diff --git a/.github/workflows/pr-build.yml b/.github/workflows/pr-build.yml
index 856a12fff..23d842532 100644
--- a/.github/workflows/pr-build.yml
+++ b/.github/workflows/pr-build.yml
@@ -176,6 +176,8 @@ jobs:
           if ($LASTEXITCODE) { exit $LASTEXITCODE }
           node scripts/test-session-fork-native.mjs $cli
           if ($LASTEXITCODE) { exit $LASTEXITCODE }
+          node scripts/test-session-aside-native.mjs $cli
+          if ($LASTEXITCODE) { exit $LASTEXITCODE }
 
   tests:
     needs: authorize
diff --git a/AGENTS.md b/AGENTS.md
index d3dad8fa6..060cc8e93 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -8,6 +8,7 @@
 - Co-locate reusable UI patterns (buttons, selectors, dropdowns, etc.) under `src/styles/components/` and avoid redefining the same utility classes elsewhere.
 - Use the shared `.window-*` primitives from `src/styles/components/window.css` for dialog, popover, and floating-window headers, toolbars, bodies, footers, titles, and actions.
 - Persistent command/search utility windows use `components/dismissible-window.tsx`: non-modal, no scrim, outside interactions keep them open, and explicit toggles use `.icon-toggle` with `aria-expanded`/`aria-controls`. Keep search state scoped to its instance/session and close it when that view becomes inactive.
+- The composer reserves `/btw` for native `session.generate`, outside ordinary prompt/command submission. Its ephemeral question/answer window uses `DismissibleWindow` and `styles/components/session-aside.css`; cancellation and inactive/session transitions fence late results without interrupting the main session.
 - OpenCode settings show a compact runtime summary with update availability and a direct install/update action; additional management actions live in the shared setup dialog, also mounted by the native preferences window. Keep diagnostics, executable selection and configuration reload in explicit disclosure sections styled in `styles/components/opencode-setup.css`.
 - Full-history search/counts use the bundled pruning plugin's bounded queries, outside the transcript store. Current-session results and the global timeline navigate through bounded anchor windows; other-session previews fetch one native message only. Search/results/progress styles live in `styles/components/history-search.css`. See `dev-docs/SESSION_HISTORY_QUERIES.md` for scope, snapshot and cleanup semantics.
 - Keep agent, model, and thinking controls in the composer footer via `PromptContextControls`; adapt that footer with the named `prompt-composer` container rather than viewport-only breakpoints.
diff --git a/dev-docs/OPENCODE_V2_COMPATIBILITY.md b/dev-docs/OPENCODE_V2_COMPATIBILITY.md
index 79d3da76e..fd1da1d72 100644
--- a/dev-docs/OPENCODE_V2_COMPATIBILITY.md
+++ b/dev-docs/OPENCODE_V2_COMPATIBILITY.md
@@ -28,6 +28,19 @@ The isolated 2.0.3→2.0.11 seed confirms native workspace-selector collapse whi
 preserving session IDs and complete history. See the
 [transition register](OPENCODE_V2_POST_BETA.md) for actual coverage and release gates.
 
+### Side questions (`/btw`)
+
+The composer-owned command calls native `session.generate({ sessionID, prompt })`
+through the ownership-checked session proxy. It does not use `session.command`,
+create a fork, replace session environment, or add transcript/inbox records.
+The native API returns one text answer; there is no client-side context assembly
+or tool loop. Closing/cancelling the ephemeral window aborts that request only.
+`scripts/test-session-aside-native.mjs` uses a private daemon and local provider
+to verify context reuse, unchanged idle history and generation during an active
+main turn. It passes on 2.0.7 (technical minimum), 2.0.11 and 2.0.12, and runs in
+the minimum/latest-stable CI matrix. These targeted results do not change the
+global minimum, dependency pins or recommended release-tested version.
+
 The audit and implementation record below is historical context, not a runtime
 qualification matrix to maintain.
 
diff --git a/packages/server/src/server/__tests__/instance-proxy.test.ts b/packages/server/src/server/__tests__/instance-proxy.test.ts
index eba0249d3..351ef220f 100644
--- a/packages/server/src/server/__tests__/instance-proxy.test.ts
+++ b/packages/server/src/server/__tests__/instance-proxy.test.ts
@@ -172,6 +172,38 @@ async function harness(
 }
 
 describe("instance proxy location enforcement", () => {
+  it("forwards side generation for an owned busy session without mutating its environment", async () => {
+    const { app, manager, sessionGets, requestCount } = await harness("/repo/worktree", { owned: { type: "running" } })
+    manager.getSessionEnvironment = async () => { throw new Error("Side generation must not replace the session environment") }
+    const response = await app.inject({ method: "POST", url: "/workspaces/workspace/instance/api/session/owned/generate",
+      payload: { prompt: "Explain the current approach" } })
+    assert.equal(response.statusCode, 200)
+    assert.deepEqual(sessionGets, ["owned"])
+    assert.deepEqual(response.json().body, { prompt: "Explain the current approach" })
+    assert.equal(response.json().url, "/api/session/owned/generate")
+    assert.equal(requestCount(), 1)
+    // Only the reviewed session-scoped route is opened, never generic generation.
+    const generic = await app.inject({ method: "POST", url: "/workspaces/workspace/instance/api/experimental/generate", payload: { prompt: "test" } })
+    assert.equal(generic.statusCode, 403)
+    assert.equal(requestCount(), 1)
+  })
+
+  it("rejects side generation for a foreign session and during worktree deletion", async () => {
+    const { app, requestCount, worktreeDeletionFence } = await harness("/repo/worktree", {}, { foreign: "/other" })
+    const foreign = await app.inject({ method: "POST", url: "/workspaces/workspace/instance/api/session/foreign/generate", payload: { prompt: "test" } })
+    assert.equal(foreign.statusCode, 403)
+    assert.equal(requestCount(), 0)
+    let release!: () => void
+    const deletion = worktreeDeletionFence.run("workspace:worktree", ["workspace:worktree"], () => (
+      new Promise<void>(resolve => { release = resolve })
+    ))
+    try {
+      const deleting = await app.inject({ method: "POST", url: "/workspaces/workspace/instance/api/session/owned/generate", payload: { prompt: "test" } })
+      assert.equal(deleting.statusCode, 409)
+      assert.equal(requestCount(), 0)
+    } finally { release(); await deletion }
+  })
+
   it("authorizes transcript reads without resolving the mutation checkout identity", async () => {
     const { app, manager, sessionGets } = await harness()
     manager.getWorktreeIdentityForPath = async () => { throw new Error("Unexpected Git mutation identity read") }
diff --git a/packages/server/src/server/http-server.ts b/packages/server/src/server/http-server.ts
index 48bd43d82..4922cf44c 100644
--- a/packages/server/src/server/http-server.ts
+++ b/packages/server/src/server/http-server.ts
@@ -1271,7 +1271,7 @@ function isAllowedInstanceApiRoute(method: string, pathname: string): boolean {
     ["DELETE", /^\/api\/session\/[^/]+$/],
     ["DELETE", /^\/api\/session\/[^/]+\/inbox\/[^/]+$/],
     ["PATCH", /^\/api\/session\/[^/]+$/],
-    ["POST", /^\/api\/session\/[^/]+\/(?:agent|model|move|prompt|command|shell|compact|interrupt|background|fork)$/],
+    ["POST", /^\/api\/session\/[^/]+\/(?:agent|model|move|prompt|command|shell|compact|interrupt|background|fork|generate)$/],
     ["POST", /^\/api\/experimental\/session\/[^/]+\/wait$/],
     ["PATCH", /^\/api\/session\/[^/]+\/inbox\/[^/]+$/],
     ["POST", /^\/api\/session\/[^/]+\/revert\/stage$/],
diff --git a/packages/ui/src/components/prompt-input.tsx b/packages/ui/src/components/prompt-input.tsx
index cf35f8077..53d5b663c 100644
--- a/packages/ui/src/components/prompt-input.tsx
+++ b/packages/ui/src/components/prompt-input.tsx
@@ -1,4 +1,4 @@
-import { Suspense, createEffect, createSignal, lazy, on, onCleanup, onMount, Show } from "solid-js"
+import { Suspense, createEffect, createSignal, createUniqueId, lazy, on, onCleanup, onMount, Show } from "solid-js"
 import { Loader2, Mic, Paperclip, Volume2, X } from "lucide-solid"
 import { addAttachment, clearAttachments, removeAttachment } from "../stores/attachments"
 import { createPastedPlaceholderRegex, pastedDisplayCounterRegex } from "./prompt-input/attachmentPlaceholders"
@@ -12,6 +12,7 @@ import { showAlertDialog } from "../stores/alerts"
 import { useI18n } from "../lib/i18n"
 import { getLogger } from "../lib/logger"
 import { getOpencodeErrorMessage } from "../lib/opencode-api"
+import { createAttachmentPlaceholderRegex, getAttachmentPlaceholder } from "../lib/attachment-placeholders"
 import { serverApi } from "../lib/api-client"
 import { isDesktopHost, isLocalWindow } from "../lib/runtime-env"
 import { preferences } from "../stores/preferences"
@@ -24,6 +25,8 @@ import { usePromptAttachments } from "./prompt-input/usePromptAttachments"
 import { usePromptPicker } from "./prompt-input/usePromptPicker"
 import { usePromptKeyDown } from "./prompt-input/usePromptKeyDown"
 import { usePromptVoiceInput } from "./prompt-input/usePromptVoiceInput"
+import { usePromptAside } from "./prompt-input/usePromptAside"
+import PromptAsideWindow from "./prompt-input/PromptAsideWindow"
 import {
   initializePromptInputHeight,
   persistPromptInputHeight,
@@ -86,6 +89,17 @@ function getConsumedPastedTextAttachmentIds(text: string, attachments: Attachmen
 
 export default function PromptInput(props: PromptInputProps) {
   const { t } = useI18n()
+  const asideId = createUniqueId()
+  const aside = usePromptAside({
+    instanceId: () => props.instanceId,
+    sessionId: () => props.sessionId,
+    active: () => props.isActive !== false,
+  })
+  // /btw is a local UI command, like OpenCode's TUI command of the same name.
+  const promptCommands = () => [
+    { name: "btw", description: t("promptInput.btw.commandDescription") },
+    ...getCommands(props.instanceId).filter(command => command.name !== "btw"),
+  ]
   initializePromptInputHeight()
   const [, setIsFocused] = createSignal(false)
   const [mode, setMode] = createSignal<PromptMode>("normal")
@@ -320,7 +334,7 @@ export default function PromptInput(props: PromptInputProps) {
     setPrompt,
     getTextarea: () => textareaRef ?? null,
     instanceAgents,
-    commands: () => getCommands(props.instanceId),
+    commands: promptCommands,
   })
 
   const {
@@ -522,6 +536,28 @@ export default function PromptInput(props: PromptInputProps) {
 
     const isShellMode = mode() === "shell"
 
+    const asideMatch = !isShellMode && /^\/btw(?:\s+([\s\S]*))?$/.exec(text)
+    if (asideMatch) {
+      const question = preparePromptSubmission({ mode: "slash", text, attachments: currentAttachments,
+        commandToken: "/btw", commandArgs: asideMatch[1] ?? "" }).resolvedCommandArgs
+      if (aside.launch(question)) {
+        // Pasted text is consumed by the side question; image/file attachments
+        // are not sent. Retain image tokens so normal attachment cleanup keeps them.
+        const imageTokens = currentAttachments.flatMap(attachment => {
+          if (attachment.source.type !== "file") return []
+          const placeholder = getAttachmentPlaceholder(attachment.display)
+          return placeholder?.kind === "image"
+            ? draftText.match(createAttachmentPlaceholderRegex("image", placeholder.counter)) ?? [] : []
+        })
+        if (imageTokens.length) setPrompt(imageTokens.join(" "))
+        else clearPrompt()
+        clearHistoryDraft()
+        setShowPicker(false)
+        restoredQueuedPayload = undefined
+      }
+      return
+    }
+
     // Slash command routing (match OpenCode TUI): only run if the command exists.
     const isSlashCandidate = !isShellMode && text.startsWith("/")
     const firstSpace = isSlashCandidate ? text.indexOf(" ") : -1
@@ -973,7 +1009,7 @@ export default function PromptInput(props: PromptInputProps) {
                 void handleSend()
               }}
               agents={instanceAgents()}
-              commands={getCommands(props.instanceId)}
+              commands={promptCommands()}
               searchQuery={searchQuery()}
               textareaRef={textareaRef}
               workspaceId={props.instanceId}
@@ -1119,6 +1155,7 @@ export default function PromptInput(props: PromptInputProps) {
         </div>
       </div>
 
+      <PromptAsideWindow id={asideId} controller={aside} />
       <DirectoryBrowserDialog
         open={isFileBrowserOpen()}
         mode="files"
diff --git a/packages/ui/src/components/prompt-input/PromptAsideWindow.tsx b/packages/ui/src/components/prompt-input/PromptAsideWindow.tsx
new file mode 100644
index 000000000..8279db03c
--- /dev/null
+++ b/packages/ui/src/components/prompt-input/PromptAsideWindow.tsx
@@ -0,0 +1,74 @@
+import { createEffect, createSignal, Show } from "solid-js"
+import { Copy, Loader2, X } from "lucide-solid"
+import DismissibleWindow from "../dismissible-window"
+import { Markdown } from "../markdown"
+import { copyToClipboard } from "../../lib/clipboard"
+import { useI18n } from "../../lib/i18n"
+import type { PromptAsideController } from "./usePromptAside"
+
+export default function PromptAsideWindow(props: { id: string; controller: PromptAsideController }) {
+  const { t } = useI18n()
+  const aside = props.controller
+  const [copied, setCopied] = createSignal(false)
+  const [copyFailed, setCopyFailed] = createSignal(false)
+  let input: HTMLTextAreaElement | undefined
+  let closeButton: HTMLButtonElement | undefined
+  createEffect(() => {
+    aside.answer()
+    aside.open()
+    setCopied(false)
+    setCopyFailed(false)
+  })
+
+  return (
+    <DismissibleWindow id={props.id} open={aside.open()} onClose={aside.close}
+      title={t("promptInput.btw.title")} description={t("promptInput.btw.description")}
+      class="session-aside-window" initialFocus={() => aside.pending() ? closeButton : input}>
+      <div class="window-header">
+        <h2 class="window-title">{t("promptInput.btw.title")}</h2>
+        <button ref={closeButton} type="button" class="window-icon-button" aria-label={t("promptInput.btw.close")} onClick={aside.close}><X /></button>
+      </div>
+      <div class="window-body">
+        <p class="session-aside-description">{t("promptInput.btw.description")}</p>
+        <form onSubmit={event => { event.preventDefault(); void aside.ask() }}>
+          <label class="session-aside-label" for={`${props.id}-question`}>{t("promptInput.btw.question")}</label>
+          <textarea ref={input} id={`${props.id}-question`} class="session-aside-question" rows={3}
+            value={aside.question()} disabled={aside.pending()}
+            onInput={event => aside.setQuestion(event.currentTarget.value)}
+            onKeyDown={event => {
+              if (event.key === "Enter" && (event.ctrlKey || event.metaKey) && !event.isComposing) {
+                event.preventDefault()
+                void aside.ask()
+              }
+            }} />
+          <div class="session-aside-actions">
+            <Show when={aside.pending()} fallback={
+              <button type="submit" class="window-action" disabled={!aside.question().trim()}>{t("promptInput.btw.ask")}</button>
+            }>
+              <span role="status" class="session-aside-loading"><Loader2 class="animate-spin" size={14} />{t("promptInput.btw.loading")}</span>
+              <button type="button" class="window-action" onClick={aside.cancel}>{t("promptInput.btw.cancel")}</button>
+            </Show>
+          </div>
+        </form>
+        <Show when={aside.error()}><p role="alert">{aside.error()}</p></Show>
+        <Show when={aside.answer()}>
+          <section class="session-aside-answer" aria-label={t("promptInput.btw.answer")} tabindex="0">
+            <Markdown part={{ type: "text", text: aside.answer() }} escapeRawHtml />
+          </section>
+        </Show>
+      </div>
+      <Show when={aside.answer()}>
+        <div class="window-footer">
+          <Show when={copyFailed()}><span role="alert">{t("promptInput.btw.copyFailed")}</span></Show>
+          <button type="button" class="window-action" onClick={async () => {
+            const answer = aside.answer()
+            const success = await copyToClipboard(answer)
+            if (aside.answer() !== answer) return
+            setCopied(success)
+            setCopyFailed(!success)
+          }}><Copy size={14} />{t(copied() ? "promptInput.btw.copied" : "promptInput.btw.copy")}</button>
+        </div>
+      </Show>
+    </DismissibleWindow>
+  )
+}
diff --git a/packages/ui/src/components/prompt-input/usePromptAside.ts b/packages/ui/src/components/prompt-input/usePromptAside.ts
new file mode 100644
index 000000000..6da8a1473
--- /dev/null
+++ b/packages/ui/src/components/prompt-input/usePromptAside.ts
@@ -0,0 +1,96 @@
+import { createEffect, createSignal, on, onCleanup, type Accessor } from "solid-js"
+import { useI18n } from "../../lib/i18n"
+import { getOpencodeErrorMessage } from "../../lib/opencode-api"
+import { getRootClient } from "../../stores/opencode-client"
+
+// Match OpenCode's native /btw instructions. session.generate performs a single
+// generation with session context; it does not run a tool loop or append messages.
+const instructions = [
+  "The user is asking a quick side question about the conversation so far.",
+  "Answer directly and concisely in markdown from what you already know.",
+  "Do not call any tools and do not take any actions.",
+].join(" ")
+
+export function usePromptAside(options: {
+  instanceId: Accessor<string>
+  sessionId: Accessor<string>
+  active: Accessor<boolean>
+}) {
+  const { t } = useI18n()
+  const [open, setOpen] = createSignal(false)
+  const [question, setQuestion] = createSignal("")
+  const [answer, setAnswer] = createSignal("")
+  const [error, setError] = createSignal("")
+  const [pending, setPending] = createSignal(false)
+  let request: AbortController | undefined
+
+  function cancel() {
+    const previous = request
+    request = undefined
+    previous?.abort()
+    setPending(false)
+  }
+
+  function close() {
+    cancel()
+    setOpen(false)
+  }
+
+  createEffect(on(
+    [options.instanceId, options.sessionId, options.active],
+    () => {
+      close()
+      setQuestion("")
+      setAnswer("")
+      setError("")
+    },
+  ))
+  onCleanup(cancel)
+
+  async function ask() {
+    const text = question().trim()
+    if (!text || pending() || !options.active()) return
+    const controller = new AbortController()
+    const instanceId = options.instanceId()
+    const sessionId = options.sessionId()
+    request = controller
+    const current = () => request === controller && !controller.signal.aborted
+      && options.active() && options.instanceId() === instanceId && options.sessionId() === sessionId
+    setPending(true)
+    setAnswer("")
+    setError("")
+    try {
+      const result = await getRootClient(instanceId).session.generate({
+        sessionID: sessionId,
+        prompt: `${instructions}\n\n${text}`,
+      }, { signal: controller.signal })
+      if (!current()) return
+      const textAnswer = result.text.trim()
+      if (!textAnswer) setError(t("promptInput.btw.empty"))
+      else setAnswer(textAnswer)
+    } catch (cause) {
+      if (current()) setError(getOpencodeErrorMessage(cause, t("promptInput.btw.failed")))
+    } finally {
+      if (current()) {
+        request = undefined
+        setPending(false)
+      }
+    }
+  }
+
+  // Return false when a second question cannot be accepted yet, so the composer
+  // keeps its draft. Bare /btw reopens the window without another model request.
+  function launch(text: string): boolean {
+    if (!options.active()) return false
+    setOpen(true)
+    if (!text.trim()) return true
+    if (pending()) return false
+    setQuestion(text.trim())
+    void ask()
+    return true
+  }
+
+  return { open, question, setQuestion, answer, error, pending, launch, ask, cancel, close }
+}
+
+export type PromptAsideController = ReturnType<typeof usePromptAside>
diff --git a/packages/ui/src/lib/i18n/messages/de/messaging.ts b/packages/ui/src/lib/i18n/messages/de/messaging.ts
index bf63e3583..5e3ca9d2e 100644
--- a/packages/ui/src/lib/i18n/messages/de/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/de/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — Zwischenfrage",
+  "promptInput.btw.commandDescription": "Eine Zwischenfrage stellen, ohne sie zum Gespräch hinzuzufügen",
+  "promptInput.btw.description": "Verwendet den Sitzungskontext in einer separaten Modellanfrage. Frage und Antwort werden nicht zum Gespräch hinzugefügt. Anhänge bleiben im Eingabefeld.",
+  "promptInput.btw.question": "Ihre Frage",
+  "promptInput.btw.answer": "Antwort",
+  "promptInput.btw.ask": "Fragen",
+  "promptInput.btw.loading": "Denkt nach…",
+  "promptInput.btw.cancel": "Frage abbrechen",
+  "promptInput.btw.close": "Zwischenfrage schließen",
+  "promptInput.btw.empty": "Das Modell hat keinen Text zurückgegeben. Formulieren Sie Ihre Frage neu.",
+  "promptInput.btw.failed": "Die Zwischenfrage konnte nicht beantwortet werden.",
+  "promptInput.btw.copy": "Antwort kopieren",
+  "promptInput.btw.copied": "Kopiert",
+  "promptInput.btw.copyFailed": "Die Antwort konnte nicht kopiert werden.",
   "history.navigation.outlineLoading": "Vollständige Zeitleiste wird geladen… {loaded} / {total}",
   "history.navigation.loading": "Abschnitt wird geladen…",
   "history.navigation.outlineUnavailable": "Vollständige Zeitleiste nicht verfügbar",
diff --git a/packages/ui/src/lib/i18n/messages/en/messaging.ts b/packages/ui/src/lib/i18n/messages/en/messaging.ts
index 6b1b0c7b9..0077be19d 100644
--- a/packages/ui/src/lib/i18n/messages/en/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/en/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — Side question",
+  "promptInput.btw.commandDescription": "Ask a side question without adding to the conversation",
+  "promptInput.btw.description": "Uses this session's context in a separate model request. Questions and answers are not added to the conversation. Attachments stay in the composer.",
+  "promptInput.btw.question": "Your question",
+  "promptInput.btw.answer": "Answer",
+  "promptInput.btw.ask": "Ask",
+  "promptInput.btw.loading": "Thinking…",
+  "promptInput.btw.cancel": "Cancel question",
+  "promptInput.btw.close": "Close side question",
+  "promptInput.btw.empty": "The model returned no text. Try rephrasing your question.",
+  "promptInput.btw.failed": "Unable to answer the side question.",
+  "promptInput.btw.copy": "Copy answer",
+  "promptInput.btw.copied": "Copied",
+  "promptInput.btw.copyFailed": "Unable to copy the answer.",
   "history.navigation.outlineLoading": "Loading full timeline… {loaded} / {total}",
   "history.navigation.loading": "Loading passage…",
   "history.navigation.outlineUnavailable": "Full timeline unavailable",
diff --git a/packages/ui/src/lib/i18n/messages/es/messaging.ts b/packages/ui/src/lib/i18n/messages/es/messaging.ts
index 4303f2d85..c2201013e 100644
--- a/packages/ui/src/lib/i18n/messages/es/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/es/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — Pregunta aparte",
+  "promptInput.btw.commandDescription": "Hacer una pregunta aparte sin añadirla a la conversación",
+  "promptInput.btw.description": "Usa el contexto de esta sesión en una solicitud independiente al modelo. La pregunta y la respuesta no se añaden a la conversación. Los adjuntos permanecen en el editor.",
+  "promptInput.btw.question": "Tu pregunta",
+  "promptInput.btw.answer": "Respuesta",
+  "promptInput.btw.ask": "Preguntar",
+  "promptInput.btw.loading": "Pensando…",
+  "promptInput.btw.cancel": "Cancelar pregunta",
+  "promptInput.btw.close": "Cerrar pregunta aparte",
+  "promptInput.btw.empty": "El modelo no devolvió texto. Intenta reformular la pregunta.",
+  "promptInput.btw.failed": "No se pudo responder a la pregunta aparte.",
+  "promptInput.btw.copy": "Copiar respuesta",
+  "promptInput.btw.copied": "Copiado",
+  "promptInput.btw.copyFailed": "No se pudo copiar la respuesta.",
   "history.navigation.outlineLoading": "Cargando la cronología completa… {loaded} / {total}",
   "history.navigation.loading": "Cargando el pasaje…",
   "history.navigation.outlineUnavailable": "Cronología completa no disponible",
diff --git a/packages/ui/src/lib/i18n/messages/fr/messaging.ts b/packages/ui/src/lib/i18n/messages/fr/messaging.ts
index aa319286b..3ef0d23eb 100644
--- a/packages/ui/src/lib/i18n/messages/fr/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/fr/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — Question à part",
+  "promptInput.btw.commandDescription": "Poser une question à part sans l'ajouter à la conversation",
+  "promptInput.btw.description": "Utilise le contexte de cette session dans une requête distincte au modèle. La question et la réponse ne sont pas ajoutées à la conversation. Les pièces jointes restent dans le composeur.",
+  "promptInput.btw.question": "Votre question",
+  "promptInput.btw.answer": "Réponse",
+  "promptInput.btw.ask": "Demander",
+  "promptInput.btw.loading": "Réflexion…",
+  "promptInput.btw.cancel": "Annuler la question",
+  "promptInput.btw.close": "Fermer la question à part",
+  "promptInput.btw.empty": "Le modèle n'a renvoyé aucun texte. Essayez de reformuler votre question.",
+  "promptInput.btw.failed": "Impossible de répondre à la question à part.",
+  "promptInput.btw.copy": "Copier la réponse",
+  "promptInput.btw.copied": "Copié",
+  "promptInput.btw.copyFailed": "Impossible de copier la réponse.",
   "history.navigation.outlineLoading": "Chargement de la timeline complète… {loaded} / {total}",
   "history.navigation.loading": "Chargement du passage…",
   "history.navigation.outlineUnavailable": "Timeline complète indisponible",
diff --git a/packages/ui/src/lib/i18n/messages/he/messaging.ts b/packages/ui/src/lib/i18n/messages/he/messaging.ts
index a1149828c..8ce8f8f14 100644
--- a/packages/ui/src/lib/i18n/messages/he/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/he/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — שאלה צדדית",
+  "promptInput.btw.commandDescription": "שאל שאלה צדדית בלי להוסיף אותה לשיחה",
+  "promptInput.btw.description": "משתמש בהקשר ההפעלה בבקשה נפרדת למודל. השאלה והתשובה אינן מתווספות לשיחה. הקבצים המצורפים נשארים בעורך ההודעה.",
+  "promptInput.btw.question": "השאלה שלך",
+  "promptInput.btw.answer": "תשובה",
+  "promptInput.btw.ask": "שאל",
+  "promptInput.btw.loading": "חושב…",
+  "promptInput.btw.cancel": "בטל שאלה",
+  "promptInput.btw.close": "סגור שאלה צדדית",
+  "promptInput.btw.empty": "המודל לא החזיר טקסט. נסה לנסח מחדש את השאלה.",
+  "promptInput.btw.failed": "לא ניתן לענות על השאלה הצדדית.",
+  "promptInput.btw.copy": "העתק תשובה",
+  "promptInput.btw.copied": "הועתק",
+  "promptInput.btw.copyFailed": "לא ניתן להעתיק את התשובה.",
   "history.navigation.outlineLoading": "טוען את ציר הזמן המלא… {loaded} / {total}",
   "history.navigation.loading": "טוען קטע…",
   "history.navigation.outlineUnavailable": "ציר הזמן המלא אינו זמין",
diff --git a/packages/ui/src/lib/i18n/messages/ja/messaging.ts b/packages/ui/src/lib/i18n/messages/ja/messaging.ts
index 39cee089d..824f5ee05 100644
--- a/packages/ui/src/lib/i18n/messages/ja/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/ja/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — 補足の質問",
+  "promptInput.btw.commandDescription": "会話に追加せずに補足の質問をする",
+  "promptInput.btw.description": "このセッションのコンテキストを使い、モデルに個別に問い合わせます。質問と回答は会話に追加されません。添付ファイルは入力欄に残ります。",
+  "promptInput.btw.question": "質問",
+  "promptInput.btw.answer": "回答",
+  "promptInput.btw.ask": "質問する",
+  "promptInput.btw.loading": "考え中…",
+  "promptInput.btw.cancel": "質問をキャンセル",
+  "promptInput.btw.close": "補足の質問を閉じる",
+  "promptInput.btw.empty": "モデルからテキストが返されませんでした。質問を言い換えてみてください。",
+  "promptInput.btw.failed": "補足の質問に回答できませんでした。",
+  "promptInput.btw.copy": "回答をコピー",
+  "promptInput.btw.copied": "コピーしました",
+  "promptInput.btw.copyFailed": "回答をコピーできませんでした。",
   "history.navigation.outlineLoading": "タイムライン全体を読み込み中… {loaded} / {total}",
   "history.navigation.loading": "該当箇所を読み込み中…",
   "history.navigation.outlineUnavailable": "全体のタイムラインを取得できません",
diff --git a/packages/ui/src/lib/i18n/messages/ne/messaging.ts b/packages/ui/src/lib/i18n/messages/ne/messaging.ts
index b87433ffa..bda6e234f 100644
--- a/packages/ui/src/lib/i18n/messages/ne/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/ne/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — छुट्टै प्रश्न",
+  "promptInput.btw.commandDescription": "कुराकानीमा नथपी छुट्टै प्रश्न सोध्नुहोस्",
+  "promptInput.btw.description": "यस सत्रको सन्दर्भ प्रयोग गरी मोडेललाई छुट्टै अनुरोध पठाउँछ। प्रश्न र उत्तर कुराकानीमा थपिँदैनन्। संलग्न फाइलहरू लेखन क्षेत्रमा रहन्छन्।",
+  "promptInput.btw.question": "तपाईंको प्रश्न",
+  "promptInput.btw.answer": "उत्तर",
+  "promptInput.btw.ask": "सोध्नुहोस्",
+  "promptInput.btw.loading": "सोच्दै…",
+  "promptInput.btw.cancel": "प्रश्न रद्द गर्नुहोस्",
+  "promptInput.btw.close": "छुट्टै प्रश्न बन्द गर्नुहोस्",
+  "promptInput.btw.empty": "मोडेलले कुनै पाठ फर्काएन। प्रश्नलाई अर्को तरिकाले सोध्नुहोस्।",
+  "promptInput.btw.failed": "छुट्टै प्रश्नको उत्तर दिन सकिएन।",
+  "promptInput.btw.copy": "उत्तर प्रतिलिपि गर्नुहोस्",
+  "promptInput.btw.copied": "प्रतिलिपि गरियो",
+  "promptInput.btw.copyFailed": "उत्तर प्रतिलिपि गर्न सकिएन।",
   "history.navigation.outlineLoading": "पूरा समयरेखा लोड हुँदैछ… {loaded} / {total}",
   "history.navigation.loading": "अंश लोड हुँदैछ…",
   "history.navigation.outlineUnavailable": "पूरा टाइमलाइन उपलब्ध छैन",
diff --git a/packages/ui/src/lib/i18n/messages/ru/messaging.ts b/packages/ui/src/lib/i18n/messages/ru/messaging.ts
index 53e186391..3323d06ad 100644
--- a/packages/ui/src/lib/i18n/messages/ru/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/ru/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — Отдельный вопрос",
+  "promptInput.btw.commandDescription": "Задать отдельный вопрос, не добавляя его в разговор",
+  "promptInput.btw.description": "Использует контекст сессии в отдельном запросе к модели. Вопрос и ответ не добавляются в разговор. Вложения остаются в поле ввода.",
+  "promptInput.btw.question": "Ваш вопрос",
+  "promptInput.btw.answer": "Ответ",
+  "promptInput.btw.ask": "Спросить",
+  "promptInput.btw.loading": "Думает…",
+  "promptInput.btw.cancel": "Отменить вопрос",
+  "promptInput.btw.close": "Закрыть отдельный вопрос",
+  "promptInput.btw.empty": "Модель не вернула текст. Попробуйте переформулировать вопрос.",
+  "promptInput.btw.failed": "Не удалось ответить на отдельный вопрос.",
+  "promptInput.btw.copy": "Копировать ответ",
+  "promptInput.btw.copied": "Скопировано",
+  "promptInput.btw.copyFailed": "Не удалось скопировать ответ.",
   "history.navigation.outlineLoading": "Загрузка полной шкалы времени… {loaded} / {total}",
   "history.navigation.loading": "Загрузка фрагмента…",
   "history.navigation.outlineUnavailable": "Полная шкала времени недоступна",
diff --git a/packages/ui/src/lib/i18n/messages/tr/messaging.ts b/packages/ui/src/lib/i18n/messages/tr/messaging.ts
index b317b0231..7f82cd980 100644
--- a/packages/ui/src/lib/i18n/messages/tr/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/tr/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — Yan soru",
+  "promptInput.btw.commandDescription": "Konuşmaya eklemeden yan bir soru sorun",
+  "promptInput.btw.description": "Bu oturumun bağlamını ayrı bir model isteğinde kullanır. Soru ve yanıt konuşmaya eklenmez. Ekler yazma alanında kalır.",
+  "promptInput.btw.question": "Sorunuz",
+  "promptInput.btw.answer": "Yanıt",
+  "promptInput.btw.ask": "Sor",
+  "promptInput.btw.loading": "Düşünüyor…",
+  "promptInput.btw.cancel": "Soruyu iptal et",
+  "promptInput.btw.close": "Yan soruyu kapat",
+  "promptInput.btw.empty": "Model metin döndürmedi. Sorunuzu yeniden ifade etmeyi deneyin.",
+  "promptInput.btw.failed": "Yan soru yanıtlanamadı.",
+  "promptInput.btw.copy": "Yanıtı kopyala",
+  "promptInput.btw.copied": "Kopyalandı",
+  "promptInput.btw.copyFailed": "Yanıt kopyalanamadı.",
   "history.navigation.loading": "Bölüm yükleniyor…",
   "history.navigation.outlineUnavailable": "Tam zaman çizelgesi kullanılamıyor",
   "history.scope": "Arama kapsamı",
diff --git a/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts b/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts
index cc8a6b205..a1babdbbc 100644
--- a/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts
@@ -1,4 +1,18 @@
 export const messagingMessages = {
+  "promptInput.btw.title": "/btw — 旁支问题",
+  "promptInput.btw.commandDescription": "提出旁支问题，不添加到对话中",
+  "promptInput.btw.description": "使用此会话的上下文单独请求模型。问题和回答不会添加到对话中。附件保留在输入框中。",
+  "promptInput.btw.question": "你的问题",
+  "promptInput.btw.answer": "回答",
+  "promptInput.btw.ask": "提问",
+  "promptInput.btw.loading": "思考中…",
+  "promptInput.btw.cancel": "取消提问",
+  "promptInput.btw.close": "关闭旁支问题",
+  "promptInput.btw.empty": "模型未返回文本。请尝试换一种方式提问。",
+  "promptInput.btw.failed": "无法回答旁支问题。",
+  "promptInput.btw.copy": "复制回答",
+  "promptInput.btw.copied": "已复制",
+  "promptInput.btw.copyFailed": "无法复制回答。",
   "history.navigation.outlineLoading": "正在加载完整时间线… {loaded} / {total}",
   "history.navigation.loading": "正在加载所选段落…",
   "history.navigation.outlineUnavailable": "完整时间线暂不可用",
diff --git a/packages/ui/src/styles/components/session-aside.css b/packages/ui/src/styles/components/session-aside.css
new file mode 100644
index 000000000..6f9f71052
--- /dev/null
+++ b/packages/ui/src/styles/components/session-aside.css
@@ -0,0 +1,61 @@
+.session-aside-window {
+  position: fixed;
+  z-index: 50;
+  inset-block-start: 15vh;
+  inset-inline-end: var(--space-md);
+  display: flex;
+  flex-direction: column;
+  width: min(36rem, calc(100vw - 2 * var(--space-md)));
+  max-height: 75dvh;
+}
+
+.session-aside-description {
+  margin: 0 0 var(--space-md);
+  color: var(--text-muted);
+  font-size: var(--font-size-sm);
+}
+
+.session-aside-label {
+  display: block;
+  margin-block-end: var(--space-xs);
+  font-size: var(--font-size-sm);
+}
+
+.session-aside-question {
+  display: block;
+  width: 100%;
+  min-height: 4rem;
+  max-height: 25dvh;
+  resize: vertical;
+  padding: var(--space-sm);
+  border: 1px solid var(--border-base);
+  background: var(--surface-base);
+  color: var(--text-primary);
+}
+
+.session-aside-question:focus-visible {
+  outline: 1px solid var(--focus-ring-color);
+}
+
+.session-aside-actions,
+.session-aside-loading {
+  display: flex;
+  align-items: center;
+  gap: var(--space-sm);
+}
+
+.session-aside-actions {
+  justify-content: flex-end;
+  margin-block: var(--space-sm);
+}
+
+.session-aside-loading {
+  color: var(--text-muted);
+  font-size: var(--font-size-sm);
+}
+
+.session-aside-answer {
+  border-top: 1px solid var(--border-divider);
+  padding-block-start: var(--space-md);
+  overflow-wrap: anywhere;
+}
diff --git a/packages/ui/src/styles/controls.css b/packages/ui/src/styles/controls.css
index 9b4b745af..cd2bc34c6 100644
--- a/packages/ui/src/styles/controls.css
+++ b/packages/ui/src/styles/controls.css
@@ -1,6 +1,7 @@
 @import "./components/buttons.css";
 @import "./components/switches.css";
 @import "./components/window.css";
+@import "./components/session-aside.css";
 @import "./components/history-search.css";
 @import "./components/native-titlebar.css";
 @import "./components/badges.css";
diff --git a/packages/ui/tests/browser/fixtures/session-aside.tsx b/packages/ui/tests/browser/fixtures/session-aside.tsx
new file mode 100644
index 000000000..452fc919b
--- /dev/null
+++ b/packages/ui/tests/browser/fixtures/session-aside.tsx
@@ -0,0 +1,45 @@
+import { createSignal, Show } from "solid-js"
+import { render } from "solid-js/web"
+import PromptInput from "../../../src/components/prompt-input"
+import { ConfigProvider } from "../../../src/stores/preferences"
+import { I18nProvider } from "../../../src/lib/i18n"
+import { ThemeProvider } from "../../../src/lib/theme"
+import { getRootClient } from "../../../src/stores/opencode-client"
+import { addInstance, setActiveInstanceId } from "../../../src/stores/instances"
+import { fetchCommands } from "../../../src/stores/commands"
+import { addAttachment, getAttachments } from "../../../src/stores/attachments"
+import { createFileAttachment, createTextAttachment } from "../../../src/types/attachment"
+import "../../../src/index.css"
+
+const instanceId = "aside-instance"
+const [session, setSession] = createSignal("source")
+const [active, setActive] = createSignal(true)
+const [mounted, setMounted] = createSignal(true)
+const sends: unknown[] = [], commands: unknown[] = []
+let interrupts = 0
+const client = getRootClient(instanceId)
+addInstance({ id: instanceId, folder: "/fixture", port: 0, pid: 0, proxyPath: "", status: "ready", client })
+setActiveInstanceId(instanceId)
+await fetchCommands(instanceId, client)
+render(() => <ConfigProvider><I18nProvider><ThemeProvider>
+  <button id="outside">Outside control</button>
+  <Show when={mounted()}><PromptInput instanceId={instanceId} instanceFolder="/fixture" sessionId={session()} isActive={active()}
+    isSessionBusy={true} onAbortSession={async () => { interrupts++ }}
+    onSend={async (text, attachments, delivery) => { sends.push({ text, attachments, delivery }) }}
+    onCommand={async (name, text) => { commands.push({ name, text }) }} /></Show>
+</ThemeProvider></I18nProvider></ConfigProvider>, document.getElementById("root")!)
+;(window as any).fixture = {
+  switch: setSession, active: setActive, unmount: () => setMounted(false),
+  attach: () => {
+    addAttachment(instanceId, session(), createFileAttachment("/fixture/notes.txt", "notes.txt"))
+  },
+  paste: () => {
+    addAttachment(instanceId, session(), createTextAttachment("PASTED_QUESTION", "[Pasted #1]", "paste.txt"))
+  },
+  image: () => {
+    const image = createFileAttachment("image.png", "image.png", "image/png")
+    image.display = "[Image #1]"
+    addAttachment(instanceId, session(), image)
+  },
+  snapshot: () => ({ sends, commands, interrupts, session: session(), attachments: getAttachments(instanceId, session()).length }),
+}
diff --git a/packages/ui/tests/browser/session-aside.test.ts b/packages/ui/tests/browser/session-aside.test.ts
new file mode 100644
index 000000000..1b2293eac
--- /dev/null
+++ b/packages/ui/tests/browser/session-aside.test.ts
@@ -0,0 +1,199 @@
+import assert from "node:assert/strict"
+import { after, before, test } from "node:test"
+import { fileURLToPath } from "node:url"
+import { chromium, type Browser, type Page, type Route } from "playwright"
+import { createServer, type ViteDevServer } from "vite"
+import solid from "vite-plugin-solid"
+
+let server: ViteDevServer, browser: Browser, url: string
+before(async () => {
+  server = await createServer({ configFile: false, root: fileURLToPath(new URL("../..", import.meta.url)), logLevel: "error",
+    plugins: [solid(), { name: "aside-fixture", configureServer(s) {
+      s.middlewares.use("/aside-fixture", async (_req, res) => {
+        res.setHeader("Content-Type", "text/html")
+        res.end(await s.transformIndexHtml("/aside-fixture", '<html><body><div id="root" style="margin:24px;max-width:1100px"></div><script type="module" src="/tests/browser/fixtures/session-aside.tsx"></script></body></html>'))
+      })
+    } }], resolve: { dedupe: ["solid-js"] }, optimizeDeps: { exclude: ["lucide-solid"] },
+    server: { host: "127.0.0.1", port: 0, hmr: false, watch: null },
+  })
+  await server.listen()
+  url = `http://127.0.0.1:${(server.httpServer!.address() as { port: number }).port}/aside-fixture`
+  browser = await chromium.launch({ executablePath: process.env.CODENOMAD_BROWSER_PATH || undefined })
+})
+after(async () => { await browser?.close(); await server?.close() })
+
+async function setup(respond?: (route: Route) => Promise<void>, width = 1200) {
+  const page = await browser.newPage({ viewport: { width, height: 850 }, locale: "en-US" })
+  const requests: { url: string; body: any }[] = [], errors: string[] = []
+  page.on("pageerror", error => errors.push(error.message))
+  await page.addInitScript(`Object.defineProperty(navigator, 'clipboard', {value: {writeText: async (text) => {window.copiedText = text}}})`)
+  await page.route("**/api/**", async route => {
+    const request = route.request()
+    if (request.url().endsWith("/generate")) {
+      requests.push({ url: request.url(), body: request.postDataJSON() })
+      return respond ? respond(route) : route.fulfill({ json: { data: { text: "## Side answer\n\n**Only in this window.**" } } })
+    }
+    if (request.url().endsWith("/api/command")) return route.fulfill({ json: { data: [{ name: "btw", description: "Server collision" }, { name: "review", description: "Review code" }] } })
+    return route.fulfill({ json: {} })
+  })
+  await page.goto(url)
+  await page.waitForFunction(() => Boolean((window as any).fixture))
+  return { page, requests, errors }
+}
+const composer = (page: Page) => page.locator(".prompt-input-container textarea").first()
+async function send(page: Page, text: string) {
+  await composer(page).fill(text)
+  // Clicking Send also covers routing while the slash picker is still open.
+  await page.locator(".send-button").click()
+}
+
+test("/btw is discoverable, uses native generation, renders Markdown and leaves the conversation untouched", async () => {
+  const { page, requests, errors } = await setup()
+  try {
+    await composer(page).fill("/btw")
+    await page.getByText("Ask a side question without adding to the conversation", { exact: true }).waitFor()
+    assert.equal(await page.getByText("Server collision", { exact: true }).count(), 0)
+    await send(page, "/btw\nWhy this approach?")
+    const dialog = page.getByRole("dialog")
+    await dialog.getByRole("heading", { name: "Side answer", exact: true }).waitFor({ timeout: 10000 }).catch(async cause => {
+      throw new Error(JSON.stringify({ requests, errors, body: await page.locator("body").innerText(), cause: String(cause) }))
+    })
+    assert.equal(requests.length, 1)
+    assert.ok(requests[0].url.endsWith("/api/session/source/generate"))
+    assert.ok(requests[0].body.prompt.endsWith("\n\nWhy this approach?"))
+    assert.match(requests[0].body.prompt, /Do not call any tools/)
+    assert.deepEqual(Object.keys(requests[0].body), ["prompt"])
+    await dialog.getByRole("button", { name: "Copy answer", exact: true }).click()
+    assert.equal(await page.evaluate(() => (window as any).copiedText), "## Side answer\n\n**Only in this window.**")
+    await page.locator("#outside").click()
+    assert.equal(await dialog.isVisible(), true)
+    const snapshot = await page.evaluate(() => (window as any).fixture.snapshot())
+    assert.deepEqual(snapshot.sends, [])
+    assert.deepEqual(snapshot.commands, [])
+    assert.equal(snapshot.interrupts, 0)
+    if (process.env.CODENOMAD_ASIDE_CAPTURE) await page.screenshot({ path: process.env.CODENOMAD_ASIDE_CAPTURE })
+    assert.deepEqual(errors, [])
+  } finally { await page.close() }
+})
+
+test("bare /btw opens a question form; attachments and normal command routing survive", async () => {
+  const { page, requests } = await setup()
+  try {
+    await page.evaluate(() => (window as any).fixture.attach())
+    await send(page, "/btw")
+    const dialog = page.getByRole("dialog")
+    await dialog.waitFor()
+    assert.equal(requests.length, 0)
+    assert.equal(await dialog.getByRole("button", { name: "Ask", exact: true }).isDisabled(), true)
+    await dialog.getByLabel("Your question", { exact: true }).fill("What happened?")
+    await dialog.getByRole("button", { name: "Ask", exact: true }).click()
+    await dialog.getByRole("heading", { name: "Side answer" }).waitFor()
+    assert.equal((await page.evaluate(() => (window as any).fixture.snapshot())).attachments, 1)
+    await dialog.getByRole("button", { name: "Close side question" }).click()
+    await composer(page).fill("/btw [Pasted #1] [Image #1]")
+    await page.evaluate(() => { (window as any).fixture.paste(); (window as any).fixture.image() })
+    await page.locator(".send-button").click()
+    await page.getByRole("heading", { name: "Side answer" }).waitFor()
+    assert.ok(requests[1].body.prompt.includes("PASTED_QUESTION"))
+    assert.equal((await page.evaluate(() => (window as any).fixture.snapshot())).attachments, 2)
+    assert.equal(await composer(page).inputValue(), "[Image #1]")
+    await page.getByRole("button", { name: "Close side question" }).click()
+    await send(page, "/review module")
+    await send(page, "/btwExtra is normal text")
+    const snapshot = await page.evaluate(() => (window as any).fixture.snapshot())
+    assert.deepEqual(snapshot.commands, [{ name: "review", text: "module" }])
+    assert.equal(snapshot.sends[0].text, "/btwExtra is normal text")
+    assert.equal(requests.length, 2)
+  } finally { await page.close() }
+})
+
+test("pending side questions do not block normal prompts; a second side question keeps its draft", async () => {
+  let release!: () => Promise<void>
+  const { page, requests } = await setup(route => new Promise<void>(resolve => {
+    release = async () => { await route.fulfill({ json: { data: { text: "Delayed answer" } } }); resolve() }
+  }))
+  try {
+    await send(page, "/btw First question")
+    await page.getByRole("status").filter({ hasText: "Thinking" }).waitFor()
+    await send(page, "/btw Second question")
+    assert.equal(await composer(page).inputValue(), "/btw Second question")
+    assert.equal(requests.length, 1)
+    await send(page, "Continue the main task")
+    assert.equal((await page.evaluate(() => (window as any).fixture.snapshot())).sends.length, 1)
+    await release()
+    await page.getByText("Delayed answer", { exact: true }).waitFor()
+  } finally { await page.close() }
+})
+
+for (const exit of ["close", "cancel", "switch", "inactive", "unmount"] as const) {
+  test(`${exit} fences pending results without interrupting the main session`, async () => {
+    const { page } = await setup()
+    try {
+      // A deliberately non-cooperative transport resolves even after abort.
+      // This exercises the result fence, not just the browser's fetch cancellation.
+      await page.evaluate(async () => {
+        const { getRootClient } = await import("/src/stores/opencode-client.ts")
+        getRootClient("aside-instance").session.generate = (_input: any, options: any) => new Promise(resolve => {
+          ;(window as any).asideSignal = options.signal
+          ;(window as any).resolveAside = resolve
+        })
+      })
+      await send(page, "/btw delayed")
+      await page.getByRole("status").filter({ hasText: "Thinking" }).waitFor()
+      if (exit === "close") await page.keyboard.press("Escape")
+      if (exit === "cancel") await page.getByRole("button", { name: "Cancel question" }).click()
+      if (exit === "switch") await page.evaluate(() => (window as any).fixture.switch("other"))
+      if (exit === "inactive") await page.evaluate(() => (window as any).fixture.active(false))
+      if (exit === "unmount") await page.evaluate(() => (window as any).fixture.unmount())
+      await page.waitForFunction(() => (window as any).asideSignal.aborted)
+      await page.evaluate(() => (window as any).resolveAside({ text: "STALE_RESPONSE" }))
+      await page.evaluate(() => new Promise(requestAnimationFrame))
+      assert.equal(await page.getByText("STALE_RESPONSE", { exact: true }).count(), 0)
+      assert.equal((await page.evaluate(() => (window as any).fixture.snapshot())).interrupts, 0)
+      assert.equal(await page.getByRole("dialog").count(), exit === "cancel" ? 1 : 0)
+    } finally { await page.close() }
+  })
+}
+
+test("a cancelled request cannot overwrite or settle a newer side question", async () => {
+  const { page } = await setup()
+  try {
+    await page.evaluate(async () => {
+      const { getRootClient } = await import("/src/stores/opencode-client.ts")
+      ;(window as any).asideResolvers = []
+      getRootClient("aside-instance").session.generate = () => new Promise(resolve => (window as any).asideResolvers.push(resolve))
+    })
+    await send(page, "/btw Old question")
+    await page.getByRole("button", { name: "Cancel question" }).click()
+    await page.getByLabel("Your question").fill("New question")
+    await page.getByLabel("Your question").press("Control+Enter")
+    await page.waitForFunction(() => (window as any).asideResolvers.length === 2)
+    await page.evaluate(() => (window as any).asideResolvers[0]({ text: "STALE_RESPONSE" }))
+    await page.evaluate(() => new Promise(requestAnimationFrame))
+    assert.equal(await page.getByRole("button", { name: "Cancel question" }).isVisible(), true)
+    assert.equal(await page.getByText("STALE_RESPONSE", { exact: true }).count(), 0)
+    await page.evaluate(() => (window as any).asideResolvers[1]({ text: "NEW_RESPONSE" }))
+    await page.getByText("NEW_RESPONSE", { exact: true }).waitFor()
+  } finally { await page.close() }
+})
+
+test("errors and empty answers remain retryable; the window fits a narrow screen", async () => {
+  let calls = 0
+  const { page } = await setup(route => {
+    calls++
+    return calls === 1 ? route.fulfill({ status: 503, json: { message: "Provider unavailable" } })
+      : route.fulfill({ json: { data: { text: calls === 2 ? "   " : "Recovered answer" } } })
+  }, 360)
+  try {
+    await send(page, "/btw Try this question")
+    const dialog = page.getByRole("dialog")
+    await dialog.getByRole("alert").waitFor()
+    assert.equal(await dialog.getByLabel("Your question").inputValue(), "Try this question")
+    await dialog.getByRole("button", { name: "Ask", exact: true }).click()
+    await dialog.getByText(/The model returned no text/).waitFor()
+    await dialog.getByRole("button", { name: "Ask", exact: true }).click()
+    await dialog.getByText("Recovered answer", { exact: true }).waitFor()
+    const bounds = await dialog.boundingBox()
+    assert.ok(bounds && bounds.x >= 0 && bounds.x + bounds.width <= 360 && bounds.height < 850)
+  } finally { await page.close() }
+})
diff --git a/scripts/test-session-aside-native.mjs b/scripts/test-session-aside-native.mjs
new file mode 100644
index 000000000..ace84b301
--- /dev/null
+++ b/scripts/test-session-aside-native.mjs
@@ -0,0 +1,117 @@
+import assert from "node:assert/strict"
+import { spawn } from "node:child_process"
+import { randomUUID } from "node:crypto"
+import { mkdtemp, mkdir, writeFile } from "node:fs/promises"
+import { createServer } from "node:http"
+import os from "node:os"
+import path from "node:path"
+import { setTimeout as delay } from "node:timers/promises"
+import { OpenCode } from "@opencode/client"
+import { stopFixtureChild } from "./native-fixture-guards.mjs"
+
+// Explicit CLI, private home/database, local fake provider. Never discovers or
+// connects to the shared daemon and never uses real provider credentials.
+const cli = process.argv[2]
+assert(cli && path.isAbsolute(cli), "Pass an absolute isolated CLI executable path")
+const parent = path.join(os.tmpdir(), "opencode")
+await mkdir(parent, { recursive: true })
+const root = await mkdtemp(path.join(parent, "aside-native-"))
+const env = Object.fromEntries(Object.entries(process.env).filter(([key]) => !/^(OPENCODE_|XDG_)/i.test(key)))
+for (const key of ["XDG_CONFIG_HOME", "XDG_DATA_HOME", "XDG_STATE_HOME", "XDG_CACHE_HOME"]) env[key] = path.join(root, key)
+const password = randomUUID()
+Object.assign(env, { HOME: root, USERPROFILE: root, OPENCODE_TEST_HOME: root,
+  OPENCODE_CONFIG_DIR: path.join(root, "config"), OPENCODE_DB: path.join(root, "fixture.db"),
+  OPENCODE_CONFIG_PROJECT_DISABLE: "1", OPENCODE_DISABLE_MODELS_FETCH: "1", OPENCODE_DISABLE_FFF: "1",
+  OPENCODE_SERVER_PASSWORD: password })
+await mkdir(env.OPENCODE_CONFIG_DIR)
+let finishMain, providerError
+const sideRequests = []
+const provider = createServer(async (req, res) => {
+  try {
+    let raw = ""
+    for await (const chunk of req) raw += chunk
+    const body = JSON.parse(raw)
+    const last = JSON.stringify(body.messages.at(-1))
+    const side = last.includes("SIDE_QUESTION")
+    if (side) sideRequests.push(body)
+    if (!body.stream) {
+      res.setHeader("Content-Type", "application/json")
+      res.end(JSON.stringify({ id: "fixture", choices: [{ message: { role: "assistant", content: last.includes("SIDE_QUESTION") ? "SIDE_ANSWER" : "Fixture" }, finish_reason: "stop" }], usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15 } }))
+      return
+    }
+    res.setHeader("Content-Type", "text/event-stream")
+    const chunk = (delta, finish_reason = null) => res.write("data: " + JSON.stringify({
+      id: "fixture", object: "chat.completion.chunk", model: "fixture", choices: [{ index: 0, delta, finish_reason }],
+      ...(finish_reason ? { usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15 } } : {}),
+    }) + "\n\n")
+    chunk({ role: "assistant" })
+    const finish = () => { chunk({ content: side ? "SIDE_ANSWER" : "MAIN_ANSWER" }); chunk({}, "stop"); res.end("data: [DONE]\n\n") }
+    if (last.includes("MAIN_PENDING")) finishMain = finish
+    else finish()
+  } catch (error) { providerError = error; res.destroy(error) }
+})
+await new Promise(resolve => provider.listen(0, "127.0.0.1", resolve))
+env.OPENCODE_CONFIG_CONTENT = JSON.stringify({ model: "fixture/fixture", providers: { fixture: {
+  package: "@opencode/ai/providers/openai-compatible", settings: { baseURL: `http://127.0.0.1:${provider.address().port}/v1`, apiKey: "fixture-only" }, models: { fixture: {} },
+} } })
+let output = "", spawnError
+const child = spawn(cli, ["serve", "--hostname", "127.0.0.1", "--port", "0", "--print-logs"], { cwd: root, env, windowsHide: true })
+const stopped = new Promise(resolve => child.once("close", resolve))
+child.on("error", error => { spawnError = error })
+child.stdout.on("data", data => { output += data })
+child.stderr.on("data", data => { output += data })
+async function until(predicate) {
+  const deadline = Date.now() + 30_000
+  while (Date.now() < deadline) {
+    if (providerError) throw providerError
+    if (spawnError) throw spawnError
+    if (child.exitCode !== null) throw new Error(output)
+    if (await predicate()) return
+    await delay(25)
+  }
+  throw new Error(`Timed out: ${output.slice(-3000)}`)
+}
+try {
+  await until(() => /http:\/\/127\.0\.0\.1:\d+/.test(output))
+  const client = OpenCode.make({ baseUrl: output.match(/http:\/\/127\.0\.0\.1:\d+/)[0],
+    headers: { authorization: `Basic ${Buffer.from(`opencode:${password}`).toString("base64")}` },
+    fetch: (url, init) => fetch(url, { ...init, signal: AbortSignal.any([...(init?.signal ? [init.signal] : []), AbortSignal.timeout(30_000)]) }),
+  })
+  const session = await client.session.create({ location: { directory: root } })
+  await client.session.prompt({ sessionID: session.id, text: "CONTEXT_CANARY: explain the repository" })
+  await client.session.wait({ sessionID: session.id })
+  const messages = () => client.message.list({ sessionID: session.id, order: "asc", limit: 100 })
+  const before = await messages()
+  const answer = await client.session.generate({ sessionID: session.id, prompt: "SIDE_QUESTION: summarize the context without tools" })
+  assert.equal(answer.text, "SIDE_ANSWER")
+  assert.deepEqual(await messages(), before, "Side generation must not mutate history")
+  assert.equal(sideRequests.length, 1)
+  assert.match(JSON.stringify(sideRequests[0]), /CONTEXT_CANARY/)
+  assert.match(JSON.stringify(sideRequests[0]), /MAIN_ANSWER/)
+  console.log("PASS idle: native context reused, history unchanged")
+
+  await client.session.prompt({ sessionID: session.id, text: "MAIN_PENDING: keep working" })
+  await until(() => Boolean(finishMain))
+  const activeBefore = await client.session.active()
+  assert.ok(activeBefore[session.id], "Main session should still be active")
+  assert.equal((await client.session.generate({ sessionID: session.id, prompt: "SIDE_QUESTION: what are you doing?" })).text, "SIDE_ANSWER")
+  assert.ok((await client.session.active())[session.id], "Side generation must not interrupt the main session")
+  assert.equal(sideRequests.length, 2)
+  assert.match(JSON.stringify(sideRequests[1]), /MAIN_PENDING/)
+  finishMain()
+  finishMain = undefined
+  await client.session.wait({ sessionID: session.id })
+  assert.doesNotMatch(JSON.stringify(await messages()), /SIDE_QUESTION|SIDE_ANSWER/)
+  const sessions = await client.session.list({ location: { directory: root } })
+  assert.equal(sessions.data.length, 1, "No temporary or child session should be created")
+  assert.equal((await client.session.inbox.list({ sessionID: session.id })).length, 0)
+  console.log("PASS busy: main execution continues, no side messages, inbox entries or child sessions")
+} finally {
+  finishMain?.()
+  try { await stopFixtureChild(child, stopped) } finally {
+    provider.closeAllConnections()
+    await new Promise(resolve => provider.close(resolve))
+    await writeFile(path.join(root, "daemon.log"), output)
+    console.log(`Isolated fixture: ${root}`)
+  }
+}