diff --git a/src/base_media_func.h b/src/base_media_func.h
index ea19f10c4b..cdcf3c2200 100644
--- a/src/base_media_func.h
+++ b/src/base_media_func.h
@@ -14,6 +14,7 @@
 #include "ini_type.h"
 #include "string_func.h"
 #include "error_func.h"
+#include "core/string_consumer.hpp"
 
 extern void CheckExternalFiles();
 
@@ -70,7 +71,12 @@ bool BaseSet<T>::FillSetDetails(const IniFile &ini, const std::string &path, con
 	}
 
 	fetch_metadata("version");
-	this->version = atoi(item->value->c_str());
+	auto value = ParseInteger(*item->value);
+	if (!value.has_value()) {
+		Debug(grf, 0, "Base {}set detail loading: {} field is invalid: {}", BaseSet<T>::SET_TYPE, item->name, *item->value);
+		return false;
+	}
+	this->version = *value;
 
 	item = metadata->GetItem("fallback");
 	this->fallback = (item != nullptr && item->value && *item->value != "0" && *item->value != "false");
diff --git a/src/cheat_gui.cpp b/src/cheat_gui.cpp
index 7583da680b..0b2c502d8a 100644
--- a/src/cheat_gui.cpp
+++ b/src/cheat_gui.cpp
@@ -35,6 +35,7 @@
 #include "timer/timer.h"
 #include "timer/timer_game_calendar.h"
 #include "timer/timer_game_economy.h"
+#include "core/string_consumer.hpp"
 
 #include "widgets/cheat_widget.h"
 
@@ -607,12 +608,13 @@ struct CheatWindow : Window {
 
 			int32_t value;
 			if (!str->empty()) {
-				long long llvalue = atoll(str->c_str());
+				auto llvalue = ParseInteger<int64_t>(*str);
+				if (!llvalue.has_value()) return;
 
 				/* Save the correct currency-translated value */
-				if (sd->flags.Test(SettingFlag::GuiCurrency)) llvalue /= GetCurrency().rate;
+				if (sd->flags.Test(SettingFlag::GuiCurrency)) llvalue = *llvalue / GetCurrency().rate;
 
-				value = ClampTo<int32_t>(llvalue);
+				value = ClampTo<int32_t>(*llvalue);
 			} else {
 				value = sd->GetDefaultValue();
 			}
@@ -621,11 +623,12 @@ struct CheatWindow : Window {
 		} else {
 			const CheatEntry *ce = &_cheats_ui[clicked_cheat];
 			int oldvalue = static_cast<int32_t>(ReadValue(ce->variable, ce->type));
-			int value = atoi(str->c_str());
+			auto value = ParseInteger<int32_t>(*str);
+			if (!value.has_value()) return;
 			*ce->been_used = true;
-			value = ce->proc(value, value - oldvalue);
+			value = ce->proc(*value, *value - oldvalue);
 
-			if (value != oldvalue) WriteValue(ce->variable, ce->type, static_cast<int64_t>(value));
+			if (*value != oldvalue) WriteValue(ce->variable, ce->type, static_cast<int64_t>(*value));
 		}
 
 		this->valuewindow_entry = nullptr;
diff --git a/src/driver.cpp b/src/driver.cpp
index c790c9ebbb..4daa3ce1e1 100644
--- a/src/driver.cpp
+++ b/src/driver.cpp
@@ -17,6 +17,7 @@
 #include "video/video_driver.hpp"
 #include "string_func.h"
 #include "fileio_func.h"
+#include "core/string_consumer.hpp"
 
 #include "table/strings.h"
 
@@ -77,7 +78,10 @@ bool GetDriverParamBool(const StringList &parm, const char *name)
 int GetDriverParamInt(const StringList &parm, const char *name, int def)
 {
 	const char *p = GetDriverParam(parm, name);
-	return p != nullptr ? atoi(p) : def;
+	if (p == nullptr) return def;
+	auto value = ParseInteger<int>(p);
+	if (value.has_value()) return *value;
+	UserError("Invalid value for driver parameter {}: {}", name, p);
 }
 
 /**
diff --git a/src/game/game_gui.cpp b/src/game/game_gui.cpp
index 0fe11b1eba..4a0b352a20 100644
--- a/src/game/game_gui.cpp
+++ b/src/game/game_gui.cpp
@@ -18,6 +18,7 @@
 #include "../dropdown_func.h"
 #include "../timer/timer.h"
 #include "../timer/timer_window.h"
+#include "../core/string_consumer.hpp"
 
 #include "game.hpp"
 #include "game_gui.hpp"
@@ -346,9 +347,10 @@ struct GSConfigWindow : public Window {
 
 	void OnQueryTextFinished(std::optional<std::string> str) override
 	{
-		if (!str.has_value() || str->empty()) return;
-		int32_t value = atoi(str->c_str());
-		SetValue(value);
+		if (!str.has_value()) return;
+		auto value = ParseInteger<int32_t>(*str);
+		if (!value.has_value()) return;
+		SetValue(*value);
 	}
 
 	void OnDropdownSelect(WidgetID widget, int index) override
diff --git a/src/genworld_gui.cpp b/src/genworld_gui.cpp
index 854ae3d226..7b911ff6d7 100644
--- a/src/genworld_gui.cpp
+++ b/src/genworld_gui.cpp
@@ -33,6 +33,7 @@
 #include "ai/ai_gui.hpp"
 #include "game/game_gui.hpp"
 #include "industry.h"
+#include "core/string_consumer.hpp"
 
 #include "widgets/genworld_widget.h"
 
@@ -913,7 +914,9 @@ struct GenerateLandscapeWindow : public Window {
 
 		int32_t value;
 		if (!str->empty()) {
-			value = atoi(str->c_str());
+			auto val = ParseInteger<int32_t>(*str);
+			if (!val.has_value()) return;
+			value = *val;
 		} else {
 			/* An empty string means revert to the default */
 			switch (this->widget_id) {
@@ -1198,19 +1201,20 @@ struct CreateScenarioWindow : public Window
 
 	void OnQueryTextFinished(std::optional<std::string> str) override
 	{
-		if (!str.has_value() || str->empty()) return;
+		if (!str.has_value()) return;
 
-		int32_t value = atoi(str->c_str());
+		auto value = ParseInteger<int32_t>(*str);
+		if (!value.has_value()) return;
 
 		switch (this->widget_id) {
 			case WID_CS_START_DATE_TEXT:
 				this->SetWidgetDirty(WID_CS_START_DATE_TEXT);
-				_settings_newgame.game_creation.starting_year = Clamp(TimerGameCalendar::Year(value), CalendarTime::MIN_YEAR, CalendarTime::MAX_YEAR);
+				_settings_newgame.game_creation.starting_year = Clamp(TimerGameCalendar::Year(*value), CalendarTime::MIN_YEAR, CalendarTime::MAX_YEAR);
 				break;
 
 			case WID_CS_FLAT_LAND_HEIGHT_TEXT:
 				this->SetWidgetDirty(WID_CS_FLAT_LAND_HEIGHT_TEXT);
-				_settings_newgame.game_creation.se_flat_world_height = Clamp(value, 0, GetMapHeightLimit());
+				_settings_newgame.game_creation.se_flat_world_height = Clamp(*value, 0, GetMapHeightLimit());
 				break;
 		}
 
diff --git a/src/industry_gui.cpp b/src/industry_gui.cpp
index 8ea8d2339a..d4bcdce7f9 100644
--- a/src/industry_gui.cpp
+++ b/src/industry_gui.cpp
@@ -48,6 +48,7 @@
 #include "timer/timer.h"
 #include "timer/timer_window.h"
 #include "hotkeys.h"
+#include "core/string_consumer.hpp"
 
 #include "widgets/industry_widget.h"
 
@@ -1142,16 +1143,17 @@ public:
 		if (!str.has_value() || str->empty()) return;
 
 		Industry *i = Industry::Get(this->window_number);
-		uint value = atoi(str->c_str());
+		auto value = ParseInteger(*str);
+		if (!value.has_value()) return;
 		switch (this->editbox_line) {
 			case IL_NONE: NOT_REACHED();
 
 			case IL_MULTIPLIER:
-				i->prod_level = ClampU(RoundDivSU(value * PRODLEVEL_DEFAULT, 100), PRODLEVEL_MINIMUM, PRODLEVEL_MAXIMUM);
+				i->prod_level = ClampU(RoundDivSU(*value * PRODLEVEL_DEFAULT, 100), PRODLEVEL_MINIMUM, PRODLEVEL_MAXIMUM);
 				break;
 
 			default:
-				i->produced[this->editbox_line - IL_RATE1].rate = ClampU(RoundDivSU(value, 8), 0, 255);
+				i->produced[this->editbox_line - IL_RATE1].rate = ClampU(RoundDivSU(*value, 8), 0, 255);
 				break;
 		}
 		UpdateIndustryProduction(i);
diff --git a/src/music.cpp b/src/music.cpp
index c30e286894..586b17029e 100644
--- a/src/music.cpp
+++ b/src/music.cpp
@@ -134,7 +134,12 @@ bool MusicSet::FillSetDetails(const IniFile &ini, const std::string &path, const
 			if (item != nullptr && item->value.has_value() && !item->value->empty()) {
 				/* Song has a CAT file index, assume it's MPS MIDI format */
 				this->songinfo[i].filetype = MTT_MPSMIDI;
-				this->songinfo[i].cat_index = atoi(item->value->c_str());
+				auto value = ParseInteger(*item->value);
+				if (!value.has_value()) {
+					Debug(grf, 0, "Invalid base music set song index: {}/{}", filename, *item->value);
+					continue;
+				}
+				this->songinfo[i].cat_index = *value;
 				auto songname = GetMusicCatEntryName(filename, this->songinfo[i].cat_index);
 				if (!songname.has_value()) {
 					Debug(grf, 0, "Base music set song missing from CAT file: {}/{}", filename, this->songinfo[i].cat_index);
diff --git a/src/network/network_gui.cpp b/src/network/network_gui.cpp
index b2ca4ab126..f24791d449 100644
--- a/src/network/network_gui.cpp
+++ b/src/network/network_gui.cpp
@@ -41,6 +41,7 @@
 #include "../timer/timer_game_calendar.h"
 #include "../textfile_gui.h"
 #include "../stringfilter_type.h"
+#include "../core/string_consumer.hpp"
 
 #include "../widgets/network_widget.h"
 
@@ -1120,12 +1121,13 @@ struct NetworkStartServerWindow : public Window {
 		if (this->widget_id == WID_NSS_SETPWD) {
 			_settings_client.network.server_password = std::move(*str);
 		} else {
-			int32_t value = atoi(str->c_str());
+			auto value = ParseInteger<int32_t>(*str);
+			if (!value.has_value()) return;
 			this->SetWidgetDirty(this->widget_id);
 			switch (this->widget_id) {
 				default: NOT_REACHED();
-				case WID_NSS_CLIENTS_TXT:    _settings_client.network.max_clients    = Clamp(value, 2, MAX_CLIENTS); break;
-				case WID_NSS_COMPANIES_TXT:  _settings_client.network.max_companies  = Clamp(value, 1, MAX_COMPANIES); break;
+				case WID_NSS_CLIENTS_TXT:    _settings_client.network.max_clients    = Clamp(*value, 2, MAX_CLIENTS); break;
+				case WID_NSS_COMPANIES_TXT:  _settings_client.network.max_companies  = Clamp(*value, 1, MAX_COMPANIES); break;
 			}
 		}
 
diff --git a/src/newgrf_debug_gui.cpp b/src/newgrf_debug_gui.cpp
index 1f83160969..c9df8d3cba 100644
--- a/src/newgrf_debug_gui.cpp
+++ b/src/newgrf_debug_gui.cpp
@@ -19,6 +19,7 @@
 #include "textbuf_gui.h"
 #include "vehicle_gui.h"
 #include "zoom_func.h"
+#include "core/string_consumer.hpp"
 
 #include "engine_base.h"
 #include "industry.h"
@@ -1078,9 +1079,11 @@ struct SpriteAlignerWindow : Window {
 
 	void OnQueryTextFinished(std::optional<std::string> str) override
 	{
-		if (!str.has_value() || str->empty()) return;
+		if (!str.has_value()) return;
 
-		this->current_sprite = atoi(str->c_str());
+		auto value = ParseInteger(*str);
+		if (!value.has_value()) return;
+		this->current_sprite = *value;
 		if (this->current_sprite >= GetMaxSpriteID()) this->current_sprite = 0;
 		while (GetSpriteType(this->current_sprite) != SpriteType::Normal) {
 			this->current_sprite = (this->current_sprite + 1) % GetMaxSpriteID();
diff --git a/src/newgrf_gui.cpp b/src/newgrf_gui.cpp
index 7cbc3d064a..2005d6de38 100644
--- a/src/newgrf_gui.cpp
+++ b/src/newgrf_gui.cpp
@@ -31,6 +31,7 @@
 #include "timer/timer.h"
 #include "timer/timer_window.h"
 #include "zoom_func.h"
+#include "core/string_consumer.hpp"
 
 #include "widgets/newgrf_widget.h"
 #include "widgets/misc_widget.h"
@@ -438,9 +439,10 @@ struct NewGRFParametersWindow : public Window {
 	void OnQueryTextFinished(std::optional<std::string> str) override
 	{
 		if (!str.has_value() || str->empty()) return;
-		int32_t value = atoi(str->c_str());
+		auto value = ParseInteger<int32_t>(*str);
+		if (!value.has_value()) return;
 		GRFParameterInfo &par_info = this->GetParameterInfo(this->clicked_row);
-		this->grf_config.SetValue(par_info, value);
+		this->grf_config.SetValue(par_info, *value);
 		this->SetDirty();
 	}
 
diff --git a/src/openttd.cpp b/src/openttd.cpp
index 6c7f14462c..1a6bcce33d 100644
--- a/src/openttd.cpp
+++ b/src/openttd.cpp
@@ -553,7 +553,13 @@ int openttd_main(std::span<char * const> arguments)
 			scanner->join_server_password = mgo.opt;
 			break;
 		case 'r': ParseResolution(&resolution, mgo.opt); break;
-		case 't': scanner->startyear = TimerGameCalendar::Year(atoi(mgo.opt)); break;
+		case 't':
+			if (auto value = ParseInteger(mgo.opt); value.has_value()) {
+				scanner->startyear = TimerGameCalendar::Year(*value);
+			} else {
+				fmt::print(stderr, "Invalid start year: {}\n", mgo.opt);
+			}
+			break;
 		case 'd': {
 #if defined(_WIN32)
 				CreateConsole();
diff --git a/src/order_gui.cpp b/src/order_gui.cpp
index a3a285ca35..80bb8474ea 100644
--- a/src/order_gui.cpp
+++ b/src/order_gui.cpp
@@ -34,6 +34,7 @@
 #include "error.h"
 #include "order_cmd.h"
 #include "company_cmd.h"
+#include "core/string_consumer.hpp"
 
 #include "widgets/order_widget.h"
 
@@ -1362,22 +1363,23 @@ public:
 		if (!str.has_value() || str->empty()) return;
 
 		VehicleOrderID sel = this->OrderGetSel();
-		uint value = atoi(str->c_str());
+		auto value = ParseInteger(*str);
+		if (!value.has_value()) return;
 
 		switch (this->vehicle->GetOrder(sel)->GetConditionVariable()) {
 			case OCV_MAX_SPEED:
-				value = ConvertDisplaySpeedToSpeed(value, this->vehicle->type);
+				value = ConvertDisplaySpeedToSpeed(*value, this->vehicle->type);
 				break;
 
 			case OCV_RELIABILITY:
 			case OCV_LOAD_PERCENTAGE:
-				value = Clamp(value, 0, 100);
+				value = Clamp(*value, 0, 100);
 				break;
 
 			default:
 				break;
 		}
-		Command<CMD_MODIFY_ORDER>::Post(STR_ERROR_CAN_T_MODIFY_THIS_ORDER, this->vehicle->tile, this->vehicle->index, sel, MOF_COND_VALUE, Clamp(value, 0, 2047));
+		Command<CMD_MODIFY_ORDER>::Post(STR_ERROR_CAN_T_MODIFY_THIS_ORDER, this->vehicle->tile, this->vehicle->index, sel, MOF_COND_VALUE, Clamp(*value, 0, 2047));
 	}
 
 	void OnDropdownSelect(WidgetID widget, int index) override
diff --git a/src/safeguards.h b/src/safeguards.h
index 1ab12a65e9..e5919c3de3 100644
--- a/src/safeguards.h
+++ b/src/safeguards.h
@@ -51,6 +51,11 @@
 #define sscanf    SAFEGUARD_DO_NOT_USE_THIS_METHOD
 #define from_string SAFEGUARD_DO_NOT_USE_THIS_METHOD
 
+/* Use ParseInteger or StringConsumer instead. */
+#define atoi      SAFEGUARD_DO_NOT_USE_THIS_METHOD
+#define atol      SAFEGUARD_DO_NOT_USE_THIS_METHOD
+#define atoll     SAFEGUARD_DO_NOT_USE_THIS_METHOD
+
 /* Use fmt::print instead. */
 #define printf    SAFEGUARD_DO_NOT_USE_THIS_METHOD
 #define fprintf   SAFEGUARD_DO_NOT_USE_THIS_METHOD
diff --git a/src/script/script_gui.cpp b/src/script/script_gui.cpp
index c2f4a02a74..601cc43eff 100644
--- a/src/script/script_gui.cpp
+++ b/src/script/script_gui.cpp
@@ -25,6 +25,7 @@
 #include "../strings_func.h"
 #include "../timer/timer.h"
 #include "../timer/timer_window.h"
+#include "../core/string_consumer.hpp"
 
 #include "script_gui.h"
 #include "script_log.hpp"
@@ -495,10 +496,10 @@ struct ScriptSettingsWindow : public Window {
 
 	void OnQueryTextFinished(std::optional<std::string> str) override
 	{
-		if (!str.has_value() || str->empty()) return;
-		int32_t value = atoi(str->c_str());
-
-		SetValue(value);
+		if (!str.has_value()) return;
+		auto value = ParseInteger<int32_t>(*str);
+		if (!value.has_value()) return;
+		SetValue(*value);
 	}
 
 	void OnDropdownSelect(WidgetID widget, int index) override
diff --git a/src/script/script_info.cpp b/src/script/script_info.cpp
index 57a1b92a9c..1fcdf3767e 100644
--- a/src/script/script_info.cpp
+++ b/src/script/script_info.cpp
@@ -14,6 +14,7 @@
 
 #include "script_info.hpp"
 #include "script_scanner.hpp"
+#include "../core/string_consumer.hpp"
 #include "../3rdparty/fmt/format.h"
 
 #include "../safeguards.h"
@@ -229,8 +230,9 @@ SQInteger ScriptInfo::AddLabels(HSQUIRRELVM vm)
 			sign = -1;
 			key_string++;
 		}
-		int key = atoi(key_string) * sign;
-		config->labels[key] = StrMakeValid(label);
+		auto key = ParseInteger<int>(key_string);
+		if (!key.has_value()) return SQ_ERROR;
+		config->labels[*key * sign] = StrMakeValid(label);
 
 		sq_pop(vm, 2);
 	}
diff --git a/src/settings_gui.cpp b/src/settings_gui.cpp
index 8004abaa75..f0c6861a82 100644
--- a/src/settings_gui.cpp
+++ b/src/settings_gui.cpp
@@ -53,6 +53,7 @@
 #include "social_integration.h"
 #include "sound_func.h"
 #include "settingentry_gui.h"
+#include "core/string_consumer.hpp"
 
 #include "table/strings.h"
 
@@ -1371,12 +1372,13 @@ struct GameOptionsWindow : Window {
 
 		int32_t value;
 		if (!str->empty()) {
-			long long llvalue = atoll(str->c_str());
+			auto llvalue = ParseInteger<int64_t>(*str);
+			if (!llvalue.has_value()) return;
 
 			/* Save the correct currency-translated value */
-			if (sd->flags.Test(SettingFlag::GuiCurrency)) llvalue /= GetCurrency().rate;
+			if (sd->flags.Test(SettingFlag::GuiCurrency)) llvalue = *llvalue / GetCurrency().rate;
 
-			value = ClampTo<int32_t>(llvalue);
+			value = ClampTo<int32_t>(*llvalue);
 		} else {
 			value = sd->GetDefaultValue();
 		}
@@ -2065,9 +2067,12 @@ struct CustomCurrencyWindow : Window {
 		if (!str.has_value()) return;
 
 		switch (this->query_widget) {
-			case WID_CC_RATE:
-				GetCustomCurrency().rate = Clamp(atoi(str->c_str()), 1, UINT16_MAX);
+			case WID_CC_RATE: {
+				auto val = ParseInteger(*str);
+				if (!val.has_value()) return;
+				GetCustomCurrency().rate = Clamp(*val, 1, UINT16_MAX);
 				break;
+			}
 
 			case WID_CC_SEPARATOR: // Thousands separator
 				GetCustomCurrency().separator = std::move(*str);
@@ -2082,9 +2087,13 @@ struct CustomCurrencyWindow : Window {
 				break;
 
 			case WID_CC_YEAR: { // Year to switch to euro
-				TimerGameCalendar::Year val{atoi(str->c_str())};
-
-				GetCustomCurrency().to_euro = (val < MIN_EURO_YEAR ? CF_NOEURO : std::min(val, CalendarTime::MAX_YEAR));
+				TimerGameCalendar::Year year = CF_NOEURO;
+				if (!str->empty()) {
+					auto val = ParseInteger(*str);
+					if (!val.has_value()) return;
+					year = Clamp(static_cast<TimerGameCalendar::Year>(*val), MIN_EURO_YEAR, CalendarTime::MAX_YEAR);
+				}
+				GetCustomCurrency().to_euro = year;
 				break;
 			}
 		}
diff --git a/src/toolbar_gui.cpp b/src/toolbar_gui.cpp
index a1541b77e9..16a1949c12 100644
--- a/src/toolbar_gui.cpp
+++ b/src/toolbar_gui.cpp
@@ -59,6 +59,7 @@
 #include "timer/timer_window.h"
 #include "timer/timer_game_calendar.h"
 #include "help_gui.h"
+#include "core/string_consumer.hpp"
 
 #include "widgets/toolbar_widget.h"
 
@@ -2500,7 +2501,9 @@ struct ScenarioEditorToolbarWindow : Window {
 
 		TimerGameCalendar::Year value;
 		if (!str->empty()) {
-			value = TimerGameCalendar::Year{atoi(str->c_str())};
+			auto val = ParseInteger(*str);
+			if (!val.has_value()) return;
+			value = static_cast<TimerGameCalendar::Year>(*val);
 		} else {
 			/* An empty string means revert to the default */
 			value = TimerGameCalendar::Year{CalendarTime::DEF_START_YEAR.base()};