feat(web): /skill slash command with autocomplete

Trigger /<name>, dropdown lists all skills filtered by name prefix,
arg passthrough sends the rest as the user message. Synthetic
skill_use tool_use renders identically to model-invoked skills.

apps/web/src/components/ChatInput.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useRef, useState, type DragEvent, type KeyboardEvent } from 'react';
+import { useCallback, useEffect, useMemo, useRef, useState, type DragEvent, type KeyboardEvent } from 'react';
 import { Check, Plus, Send } from 'lucide-react';
 import { toast } from 'sonner';
 import { Textarea } from '@/components/ui/textarea';
@@ -22,8 +22,10 @@ import { AttachmentPreviewModal } from '@/components/AttachmentPreviewModal';
 import { FileMentionPopover } from '@/components/FileMentionPopover';
 import { DropOverlay } from '@/components/DropOverlay';
 import { AgentPicker } from '@/components/AgentPicker';
+import { SkillSlashCommand } from '@/components/SkillSlashCommand';
 import { api } from '@/api/client';
 import { sessionEvents } from '@/hooks/sessionEvents';
+import { useSkills } from '@/hooks/useSkills';
 import { useViewport } from '@/hooks/useViewport';
 
 const MAX_ATTACHMENTS = 10;
@@ -44,9 +46,14 @@ interface Props {
   webSearchEnabled?: boolean | null;
   onSend: (content: string) => void | Promise<void>;
   onForceSend?: (content: string) => void | Promise<void>;
+  // Batch 9.6: slash-command dispatch. When the input parses to a known skill,
+  // ChatInput calls this with the skill name + the post-name args (possibly
+  // empty). Callers wire this to api.chats.skillInvoke. Omitting the prop
+  // disables slash-command dispatch (input is sent as literal text).
+  onSlashCommand?: (skillName: string, userMessage: string) => void | Promise<void>;
 }
 
-export function ChatInput({ disabled, projectId, agentId, onAgentChange, sessionId, webSearchEnabled, onSend, onForceSend }: Props) {
+export function ChatInput({ disabled, projectId, agentId, onAgentChange, sessionId, webSearchEnabled, onSend, onForceSend, onSlashCommand }: Props) {
   const { isMobile } = useViewport();
   const [value, setValue] = useState('');
   const [busy, setBusy] = useState(false);
@@ -61,6 +68,19 @@ export function ChatInput({ disabled, projectId, agentId, onAgentChange, session
     atIdx: number;
     anchorRect: { top: number; left: number };
   } | null>(null);
+  // Batch 9.6: slash-command dropdown. Opens when `/` is the first char of
+  // the input and stays open while the input is `/<word>` with no whitespace.
+  // Disabled entirely when the caller doesn't pass onSlashCommand.
+  const [slashState, setSlashState] = useState<{
+    query: string;
+    anchorRect: { top: number; left: number };
+  } | null>(null);
+  const { skills } = useSkills();
+  const skillsLookup = useMemo(() => {
+    const m = new Map<string, true>();
+    for (const s of skills) m.set(s.name, true);
+    return m;
+  }, [skills]);
   const [fileIndex, setFileIndex] = useState<string[] | null>(null);
   const textareaRef = useRef<HTMLTextAreaElement | null>(null);
 
@@ -95,6 +115,31 @@ export function ChatInput({ disabled, projectId, agentId, onAgentChange, session
     const text = value.trim();
     if (!text && attachments.length === 0) return;
     if (disabled || busy) return;
+
+    // Batch 9.6: slash-command dispatch. Only when no attachments and the
+    // input parses to a known skill. Falls through to onSend for unknown
+    // slash names (literal text) or when slash dispatch isn't wired.
+    if (onSlashCommand && attachments.length === 0 && text.startsWith('/')) {
+      const match = text.match(/^\/(\S+)\s*([\s\S]*)$/);
+      if (match && skillsLookup.has(match[1]!)) {
+        const skillName = match[1]!;
+        const args = (match[2] ?? '').trim();
+        setBusy(true);
+        try {
+          await onSlashCommand(skillName, args);
+          setValue('');
+          setAttachments([]);
+          setSlashState(null);
+        } catch (err) {
+          toast.error(err instanceof Error ? err.message : 'skill invocation failed');
+        } finally {
+          setBusy(false);
+        }
+        return;
+      }
+      // Unknown skill name — fall through and send as literal text.
+    }
+
     setBusy(true);
     try {
       const body = flattenToMessage(attachments, text);
@@ -108,6 +153,19 @@ export function ChatInput({ disabled, projectId, agentId, onAgentChange, session
     }
   }
 
+  function handleSlashSelect(skillName: string) {
+    const next = `/${skillName} `;
+    setValue(next);
+    setSlashState(null);
+    requestAnimationFrame(() => {
+      const ta = textareaRef.current;
+      if (ta) {
+        ta.selectionStart = ta.selectionEnd = next.length;
+        ta.focus();
+      }
+    });
+  }
+
   function getCaretCoords(textarea: HTMLTextAreaElement): { top: number; left: number } {
     const mirror = document.createElement('div');
     const style = window.getComputedStyle(textarea);
@@ -158,6 +216,23 @@ export function ChatInput({ disabled, projectId, agentId, onAgentChange, session
     const ta = e.target;
     const pos = ta.selectionStart;
 
+    // Batch 9.6: slash-command trigger. Active while the input is a single
+    // slash-prefixed token with no whitespace (i.e. user is still typing the
+    // skill name). Hand off to args mode the moment a space appears or the
+    // slash leaves position 0.
+    if (onSlashCommand && /^\/[^\s]*$/.test(newValue)) {
+      const query = newValue.slice(1);
+      if (!slashState) {
+        const rect = ta.getBoundingClientRect();
+        setSlashState({ query, anchorRect: { top: rect.top, left: rect.left } });
+      } else if (slashState.query !== query) {
+        setSlashState({ ...slashState, query });
+      }
+      if (mentionState?.open) setMentionState(null);
+      return;
+    }
+    if (slashState) setSlashState(null);
+
     // Check for @ trigger
     if (pos > 0 && newValue[pos - 1] === '@') {
       const charBefore = pos >= 2 ? newValue[pos - 2] : null;
@@ -374,6 +449,9 @@ export function ChatInput({ disabled, projectId, agentId, onAgentChange, session
 
   function onKeyDown(e: KeyboardEvent<HTMLTextAreaElement>) {
     if (mentionState?.open) return;
+    // SkillSlashCommand owns Arrow/Enter/Tab/Esc via a document listener; let
+    // it consume them so the textarea doesn't also submit on Enter.
+    if (slashState) return;
     // IME safety: never act on Enter while an IME composition is in flight
     // (CJK input methods commit composition via Enter). Without this, the
     // first Enter of a Japanese/Chinese/Korean composition would submit
@@ -524,6 +602,15 @@ export function ChatInput({ disabled, projectId, agentId, onAgentChange, session
           onClose={closeMention}
         />
       )}
+      {slashState && (
+        <SkillSlashCommand
+          query={slashState.query}
+          skills={skills}
+          anchorRect={slashState.anchorRect}
+          onSelect={handleSlashSelect}
+          onClose={() => setSlashState(null)}
+        />
+      )}
     </div>
   );
 }

apps/web/src/components/SkillSlashCommand.tsx

@@ -0,0 +1,137 @@
+import { useEffect, useMemo, useRef, useState } from 'react';
+import { cn } from '@/lib/utils';
+import type { Skill } from '@/api/types';
+
+interface Props {
+  query: string;
+  skills: Skill[];
+  anchorRect: { top: number; left: number };
+  onSelect: (skillName: string) => void;
+  onClose: () => void;
+}
+
+// Batch 9.6: slash-command dropdown. Models FileMentionPopover's pattern —
+// fixed-positioned popover, keyboard nav, click-outside-to-close. shadcn
+// `Command` (cmdk) isn't installed in this project; per the addendum we use
+// a plain div + Tailwind instead of pulling a new primitive autonomously.
+
+// Case-insensitive prefix match on `name` only. Description is display-only
+// in v1 (substring search across description is deferred to a polish batch).
+function filterByPrefix(skills: Skill[], query: string): Skill[] {
+  const q = query.toLowerCase();
+  const filtered = q
+    ? skills.filter((s) => s.name.toLowerCase().startsWith(q))
+    : skills;
+  // Stable alphabetical ordering matches the server's cache order (skills.ts
+  // sorts on name asc) but we re-sort here so a stale client cache doesn't
+  // surprise the user.
+  return [...filtered].sort((a, b) => a.name.localeCompare(b.name));
+}
+
+export function SkillSlashCommand({ query, skills, anchorRect, onSelect, onClose }: Props) {
+  const [highlightIndex, setHighlightIndex] = useState(0);
+  const popoverRef = useRef<HTMLDivElement>(null);
+  const filtered = useMemo(() => filterByPrefix(skills, query), [skills, query]);
+
+  useEffect(() => { setHighlightIndex(0); }, [query]);
+
+  // Arrow / Enter / Tab / Escape. Bound on document so keystrokes from the
+  // textarea reach the popover even though focus stays in the textarea.
+  useEffect(() => {
+    function handleKeyDown(e: KeyboardEvent) {
+      if (e.key === 'ArrowDown') {
+        e.preventDefault();
+        setHighlightIndex((prev) => (prev < filtered.length - 1 ? prev + 1 : 0));
+      } else if (e.key === 'ArrowUp') {
+        e.preventDefault();
+        setHighlightIndex((prev) => (prev > 0 ? prev - 1 : filtered.length - 1));
+      } else if (e.key === 'Enter' || e.key === 'Tab') {
+        if (filtered.length === 0) return;
+        e.preventDefault();
+        const target = filtered[highlightIndex] ?? filtered[0];
+        if (target) onSelect(target.name);
+      } else if (e.key === 'Escape') {
+        e.preventDefault();
+        onClose();
+      }
+    }
+    document.addEventListener('keydown', handleKeyDown);
+    return () => document.removeEventListener('keydown', handleKeyDown);
+  }, [filtered, highlightIndex, onSelect, onClose]);
+
+  useEffect(() => {
+    function handleMouseDown(e: MouseEvent) {
+      if (popoverRef.current && !popoverRef.current.contains(e.target as Node)) {
+        onClose();
+      }
+    }
+    document.addEventListener('mousedown', handleMouseDown);
+    return () => document.removeEventListener('mousedown', handleMouseDown);
+  }, [onClose]);
+
+  useEffect(() => {
+    const el = popoverRef.current?.querySelector('[data-highlighted="true"]');
+    if (el) el.scrollIntoView({ block: 'nearest' });
+  }, [highlightIndex]);
+
+  // Anchor sits above the input — translate(-100%) on Y so the dropdown
+  // expands upward from the anchor point rather than over the textarea.
+  const style = {
+    top: anchorRect.top,
+    left: anchorRect.left,
+    transform: 'translateY(-100%)',
+  } as const;
+
+  if (filtered.length === 0) {
+    return (
+      <div
+        ref={popoverRef}
+        className="fixed z-50 bg-popover border border-border rounded-md shadow min-w-[320px] p-2"
+        style={style}
+      >
+        <div className="text-xs text-muted-foreground px-2 py-1">
+          {query ? `No skill starts with "/${query}"` : 'No skills available'}
+        </div>
+      </div>
+    );
+  }
+
+  return (
+    <div
+      ref={popoverRef}
+      className="fixed z-50 bg-popover border border-border rounded-md shadow min-w-[320px] max-w-[420px] max-h-[320px] overflow-y-auto"
+      style={style}
+    >
+      {filtered.map((skill, i) => (
+        <button
+          key={skill.name}
+          type="button"
+          data-highlighted={i === highlightIndex}
+          className={cn(
+            'w-full text-left px-2.5 py-2 cursor-pointer block',
+            i === highlightIndex && 'bg-muted',
+          )}
+          onMouseEnter={() => setHighlightIndex(i)}
+          onMouseDown={(e) => {
+            // mousedown not click — click runs after blur/focus shuffles which
+            // can race with the textarea's onBlur close path.
+            e.preventDefault();
+            onSelect(skill.name);
+          }}
+        >
+          <div className="font-mono text-xs font-bold text-foreground">/{skill.name}</div>
+          <div
+            className="text-xs text-muted-foreground overflow-hidden"
+            style={{
+              display: '-webkit-box',
+              WebkitLineClamp: 2,
+              WebkitBoxOrient: 'vertical',
+            }}
+          >
+            {skill.description}
+          </div>
+        </button>
+      ))}
+    </div>
+  );
+}

apps/web/src/components/panes/ChatPane.tsx

@@ -96,6 +96,18 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
     }
   }, [chatId]);
 
+  // Batch 9.6: slash-command dispatch. Sent regardless of streaming state —
+  // matches the existing /compact precedent (which also fires immediately).
+  // Empty args go to the server as null; the server fills in a default user
+  // message ("Apply this skill.") so the model has something to act on.
+  const handleSlashCommand = useCallback(async (skillName: string, userMessage: string) => {
+    try {
+      await api.chats.skillInvoke(chatId, skillName, userMessage.length > 0 ? userMessage : null);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : `/${skillName} failed`);
+    }
+  }, [chatId]);
+
   function removeQueued(idx: number) {
     setQueue((prev) => prev.filter((_, i) => i !== idx));
   }
@@ -183,6 +195,7 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
           webSearchEnabled={webSearchEnabled}
           onSend={handleSend}
           onForceSend={streaming ? handleForceSend : undefined}
+          onSlashCommand={handleSlashCommand}
         />
       </div>
     </div>

apps/web/src/hooks/useSkills.ts

@@ -0,0 +1,43 @@
+import { useEffect, useState } from 'react';
+import { api } from '@/api/client';
+import type { Skill } from '@/api/types';
+
+// Batch 9.6: shared in-memory cache for the slash-command dropdown. One fetch
+// per process; subsequent mounts of useSkills() return the cached list and
+// don't re-hit /api/skills. Matches the useSidebar / useChatStatus module-
+// singleton pattern so the dropdown stays cheap even with many ChatInputs
+// mounted at once.
+
+let cachedSkills: Skill[] | null = null;
+let inflight: Promise<Skill[]> | null = null;
+const subscribers = new Set<(s: Skill[]) => void>();
+
+async function loadSkills(): Promise<Skill[]> {
+  if (inflight) return inflight;
+  inflight = api.skills
+    .list()
+    .then((r) => {
+      cachedSkills = r.skills;
+      for (const sub of subscribers) {
+        try { sub(cachedSkills); } catch { /* swallow */ }
+      }
+      return cachedSkills;
+    })
+    .finally(() => { inflight = null; });
+  return inflight;
+}
+
+export function useSkills(): { skills: Skill[]; loaded: boolean } {
+  const [skills, setSkills] = useState<Skill[]>(cachedSkills ?? []);
+  const [loaded, setLoaded] = useState<boolean>(cachedSkills !== null);
+
+  useEffect(() => {
+    subscribers.add(setSkills);
+    if (cachedSkills === null) {
+      void loadSkills().then(() => setLoaded(true)).catch(() => setLoaded(true));
+    }
+    return () => { subscribers.delete(setSkills); };
+  }, []);
+
+  return { skills, loaded };
+}