@@ -0,0 +1,145 @@
+import { base64Encode } from "@opencode/util/encode"
+import { expect, test } from "@playwright/test"
+import { mockOpenCodeServer } from "../utils/mock-server"
+import { expectSessionTitle } from "../utils/waits"
+
+const directory = "C:/OpenCode/BtwSidebar"
+const projectID = "proj_btw_sidebar"
+const sessionID = "ses_btw_sidebar"
+const otherSessionID = "ses_btw_sidebar_other"
+const title = "Side question session"
+const otherTitle = "Other side question session"
+const server = `http://${process.env.PLAYWRIGHT_SERVER_HOST ?? "127.0.0.1"}:${process.env.PLAYWRIGHT_SERVER_PORT ?? "4096"}`
+const sessionHref = (id: string) => `/server/${base64Encode(server)}/session/${id}`
+
+test.use({ viewport: { width: 1440, height: 900 } })
+
+test("answers /btw in the side panel without admitting a prompt", async ({ page }) => {
+  const generations: { sessionID: string; prompt: string }[] = []
+  const prompts: unknown[] = []
+  const generated = Promise.withResolvers<void>()
+  await mockOpenCodeServer(page, {
+    directory,
+    project: {
+      id: projectID,
+      worktree: directory,
+      vcs: "git",
+      name: "btw-sidebar",
+      time: { created: 1700000000000, updated: 1700000000000 },
+      sandboxes: [],
+    },
+    provider: {
+      all: [
+        {
+          id: "opencode",
+          name: "OpenCode",
+          models: { test: { id: "test", name: "Test", limit: { context: 200_000 } } },
+        },
+      ],
+      connected: ["opencode"],
+      default: { providerID: "opencode", modelID: "test" },
+    },
+    sessions: [
+      {
+        id: sessionID,
+        slug: sessionID,
+        projectID,
+        directory,
+        title,
+        version: "dev",
+        time: { created: 1700000000000, updated: 1700000000000 },
+      },
+      {
+        id: otherSessionID,
+        slug: otherSessionID,
+        projectID,
+        directory,
+        title: otherTitle,
+        version: "dev",
+        time: { created: 1700000001000, updated: 1700000001000 },
+      },
+    ],
+    pageMessages: () => ({ items: [] }),
+    vcsDiff: [],
+    onPrompt: (input) => prompts.push(input),
+    generate: async (input) => {
+      generations.push(input)
+      if (input.sessionID === otherSessionID) return { text: "This answer belongs to the **other session**." }
+      await generated.promise
+      return {
+        text: "The retry loop uses **exponential backoff** and stops after three attempts.\n\n```ts\nconst delay = 2 ** attempt\n```",
+      }
+    },
+  })
+  await page.addInitScript(
+    ({ directory, server, sessionID, otherSessionID }) => {
+      localStorage.setItem(
+        "opencode.global.dat:server",
+        JSON.stringify({
+          projects: { local: [{ worktree: directory, expanded: true }] },
+          lastProject: { local: directory },
+        }),
+      )
+      localStorage.setItem(
+        "opencode.window.browser.dat:tabs",
+        JSON.stringify([
+          { type: "session", server, sessionId: sessionID },
+          { type: "session", server, sessionId: otherSessionID },
+        ]),
+      )
+    },
+    { directory, server, sessionID, otherSessionID },
+  )
+
+  await page.goto(sessionHref(sessionID))
+  await expectSessionTitle(page, title)
+  const editor = page.locator('[data-component="composer-editor"]')
+  await expect(editor).toBeEditable()
+
+  await editor.fill("/btw")
+  const suggestion = page.locator('[data-suggestion-id="session.btw"]')
+  await expect(suggestion).toBeVisible()
+  await suggestion.click()
+  await expect(editor).toHaveText("/btw ")
+  await editor.press("Enter")
+
+  const panel = page.locator('[data-slot="session-btw-panel"]')
+  await expect(panel).toBeHidden()
+  await expect(page.getByText("Add a question after /btw", { exact: true })).toBeVisible()
+  expect(generations).toEqual([])
+  expect(prompts).toEqual([])
+
+  await editor.fill("/btw how does the retry loop work?")
+  await editor.press("Enter")
+
+  const tab = page.getByRole("tab", { name: "/btw" })
+  await expect(panel).toBeVisible()
+  await expect(panel.getByRole("textbox")).toHaveCount(0)
+  await expect(panel.getByRole("status")).toContainText("Working")
+  await expect(tab).toHaveAttribute("data-selected", "")
+  generated.resolve()
+  await expect(panel.getByText("how does the retry loop work?", { exact: true })).toBeVisible()
+  await expect(panel.getByText("exponential backoff", { exact: false })).toBeVisible()
+  await expect(panel.getByText("const delay = 2 ** attempt", { exact: true })).toBeVisible()
+  expect(generations).toHaveLength(1)
+  expect(generations[0]?.sessionID).toBe(sessionID)
+  expect(generations[0]?.prompt).toContain("how does the retry loop work?")
+  expect(prompts).toEqual([])
+  await expect(editor).toHaveText("")
+
+  await page.locator(`[data-titlebar-tab-link][href="${sessionHref(otherSessionID)}"]`).click()
+  await expectSessionTitle(page, otherTitle)
+  await editor.fill("/btw what belongs here?")
+  await editor.press("Enter")
+  await expect(panel.getByText("other session", { exact: false })).toBeVisible()
+
+  await page.locator(`[data-titlebar-tab-link][href="${sessionHref(sessionID)}"]`).click()
+  await expectSessionTitle(page, title)
+  await expect(panel.getByText("exponential backoff", { exact: false })).toBeVisible()
+  await expect(panel.getByText("other session", { exact: false })).toHaveCount(0)
+
+  await page.reload()
+  await expectSessionTitle(page, title)
+  await expect(page.getByRole("tab", { name: "/btw" })).toHaveCount(0)
+  await expect(page.locator('[data-slot="session-btw-panel"]')).toHaveCount(0)
+})
@@ -197,6 +197,13 @@ const Group = HttpApiGroup.make("mock")
       success: Json,
     }),
   )
+  .add(
+    HttpApiEndpoint.post("sessionGenerate", "/api/session/:sessionID/generate", {
+      params: SessionParams,
+      payload: Schema.Struct({ prompt: Schema.String }),
+      success: Json,
+    }),
+  )
   .add(
     HttpApiEndpoint.post("sessionSwitchAgent", "/api/session/:sessionID/agent", {
       params: SessionParams,
 
@@ -42,6 +42,7 @@ export interface MockServerConfig {
   sessionStatus?: Record<string, unknown> | (() => Record<string, unknown>)
   inbox?: unknown[] | (() => unknown[])
   onPrompt?: (input: { sessionID: string; body: Record<string, unknown> }) => void
+  generate?: (input: { sessionID: string; prompt: string }) => { text: string } | Promise<{ text: string }>
   onInboxChange?: (input: { sessionID: string; inboxID: string; action: "cancel" | "steer" | "queue" }) => void
 }
 
@@ -456,6 +457,12 @@ function mockHandlers(config: MockServerConfig, state: { cursors: Map<string, st
               },
             }
           }),
+        sessionGenerate: (ctx) =>
+          Effect.promise(async () => ({
+            data: (await config.generate?.({ sessionID: ctx.params.sessionID, prompt: ctx.payload.prompt })) ?? {
+              text: "Side-question answer",
+            },
+          })),
         sessionInboxCancel: (ctx) =>
           Effect.sync(() =>
             config.onInboxChange?.({ sessionID: ctx.params.sessionID, inboxID: ctx.params.inboxID, action: "cancel" }),
 
@@ -0,0 +1,32 @@
+import { describe, expect, test } from "bun:test"
+import { parseClientSlashCommand } from "./client-slash-command"
+
+const options = [
+  { id: "session.btw", trigger: "btw", arguments: true, type: "builtin" as const },
+  { id: "custom.btw", trigger: "custom", type: "custom" as const },
+  { id: "model.choose", trigger: "model", type: "builtin" as const },
+]
+
+describe("parseClientSlashCommand", () => {
+  test("parses inline and multiline arguments", () => {
+    expect(parseClientSlashCommand(options, "/btw why this approach?")).toEqual({
+      id: "session.btw",
+      input: "why this approach?",
+    })
+    expect(parseClientSlashCommand(options, "/btw\nwhy this approach?")).toEqual({
+      id: "session.btw",
+      input: "why this approach?",
+    })
+  })
+
+  test("accepts a bare argument command", () => {
+    expect(parseClientSlashCommand(options, "/btw")).toEqual({ id: "session.btw", input: "" })
+  })
+
+  test("rejects prefixes, custom commands, and ordinary slash commands", () => {
+    expect(parseClientSlashCommand(options, "/btwx nope")).toBeUndefined()
+    expect(parseClientSlashCommand(options, "/custom nope")).toBeUndefined()
+    expect(parseClientSlashCommand(options, "/model opus")).toBeUndefined()
+    expect(parseClientSlashCommand(options, "ask /btw later")).toBeUndefined()
+  })
+})
@@ -0,0 +1,24 @@
+type ClientSlashCommand = {
+  id: string
+  trigger: string
+  arguments?: boolean
+  type: "builtin" | "custom"
+}
+
+export function parseSlashCommand(text: string) {
+  if (!text.startsWith("/")) return
+  const separator = text.search(/\s/)
+  const name = text.slice(1, separator === -1 ? undefined : separator)
+  return { name, input: separator === -1 ? "" : text.slice(separator).trim() }
+}
+
+export function parseClientSlashCommand(options: readonly ClientSlashCommand[], text: string) {
+  const command = parseSlashCommand(text)
+  if (!command) return
+  const option = options.find((item) => item.type === "builtin" && item.arguments && item.trigger === command.name)
+  if (!option) return
+  return {
+    id: option.id,
+    input: command.input,
+  }
+}
@@ -24,6 +24,7 @@ import { createComposerHistory } from "./history/store"
 import { composerPlaceholder } from "./placeholder"
 import { createComposerSubmit } from "./submit"
 import { useAttachmentDestination } from "./attachments/destination"
+import { parseClientSlashCommand } from "./client-slash-command"
 
 export type ComposerModel = ComposerEditorModel & {
   readonly model: ComposerControls["model"]
@@ -73,9 +74,7 @@ export function createComposerModel(adapter: ComposerAdapter, options?: { queue?
       return [...result, path]
     }, [])
   })
-  const attachments = createMemo(() =>
-    prompt.current().filter(isAttachment),
-  )
+  const attachments = createMemo(() => prompt.current().filter(isAttachment))
   const commentCount = createMemo(() => {
     if (mode() === "shell") return 0
     return prompt.context.items().filter((item) => !!item.comment?.trim()).length
@@ -242,6 +241,7 @@ export function createComposerModel(adapter: ComposerAdapter, options?: { queue?
         trigger: item.slash!,
         title: item.title,
         description: item.description,
+        arguments: item.slashArguments,
         type: "builtin" as const,
       })),
   ])
@@ -299,6 +299,11 @@ export function createComposerModel(adapter: ComposerAdapter, options?: { queue?
       clear: comments.clear,
       restore: restoreHistoryComments,
     },
+    clientCommand: (text) => {
+      const selected = parseClientSlashCommand(slashCommands(), text)
+      if (!selected) return
+      return () => command.trigger(selected.id, "slash", selected.input)
+    },
   })
   const controller = createComposerEditor({
     store: prompt.store,
@@ -340,6 +345,7 @@ export function createComposerModel(adapter: ComposerAdapter, options?: { queue?
       if (item.kind !== "command") return
       const selected = slashCommands().find((entry) => entry.id === item.id)
       if (!selected || selected.type === "custom") return
+      if (selected.arguments) return
       return () => command.trigger(selected.id, "slash")
     },
     attachments: {
 
@@ -54,14 +54,17 @@ function submitInput(
   mode: "normal" | "shell" = "normal",
   commands: () => readonly { name: string }[] | undefined = () => [],
   history: string[] = [],
+  clientCommand?: (text: string) => (() => void | Promise<void>) | undefined,
 ) {
   return createComposerSubmit({
     adapter,
     mode: () => mode,
     commands,
+    clientCommand,
     editor: () => undefined,
     queueScroll() {},
-    addToHistory: (prompt) => history.push(`add:${prompt.map((part) => ("content" in part ? part.content : part.type)).join("")}`),
+    addToHistory: (prompt) =>
+      history.push(`add:${prompt.map((part) => ("content" in part ? part.content : part.type)).join("")}`),
     removeFromHistory: (prompt) =>
       history.push(`remove:${prompt.map((part) => ("content" in part ? part.content : part.type)).join("")}`),
     resetHistory() {},
@@ -118,6 +121,61 @@ function session(input: {
 }
 
 describe("Composer submission", () => {
+  test("runs a client argument command without admitting it to the session", async () => {
+    const state = createMemoryComposerState().capture()
+    state.set([
+      { type: "text", content: "/btw why this approach?", start: 0, end: 23 },
+      {
+        type: "image",
+        id: "attachment",
+        filename: "diagram.png",
+        mime: "image/png",
+        blob: { id: "attachment", url: "data:image/png;base64,YQ==" },
+      },
+    ])
+    state.context.add({ type: "file", path: "src/retry.ts" })
+    const calls: string[] = []
+    const target = session({
+      calls,
+      prompt: async () => {
+        throw new Error("client command must not call prompt")
+      },
+    })
+    const adapter: ActiveComposerAdapter = {
+      kind: "active-session",
+      state,
+      ready: () => true,
+      controls,
+      working: () => false,
+      session: () => target,
+      interrupt: async () => undefined,
+      submitted() {},
+      setEditor() {},
+    }
+
+    const history: string[] = []
+    await submitInput(adapter, undefined, "normal", undefined, history, (text) => {
+      expect(text).toBe("/btw why this approach?")
+      return () => {
+        calls.push("btw")
+      }
+    }).submit(new Event("submit"))
+
+    expect(calls).toEqual(["btw"])
+    expect(history).toEqual([])
+    expect(state.current()).toEqual([
+      { type: "text", content: "", start: 0, end: 0 },
+      {
+        type: "image",
+        id: "attachment",
+        filename: "diagram.png",
+        mime: "image/png",
+        blob: { id: "attachment", url: "data:image/png;base64,YQ==" },
+      },
+    ])
+    expect(state.context.items()).toHaveLength(1)
+  })
+
   test("applies the captured agent and model before a custom command without passing over its overrides", async () => {
     const state = createMemoryComposerState({ prompt: "/review changes" }).capture()
     const calls: string[] = []
@@ -625,12 +683,7 @@ describe("Composer submission", () => {
       },
     }
 
-    await submitInput(
-      adapter,
-      undefined,
-      "normal",
-      () => catalog,
-    ).submit(new Event("submit"))
+    await submitInput(adapter, undefined, "normal", () => catalog).submit(new Event("submit"))
 
     expect(await sent.promise).toBe("command")
     expect(requests).toEqual([