diff --git a/src/auto-reply/envelope.ts b/src/auto-reply/envelope.ts
index 628e13e54..0130ed59d 100644
--- a/src/auto-reply/envelope.ts
+++ b/src/auto-reply/envelope.ts
@@ -34,3 +34,17 @@ export function formatAgentEnvelope(params: AgentEnvelopeParams): string {
   const header = `[${parts.join(" ")}]`;
   return `${header} ${params.body}`;
 }
+
+export function formatThreadStarterEnvelope(params: {
+  provider: string;
+  author?: string;
+  timestamp?: number | Date;
+  body: string;
+}): string {
+  return formatAgentEnvelope({
+    provider: params.provider,
+    from: params.author,
+    timestamp: params.timestamp,
+    body: params.body,
+  });
+}
diff --git a/src/discord/monitor.ts b/src/discord/monitor.ts
index 01ac99d50..11482199d 100644
--- a/src/discord/monitor.ts
+++ b/src/discord/monitor.ts
@@ -27,7 +27,10 @@ import {
   listNativeCommandSpecs,
   shouldHandleTextCommands,
 } from "../auto-reply/commands-registry.js";
-import { formatAgentEnvelope } from "../auto-reply/envelope.js";
+import {
+  formatAgentEnvelope,
+  formatThreadStarterEnvelope,
+} from "../auto-reply/envelope.js";
 import { dispatchReplyFromConfig } from "../auto-reply/reply/dispatch-from-config.js";
 import {
   buildMentionRegexes,
@@ -55,6 +58,7 @@ import {
   buildAgentSessionKey,
   resolveAgentRoute,
 } from "../routing/resolve-route.js";
+import { resolveThreadSessionKeys } from "../routing/session-key.js";
 import type { RuntimeEnv } from "../runtime.js";
 import { loadWebMedia } from "../web/media.js";
 import { fetchDiscordApplicationId } from "./probe.js";
@@ -863,9 +867,9 @@ export function createDiscordMessageHandler(params: {
           parentId: threadParentId,
         });
         if (starter?.text) {
-          const starterEnvelope = formatAgentEnvelope({
+          const starterEnvelope = formatThreadStarterEnvelope({
             provider: "Discord",
-            from: starter.author,
+            author: starter.author,
             timestamp: starter.timestamp,
             body: starter.text,
           });
@@ -885,13 +889,19 @@ export function createDiscordMessageHandler(params: {
       }
       const mediaPayload = buildDiscordMediaPayload(mediaList);
       const discordTo = `channel:${message.channelId}`;
+      const threadKeys = resolveThreadSessionKeys({
+        baseSessionKey,
+        threadId: threadChannel ? message.channelId : undefined,
+        parentSessionKey,
+        useSuffix: false,
+      });
       const ctxPayload = {
         Body: combinedBody,
         From: isDirectMessage
           ? `discord:${author.id}`
           : `group:${message.channelId}`,
         To: discordTo,
-        SessionKey: baseSessionKey,
+        SessionKey: threadKeys.sessionKey,
         AccountId: route.accountId,
         ChatType: isDirectMessage ? "direct" : "group",
         SenderName:
@@ -909,7 +919,7 @@ export function createDiscordMessageHandler(params: {
         Surface: "discord" as const,
         WasMentioned: wasMentioned,
         MessageSid: message.id,
-        ParentSessionKey: parentSessionKey,
+        ParentSessionKey: threadKeys.parentSessionKey,
         ThreadStarterBody: threadStarterBody,
         ThreadLabel: threadLabel,
         Timestamp: resolveTimestampMs(message.timestamp),
diff --git a/src/routing/session-key.ts b/src/routing/session-key.ts
index 52563936f..f0efb1004 100644
--- a/src/routing/session-key.ts
+++ b/src/routing/session-key.ts
@@ -89,3 +89,20 @@ export function buildAgentPeerSessionKey(params: {
   const peerId = (params.peerId ?? "").trim() || "unknown";
   return `agent:${normalizeAgentId(params.agentId)}:${provider}:${peerKind}:${peerId}`;
 }
+
+export function resolveThreadSessionKeys(params: {
+  baseSessionKey: string;
+  threadId?: string | null;
+  parentSessionKey?: string;
+  useSuffix?: boolean;
+}): { sessionKey: string; parentSessionKey?: string } {
+  const threadId = (params.threadId ?? "").trim();
+  if (!threadId) {
+    return { sessionKey: params.baseSessionKey, parentSessionKey: undefined };
+  }
+  const useSuffix = params.useSuffix ?? true;
+  const sessionKey = useSuffix
+    ? `${params.baseSessionKey}:thread:${threadId}`
+    : params.baseSessionKey;
+  return { sessionKey, parentSessionKey: params.parentSessionKey };
+}
diff --git a/src/slack/monitor.ts b/src/slack/monitor.ts
index 3af63cff7..d5ede54d1 100644
--- a/src/slack/monitor.ts
+++ b/src/slack/monitor.ts
@@ -14,7 +14,10 @@ import {
   listNativeCommandSpecs,
   shouldHandleTextCommands,
 } from "../auto-reply/commands-registry.js";
-import { formatAgentEnvelope } from "../auto-reply/envelope.js";
+import {
+  formatAgentEnvelope,
+  formatThreadStarterEnvelope,
+} from "../auto-reply/envelope.js";
 import { dispatchReplyFromConfig } from "../auto-reply/reply/dispatch-from-config.js";
 import {
   buildMentionRegexes,
@@ -34,6 +37,7 @@ import {
   resolveStorePath,
   updateLastRoute,
 } from "../config/sessions.js";
+import { resolveThreadSessionKeys } from "../routing/session-key.js";
 import { danger, logVerbose, shouldLogVerbose } from "../globals.js";
 import { enqueueSystemEvent } from "../infra/system-events.js";
 import { getChildLogger } from "../logging.js";
@@ -930,12 +934,12 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
     const isThreadReply =
       hasThreadTs &&
       (threadTs !== message.ts || Boolean(message.parent_user_id));
-    const threadSessionKey =
-      isThreadReply && threadTs
-        ? `${baseSessionKey}:thread:${threadTs}`
-        : undefined;
-    const parentSessionKey = isThreadReply ? baseSessionKey : undefined;
-    const sessionKey = threadSessionKey ?? baseSessionKey;
+    const threadKeys = resolveThreadSessionKeys({
+      baseSessionKey,
+      threadId: isThreadReply ? threadTs : undefined,
+      parentSessionKey: isThreadReply ? baseSessionKey : undefined,
+    });
+    const sessionKey = threadKeys.sessionKey;
     enqueueSystemEvent(`${inboundLabel}: ${preview}`, {
       sessionKey,
       contextKey: `slack:message:${message.channel}:${message.ts ?? "unknown"}`,
@@ -978,9 +982,9 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
           : null;
         const starterName = starterUser?.name ?? starter.userId ?? "Unknown";
         const starterWithId = `${starter.text}\n[slack message id: ${starter.ts ?? threadTs} channel: ${message.channel}]`;
-        threadStarterBody = formatAgentEnvelope({
+        threadStarterBody = formatThreadStarterEnvelope({
           provider: "Slack",
-          from: starterName,
+          author: starterName,
           timestamp: starter.ts
             ? Math.round(Number(starter.ts) * 1000)
             : undefined,
@@ -1007,7 +1011,7 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
       Surface: "slack" as const,
       MessageSid: message.ts,
       ReplyToId: message.thread_ts ?? message.ts,
-      ParentSessionKey: parentSessionKey,
+      ParentSessionKey: threadKeys.parentSessionKey,
       ThreadStarterBody: threadStarterBody,
       ThreadLabel: threadLabel,
       Timestamp: message.ts ? Math.round(Number(message.ts) * 1000) : undefined,