diff --git a/open-sse/handlers/chatCore/streamingPipeline.ts b/open-sse/handlers/chatCore/streamingPipeline.ts
index d67803918b..429b3d79de 100644
--- a/open-sse/handlers/chatCore/streamingPipeline.ts
+++ b/open-sse/handlers/chatCore/streamingPipeline.ts
@@ -23,6 +23,17 @@ import { createPiiSseTransform as defaultPiiSse } from "@/lib/streamingPiiTransf
 import { isFeatureFlagEnabled as defaultFeatureFlag } from "@/shared/utils/featureFlags";
 import { OMNIROUTE_RESPONSE_HEADERS } from "@/shared/constants/headers";
 import { SSE_HEARTBEAT_INTERVAL_MS } from "../../config/constants.ts";
+/**
+ * Pipeline assembly instrumentation — performance.mark() along the SSE hot path.
+ * Marks are visible to Node.js perf_hooks consumers and DevTools' Performance
+ * panel when NODE_OPTIONS=--enable-node-performance-clinician or similar.
+ *
+ * Each call to assembleStreamingPipeline creates one measure record:
+ *   "omni-pipeline" — wall-clock duration of the full transform chain assembly.
+ */
+const PIPELINE_START = "omni-pipeline-start";
+const PIPELINE_END = "omni-pipeline-end";
+const PIPELINE_MEASURE = "omni-pipeline";
 
 type HeadersLike = Headers | Record<string, unknown> | null | undefined;
 
@@ -61,6 +72,7 @@ export function assembleStreamingPipeline(
   },
   deps: StreamingPipelineDeps = DEFAULT_DEPS
 ) {
+  performance.mark(PIPELINE_START);
   // ── Phase 9.3: Progress tracking (opt-in) ──
   const progressEnabled = deps.wantsProgress(args.clientRawRequestHeaders);
   let finalStream;
@@ -77,7 +89,9 @@ export function assembleStreamingPipeline(
   }
 
   if (progressEnabled) {
-    const progressTransform = deps.createProgressTransform({ signal: args.streamController.signal });
+    const progressTransform = deps.createProgressTransform({
+      signal: args.streamController.signal,
+    });
     // Chain: provider → transform → progress → client
     finalStream = piiStream.pipeThrough(progressTransform);
     args.responseHeaders[OMNIROUTE_RESPONSE_HEADERS.progress] = "enabled";
@@ -95,5 +109,7 @@ export function assembleStreamingPipeline(
   if (args.echoModel) {
     finalStream = finalStream.pipeThrough(deps.createModelEchoTransform(args.echoModel));
   }
+  performance.mark(PIPELINE_END);
+  performance.measure(PIPELINE_MEASURE, PIPELINE_START, PIPELINE_END);
   return finalStream;
 }
diff --git a/open-sse/utils/stream.ts b/open-sse/utils/stream.ts
index 581f1f6342..e145ab4290 100644
--- a/open-sse/utils/stream.ts
+++ b/open-sse/utils/stream.ts
@@ -639,6 +639,16 @@ export function createSSEStream(options: StreamOptions = {}) {
     dropResponsesCommentary,
   } = options;
   const signatureNamespace = connectionId;
+  // Request-body-size metric (for monitoring payload size distribution & correlation with TTFT).
+  // The size is JSON-serialised byte count; stored as a performance mark detail so monitoring
+  // tools can query performance.getEntriesByType("mark") filtered by name.
+  let bodySize = 0;
+  try {
+    bodySize = body ? new TextEncoder().encode(JSON.stringify(body)).length : 0;
+  } catch {
+    /* body may not be JSON-serialisable (e.g. FormData, Blob) — metric stays 0 */
+  }
+  if (bodySize > 0) performance.mark("omni-request-body-size", { detail: bodySize });
 
   // Drop internal commentary-phase Responses output before forwarding (#6199).
   // Explicit option wins; otherwise read the feature flag (default on). Resolved