diff --git a/changelog.d/fixes/10156-responses-commentary-completed-snapshot.md b/changelog.d/fixes/10156-responses-commentary-completed-snapshot.md
new file mode 100644
index 0000000000..7a976ab85d
--- /dev/null
+++ b/changelog.d/fixes/10156-responses-commentary-completed-snapshot.md
@@ -0,0 +1 @@
+- **fix(sse):** Responses-passthrough `response.completed` snapshots now drop `phase:"commentary"` items the same way live SSE frames already do, so the terminal `response.output` array no longer echoes internal commentary text that was already suppressed from the stream (#10156).
diff --git a/open-sse/utils/responsesStreamHelpers.ts b/open-sse/utils/responsesStreamHelpers.ts
index ce40999eb8..a2cba80fc1 100644
--- a/open-sse/utils/responsesStreamHelpers.ts
+++ b/open-sse/utils/responsesStreamHelpers.ts
@@ -121,6 +121,29 @@ export function pushUniqueResponsesOutputItems(target: unknown[], items: readonl
   }
 }
 
+/**
+ * #10156 — strip items matched by `isCommentaryItem` (the same predicate used
+ * to drop live commentary-phase SSE frames, #6199) from a `response.completed`
+ * output array before it is forwarded or buffered for backfill. Upstreams may
+ * echo an already-dropped commentary item back inside a non-empty terminal
+ * `output` array; without this, the live stream and the terminal snapshot
+ * silently disagree about what the client actually saw.
+ */
+export function filterResponsesCommentaryFromItems(
+  items: readonly unknown[],
+  isCommentaryItem: (item: unknown) => boolean
+): { items: unknown[]; changed: boolean } {
+  let changed = false;
+  const filtered = items.filter((item) => {
+    if (isCommentaryItem(item)) {
+      changed = true;
+      return false;
+    }
+    return true;
+  });
+  return { items: filtered, changed };
+}
+
 export function backfillResponsesCompletedOutput(
   parsed: unknown,
   collectedItems: readonly unknown[]
diff --git a/open-sse/utils/stream.ts b/open-sse/utils/stream.ts
index 275669e983..4eab8ea7fc 100644
--- a/open-sse/utils/stream.ts
+++ b/open-sse/utils/stream.ts
@@ -36,6 +36,7 @@ import {
 import { STREAM_IDLE_TIMEOUT_MS, FETCH_BODY_TIMEOUT_MS, HTTP_STATUS } from "../config/constants.ts";
 import {
   OMIT_STREAMING_CHUNK_MARKER,
+  isResponsesCommentaryMessageItem,
   sanitizeStreamingChunk,
 } from "../handlers/responseSanitizer.ts";
 import { isFeatureFlagEnabled } from "@/shared/utils/featureFlags";
@@ -59,6 +60,7 @@ import {
 } from "../services/sessionManager.ts";
 import {
   backfillResponsesCompletedOutput,
+  filterResponsesCommentaryFromItems,
   normalizeResponsesCompletedUsage as normalizeUsage,
   normalizeResponsesSseIds,
   pushUniqueResponsesOutputItems,
@@ -1565,11 +1567,26 @@ export function createSSEStream(options: StreamOptions = {}) {
                       }
                     }
                   }
+                  let responsesCommentaryStrippedFromCompleted = false;
                   if (
                     parsed.type === "response.completed" &&
                     Array.isArray(parsed.response?.output) &&
                     parsed.response.output.length > 0
                   ) {
+                    // #10156 — an upstream may echo a `phase:"commentary"` item back
+                    // inside a non-empty terminal `output` array even though its live
+                    // SSE frames were already dropped above. Keep both representations
+                    // consistent by applying the same drop here.
+                    if (shouldDropResponsesCommentary) {
+                      const { items, changed } = filterResponsesCommentaryFromItems(
+                        parsed.response.output,
+                        isResponsesCommentaryMessageItem
+                      );
+                      if (changed) {
+                        parsed.response.output = items;
+                        responsesCommentaryStrippedFromCompleted = true;
+                      }
+                    }
                     pushUniqueResponsesOutputItems(
                       passthroughResponsesOutputItems,
                       parsed.response.output
@@ -1613,9 +1630,19 @@ export function createSSEStream(options: StreamOptions = {}) {
                     ]) as typeof parsed;
                   }
                   const stripped = stripResponsesLifecycleEcho(parsed);
+                  // Belt-and-suspenders for #10156: filter the backfill buffer itself
+                  // before it can seed an empty `response.completed.response.output`,
+                  // in case a future code path pushes a commentary item into it
+                  // without going through the response.completed branch above.
+                  const backfillCandidates = shouldDropResponsesCommentary
+                    ? filterResponsesCommentaryFromItems(
+                        passthroughResponsesOutputItems,
+                        isResponsesCommentaryMessageItem
+                      ).items
+                    : passthroughResponsesOutputItems;
                   const backfilled = backfillResponsesCompletedOutput(
                     parsed,
-                    passthroughResponsesOutputItems
+                    backfillCandidates
                   );
                   const usageNormalized = normalizeUsage(parsed);
                   if (
@@ -1623,7 +1650,8 @@ export function createSSEStream(options: StreamOptions = {}) {
                     backfilled ||
                     textualToolCallBackfilled ||
                     responsesIdsNormalized ||
-                    usageNormalized
+                    usageNormalized ||
+                    responsesCommentaryStrippedFromCompleted
                   ) {
                     output = `data: ${JSON.stringify(parsed)}\n\n`;
                     injectedUsage = true;
diff --git a/tests/unit/responses-commentary-passthrough-6199.test.ts b/tests/unit/responses-commentary-passthrough-6199.test.ts
index 2b041c48d8..c1fce80786 100644
--- a/tests/unit/responses-commentary-passthrough-6199.test.ts
+++ b/tests/unit/responses-commentary-passthrough-6199.test.ts
@@ -364,3 +364,63 @@ test("Claude to Responses translation includes canonical Codex usage", async ()
   assert.equal(completed.response.usage.output_tokens, 6);
   assert.equal(completed.response.usage.total_tokens, 94);
 });
+
+// #10156 — the live-frame drop above works correctly, but real upstreams (as in
+// the issue's repro) echo the ALREADY-DROPPED commentary item back inside the
+// terminal `response.completed.response.output` array. Because that array is
+// non-empty, `backfillResponsesCompletedOutput` never touches it, so the
+// terminal snapshot silently disagreed with the events already delivered to
+// the client. This must stay filtered too.
+test("response.completed strips a commentary item the upstream echoes back non-empty (#10156)", async () => {
+  const output = await readTransformed(
+    [
+      ...buildResponsesStream().slice(0, -1),
+      sse({
+        type: "response.completed",
+        response: {
+          id: "resp_10156",
+          output: [
+            {
+              id: "msg_commentary",
+              type: "message",
+              role: "assistant",
+              phase: "commentary",
+              content: [{ type: "output_text", text: COMMENTARY_TEXT }],
+            },
+            {
+              id: "msg_final",
+              type: "message",
+              role: "assistant",
+              phase: "final",
+              content: [{ type: "output_text", text: FINAL_TEXT }],
+            },
+          ],
+        },
+      }),
+    ],
+    { ...PASSTHROUGH_RESPONSES_OPTIONS, dropResponsesCommentary: true }
+  );
+
+  assert.ok(
+    !output.includes(COMMENTARY_TEXT),
+    "commentary text must never reach the client, live or in the terminal snapshot"
+  );
+  assert.ok(
+    !output.includes("msg_commentary"),
+    "the commentary item id must not appear anywhere in the forwarded stream"
+  );
+
+  const completedLine = output
+    .split(/\r?\n/)
+    .find((line) => line.startsWith("data:") && line.includes('"response.completed"'));
+  assert.ok(completedLine, "the terminal Responses event must be forwarded");
+  const completed = JSON.parse(completedLine.slice(5).trim());
+  assert.ok(
+    !completed.response.output.some((item: { phase?: string }) => item.phase === "commentary"),
+    "BUG #10156: response.completed.response.output must not retain the commentary item once its live SSE frames were suppressed — live stream and terminal snapshot must stay consistent"
+  );
+  assert.ok(
+    completed.response.output.some((item: { id?: string }) => item.id === "msg_final"),
+    "the final answer item must still be present in the terminal snapshot"
+  );
+});