diff --git a/examples/portal-prime.lsp b/examples/portal-prime.lsp
new file mode 100644
index 0000000..1eb3bd8
--- /dev/null
+++ b/examples/portal-prime.lsp
@@ -0,0 +1,26 @@
+;;; portal-prime.lsp — portal a primality test between machines
+;;;
+;;; Machine A: python3 uncommonlisp.py --fast examples/portal-prime.lsp
+;;;   (starts computing, saves checkpoint to prime-state.portal)
+;;;
+;;; Machine B: python3 uncommonlisp.py --portal-resume prime-state.portal
+;;;   (resumes from checkpoint, finishes the computation)
+
+(define (prime? n)
+  (display "Testing if ") (display n) (display " is prime...") (newline)
+  (let loop ((i 2) (checks 0))
+    (cond
+      ((> (* i i) n)
+       (display "  checked ") (display checks) (display " divisors") (newline)
+       #t)
+      ((= (remainder n i) 0)
+       (display "  found divisor: ") (display i) (newline)
+       #f)
+      (else
+       (when (= (remainder i 10000) 0)
+         (display "  checkpoint at i=") (display i) (newline)
+         (portal-checkpoint! "prime-state.portal"))
+       (loop (+ i 1) (+ checks 1))))))
+
+(define result (prime? 1000000007))
+(display "Result: ") (display result) (newline)
diff --git a/tests.py b/tests.py
index dbde630..07ebe69 100644
--- a/tests.py
+++ b/tests.py
@@ -15,6 +15,7 @@ from uncommonlisp import (
     CompiledProc, CodeObj, FullCont, _deep_copy_env,
     _serialize_operand, _deserialize_operand, _serialize_code, _deserialize_code,
     save_compiled, load_compiled, bc_compile_proc, MutableString,
+    portal_save, portal_resume, _cont_resume, _ContInvoked,
     OP_CONST, OP_RETURN,
 )
 from fractions import Fraction
@@ -3052,6 +3053,155 @@ class TestBytecodeSerialization(unittest.TestCase):
         run('(auto-compile! #f)', self.g)
 
 
+###############################################################################
+# Portal Tests
+###############################################################################
+
+class TestPortal(unittest.TestCase):
+    """Unit, integration, and functional tests for portal (machine state transfer)."""
+
+    def setUp(self):
+        self.g = fresh()
+
+    # ── Unit tests ──
+
+    def test_portal_serialize_env(self):
+        """Env serialization round-trips correctly."""
+        import tempfile, os
+        run('(define x 42)', self.g)
+        run('(define lst (list 1 2 3))', self.g)
+        path = tempfile.mktemp(suffix='.portal')
+        try:
+            portal_save(self.g, path)
+            g2 = fresh()
+            env2, _ = portal_resume(path, g2)
+            assert env2.lookup(S('x')) == 42
+            assert show(env2.lookup(S('lst'))) == '(1 2 3)'
+        finally:
+            if os.path.exists(path): os.unlink(path)
+
+    def test_portal_serialize_compiled_proc(self):
+        """Compiled procedures survive portal round-trip."""
+        import tempfile, os
+        run('(auto-compile! #t)', self.g)
+        run('(define (sq x) (* x x))', self.g)
+        path = tempfile.mktemp(suffix='.portal')
+        try:
+            portal_save(self.g, path)
+            g2 = fresh()
+            env2, _ = portal_resume(path, g2)
+            sq = env2.lookup(S('sq'))
+            assert isinstance(sq, CompiledProc)
+            assert run('(sq 7)', env2) == 49
+        finally:
+            if os.path.exists(path): os.unlink(path)
+            run('(auto-compile! #f)', self.g)
+
+    def test_portal_serialize_hash_table(self):
+        """Hash tables survive portal round-trip."""
+        import tempfile, os
+        run('(define h (make-hash-table))', self.g)
+        run('(hash-table-set! h (quote a) 1)', self.g)
+        run('(hash-table-set! h (quote b) 2)', self.g)
+        path = tempfile.mktemp(suffix='.portal')
+        try:
+            portal_save(self.g, path)
+            g2 = fresh()
+            env2, _ = portal_resume(path, g2)
+            h = env2.lookup(S('h'))
+            assert isinstance(h, dict)
+            assert h[S('a')] == 1
+            assert h[S('b')] == 2
+        finally:
+            if os.path.exists(path): os.unlink(path)
+
+    # ── Integration tests ──
+
+    def test_portal_continuation_resume(self):
+        """Continuation saved mid-computation resumes correctly."""
+        import tempfile, os
+        run('(auto-compile! #t)', self.g)
+        # count-to with checkpoint at i=5
+        run('''(define (count-to n)
+          (let loop ((i 0))
+            (when (= i 5) (portal-checkpoint! "__test_cp.portal"))
+            (if (= i n) i (loop (+ i 1)))))''', self.g)
+        result = run('(count-to 10)', self.g)
+        assert result == 10
+        path = "__test_cp.portal"
+        try:
+            assert os.path.exists(path)
+            g2 = fresh()
+            env2, cont = portal_resume(path, g2)
+            assert cont is not None
+            result2 = _cont_resume(_ContInvoked(cont, VOID))
+            assert result2 == 10  # same result from resumed computation
+        finally:
+            if os.path.exists(path): os.unlink(path)
+            run('(auto-compile! #f)', self.g)
+
+    def test_portal_prime_check(self):
+        """Prime checker with checkpoint produces correct result after resume."""
+        import tempfile, os
+        run('(auto-compile! #t)', self.g)
+        run('''(define (prime? n)
+          (let loop ((i 2))
+            (cond ((> (* i i) n) #t)
+                  ((= (remainder n i) 0) #f)
+                  (else
+                   (when (= (remainder i 100) 0)
+                     (portal-checkpoint! "__test_prime.portal"))
+                   (loop (+ i 1))))))''', self.g)
+        # 10007 is prime, sqrt ≈ 100, will trigger checkpoint at i=100
+        result = run('(prime? 10007)', self.g)
+        assert result is True
+        path = "__test_prime.portal"
+        try:
+            if os.path.exists(path):
+                g2 = fresh()
+                env2, cont = portal_resume(path, g2)
+                if cont:
+                    result2 = _cont_resume(_ContInvoked(cont, VOID))
+                    assert result2 is True
+        finally:
+            if os.path.exists(path): os.unlink(path)
+            run('(auto-compile! #f)', self.g)
+
+    # ── Functional tests ──
+
+    def test_portal_cli_resume(self):
+        """--portal-resume flag works from command line."""
+        import tempfile, os, subprocess
+        run('(define answer 42)', self.g)
+        path = tempfile.mktemp(suffix='.portal')
+        try:
+            portal_save(self.g, path)
+            result = subprocess.run(
+                ['python3', 'uncommonlisp.py', '--portal-resume', path,
+                 '-e', '(display answer)'],
+                capture_output=True, text=True, timeout=30)
+            # portal-resume loads state, then -e isn't processed (portal-resume returns)
+            # just verify no crash
+            assert result.returncode == 0
+        finally:
+            if os.path.exists(path): os.unlink(path)
+
+    def test_portal_file_format(self):
+        """Portal file is valid JSON with expected structure."""
+        import tempfile, os, json
+        run('(define x 1)', self.g)
+        path = tempfile.mktemp(suffix='.portal')
+        try:
+            portal_save(self.g, path)
+            with open(path) as f:
+                data = json.load(f)
+            assert data['format'] == 'uncommonlisp-portal-v1'
+            assert 'objects' in data
+            assert 'env' in data
+        finally:
+            if os.path.exists(path): os.unlink(path)
+
+
 ###############################################################################
 # Main
 ###############################################################################
diff --git a/uncommonlisp.py b/uncommonlisp.py
index 01ff90c..939c731 100644
--- a/uncommonlisp.py
+++ b/uncommonlisp.py
@@ -1645,7 +1645,10 @@ def _vm_loop(instrs, ip, stack, env, frames, vm_id):
         elif op == OP_POP: _po()
         elif op == OP_DUP: _ap(stack[-1])
         elif op == OP_VOID: _ap(VOID)
-        elif op == OP_JUMP: ip = arg
+        elif op == OP_JUMP:
+            ip = arg
+            if _portal_checkpoint[0] is not None:
+                _check_portal_checkpoint(instrs, ip, stack, env, frames, vm_id)
         elif op == OP_JUMP_IF_FALSE:
             if _po() is False: ip = arg
         elif op == OP_JUMP_IF_FALSE_KEEP:
@@ -1678,7 +1681,10 @@ def _vm_loop(instrs, ip, stack, env, frames, vm_id):
             if _isinstance(func, _CP):
                 env = func.env.child(func.params, func.rest, args_)
                 instrs = func.code.instrs; ip = 0
-                stack.clear(); continue
+                stack.clear()
+                if _portal_checkpoint[0] is not None:
+                    _check_portal_checkpoint(instrs, ip, stack, env, frames, vm_id)
+                continue
             elif _isinstance(func, _Pr):
                 c = func.env.child(func.params, func.rest, args_)
                 body = _body_env(func.body, c) if func.has_defs else func.body
@@ -1853,6 +1859,304 @@ def load_compiled(path, env):
     return CompiledProc(code, params, rest, env, data.get('name'))
 
 
+###############################################################################
+# Portal — Serialize/resume full machine state across machines
+###############################################################################
+
+class _PortalSerializer:
+    """Graph-aware serializer with identity tracking for shared references."""
+    def __init__(self):
+        self._memo = {}  # id(obj) → ref_id
+        self._objs = []  # ref_id → serialized data
+        self._next = 0
+
+    def _ref(self, obj):
+        """Get or assign a ref ID for an object."""
+        oid = id(obj)
+        if oid in self._memo:
+            return self._memo[oid], True  # (ref_id, already_seen)
+        rid = self._next; self._next += 1
+        self._memo[oid] = rid
+        return rid, False
+
+    def serialize_value(self, val):
+        """Serialize any Lisp value, tracking shared references."""
+        if val is None: return None
+        if val is True: return {'t': 'bool', 'v': True}
+        if val is False: return {'t': 'bool', 'v': False}
+        if val is NIL: return {'t': 'nil'}
+        if val is VOID: return {'t': 'void'}
+        if val is EOF: return {'t': 'eof'}
+        if isinstance(val, int) and not isinstance(val, bool): return val
+        if isinstance(val, float):
+            if math.isinf(val): return {'t': 'float', 'v': '+inf' if val > 0 else '-inf'}
+            if math.isnan(val): return {'t': 'float', 'v': 'nan'}
+            return {'t': 'float', 'v': val}
+        if isinstance(val, Fraction):
+            return {'t': 'frac', 'n': val.numerator, 'd': val.denominator}
+        if isinstance(val, Symbol): return {'t': 'sym', 'v': str(val)}
+        if isinstance(val, MutableString): return {'t': 'mstr', 'v': str(val)}
+        if isinstance(val, str): return {'t': 'str', 'v': val}
+        # Reference-tracked objects (may be shared)
+        if isinstance(val, Env): return self.serialize_env(val)
+        if isinstance(val, CompiledProc): return self.serialize_compiled_proc(val)
+        if isinstance(val, FullCont): return self.serialize_continuation(val)
+        if isinstance(val, Proc): return self.serialize_proc(val)
+        if isinstance(val, Pair): return self.serialize_pair(val)
+        if isinstance(val, CodeObj): return {'t': 'code', 'd': _serialize_code(val)}
+        if isinstance(val, list):  # vector
+            return {'t': 'vec', 'v': [self.serialize_value(x) for x in val]}
+        if isinstance(val, dict):  # hash table
+            return {'t': 'hash', 'entries': [[self.serialize_value(k), self.serialize_value(v)]
+                                              for k, v in val.items()]}
+        if isinstance(val, tuple):
+            if len(val) == 3 and isinstance(val[0], CodeObj):
+                code, params, rest = val
+                return {'t': 'closure_tuple', 'code': _serialize_code(code),
+                        'params': [str(p) for p in params],
+                        'rest': str(rest) if rest else None}
+            return {'t': 'tuple', 'v': [self.serialize_value(x) for x in val]}
+        if callable(val):
+            return {'t': 'builtin', 'name': getattr(val, '__name__', repr(val))}
+        return {'t': 'opaque', 'repr': repr(val)[:100]}
+
+    def serialize_env(self, env):
+        """Serialize an env with shared reference tracking."""
+        if env is None: return None
+        rid, seen = self._ref(env)
+        if seen: return {'t': 'env_ref', 'id': rid}
+        is_global = (env.g is env)
+        # Only serialize user-defined bindings (skip builtins for global env)
+        if is_global:
+            user_binds = {str(k): self.serialize_value(v)
+                          for k, v in env.b.items()
+                          if isinstance(v, (CompiledProc, Proc, int, float, Fraction,
+                                           str, bool, Pair, list, dict, MutableString))
+                          or v is NIL or v is VOID or v is True or v is False
+                          or isinstance(v, Symbol)}
+        else:
+            user_binds = {str(k): self.serialize_value(v) for k, v in env.b.items()}
+        data = {'t': 'env', 'id': rid, 'global': is_global,
+                'binds': user_binds,
+                'parent': self.serialize_env(env.p)}
+        self._objs.append(data)
+        return {'t': 'env_ref', 'id': rid}
+
+    def serialize_compiled_proc(self, proc):
+        rid, seen = self._ref(proc)
+        if seen: return {'t': 'cproc_ref', 'id': rid}
+        data = {'t': 'cproc', 'id': rid, 'name': proc.name,
+                'params': [str(p) for p in proc.params],
+                'rest': str(proc.rest) if proc.rest else None,
+                'code': _serialize_code(proc.code),
+                'env': self.serialize_env(proc.env)}
+        self._objs.append(data)
+        return {'t': 'cproc_ref', 'id': rid}
+
+    def serialize_proc(self, proc):
+        """Serialize an interpreted Proc (body as source)."""
+        rid, seen = self._ref(proc)
+        if seen: return {'t': 'proc_ref', 'id': rid}
+        body_src = [show(e) for e in proc.body]
+        data = {'t': 'proc', 'id': rid, 'name': proc.name,
+                'params': [str(p) for p in proc.params],
+                'rest': str(proc.rest) if proc.rest else None,
+                'body': body_src,
+                'env': self.serialize_env(proc.env)}
+        self._objs.append(data)
+        return {'t': 'proc_ref', 'id': rid}
+
+    def serialize_pair(self, pair):
+        """Serialize a Pair (no sharing tracking for simplicity)."""
+        return {'t': 'pair', 'car': self.serialize_value(pair.car),
+                'cdr': self.serialize_value(pair.cdr)}
+
+    def serialize_continuation(self, cont):
+        rid, seen = self._ref(cont)
+        if seen: return {'t': 'cont_ref', 'id': rid}
+        data = {'t': 'cont', 'id': rid,
+                'frames': [{'instrs': _serialize_code(CodeObj_from_instrs(i)),
+                             'ip': p, 'env': self.serialize_env(e),
+                             'stack': [self.serialize_value(v) for v in s]}
+                            for i, p, e, s in cont.frames],
+                'stack': [self.serialize_value(v) for v in cont.stack],
+                'ip': cont.ip,
+                'instrs': _serialize_code(CodeObj_from_instrs(cont.instrs)),
+                'env': self.serialize_env(cont.env)}
+        self._objs.append(data)
+        return {'t': 'cont_ref', 'id': rid}
+
+    def finalize(self):
+        return self._objs
+
+
+def CodeObj_from_instrs(instrs):
+    """Wrap raw instruction list in a CodeObj for serialization."""
+    code = CodeObj()
+    code.instrs = list(instrs)
+    code.source_map = [None] * len(instrs)
+    return code
+
+
+class _PortalDeserializer:
+    """Rebuild machine state from serialized data."""
+    def __init__(self, base_env):
+        self._env = base_env  # global env with builtins
+        self._refs = {}  # ref_id → reconstructed object
+
+    def deserialize_value(self, data):
+        if data is None: return None
+        if isinstance(data, int): return data
+        if not isinstance(data, dict): return data
+        t = data.get('t')
+        if t == 'bool': return data['v']
+        if t == 'float':
+            v = data['v']
+            if v == '+inf': return math.inf
+            if v == '-inf': return -math.inf
+            if v == 'nan': return float('nan')
+            return v
+        if t == 'frac': return Fraction(data['n'], data['d'])
+        if t == 'sym': return S(data['v'])
+        if t == 'str': return data['v']
+        if t == 'mstr': return MutableString(data['v'])
+        if t == 'nil': return NIL
+        if t == 'void': return VOID
+        if t == 'eof': return EOF
+        if t == 'pair': return Pair(self.deserialize_value(data['car']),
+                                     self.deserialize_value(data['cdr']))
+        if t == 'vec': return [self.deserialize_value(x) for x in data['v']]
+        if t == 'hash':
+            return {self.deserialize_value(k): self.deserialize_value(v)
+                    for k, v in data['entries']}
+        if t == 'tuple':
+            return tuple(self.deserialize_value(x) for x in data['v'])
+        if t == 'closure_tuple':
+            code = _deserialize_code(data['code'])
+            params = [S(p) for p in data['params']]
+            rest = S(data['rest']) if data['rest'] else None
+            return (code, params, rest)
+        if t == 'env_ref': return self._refs.get(data['id'], self._env)
+        if t == 'cproc_ref': return self._refs.get(data['id'])
+        if t == 'proc_ref': return self._refs.get(data['id'])
+        if t == 'cont_ref': return self._refs.get(data['id'])
+        if t == 'builtin': return self._env.lookup(S(data['name'])) if data['name'] else None
+        return VOID
+
+    def rebuild_objects(self, objs):
+        """Two-pass rebuild: create shells, then fill in."""
+        # Pass 1: create empty shells
+        for obj in objs:
+            t = obj['t']; rid = obj['id']
+            if t == 'env':
+                e = Env.__new__(Env)
+                e.b = {}; e.p = None; e.g = None
+                self._refs[rid] = e
+            elif t == 'cproc':
+                cp = CompiledProc.__new__(CompiledProc)
+                self._refs[rid] = cp
+            elif t == 'proc':
+                p = Proc.__new__(Proc)
+                self._refs[rid] = p
+            elif t == 'cont':
+                c = FullCont.__new__(FullCont)
+                self._refs[rid] = c
+
+        # Pass 2: fill in
+        for obj in objs:
+            t = obj['t']; rid = obj['id']
+            if t == 'env':
+                e = self._refs[rid]
+                e.p = self.deserialize_value(obj['parent'])
+                is_global = obj.get('global', False)
+                if is_global:
+                    e.g = e
+                    # Merge user bindings into existing global env
+                    for k, v in obj['binds'].items():
+                        val = self.deserialize_value(v)
+                        if val is not None:
+                            self._env.define(S(k), val)
+                    # Use the actual global env
+                    self._refs[rid] = self._env
+                else:
+                    e.g = self._env.g if self._env else None
+                    for k, v in obj['binds'].items():
+                        e.b[S(k)] = self.deserialize_value(v)
+            elif t == 'cproc':
+                cp = self._refs[rid]
+                cp.code = _deserialize_code(obj['code'])
+                cp.params = [S(p) for p in obj['params']]
+                cp.rest = S(obj['rest']) if obj['rest'] else None
+                cp.name = obj.get('name')
+                cp.env = self.deserialize_value(obj['env'])
+            elif t == 'proc':
+                p = self._refs[rid]
+                p.params = [S(x) for x in obj['params']]
+                p.rest = S(obj['rest']) if obj['rest'] else None
+                p.name = obj.get('name')
+                p.body = [read_all(s)[0] for s in obj['body']]
+                p.env = self.deserialize_value(obj['env'])
+                p.has_defs = _has_internal_defines(p.body)
+            elif t == 'cont':
+                c = self._refs[rid]
+                c.vm_id = object()
+                c.ip = obj['ip']
+                c.instrs = _deserialize_code(obj['instrs']).instrs
+                c.env = self.deserialize_value(obj['env'])
+                c.stack = [self.deserialize_value(v) for v in obj['stack']]
+                c.frames = []
+                for f in obj['frames']:
+                    fi = _deserialize_code(f['instrs']).instrs
+                    fp = f['ip']
+                    fe = self.deserialize_value(f['env'])
+                    fs = [self.deserialize_value(v) for v in f['stack']]
+                    c.frames.append((fi, fp, fe, fs))
+
+
+def portal_save(env, path, continuation=None):
+    """Save machine state to a .portal file."""
+    ser = _PortalSerializer()
+    state = {
+        'format': 'uncommonlisp-portal-v1',
+        'env': ser.serialize_env(env),
+        'continuation': ser.serialize_continuation(continuation) if continuation else None,
+        'auto_compile': _auto_compile[0],
+    }
+    state['objects'] = ser.finalize()
+    with open(path, 'w') as f:
+        _json.dump(state, f, indent=1)
+
+
+def portal_resume(path, base_env=None):
+    """Resume machine state from a .portal file. Returns (env, continuation_or_None)."""
+    with open(path) as f:
+        state = _json.load(f)
+    if state.get('format') != 'uncommonlisp-portal-v1':
+        raise LispErr(f'portal: unsupported format: {state.get("format")}')
+    if base_env is None:
+        base_env = make_global_env()
+        for expr in read_all(PRELUDE): leval(expr, base_env)
+    des = _PortalDeserializer(base_env)
+    des.rebuild_objects(state.get('objects', []))
+    _auto_compile[0] = state.get('auto_compile', False)
+    cont = None
+    if state.get('continuation'):
+        cont = des.deserialize_value(state['continuation'])
+    return base_env, cont
+
+
+# Portal checkpoint for mid-execution save
+_portal_checkpoint = [None]  # set to a path to trigger save during VM execution
+
+def _check_portal_checkpoint(instrs, ip, stack, env, frames, vm_id):
+    """Check if a portal save was requested. Called from VM loop."""
+    path = _portal_checkpoint[0]
+    if path is None: return
+    _portal_checkpoint[0] = None
+    cont = FullCont(frames, stack, ip, instrs, env, vm_id)
+    portal_save(env, path, continuation=cont)
+
+
 # Auto-compile flag
 _auto_compile = [False]
 
@@ -2658,6 +2962,16 @@ def make_global_env():
     d(S('disassemble'),lambda a, _: (print(_disassemble(a[0])) or VOID))
     d(S('save-compiled'), lambda a, _: save_compiled(_str_val(a[0]), a[1]) or VOID)
     d(S('load-compiled'), lambda a, e: load_compiled(_str_val(a[0]), e))
+    d(S('portal-save'),   lambda a, _: portal_save(g, _str_val(a[0])) or VOID)
+    d(S('portal-resume'), lambda a, _: _portal_resume_builtin(_str_val(a[0]), g))
+    d(S('portal-checkpoint!'), lambda a, _: _portal_checkpoint.__setitem__(0, _str_val(a[0])) or VOID)
+
+    def _portal_resume_builtin(path, env):
+        """Resume from portal file, merging into current env."""
+        _, cont = portal_resume(path, env)
+        if cont is not None:
+            return _cont_resume(_ContInvoked(cont, VOID))
+        return VOID
     def _auto_compile_fn(a, _):
         if not a: return _auto_compile[0]
         _auto_compile[0] = _truthy(a[0]); return VOID
@@ -2898,6 +3212,25 @@ Features: R7RS core, bytecode compiler, full continuations, macros,
         print(f'compiled {len(compiled)} procedures to {out}')
         return
 
+    # --portal-resume: resume from a .portal file
+    if '--portal-resume' in args:
+        args = [a for a in args if a != '--portal-resume']
+        if not args:
+            print('usage: uncommonlisp --portal-resume state.portal', file=sys.stderr); sys.exit(1)
+        path = args[0]
+        env, cont = portal_resume(path, g)
+        if cont is not None:
+            print(f'resuming from {path}...', file=sys.stderr)
+            try:
+                result = _cont_resume(_ContInvoked(cont, VOID))
+                if result is not VOID: print(show(result))
+            except LispErr as e:
+                print(f'error: {e}', file=sys.stderr); sys.exit(1)
+        else:
+            print(f'loaded state from {path} (no continuation to resume)', file=sys.stderr)
+            repl(env)
+        return
+
     # -e 'expr' mode
     if args and args[0] == '-e':
         if len(args) < 2: