diff --git a/src/cli/App.tsx b/src/cli/App.tsx
index f4c1388..d6eb9fb 100644
--- a/src/cli/App.tsx
+++ b/src/cli/App.tsx
@@ -1250,6 +1250,10 @@ export default function App({
     llmConfigRef.current = llmConfig;
   }, [llmConfig]);
   const [currentModelId, setCurrentModelId] = useState<string | null>(null);
+  // Full model handle for API calls (e.g., "anthropic/claude-sonnet-4-5-20251101")
+  const [currentModelHandle, setCurrentModelHandle] = useState<string | null>(
+    null,
+  );
   // Derive agentName from agentState (single source of truth)
   const agentName = agentState?.name ?? null;
   const [agentDescription, setAgentDescription] = useState<string | null>(null);
@@ -2605,6 +2609,8 @@ export default function App({
           } else {
             setCurrentModelId(agentModelHandle || null);
           }
+          // Store full handle for API calls (e.g., compaction)
+          setCurrentModelHandle(agentModelHandle || null);
 
           // Derive toolset from agent's model (not persisted, computed on resume)
           if (agentModelHandle) {
@@ -6423,10 +6429,28 @@ export default function App({
         }
 
         // Special handling for /compact command - summarize conversation history
-        if (msg.trim() === "/compact") {
+        // Supports: /compact, /compact all, /compact sliding_window
+        if (msg.trim().startsWith("/compact")) {
+          const parts = msg.trim().split(/\s+/);
+          const modeArg = parts[1] as "all" | "sliding_window" | undefined;
+          const validModes = ["all", "sliding_window"];
+
+          // Validate mode if provided
+          if (modeArg && !validModes.includes(modeArg)) {
+            const cmd = commandRunner.start(
+              msg.trim(),
+              `Invalid mode "${modeArg}". Valid modes: ${validModes.join(", ")}`,
+            );
+            cmd.fail(
+              `Invalid mode "${modeArg}". Valid modes: ${validModes.join(", ")}`,
+            );
+            return { submitted: true };
+          }
+
+          const modeDisplay = modeArg ? ` (mode: ${modeArg})` : "";
           const cmd = commandRunner.start(
             msg.trim(),
-            "Compacting conversation history...",
+            `Compacting conversation history${modeDisplay}...`,
           );
 
           setCommandRunning(true);
@@ -6448,18 +6472,38 @@ export default function App({
             }
 
             const client = await getClient();
+
+            // Compute model handle from llmConfig
+            const modelHandle =
+              llmConfig?.model_endpoint_type && llmConfig?.model
+                ? `${llmConfig.model_endpoint_type}/${llmConfig.model}`
+                : llmConfig?.model || null;
+
+            // Build compaction settings if mode was specified
+            // Pass mode-specific prompt to override any agent defaults
+            const compactParams =
+              modeArg && modelHandle
+                ? {
+                    compaction_settings: {
+                      mode: modeArg,
+                      model: modelHandle,
+                    },
+                  }
+                : undefined;
+
             // Use agent-level compact API for "default" conversation,
             // otherwise use conversation-level API
             const result =
               conversationIdRef.current === "default"
-                ? await client.agents.messages.compact(agentId)
+                ? await client.agents.messages.compact(agentId, compactParams)
                 : await client.conversations.messages.compact(
                     conversationIdRef.current,
+                    compactParams,
                   );
 
             // Format success message with before/after counts and summary
             const outputLines = [
-              `Compaction completed. Message buffer length reduced from ${result.num_messages_before} to ${result.num_messages_after}.`,
+              `Compaction completed${modeDisplay}. Message buffer length reduced from ${result.num_messages_before} to ${result.num_messages_after}.`,
               "",
               `Summary: ${result.summary}`,
             ];
diff --git a/src/cli/commands/registry.ts b/src/cli/commands/registry.ts
index c4341ab..0e34a55 100644
--- a/src/cli/commands/registry.ts
+++ b/src/cli/commands/registry.ts
@@ -366,7 +366,8 @@ export const commands: Record<string, Command> = {
     },
   },
   "/compact": {
-    desc: "Summarize conversation history (compaction)",
+    desc: "Summarize conversation history (compaction) with optional mode",
+    args: "[all|sliding_window]",
     handler: () => {
       // Handled specially in App.tsx to access client and agent ID
       return "Compacting conversation...";