feat(routing): add thread parent binding inheritance for Discord

When a Discord thread message doesn't match a direct peer binding,
now checks if the parent channel has a binding and uses that agent.

This enables multi-agent setups where threads inherit their parent
channel's agent binding automatically.

Changes:
- Add parentPeer parameter to ResolveAgentRouteInput
- Add binding.peer.parent match type
- Resolve thread parent early in Discord preflight
- Pass parentPeer to resolveAgentRoute for threads

Fixes thread routing in Discord multi-agent configurations where
threads were incorrectly routed to the default agent instead of
inheriting from their parent channel's binding.

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

@@ -166,6 +166,32 @@ export async function preflightDiscordMessage(
     accountId: params.accountId,
     direction: "inbound",
   });
+
+  // Resolve thread parent early for binding inheritance
+  const channelName =
+    channelInfo?.name ??
+    ((isGuildMessage || isGroupDm) && message.channel && "name" in message.channel
+      ? message.channel.name
+      : undefined);
+  const earlyThreadChannel = resolveDiscordThreadChannel({
+    isGuildMessage,
+    message,
+    channelInfo,
+  });
+  let earlyThreadParentId: string | undefined;
+  let earlyThreadParentName: string | undefined;
+  let earlyThreadParentType: ChannelType | undefined;
+  if (earlyThreadChannel) {
+    const parentInfo = await resolveDiscordThreadParentInfo({
+      client: params.client,
+      threadChannel: earlyThreadChannel,
+      channelInfo,
+    });
+    earlyThreadParentId = parentInfo.id;
+    earlyThreadParentName = parentInfo.name;
+    earlyThreadParentType = parentInfo.type;
+  }
+
   const route = resolveAgentRoute({
     cfg: params.cfg,
     channel: "discord",
@@ -175,6 +201,8 @@ export async function preflightDiscordMessage(
       kind: isDirectMessage ? "dm" : isGroupDm ? "group" : "channel",
       id: isDirectMessage ? author.id : message.channelId,
     },
+    // Pass parent peer for thread binding inheritance
+    parentPeer: earlyThreadParentId ? { kind: "channel", id: earlyThreadParentId } : undefined,
   });
   const mentionRegexes = buildMentionRegexes(params.cfg, route.agentId);
   const explicitlyMentioned = Boolean(
@@ -236,29 +264,11 @@ export async function preflightDiscordMessage(
     return null;
   }
 
-  const channelName =
-    channelInfo?.name ??
-    ((isGuildMessage || isGroupDm) && message.channel && "name" in message.channel
-      ? message.channel.name
-      : undefined);
-  const threadChannel = resolveDiscordThreadChannel({
-    isGuildMessage,
-    message,
-    channelInfo,
-  });
-  let threadParentId: string | undefined;
-  let threadParentName: string | undefined;
-  let threadParentType: ChannelType | undefined;
-  if (threadChannel) {
-    const parentInfo = await resolveDiscordThreadParentInfo({
-      client: params.client,
-      threadChannel,
-      channelInfo,
-    });
-    threadParentId = parentInfo.id;
-    threadParentName = parentInfo.name;
-    threadParentType = parentInfo.type;
-  }
+  // Reuse early thread resolution from above (for binding inheritance)
+  const threadChannel = earlyThreadChannel;
+  const threadParentId = earlyThreadParentId;
+  const threadParentName = earlyThreadParentName;
+  const threadParentType = earlyThreadParentType;
   const threadName = threadChannel?.name;
   const configChannelName = threadParentName ?? channelName;
   const configChannelSlug = configChannelName ? normalizeDiscordSlug(configChannelName) : "";

src/routing/resolve-route.test.ts

@@ -253,3 +253,160 @@ test("dmScope=per-account-channel-peer uses default accountId when not provided"
   });
   expect(route.sessionKey).toBe("agent:main:telegram:default:dm:7550356539");
 });
+
+describe("parentPeer binding inheritance (thread support)", () => {
+  test("thread inherits binding from parent channel when no direct match", () => {
+    const cfg: MoltbotConfig = {
+      bindings: [
+        {
+          agentId: "adecco",
+          match: {
+            channel: "discord",
+            peer: { kind: "channel", id: "parent-channel-123" },
+          },
+        },
+      ],
+    };
+    const route = resolveAgentRoute({
+      cfg,
+      channel: "discord",
+      peer: { kind: "channel", id: "thread-456" },
+      parentPeer: { kind: "channel", id: "parent-channel-123" },
+    });
+    expect(route.agentId).toBe("adecco");
+    expect(route.matchedBy).toBe("binding.peer.parent");
+  });
+
+  test("direct peer binding wins over parent peer binding", () => {
+    const cfg: MoltbotConfig = {
+      bindings: [
+        {
+          agentId: "thread-agent",
+          match: {
+            channel: "discord",
+            peer: { kind: "channel", id: "thread-456" },
+          },
+        },
+        {
+          agentId: "parent-agent",
+          match: {
+            channel: "discord",
+            peer: { kind: "channel", id: "parent-channel-123" },
+          },
+        },
+      ],
+    };
+    const route = resolveAgentRoute({
+      cfg,
+      channel: "discord",
+      peer: { kind: "channel", id: "thread-456" },
+      parentPeer: { kind: "channel", id: "parent-channel-123" },
+    });
+    expect(route.agentId).toBe("thread-agent");
+    expect(route.matchedBy).toBe("binding.peer");
+  });
+
+  test("parent peer binding wins over guild binding", () => {
+    const cfg: MoltbotConfig = {
+      bindings: [
+        {
+          agentId: "parent-agent",
+          match: {
+            channel: "discord",
+            peer: { kind: "channel", id: "parent-channel-123" },
+          },
+        },
+        {
+          agentId: "guild-agent",
+          match: {
+            channel: "discord",
+            guildId: "guild-789",
+          },
+        },
+      ],
+    };
+    const route = resolveAgentRoute({
+      cfg,
+      channel: "discord",
+      peer: { kind: "channel", id: "thread-456" },
+      parentPeer: { kind: "channel", id: "parent-channel-123" },
+      guildId: "guild-789",
+    });
+    expect(route.agentId).toBe("parent-agent");
+    expect(route.matchedBy).toBe("binding.peer.parent");
+  });
+
+  test("falls back to guild binding when no parent peer match", () => {
+    const cfg: MoltbotConfig = {
+      bindings: [
+        {
+          agentId: "other-parent-agent",
+          match: {
+            channel: "discord",
+            peer: { kind: "channel", id: "other-parent-999" },
+          },
+        },
+        {
+          agentId: "guild-agent",
+          match: {
+            channel: "discord",
+            guildId: "guild-789",
+          },
+        },
+      ],
+    };
+    const route = resolveAgentRoute({
+      cfg,
+      channel: "discord",
+      peer: { kind: "channel", id: "thread-456" },
+      parentPeer: { kind: "channel", id: "parent-channel-123" },
+      guildId: "guild-789",
+    });
+    expect(route.agentId).toBe("guild-agent");
+    expect(route.matchedBy).toBe("binding.guild");
+  });
+
+  test("parentPeer with empty id is ignored", () => {
+    const cfg: MoltbotConfig = {
+      bindings: [
+        {
+          agentId: "parent-agent",
+          match: {
+            channel: "discord",
+            peer: { kind: "channel", id: "parent-channel-123" },
+          },
+        },
+      ],
+    };
+    const route = resolveAgentRoute({
+      cfg,
+      channel: "discord",
+      peer: { kind: "channel", id: "thread-456" },
+      parentPeer: { kind: "channel", id: "" },
+    });
+    expect(route.agentId).toBe("main");
+    expect(route.matchedBy).toBe("default");
+  });
+
+  test("null parentPeer is handled gracefully", () => {
+    const cfg: MoltbotConfig = {
+      bindings: [
+        {
+          agentId: "parent-agent",
+          match: {
+            channel: "discord",
+            peer: { kind: "channel", id: "parent-channel-123" },
+          },
+        },
+      ],
+    };
+    const route = resolveAgentRoute({
+      cfg,
+      channel: "discord",
+      peer: { kind: "channel", id: "thread-456" },
+      parentPeer: null,
+    });
+    expect(route.agentId).toBe("main");
+    expect(route.matchedBy).toBe("default");
+  });
+});

src/routing/resolve-route.ts

@@ -22,6 +22,8 @@ export type ResolveAgentRouteInput = {
   channel: string;
   accountId?: string | null;
   peer?: RoutePeer | null;
+  /** Parent peer for threads — used for binding inheritance when peer doesn't match directly. */
+  parentPeer?: RoutePeer | null;
   guildId?: string | null;
   teamId?: string | null;
 };
@@ -37,6 +39,7 @@ export type ResolvedAgentRoute = {
   /** Match description for debugging/logging. */
   matchedBy:
     | "binding.peer"
+    | "binding.peer.parent"
     | "binding.guild"
     | "binding.team"
     | "binding.account"
@@ -186,6 +189,15 @@ export function resolveAgentRoute(input: ResolveAgentRouteInput): ResolvedAgentR
     if (peerMatch) return choose(peerMatch.agentId, "binding.peer");
   }
 
+  // Thread parent inheritance: if peer (thread) didn't match, check parent peer binding
+  const parentPeer = input.parentPeer
+    ? { kind: input.parentPeer.kind, id: normalizeId(input.parentPeer.id) }
+    : null;
+  if (parentPeer && parentPeer.id) {
+    const parentPeerMatch = bindings.find((b) => matchesPeer(b.match, parentPeer));
+    if (parentPeerMatch) return choose(parentPeerMatch.agentId, "binding.peer.parent");
+  }
+
   if (guildId) {
     const guildMatch = bindings.find((b) => matchesGuild(b.match, guildId));
     if (guildMatch) return choose(guildMatch.agentId, "binding.guild");