fix(discord): strip reasoning tags from partial stream preview

When streamMode is "partial", reasoning/thinking block content can leak
into the Discord draft preview because the partial text is forwarded to
the draft stream without filtering.  Apply `stripReasoningTagsFromText`
before updating the draft and skip pure-reasoning messages (those
starting with "Reasoning:\n") so internal thinking traces never reach
the user-visible preview.

Fixes #24532

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/discord/monitor/message-handler.process.ts

@@ -30,6 +30,7 @@ import { convertMarkdownTables } from "../../markdown/tables.js";
 import { buildAgentSessionKey } from "../../routing/resolve-route.js";
 import { resolveThreadSessionKeys } from "../../routing/session-key.js";
 import { buildUntrustedChannelMetadata } from "../../security/channel-metadata.js";
+import { stripReasoningTagsFromText } from "../../shared/text/reasoning-tags.js";
 import { truncateUtf16Safe } from "../../utils.js";
 import { chunkDiscordTextWithMode } from "../chunk.js";
 import { resolveDiscordDraftStreamingChunking } from "../draft-chunking.js";
@@ -485,7 +486,13 @@ export async function processDiscordMessage(ctx: DiscordMessagePreflightContext)
     if (!draftStream || !text) {
       return;
     }
-    if (text === lastPartialText) {
+    // Strip reasoning/thinking tags that may leak through the stream.
+    const cleaned = stripReasoningTagsFromText(text, { mode: "strict", trim: "both" });
+    // Skip pure-reasoning messages (e.g. "Reasoning:\n…") that contain no answer text.
+    if (!cleaned || cleaned.startsWith("Reasoning:\n")) {
+      return;
+    }
+    if (cleaned === lastPartialText) {
       return;
     }
     hasStreamedMessage = true;
@@ -493,30 +500,30 @@ export async function processDiscordMessage(ctx: DiscordMessagePreflightContext)
       // Keep the longer preview to avoid visible punctuation flicker.
       if (
         lastPartialText &&
-        lastPartialText.startsWith(text) &&
-        text.length < lastPartialText.length
+        lastPartialText.startsWith(cleaned) &&
+        cleaned.length < lastPartialText.length
       ) {
         return;
       }
-      lastPartialText = text;
-      draftStream.update(text);
+      lastPartialText = cleaned;
+      draftStream.update(cleaned);
       return;
     }
 
-    let delta = text;
-    if (text.startsWith(lastPartialText)) {
-      delta = text.slice(lastPartialText.length);
+    let delta = cleaned;
+    if (cleaned.startsWith(lastPartialText)) {
+      delta = cleaned.slice(lastPartialText.length);
     } else {
       // Streaming buffer reset (or non-monotonic stream). Start fresh.
       draftChunker?.reset();
       draftText = "";
     }
-    lastPartialText = text;
+    lastPartialText = cleaned;
     if (!delta) {
       return;
     }
     if (!draftChunker) {
-      draftText = text;
+      draftText = cleaned;
       draftStream.update(draftText);
       return;
     }