diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 30580b1..3ad9f8f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -15,6 +15,9 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref != 'refs/heads/main' || startsWith(github.ref, 'refs/heads/release-') || github.run_number }}
   cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
 
+permissions:
+  contents: read
+
 jobs:
   test:
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }} - ${{ github.event_name }}
@@ -45,6 +48,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v6
+        with:
+          persist-credentials: false
       - uses: julia-actions/setup-julia@v2
         with:
           version: ${{ matrix.version }}
@@ -73,6 +78,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
+        with:
+          persist-credentials: false
       - uses: julia-actions/setup-julia@latest
         with:
           version: '1'
@@ -82,3 +89,17 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: julia --project=docs/ docs/make.jl
+
+  jet:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          persist-credentials: false
+      - uses: julia-actions/setup-julia@latest
+        with:
+          version: '1.12'
+          # version: 'nightly'
+      - run: julia --color=yes --project=ci/jet -e 'import Pkg; Pkg.instantiate()'
+      - name: Run the JET tests
+        run: julia --color=yes --project=ci/jet ci/jet/check.jl
diff --git a/ci/jet/Project.toml b/ci/jet/Project.toml
new file mode 100644
index 0000000..15df17d
--- /dev/null
+++ b/ci/jet/Project.toml
@@ -0,0 +1,11 @@
+[deps]
+DistributedNext = "fab6aee4-877b-4bac-a744-3eca44acbb6f"
+JET = "c3a54625-cd67-489e-a8e7-0a5a0ff4e31b"
+Serialization = "9e88b42a-f829-5b0c-bbe9-9e923198166b"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+
+[sources]
+DistributedNext = {path = "../.."}
+
+[compat]
+JET = "0.11"
diff --git a/ci/jet/check.jl b/ci/jet/check.jl
new file mode 100644
index 0000000..11e615c
--- /dev/null
+++ b/ci/jet/check.jl
@@ -0,0 +1,20 @@
+using DistributedNext: DistributedNext
+
+using JET: JET
+using Serialization: Serialization
+using Test: Test, @testset
+
+# We don't want to fail PkgEval because of a JET failure
+# Therefore, we don't put the JET tests in the regular DistributedNext test suite
+# Instead, we put it in a separate CI job, which runs on the DistributedNext repo
+
+@testset "JET" begin
+    ignored_modules = (
+        # We will ignore Base:
+        Base,
+
+        # We'll ignore the Serialization stdlib:
+        Serialization,
+    )
+    JET.test_package(DistributedNext; ignored_modules)
+end
diff --git a/src/cluster.jl b/src/cluster.jl
index 82a355f..d3a903a 100644
--- a/src/cluster.jl
+++ b/src/cluster.jl
@@ -9,6 +9,14 @@ Cluster managers implement how workers can be added, removed and communicated wi
 """
 abstract type ClusterManager end
 
+# cluster_manager is a global constant
+const cluster_manager = Ref{ClusterManager}()
+
+function throw_if_cluster_manager_unassigned()
+    isassigned(cluster_manager) || error("cluster_manager is unassigned")
+    return nothing
+end
+
 """
     WorkerConfig
 
@@ -390,8 +398,7 @@ function init_worker(cookie::AbstractString, manager::ClusterManager=DefaultClus
 
     # On workers, the default cluster manager connects via TCP sockets. Custom
     # transports will need to call this function with their own manager.
-    global cluster_manager
-    cluster_manager = manager
+    cluster_manager[] = manager
 
     # Since our pid has yet to be set, ensure no RemoteChannel / Future  have been created or addprocs() called.
     @assert nprocs() <= 1
@@ -569,7 +576,7 @@ function setup_launched_worker(manager, wconfig, launched_q)
     # same type. This is done by setting an appropriate value to `WorkerConfig.cnt`.
     cnt = something(wconfig.count, 1)
     if cnt === :auto
-        cnt = wconfig.environ[:cpu_threads]
+        cnt = (wconfig.environ::AbstractDict)[:cpu_threads]
     end
     cnt = cnt - 1   # Removing self from the requested number
 
@@ -607,7 +614,7 @@ function launch_n_additional_processes(manager, frompid, fromconfig, cnt, launch
     end
 end
 
-function create_worker(manager, wconfig)
+function create_worker(manager::ClusterManager, wconfig::WorkerConfig)
     # only node 1 can add new nodes, since nobody else has the full list of address:port
     @assert LPROC.id == 1
     timeout = worker_timeout()
diff --git a/src/macros.jl b/src/macros.jl
index c58faf6..ea98c60 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -333,6 +333,7 @@ completion. To wait for completion, prefix the call with [`@sync`](@ref), like :
 macro distributed(args...)
     na = length(args)
     if na==1
+        reducer = identity
         loop = args[1]
     elseif na==2
         reducer = args[1]
diff --git a/src/managers.jl b/src/managers.jl
index ab79abe..2ce1a3b 100644
--- a/src/managers.jl
+++ b/src/managers.jl
@@ -347,7 +347,7 @@ function launch_on_machine(manager::SSHManager, machine::AbstractString, cnt, pa
 
         any(c -> c == '"', exename) && throw(ArgumentError("invalid exename"))
 
-        remotecmd = shell_escape_wincmd(escape_microsoft_c_args(exename, exeflags...))
+        remotecmd = shell_escape_wincmd(escape_microsoft_c_args(exename, exeflags...)::AbstractString)
         # change working directory
         if dir !== nothing && dir != ""
             any(c -> c == '"', dir) && throw(ArgumentError("invalid dir"))
@@ -553,7 +553,7 @@ end
 
 function manage(manager::LocalManager, id::Integer, config::WorkerConfig, op::Symbol)
     if op === :interrupt
-        kill(config.process, 2)
+        kill(config.process::Process, 2)
     end
 end
 
@@ -606,7 +606,7 @@ function connect(manager::ClusterManager, pid::Int, config::WorkerConfig)
 
     # master connecting to workers
     if config.io !== nothing
-        (bind_addr, port::Int) = read_worker_host_port(config.io)
+        (bind_addr, port::Int) = read_worker_host_port(config.io::IO)
         pubhost = something(config.host, bind_addr)
         config.host = pubhost
         config.port = port
@@ -776,21 +776,22 @@ function kill(manager::LocalManager, pid::Int, config::WorkerConfig; profile_wai
         sleep(exit_timeout)
 
         # Check to see if our child exited, and if not, send an actual kill signal
-        if !process_exited(config.process)
+        process = config.process::Process
+        if !process_exited(process)
             @warn "Failed to gracefully kill worker $(pid)"
             profile_sig = Sys.iswindows() ? nothing : Sys.isbsd() ? ("SIGINFO", 29) : ("SIGUSR1" , 10)
             if profile_sig !== nothing
                 @warn("Sending profile $(profile_sig[1]) to worker $(pid)")
-                kill(config.process, profile_sig[2])
+                kill(process, profile_sig[2])
                 sleep(profile_wait)
             end
             @warn("Sending SIGQUIT to worker $(pid)")
-            kill(config.process, Base.SIGQUIT)
+            kill(process, Base.SIGQUIT)
 
             sleep(term_timeout)
-            if !process_exited(config.process)
+            if !process_exited(process)
                 @warn("Worker $(pid) ignored SIGQUIT, sending SIGKILL")
-                kill(config.process, Base.SIGKILL)
+                kill(process, Base.SIGKILL)
             end
         end
     end
diff --git a/src/messages.jl b/src/messages.jl
index 6e895f0..1a5dd82 100644
--- a/src/messages.jl
+++ b/src/messages.jl
@@ -102,7 +102,7 @@ end
 function send_msg(s::IO, header, msg)
     id = worker_id_from_socket(s)
     if id > -1
-        return send_msg(worker_from_id(id), header, msg)
+        return send_msg(worker_from_id(id)::Worker, header, msg)
     end
     send_msg_unknown(s, header, msg)
 end
@@ -110,7 +110,7 @@ end
 function send_msg_now(s::IO, header, msg::AbstractMsg)
     id = worker_id_from_socket(s)
     if id > -1
-        return send_msg_now(worker_from_id(id), header, msg)
+        return send_msg_now(worker_from_id(id)::Worker, header, msg)
     end
     send_msg_unknown(s, header, msg)
 end
diff --git a/src/process_messages.jl b/src/process_messages.jl
index a444651..211c225 100644
--- a/src/process_messages.jl
+++ b/src/process_messages.jl
@@ -75,7 +75,7 @@ function run_work_thunk(thunk::Function, print_error::Bool)
     end
     return result
 end
-function run_work_thunk(rv::RemoteValue, thunk)
+function run_work_thunk_remotevalue(rv::RemoteValue, thunk)
     put!(rv, run_work_thunk(thunk, false))
     nothing
 end
@@ -85,7 +85,7 @@ function schedule_call(rid, thunk)
         rv = RemoteValue(def_rv_channel())
         (PGRP::ProcessGroup).refs[rid] = rv
         push!(rv.clientset, rid.whence)
-        errormonitor(@async run_work_thunk(rv, thunk))
+        errormonitor(@async run_work_thunk_remotevalue(rv, thunk))
         return rv
     end
 end
@@ -289,7 +289,7 @@ function handle_msg(msg::CallMsg{:call_fetch}, header, r_stream, w_stream, versi
             try
                 deliver_result(w_stream, :call_fetch, header.notify_oid, v.v)
             finally
-                unlock(v.rv.synctake)
+                unlock(v.rv.synctake::ReentrantLock)
             end
         else
             deliver_result(w_stream, :call_fetch, header.notify_oid, v)
@@ -315,8 +315,10 @@ function handle_msg(msg::ResultMsg, header, r_stream, w_stream, version)
 end
 
 function handle_msg(msg::IdentifySocketMsg, header, r_stream, w_stream, version)
+    throw_if_cluster_manager_unassigned()
+
     # register a new peer worker connection
-    w = Worker(msg.from_pid, r_stream, w_stream, cluster_manager; version=version)
+    w = Worker(msg.from_pid, r_stream, w_stream, cluster_manager[]; version=version)::Worker
     send_connection_hdr(w, false)
     send_msg_now(w, MsgHeader(), IdentifySocketAckMsg())
     notify(w.initialized)
@@ -328,8 +330,10 @@ function handle_msg(msg::IdentifySocketAckMsg, header, r_stream, w_stream, versi
 end
 
 function handle_msg(msg::JoinPGRPMsg, header, r_stream, w_stream, version)
+    throw_if_cluster_manager_unassigned()
+
     LPROC.id = msg.self_pid
-    controller = Worker(1, r_stream, w_stream, cluster_manager; version=version)
+    controller = Worker(1, r_stream, w_stream, cluster_manager[]; version=version)::Worker
     notify(controller.initialized)
     register_worker(LPROC)
     topology(msg.topology)
@@ -348,9 +352,9 @@ function handle_msg(msg::JoinPGRPMsg, header, r_stream, w_stream, version)
         let rpid=rpid, wconfig=wconfig
             if lazy
                 # The constructor registers the object with a global registry.
-                Worker(rpid, ()->connect_to_peer(cluster_manager, rpid, wconfig))
+                Worker(rpid, ()->connect_to_peer(cluster_manager[], rpid, wconfig))
             else
-                @async connect_to_peer(cluster_manager, rpid, wconfig)
+                @async connect_to_peer(cluster_manager[], rpid, wconfig)
             end
         end
     end
@@ -362,7 +366,7 @@ end
 function connect_to_peer(manager::ClusterManager, rpid::Int, wconfig::WorkerConfig)
     try
         (r_s, w_s) = connect(manager, rpid, wconfig)
-        w = Worker(rpid, r_s, w_s, manager; config=wconfig)
+        w = Worker(rpid, r_s, w_s, manager; config=wconfig)::Worker
         process_messages(w.r_stream, w.w_stream, false)
         send_connection_hdr(w, true)
         send_msg_now(w, MsgHeader(), IdentifySocketMsg(myid()))
diff --git a/src/remotecall.jl b/src/remotecall.jl
index 644ff04..d0ac719 100644
--- a/src/remotecall.jl
+++ b/src/remotecall.jl
@@ -706,8 +706,8 @@ function put_ref(rid, caller, args...)
     put!(rv, args...)
     if myid() == caller && rv.synctake !== nothing
         # Wait till a "taken" value is serialized out - github issue #29932
-        lock(rv.synctake)
-        unlock(rv.synctake)
+        lock(rv.synctake::ReentrantLock)
+        unlock(rv.synctake::ReentrantLock)
     end
     nothing
 end
@@ -731,7 +731,7 @@ function take_ref(rid, caller, args...)
         # special handling for local put! / remote take! on unbuffered channel
         # github issue #29932
         synctake = true
-        lock(rv.synctake)
+        lock(rv.synctake::ReentrantLock)
     end
 
     v = try
@@ -739,7 +739,7 @@ function take_ref(rid, caller, args...)
     catch e
         # avoid unmatched unlock when exception occurs
         # github issue #33972
-        synctake && unlock(rv.synctake)
+        synctake && unlock(rv.synctake::ReentrantLock)
         rethrow(e)
     end
 
diff --git a/src/workerpool.jl b/src/workerpool.jl
index b28c726..aa73a61 100644
--- a/src/workerpool.jl
+++ b/src/workerpool.jl
@@ -289,7 +289,7 @@ julia> default_worker_pool()
 WorkerPool(Channel{Int64}(sz_max:9223372036854775807,sz_curr:3), Set([4, 2, 3]), RemoteChannel{Channel{Any}}(1, 1, 4))
 ```
 """
-function default_worker_pool()
+function default_worker_pool()::AbstractWorkerPool
     # On workers retrieve the default worker pool from the master when accessed
     # for the first time
     if _default_worker_pool[] === nothing
@@ -299,7 +299,7 @@ function default_worker_pool()
             _default_worker_pool[] = remotecall_fetch(()->default_worker_pool(), 1)
         end
     end
-    return _default_worker_pool[]
+    return _default_worker_pool[]::AbstractWorkerPool
 end
 
 """