diff --git a/extensions/slack/src/channel.ts b/extensions/slack/src/channel.ts
index 8323a9ce0..b738c9c79 100644
--- a/extensions/slack/src/channel.ts
+++ b/extensions/slack/src/channel.ts
@@ -263,7 +263,11 @@ export const slackPlugin: ChannelPlugin<ResolvedSlackAccount> = {
       const to = typeof args.to === "string" ? args.to : undefined;
       if (!to) return null;
       const accountId = typeof args.accountId === "string" ? args.accountId.trim() : undefined;
-      return { to, accountId };
+      const threadIdRaw = typeof args.threadId === "string" ? args.threadId.trim() : "";
+      const replyToRaw = typeof args.replyTo === "string" ? args.replyTo.trim() : "";
+      const threadTsRaw = typeof args.threadTs === "string" ? args.threadTs.trim() : "";
+      const threadId = threadIdRaw || replyToRaw || threadTsRaw || undefined;
+      return { to, accountId, threadId };
     },
     handleAction: async ({ action, params, cfg, accountId, toolContext }) => {
       const resolveChannelId = () =>
diff --git a/src/agents/pi-embedded-messaging.ts b/src/agents/pi-embedded-messaging.ts
index 5aae66fd4..8d91f7486 100644
--- a/src/agents/pi-embedded-messaging.ts
+++ b/src/agents/pi-embedded-messaging.ts
@@ -5,6 +5,7 @@ export type MessagingToolSend = {
   provider: string;
   accountId?: string;
   to?: string;
+  threadId?: string | number;
 };
 
 const CORE_MESSAGING_TOOLS = new Set(["sessions_send", "message"]);
diff --git a/src/agents/pi-embedded-subscribe.tools.ts b/src/agents/pi-embedded-subscribe.tools.ts
index 195a70a64..232872e40 100644
--- a/src/agents/pi-embedded-subscribe.tools.ts
+++ b/src/agents/pi-embedded-subscribe.tools.ts
@@ -118,6 +118,14 @@ export function extractMessagingToolSend(
   toolName: string,
   args: Record<string, unknown>,
 ): MessagingToolSend | undefined {
+  const readThreadId = (value: unknown): string | number | undefined => {
+    if (typeof value === "string") {
+      const trimmed = value.trim();
+      return trimmed ? trimmed : undefined;
+    }
+    if (typeof value === "number" && Number.isFinite(value)) return value;
+    return undefined;
+  };
   // Provider docking: new provider tools must implement plugin.actions.extractToolSend.
   const action = typeof args.action === "string" ? args.action.trim() : "";
   const accountIdRaw = typeof args.accountId === "string" ? args.accountId.trim() : undefined;
@@ -132,7 +140,11 @@ export function extractMessagingToolSend(
     const providerId = providerHint ? normalizeChannelId(providerHint) : null;
     const provider = providerId ?? (providerHint ? providerHint.toLowerCase() : "message");
     const to = normalizeTargetForProvider(provider, toRaw);
-    return to ? { tool: toolName, provider, accountId, to } : undefined;
+    let threadId = readThreadId(args.threadId);
+    if (!threadId && provider === "slack") {
+      threadId = readThreadId(args.replyTo ?? args.threadTs);
+    }
+    return to ? { tool: toolName, provider, accountId, to, threadId } : undefined;
   }
   const providerId = normalizeChannelId(toolName);
   if (!providerId) return undefined;
@@ -146,6 +158,7 @@ export function extractMessagingToolSend(
         provider: providerId,
         accountId: extracted.accountId ?? accountId,
         to,
+        threadId: extracted.threadId,
       }
     : undefined;
 }
diff --git a/src/auto-reply/reply/agent-runner-payloads.ts b/src/auto-reply/reply/agent-runner-payloads.ts
index 60d6fa763..6e03e37b8 100644
--- a/src/auto-reply/reply/agent-runner-payloads.ts
+++ b/src/auto-reply/reply/agent-runner-payloads.ts
@@ -31,6 +31,7 @@ export function buildReplyPayloads(params: {
     typeof shouldSuppressMessagingToolReplies
   >[0]["messagingToolSentTargets"];
   originatingTo?: string;
+  originatingThreadId?: string | number;
   accountId?: string;
 }): { replyPayloads: ReplyPayload[]; didLogHeartbeatStrip: boolean } {
   let didLogHeartbeatStrip = params.didLogHeartbeatStrip;
@@ -94,6 +95,7 @@ export function buildReplyPayloads(params: {
     messageProvider: params.messageProvider,
     messagingToolSentTargets,
     originatingTo: params.originatingTo,
+    originatingThreadId: params.originatingThreadId,
     accountId: params.accountId,
   });
   const dedupedPayloads = filterMessagingToolDuplicates({
diff --git a/src/auto-reply/reply/agent-runner.ts b/src/auto-reply/reply/agent-runner.ts
index 227e6f17e..95897b4bb 100644
--- a/src/auto-reply/reply/agent-runner.ts
+++ b/src/auto-reply/reply/agent-runner.ts
@@ -408,6 +408,7 @@ export async function runReplyAgent(params: {
       messagingToolSentTexts: runResult.messagingToolSentTexts,
       messagingToolSentTargets: runResult.messagingToolSentTargets,
       originatingTo: sessionCtx.OriginatingTo ?? sessionCtx.To,
+      originatingThreadId: sessionCtx.MessageThreadId,
       accountId: sessionCtx.AccountId,
     });
     const { replyPayloads } = payloadResult;
diff --git a/src/auto-reply/reply/reply-payloads.ts b/src/auto-reply/reply/reply-payloads.ts
index ecb28cf00..6afe2cbb4 100644
--- a/src/auto-reply/reply/reply-payloads.ts
+++ b/src/auto-reply/reply/reply-payloads.ts
@@ -82,12 +82,22 @@ export function shouldSuppressMessagingToolReplies(params: {
   messageProvider?: string;
   messagingToolSentTargets?: MessagingToolSend[];
   originatingTo?: string;
+  originatingThreadId?: string | number;
   accountId?: string;
 }): boolean {
   const provider = params.messageProvider?.trim().toLowerCase();
   if (!provider) return false;
   const originTarget = normalizeTargetForProvider(provider, params.originatingTo);
   if (!originTarget) return false;
+  const normalizeThreadId = (value?: string | number) => {
+    if (value === undefined || value === null) return undefined;
+    if (typeof value === "string") {
+      const trimmed = value.trim();
+      return trimmed ? trimmed : undefined;
+    }
+    return Number.isFinite(value) ? String(value) : undefined;
+  };
+  const originThreadId = normalizeThreadId(params.originatingThreadId);
   const originAccount = normalizeAccountId(params.accountId);
   const sentTargets = params.messagingToolSentTargets ?? [];
   if (sentTargets.length === 0) return false;
@@ -95,11 +105,15 @@ export function shouldSuppressMessagingToolReplies(params: {
     if (!target?.provider) return false;
     if (target.provider.trim().toLowerCase() !== provider) return false;
     const targetKey = normalizeTargetForProvider(provider, target.to);
-    if (!targetKey) return false;
+    if (!targetKey || targetKey !== originTarget) return false;
     const targetAccount = normalizeAccountId(target.accountId);
     if (originAccount && targetAccount && originAccount !== targetAccount) {
       return false;
     }
-    return targetKey === originTarget;
+    const targetThreadId = normalizeThreadId(target.threadId);
+    if (originThreadId || targetThreadId) {
+      return originThreadId === targetThreadId;
+    }
+    return true;
   });
 }
diff --git a/src/channels/plugins/actions/telegram.ts b/src/channels/plugins/actions/telegram.ts
index 364707e0a..cf7b7b335 100644
--- a/src/channels/plugins/actions/telegram.ts
+++ b/src/channels/plugins/actions/telegram.ts
@@ -62,7 +62,9 @@ export const telegramMessageActions: ChannelMessageActionAdapter = {
     const to = typeof args.to === "string" ? args.to : undefined;
     if (!to) return null;
     const accountId = typeof args.accountId === "string" ? args.accountId.trim() : undefined;
-    return { to, accountId };
+    const threadIdRaw = typeof args.threadId === "string" ? args.threadId.trim() : "";
+    const threadId = threadIdRaw || undefined;
+    return { to, accountId, threadId };
   },
   handleAction: async ({ action, params, cfg, accountId }) => {
     if (action === "send") {
diff --git a/src/channels/plugins/slack.actions.ts b/src/channels/plugins/slack.actions.ts
index ca8aa6fb8..fbb8ff617 100644
--- a/src/channels/plugins/slack.actions.ts
+++ b/src/channels/plugins/slack.actions.ts
@@ -54,7 +54,11 @@ export function createSlackActions(providerId: string): ChannelMessageActionAdap
       const to = typeof args.to === "string" ? args.to : undefined;
       if (!to) return null;
       const accountId = typeof args.accountId === "string" ? args.accountId.trim() : undefined;
-      return { to, accountId };
+      const threadIdRaw = typeof args.threadId === "string" ? args.threadId.trim() : "";
+      const replyToRaw = typeof args.replyTo === "string" ? args.replyTo.trim() : "";
+      const threadTsRaw = typeof args.threadTs === "string" ? args.threadTs.trim() : "";
+      const threadId = threadIdRaw || replyToRaw || threadTsRaw || undefined;
+      return { to, accountId, threadId };
     },
     handleAction: async (ctx: ChannelMessageActionContext) => {
       const { action, params, cfg } = ctx;
diff --git a/src/channels/plugins/types.core.ts b/src/channels/plugins/types.core.ts
index 6a76743f2..790e01ab0 100644
--- a/src/channels/plugins/types.core.ts
+++ b/src/channels/plugins/types.core.ts
@@ -300,6 +300,7 @@ export type ChannelMessageActionContext = {
 export type ChannelToolSend = {
   to: string;
   accountId?: string | null;
+  threadId?: string | number;
 };
 
 export type ChannelMessageActionAdapter = {
diff --git a/src/slack/monitor/message-handler/dispatch.ts b/src/slack/monitor/message-handler/dispatch.ts
index 38b69f049..1df3deaee 100644
--- a/src/slack/monitor/message-handler/dispatch.ts
+++ b/src/slack/monitor/message-handler/dispatch.ts
@@ -44,6 +44,7 @@ export async function dispatchPreparedSlackMessage(prepared: PreparedSlackMessag
 
   const messageTs = message.ts ?? message.event_ts;
   const incomingThreadTs = message.thread_ts;
+  const toolThreadTs = incomingThreadTs ?? (ctx.replyToMode === "all" ? messageTs : undefined);
   let didSetStatus = false;
 
   // Shared mutable ref for "replyToMode=first". Both tool + auto-reply flows
@@ -101,8 +102,13 @@ export async function dispatchPreparedSlackMessage(prepared: PreparedSlackMessag
     responsePrefix: prefixContext.responsePrefix,
     responsePrefixContextProvider: prefixContext.responsePrefixContextProvider,
     humanDelay: resolveHumanDelayConfig(cfg, route.agentId),
-    deliver: async (payload) => {
-      const replyThreadTs = replyPlan.nextThreadTs();
+    deliver: async (payload, info) => {
+      const replyThreadTs = info.kind === "tool" ? toolThreadTs : replyPlan.nextThreadTs();
+      if (shouldLogVerbose()) {
+        logVerbose(
+          `slack threading: deliver kind=${info.kind} target=${prepared.replyTarget} thread_ts=${replyThreadTs ?? "none"} payload.replyToId=${payload.replyToId ?? "none"}`,
+        );
+      }
       await deliverReplies({
         replies: [payload],
         target: prepared.replyTarget,
@@ -112,7 +118,9 @@ export async function dispatchPreparedSlackMessage(prepared: PreparedSlackMessag
         textLimit: ctx.textLimit,
         replyThreadTs,
       });
-      replyPlan.markSent();
+      if (info.kind !== "tool") {
+        replyPlan.markSent();
+      }
     },
     onError: (err, info) => {
       runtime.error?.(danger(`slack ${info.kind} reply failed: ${String(err)}`));
diff --git a/src/slack/monitor/message-handler/prepare.ts b/src/slack/monitor/message-handler/prepare.ts
index 8a2a9e111..5073d262e 100644
--- a/src/slack/monitor/message-handler/prepare.ts
+++ b/src/slack/monitor/message-handler/prepare.ts
@@ -197,14 +197,17 @@ export async function prepareSlackMessage(params: {
   const threadContext = resolveSlackThreadContext({ message, replyToMode: ctx.replyToMode });
   const threadTs = threadContext.incomingThreadTs;
   const isThreadReply = threadContext.isThreadReply;
+  const autoThreadId =
+    !isThreadReply && ctx.replyToMode === "all" ? threadContext.messageThreadId : undefined;
+  const threadSessionId = isThreadReply ? threadTs : autoThreadId;
   const threadKeys = resolveThreadSessionKeys({
     baseSessionKey,
-    threadId: isThreadReply ? threadTs : undefined,
-    parentSessionKey: isThreadReply && ctx.threadInheritParent ? baseSessionKey : undefined,
+    threadId: threadSessionId,
+    parentSessionKey: threadSessionId && ctx.threadInheritParent ? baseSessionKey : undefined,
   });
   const sessionKey = threadKeys.sessionKey;
   const historyKey =
-    isThreadReply && ctx.threadHistoryScope === "thread" ? sessionKey : message.channel;
+    threadSessionId && ctx.threadHistoryScope === "thread" ? sessionKey : message.channel;
 
   const mentionRegexes = buildMentionRegexes(cfg, route.agentId);
   const hasAnyMention = /<@[^>]+>/.test(message.text ?? "");
@@ -402,7 +405,7 @@ export async function prepareSlackMessage(params: {
   const envelopeOptions = resolveEnvelopeFormatOptions(ctx.cfg);
   const previousTimestamp = readSessionUpdatedAt({
     storePath,
-    sessionKey: route.sessionKey,
+    sessionKey,
   });
   const body = formatInboundEnvelope({
     channel: "Slack",