diff --git a/src/channels/streaming-flat-key-deprecation.test.ts b/src/channels/streaming-flat-key-deprecation.test.ts
index bc237fcc4f3e..2a613ac2593e 100644
--- a/src/channels/streaming-flat-key-deprecation.test.ts
+++ b/src/channels/streaming-flat-key-deprecation.test.ts
@@ -2,6 +2,7 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { resetFlatStreamingKeyDeprecationWarningsForTest } from "./streaming-flat-key-deprecation.js";
 import {
+  resolveChannelPreviewStreamMode,
   resolveChannelStreamingBlockCoalesce,
   resolveChannelStreamingBlockEnabled,
   resolveChannelStreamingChunkMode,
@@ -56,4 +57,15 @@ describe("flat streaming key deprecation warning", () => {
     expect(resolveChannelStreamingBlockCoalesce({})).toBeUndefined();
     expect(loggerMocks.warn).not.toHaveBeenCalled();
   });
+
+  it.each([
+    { scalar: "block", expected: "block" },
+    { scalar: true, expected: "partial" },
+  ])("warns once when the scalar streaming fallback is used", ({ scalar, expected }) => {
+    expect(resolveChannelPreviewStreamMode({ streaming: scalar }, "off")).toBe(expected);
+    expect(resolveChannelPreviewStreamMode({ streaming: scalar }, "off")).toBe(expected);
+    expect(loggerMocks.warn).toHaveBeenCalledTimes(1);
+    expect(loggerMocks.warn.mock.calls[0]?.[0]).toContain('"streaming"');
+    expect(loggerMocks.warn.mock.calls[0]?.[0]).toContain("streaming.mode");
+  });
 });
diff --git a/src/channels/streaming.test.ts b/src/channels/streaming.test.ts
index 9eeb57541c91..bef1379f1fa6 100644
--- a/src/channels/streaming.test.ts
+++ b/src/channels/streaming.test.ts
@@ -95,9 +95,8 @@ describe("buildChannelProgressDraftLine", () => {
 });
 
 describe("streaming config resolution", () => {
-  // Flat delivery keys stay canonical for channels without a nested streaming
-  // schema and for SDK plugins; mode-family aliases (streamMode, scalar
-  // streaming, nativeStreaming) are doctor-migrated and unread at runtime.
+  // Flat delivery keys remain external SDK compatibility fallbacks. Bundled
+  // schemas are nested-only; mode-family aliases stay doctor-only.
   it("resolves flat delivery keys while ignoring mode-family aliases", () => {
     const legacyEntry = {
       streamMode: "block",
@@ -135,8 +134,8 @@ describe("streaming config resolution", () => {
     expect(resolveChannelStreamingNativeTransport(entry)).toBe(false);
   });
 
-  it("keeps scalar streaming support for channels whose schema allows it", () => {
-    // Mattermost's schema accepts a scalar mode string or boolean as canonical.
+  it("keeps the scalar streaming fallback for external SDK plugin configs", () => {
+    // Bundled schemas are nested-only; this compatibility path is deprecated.
     expect(resolveChannelPreviewStreamMode({ streaming: "block" }, "partial")).toBe("block");
     expect(resolveChannelPreviewStreamMode({ streaming: true }, "off")).toBe("partial");
     expect(resolveChannelPreviewStreamMode({ streaming: false }, "partial")).toBe("off");
diff --git a/src/channels/streaming.ts b/src/channels/streaming.ts
index 9a7bb0c6cca7..65e73dadcce6 100644
--- a/src/channels/streaming.ts
+++ b/src/channels/streaming.ts
@@ -41,13 +41,10 @@ export type StreamingCompatEntry = {
   draftChunk?: unknown;
 };
 
-// Nested streaming config wins. Every bundled channel now uses a nested-only
-// streaming schema with doctor migrating the flat spellings, so in-tree the
-// flat delivery keys (chunkMode, blockStreaming, blockStreamingCoalesce,
-// draftChunk) are legacy config. The fallback reads below serve external SDK
-// plugin configs only and emit a once-per-key deprecation warning; remove
-// them (and the flat StreamingCompatEntry fields) when the next release train
-// closes the SDK deprecation window.
+// Bundled schemas are nested-only; doctor migrates flat delivery keys
+// (chunkMode, blockStreaming, blockStreamingCoalesce, draftChunk) and scalar
+// `streaming`. External SDK fallbacks warn once; remove them, the flat
+// StreamingCompatEntry fields, and scalar support after the next release train.
 // Mode-family aliases (streamMode) are doctor-only and stay unread here.
 
 function asObjectRecord(value: unknown): Record<string, unknown> | null {
@@ -922,13 +919,16 @@ export function resolveChannelPreviewStreamMode(
   // Scalar `streaming` (mode string or boolean) is rejected by every bundled
   // channel schema and doctor-migrated to streaming.mode; the read here stays
   // only for external SDK plugin configs that predate the nested shape.
-  const parsedStreaming = parsePreviewStreamingMode(
-    getChannelStreamingConfigObject(entry)?.mode ?? entry?.streaming,
-  );
+  const streamingConfig = getChannelStreamingConfigObject(entry);
+  const parsedStreaming = parsePreviewStreamingMode(streamingConfig?.mode ?? entry?.streaming);
   if (parsedStreaming) {
+    if (!streamingConfig) {
+      warnFlatStreamingKeyFallback("streaming", "mode");
+    }
     return parsedStreaming;
   }
   if (typeof entry?.streaming === "boolean") {
+    warnFlatStreamingKeyFallback("streaming", "mode");
     return entry.streaming ? "partial" : "off";
   }
   return defaultMode;
diff --git a/src/plugin-sdk/channel-streaming.test.ts b/src/plugin-sdk/channel-streaming.test.ts
index 2f8a504210bb..bf80ba4e61a2 100644
--- a/src/plugin-sdk/channel-streaming.test.ts
+++ b/src/plugin-sdk/channel-streaming.test.ts
@@ -121,10 +121,9 @@ describe("channel-streaming", () => {
   });
 
   it("resolves flat delivery keys when no nested streaming config exists", () => {
-    // Flat delivery keys stay canonical for channels without a nested
-    // streaming schema (Mattermost, WhatsApp, Google Chat, IRC, Signal) and
-    // for external SDK plugins; mode-family aliases (streamMode, scalar
-    // streaming, nativeStreaming) are doctor-only and stay unread.
+    // Bundled channel schemas are nested-only. Flat delivery keys remain
+    // compatibility fallbacks for external SDK plugins; mode-family aliases
+    // are doctor-only and stay unread.
     const entry = {
       chunkMode: "newline",
       blockStreaming: true,