diff --git a/control_plane/src/planner_selection.rs b/control_plane/src/planner_selection.rs
index 0b2fc92c..fe902cb4 100644
--- a/control_plane/src/planner_selection.rs
+++ b/control_plane/src/planner_selection.rs
@@ -28,6 +28,30 @@ pub enum SelectionError {
     NoLegalCandidate,
     #[error("ASAPPlanner sketch strategy produced a logical rewrite instead of a summary")]
     UnexpectedRewrite,
+    /// A root offered materially different legal alternatives and the cost
+    /// model priced none of them. Selection must not resolve that group from
+    /// candidate discovery order — registration order is not optimizer policy.
+    #[error(
+        "cost model reported no comparable cost for target {target_id}: \
+         {candidate_count} legal alternatives ({strategies}) are unpriced, so selection \
+         has no evidence to rank them"
+    )]
+    CostUnavailable {
+        target_id: String,
+        candidate_count: usize,
+        strategies: String,
+        /// Per-candidate diagnostic, mirroring the selection trace entries.
+        candidates: Vec<CostUnavailableCandidate>,
+    },
+}
+
+/// One unpriced alternative in a [`SelectionError::CostUnavailable`] group.
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize)]
+pub struct CostUnavailableCandidate {
+    pub candidate_id: String,
+    pub strategy: String,
+    pub replacement_kind: String,
+    pub provenance: String,
 }
 
 /// Versioned diagnostic identity over existing canonical IR, never Rc or rank IDs.
@@ -210,6 +234,17 @@ pub fn archive_only(intent: &AggIntent) -> bool {
     )
 }
 
+/// Whether this alternative keeps the target pre-ASAP — the raw/exact
+/// fallback the planner emits when an intent has no summary realization.
+/// Matched on the post-ASAP IR rather than on rationale text, so the check
+/// survives rewording upstream.
+fn keeps_pre_asap(candidate: &asap_aware_mapping::ReplacementSubDAG) -> bool {
+    let Replacement::Summary(node) = &candidate.replacement else {
+        return false;
+    };
+    matches!(node.expr, SummaryExpr::KeepPreAsap(_))
+}
+
 /// Preserve an unsupported subtree explicitly at the post-ASAP boundary.
 pub fn keep_pre_asap(expr: &QueryExpr) -> Result<Rc<SummaryNode>, SelectionError> {
     let schema = expr.output_schema()?;
@@ -427,6 +462,75 @@ fn select_workload_impl(
         }).collect::<Vec<_>>();
         *trace = serde_json::json!({ "schema_version": 1, "group_id_scope": "this_selection", "groups": groups });
     }
+    // A group with two or more materially different legal alternatives and no
+    // comparable cost cannot be resolved on evidence. Selection would fall back
+    // to candidate discovery order, which makes strategy registration order an
+    // undeclared optimizer policy — so fail with the candidate identities and
+    // let the caller supply costs or pick a documented policy instead.
+    if let Some(unpriced) = space
+        .cost_sorted(cost_model)
+        .iter()
+        .find(|group| {
+            group.candidates.len() > 1
+                && group
+                    .candidates
+                    .iter()
+                    .all(|candidate| cost_model.candidate_cost(candidate, &TargetSubDAG::with_consumer_count(group.target, group.consumer_count)).is_none_or(|cost| !cost.0.is_finite()))
+                // The complaint is specifically a *silent raw fallback*: the
+                // discovery-order winner keeps the subtree pre-ASAP while a
+                // realizable alternative sits behind it, unranked. A group
+                // whose order-chosen candidate is already realizable is not
+                // resolved by registration order in any way a reader would
+                // call raw, so it keeps planning.
+                && keeps_pre_asap(group.candidates[0])
+                && group.candidates[1..]
+                    .iter()
+                    .any(|candidate| !keeps_pre_asap(candidate))
+        })
+        .map(|group| {
+            let candidates = group
+                .candidates
+                .iter()
+                .map(|candidate| CostUnavailableCandidate {
+                    candidate_id: replacement_identity(
+                        group.target,
+                        &candidate.replacement,
+                        &accuracy,
+                    )
+                    .unwrap_or_else(|| "unidentified".into()),
+                    strategy: candidate.strategy.to_string(),
+                    replacement_kind: match &candidate.replacement {
+                        Replacement::Summary(_) => "summary",
+                        Replacement::Rewrite(_) => "rewrite",
+                        Replacement::ExactComposition(_) => "exact_composition",
+                    }
+                    .to_string(),
+                    provenance: format!("{:?}", candidate.provenance),
+                })
+                .collect::<Vec<_>>();
+            SelectionError::CostUnavailable {
+                target_id: target_identity(group.target, &accuracy)
+                    .unwrap_or_else(|| "unidentified".into()),
+                candidate_count: candidates.len(),
+                strategies: candidates
+                    .iter()
+                    .map(|candidate| candidate.strategy.as_str())
+                    .collect::<Vec<_>>()
+                    .join(", "),
+                candidates,
+            }
+        })
+    {
+        if let Some(trace) = trace.as_deref_mut() {
+            trace["unresolved_group"] = serde_json::json!({
+                "reason": "cost_unavailable",
+                "policy": "fail_loudly",
+                "detail": unpriced.to_string(),
+            });
+        }
+        return Err(unpriced);
+    }
+
     let roots = space
         .roots
         .iter()
@@ -819,3 +923,154 @@ mod workload_tests {
         );
     }
 }
+
+#[cfg(test)]
+mod cost_unavailable_selection {
+    use super::*;
+    use crate::physical::post_asap::cost_model::ControlPlaneCostModel;
+
+    fn select(
+        query: &str,
+    ) -> Result<(Vec<(usize, Rc<SummaryNode>)>, serde_json::Value), SelectionError> {
+        let accuracy = AccuracyTarget::Epsilon(0.05);
+        let root =
+            crate::query_parser::parse_query_expr_canonical(query, accuracy.clone()).unwrap();
+        select_workload_with_accuracy_model_and_trace(
+            vec![(0, Rc::new(root))],
+            accuracy.clone(),
+            &ControlPlaneCostModel::new(accuracy),
+            &asap_aware_mapping::NoAccuracyEvidence,
+            &asap_aware_mapping::DefaultAccuracyModel,
+        )
+    }
+
+    /// `avg by (job) (data)` offers two materially different legal
+    /// alternatives for the same root: `SketchAlgorithmStrategy`'s `Avg`
+    /// pass-through (no summary realization exists, so it degrades to raw)
+    /// and `SemanticEquivalentRewriteStrategy`'s realizable `sum / count`
+    /// rewrite. Neither is priced.
+    ///
+    /// Before this guard, selection preserved candidate discovery order, and
+    /// the deployment registers the sketch strategy first — so the raw
+    /// pass-through won without any cost evidence, making strategy
+    /// registration order an undeclared optimizer policy.
+    #[test]
+    fn unpriced_alternatives_fail_instead_of_resolving_on_discovery_order() {
+        let error = select("avg by (job) (data)").expect_err("must not silently select a root");
+        let SelectionError::CostUnavailable {
+            target_id,
+            candidate_count,
+            candidates,
+            ..
+        } = &error
+        else {
+            panic!("expected a typed cost-unavailable error, got: {error}");
+        };
+        assert!(target_id.starts_with("asap-explain-v1:target:"), "{error}");
+        assert_eq!(*candidate_count, 2, "{error}");
+
+        // The diagnostic has to name both alternatives, so a reader can tell
+        // which inputs the cost model owes rather than only that ranking failed.
+        let strategies: Vec<&str> = candidates
+            .iter()
+            .map(|candidate| candidate.strategy.as_str())
+            .collect();
+        assert!(
+            strategies.contains(&"SketchAlgorithmStrategy")
+                && strategies.contains(&"SemanticEquivalentRewriteStrategy"),
+            "{strategies:?}"
+        );
+        let kinds: Vec<&str> = candidates
+            .iter()
+            .map(|candidate| candidate.replacement_kind.as_str())
+            .collect();
+        assert!(
+            kinds.contains(&"summary") && kinds.contains(&"rewrite"),
+            "the group must be materially different, not two rankings of one shape: {kinds:?}"
+        );
+        assert!(
+            candidates.iter().all(|candidate| candidate
+                .candidate_id
+                .starts_with("asap-explain-v1:candidate:")),
+            "{candidates:?}"
+        );
+    }
+
+    /// A root whose alternatives the model does price still plans. The guard
+    /// must fire on missing evidence, not on every unpriced candidate.
+    #[test]
+    fn priced_roots_still_select() {
+        let (roots, trace) =
+            select("quantile_over_time(0.9, m[1m])").expect("a priced root still plans");
+        assert_eq!(roots.len(), 1);
+        assert!(
+            trace.get("unresolved_group").is_none(),
+            "a resolved selection must not carry an unresolved-group diagnostic: {trace}"
+        );
+    }
+}
+
+#[cfg(test)]
+mod probe_721_scope {
+    use super::*;
+    use crate::physical::post_asap::cost_model::ControlPlaneCostModel;
+
+    #[test]
+    fn survey() {
+        let queries = [
+            "avg by (job) (data)",
+            "rate(asap_demo_counter_total[5s])",
+            "increase(asap_demo_counter_total[5s])",
+            "sum(sum_over_time(asap_demo_gauge[5s]))",
+            "quantile_over_time(0.5, asap_demo_latency_ms[5s])",
+            "topk(1, sum_over_time(asap_demo_gauge[5s]))",
+            "topk(1, count_over_time(asap_demo_gauge[5s]))",
+            "sum by (zone) (http_requests_total)",
+            "count_over_time(m[1m])",
+        ];
+        let accuracy = AccuracyTarget::Epsilon(0.05);
+        for q in queries {
+            let Ok(root) = crate::query_parser::parse_query_expr_canonical(q, accuracy.clone())
+            else {
+                eprintln!("SURVEY {q} -> parse error");
+                continue;
+            };
+            let cost_model = ControlPlaneCostModel::new(accuracy.clone());
+            let strategies = replacement_strategies(
+                &cost_model,
+                &asap_aware_mapping::NoAccuracyEvidence,
+                &asap_aware_mapping::DefaultAccuracyModel,
+            );
+            let space = asap_aware_mapping::search_workload_with_targets(
+                vec![(0usize, Rc::new(root), Some(accuracy.clone()))],
+                &strategies,
+                &asap_aware_mapping::DefaultAccuracyModel,
+            );
+            let mut mixed_unpriced = 0;
+            let mut rank0_passthrough = 0;
+            for group in space.cost_sorted(&cost_model) {
+                if group.candidates.len() < 2 {
+                    continue;
+                }
+                let target = TargetSubDAG::with_consumer_count(group.target, group.consumer_count);
+                let all_unpriced = group.candidates.iter().all(|c| {
+                    cost_model
+                        .candidate_cost(c, &target)
+                        .is_none_or(|x| !x.0.is_finite())
+                });
+                let kinds: std::collections::HashSet<_> = group
+                    .candidates
+                    .iter()
+                    .map(|c| std::mem::discriminant(&c.replacement))
+                    .collect();
+                if all_unpriced && kinds.len() > 1 {
+                    mixed_unpriced += 1;
+                    if group.candidates[0].rationale.contains("pass-through") {
+                        rank0_passthrough += 1;
+                    }
+                }
+            }
+            eprintln!("SURVEY {q} -> mixed_unpriced_groups={mixed_unpriced} rank0_is_passthrough={rank0_passthrough}");
+        }
+    }
+}
diff --git a/data_plane/tests/support/current_series_process.rs b/data_plane/tests/support/current_series_process.rs
index cd383322..cc10b4e2 100644
--- a/data_plane/tests/support/current_series_process.rs
+++ b/data_plane/tests/support/current_series_process.rs
@@ -37,7 +37,12 @@ async fn current_series_quantiles_topk_share_and_replace_values() {
         queries.push(format!("topk({k}, a)"));
         queries.push(format!("topk by (job) ({k}, a)"));
     }
-    for operation in ["sum", "count", "avg"] {
+    // `avg` has no summary realization, so its root offers a raw pass-through
+    // and a realizable sum/count rewrite with no comparable cost between them.
+    // Planning now fails loudly on that rather than resolving it from strategy
+    // registration order (#721); the behaviour is asserted in
+    // `control_plane::planner_selection`'s cost-unavailable regression tests.
+    for operation in ["sum", "count"] {
         queries.push(format!("{operation}(a)"));
         queries.push(format!("{operation} by (job) (a)"));
     }
@@ -252,7 +257,7 @@ async fn current_series_quantiles_topk_share_and_replace_values() {
             "{body}"
         );
     }
-    for operation in ["sum", "count", "avg"] {
+    for operation in ["sum", "count"] {
         for text in [
             format!("{operation}(a)"),
             format!("{operation} by (job) (a)"),
@@ -314,7 +319,7 @@ async fn current_series_quantiles_topk_share_and_replace_values() {
             &body,
         )
         .await;
-        for operation in ["sum", "count", "avg"] {
+        for operation in ["sum", "count"] {
             for text in [
                 format!("{operation}(a)"),
                 format!("{operation} by (job) (a)"),
diff --git a/data_plane/tests/support/issue_701_702_process.rs b/data_plane/tests/support/issue_701_702_process.rs
index 9a0420f5..b732116d 100644
--- a/data_plane/tests/support/issue_701_702_process.rs
+++ b/data_plane/tests/support/issue_701_702_process.rs
@@ -52,10 +52,12 @@ fn queries() -> Vec<(String, u64, u64)> {
         ));
         queries.push((format!("quantile by(job)({q}, issue701_data)"), 1, 1));
     }
-    for operation in ["sum", "count", "avg", "min", "max"] {
+    // `avg` is excluded: it plans to a raw pass-through against a realizable
+    // rewrite with no comparable cost, which now fails loudly (#721).
+    for operation in ["sum", "count", "min", "max"] {
         queries.push((format!("{operation}_over_time(issue701_data[5m])"), 300, 30));
     }
-    for operation in ["sum", "count", "avg"] {
+    for operation in ["sum", "count"] {
         queries.push((format!("{operation}(issue701_data)"), 1, 1));
         queries.push((format!("{operation} by(job)(issue701_data)"), 1, 1));
     }
@@ -77,11 +79,6 @@ fn queries() -> Vec<(String, u64, u64)> {
         300,
         60,
     ));
-    queries.push((
-        "avg_over_time(issue701_data[5m]) / quantile_over_time(0.5, issue701_data[5m])".into(),
-        300,
-        30,
-    ));
     queries
 }