diff --git a/system/abi.zig b/system/abi.zig
index d005fbc..18adda2 100644
--- a/system/abi.zig
+++ b/system/abi.zig
@@ -186,6 +186,7 @@ pub const ProcessState = enum(u32) {
 pub const ProcessDescriptor = extern struct {
     id: u32, // kernel-assigned process id; never reused (monotonic)
     supervisor: u32, // id of the process that spawned it (0 = the kernel)
+    leader: u32, // process-leader id: == id for a main task, the main task's id for a thread
     state: u32, // a ProcessState value
     priority: u32,
     name_length: u32,
diff --git a/system/kernel/process.zig b/system/kernel/process.zig
index da1546a..2fb4bba 100644
--- a/system/kernel/process.zig
+++ b/system/kernel/process.zig
@@ -715,17 +715,17 @@ fn systemThreadSpawn(state: *architecture.CpuState) void {
         null
     else
         ipc.resolveHandle(t, exit_handle) orelse return failErr(state, ipc.EBADF);
-    const tid = spawnThreadSupervised(t.address_space, entry, stack_top, arg, t.priority, t.id, exit_endpoint) orelse return fail(state);
+    const tid = spawnThreadSupervised(t.address_space, entry, stack_top, arg, t.priority, t.id, exit_endpoint, t.leader) orelse return fail(state);
     architecture.setSystemCallResult(state, tid);
 }
 
 /// Spawn a thread sharing `address_space`, taking the exit-endpoint reference under the **same**
 /// lock as the spawn (as `spawnProcessSupervised` does), so the thread cannot die before
 /// its reference exists. Returns the new thread id, or null on resource exhaustion.
-fn spawnThreadSupervised(address_space: u64, entry: u64, stack_top: u64, arg: u64, priority: scheduler.Priority, supervisor: u32, exit_endpoint: ?*ipc.Endpoint) ?u32 {
+fn spawnThreadSupervised(address_space: u64, entry: u64, stack_top: u64, arg: u64, priority: scheduler.Priority, supervisor: u32, exit_endpoint: ?*ipc.Endpoint, leader: u32) ?u32 {
     const flags = sync.enter();
     defer sync.leave(flags);
-    const tid = scheduler.spawnUserLocked(address_space, entry, stack_top, arg, priority, "thread", supervisor, if (exit_endpoint) |e| @ptrCast(e) else null) orelse return null;
+    const tid = scheduler.spawnUserLocked(address_space, entry, stack_top, arg, priority, "thread", supervisor, if (exit_endpoint) |e| @ptrCast(e) else null, leader) orelse return null;
     if (exit_endpoint) |endpoint| endpoint.refcount += 1; // the thread holds it birth-to-death
     return tid;
 }
@@ -972,7 +972,16 @@ pub fn killProcess(caller_id: u32, target_id: u32) i64 {
     defer sync.leave(flags);
     const target = scheduler.taskByIdLocked(target_id) orelse return -ipc.ESRCH;
     if (target.address_space == 0) return -ipc.ESRCH; // kernel tasks are not processes
-    if (target.supervisor != caller_id) return -ipc.EPERM;
+    // The kill authority is the supervision link of the process's LEADER, so the
+    // capability is per-process, aimed at any member id — a thread's own
+    // `supervisor` (the task that spawned it) grants nothing here
+    // (docs/shared-fate-plan.md M1). Kernel tasks were -ESRCH'd above, so a
+    // leader of 0 is unreachable.
+    const leader = if (target.leader == target.id)
+        target
+    else
+        scheduler.taskByIdLocked(target.leader) orelse return -ipc.ESRCH;
+    if (leader.supervisor != caller_id) return -ipc.EPERM;
     target.exit_reason = .killed;
     if (target.state == .running) {
         target.kill_pending = true;
@@ -1088,9 +1097,17 @@ fn systemProcessSignal(state: *architecture.CpuState) void {
     if (signal > 31) return failErr(state, ipc.EBADF); // not a Signal bit position
     const flags = sync.enter();
     defer sync.leave(flags);
-    const target = scheduler.taskByIdLocked(@intCast(id)) orelse return failErr(state, ipc.ESRCH);
-    if (target.address_space == 0) return failErr(state, ipc.ESRCH);
-    if (target.supervisor != t.id and target.id != t.id) return failErr(state, ipc.EPERM);
+    const member = scheduler.taskByIdLocked(@intCast(id)) orelse return failErr(state, ipc.ESRCH);
+    if (member.address_space == 0) return failErr(state, ipc.ESRCH);
+    // Signals address the process: any member id resolves to the LEADER, whose
+    // endpoint the service harness binds. Authority mirrors process_kill (the
+    // leader's supervisor), plus the group may signal itself
+    // (docs/shared-fate-plan.md M1).
+    const target = if (member.leader == member.id)
+        member
+    else
+        scheduler.taskByIdLocked(member.leader) orelse return failErr(state, ipc.ESRCH);
+    if (target.supervisor != t.id and target.id != t.leader) return failErr(state, ipc.EPERM);
     target.pending_signals |= @as(u32, 1) << @intCast(signal);
     if (target.signal_endpoint) |raw| {
         const endpoint: *ipc.Endpoint = @ptrCast(@alignCast(raw));
@@ -1765,7 +1782,7 @@ pub fn spawnProcessSupervised(image: []const u8, priority: u3, argv: []const []c
         architecture.mapUserPageInto(address_space, page_virtual, stack_frame, true, false); // RW + NX
     }
 
-    const child = scheduler.spawnUserLocked(address_space, parsed.entry, user_sp, 0, priority, argv[0], supervisor, if (exit_endpoint) |endpoint| @ptrCast(endpoint) else null) orelse
+    const child = scheduler.spawnUserLocked(address_space, parsed.entry, user_sp, 0, priority, argv[0], supervisor, if (exit_endpoint) |endpoint| @ptrCast(endpoint) else null, 0) orelse
         return error.OutOfMemory;
     // The child holds a reference to its exit endpoint from birth to death. Taken
     // only now, after nothing can fail; the lock is still held, so the child
diff --git a/system/kernel/scheduler.zig b/system/kernel/scheduler.zig
index 7fef809..738e313 100644
--- a/system/kernel/scheduler.zig
+++ b/system/kernel/scheduler.zig
@@ -49,6 +49,12 @@ pub const Task = struct {
     // Id of the process that spawned this one (0 = the kernel). The supervision
     // link is the kill authority: only the supervisor may process_kill a child.
     supervisor: u32 = 0,
+    // Id of this task's process leader — the main task's own id, copied to every
+    // thread it (transitively) spawns; 0 for kernel tasks and never followed.
+    // Equal `leader` is what makes two tasks one process; the leader's id is the
+    // id `system_spawn` returned, so it is the process id the supervisor speaks
+    // (docs/shared-fate-plan.md).
+    leader: u32 = 0,
     // Endpoint to notify when this process ends (any way: exit, fault, kill), or
     // null. Holds its own reference, dropped when the notification is posted.
     // Opaque here for the same reason as `handles` below.
@@ -460,14 +466,16 @@ pub fn spawnOn(entry: *const fn () void, priority: Priority, cpu: u32) bool {
 /// in user mode at `entry` on `user_sp`, recorded under `name` (its argv[0]).
 /// `supervisor` is the id of the spawning process (0 = the kernel) — the kill
 /// authority — and `exit_endpoint` (an *ipc.Endpoint whose reference the caller
-/// has already taken, or null) is notified when this process ends.
+/// has already taken, or null) is notified when this process ends. `leader` is
+/// the process leader's id for a thread, or 0 to make the new task its own
+/// leader (a process spawn).
 /// It gets a fresh kernel stack for syscalls/interrupts, and its first switch-in
 /// lands in `user_task_trampoline`.
 /// Returns the new process id, or null (creating nothing) if the table is full or
 /// out of memory.
 /// **Caller must hold the kernel lock** (the loader that builds `address_space` holds it
 /// across the whole spawn, so the address space and the task appear atomically).
-pub fn spawnUserLocked(address_space: u64, entry: u64, user_sp: u64, user_arg: u64, priority: Priority, task_name: []const u8, supervisor: u32, exit_endpoint: ?*anyopaque) ?u32 {
+pub fn spawnUserLocked(address_space: u64, entry: u64, user_sp: u64, user_arg: u64, priority: Priority, task_name: []const u8, supervisor: u32, exit_endpoint: ?*anyopaque, leader: u32) ?u32 {
     const t = freeSlot() orelse return null;
     const stack = heap.allocator().alloc(u8, stack_size) catch return null;
     // Take this task's reference to the address space before we commit the slot, so a
@@ -488,6 +496,7 @@ pub fn spawnUserLocked(address_space: u64, entry: u64, user_sp: u64, user_arg: u
         .user_arg = user_arg,
         .supervisor = supervisor,
         .exit_endpoint = exit_endpoint,
+        .leader = if (leader == 0) next_id else leader,
     };
     const name_length = @min(task_name.len, maximum_task_name);
     @memcpy(t.name_buffer[0..name_length], task_name[0..name_length]);
@@ -1035,6 +1044,7 @@ pub fn enumerate(out: []abi.ProcessDescriptor) u64 {
             d.* = .{
                 .id = t.id,
                 .supervisor = t.supervisor,
+                .leader = t.leader,
                 .state = @intFromEnum(@as(abi.ProcessState, switch (t.state) {
                     .ready => .ready,
                     .running => .running,
diff --git a/system/kernel/tests.zig b/system/kernel/tests.zig
index 50e0648..476ff03 100644
--- a/system/kernel/tests.zig
+++ b/system/kernel/tests.zig
@@ -1430,7 +1430,7 @@ fn spawnFaultingProcess() ?u32 {
     architecture.mapUserPageInto(address_space, process.stack_base_virtual, stack_frame, true, false); // RW + NX
 
     // Supervised by the calling test task, so exitReasonOf can read the verdict.
-    const id = scheduler.spawnUserLocked(address_space, process.code_virtual, process.stack_base_virtual + abi.page_size, 0, 4, "fault-probe", scheduler.currentId(), null) orelse {
+    const id = scheduler.spawnUserLocked(address_space, process.code_virtual, process.stack_base_virtual + abi.page_size, 0, 4, "fault-probe", scheduler.currentId(), null, 0) orelse {
         architecture.destroyAddressSpace(address_space);
         return null;
     };