diff --git a/.changeset/fast-subagent-transcripts.md b/.changeset/fast-subagent-transcripts.md
new file mode 100644
index 000000000000..9db283c90b47
--- /dev/null
+++ b/.changeset/fast-subagent-transcripts.md
@@ -0,0 +1,5 @@
+---
+"kilo-code": patch
+---
+
+Improve live subagent transcript performance in editor tabs and Agent Manager, especially with many parallel agents and shared-board messages.
diff --git a/packages/kilo-ui/src/components/message-part.tsx b/packages/kilo-ui/src/components/message-part.tsx
index 151295dadda4..738165661979 100644
--- a/packages/kilo-ui/src/components/message-part.tsx
+++ b/packages/kilo-ui/src/components/message-part.tsx
@@ -1287,7 +1287,7 @@ function McpTool(props: ToolProps) {
   })
 
   const formattedOutput = createMemo(() => {
-    if (!props.output) return undefined
+    if (messages() || !props.output) return undefined
     try {
       const parsed = JSON.parse(props.output)
       return "```json\n" + JSON.stringify(parsed, null, 2) + "\n```"
@@ -1303,6 +1303,7 @@ function McpTool(props: ToolProps) {
     >
       <BasicTool
         icon={board() ? "task" : "mcp"}
+        defer={board()}
         status={props.status}
         tool={props.tool}
         partID={props.partID}
diff --git a/packages/kilo-vscode/tests/fixtures/board-tool-render.tsx b/packages/kilo-vscode/tests/fixtures/board-tool-render.tsx
new file mode 100644
index 000000000000..2f017b65c8f1
--- /dev/null
+++ b/packages/kilo-vscode/tests/fixtures/board-tool-render.tsx
@@ -0,0 +1,155 @@
+import assert from "node:assert/strict"
+import { Window } from "happy-dom"
+import type { AssistantMessage, ToolPart } from "@kilocode/sdk/v2"
+
+const window = new Window({ url: "http://localhost" })
+Object.assign(globalThis, {
+  window,
+  document: window.document,
+  navigator: window.navigator,
+  Node: window.Node,
+  Element: window.Element,
+  HTMLElement: window.HTMLElement,
+  HTMLAnchorElement: window.HTMLAnchorElement,
+  HTMLButtonElement: window.HTMLButtonElement,
+  HTMLDivElement: window.HTMLDivElement,
+  HTMLPreElement: window.HTMLPreElement,
+  SVGElement: window.SVGElement,
+  MutationObserver: window.MutationObserver,
+  ResizeObserver: window.ResizeObserver,
+  CustomEvent: window.CustomEvent,
+  Event: window.Event,
+  MouseEvent: window.MouseEvent,
+  requestAnimationFrame: window.requestAnimationFrame.bind(window),
+  cancelAnimationFrame: window.cancelAnimationFrame.bind(window),
+  getComputedStyle: window.getComputedStyle.bind(window),
+})
+
+const { createSignal } = await import("solid-js")
+const { createStore } = await import("solid-js/store")
+const { render } = await import("solid-js/web")
+const { Part } = await import("@kilocode/kilo-ui/message-part")
+const { MarkedProvider, createMarkedParser } = await import("@kilocode/kilo-ui/context/marked")
+
+const labels = ["initial", "hidden", "latest", "reopened", "search", "search-updated"]
+const outputs = labels.map((label) =>
+  JSON.stringify({
+    messages: [
+      { from: "worker", to: "main", fromLabel: `Worker ${label}`, toLabel: "Coordinator", body: `**${label}** body` },
+    ],
+    hasMore: false,
+  }),
+)
+const message: AssistantMessage = {
+  id: "assistant",
+  sessionID: "child",
+  role: "assistant",
+  parentID: "prompt",
+  modelID: "test",
+  providerID: "test",
+  mode: "code",
+  agent: "code",
+  path: { cwd: "/test", root: "/test" },
+  time: { created: 1, completed: 2 },
+  cost: 0,
+  tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+}
+const [part, setPart] = createStore({
+  id: "board-read",
+  sessionID: message.sessionID,
+  messageID: message.id,
+  type: "tool",
+  callID: "board-read-call",
+  tool: "board_read",
+  state: {
+    status: "completed",
+    input: { limit: 1 },
+    output: outputs.at(0)!,
+    metadata: {},
+    title: "Board messages",
+    time: { start: 1, end: 2 },
+  },
+} satisfies ToolPart)
+const [search, setSearch] = createSignal(false)
+const parsed: string[] = []
+const decoded: string[] = []
+const parser = createMarkedParser({})
+const decode = JSON.parse
+JSON.parse = (text, reviver) => {
+  if (outputs.includes(text)) decoded.push(text)
+  return decode(text, reviver)
+}
+const root = document.createElement("div")
+document.body.append(root)
+const dispose = render(
+  () => (
+    <MarkedProvider
+      nativeParser={async (text) => {
+        parsed.push(text)
+        return parser.parse(text)
+      }}
+    >
+      <Part part={part} message={message} forceOpen={search()} />
+    </MarkedProvider>
+  ),
+  root,
+)
+const settle = async () => {
+  await Promise.resolve()
+  await window.happyDOM.waitUntilComplete()
+}
+const trigger = () => {
+  const button = root.querySelector<HTMLButtonElement>('[data-slot="collapsible-trigger"]')
+  assert(button)
+  return button
+}
+const update = async (index: number) => {
+  setPart("state", "output", outputs.at(index)!)
+  await settle()
+}
+const visible = (label: string) => {
+  assert.equal(trigger().getAttribute("aria-expanded"), "true")
+  assert.equal(root.querySelector('[data-slot="board-message-body"] strong')?.textContent, label)
+  assert.equal(root.querySelector(".board-route-sender")?.textContent, `Worker ${label}`)
+  assert.equal(root.querySelector(".board-route-recipient")?.textContent, "Coordinator")
+  assert(parsed.includes(`**${label}** body`))
+}
+
+try {
+  await settle()
+  for (const index of [0, 1, 2]) {
+    if (index) await update(index)
+    assert.equal(trigger().getAttribute("aria-expanded"), "false")
+    assert.equal(root.querySelector('[data-component="board-messages"]'), null)
+    assert.equal(root.querySelector('[data-component="markdown"]'), null)
+    assert.deepEqual(parsed, [])
+    assert.deepEqual(decoded, outputs.slice(0, index + 1))
+  }
+
+  trigger().click()
+  await settle()
+  visible("latest")
+  assert.deepEqual(parsed, ["**latest** body"])
+
+  trigger().click()
+  await settle()
+  await update(3)
+  trigger().click()
+  await settle()
+  visible("reopened")
+
+  trigger().click()
+  await settle()
+  await update(4)
+  setSearch(true)
+  await settle()
+  visible("search")
+  await update(5)
+  visible("search-updated")
+  assert.deepEqual(decoded, outputs)
+} finally {
+  dispose()
+  JSON.parse = decode
+  await window.happyDOM.cancelAsync()
+  await window.happyDOM.close()
+}
diff --git a/packages/kilo-vscode/tests/fixtures/session-provider-activity.tsx b/packages/kilo-vscode/tests/fixtures/session-provider-activity.tsx
index b5d12e4ee82c..e7dd38445af4 100644
--- a/packages/kilo-vscode/tests/fixtures/session-provider-activity.tsx
+++ b/packages/kilo-vscode/tests/fixtures/session-provider-activity.tsx
@@ -45,10 +45,11 @@ Object.assign(globalThis, {
 })
 
 const { render } = await import("solid-js/web")
-const { For, Show, createEffect, createSignal } = await import("solid-js")
+const { For, Show, createEffect, createRoot, createSignal } = await import("solid-js")
 const { unwrap } = await import("solid-js/store")
 const { WorktreeItem } = await import("../../webview-ui/agent-manager/WorktreeItem")
 const { SubagentPanel } = await import("../../webview-ui/agent-manager/SubagentPanel")
+const { createSubagentController } = await import("../../webview-ui/agent-manager/subagent-tabs")
 const { DragDropProvider, SortableProvider } = await import("@thisbeyond/solid-dnd")
 const { renderTab } = await import("../../webview-ui/agent-manager/tab-rendering")
 const { VSCodeProvider } = await import("../../webview-ui/src/context/vscode")
@@ -1079,6 +1080,103 @@ try {
   await emit({ type: "sessionStatus", sessionID: "root", status: "idle" })
   await check("root", "idle")
 
+  setInspector(false)
+  setInspected(["inspector-child", "inspector-sibling"])
+  setActive("inspector-child")
+  const start = sent.length
+  const loads = () => sent.slice(start).filter((message) => message.type === "loadMessages")
+  setInspector(true)
+  await settle()
+  assert.deepEqual(loads(), [
+    { type: "loadMessages", sessionID: "inspector-child", mode: "replace", focus: false, limit: 80 },
+  ])
+  for (const id of inspected()) {
+    await emit({ type: "messagesLoaded", sessionID: id, messages: [], mode: "replace" })
+    assert.equal(loads().length, 1, `${id} snapshot reloaded the selected inspector`)
+    for (const status of ["busy", "idle"] as const) {
+      await emit({ type: "sessionStatus", sessionID: id, status })
+      assert.equal(loads().length, 1, `${id} ${status} reloaded the selected inspector`)
+      assert.equal(active(), "inspector-child")
+      assert.equal(value.currentSessionID(), "root")
+    }
+  }
+  for (const id of ["inspector-sibling", "inspector-child"]) {
+    const tab = host.querySelector<HTMLElement>(`[data-tab-id="${id}"] [role="tab"]`)
+    assert(tab)
+    tab.click()
+    await settle()
+    assert.equal(active(), id)
+    assert.deepEqual(loads().at(-1), {
+      type: "loadMessages",
+      sessionID: id,
+      mode: "reconcile",
+      focus: false,
+      limit: 80,
+    })
+    assert.equal(value.currentSessionID(), "root")
+  }
+  assert.equal(loads().length, 3)
+  setInspector(false)
+  await settle()
+
+  const family = createRoot((dispose) => {
+    const state = { reads: 0 }
+    createEffect(() => {
+      value.scopedPermissions("root")
+      state.reads++
+    })
+    return { state, dispose }
+  })
+  try {
+    await settle()
+    assert.equal(family.state.reads, 1)
+    for (const id of inspected()) {
+      await emit({ type: "sessionStatus", sessionID: id, status: "busy" })
+      await emit({ type: "sessionStatus", sessionID: id, status: "busy" })
+    }
+    assert.equal(family.state.reads, 1, "Busy updates rebuilt unchanged session ancestry")
+  } finally {
+    family.dispose()
+  }
+
+  const opened = createRoot((dispose) => {
+    const [visible, setVisible] = createSignal(false)
+    const selected: (string | undefined)[] = []
+    const controller = createSubagentController({
+      project: () => undefined,
+      current: () => "root",
+      selection: () => null,
+      parts: () =>
+        inspected().map((id) => ({
+          id,
+          type: "tool",
+          tool: "task",
+          state: { status: "running", input: {} },
+          metadata: { sessionId: id },
+        })),
+      visible,
+      show: () => setVisible(true),
+      hide: () => setVisible(false),
+      sync: () => {},
+      unsync: () => {},
+    })
+    createEffect(() => selected.push(controller.tabs.active()))
+    return { ...controller, selected, dispose }
+  })
+  try {
+    await settle()
+    assert.deepEqual(opened.selected, [undefined])
+    opened.toolbar.toggle()
+    await settle()
+    assert.deepEqual(
+      opened.tabs.tabs().map((tab) => tab.id),
+      inspected(),
+    )
+    assert.deepEqual(opened.selected, [undefined, "inspector-sibling"])
+  } finally {
+    opened.dispose()
+  }
+
   await emit({ type: "sessionStatus", sessionID: "root", status: "busy" })
   await emit({
     type: "suggestionRequest",
diff --git a/packages/kilo-vscode/tests/unit/board-tool-render.test.ts b/packages/kilo-vscode/tests/unit/board-tool-render.test.ts
new file mode 100644
index 000000000000..9840092ecde5
--- /dev/null
+++ b/packages/kilo-vscode/tests/unit/board-tool-render.test.ts
@@ -0,0 +1,51 @@
+import { describe, expect, it } from "bun:test"
+import { unlinkSync } from "node:fs"
+import path from "node:path"
+import { build } from "esbuild"
+import { solidPlugin } from "esbuild-plugin-solid"
+
+const root = path.resolve(import.meta.dir, "../..")
+const webview = path.join(root, "webview-ui")
+
+describe("Board tool transcript rendering", () => {
+  it("defers collapsed bodies and shows current routes and Markdown when opened", async () => {
+    const solid = path.dirname(Bun.resolveSync("solid-js/package.json", webview))
+    const aliases: Record<string, string> = {
+      "solid-js": path.join(solid, "dist/solid.js"),
+      "solid-js/web": path.join(solid, "web/dist/web.js"),
+      "solid-js/store": path.join(solid, "store/dist/store.js"),
+    }
+    const dedupe = {
+      name: "solid-dedupe",
+      setup(ctx: Parameters<NonNullable<Parameters<typeof build>[0]["plugins"]>[number]["setup"]>[0]) {
+        ctx.onResolve({ filter: /^solid-js(\/web|\/store)?$/ }, (args) => ({ path: aliases[args.path] }))
+        ctx.onResolve({ filter: /\?worker&url$/ }, (args) => ({ path: args.path, namespace: "worker-url" }))
+        ctx.onLoad({ filter: /.*/, namespace: "worker-url" }, () => ({
+          contents: "export default undefined",
+          loader: "js",
+        }))
+      },
+    }
+    const result = await build({
+      entryPoints: [path.join(root, "tests/fixtures/board-tool-render.tsx")],
+      bundle: true,
+      conditions: ["browser"],
+      external: ["happy-dom"],
+      format: "esm",
+      logLevel: "silent",
+      loader: { ".css": "empty", ".svg": "dataurl" },
+      platform: "node",
+      plugins: [dedupe, solidPlugin()],
+      target: "es2022",
+      write: false,
+    })
+    const file = path.join(root, `.board-tool-render-${crypto.randomUUID()}.mjs`)
+    await Bun.write(file, result.outputFiles.at(0)!.contents)
+    try {
+      const child = Bun.spawnSync(["bun", file], { cwd: webview, stdout: "pipe", stderr: "pipe" })
+      expect(child.exitCode, child.stdout.toString() + child.stderr.toString()).toBe(0)
+    } finally {
+      unlinkSync(file)
+    }
+  }, 15_000)
+})
diff --git a/packages/kilo-vscode/webview-ui/agent-manager/SubagentPanel.tsx b/packages/kilo-vscode/webview-ui/agent-manager/SubagentPanel.tsx
index 763752d8f346..a70b7c401376 100644
--- a/packages/kilo-vscode/webview-ui/agent-manager/SubagentPanel.tsx
+++ b/packages/kilo-vscode/webview-ui/agent-manager/SubagentPanel.tsx
@@ -7,7 +7,7 @@
 
 import { Icon } from "@kilocode/kilo-ui/icon"
 import { IconButton } from "@kilocode/kilo-ui/icon-button"
-import { createEffect, createMemo, type Accessor, type Component } from "solid-js"
+import { createEffect, createMemo, on, type Accessor, type Component } from "solid-js"
 import { DataBridge } from "../src/App"
 import { ChatView } from "../src/components/chat"
 import { ActivityIcon } from "../src/components/shared/ActivityIcon"
@@ -34,11 +34,12 @@ interface Props {
 const SubagentChat: Component<{ active: Accessor<string | undefined> }> = (props) => {
   const session = useSession()
 
-  createEffect(() => {
-    const id = props.active()
-    if (!id) return
-    session.selectSession(id, { focus: false })
-  })
+  createEffect(
+    on(props.active, (id) => {
+      if (!id) return
+      session.selectSession(id, { focus: false })
+    }),
+  )
 
   return (
     <DataBridge>
diff --git a/packages/kilo-vscode/webview-ui/agent-manager/subagent-tabs.ts b/packages/kilo-vscode/webview-ui/agent-manager/subagent-tabs.ts
index 03b5df55ac09..0f8c96c86f9d 100644
--- a/packages/kilo-vscode/webview-ui/agent-manager/subagent-tabs.ts
+++ b/packages/kilo-vscode/webview-ui/agent-manager/subagent-tabs.ts
@@ -162,7 +162,9 @@ export function createSubagentToolbar(opts: {
     }
     const id = opts.current()
     if (!id) return
-    for (const tab of available()) opts.open(tab.id, tab.title, id)
+    batch(() => {
+      for (const tab of available()) opts.open(tab.id, tab.title, id)
+    })
   }
   createEffect(
     on(
diff --git a/packages/kilo-vscode/webview-ui/src/context/session.tsx b/packages/kilo-vscode/webview-ui/src/context/session.tsx
index e3e1ad4cd1ef..4598eb3c4432 100644
--- a/packages/kilo-vscode/webview-ui/src/context/session.tsx
+++ b/packages/kilo-vscode/webview-ui/src/context/session.tsx
@@ -174,6 +174,7 @@ export const SessionProvider: ParentComponent = (props) => {
   }
   const clearClose = (id: string) => {
     recoveries.delete(id)
+    if (!closeMap[id]) return
     setCloseMap(
       produce((map) => {
         delete map[id]
@@ -2553,7 +2554,7 @@ export const SessionProvider: ParentComponent = (props) => {
       vscode.postMessage({
         type: "loadMessages",
         sessionID: id,
-        mode: "replace",
+        mode: ready ? "reconcile" : "replace",
         focus: false,
         limit: MESSAGE_PAGE_LIMIT,
       })