dir, "campaign-validate-selfreeonly-seed");
+    let proc_id =
+        register_process_doc(dir, "selfreeonly.process.json", SELECTION_FREE_SWEEP_ONLY_PROCESS_DOC);
+    write_doc(
+        dir,
+        "selfreeonly.campaign.json",
+        &campaign_doc_json(&bp_id, &proc_id, (1, 2), "", ""),
+    );
+    let (out, code) = run_code_in(dir, &["campaign", "validate", "selfreeonly.campaign.json"]);
+    assert_eq!(code, Some(0), "a terminal selection-free sweep validates as executable: {out}");
+    assert!(
+        out.contains("campaign document valid (executable): pipeline shape and static guards pass"),
+        "stdout/stderr: {out}"
+    );
+}
+
 /// The v2 boundary, campaign side: a generalize-bearing process is an
 /// executable shape, but `std::generalize` needs >= 2 instruments in the
 /// campaign — a STATIC preflight fact (the referential gate has run, no data

crates/aura-research/src/lib.rs

@@ -47,10 +47,14 @@ pub struct ProcessDoc {
 pub enum StageBlock {
     #[serde(rename = "std::sweep")]
     Sweep {
-        metric: String,
-        select: SelectRule,
-        #[serde(default, skip_serializing_if = "is_false")]
-        deflate: bool,
+        /// The selection triple is one optional group: `None` is a
+        /// selection-free sweep (permitted only as the terminal stage — the
+        /// executor's preflight rule), `Some` carries metric+select(+deflate).
+        /// Flattened so the wire form stays the flat `metric`/`select`/
+        /// `deflate` slots and every existing document's canonical bytes —
+        /// and content id — are unchanged.
+        #[serde(flatten, skip_serializing_if = "Option::is_none")]
+        selection: Option<SweepSelection>,
     },
     #[serde(rename = "std::gate")]
     Gate { all: Vec<Predicate> },
@@ -73,6 +77,16 @@ fn is_false(b: &bool) -> bool {
     !*b
 }
 
+/// The sweep stage's selection group — all-or-nothing (a half-populated
+/// group is unrepresentable; the schema-strict parser refuses it).
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct SweepSelection {
+    pub metric: String,
+    pub select: SelectRule,
+    #[serde(default, skip_serializing_if = "is_false")]
+    pub deflate: bool,
+}
+
 /// Winner-selection rule; wire strings mirror the CLI `--select` values.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub enum SelectRule {
@@ -164,10 +178,12 @@ pub struct BlockSchema {
 pub const PROCESS_BLOCKS: &[BlockSchema] = &[
     BlockSchema {
         id: "std::sweep",
-        doc: "evaluate the campaign's axes-space; reduce members to R metrics; select a winner",
+        doc: "evaluate the campaign's axes-space; reduce members to R metrics; \
+              optionally select a winner (the selection group metric+select is \
+              all-or-nothing; omitted = selection-free, terminal-stage-only)",
         slots: &[
-            SlotInfo { name: "metric", kind: SlotKind::MetricName, required: true },
-            SlotInfo { name: "select", kind: SlotKind::SelectRule, required: true },
+            SlotInfo { name: "metric", kind: SlotKind::MetricName, required: false },
+            SlotInfo { name: "select", kind: SlotKind::SelectRule, required: false },
             SlotInfo { name: "deflate", kind: SlotKind::Bool, required: false },
         ],
     },
@@ -300,11 +316,39 @@ fn stage_from_value(v: &serde_json::Value) -> Result<StageBlock, String> {
         }
     }
     match id.as_str() {
-        "std::sweep" => Ok(StageBlock::Sweep {
-            metric: require_str(m, "metric", &id)?,
-            select: select_from(m, &id)?,
-            deflate: optional_bool(m, "deflate", &id)?,
-        }),
+        "std::sweep" => {
+            let has_metric = m.contains_key("metric");
+            let has_select = m.contains_key("select");
+            let selection = match (has_metric, has_select) {
+                (true, true) => Some(SweepSelection {
+                    metric: require_str(m, "metric", &id)?,
+                    select: select_from(m, &id)?,
+                    deflate: optional_bool(m, "deflate", &id)?,
+                }),
+                (false, false) => {
+                    if m.contains_key("deflate") {
+                        return Err(format!(
+                            "block {id}: slot \"deflate\" needs the selection \
+                             group (\"metric\" + \"select\")"
+                        ));
+                    }
+                    None
+                }
+                (true, false) => {
+                    return Err(format!(
+                        "block {id}: slot \"metric\" without \"select\" — the \
+                         selection group is all-or-nothing"
+                    ));
+                }
+                (false, true) => {
+                    return Err(format!(
+                        "block {id}: slot \"select\" without \"metric\" — the \
+                         selection group is all-or-nothing"
+                    ));
+                }
+            };
+            Ok(StageBlock::Sweep { selection })
+        }
         "std::gate" => {
             let all = m
                 .get("all")
@@ -634,7 +678,14 @@ pub fn validate_process(doc: &ProcessDoc) -> Vec<DocFault> {
     let mut terminal_seen = false;
     for (i, stage) in doc.pipeline.iter().enumerate() {
         match stage {
-            StageBlock::Sweep { metric, .. } | StageBlock::Generalize { metric } => {
+            StageBlock::Sweep { selection } => {
+                if let Some(sel) = selection
+                    && !is_known_metric(&sel.metric)
+                {
+                    faults.push(DocFault::UnknownMetric { stage: i, metric: sel.metric.clone() });
+                }
+            }
+            StageBlock::Generalize { metric } => {
                 if !is_known_metric(metric) {
                     faults.push(DocFault::UnknownMetric { stage: i, metric: metric.clone() });
                 }
@@ -981,10 +1032,11 @@ mod tests {
         assert_eq!(doc.name, "wf-deflated-screen");
         assert_eq!(doc.pipeline.len(), 3);
         match &doc.pipeline[0] {
-            StageBlock::Sweep { metric, select, deflate } => {
-                assert_eq!(metric, "net_expectancy_r");
-                assert_eq!(*select, SelectRule::PlateauWorst);
-                assert!(*deflate);
+            StageBlock::Sweep { selection } => {
+                let sel = selection.as_ref().expect("fixture sweep carries a selection");
+                assert_eq!(sel.metric, "net_expectancy_r");
+                assert_eq!(sel.select, SelectRule::PlateauWorst);
+                assert!(sel.deflate);
             }
             other => panic!("stage 0 is not a sweep: {other:?}"),
         }
@@ -1262,6 +1314,22 @@ mod tests {
             .contains(&DocFault::GateAfterTerminalStage { stage: 2 }));
     }
 
+    /// The intrinsic tier's metric-validation branch for `std::sweep` must
+    /// skip a `None` selection cleanly (no metric to resolve, so no spurious
+    /// `UnknownMetric`) — the terminal-placement rule itself is the
+    /// executor preflight's job, not this validator's.
+    #[test]
+    fn validate_process_accepts_a_selection_free_sweep() {
+        let doc = ProcessDoc {
+            format_version: FORMAT_VERSION,
+            kind: DocKind::Process,
+            name: "sweep-only".to_string(),
+            description: None,
+            pipeline: vec![StageBlock::Sweep { selection: None }],
+        };
+        assert_eq!(validate_process(&doc), Vec::new());
+    }
+
     #[test]
     fn walk_forward_parses_both_modes_and_refuses_bad_slots() {
         // "anchored" is the other accepted wire mode
@@ -1474,7 +1542,7 @@ mod tests {
             }
         }
         let sweep = describe_block("std::sweep").expect("sweep describable");
-        assert!(sweep.slots.iter().any(|s| s.name == "metric" && s.required));
+        assert!(sweep.slots.iter().any(|s| s.name == "metric" && !s.required));
         assert!(sweep.slots.iter().any(|s| s.name == "deflate" && !s.required));
         let wf = describe_block("std::walk_forward").expect("walk_forward describable");
         let names: Vec<&str> = wf.slots.iter().map(|s| s.name).collect();
@@ -1532,7 +1600,7 @@ mod tests {
             "pipeline": [ { "block": "std::sweep", "metric": "sqn" } ] }"#;
         let slots = open_slots_process(partial).unwrap();
         assert!(slots.iter().any(|s| s.path == "name"));
-        assert!(slots.iter().any(|s| s.path == "pipeline[0].select"));
+        assert!(!slots.iter().any(|s| s.path == "pipeline[0].select"));
         assert!(!slots.iter().any(|s| s.path == "pipeline[0].metric"));
 
         let draft = r#"{ "format_version": 1, "kind": "campaign", "name": "draft",
@@ -1583,4 +1651,52 @@ mod tests {
             "process `ref: {{}}` placeholder not reported as an open slot: {slots:?}"
         );
     }
+
+    #[test]
+    fn sweep_selection_group_is_all_or_nothing() {
+        let free = serde_json::json!({ "block": "std::sweep" });
+        assert_eq!(
+            stage_from_value(&free).unwrap(),
+            StageBlock::Sweep { selection: None }
+        );
+        let metric_only = serde_json::json!({ "block": "std::sweep", "metric": "sqn" });
+        let err = stage_from_value(&metric_only).unwrap_err();
+        assert!(err.contains("all-or-nothing"), "{err}");
+        let select_only = serde_json::json!({ "block": "std::sweep", "select": "argmax" });
+        let err = stage_from_value(&select_only).unwrap_err();
+        assert!(err.contains("all-or-nothing"), "{err}");
+        let deflate_only = serde_json::json!({ "block": "std::sweep", "deflate": true });
+        let err = stage_from_value(&deflate_only).unwrap_err();
+        assert!(err.contains("needs the selection group"), "{err}");
+    }
+
+    #[test]
+    fn selection_free_sweep_round_trips_and_hashes_distinctly() {
+        let doc = ProcessDoc {
+            format_version: FORMAT_VERSION,
+            kind: DocKind::Process,
+            name: "sweep".to_string(),
+            description: None,
+            pipeline: vec![StageBlock::Sweep { selection: None }],
+        };
+        let text = process_to_json(&doc);
+        // the absent group leaves ONLY the block id on the wire
+        assert!(text.contains("\"block\":\"std::sweep\""), "{text}");
+        assert!(!text.contains("metric"), "{text}");
+        assert!(!text.contains("select"), "{text}");
+        let back = parse_process(&text).expect("selection-free sweep parses");
+        assert_eq!(back, doc);
+        // a selected sibling hashes differently
+        let selected = ProcessDoc {
+            pipeline: vec![StageBlock::Sweep {
+                selection: Some(SweepSelection {
+                    metric: "sqn_normalized".to_string(),
+                    select: SelectRule::Argmax,
+                    deflate: false,
+                }),
+            }],
+            ..doc.clone()
+        };
+        assert_ne!(content_id_of(&text), content_id_of(&process_to_json(&selected)));
+    }
 }