fix(server): messages from a linked environment name the sending thread

apps/mobile/src/features/threads/ThreadFeed.tsx

@@ -1333,6 +1333,7 @@ function useMarkdownStyles(): MarkdownStyleSets {
 function AgentMessageFoldHeader(props: {
   readonly environmentId: EnvironmentId;
   readonly senderThreadId?: ThreadId;
+  readonly senderTitle?: string;
   readonly summary: string;
   readonly expanded: boolean;
   readonly onToggle: () => void;
@@ -1341,7 +1342,7 @@ function AgentMessageFoldHeader(props: {
   const senderShell = useQuietThreadShell(
     props.senderThreadId ? scopeThreadRef(props.environmentId, props.senderThreadId) : null,
   );
-  const label = senderShell?.title.trim() || "Agent";
+  const label = senderShell?.title.trim() || props.senderTitle?.trim() || "Agent";
   return (
     <Pressable
       accessibilityRole="button"
@@ -2036,8 +2037,9 @@ function renderFeedEntry(
               {presentation.attribution === "guest" ? <GuestBubbleTint /> : null}
               {foldRowId !== null ? (
                 <AgentMessageFoldHeader
-                  environmentId={props.environmentId}
+                  environmentId={message.senderEnvironmentId ?? props.environmentId}
                   senderThreadId={message.senderThreadId}
+                  senderTitle={message.senderTitle}
                   summary={
                     replaceComposerContextReferences(presentation.text, (ref) => ref.label)
                       .split(/\r?\n/)
@@ -2146,7 +2148,7 @@ function renderFeedEntry(
             <View className="mt-1 flex-row items-center justify-end gap-1 pr-0.5">
               {foldRowId !== null && message.senderThreadId ? (
                 <OpenSenderThreadButton
-                  environmentId={props.environmentId}
+                  environmentId={message.senderEnvironmentId ?? props.environmentId}
                   senderThreadId={message.senderThreadId}
                 />
               ) : null}

apps/mobile/src/features/threads/ThreadQueueControl.tsx

@@ -71,7 +71,9 @@ function QueuedRunSender(props: {
   const { attribution, scheduledTaskId } = resolveUserMessagePresentation(props.message);
   const senderThreadId = attribution === "agent" ? props.message.senderThreadId : undefined;
   const senderShell = useQuietThreadShell(
-    senderThreadId ? scopeThreadRef(props.environmentId, senderThreadId) : null,
+    senderThreadId
+      ? scopeThreadRef(props.message.senderEnvironmentId ?? props.environmentId, senderThreadId)
+      : null,
   );
   const tasks = useEnvironmentQuery(
     scheduledTaskId
@@ -88,7 +90,7 @@ function QueuedRunSender(props: {
       : attribution === "agent"
         ? {
             icon: "bubble.left.and.bubble.right" as const,
-            label: senderShell?.title.trim() || "Agent",
+            label: senderShell?.title.trim() || props.message.senderTitle?.trim() || "Agent",
           }
         : attribution === "t3code"
           ? { icon: null, label: "Sent by T3 Code" }

apps/mobile/src/lib/threadActivity.ts

@@ -42,6 +42,7 @@ import {
 } from "@t3tools/shared/t3McpToolPresentation";
 import type {
   ChatAttachment,
+  EnvironmentId,
   MessageId,
   OrchestrationV2Actor,
   OrchestrationV2CreationSource,
@@ -140,6 +141,8 @@ export interface ThreadFeedMessage {
   readonly creationSource?: OrchestrationV2CreationSource;
   readonly scheduledTaskId?: ScheduledTaskId;
   readonly senderThreadId?: ThreadId;
+  readonly senderEnvironmentId?: EnvironmentId;
+  readonly senderTitle?: string;
   readonly idleCompaction?: import("@t3tools/contracts").OrchestrationV2IdleCompaction;
   readonly visibility: OrchestrationV2ProjectedTurnItem["visibility"];
   readonly sourceThreadId: ThreadId;
@@ -2387,6 +2390,10 @@ export function buildThreadFeed(
                 creationSource: item.creationSource,
                 ...(item.scheduledTaskId ? { scheduledTaskId: item.scheduledTaskId } : {}),
                 ...(item.senderThreadId ? { senderThreadId: item.senderThreadId } : {}),
+                ...(item.senderEnvironmentId
+                  ? { senderEnvironmentId: item.senderEnvironmentId }
+                  : {}),
+                ...(item.senderTitle ? { senderTitle: item.senderTitle } : {}),
                 ...(item.idleCompaction ? { idleCompaction: item.idleCompaction } : {}),
               }
             : {}),

apps/server/src/mcp/McpHttpServer.test.ts

@@ -999,6 +999,7 @@ it.effect("admits provider and OAuth client credentials and points only clients
     Effect.gen(function* () {
       const providerToken = "providerTokenWithoutDots";
       const clientToken = "client-payload.client-signature";
+      const linkToken = "link-payload.link-signature";
       const providerScope: McpInvocationContext.McpInvocationScope = {
         environmentId,
         requestNamespace: "provider-session",
@@ -1019,6 +1020,11 @@ it.effect("admits provider and OAuth client credentials and points only clients
         capabilities: new Set(["orchestration"]),
         issuedAt: 1,
       };
+      const linkScope: McpInvocationContext.McpInvocationScope = {
+        ...clientScope,
+        requestNamespace: "client:link",
+        client: { sessionId: "link", label: "T3 Code · laptop", access: "auto", linked: true },
+      };
       const seen: Array<McpInvocationContext.McpInvocationScope> = [];
       const ProbeToolkit = Toolkit.make(
         Tool.make("probe", {
@@ -1042,7 +1048,7 @@ it.effect("admits provider and OAuth client credentials and points only clients
           }),
         ),
       ).pipe(
-        Layer.provide(ServerSettings.layerTest()),
+        Layer.provide(ServerSettings.layerTest({ linkedEnvironmentsEnabled: true })),
         Layer.provide(
           Layer.mock(McpSessionRegistry.McpSessionRegistry)({
             resolve: (token) =>
@@ -1057,7 +1063,11 @@ it.effect("admits provider and OAuth client credentials and points only clients
           Layer.succeed(McpHttpServer.McpClientAuthenticator, {
             authenticate: (request) =>
               Effect.succeed(
-                request.headers.authorization === `Bearer ${clientToken}` ? clientScope : undefined,
+                request.headers.authorization === `Bearer ${clientToken}`
+                  ? clientScope
+                  : request.headers.authorization === `Bearer ${linkToken}`
+                    ? linkScope
+                    : undefined,
               ),
           }),
         ),
@@ -1068,11 +1078,12 @@ it.effect("admits provider and OAuth client credentials and points only clients
         Effect.withTracer(traced.tracer),
       );
       const httpClient = yield* HttpClient.HttpClient;
-      const call = (token: string | undefined) =>
+      const call = (token: string | undefined, extraHeaders: Record<string, string> = {}) =>
         Effect.gen(function* () {
           const headers = {
             accept: "application/json, text/event-stream",
             ...(token === undefined ? {} : { authorization: `Bearer ${token}` }),
+            ...extraHeaders,
           };
           const initialize = yield* httpClient.post("/mcp", {
             headers,
@@ -1117,6 +1128,19 @@ it.effect("admits provider and OAuth client credentials and points only clients
         "client:session-1",
       ]);
 
+      // Only a peer link may name the thread it calls for.
+      const senderHeaders = {
+        "t3-sender-environment": "environment-laptop",
+        "t3-sender-thread": "thread-laptop",
+        "t3-sender-title": encodeURIComponent("Café build"),
+      };
+      expect((yield* call(clientToken, senderHeaders)).status).toBe(200);
+      expect((yield* call(linkToken, senderHeaders)).status).toBe(200);
+      expect(seen.slice(2).map((scope) => scope.client?.sender)).toEqual([
+        undefined,
+        { environmentId: "environment-laptop", threadId: "thread-laptop", title: "Café build" },
+      ]);
+
       const missing = yield* call(undefined);
       expect(missing.status).toBe(401);
       expect(missing.headers["www-authenticate"]).toMatch(

apps/server/src/mcp/McpHttpServer.ts

@@ -16,6 +16,7 @@ import { AiError, McpProtocol, McpSchema, McpServer, Tool, Toolkit } from "effec
 import { HttpRouter, HttpServerRequest, HttpServerResponse } from "effect/http";
 import {
   DeviceToolError,
+  EnvironmentId,
   OrchestratorMcpFailure,
   PreviewAutomationError,
   ProviderInteractionMode,
@@ -190,6 +191,40 @@ const withModeLimit = (
     : { ...invocation, client: { ...invocation.client, narrowedTo: limit.value } };
 };
 
+const decodeSender = Schema.decodeUnknownOption(
+  Schema.Struct({ environmentId: EnvironmentId, threadId: ThreadId }),
+);
+
+/** Names the thread a peer link calls for; other clients cannot claim a sender. */
+const withSender = (
+  invocation: McpInvocationContext.McpInvocationScope,
+  request: HttpServerRequest.HttpServerRequest,
+): McpInvocationContext.McpInvocationScope => {
+  if (invocation.client?.linked !== true) return invocation;
+  const sender = decodeSender({
+    environmentId: request.headers[McpInvocationContext.SENDER_ENVIRONMENT_HEADER],
+    threadId: request.headers[McpInvocationContext.SENDER_THREAD_HEADER],
+  });
+  if (Option.isNone(sender)) return invocation;
+  const title = decodeTitle(request.headers[McpInvocationContext.SENDER_TITLE_HEADER]);
+  return {
+    ...invocation,
+    client: {
+      ...invocation.client,
+      sender: title === undefined ? sender.value : { ...sender.value, title },
+    },
+  };
+};
+
+const decodeTitle = (header: string | undefined) => {
+  if (header === undefined) return undefined;
+  try {
+    return decodeURIComponent(header);
+  } catch {
+    return undefined;
+  }
+};
+
 // The Linked environments beta is off here: links wait until it is on again.
 const linkedEnvironmentsOff = HttpServerResponse.jsonUnsafe(
   {
@@ -235,7 +270,7 @@ const makeMcpAuthMiddleware = Effect.gen(function* () {
         offerOAuth: Option.isSome(clients) && !looksLikeProviderToken(token),
       });
     }
-    const asked = withModeLimit(invocation, request);
+    const asked = withModeLimit(withSender(invocation, request), request);
     if (asked === undefined) return invalidModeLimit;
     const limited = McpInvocationContext.withRuntimeModeCeiling(asked, runtimeModeCeiling);
     const linkedEnvironments = yield* linksEnabled;

apps/server/src/mcp/McpInvocationContext.ts

@@ -45,6 +45,8 @@ export interface McpClientCaller {
    * another environment passing on the limits of the agent it calls for.
    */
   readonly narrowedTo?: ClientModes | undefined;
+  /** The thread a peer link calls for (`T3-Sender-*`), so what it sends here names its sender. */
+  readonly sender?: McpRemoteSender | undefined;
   /** An outside client's thread links go through this hosted app, as Copy thread's do. */
   readonly hostedAppUrl?: string | undefined;
 }
@@ -57,6 +59,19 @@ export interface McpClientCaller {
  */
 export const MODE_LIMIT_HEADER = "t3-mode-limit";
 
+/** The calling thread a peer link passes on; honored only from peer links. */
+export const SENDER_ENVIRONMENT_HEADER = "t3-sender-environment";
+export const SENDER_THREAD_HEADER = "t3-sender-thread";
+/** URI-encoded, since a header carries only Latin-1. */
+export const SENDER_TITLE_HEADER = "t3-sender-title";
+
+export interface McpRemoteSender {
+  readonly environmentId: EnvironmentId;
+  readonly threadId: ThreadId;
+  /** The sender's title when sent, for clients that cannot read the sender's thread. */
+  readonly title?: string | undefined;
+}
+
 export interface ClientModes {
   readonly runtimeMode: RuntimeMode;
   readonly interactionMode: ProviderInteractionMode;

apps/server/src/mcp/OrchestratorMcpService.ts

@@ -7,6 +7,7 @@ import {
   NodeId,
   ORCHESTRATOR_MCP_RUNNING_TASK_NEXT,
   TurnItemId,
+  type EnvironmentId,
   type OrchestrationV2Run,
   type OrchestrationV2Subagent,
   type OrchestrationV2ThreadProjection,
@@ -253,6 +254,22 @@ function threadManagementFailure(error: unknown): OrchestratorMcpFailure {
 }
 
 // Without a reason, a Stop reads as the user's (see RunExecutionService.USER_STOP_REASON).
+/** The thread a message comes from: the caller's own, or the one a peer link calls for. */
+function senderOf(
+  scope: McpInvocationScope,
+  parent: Pick<OrchestrationV2ThreadProjection, "thread"> | undefined,
+): { senderThreadId?: ThreadId; senderEnvironmentId?: EnvironmentId; senderTitle?: string } {
+  if (parent !== undefined) return { senderThreadId: parent.thread.id };
+  const sender = scope.client?.sender;
+  return sender === undefined || sender.environmentId === scope.environmentId
+    ? {}
+    : {
+        senderThreadId: sender.threadId,
+        senderEnvironmentId: sender.environmentId,
+        ...(sender.title === undefined ? {} : { senderTitle: sender.title }),
+      };
+}
+
 function agentStopReason(reason: string | undefined): string {
   return reason === undefined ? "Stopped by an agent" : `Stopped by an agent: ${reason}`;
 }
@@ -3143,7 +3160,7 @@ const make = Effect.gen(function* () {
             operation: "thread-send",
           }),
           threadId: input.threadId,
-          ...(parent === undefined ? {} : { senderThreadId: parent.thread.id }),
+          ...senderOf(scope, parent),
           messageId,
           text: input.message,
           attachments: [],