diff --git a/web/hwlab-cloud-web/scripts/workbench-message-projection-budget.test.ts b/web/hwlab-cloud-web/scripts/workbench-message-projection-budget.test.ts
new file mode 100644
index 00000000..71f7aa05
--- /dev/null
+++ b/web/hwlab-cloud-web/scripts/workbench-message-projection-budget.test.ts
@@ -0,0 +1,79 @@
+// SPEC: pikasTech/HWLAB#2351 Workbench JSON parse/projection merge cost reduction.
+// Responsibility: Regression tests for bounded Workbench message projection refresh planning.
+
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import type { ChatMessage } from "../src/types/index.ts";
+import { boundedProjectionMessageLimit, mergeBoundedProjectionMessages, selectProjectionMessageWindow, traceProjectionIsTerminalSealed } from "../src/stores/workbench-message-projection-budget.ts";
+
+test("projection window keeps only bounded tail for automatic session refresh", () => {
+  const messages = Array.from({ length: 8 }, (_, index) => message(`msg_${index}`, `trc_${index}`));
+  const selected = selectProjectionMessageWindow(messages, { limit: 3 });
+
+  assert.deepEqual(selected.map((item) => item.id), ["msg_5", "msg_6", "msg_7"]);
+});
+
+test("projection window keeps active trace outside the bounded tail", () => {
+  const messages = Array.from({ length: 8 }, (_, index) => message(`msg_${index}`, `trc_${index}`));
+  const selected = selectProjectionMessageWindow(messages, { traceId: "trc_2", limit: 3 });
+
+  assert.deepEqual(selected.map((item) => item.id), ["msg_2", "msg_5", "msg_6", "msg_7"]);
+});
+
+test("bounded projection merge updates window messages without dropping history", () => {
+  const existing = [
+    message("msg_1", "trc_1", { text: "old one" }),
+    message("msg_2", "trc_2", { text: "old two" }),
+    message("msg_3", "trc_3", { text: "old three" })
+  ];
+  const incoming = [
+    message("msg_2", "trc_2", { text: "new two" }),
+    message("msg_4", "trc_4", { text: "new four" })
+  ];
+
+  const merged = mergeBoundedProjectionMessages(existing, incoming);
+
+  assert.deepEqual(merged.map((item) => item.id), ["msg_1", "msg_2", "msg_3", "msg_4"]);
+  assert.equal(merged[0]?.text, "old one");
+  assert.equal(merged[1]?.text, "new two");
+  assert.equal(merged[3]?.text, "new four");
+});
+
+test("bounded projection merge replaces optimistic agent row by trace when message ids differ", () => {
+  const existing = [message("msg_optimistic", "trc_same", { text: "optimistic" })];
+  const incoming = [message("msg_canonical", "trc_same", { text: "canonical" })];
+
+  const merged = mergeBoundedProjectionMessages(existing, incoming);
+
+  assert.equal(merged.length, 1);
+  assert.equal(merged[0]?.id, "msg_canonical");
+  assert.equal(merged[0]?.text, "canonical");
+});
+
+test("terminal sealed trace skips redundant message projection refresh", () => {
+  const sealed = message("msg_terminal", "trc_terminal", {
+    status: "completed",
+    text: "final answer",
+    finalResponse: { text: "final answer" }
+  });
+  const unsealed = message("msg_unsealed", "trc_unsealed", { status: "completed", text: "" });
+
+  assert.equal(traceProjectionIsTerminalSealed("trc_terminal", [sealed, unsealed]), true);
+  assert.equal(traceProjectionIsTerminalSealed("trc_unsealed", [sealed, unsealed]), false);
+  assert.equal(boundedProjectionMessageLimit(0, 5), 5);
+});
+
+function message(id: string, traceId: string, extra: Partial<ChatMessage> = {}): ChatMessage {
+  return {
+    id,
+    messageId: id,
+    role: "agent",
+    title: "Code Agent",
+    text: "",
+    status: "running",
+    traceId,
+    createdAt: "2026-07-02T00:00:00.000Z",
+    ...extra
+  };
+}
diff --git a/web/hwlab-cloud-web/scripts/workbench-realtime-runtime.test.ts b/web/hwlab-cloud-web/scripts/workbench-realtime-runtime.test.ts
index 04f399ad..45fe78b6 100644
--- a/web/hwlab-cloud-web/scripts/workbench-realtime-runtime.test.ts
+++ b/web/hwlab-cloud-web/scripts/workbench-realtime-runtime.test.ts
@@ -34,6 +34,8 @@ test("Workbench runtime policy reads injected config while preserving defaults",
     sessionListPageLimit: 7,
     traceHydrationMaxPages: 2,
     workbenchSessionDetailMinRefreshMs: 1234,
+    workbenchSessionMessagesWindowLimit: 9,
+    workbenchTraceMessagesWindowLimit: 4,
     workbenchRealtimeErrorGapFillMinMs: 0,
     defaultGatewayTimeoutMs: "bad"
   });
@@ -41,6 +43,8 @@ test("Workbench runtime policy reads injected config while preserving defaults",
   assert.equal(policy.sessionListPageLimit, 7);
   assert.equal(policy.traceHydrationMaxPages, 2);
   assert.equal(policy.workbenchSessionDetailMinRefreshMs, 1234);
+  assert.equal(policy.workbenchSessionMessagesWindowLimit, 9);
+  assert.equal(policy.workbenchTraceMessagesWindowLimit, 4);
   assert.equal(policy.workbenchRealtimeErrorGapFillMinMs, 0);
   assert.equal(policy.defaultGatewayTimeoutMs, 120_000);
 });
@@ -450,4 +454,3 @@ function recoveryEvent(actions: WorkbenchStreamTransportRecovery["actions"]): Wo
     diagnostic: { code: "workbench_sse_recovery", valuesRedacted: true }
   };
 }
-
diff --git a/web/hwlab-cloud-web/src/config/workbench-runtime-policy.ts b/web/hwlab-cloud-web/src/config/workbench-runtime-policy.ts
index d90b2791..6ec40787 100644
--- a/web/hwlab-cloud-web/src/config/workbench-runtime-policy.ts
+++ b/web/hwlab-cloud-web/src/config/workbench-runtime-policy.ts
@@ -18,6 +18,8 @@ export interface WorkbenchRuntimePolicy {
   workbenchTraceEventsMinRefreshMs: number;
   workbenchSessionDetailMinRefreshMs: number;
   workbenchSessionMessagesMinRefreshMs: number;
+  workbenchSessionMessagesWindowLimit: number;
+  workbenchTraceMessagesWindowLimit: number;
   workbenchRealtimeSessionMessagesMinRefreshMs: number;
   workbenchTraceEventsTimeoutMs: number;
   sessionListRealtimeRefreshDelayMs: number;
@@ -45,6 +47,8 @@ const DEFAULT_WORKBENCH_RUNTIME_POLICY: WorkbenchRuntimePolicy = Object.freeze({
   workbenchTraceEventsMinRefreshMs: 4_000,
   workbenchSessionDetailMinRefreshMs: 5_000,
   workbenchSessionMessagesMinRefreshMs: 5_000,
+  workbenchSessionMessagesWindowLimit: 20,
+  workbenchTraceMessagesWindowLimit: 8,
   workbenchRealtimeSessionMessagesMinRefreshMs: 1_000,
   workbenchTraceEventsTimeoutMs: 5_000,
   sessionListRealtimeRefreshDelayMs: 5_000,
@@ -74,6 +78,8 @@ export function workbenchRuntimePolicy(input: unknown = runtimePolicyConfig()):
     workbenchTraceEventsMinRefreshMs: nonNegativeNumber(source.workbenchTraceEventsMinRefreshMs, DEFAULT_WORKBENCH_RUNTIME_POLICY.workbenchTraceEventsMinRefreshMs),
     workbenchSessionDetailMinRefreshMs: nonNegativeNumber(source.workbenchSessionDetailMinRefreshMs, DEFAULT_WORKBENCH_RUNTIME_POLICY.workbenchSessionDetailMinRefreshMs),
     workbenchSessionMessagesMinRefreshMs: nonNegativeNumber(source.workbenchSessionMessagesMinRefreshMs, DEFAULT_WORKBENCH_RUNTIME_POLICY.workbenchSessionMessagesMinRefreshMs),
+    workbenchSessionMessagesWindowLimit: positiveInteger(source.workbenchSessionMessagesWindowLimit, DEFAULT_WORKBENCH_RUNTIME_POLICY.workbenchSessionMessagesWindowLimit),
+    workbenchTraceMessagesWindowLimit: positiveInteger(source.workbenchTraceMessagesWindowLimit, DEFAULT_WORKBENCH_RUNTIME_POLICY.workbenchTraceMessagesWindowLimit),
     workbenchRealtimeSessionMessagesMinRefreshMs: nonNegativeNumber(source.workbenchRealtimeSessionMessagesMinRefreshMs, DEFAULT_WORKBENCH_RUNTIME_POLICY.workbenchRealtimeSessionMessagesMinRefreshMs),
     workbenchTraceEventsTimeoutMs: positiveNumber(source.workbenchTraceEventsTimeoutMs, DEFAULT_WORKBENCH_RUNTIME_POLICY.workbenchTraceEventsTimeoutMs),
     sessionListRealtimeRefreshDelayMs: nonNegativeNumber(source.sessionListRealtimeRefreshDelayMs, DEFAULT_WORKBENCH_RUNTIME_POLICY.sessionListRealtimeRefreshDelayMs),
diff --git a/web/hwlab-cloud-web/src/stores/workbench-message-projection-budget.ts b/web/hwlab-cloud-web/src/stores/workbench-message-projection-budget.ts
new file mode 100644
index 00000000..eeb23e43
--- /dev/null
+++ b/web/hwlab-cloud-web/src/stores/workbench-message-projection-budget.ts
@@ -0,0 +1,72 @@
+// SPEC: pikasTech/HWLAB#2351 Workbench JSON parse/projection merge cost reduction.
+// Responsibility: Pure bounded-window planning for Workbench session message projection refreshes.
+
+import type { ChatMessage } from "@/types";
+import { firstNonEmptyString } from "@/utils";
+import { isTerminalMessageStatus, messageHasTerminalResponse } from "./workbench-message-projection-runtime";
+
+export interface ProjectionWindowOptions {
+  traceId?: string | null;
+  limit: number;
+}
+
+export function boundedProjectionMessageLimit(value: number | null | undefined, fallback: number): number {
+  const number = Number(value);
+  if (Number.isFinite(number) && number > 0) return Math.max(1, Math.trunc(number));
+  return Math.max(1, Math.trunc(fallback));
+}
+
+export function selectProjectionMessageWindow(source: ChatMessage[], options: ProjectionWindowOptions): ChatMessage[] {
+  const limit = boundedProjectionMessageLimit(options.limit, source.length || 1);
+  const tail = source.slice(-limit);
+  const traceId = firstNonEmptyString(options.traceId);
+  if (!traceId) return tail;
+  const selected = new Set(tail);
+  for (const message of source) {
+    if (messageTraceId(message) === traceId) selected.add(message);
+  }
+  return source.filter((message) => selected.has(message));
+}
+
+export function mergeBoundedProjectionMessages(existing: ChatMessage[], incoming: ChatMessage[]): ChatMessage[] {
+  if (existing.length === 0) return incoming;
+  if (incoming.length === 0) return existing;
+  const incomingByKey = new Map<string, ChatMessage>();
+  for (const message of incoming) {
+    const key = messageProjectionKey(message);
+    if (key) incomingByKey.set(key, message);
+  }
+  if (incomingByKey.size === 0) return existing;
+  const seen = new Set<string>();
+  const merged = existing.map((message) => {
+    const key = messageProjectionKey(message);
+    if (!key) return message;
+    seen.add(key);
+    return incomingByKey.get(key) ?? message;
+  });
+  for (const message of incoming) {
+    const key = messageProjectionKey(message);
+    if (!key || seen.has(key)) continue;
+    seen.add(key);
+    merged.push(message);
+  }
+  return merged;
+}
+
+export function traceProjectionIsTerminalSealed(traceId: string | null | undefined, source: ChatMessage[]): boolean {
+  const id = firstNonEmptyString(traceId);
+  if (!id) return false;
+  const message = [...source].reverse().find((item) => item.role === "agent" && messageTraceId(item) === id) ?? null;
+  return Boolean(message && isTerminalMessageStatus(message.status) && messageHasTerminalResponse(message));
+}
+
+function messageProjectionKey(message: ChatMessage): string | null {
+  const traceId = messageTraceId(message);
+  if (traceId) return `trace:${message.role}:${traceId}`;
+  const messageId = firstNonEmptyString(message.messageId, message.id);
+  return messageId ? `message:${messageId}` : null;
+}
+
+function messageTraceId(message: ChatMessage): string | null {
+  return firstNonEmptyString(message.traceId, message.runnerTrace?.traceId) ?? null;
+}
diff --git a/web/hwlab-cloud-web/src/stores/workbench.ts b/web/hwlab-cloud-web/src/stores/workbench.ts
index 75827dbc..8e2a7639 100644
--- a/web/hwlab-cloud-web/src/stores/workbench.ts
+++ b/web/hwlab-cloud-web/src/stores/workbench.ts
@@ -20,6 +20,7 @@ import { cleanupWorkbenchServerStateSessions, selectActiveMessages, selectActive
 import { cleanupDroppedWorkbenchSessionCaches, trimWorkbenchSessionCache } from "./workbench-session-cache";
 import { reduceWorkbenchRealtimeEvent, type WorkbenchRealtimeAction } from "./workbench-event-reducer";
 import { messageHasSealedTerminalResult, messageIsSealedTerminal, traceAuthorityIsSealed } from "./workbench-terminal-authority";
+import { boundedProjectionMessageLimit, mergeBoundedProjectionMessages, selectProjectionMessageWindow, traceProjectionIsTerminalSealed } from "./workbench-message-projection-budget";
 import {
   agentErrorDisplayText,
   agentErrorFromApiFailure,
@@ -491,10 +492,10 @@ export const useWorkbenchStore = defineStore("workbench", () => {
   async function refreshSessionMessageProjectionPage(sessionId: string | null | undefined, options: { force?: boolean } = {}): Promise<void> {
     const id = normalizeWorkbenchSessionId(sessionId);
     if (!id) return;
-    const response = await workbenchColadaQueries.fetchSessionMessages(id, { limit: 100, minIntervalMs: runtimePolicy.workbenchSessionMessagesMinRefreshMs, force: options.force });
+    const response = await workbenchColadaQueries.fetchSessionMessages(id, { limit: sessionMessageProjectionWindowLimit(), minIntervalMs: runtimePolicy.workbenchSessionMessagesMinRefreshMs, force: options.force });
     if (!response.ok || !response.data) return;
     const pageMessages = Array.isArray(response.data.messages) ? response.data.messages.map((message) => normalizeChatMessage(message as ChatMessage)) : [];
-    const merged = mergeMessageProjectionPage(id, pageMessages);
+    const merged = mergeMessageProjectionPage(id, pageMessages, { limit: sessionMessageProjectionWindowLimit() });
     rememberSessionMessages(id, merged);
     await hydrateTurnStatusAuthority(merged);
     hydrateTerminalTraceGaps(merged, "session-message-page");
@@ -510,7 +511,9 @@ export const useWorkbenchStore = defineStore("workbench", () => {
   async function refreshMessageProjectionForTrace(sessionId: string | null | undefined, traceId: string, options: { force?: boolean } = {}): Promise<void> {
     const id = normalizeWorkbenchSessionId(sessionId);
     if (!id) return;
-    const response = await workbenchColadaQueries.fetchSessionMessages(id, { limit: 100, minIntervalMs: runtimePolicy.workbenchSessionMessagesMinRefreshMs, force: options.force });
+    const existing = serverState.value.messagesBySessionId[id] ?? [];
+    if (traceProjectionIsTerminalSealed(traceId, existing)) return;
+    const response = await workbenchColadaQueries.fetchSessionMessages(id, { limit: traceMessageProjectionWindowLimit(), minIntervalMs: runtimePolicy.workbenchSessionMessagesMinRefreshMs, force: options.force });
     if (!response.ok || !response.data) {
       if (shouldSuppressTransientWorkbenchReadFailure(response)) return;
       if (traceHasTerminalResponse(traceId, messages.value)) return;
@@ -518,15 +521,25 @@ export const useWorkbenchStore = defineStore("workbench", () => {
       return;
     }
     const pageMessages = Array.isArray(response.data.messages) ? response.data.messages.map((message) => normalizeChatMessage(message as ChatMessage)) : [];
-    const merged = mergeMessageProjectionPage(id, pageMessages);
+    const merged = mergeMessageProjectionPage(id, pageMessages, { traceId, limit: traceMessageProjectionWindowLimit() });
     rememberSessionMessages(id, merged);
     await hydrateTurnStatusAuthority(merged);
-    hydrateTerminalTraceGaps(merged, `trace-message-page:${traceId}`);
+    if (!traceProjectionIsTerminalSealed(traceId, merged)) hydrateTerminalTraceGaps(merged, `trace-message-page:${traceId}`);
   }
 
-  function mergeMessageProjectionPage(sessionId: string, pageMessages: ChatMessage[]): ChatMessage[] {
+  function sessionMessageProjectionWindowLimit(): number {
+    return boundedProjectionMessageLimit(runtimePolicy.workbenchSessionMessagesWindowLimit, runtimePolicy.sessionListPageLimit);
+  }
+
+  function traceMessageProjectionWindowLimit(): number {
+    return boundedProjectionMessageLimit(runtimePolicy.workbenchTraceMessagesWindowLimit, runtimePolicy.traceHydrationAutoQueueLimit);
+  }
+
+  function mergeMessageProjectionPage(sessionId: string, pageMessages: ChatMessage[], options: { traceId?: string | null; limit: number }): ChatMessage[] {
     const existing = serverState.value.messagesBySessionId[sessionId] ?? [];
-    return pageMessages.map((message) => mergeMessageProjectionMessage(message, existing));
+    const windowMessages = selectProjectionMessageWindow(pageMessages, options);
+    const incoming = windowMessages.map((message) => mergeMessageProjectionMessage(message, existing));
+    return mergeBoundedProjectionMessages(existing, incoming);
   }
 
   function mergeMessageProjectionMessage(message: ChatMessage, existing: ChatMessage[]): ChatMessage {
@@ -999,6 +1012,11 @@ export const useWorkbenchStore = defineStore("workbench", () => {
     if (ownerBefore === activeSessionId.value) recordActivity(reason);
     await refreshTurnStatusByTraceId(id);
     const ownerSessionId = traceOwnerSessionId(id, turnStatusAuthority.value[id]?.sessionId ?? null) ?? ownerBefore;
+    const ownerMessagesBeforeProjection = ownerSessionId ? serverState.value.messagesBySessionId[ownerSessionId] ?? [] : messages.value;
+    if (traceProjectionIsTerminalSealed(id, ownerMessagesBeforeProjection)) {
+      clearActiveTraceRestGapFill(id);
+      return;
+    }
     if (ownerSessionId) {
       scheduleSessionListRefresh(ownerSessionId, runtimePolicy.sessionListRealtimeRefreshDelayMs);
       await refreshMessageProjectionForTrace(ownerSessionId, id);
@@ -1288,10 +1306,10 @@ export const useWorkbenchStore = defineStore("workbench", () => {
     rememberTurnStatus(traceId, result);
     markWorkbenchTraceProjected(traceId);
     clearActiveTraceRestGapFill(traceId);
-    void refreshMessageProjectionForTrace(ownerSessionId, traceId, { force: options.forceRead });
     const ownerMessages = serverState.value.messagesBySessionId[ownerSessionId] ?? [];
     const terminalMessage = latestMessageForTrace(traceId, ownerMessages);
-    if (options.forceRead && terminalMessage) void hydrateTraceEventsForMessage(terminalMessage, { force: true });
+    if (!traceProjectionIsTerminalSealed(traceId, ownerMessages)) void refreshMessageProjectionForTrace(ownerSessionId, traceId, { force: options.forceRead });
+    if (options.forceRead && terminalMessage && !traceProjectionIsTerminalSealed(traceId, ownerMessages)) void hydrateTraceEventsForMessage(terminalMessage, { force: true });
     if (ownerSessionId === activeSessionId.value) {
       chatPending.value = false;
       currentRequest.value = null;
diff --git a/web/hwlab-cloud-web/src/types/global.d.ts b/web/hwlab-cloud-web/src/types/global.d.ts
index 108d7d5e..651a490a 100644
--- a/web/hwlab-cloud-web/src/types/global.d.ts
+++ b/web/hwlab-cloud-web/src/types/global.d.ts
@@ -30,6 +30,8 @@ declare global {
           workbenchTraceEventsMinRefreshMs?: number;
           workbenchSessionDetailMinRefreshMs?: number;
           workbenchSessionMessagesMinRefreshMs?: number;
+          workbenchSessionMessagesWindowLimit?: number;
+          workbenchTraceMessagesWindowLimit?: number;
           workbenchRealtimeSessionMessagesMinRefreshMs?: number;
           workbenchTraceEventsTimeoutMs?: number;
           sessionListRealtimeRefreshDelayMs?: number;