feat(runtime): reuse session reservations inside the registry

Extend the runtime reservation contract with an optional owner id so
repeated acquire calls can converge on the same backend-owned reservation
inside pantograph-runtime-registry instead of depending on adapter-local
contains-check guards.

Add owner-conflict detection when the same owner attempts to bind to a different
runtime, persist the owner id on reservation leases, and update the embedded
workflow host to pass the session id as the reservation owner while removing
its duplicate-load precheck.

Update the runtime-registry README and Milestone 3 implementation plan to
record that session-load reuse now belongs to the backend registry boundary.

Verification:
- cargo test -p pantograph-runtime-registry
- cargo test -p pantograph-embedded-runtime keep_alive_session_load_tracks_registry_reservation_lifecycle -- --nocapture
- cargo check --manifest-path src-tauri/Cargo.toml

Author: MrScripty

IMPLEMENTATION-PLAN-pantograph-runtime-registry-technical-fit-selection.md

@@ -452,6 +452,10 @@ runtime callers.
 - 2026-04-13: The registry now also exposes an idempotent reservation-release
   path so overlapping cleanup or retry flows can remain concurrency-safe
   without adapter-local duplicate-release suppression logic.
+- 2026-04-13: Session runtime acquire now has a backend-owned owner-key reuse
+  path so repeated session loads can converge on the same reservation inside
+  the registry lock instead of depending on adapter-local duplicate-load
+  guards.
 
 **Verification:**
 - `cargo test -p pantograph-runtime-registry`

crates/pantograph-embedded-runtime/src/lib.rs

@@ -769,17 +769,6 @@ impl EmbeddedWorkflowHost {
             return Ok(());
         };
 
-        {
-            let reservations = self.session_runtime_reservations.lock().map_err(|_| {
-                WorkflowServiceError::Internal(
-                    "session runtime reservation lock poisoned".to_string(),
-                )
-            })?;
-            if reservations.contains_key(session_id) {
-                return Ok(());
-            }
-        }
-
         let mode_info = self.gateway.mode_info().await;
         let runtime_id = mode_info
             .active_runtime
@@ -807,6 +796,7 @@ impl EmbeddedWorkflowHost {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id,
                 workflow_id: workflow_id.to_string(),
+                reservation_owner_id: Some(session_id.to_string()),
                 usage_profile: Self::trimmed_optional(usage_profile),
                 model_id: mode_info.active_model_target.clone(),
                 pin_runtime: false,

crates/pantograph-runtime-registry/src/README.md

@@ -67,6 +67,9 @@ desktop app wiring.
 - Hosts may supply runtime admission budgets and reservation memory
   requirements, but the resulting acceptance or rejection decision belongs to
   this crate.
+- Hosts may also supply a stable reservation owner id when repeated acquire
+  calls should reuse the same backend-owned reservation instead of duplicating
+  session-local prechecks in adapters.
 - Hosts may also supply a reservation retention hint such as keep-alive intent,
   but this crate remains the owner of how that hint affects retention
   disposition and future eviction policy.

crates/pantograph-runtime-registry/src/lib.rs

@@ -47,6 +47,15 @@ pub enum RuntimeRegistryError {
     #[error("runtime '{0}' cannot accept reservations while stopping or failed")]
     ReservationRejected(String),
 
+    #[error(
+        "reservation owner '{owner_id}' is already bound to runtime '{existing_runtime_id}', not '{requested_runtime_id}'"
+    )]
+    ReservationOwnerConflict {
+        owner_id: String,
+        existing_runtime_id: String,
+        requested_runtime_id: String,
+    },
+
     #[error("runtime '{runtime_id}' admission rejected reservation: {failure}")]
     AdmissionRejected {
         runtime_id: String,
@@ -202,6 +211,23 @@ impl RuntimeRegistry {
             .state
             .lock()
             .expect("runtime registry state lock poisoned");
+        if let Some(owner_id) = request.reservation_owner_id.as_deref() {
+            if let Some(existing_reservation) = guard
+                .reservations
+                .values()
+                .find(|reservation| reservation.reservation_owner_id.as_deref() == Some(owner_id))
+            {
+                if existing_reservation.runtime_id == runtime_id {
+                    return Ok(existing_reservation.clone().into_lease());
+                }
+
+                return Err(RuntimeRegistryError::ReservationOwnerConflict {
+                    owner_id: owner_id.to_string(),
+                    existing_runtime_id: existing_reservation.runtime_id.clone(),
+                    requested_runtime_id: runtime_id,
+                });
+            }
+        }
         let record = guard
             .runtimes
             .get(&runtime_id)
@@ -231,6 +257,7 @@ impl RuntimeRegistry {
             reservation_id,
             runtime_id: runtime_id.clone(),
             workflow_id: request.workflow_id,
+            reservation_owner_id: request.reservation_owner_id,
             usage_profile: request.usage_profile,
             model_id: request.model_id,
             pin_runtime: request.pin_runtime,
@@ -704,6 +731,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "onnxruntime".to_string(),
                 workflow_id: "wf-1".to_string(),
+                reservation_owner_id: None,
                 usage_profile: Some("audio".to_string()),
                 model_id: Some("model-a".to_string()),
                 pin_runtime: true,
@@ -776,6 +804,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "llama.cpp".to_string(),
                 workflow_id: "wf-budget".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: None,
                 pin_runtime: false,
@@ -824,6 +853,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "llama.cpp".to_string(),
                 workflow_id: "wf-stop".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: None,
                 pin_runtime: false,
@@ -957,6 +987,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "reserved-ready".to_string(),
                 workflow_id: "wf-reserved".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: Some("model-b".to_string()),
                 pin_runtime: false,
@@ -1007,6 +1038,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "shared-runtime".to_string(),
                 workflow_id: "wf-keep-alive".to_string(),
+                reservation_owner_id: None,
                 usage_profile: Some("interactive".to_string()),
                 model_id: Some("model-a".to_string()),
                 pin_runtime: false,
@@ -1018,6 +1050,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "shared-runtime".to_string(),
                 workflow_id: "wf-ephemeral".to_string(),
+                reservation_owner_id: None,
                 usage_profile: Some("batch".to_string()),
                 model_id: Some("model-a".to_string()),
                 pin_runtime: false,
@@ -1090,6 +1123,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "idempotent-runtime".to_string(),
                 workflow_id: "wf-idempotent".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: Some("model-a".to_string()),
                 pin_runtime: false,
@@ -1112,6 +1146,86 @@ mod tests {
         );
     }
 
+    #[test]
+    fn acquire_reservation_reuses_existing_owner_binding() {
+        let registry = RuntimeRegistry::new();
+        registry.observe_runtimes(vec![
+            RuntimeObservation {
+                runtime_id: "owner-runtime-a".to_string(),
+                display_name: "owner-runtime-a".to_string(),
+                backend_keys: vec!["llama_cpp".to_string()],
+                model_id: Some("model-a".to_string()),
+                status: RuntimeRegistryStatus::Ready,
+                runtime_instance_id: Some("owner-runtime-a-1".to_string()),
+                last_error: None,
+            },
+            RuntimeObservation {
+                runtime_id: "owner-runtime-b".to_string(),
+                display_name: "owner-runtime-b".to_string(),
+                backend_keys: vec!["llama_cpp".to_string()],
+                model_id: Some("model-b".to_string()),
+                status: RuntimeRegistryStatus::Ready,
+                runtime_instance_id: Some("owner-runtime-b-1".to_string()),
+                last_error: None,
+            },
+        ]);
+
+        let first = registry
+            .acquire_reservation(RuntimeReservationRequest {
+                runtime_id: "owner-runtime-a".to_string(),
+                workflow_id: "wf-owner".to_string(),
+                reservation_owner_id: Some("session-owner".to_string()),
+                usage_profile: Some("interactive".to_string()),
+                model_id: Some("model-a".to_string()),
+                pin_runtime: false,
+                requirements: None,
+                retention_hint: RuntimeRetentionHint::KeepAlive,
+            })
+            .expect("first owner reservation");
+
+        let reused = registry
+            .acquire_reservation(RuntimeReservationRequest {
+                runtime_id: "owner-runtime-a".to_string(),
+                workflow_id: "wf-owner".to_string(),
+                reservation_owner_id: Some("session-owner".to_string()),
+                usage_profile: None,
+                model_id: None,
+                pin_runtime: false,
+                requirements: None,
+                retention_hint: RuntimeRetentionHint::Ephemeral,
+            })
+            .expect("second owner reservation should reuse existing lease");
+
+        assert_eq!(first.reservation_id, reused.reservation_id);
+        assert_eq!(
+            reused.reservation_owner_id.as_deref(),
+            Some("session-owner")
+        );
+        assert_eq!(registry.snapshot().reservations.len(), 1);
+
+        let err = registry
+            .acquire_reservation(RuntimeReservationRequest {
+                runtime_id: "owner-runtime-b".to_string(),
+                workflow_id: "wf-owner".to_string(),
+                reservation_owner_id: Some("session-owner".to_string()),
+                usage_profile: None,
+                model_id: None,
+                pin_runtime: false,
+                requirements: None,
+                retention_hint: RuntimeRetentionHint::Ephemeral,
+            })
+            .expect_err("owner should not bind to another runtime");
+
+        assert_eq!(
+            err,
+            RuntimeRegistryError::ReservationOwnerConflict {
+                owner_id: "session-owner".to_string(),
+                existing_runtime_id: "owner-runtime-a".to_string(),
+                requested_runtime_id: "owner-runtime-b".to_string(),
+            }
+        );
+    }
+
     #[test]
     fn eviction_candidates_use_deterministic_status_and_age_ordering() {
         let registry = RuntimeRegistry::new();
@@ -1187,6 +1301,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "llama.cpp".to_string(),
                 workflow_id: "wf-1".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: Some("model-a".to_string()),
                 pin_runtime: false,
@@ -1204,6 +1319,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "llama.cpp".to_string(),
                 workflow_id: "wf-2".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: Some("model-b".to_string()),
                 pin_runtime: false,
@@ -1253,6 +1369,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "pytorch".to_string(),
                 workflow_id: "wf-ram-1".to_string(),
+                reservation_owner_id: None,
                 usage_profile: Some("interactive".to_string()),
                 model_id: Some("model-ram-a".to_string()),
                 pin_runtime: false,
@@ -1270,6 +1387,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "pytorch".to_string(),
                 workflow_id: "wf-ram-2".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: Some("model-ram-b".to_string()),
                 pin_runtime: false,
@@ -1305,6 +1423,7 @@ mod tests {
             .acquire_reservation(RuntimeReservationRequest {
                 runtime_id: "pytorch".to_string(),
                 workflow_id: "wf-ram-3".to_string(),
+                reservation_owner_id: None,
                 usage_profile: None,
                 model_id: Some("model-ram-c".to_string()),
                 pin_runtime: false,

crates/pantograph-runtime-registry/src/reservation.rs

@@ -15,6 +15,8 @@ pub struct RuntimeReservationRequest {
     pub runtime_id: String,
     pub workflow_id: String,
     #[serde(default)]
+    pub reservation_owner_id: Option<String>,
+    #[serde(default)]
     pub usage_profile: Option<String>,
     #[serde(default)]
     pub model_id: Option<String>,
@@ -33,6 +35,8 @@ pub struct RuntimeReservationLease {
     pub runtime_id: String,
     pub workflow_id: String,
     #[serde(default)]
+    pub reservation_owner_id: Option<String>,
+    #[serde(default)]
     pub usage_profile: Option<String>,
     #[serde(default)]
     pub model_id: Option<String>,
@@ -47,6 +51,7 @@ pub(crate) struct RuntimeReservationRecord {
     pub reservation_id: u64,
     pub runtime_id: String,
     pub workflow_id: String,
+    pub reservation_owner_id: Option<String>,
     pub usage_profile: Option<String>,
     pub model_id: Option<String>,
     pub pin_runtime: bool,
@@ -61,6 +66,7 @@ impl RuntimeReservationRecord {
             reservation_id: self.reservation_id,
             runtime_id: self.runtime_id,
             workflow_id: self.workflow_id,
+            reservation_owner_id: self.reservation_owner_id,
             usage_profile: self.usage_profile,
             model_id: self.model_id,
             pin_runtime: self.pin_runtime,