diff --git a/src/gateway/server-methods/agent.ts b/src/gateway/server-methods/agent.ts
index ee08425b7fd..5a7507345df 100644
--- a/src/gateway/server-methods/agent.ts
+++ b/src/gateway/server-methods/agent.ts
@@ -50,8 +50,7 @@ import { performGatewaySessionReset } from "../session-reset-service.js";
 import {
   canonicalizeSpawnedByForAgent,
   loadSessionEntry,
-  pruneLegacyStoreKeys,
-  resolveGatewaySessionStoreTarget,
+  migrateAndPruneGatewaySessionStoreKey,
 } from "../session-utils.js";
 import { formatForLog } from "../ws-log.js";
 import { waitForAgentJob } from "./agent-job.js";
@@ -425,18 +424,13 @@ export const agentHandlers: GatewayRequestHandlers = {
       const mainSessionKey = resolveAgentMainSessionKey({ cfg, agentId });
       if (storePath) {
         const persisted = await updateSessionStore(storePath, (store) => {
-          const target = resolveGatewaySessionStoreTarget({
+          const { primaryKey } = migrateAndPruneGatewaySessionStoreKey({
             cfg,
             key: requestedSessionKey,
             store,
           });
-          pruneLegacyStoreKeys({
-            store,
-            canonicalKey: target.canonicalKey,
-            candidates: target.storeKeys,
-          });
-          const merged = mergeSessionEntry(store[canonicalSessionKey], nextEntryPatch);
-          store[canonicalSessionKey] = merged;
+          const merged = mergeSessionEntry(store[primaryKey], nextEntryPatch);
+          store[primaryKey] = merged;
           return merged;
         });
         sessionEntry = persisted;
diff --git a/src/gateway/server-methods/sessions.ts b/src/gateway/server-methods/sessions.ts
index f2e3817bfa6..d5244116d33 100644
--- a/src/gateway/server-methods/sessions.ts
+++ b/src/gateway/server-methods/sessions.ts
@@ -31,7 +31,7 @@ import {
   listSessionsFromStore,
   loadCombinedSessionStoreForGateway,
   loadSessionEntry,
-  pruneLegacyStoreKeys,
+  migrateAndPruneGatewaySessionStoreKey,
   readSessionPreviewItemsFromTranscript,
   resolveGatewaySessionStoreTarget,
   resolveSessionModelRef,
@@ -92,31 +92,6 @@ function rejectWebchatSessionMutation(params: {
   return true;
 }
 
-function migrateAndPruneSessionStoreKey(params: {
-  cfg: ReturnType<typeof loadConfig>;
-  key: string;
-  store: Record<string, SessionEntry>;
-}) {
-  const target = resolveGatewaySessionStoreTarget({
-    cfg: params.cfg,
-    key: params.key,
-    store: params.store,
-  });
-  const primaryKey = target.canonicalKey;
-  if (!params.store[primaryKey]) {
-    const existingKey = target.storeKeys.find((candidate) => Boolean(params.store[candidate]));
-    if (existingKey) {
-      params.store[primaryKey] = params.store[existingKey];
-    }
-  }
-  pruneLegacyStoreKeys({
-    store: params.store,
-    canonicalKey: primaryKey,
-    candidates: target.storeKeys,
-  });
-  return { target, primaryKey, entry: params.store[primaryKey] };
-}
-
 export const sessionsHandlers: GatewayRequestHandlers = {
   "sessions.list": ({ params, respond }) => {
     if (!assertValidParams(params, validateSessionsListParams, "sessions.list", respond)) {
@@ -224,7 +199,7 @@ export const sessionsHandlers: GatewayRequestHandlers = {
 
     const { cfg, target, storePath } = resolveGatewaySessionTargetFromKey(key);
     const applied = await updateSessionStore(storePath, async (store) => {
-      const { primaryKey } = migrateAndPruneSessionStoreKey({ cfg, key, store });
+      const { primaryKey } = migrateAndPruneGatewaySessionStoreKey({ cfg, key, store });
       return await applySessionsPatchToStore({
         cfg,
         store,
@@ -316,7 +291,7 @@ export const sessionsHandlers: GatewayRequestHandlers = {
     }
     const sessionId = entry?.sessionId;
     const deleted = await updateSessionStore(storePath, (store) => {
-      const { primaryKey } = migrateAndPruneSessionStoreKey({ cfg, key, store });
+      const { primaryKey } = migrateAndPruneGatewaySessionStoreKey({ cfg, key, store });
       const hadEntry = Boolean(store[primaryKey]);
       if (hadEntry) {
         delete store[primaryKey];
@@ -385,7 +360,7 @@ export const sessionsHandlers: GatewayRequestHandlers = {
     const { cfg, target, storePath } = resolveGatewaySessionTargetFromKey(key);
     // Lock + read in a short critical section; transcript work happens outside.
     const compactTarget = await updateSessionStore(storePath, (store) => {
-      const { entry, primaryKey } = migrateAndPruneSessionStoreKey({ cfg, key, store });
+      const { entry, primaryKey } = migrateAndPruneGatewaySessionStoreKey({ cfg, key, store });
       return { entry, primaryKey };
     });
     const entry = compactTarget.entry;
diff --git a/src/gateway/server-node-events.ts b/src/gateway/server-node-events.ts
index b36ca9aca50..8ab24644101 100644
--- a/src/gateway/server-node-events.ts
+++ b/src/gateway/server-node-events.ts
@@ -16,11 +16,7 @@ import { defaultRuntime } from "../runtime.js";
 import { parseMessageWithAttachments } from "./chat-attachments.js";
 import { normalizeRpcAttachmentsToChatAttachments } from "./server-methods/attachment-normalize.js";
 import type { NodeEvent, NodeEventContext } from "./server-node-events-types.js";
-import {
-  loadSessionEntry,
-  pruneLegacyStoreKeys,
-  resolveGatewaySessionStoreTarget,
-} from "./session-utils.js";
+import { loadSessionEntry, migrateAndPruneGatewaySessionStoreKey } from "./session-utils.js";
 import { formatForLog } from "./ws-log.js";
 
 const MAX_EXEC_EVENT_OUTPUT_CHARS = 180;
@@ -152,17 +148,12 @@ async function touchSessionStore(params: {
     return;
   }
   await updateSessionStore(storePath, (store) => {
-    const target = resolveGatewaySessionStoreTarget({
+    const { primaryKey } = migrateAndPruneGatewaySessionStoreKey({
       cfg: params.cfg,
       key: params.sessionKey,
       store,
     });
-    pruneLegacyStoreKeys({
-      store,
-      canonicalKey: target.canonicalKey,
-      candidates: target.storeKeys,
-    });
-    store[params.canonicalKey] = {
+    store[primaryKey] = {
       sessionId: params.sessionId,
       updatedAt: params.now,
       thinkingLevel: params.entry?.thinkingLevel,
diff --git a/src/gateway/session-reset-service.ts b/src/gateway/session-reset-service.ts
index 15b9a0aa37f..b0a5b0a54f0 100644
--- a/src/gateway/session-reset-service.ts
+++ b/src/gateway/session-reset-service.ts
@@ -25,38 +25,13 @@ import { ErrorCodes, errorShape } from "./protocol/index.js";
 import {
   archiveSessionTranscripts,
   loadSessionEntry,
-  pruneLegacyStoreKeys,
+  migrateAndPruneGatewaySessionStoreKey,
   resolveGatewaySessionStoreTarget,
   resolveSessionModelRef,
 } from "./session-utils.js";
 
 const ACP_RUNTIME_CLEANUP_TIMEOUT_MS = 15_000;
 
-function migrateAndPruneSessionStoreKey(params: {
-  cfg: ReturnType<typeof loadConfig>;
-  key: string;
-  store: Record<string, SessionEntry>;
-}) {
-  const target = resolveGatewaySessionStoreTarget({
-    cfg: params.cfg,
-    key: params.key,
-    store: params.store,
-  });
-  const primaryKey = target.canonicalKey;
-  if (!params.store[primaryKey]) {
-    const existingKey = target.storeKeys.find((candidate) => Boolean(params.store[candidate]));
-    if (existingKey) {
-      params.store[primaryKey] = params.store[existingKey];
-    }
-  }
-  pruneLegacyStoreKeys({
-    store: params.store,
-    canonicalKey: primaryKey,
-    candidates: target.storeKeys,
-  });
-  return { target, primaryKey, entry: params.store[primaryKey] };
-}
-
 function stripRuntimeModelState(entry?: SessionEntry): SessionEntry | undefined {
   if (!entry) {
     return entry;
@@ -311,7 +286,11 @@ export async function performGatewaySessionReset(params: {
   let oldSessionId: string | undefined;
   let oldSessionFile: string | undefined;
   const next = await updateSessionStore(storePath, (store) => {
-    const { primaryKey } = migrateAndPruneSessionStoreKey({ cfg, key: params.key, store });
+    const { primaryKey } = migrateAndPruneGatewaySessionStoreKey({
+      cfg,
+      key: params.key,
+      store,
+    });
     const currentEntry = store[primaryKey];
     const resetEntry = stripRuntimeModelState(currentEntry);
     const parsed = parseAgentSessionKey(primaryKey);
diff --git a/src/gateway/session-utils.ts b/src/gateway/session-utils.ts
index 591799879b9..00a2cb7747e 100644
--- a/src/gateway/session-utils.ts
+++ b/src/gateway/session-utils.ts
@@ -263,6 +263,31 @@ export function pruneLegacyStoreKeys(params: {
   }
 }
 
+export function migrateAndPruneGatewaySessionStoreKey(params: {
+  cfg: ReturnType<typeof loadConfig>;
+  key: string;
+  store: Record<string, SessionEntry>;
+}) {
+  const target = resolveGatewaySessionStoreTarget({
+    cfg: params.cfg,
+    key: params.key,
+    store: params.store,
+  });
+  const primaryKey = target.canonicalKey;
+  if (!params.store[primaryKey]) {
+    const existingKey = target.storeKeys.find((candidate) => Boolean(params.store[candidate]));
+    if (existingKey) {
+      params.store[primaryKey] = params.store[existingKey];
+    }
+  }
+  pruneLegacyStoreKeys({
+    store: params.store,
+    canonicalKey: primaryKey,
+    candidates: target.storeKeys,
+  });
+  return { target, primaryKey, entry: params.store[primaryKey] };
+}
+
 export function classifySessionKey(key: string, entry?: SessionEntry): GatewaySessionRow["kind"] {
   if (key === "global") {
     return "global";
diff --git a/src/gateway/sessions-resolve.ts b/src/gateway/sessions-resolve.ts
index 21b6779573c..47ca47b86e3 100644
--- a/src/gateway/sessions-resolve.ts
+++ b/src/gateway/sessions-resolve.ts
@@ -10,7 +10,7 @@ import {
 import {
   listSessionsFromStore,
   loadCombinedSessionStoreForGateway,
-  pruneLegacyStoreKeys,
+  migrateAndPruneGatewaySessionStoreKey,
   resolveGatewaySessionStoreTarget,
 } from "./session-utils.js";
 
@@ -58,13 +58,10 @@ export async function resolveSessionKeyFromResolveParams(params: {
       };
     }
     await updateSessionStore(target.storePath, (s) => {
-      const liveTarget = resolveGatewaySessionStoreTarget({ cfg, key, store: s });
-      const canonicalKey = liveTarget.canonicalKey;
-      // Migrate the first legacy entry to the canonical key.
-      if (!s[canonicalKey] && s[legacyKey]) {
-        s[canonicalKey] = s[legacyKey];
+      const { primaryKey } = migrateAndPruneGatewaySessionStoreKey({ cfg, key, store: s });
+      if (!s[primaryKey] && s[legacyKey]) {
+        s[primaryKey] = s[legacyKey];
       }
-      pruneLegacyStoreKeys({ store: s, canonicalKey, candidates: liveTarget.storeKeys });
     });
     return { ok: true, key: target.canonicalKey };
   }