feat: thread-bound subagents on Discord (#21805)

* docs: thread-bound subagents plan

* docs: add exact thread-bound subagent implementation touchpoints

* Docs: prioritize auto thread-bound subagent flow

* Docs: add ACP harness thread-binding extensions

* Discord: add thread-bound session routing and auto-bind spawn flow

* Subagents: add focus commands and ACP/session binding lifecycle hooks

* Tests: cover thread bindings, focus commands, and ACP unbind hooks

* Docs: add plugin-hook appendix for thread-bound subagents

* Plugins: add subagent lifecycle hook events

* Core: emit subagent lifecycle hooks and decouple Discord bindings

* Discord: handle subagent bind lifecycle via plugin hooks

* Subagents: unify completion finalizer and split registry modules

* Add subagent lifecycle events module

* Hooks: fix subagent ended context key

* Discord: share thread bindings across ESM and Jiti

* Subagents: add persistent sessions_spawn mode for thread-bound sessions

* Subagents: clarify thread intro and persistent completion copy

* test(subagents): stabilize sessions_spawn lifecycle cleanup assertions

* Discord: add thread-bound session TTL with auto-unfocus

* Subagents: fail session spawns when thread bind fails

* Subagents: cover thread session failure cleanup paths

* Session: add thread binding TTL config and /session ttl controls

* Tests: align discord reaction expectations

* Agent: persist sessionFile for keyed subagent sessions

* Discord: normalize imports after conflict resolution

* Sessions: centralize sessionFile resolve/persist helper

* Discord: harden thread-bound subagent session routing

* Rebase: resolve upstream/main conflicts

* Subagents: move thread binding into hooks and split bindings modules

* Docs: add channel-agnostic subagent routing hook plan

* Agents: decouple subagent routing from Discord

* Discord: refactor thread-bound subagent flows

* Subagents: prevent duplicate end hooks and orphaned failed sessions

* Refactor: split subagent command and provider phases

* Subagents: honor hook delivery target overrides

* Discord: add thread binding kill switches and refresh plan doc

* Discord: fix thread bind channel resolution

* Routing: centralize account id normalization

* Discord: clean up thread bindings on startup failures

* Discord: add startup cleanup regression tests

* Docs: add long-term thread-bound subagent architecture

* Docs: split session binding plan and dedupe thread-bound doc

* Subagents: add channel-agnostic session binding routing

* Subagents: stabilize announce completion routing tests

* Subagents: cover multi-bound completion routing

* Subagents: suppress lifecycle hooks on failed thread bind

* tests: fix discord provider mock typing regressions

* docs/protocol: sync slash command aliases and delete param models

* fix: add changelog entry for Discord thread-bound subagents (#21805) (thanks @onutc)

---------

Co-authored-by: Shadow <hi@shadowing.dev>

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

@@ -94,6 +94,8 @@ export async function processDiscordMessage(ctx: DiscordMessagePreflightContext)
     guildSlug,
     channelConfig,
     baseSessionKey,
+    boundSessionKey,
+    threadBindings,
     route,
     commandAuthorized,
   } = ctx;
@@ -324,7 +326,7 @@ export async function processDiscordMessage(ctx: DiscordMessagePreflightContext)
     CommandBody: baseText,
     From: effectiveFrom,
     To: effectiveTo,
-    SessionKey: autoThreadContext?.SessionKey ?? threadKeys.sessionKey,
+    SessionKey: boundSessionKey ?? autoThreadContext?.SessionKey ?? threadKeys.sessionKey,
     AccountId: route.accountId,
     ChatType: isDirectMessage ? "direct" : "channel",
     ConversationLabel: fromLabel,
@@ -346,6 +348,7 @@ export async function processDiscordMessage(ctx: DiscordMessagePreflightContext)
     ReplyToBody: replyContext?.body,
     ReplyToSender: replyContext?.sender,
     ParentSessionKey: autoThreadContext?.ParentSessionKey ?? threadKeys.parentSessionKey,
+    MessageThreadId: threadChannel?.id ?? autoThreadContext?.createdThreadId ?? undefined,
     ThreadStarterBody: threadStarterBody,
     ThreadLabel: threadLabel,
     Timestamp: resolveTimestampMs(message.timestamp),
@@ -633,6 +636,8 @@ export async function processDiscordMessage(ctx: DiscordMessagePreflightContext)
         maxLinesPerMessage: discordConfig?.maxLinesPerMessage,
         tableMode,
         chunkMode,
+        sessionKey: ctxPayload.SessionKey,
+        threadBindings,
       });
       replyReference.markSent();
     },