diff --git a/internal/cloud/server-workbench-http.ts b/internal/cloud/server-workbench-http.ts
index 31638963..b20207fc 100644
--- a/internal/cloud/server-workbench-http.ts
+++ b/internal/cloud/server-workbench-http.ts
@@ -15,6 +15,7 @@ import {
 } from "./server-http-utils.ts";
 import { createWorkbenchReadModel } from "./workbench-read-model.ts";
 import { createWorkbenchRuntimeClient } from "./workbench-runtime-client.ts";
+import { buildWorkbenchSessionDetail, compactLaunchContext, includeMessagesForSessionDetail } from "./workbench-session-detail-response.ts";
 import { durableTraceStatus, RUNNING_STATUSES, terminalFinalResponse, TERMINAL_STATUSES } from "./workbench-turn-projection.ts";
 import { emitCodeAgentOtelSpan, emitHttpServerRequestSpan } from "./otel-trace.ts";
 
@@ -1018,87 +1019,13 @@ function factSessionSummary(session, facts = {}) {
 
 function factSessionDetail(session, facts = {}, options = {}) {
   const sessionId = factSessionId(session);
-  const detail = {
-    ...factSessionSummary(session, facts),
-    metadata: {
-      startedAt: session?.startedAt ?? session?.createdAt ?? null,
-      endedAt: session?.endedAt ?? null,
-      ownerUserId: session?.ownerUserId ?? null,
-      launchContext: compactLaunchContext(session?.sessionJson?.launchContext)
-    },
-    messagePageUrl: sessionId ? `/v1/workbench/sessions/${encodeURIComponent(sessionId)}/messages` : null,
-    valuesRedacted: true,
-    secretMaterialStored: false
-  };
-  return options.includeMessages === true ? { ...detail, messages: factMessagesForSession(session, facts) } : detail;
-}
-
-function compactLaunchContext(value) {
-  if (!value || typeof value !== "object" || Array.isArray(value)) return null;
-  const projectId = textValue(value.projectId) || null;
-  const taskRef = textValue(value.taskRef) || null;
-  if (!projectId && !taskRef) return null;
-  const execution = compactLaunchExecutionContext(value.executionContext ?? value.execution ?? value);
-  return {
-    source: textValue(value.source) || "project-management",
-    projectId,
-    taskRef,
-    sourceId: textValue(value.sourceId) || null,
-    sourceKind: textValue(value.sourceKind) || execution?.sourceKind || null,
-    fileRef: textValue(value.fileRef) || null,
-    relativePath: textValue(value.relativePath) || execution?.relativePath || null,
-    taskId: textValue(value.taskId) || null,
-    title: textValue(value.title) || null,
-    status: textValue(value.status) || null,
-    hwpodId: (execution?.hwpodId ?? textValue(value.hwpodId)) || null,
-    nodeId: (execution?.nodeId ?? textValue(value.nodeId)) || null,
-    mdtodoRootRef: (execution?.mdtodoRootRef ?? textValue(value.mdtodoRootRef)) || null,
-    workspaceRootHash: (execution?.workspaceRootHash ?? textValue(value.workspaceRootHash)) || null,
-    workspaceRootLabel: (execution?.workspaceRootLabel ?? textValue(value.workspaceRootLabel)) || null,
-    hwpodWorkspaceArgs: (execution?.hwpodWorkspaceArgs ?? textValue(value.hwpodWorkspaceArgs)) || null,
-    contextFingerprint: (execution?.contextFingerprint ?? textValue(value.contextFingerprint)) || null,
-    capabilities: compactPlainObject(value.capabilities),
-    executionContext: execution,
-    route: textValue(value.route) || "/projects/mdtodo",
-    valuesRedacted: true
-  };
-}
-
-function compactLaunchExecutionContext(value) {
-  if (!value || typeof value !== "object" || Array.isArray(value)) return null;
-  const sourceId = textValue(value.sourceId) || null;
-  const taskRef = textValue(value.taskRef) || null;
-  const hwpodId = textValue(value.hwpodId) || null;
-  if (!sourceId && !taskRef && !hwpodId) return null;
-  return {
-    sourceId,
-    sourceKind: textValue(value.sourceKind) || null,
-    projectId: textValue(value.projectId) || null,
-    taskRef,
-    fileRef: textValue(value.fileRef) || null,
-    relativePath: textValue(value.relativePath) || null,
-    taskId: textValue(value.taskId) || null,
-    hwpodId,
-    nodeId: textValue(value.nodeId) || null,
-    mdtodoRootRef: textValue(value.mdtodoRootRef) || null,
-    workspaceRootRef: textValue(value.workspaceRootRef) || null,
-    workspaceRootHash: textValue(value.workspaceRootHash) || null,
-    workspaceRootLabel: textValue(value.workspaceRootLabel) || null,
-    hwpodWorkspaceArgs: textValue(value.hwpodWorkspaceArgs) || null,
-    contextFingerprint: textValue(value.contextFingerprint) || null,
-    capabilities: compactPlainObject(value.capabilities),
-    valuesRedacted: true
-  };
-}
-
-function compactPlainObject(value) {
-  if (!value || typeof value !== "object" || Array.isArray(value)) return {};
-  const out = {};
-  for (const [key, item] of Object.entries(value)) {
-    if (!/^[A-Za-z0-9_.:-]{1,80}$/u.test(key)) continue;
-    if (["string", "number", "boolean"].includes(typeof item) || item === null) out[key] = item;
-  }
-  return out;
+  return buildWorkbenchSessionDetail({
+    summary: factSessionSummary(session, facts),
+    session,
+    sessionId,
+    includeMessages: options.includeMessages === true,
+    messages: factMessagesForSession(session, facts)
+  });
 }
 
 function factMessagesForSession(session, facts = {}) {
@@ -2931,17 +2858,6 @@ function boundedSessionListLimit(value) {
   return Math.min(MAX_PAGE_LIMIT, parsePositiveInteger(value, DEFAULT_SESSION_LIST_LIMIT));
 }
 
-function includeMessagesForSessionDetail(params) {
-  if (params.get("includeMessages") !== null) return truthyQueryFlag(params.get("includeMessages"));
-  if (params.get("messages") !== null) return truthyQueryFlag(params.get("messages"));
-  return false;
-}
-
-function truthyQueryFlag(value) {
-  const normalized = textValue(value).toLowerCase();
-  return normalized === "1" || normalized === "true" || normalized === "yes";
-}
-
 function cursorOffset(value) {
   const text = textValue(value);
   if (!text) return 0;
diff --git a/internal/cloud/workbench-session-detail-response.ts b/internal/cloud/workbench-session-detail-response.ts
new file mode 100644
index 00000000..2b96c35b
--- /dev/null
+++ b/internal/cloud/workbench-session-detail-response.ts
@@ -0,0 +1,120 @@
+/*
+ * SPEC: pikasTech/HWLAB#2359 Workbench session detail metadata-only contract.
+ * Responsibility: Build Workbench session detail payloads and parse detail payload mode flags.
+ */
+
+export function includeMessagesForSessionDetail(params: URLSearchParams): boolean {
+  if (params.get("includeMessages") !== null) return truthyQueryFlag(params.get("includeMessages"));
+  if (params.get("messages") !== null) return truthyQueryFlag(params.get("messages"));
+  return false;
+}
+
+export function buildWorkbenchSessionDetail(input: {
+  summary: Record<string, unknown>;
+  session: Record<string, unknown> | null | undefined;
+  sessionId: string | null | undefined;
+  includeMessages: boolean;
+  messages: unknown[];
+}): Record<string, unknown> {
+  const detail = {
+    ...input.summary,
+    metadata: {
+      startedAt: input.session?.startedAt ?? input.session?.createdAt ?? null,
+      endedAt: input.session?.endedAt ?? null,
+      ownerUserId: input.session?.ownerUserId ?? null,
+      launchContext: compactLaunchContext(sessionJson(input.session)?.launchContext)
+    },
+    messagePageUrl: input.sessionId ? `/v1/workbench/sessions/${encodeURIComponent(input.sessionId)}/messages` : null,
+    valuesRedacted: true,
+    secretMaterialStored: false
+  };
+  return input.includeMessages ? { ...detail, messages: input.messages } : detail;
+}
+
+export function compactLaunchContext(value: unknown): Record<string, unknown> | null {
+  const record = objectRecord(value);
+  if (!record) return null;
+  const projectId = textValue(record.projectId) || null;
+  const taskRef = textValue(record.taskRef) || null;
+  if (!projectId && !taskRef) return null;
+  const execution = compactLaunchExecutionContext(record.executionContext ?? record.execution ?? record);
+  return {
+    source: textValue(record.source) || "project-management",
+    projectId,
+    taskRef,
+    sourceId: textValue(record.sourceId) || null,
+    sourceKind: textValue(record.sourceKind) || execution?.sourceKind || null,
+    fileRef: textValue(record.fileRef) || null,
+    relativePath: textValue(record.relativePath) || execution?.relativePath || null,
+    taskId: textValue(record.taskId) || null,
+    title: textValue(record.title) || null,
+    status: textValue(record.status) || null,
+    hwpodId: (execution?.hwpodId ?? textValue(record.hwpodId)) || null,
+    nodeId: (execution?.nodeId ?? textValue(record.nodeId)) || null,
+    mdtodoRootRef: (execution?.mdtodoRootRef ?? textValue(record.mdtodoRootRef)) || null,
+    workspaceRootHash: (execution?.workspaceRootHash ?? textValue(record.workspaceRootHash)) || null,
+    workspaceRootLabel: (execution?.workspaceRootLabel ?? textValue(record.workspaceRootLabel)) || null,
+    hwpodWorkspaceArgs: (execution?.hwpodWorkspaceArgs ?? textValue(record.hwpodWorkspaceArgs)) || null,
+    contextFingerprint: (execution?.contextFingerprint ?? textValue(record.contextFingerprint)) || null,
+    capabilities: compactPlainObject(record.capabilities),
+    executionContext: execution,
+    route: textValue(record.route) || "/projects/mdtodo",
+    valuesRedacted: true
+  };
+}
+
+function compactLaunchExecutionContext(value: unknown): Record<string, unknown> | null {
+  const record = objectRecord(value);
+  if (!record) return null;
+  const sourceId = textValue(record.sourceId) || null;
+  const taskRef = textValue(record.taskRef) || null;
+  const hwpodId = textValue(record.hwpodId) || null;
+  if (!sourceId && !taskRef && !hwpodId) return null;
+  return {
+    sourceId,
+    sourceKind: textValue(record.sourceKind) || null,
+    projectId: textValue(record.projectId) || null,
+    taskRef,
+    fileRef: textValue(record.fileRef) || null,
+    relativePath: textValue(record.relativePath) || null,
+    taskId: textValue(record.taskId) || null,
+    hwpodId,
+    nodeId: textValue(record.nodeId) || null,
+    mdtodoRootRef: textValue(record.mdtodoRootRef) || null,
+    workspaceRootRef: textValue(record.workspaceRootRef) || null,
+    workspaceRootHash: textValue(record.workspaceRootHash) || null,
+    workspaceRootLabel: textValue(record.workspaceRootLabel) || null,
+    hwpodWorkspaceArgs: textValue(record.hwpodWorkspaceArgs) || null,
+    contextFingerprint: textValue(record.contextFingerprint) || null,
+    capabilities: compactPlainObject(record.capabilities),
+    valuesRedacted: true
+  };
+}
+
+function compactPlainObject(value: unknown): Record<string, unknown> {
+  const record = objectRecord(value);
+  if (!record) return {};
+  const out: Record<string, unknown> = {};
+  for (const [key, item] of Object.entries(record)) {
+    if (!/^[A-Za-z0-9_.:-]{1,80}$/u.test(key)) continue;
+    if (["string", "number", "boolean"].includes(typeof item) || item === null) out[key] = item;
+  }
+  return out;
+}
+
+function sessionJson(session: Record<string, unknown> | null | undefined): Record<string, unknown> | null {
+  return objectRecord(session?.sessionJson);
+}
+
+function objectRecord(value: unknown): Record<string, unknown> | null {
+  return value && typeof value === "object" && !Array.isArray(value) ? value as Record<string, unknown> : null;
+}
+
+function textValue(value: unknown): string {
+  return typeof value === "string" && value.trim() ? value.trim() : "";
+}
+
+function truthyQueryFlag(value: string | null): boolean {
+  const normalized = textValue(value).toLowerCase();
+  return normalized === "1" || normalized === "true" || normalized === "yes";
+}