diff --git a/repos/base-foc/include/foc/thread_state.h b/repos/base-foc/include/foc/thread_state.h
index 19d25d1723..b1b7a7be25 100644
--- a/repos/base-foc/include/foc/thread_state.h
+++ b/repos/base-foc/include/foc/thread_state.h
@@ -26,7 +26,7 @@ namespace Genode { struct Foc_thread_state; }
 struct Genode::Foc_thread_state : Thread_state
 {
 	Foc::l4_cap_idx_t  kcap { Foc::L4_INVALID_CAP }; /* thread's gate cap in its PD */
-	uint16_t           id   { };                     /* ID of gate capability */
+	uint32_t           id   { };                     /* ID of gate capability */
 	addr_t             utcb { };                     /* thread's UTCB in its PD */
 };
 
diff --git a/repos/base-foc/src/core/include/cap_id_alloc.h b/repos/base-foc/src/core/include/cap_id_alloc.h
index 39c34ca571..6cbeaed546 100644
--- a/repos/base-foc/src/core/include/cap_id_alloc.h
+++ b/repos/base-foc/src/core/include/cap_id_alloc.h
@@ -30,17 +30,15 @@ class Core::Cap_id_allocator
 {
 	public:
 
-		using id_t = uint16_t;
-
-		enum { ID_MASK = 0xffff };
+		using id_t = unsigned;
 
 	private:
 
 		enum {
-			CAP_ID_RANGE   = ~0UL,
-			CAP_ID_MASK    = ~3UL,
-			CAP_ID_NUM_MAX = CAP_ID_MASK >> 2,
-			CAP_ID_OFFSET  = 1 << 2
+			CAP_ID_OFFSET  = 1 << 2,
+			CAP_ID_MASK    = CAP_ID_OFFSET - 1,
+			CAP_ID_RANGE   = 1u << 28,
+			ID_MASK        = CAP_ID_RANGE - 1,
 		};
 
 		Synced_range_allocator<Allocator_avl> _id_alloc;
diff --git a/repos/base-foc/src/core/platform_thread.cc b/repos/base-foc/src/core/platform_thread.cc
index b06d35bdb5..378f232671 100644
--- a/repos/base-foc/src/core/platform_thread.cc
+++ b/repos/base-foc/src/core/platform_thread.cc
@@ -209,7 +209,7 @@ Foc_thread_state Platform_thread::state()
 		s = _pager_obj->state.state;
 
 	s.kcap = _gate.remote;
-	s.id   = (uint16_t)_gate.local.local_name();
+	s.id   = Cap_index::id_t(_gate.local.local_name());
 	s.utcb = _utcb;
 
 	return s;
diff --git a/repos/base-foc/src/core/rpc_cap_factory.cc b/repos/base-foc/src/core/rpc_cap_factory.cc
index 851ef114ef..caf7a226a9 100644
--- a/repos/base-foc/src/core/rpc_cap_factory.cc
+++ b/repos/base-foc/src/core/rpc_cap_factory.cc
@@ -190,7 +190,7 @@ Cap_id_allocator::Cap_id_allocator(Allocator &alloc)
 :
 	_id_alloc(&alloc)
 {
-	_id_alloc.add_range(CAP_ID_OFFSET, CAP_ID_RANGE);
+	_id_alloc.add_range(CAP_ID_OFFSET, unsigned(CAP_ID_RANGE) - unsigned(CAP_ID_OFFSET));
 }
 
 
@@ -213,7 +213,7 @@ void Cap_id_allocator::free(id_t id)
 	Mutex::Guard lock_guard(_mutex);
 
 	if (id < CAP_ID_RANGE)
-		_id_alloc.free((void*)(id & CAP_ID_MASK), CAP_ID_OFFSET);
+		_id_alloc.free((void*)(addr_t(id & CAP_ID_MASK)), CAP_ID_OFFSET);
 }
 
 
diff --git a/repos/base-foc/src/include/base/internal/capability_data.h b/repos/base-foc/src/include/base/internal/capability_data.h
index 2f083cf190..8c8481192a 100644
--- a/repos/base-foc/src/include/base/internal/capability_data.h
+++ b/repos/base-foc/src/include/base/internal/capability_data.h
@@ -30,12 +30,13 @@ class Genode::Native_capability::Data : public Avl_node<Data>
 {
 	public:
 
-		using id_t = uint16_t;
+		using id_t = unsigned;
+
+		constexpr static id_t INVALID_ID = ~0u;
 
 	private:
 
-		constexpr static uint16_t INVALID_ID = ~0;
-		constexpr static uint16_t UNUSED     =  0;
+		constexpr static id_t UNUSED     =  0;
 
 		uint8_t _ref_cnt; /* reference counter    */
 		id_t    _id;      /* global capability id */
@@ -46,8 +47,8 @@ class Genode::Native_capability::Data : public Avl_node<Data>
 
 		bool     valid() const   { return _id != INVALID_ID; }
 		bool     used()  const   { return _id != UNUSED;     }
-		uint16_t id()    const   { return _id;               }
-		void     id(uint16_t id) { _id = id;                 }
+		id_t     id()    const   { return _id;               }
+		void     id(id_t id)     { _id = id;                 }
 		uint8_t  inc();
 		uint8_t  dec();
 		addr_t   kcap() const;
diff --git a/repos/base-foc/src/lib/base/cap_map.cc b/repos/base-foc/src/lib/base/cap_map.cc
index f97b45766a..f076a9d5c5 100644
--- a/repos/base-foc/src/lib/base/cap_map.cc
+++ b/repos/base-foc/src/lib/base/cap_map.cc
@@ -3,11 +3,11 @@
  * \author Stefan Kalkowski
  * \date   2010-12-06
  *
- * This is a Fiasco.OC-specific addition to the process enviroment.
+ * This is a Fiasco.OC-specific addition to the process environment.
  */
 
 /*
- * Copyright (C) 2010-2017 Genode Labs GmbH
+ * Copyright (C) 2010-2025 Genode Labs GmbH
  *
  * This file is part of the Genode OS framework, which is distributed
  * under the terms of the GNU Affero General Public License version 3.
@@ -59,7 +59,7 @@ static volatile int _cap_index_spinlock = SPINLOCK_UNLOCKED;
 bool Cap_index::higher(Cap_index *n) { return n->_id > _id; }
 
 
-Cap_index* Cap_index::find_by_id(uint16_t id)
+Cap_index* Cap_index::find_by_id(id_t id)
 {
 	if (_id == id) return this;
 
@@ -116,8 +116,8 @@ Cap_index* Capability_map::insert(Cap_index::id_t id)
 {
 	Spin_lock::Guard guard(_lock);
 
-	ASSERT(!_tree.first() || !_tree.first()->find_by_id(id),
-	       "Double insertion in cap_map()!");
+	if (_tree.first() && _tree.first()->find_by_id(id))
+		return { };
 
 	Cap_index * const i = cap_idx_alloc().alloc_range(1);
 	if (i) {
@@ -184,9 +184,16 @@ Cap_index* Capability_map::insert_map(Cap_index::id_t id, addr_t kcap)
 	_tree.insert(i);
 
 	/* map the given cap to our registry entry */
-	l4_task_map(L4_BASE_TASK_CAP, L4_BASE_TASK_CAP,
-				l4_obj_fpage(kcap, 0, L4_FPAGE_RWX),
-				i->kcap() | L4_ITEM_MAP | L4_MAP_ITEM_GRANT);
+	auto const msg = l4_task_map(L4_BASE_TASK_CAP, L4_BASE_TASK_CAP,
+	                             l4_obj_fpage(kcap, 0, L4_FPAGE_RWX),
+	                             i->kcap() | L4_ITEM_MAP | L4_MAP_ITEM_GRANT);
+
+	if (l4_error(msg)) {
+		_tree.remove(i);
+		cap_idx_alloc().free(i, 1);
+		return 0;
+	}
+
 	return i;
 }
 
diff --git a/repos/base-foc/src/lib/base/ipc.cc b/repos/base-foc/src/lib/base/ipc.cc
index 1b6985d9c6..f096586385 100644
--- a/repos/base-foc/src/lib/base/ipc.cc
+++ b/repos/base-foc/src/lib/base/ipc.cc
@@ -55,9 +55,6 @@ static inline bool ipc_error(l4_msgtag_t tag, bool print)
 }
 
 
-static constexpr Cap_index::id_t INVALID_BADGE = 0xffff;
-
-
 /**
  * Representation of a capability during UTCB marshalling/unmarshalling
  */
@@ -116,7 +113,7 @@ static int extract_msg_from_utcb(l4_msgtag_t     tag,
 
 		Cap_index::id_t const badge = (Cap_index::id_t)(*msg_words++);
 
-		if (badge == INVALID_BADGE)
+		if (badge == Cap_index::INVALID_ID)
 			continue;
 
 		/* received a delegated capability */
@@ -227,7 +224,7 @@ static l4_msgtag_t copy_msgbuf_to_utcb(Msgbuf_base &snd_msg,
 	for (unsigned i = 0; i < num_caps; i++) {
 
 		/* store badge as normal message word */
-		*msg_words++ = caps[i].valid ? caps[i].badge : INVALID_BADGE;
+		*msg_words++ = caps[i].valid ? caps[i].badge : Cap_index::INVALID_ID;
 
 		/* setup flexpage for valid capability to delegate */
 		if (caps[i].valid) {