diff --git a/src/agent/check-approval.ts b/src/agent/check-approval.ts
index c4be991..d105c4a 100644
--- a/src/agent/check-approval.ts
+++ b/src/agent/check-approval.ts
@@ -474,23 +474,24 @@ export async function getResumeData(
       const retrievedMessages = await client.messages.retrieve(lastInContextId);
 
       // Fetch message history for backfill through the default conversation route.
-      // Default conversation is represented by the agent id at the conversations endpoint.
+      // Default conversation uses the "default" sentinel plus agent_id as a query param.
       // Wrapped in try/catch so backfill failures don't crash the CLI (e.g., older servers
       // may not support this pattern)
       if (includeMessageHistory && isBackfillEnabled()) {
         try {
           const messagesPage = await client.conversations.messages.list(
-            agent.id,
+            "default",
             {
               limit: BACKFILL_PAGE_LIMIT,
               order: "desc",
+              agent_id: agent.id,
             },
           );
           messages = sortChronological(messagesPage.getPaginatedItems());
 
           if (process.env.DEBUG) {
             console.log(
-              `[DEBUG] conversations.messages.list(${agent.id}) returned ${messages.length} messages`,
+              `[DEBUG] conversations.messages.list(default, agent_id=${agent.id}) returned ${messages.length} messages`,
             );
           }
         } catch (backfillError) {
diff --git a/src/cli/components/ConversationSelector.tsx b/src/cli/components/ConversationSelector.tsx
index fa7fea2..72ecde8 100644
--- a/src/cli/components/ConversationSelector.tsx
+++ b/src/cli/components/ConversationSelector.tsx
@@ -243,11 +243,11 @@ export function ConversationSelector({
         if (!afterCursor) {
           try {
             const defaultMessages = await client.conversations.messages.list(
-              // Default conversation is represented by the agent id at the conversations endpoint.
-              agentId,
+              "default",
               {
                 limit: 20,
                 order: "desc",
+                agent_id: agentId,
               },
             );
             const defaultMsgItems = defaultMessages.getPaginatedItems();
diff --git a/src/cli/subcommands/messages.ts b/src/cli/subcommands/messages.ts
index 03874de..193770c 100644
--- a/src/cli/subcommands/messages.ts
+++ b/src/cli/subcommands/messages.ts
@@ -159,12 +159,12 @@ export async function runMessagesSubcommand(argv: string[]): Promise<number> {
         return 1;
       }
 
-      // Default conversation is represented by the agent id at the conversations endpoint.
-      const response = await client.conversations.messages.list(agentId, {
+      const response = await client.conversations.messages.list("default", {
         limit: parseLimit(parsed.values.limit, 20),
         after: parsed.values.after,
         before: parsed.values.before,
         order,
+        agent_id: agentId,
       });
 
       const messages = response.getPaginatedItems() ?? [];