--- a/rts/Game/WebbarBrowser.h
+++ b/rts/Game/WebbarBrowser.h
@@ -46,6 +46,7 @@
 #include "WebbarCloak.h"
 #include "WebbarWorkAudio.h"
 #include "WebbarEconomy.h"
+#include "WebbarCatalog.h"
 
 static bool WebbarSupports(const UnitDef* def)
 {
@@ -90,9 +91,7 @@
 		}, gu->myPlayerNum, gu->myTeam, gu->myAllyTeam, int(gu->spectating));
 		for (const UnitDef& def: unitDefHandler->GetUnitDefsVec()) {
 			if (!WebbarSupports(&def)) continue;
-			MAIN_THREAD_EM_ASM({
-				if (Module['webbarCatalog']) Module['webbarCatalog']($0, UTF8ToString($1), $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12);
-			}, def.id, def.name.c_str(), def.cost.metal, def.cost.energy, def.buildTime, def.xsize * SQUARE_SIZE, def.zsize * SQUARE_SIZE, def.IsBuilderUnit(), def.IsFactoryUnit(), def.IsExtractorUnit(), !def.IsImmobileUnit(), def.CanDamage(), def.needGeo);
+			PublishWebbarCatalog(def);
 			for (const auto& option: def.buildOptions) {
 				const UnitDef* target = unitDefHandler->GetUnitDefByName(option.second);
 				if (!WebbarSupports(target)) continue;
--- /dev/null
+++ b/rts/Game/WebbarCatalog.h
@@ -0,0 +1,45 @@
+/* This file is part of the Spring engine (GPL v2 or later), see LICENSE.html */
+#pragma once
+
+// Read-only initial match definitions, after the game's definition processing.
+// They are public specifications, never unit state, occupancy or net income.
+static void PublishWebbarCatalog(const UnitDef& def)
+{
+	const float values[] = {
+		def.upkeep.metal, def.upkeep.energy, def.resourceMake.metal, def.resourceMake.energy,
+		def.makesResources.metal, def.makesResources.energy, def.windGenerator.metal, def.windGenerator.energy,
+		def.tidalGenerator.metal, def.tidalGenerator.energy, def.storage.metal, def.storage.energy,
+		def.extractsMetal, def.extractRange, def.minWaterDepth, def.maxWaterDepth, def.maxHeightDif, def.waterline,
+	};
+	std::vector<uint8_t> yardmap;
+	yardmap.reserve(def.yardmap.size());
+	for (const auto cell: def.yardmap) yardmap.push_back(uint8_t(cell));
+	const auto param = [&def](const char* key) -> const char* {
+		const auto found = def.customParams.find(key);
+		return found == def.customParams.end() ? nullptr : found->second.c_str();
+	};
+	const auto pointer = [](const void* value) { return uint32_t(reinterpret_cast<uintptr_t>(value)); };
+	const uint32_t words[] = {
+		pointer(yardmap.data()), uint32_t(yardmap.size()), uint32_t(def.xsize), uint32_t(def.zsize),
+		uint32_t(def.onoffable), uint32_t(def.activateWhenBuilt), uint32_t(def.floatOnWater),
+		pointer(param("energyconv_capacity")), pointer(param("energyconv_efficiency")),
+		pointer(param("metal_extractor")), pointer(param("energymultiplier")), uint32_t(def.levelGround), def.buildingMask,
+	};
+	MAIN_THREAD_EM_ASM({
+		if (!Module['webbarCatalog']) return;
+		const values = Array.from(HEAPF32.subarray($13 >> 2, ($13 >> 2) + 18));
+		// Keep the host-call argument count below Emscripten's 16-argument limit.
+		const words = HEAPU32.subarray($14 >> 2, ($14 >> 2) + 13);
+		const yardmap = Array.from(HEAPU8.subarray(words[0], words[0] + words[1]));
+		const nativeEconomy = ({version: 1, values, yardmap, xsize: words[2], zsize: words[3],
+			onoffable: Boolean(words[4]), activateWhenBuilt: Boolean(words[5]), floatOnWater: Boolean(words[6]),
+			conversionCapacity: words[7] ? UTF8ToString(words[7]) : null,
+			conversionEfficiency: words[8] ? UTF8ToString(words[8]) : null,
+			extractionMultiplier: words[9] ? UTF8ToString(words[9]) : null,
+			energyMultiplier: words[10] ? UTF8ToString(words[10]) : null, levelGround: Boolean(words[11]), buildingMask: words[12]});
+		Module['webbarCatalog']($0, UTF8ToString($1), $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, nativeEconomy);
+	}, def.id, def.name.c_str(), def.cost.metal, def.cost.energy, def.buildTime,
+		def.xsize * SQUARE_SIZE, def.zsize * SQUARE_SIZE, def.IsBuilderUnit(), def.IsFactoryUnit(),
+		def.IsExtractorUnit(), !def.IsImmobileUnit(), def.CanDamage(), def.needGeo,
+		values, words);
+}