diff --git a/src/browser/components/ChatInputToasts.tsx b/src/browser/components/ChatInputToasts.tsx
index 89cb5a7245..f1304828b9 100644
--- a/src/browser/components/ChatInputToasts.tsx
+++ b/src/browser/components/ChatInputToasts.tsx
@@ -130,6 +130,34 @@ export const createCommandToast = (parsed: ParsedCommand): Toast | null => {
         ),
       };
 
+    case "command-missing-args":
+      return {
+        id: Date.now().toString(),
+        type: "error",
+        title: "Missing Arguments",
+        message: `/${parsed.command} requires arguments`,
+        solution: (
+          <>
+            <SolutionLabel>Usage:</SolutionLabel>
+            {parsed.usage}
+          </>
+        ),
+      };
+
+    case "command-invalid-args":
+      return {
+        id: Date.now().toString(),
+        type: "error",
+        title: "Invalid Argument",
+        message: `'${parsed.input}' is not valid for /${parsed.command}`,
+        solution: (
+          <>
+            <SolutionLabel>Usage:</SolutionLabel>
+            {parsed.usage}
+          </>
+        ),
+      };
+
     case "unknown-command": {
       const cmd = "/" + parsed.command + (parsed.subcommand ? " " + parsed.subcommand : "");
       return {
diff --git a/src/browser/utils/slashCommands/registry.ts b/src/browser/utils/slashCommands/registry.ts
index e55ecbaada..6ed0bc8e01 100644
--- a/src/browser/utils/slashCommands/registry.ts
+++ b/src/browser/utils/slashCommands/registry.ts
@@ -186,23 +186,26 @@ const clearCommandDefinition: SlashCommandDefinition = {
   },
 };
 
+const TRUNCATE_USAGE = "/truncate <0-100> (percentage to remove)";
+
 const truncateCommandDefinition: SlashCommandDefinition = {
   key: "truncate",
   description: "Truncate conversation history by percentage (0-100)",
   handler: ({ cleanRemainingTokens }): ParsedCommand => {
     if (cleanRemainingTokens.length === 0) {
       return {
-        type: "unknown-command",
+        type: "command-missing-args",
         command: "truncate",
-        subcommand: undefined,
+        usage: TRUNCATE_USAGE,
       };
     }
 
     if (cleanRemainingTokens.length > 1) {
       return {
-        type: "unknown-command",
+        type: "command-invalid-args",
         command: "truncate",
-        subcommand: cleanRemainingTokens[1],
+        input: cleanRemainingTokens.join(" "),
+        usage: TRUNCATE_USAGE,
       };
     }
 
@@ -212,9 +215,10 @@ const truncateCommandDefinition: SlashCommandDefinition = {
 
     if (isNaN(pct) || pct < 0 || pct > 100) {
       return {
-        type: "unknown-command",
+        type: "command-invalid-args",
         command: "truncate",
-        subcommand: pctStr,
+        input: pctStr,
+        usage: TRUNCATE_USAGE,
       };
     }
 
@@ -632,6 +636,8 @@ function parseMCPNameCommand(
   return { name, command };
 }
 
+const IDLE_USAGE = "/idle <hours> or /idle off";
+
 const idleCommandDefinition: SlashCommandDefinition = {
   key: "idle",
   description: "Configure idle compaction for this project. Usage: /idle <hours> or /idle off",
@@ -639,9 +645,9 @@ const idleCommandDefinition: SlashCommandDefinition = {
   handler: ({ cleanRemainingTokens }): ParsedCommand => {
     if (cleanRemainingTokens.length === 0) {
       return {
-        type: "unknown-command",
+        type: "command-missing-args",
         command: "idle",
-        subcommand: undefined,
+        usage: IDLE_USAGE,
       };
     }
 
@@ -655,9 +661,10 @@ const idleCommandDefinition: SlashCommandDefinition = {
     const hours = parseInt(arg, 10);
     if (isNaN(hours) || hours < 1) {
       return {
-        type: "unknown-command",
+        type: "command-invalid-args",
         command: "idle",
-        subcommand: arg,
+        input: arg,
+        usage: IDLE_USAGE,
       };
     }
 
@@ -678,7 +685,11 @@ const mcpCommandDefinition: SlashCommandDefinition = {
     if (sub === "add" || sub === "edit") {
       const parsed = parseMCPNameCommand(sub, cleanRemainingTokens, rawInput);
       if (!parsed) {
-        return { type: "unknown-command", command: "mcp", subcommand: sub };
+        return {
+          type: "command-missing-args",
+          command: `mcp ${sub}`,
+          usage: `/mcp ${sub} <name> <command>`,
+        };
       }
       return { type: sub === "add" ? "mcp-add" : "mcp-edit", ...parsed };
     }
@@ -686,7 +697,11 @@ const mcpCommandDefinition: SlashCommandDefinition = {
     if (sub === "remove") {
       const name = cleanRemainingTokens[1];
       if (!name) {
-        return { type: "unknown-command", command: "mcp", subcommand: "remove" };
+        return {
+          type: "command-missing-args",
+          command: "mcp remove",
+          usage: "/mcp remove <server-name>",
+        };
       }
       return { type: "mcp-remove", name };
     }
diff --git a/src/browser/utils/slashCommands/types.ts b/src/browser/utils/slashCommands/types.ts
index 26b8680550..c5a684f51c 100644
--- a/src/browser/utils/slashCommands/types.ts
+++ b/src/browser/utils/slashCommands/types.ts
@@ -37,6 +37,8 @@ export type ParsedCommand =
   | { type: "plan-open" }
   | { type: "init" }
   | { type: "unknown-command"; command: string; subcommand?: string }
+  | { type: "command-missing-args"; command: string; usage: string }
+  | { type: "command-invalid-args"; command: string; input: string; usage: string }
   | { type: "idle-compaction"; hours: number | null }
   | null;