diff --git a/src/cli/App.tsx b/src/cli/App.tsx
index 7ce533f..e314801 100644
--- a/src/cli/App.tsx
+++ b/src/cli/App.tsx
@@ -3311,6 +3311,79 @@ ${gitContext}
           return { submitted: true };
         }
 
+        // === Custom command handling ===
+        // Check BEFORE falling through to executeCommand()
+        const { findCustomCommand, substituteArguments, expandBashCommands } =
+          await import("./commands/custom.js");
+        const commandName = trimmed.split(/\s+/)[0]?.slice(1) || ""; // e.g., "review" from "/review arg"
+        const matchedCustom = await findCustomCommand(commandName);
+
+        if (matchedCustom) {
+          const cmdId = uid("cmd");
+
+          // Extract arguments (everything after command name)
+          const args = trimmed.slice(`/${matchedCustom.id}`.length).trim();
+
+          // Build prompt: 1) substitute args, 2) expand bash commands
+          let prompt = substituteArguments(matchedCustom.content, args);
+          prompt = await expandBashCommands(prompt);
+
+          // Show command in transcript (running phase for visual feedback)
+          buffersRef.current.byId.set(cmdId, {
+            kind: "command",
+            id: cmdId,
+            input: trimmed,
+            output: `Running /${matchedCustom.id}...`,
+            phase: "running",
+          });
+          buffersRef.current.order.push(cmdId);
+          refreshDerived();
+
+          setCommandRunning(true);
+
+          try {
+            // Mark command as finished BEFORE sending to agent
+            // (matches /remember pattern - command succeeded in triggering agent)
+            buffersRef.current.byId.set(cmdId, {
+              kind: "command",
+              id: cmdId,
+              input: trimmed,
+              output: `Running custom command...`,
+              phase: "finished",
+              success: true,
+            });
+            refreshDerived();
+
+            // Send prompt to agent
+            // NOTE: Unlike /remember, we DON'T append args separately because
+            // they're already substituted into the prompt via $ARGUMENTS
+            await processConversation([
+              {
+                type: "message",
+                role: "user",
+                content: `<system-reminder>\n${prompt}\n</system-reminder>`,
+              },
+            ]);
+          } catch (error) {
+            // Only catch errors from processConversation setup, not agent execution
+            const errorDetails = formatErrorDetails(error, agentId);
+            buffersRef.current.byId.set(cmdId, {
+              kind: "command",
+              id: cmdId,
+              input: trimmed,
+              output: `Failed to run command: ${errorDetails}`,
+              phase: "finished",
+              success: false,
+            });
+            refreshDerived();
+          } finally {
+            setCommandRunning(false);
+          }
+
+          return { submitted: true };
+        }
+        // === END custom command handling ===
+
         // Immediately add command to transcript with "running" phase
         const cmdId = uid("cmd");
         buffersRef.current.byId.set(cmdId, {
diff --git a/src/cli/commands/custom.ts b/src/cli/commands/custom.ts
new file mode 100644
index 0000000..7a33325
--- /dev/null
+++ b/src/cli/commands/custom.ts
@@ -0,0 +1,259 @@
+/**
+ * Custom slash commands - user-defined commands from .commands/ and ~/.letta/commands/
+ */
+
+import { existsSync } from "node:fs";
+import { readdir, readFile } from "node:fs/promises";
+import { basename, dirname, join } from "node:path";
+import { getStringField, parseFrontmatter } from "../../utils/frontmatter.js";
+
+export const COMMANDS_DIR = ".commands";
+export const GLOBAL_COMMANDS_DIR = join(
+  process.env.HOME || process.env.USERPROFILE || "~",
+  ".letta/commands",
+);
+
+export interface CustomCommand {
+  id: string; // Command name without slash (e.g., "review")
+  description: string; // For autocomplete display
+  argumentHint?: string; // e.g., "[message]" shown after command in autocomplete
+  namespace?: string; // Subdirectory name for disambiguation
+  source: "project" | "user";
+  path: string; // Full path to .md file
+  content: string; // Prompt body (after frontmatter)
+  // Future fields (parsed but not used in MVP):
+  // allowedTools?: string[];
+  // model?: string;
+  // disableModelInvocation?: boolean;
+}
+
+// Cached commands (lazy initialized)
+let cachedCommands: CustomCommand[] | null = null;
+
+/**
+ * Get custom commands (cached after first call)
+ */
+export async function getCustomCommands(): Promise<CustomCommand[]> {
+  if (cachedCommands !== null) {
+    return cachedCommands;
+  }
+  cachedCommands = await discoverCustomCommands();
+  return cachedCommands;
+}
+
+/**
+ * Force refresh of cached commands
+ */
+export function refreshCustomCommands(): void {
+  cachedCommands = null;
+}
+
+/**
+ * Discover custom commands from project and user directories
+ */
+export async function discoverCustomCommands(
+  projectPath: string = join(process.cwd(), COMMANDS_DIR),
+): Promise<CustomCommand[]> {
+  const commandsById = new Map<string, CustomCommand[]>(); // Group by id for collision handling
+
+  // 1. Discover user commands first (lower priority)
+  const userCommands = await discoverFromDirectory(GLOBAL_COMMANDS_DIR, "user");
+  for (const cmd of userCommands) {
+    const existing = commandsById.get(cmd.id) || [];
+    existing.push(cmd);
+    commandsById.set(cmd.id, existing);
+  }
+
+  // 2. Discover project commands (higher priority - may override user)
+  const projectCommands = await discoverFromDirectory(projectPath, "project");
+  for (const cmd of projectCommands) {
+    const existing = commandsById.get(cmd.id) || [];
+    // Insert project commands at front (higher priority)
+    existing.unshift(cmd);
+    commandsById.set(cmd.id, existing);
+  }
+
+  // Flatten to array - keep all commands (for namespace disambiguation)
+  // Note: When executing, we pick the first match (project > user)
+  const result: CustomCommand[] = [];
+  for (const [_id, cmds] of commandsById) {
+    result.push(...cmds);
+  }
+
+  return result;
+}
+
+/**
+ * Discover commands from a single directory
+ */
+async function discoverFromDirectory(
+  dirPath: string,
+  source: "project" | "user",
+): Promise<CustomCommand[]> {
+  if (!existsSync(dirPath)) {
+    return [];
+  }
+
+  const commands: CustomCommand[] = [];
+  await findCommandFiles(dirPath, dirPath, commands, source);
+  return commands;
+}
+
+/**
+ * Recursively find .md files in directory
+ */
+async function findCommandFiles(
+  currentPath: string,
+  rootPath: string,
+  commands: CustomCommand[],
+  source: "project" | "user",
+): Promise<void> {
+  try {
+    const entries = await readdir(currentPath, { withFileTypes: true });
+
+    for (const entry of entries) {
+      const fullPath = join(currentPath, entry.name);
+
+      if (entry.isDirectory()) {
+        await findCommandFiles(fullPath, rootPath, commands, source);
+      } else if (entry.isFile() && entry.name.endsWith(".md")) {
+        try {
+          const cmd = await parseCommandFile(fullPath, rootPath, source);
+          if (cmd) {
+            commands.push(cmd);
+          }
+        } catch (_error) {
+          // Silently skip malformed command files
+          // In future: could track errors in a separate array for debugging
+        }
+      }
+    }
+  } catch (_error) {
+    // Directory read failed - silently continue
+    // This is expected if directory doesn't exist or lacks permissions
+  }
+}
+
+/**
+ * Parse a command markdown file
+ */
+async function parseCommandFile(
+  filePath: string,
+  rootPath: string,
+  source: "project" | "user",
+): Promise<CustomCommand | null> {
+  const content = await readFile(filePath, "utf-8");
+  const { frontmatter, body } = parseFrontmatter(content);
+
+  // Derive command ID from filename (without .md extension)
+  const id = basename(filePath, ".md");
+
+  // Derive namespace from subdirectory path
+  const relativePath = dirname(filePath).slice(rootPath.length);
+  const namespace = relativePath.replace(/^[/\\]/, "") || undefined;
+
+  // Get description from frontmatter or first line of body
+  let description = getStringField(frontmatter, "description");
+  if (!description) {
+    const firstLine = body.split("\n")[0]?.trim();
+    description = firstLine?.replace(/^#\s*/, "") || `Custom command: ${id}`;
+  }
+
+  const argumentHint = getStringField(frontmatter, "argument-hint");
+
+  return {
+    id,
+    description,
+    argumentHint,
+    namespace,
+    source,
+    path: filePath,
+    content: body,
+  };
+}
+
+/**
+ * Substitute arguments in command content
+ */
+export function substituteArguments(content: string, args: string): string {
+  let result = content;
+
+  // Replace $ARGUMENTS with all arguments
+  result = result.replace(/\$ARGUMENTS/g, args);
+
+  // Replace $1, $2, ... $9 with positional arguments
+  const argParts = args.split(/\s+/).filter(Boolean);
+  for (let i = 0; i < 9; i++) {
+    result = result.replace(new RegExp(`\\$${i + 1}`, "g"), argParts[i] || "");
+  }
+
+  return result;
+}
+
+/**
+ * Expand bash commands in content
+ * Replaces !`command` patterns with command output
+ * Uses existing spawnCommand from Bash tool for consistency
+ */
+export async function expandBashCommands(content: string): Promise<string> {
+  // Match !`command` pattern (backticks required)
+  const bashPattern = /!`([^`]+)`/g;
+  const matches = [...content.matchAll(bashPattern)];
+
+  if (matches.length === 0) {
+    return content;
+  }
+
+  // Import spawnCommand from Bash tool (same as bash mode uses)
+  const { spawnCommand } = await import("../../tools/impl/Bash.js");
+  const { getShellEnv } = await import("../../tools/impl/shellEnv.js");
+
+  let result = content;
+
+  // Execute each bash command and replace with output
+  for (const match of matches) {
+    const fullMatch = match[0]; // e.g., !`git status`
+    const command = match[1]; // e.g., git status
+
+    if (!command) continue; // Skip if no capture group match
+
+    try {
+      const cmdResult = await spawnCommand(command, {
+        cwd: process.cwd(),
+        env: getShellEnv(),
+        timeout: 10000, // 10 second timeout for inline commands
+      });
+
+      const output = (cmdResult.stdout + cmdResult.stderr).trim();
+      result = result.replace(fullMatch, output);
+    } catch (error) {
+      // On error, replace with error message
+      const errMsg = error instanceof Error ? error.message : String(error);
+      result = result.replace(
+        fullMatch,
+        `[Error executing ${command}: ${errMsg}]`,
+      );
+    }
+  }
+
+  return result;
+}
+
+/**
+ * Find a custom command by name (handles namespace disambiguation)
+ * Returns the highest priority match (project > user, then first namespace)
+ */
+export async function findCustomCommand(
+  commandName: string, // e.g., "review" or "frontend/test"
+): Promise<CustomCommand | undefined> {
+  const commands = await getCustomCommands();
+
+  // First try exact id match
+  const exactMatches = commands.filter((cmd) => cmd.id === commandName);
+  if (exactMatches.length > 0) {
+    // Return project command if available, else user
+    return exactMatches.find((c) => c.source === "project") || exactMatches[0];
+  }
+
+  return undefined;
+}
diff --git a/src/cli/components/HelpDialog.tsx b/src/cli/components/HelpDialog.tsx
index 72fe865..689a625 100644
--- a/src/cli/components/HelpDialog.tsx
+++ b/src/cli/components/HelpDialog.tsx
@@ -1,5 +1,5 @@
 import { Box, Text, useInput } from "ink";
-import { useCallback, useMemo, useState } from "react";
+import { useCallback, useEffect, useMemo, useState } from "react";
 import { getVersion } from "../../version";
 import { commands } from "../commands/registry";
 import { colors } from "./colors";
@@ -28,18 +28,34 @@ export function HelpDialog({ onClose }: HelpDialogProps) {
   const [activeTab, setActiveTab] = useState<HelpTab>("commands");
   const [currentPage, setCurrentPage] = useState(0);
   const [selectedIndex, setSelectedIndex] = useState(0);
+  const [customCommands, setCustomCommands] = useState<CommandItem[]>([]);
 
-  // Get all non-hidden commands, sorted by order
+  // Load custom commands once on mount
+  useEffect(() => {
+    import("../commands/custom.js").then(({ getCustomCommands }) => {
+      getCustomCommands().then((customs) => {
+        setCustomCommands(
+          customs.map((cmd) => ({
+            name: `/${cmd.id}`,
+            description: `${cmd.description} (${cmd.source}${cmd.namespace ? `:${cmd.namespace}` : ""})`,
+            order: 200 + (cmd.source === "project" ? 0 : 100),
+          })),
+        );
+      });
+    });
+  }, []);
+
+  // Get all non-hidden commands, sorted by order (includes custom commands)
   const allCommands = useMemo<CommandItem[]>(() => {
-    return Object.entries(commands)
+    const builtins = Object.entries(commands)
       .filter(([_, cmd]) => !cmd.hidden)
       .map(([name, cmd]) => ({
         name,
         description: cmd.desc,
         order: cmd.order ?? 100,
-      }))
-      .sort((a, b) => a.order - b.order);
-  }, []);
+      }));
+    return [...builtins, ...customCommands].sort((a, b) => a.order - b.order);
+  }, [customCommands]);
 
   // Keyboard shortcuts
   const shortcuts = useMemo<ShortcutItem[]>(() => {
diff --git a/src/cli/components/SlashCommandAutocomplete.tsx b/src/cli/components/SlashCommandAutocomplete.tsx
index d127d4f..efff385 100644
--- a/src/cli/components/SlashCommandAutocomplete.tsx
+++ b/src/cli/components/SlashCommandAutocomplete.tsx
@@ -51,37 +51,60 @@ export function SlashCommandAutocomplete({
   workingDirectory = process.cwd(),
 }: AutocompleteProps) {
   const [matches, setMatches] = useState<CommandMatch[]>([]);
+  const [customCommands, setCustomCommands] = useState<CommandMatch[]>([]);
 
-  // Check pin status to conditionally show/hide pin/unpin commands
-  const allCommands = useMemo(() => {
-    if (!agentId) return _allCommands;
-
-    try {
-      const globalPinned = settingsManager.getGlobalPinnedAgents();
-      const localPinned =
-        settingsManager.getLocalPinnedAgents(workingDirectory);
-
-      const isPinnedGlobally = globalPinned.includes(agentId);
-      const isPinnedLocally = localPinned.includes(agentId);
-      const isPinnedAnywhere = isPinnedGlobally || isPinnedLocally;
-      const isPinnedBoth = isPinnedGlobally && isPinnedLocally;
-
-      return _allCommands.filter((cmd) => {
-        // Hide /pin if agent is pinned both locally AND globally
-        if (cmd.cmd === "/pin" && isPinnedBoth) {
-          return false;
-        }
-        // Hide /unpin if agent is not pinned anywhere
-        if (cmd.cmd === "/unpin" && !isPinnedAnywhere) {
-          return false;
-        }
-        return true;
+  // Load custom commands once on mount
+  useEffect(() => {
+    import("../commands/custom.js").then(({ getCustomCommands }) => {
+      getCustomCommands().then((customs) => {
+        const matches: CommandMatch[] = customs.map((cmd) => ({
+          cmd: `/${cmd.id}`,
+          // Include source/namespace in description for disambiguation
+          desc: `${cmd.description} (${cmd.source}${cmd.namespace ? `:${cmd.namespace}` : ""})`,
+          order: 200 + (cmd.source === "project" ? 0 : 100),
+        }));
+        setCustomCommands(matches);
       });
-    } catch (_error) {
-      // If settings aren't loaded, just show all commands
-      return _allCommands;
+    });
+  }, []);
+
+  // Check pin status to conditionally show/hide pin/unpin commands, merge with custom commands
+  const allCommands = useMemo(() => {
+    let builtins = _allCommands;
+
+    if (agentId) {
+      try {
+        const globalPinned = settingsManager.getGlobalPinnedAgents();
+        const localPinned =
+          settingsManager.getLocalPinnedAgents(workingDirectory);
+
+        const isPinnedGlobally = globalPinned.includes(agentId);
+        const isPinnedLocally = localPinned.includes(agentId);
+        const isPinnedAnywhere = isPinnedGlobally || isPinnedLocally;
+        const isPinnedBoth = isPinnedGlobally && isPinnedLocally;
+
+        builtins = _allCommands.filter((cmd) => {
+          // Hide /pin if agent is pinned both locally AND globally
+          if (cmd.cmd === "/pin" && isPinnedBoth) {
+            return false;
+          }
+          // Hide /unpin if agent is not pinned anywhere
+          if (cmd.cmd === "/unpin" && !isPinnedAnywhere) {
+            return false;
+          }
+          return true;
+        });
+      } catch (_error) {
+        // If settings aren't loaded, just use all builtins
+        builtins = _allCommands;
+      }
     }
-  }, [agentId, workingDirectory]);
+
+    // Merge with custom commands and sort by order
+    return [...builtins, ...customCommands].sort(
+      (a, b) => (a.order ?? 100) - (b.order ?? 100),
+    );
+  }, [agentId, workingDirectory, customCommands]);
 
   const { selectedIndex } = useAutocompleteNavigation({
     matches,