diff --git a/crates/aura-registry/src/compat.rs b/crates/aura-registry/src/compat.rs
new file mode 100644
index 0000000..374991c
--- /dev/null
+++ b/crates/aura-registry/src/compat.rs
@@ -0,0 +1,77 @@
+//! Back-compatible read shapes for the C18 flat runs store.
+//!
+//! [`Registry::load`](crate::Registry::load) is the durable read-path and must
+//! stay tolerant of the param-value wire shape across the cycle-0047 typed-`Scalar`
+//! migration (`86746e3`). The current format tags each param value as an
+//! externally-tagged [`Scalar`] (`{"F64":2.0}` / `{"I64":10}`); a `runs.jsonl`
+//! line written *before* that migration carries the value as a **bare JSON
+//! number** (`2.0`). These mirror types deserialize either shape and convert into
+//! the canonical [`RunReport`] — the read side only. The forward write path
+//! (`Registry::append`, `RunReport::to_json`) is untouched and still emits the
+//! tagged form, so this is a one-directional widening, not a format change.
+
+use aura_engine::{RunManifest, RunMetrics, RunReport, Scalar, Timestamp};
+use serde::de::{self, Deserializer};
+use serde::Deserialize;
+
+/// A `RunReport` whose param values read back from either the tagged or the
+/// legacy bare-float shape. Field-for-field structural mirror of `RunReport`;
+/// only [`ScalarRead`] differs from the engine type.
+#[derive(Deserialize)]
+pub(crate) struct RunReportRead {
+    manifest: RunManifestRead,
+    metrics: RunMetrics,
+}
+
+#[derive(Deserialize)]
+struct RunManifestRead {
+    commit: String,
+    params: Vec<(String, ScalarRead)>,
+    window: (Timestamp, Timestamp),
+    seed: u64,
+    broker: String,
+}
+
+/// A param value that accepts BOTH the current tagged [`Scalar`] form
+/// (`{"F64":2.0}` / `{"I64":10}`) and the legacy pre-0047 bare JSON number
+/// (`2.0`, coerced to `Scalar::F64` per the documented `f64` coercion).
+struct ScalarRead(Scalar);
+
+impl<'de> Deserialize<'de> for ScalarRead {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // serde_json's untagged `Value` lets us peek the shape without committing
+        // the parse: an object/string deserializes as the tagged `Scalar`; a bare
+        // number is the legacy form, coerced to `Scalar::f64`.
+        let value = serde_json::Value::deserialize(deserializer)?;
+        match value {
+            serde_json::Value::Number(n) => {
+                let f = n.as_f64().ok_or_else(|| {
+                    de::Error::custom(format!("legacy bare-float param value out of f64 range: {n}"))
+                })?;
+                Ok(ScalarRead(Scalar::f64(f)))
+            }
+            other => Scalar::deserialize(other)
+                .map(ScalarRead)
+                .map_err(de::Error::custom),
+        }
+    }
+}
+
+impl From<RunReportRead> for RunReport {
+    fn from(r: RunReportRead) -> Self {
+        let RunManifestRead { commit, params, window, seed, broker } = r.manifest;
+        RunReport {
+            manifest: RunManifest {
+                commit,
+                params: params.into_iter().map(|(name, v)| (name, v.0)).collect(),
+                window,
+                seed,
+                broker,
+            },
+            metrics: r.metrics,
+        }
+    }
+}
diff --git a/crates/aura-registry/src/lib.rs b/crates/aura-registry/src/lib.rs
index d847202..a455fdc 100644
--- a/crates/aura-registry/src/lib.rs
+++ b/crates/aura-registry/src/lib.rs
@@ -19,6 +19,8 @@ use std::path::{Path, PathBuf};
 
 use aura_engine::{RunReport, SweepFamily, SweepPoint};
 
+mod compat;
+
 mod lineage;
 pub use lineage::{
     group_families, mc_member_reports, sweep_member_reports, walkforward_member_reports, Family,
@@ -64,8 +66,13 @@ impl Registry {
             if raw.trim().is_empty() {
                 continue;
             }
-            let report =
-                serde_json::from_str(raw).map_err(|source| RegistryError::Parse { line: i + 1, source })?;
+            // Read via the back-compat mirror: tolerant of both the current
+            // tagged-`Scalar` param shape (`{"F64":2.0}`) and the legacy pre-0047
+            // bare-float shape (`2.0`). The forward write path (`append`,
+            // `RunReport::to_json`) is unchanged — it still emits the tagged form.
+            let report: RunReport = serde_json::from_str::<compat::RunReportRead>(raw)
+                .map_err(|source| RegistryError::Parse { line: i + 1, source })?
+                .into();
             reports.push(report);
         }
         Ok(reports)