@@ -43,6 +43,12 @@ jobs:
         with:
           report-path: "**/ctrf/*.json"
         if: always()
+      - name: Validate FMI
+        run: |
+          npm run validate --workspace=@modelscript/fmi
+          npm run fetch:fmusim --workspace=@modelscript/fmi
+          npm run test:fmusim --workspace=@modelscript/fmi
+        if: always()
       - name: Run Lint
         run: npm run lint
       - name: Package VS Code Extension
 
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
+import { initBltWasm } from "@modelscript/compiler";
 import { ArenaSimulator } from "@modelscript/compiler/simulator";
 import { Context } from "@modelscript/core";
 import {
@@ -9,11 +10,9 @@ import {
   FMI2_TYPES_PLATFORM_H,
   buildFmuArchive,
   compileToWasm,
-  createZip,
   generateFmu,
   generateFmuCSources,
   generateFmuWasmSource,
-  serializeArenaToJson,
 } from "@modelscript/fmi";
 import Modelica from "@modelscript/modelica/parser";
 import { execSync } from "node:child_process";
@@ -146,6 +145,8 @@ export const Fmu: CommandModule<{}, FmuArgs> = {
       return;
     }
 
+    await initBltWasm();
+
     // Prepare simulator to get state variable info
     const simulator = new ArenaSimulator(arena);
     simulator.prepare();
@@ -200,6 +201,8 @@ export const Fmu: CommandModule<{}, FmuArgs> = {
     }
 
     // ── Full FMU archive mode ──
+    const fmiVersionStr =
+      String(args["fmi-version"]) === "3" ? "3" : String(args["fmi-version"]) === "2" ? "2" : "both";
     const archiveOptions: FmuArchiveOptions = {
       modelIdentifier,
       description: args.description,
@@ -210,7 +213,7 @@ export const Fmu: CommandModule<{}, FmuArgs> = {
       fmuType,
       includeSources: args.source !== false,
       includeModelJson: true,
-      fmiVersion: args["fmi-version"] as "2" | "3" | "both",
+      fmiVersion: fmiVersionStr,
     };
 
     profiler.start("codegen");
@@ -251,29 +254,41 @@ export const Fmu: CommandModule<{}, FmuArgs> = {
         fs.mkdirSync(srcDir, { recursive: true });
         fs.writeFileSync(path.join(srcDir, `${modelIdentifier}_model.h`), sources.modelH);
         fs.writeFileSync(path.join(srcDir, `${modelIdentifier}_model.c`), sources.modelC);
-        fs.writeFileSync(path.join(srcDir, "fmi2Functions.c"), sources.fmi2FunctionsC);
+        const fmi2c = path.join(srcDir, "fmi2Functions.c");
+        const fmi3c = path.join(srcDir, "fmi3Functions.c");
+        if (archiveOptions.fmiVersion === "2" || archiveOptions.fmiVersion === "both") {
+          fs.writeFileSync(fmi2c, sources.fmi2FunctionsC);
+        }
+        if (archiveOptions.fmiVersion === "3" || archiveOptions.fmiVersion === "both") {
+          fs.writeFileSync(fmi3c, sources.fmi3FunctionsC);
+        }
 
         // Write FMI headers from archive
         const encoder = new TextEncoder();
         for (const [name, content] of Object.entries(FMI_HEADERS)) {
           fs.writeFileSync(path.join(srcDir, name), encoder.encode(content));
         }
 
-        // Detect platform
-        const arch = os.arch() === "x64" ? "64" : "32";
-        const plat =
+        const arch2 = os.arch() === "x64" ? "64" : "32";
+        const plat2 =
           process.platform === "win32"
-            ? `win${arch}`
+            ? `win${arch2}`
             : process.platform === "darwin"
-              ? `darwin${arch}`
-              : `linux${arch}`;
+              ? `darwin${arch2}`
+              : `linux${arch2}`;
+        const arch3 = os.arch() === "x64" ? "x86_64" : os.arch() === "arm64" ? "aarch64" : "x86";
+        const plat3 =
+          process.platform === "win32"
+            ? `${arch3}-windows`
+            : process.platform === "darwin"
+              ? `${arch3}-darwin`
+              : `${arch3}-linux`;
         const ext = process.platform === "win32" ? ".dll" : process.platform === "darwin" ? ".dylib" : ".so";
 
         // Compile
         const cc = process.env.CC ?? "gcc";
-        const binDir = path.join(tmpDir, "binaries", plat);
-        fs.mkdirSync(binDir, { recursive: true });
-        const sharedLib = path.join(binDir, `${modelIdentifier}${ext}`);
+        // Just build to temp dir first
+        const sharedLib = path.join(tmpDir, `${modelIdentifier}${ext}`);
 
         const ccCmd = [
           cc,
@@ -284,16 +299,20 @@ export const Fmu: CommandModule<{}, FmuArgs> = {
           "-Wextra",
           `-I${srcDir}`,
           path.join(srcDir, `${modelIdentifier}_model.c`),
-          path.join(srcDir, "fmi2Functions.c"),
+          archiveOptions.fmiVersion === "2" || archiveOptions.fmiVersion === "both" ? fmi2c : "",
+          archiveOptions.fmiVersion === "3" || archiveOptions.fmiVersion === "both" ? fmi3c : "",
           "-o",
           sharedLib,
           "-lm",
-        ].join(" ");
+          "-pthread",
+        ]
+          .filter(Boolean)
+          .join(" ");
 
         console.log(`Compiling with: ${ccCmd}`);
         try {
           execSync(ccCmd, { stdio: "pipe", timeout: 60000 });
-          console.log(`  Compiled: binaries/${plat}/${modelIdentifier}${ext}`);
+          console.log(`  Compiled native binary`);
         } catch (compileErr) {
           const msg =
             compileErr instanceof Error && "stderr" in compileErr
@@ -306,50 +325,27 @@ export const Fmu: CommandModule<{}, FmuArgs> = {
           return;
         }
 
-        // Rebuild archive with the compiled binary
-        const finalEntries = new Map<string, Uint8Array>();
-
-        // Actually, it's easier to use the resourceFiles map for the native binary
-        if (!archiveOptions.resourceFiles) archiveOptions.resourceFiles = new Map();
-        // FMI standard expects native binaries in binaries/PLATFORM/
-        // So we hack it in via resourceFiles by using a relative path that breaks out of resources/
-        // Wait, fmu-archive adds "resources/" prefix. So we can't use resourceFiles.
-
-        // Let's just re-implement the entry copying like in export-fmu.ts
-        finalEntries.set("modelDescription.xml", encoder.encode(result.fmuResult.modelDescriptionXml));
-
-        if (archiveOptions.fmiVersion === "3" || archiveOptions.fmiVersion === "both") {
-          // Add terminalsAndIcons if present
-          // We can't easily get fmi3Result without calling generateFmi3.
-          // A cleaner way is to just call buildFmuArchive again but modify it to accept native binaries.
-        }
-
-        // Since we're replacing export-fmu, let's copy its logic directly.
-        finalEntries.set("modelDescription.xml", encoder.encode(result.fmuResult.modelDescriptionXml));
-        if (archiveOptions.includeSources !== false) {
-          finalEntries.set(`sources/${modelIdentifier}_model.h`, encoder.encode(sources.modelH));
-          finalEntries.set(`sources/${modelIdentifier}_model.c`, encoder.encode(sources.modelC));
-          finalEntries.set("sources/fmi2Functions.c", encoder.encode(sources.fmi2FunctionsC));
-          finalEntries.set("sources/CMakeLists.txt", encoder.encode(sources.cmakeLists));
-          for (const [name, content] of Object.entries(FMI_HEADERS)) {
-            finalEntries.set(`sources/${name}`, encoder.encode(content));
-          }
+        const binData = fs.readFileSync(sharedLib);
+        archiveOptions.nativeBinaries = [];
+        if (archiveOptions.fmiVersion === "2" || archiveOptions.fmiVersion === "both") {
+          archiveOptions.nativeBinaries.push({ platform: plat2, ext, binary: binData });
         }
-        if (archiveOptions.includeModelJson !== false) {
-          finalEntries.set(
-            "resources/model.json",
-            encoder.encode(JSON.stringify(serializeArenaToJson(arena), null, 2)),
-          );
+        if (archiveOptions.fmiVersion === "3" || archiveOptions.fmiVersion === "both") {
+          archiveOptions.nativeBinaries.push({ platform: plat3, ext, binary: binData });
         }
 
-        if (archiveOptions.wasmBinary && archiveOptions.wasmJsGlue) {
-          finalEntries.set(`binaries/wasm32/${modelIdentifier}.wasm`, archiveOptions.wasmBinary);
-          finalEntries.set(`binaries/wasm32/${modelIdentifier}.js`, encoder.encode(archiveOptions.wasmJsGlue));
-        }
+        console.log("FMI VERSION: ", archiveOptions.fmiVersion);
+        console.log(
+          "NATIVE BINARIES: ",
+          archiveOptions.nativeBinaries.map((b) => b.platform),
+        );
 
-        finalEntries.set(`binaries/${plat}/${modelIdentifier}${ext}`, fs.readFileSync(sharedLib));
+        // Re-build archive to include native binaries properly
+        profiler.start("codegen");
+        result = buildFmuArchive(arena, archiveOptions, stateVars);
+        profiler.end("codegen");
 
-        fs.writeFileSync(outputPath, createZip(finalEntries));
+        fs.writeFileSync(outputPath, result.archive);
       } finally {
         fs.rmSync(tmpDir, { recursive: true, force: true });
         profiler.end("compilation_c");
 
@@ -1,10 +1,11 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
-import { type ArenaDAEBuilder } from "@modelscript/compiler";
+import { initBltWasm, type ArenaDAEBuilder } from "@modelscript/compiler";
 import {
   ArenaSimulator,
   runWasmSimulation,
   simulateArena,
+  simulateArenaAsync,
   snapshotMemory,
   type MemorySnapshot,
 } from "@modelscript/compiler/simulator";
@@ -82,7 +83,7 @@ export const Simulate: CommandModule<{}, SimulateArgs> = {
       .option("solver", {
         description: "ODE solver to use",
         type: "string",
-        choices: ["rk4", "dopri5", "bdf", "auto"],
+        choices: ["euler", "rk4", "dopri5", "bdf", "auto", "cvode"],
         default: "dopri5",
       })
       .option("realtime", {
@@ -150,6 +151,8 @@ export const Simulate: CommandModule<{}, SimulateArgs> = {
       lastSnap = snap;
     }
 
+    await initBltWasm();
+
     if (!arena) {
       console.error(`'${args.name}' not found or had flattening errors.`);
       return;
@@ -168,7 +171,7 @@ export const Simulate: CommandModule<{}, SimulateArgs> = {
         await simulateC(arena, args, profiler, startTime, stopTime, step, memProfiles, lastSnap);
         break;
       case "arena":
-        simulateArenaEngine(arena, args, profiler, startTime, stopTime, step, memProfiles, lastSnap);
+        await simulateArenaEngine(arena, args, profiler, startTime, stopTime, step, memProfiles, lastSnap);
         break;
       case "js":
       default:
@@ -198,13 +201,22 @@ function simulateJs(
 ): void {
   profiler.start("simulation");
 
+  const outputStringIds: number[] = [];
+  for (let i = 0; i < arena.varCount; i++) {
+    const name = arena.getVarName(i);
+    if (!name.startsWith("$") && !name.startsWith("der(")) {
+      outputStringIds.push(arena.getVarNameId(i));
+    }
+  }
+
   // Map CLI solver to simulateArena solver
-  const solver = args.solver as "euler" | "rk4" | "dopri5" | "bdf" | "auto";
+  const solver = args.solver as "euler" | "rk4" | "dopri5" | "bdf" | "auto" | "cvode";
   const result = simulateArena(arena, {
     startTime,
     stopTime,
     step,
     solver,
+    outputStringIds,
   });
 
   profiler.end("simulation");
@@ -219,7 +231,7 @@ function simulateJs(
 
 // ── Arena Engine ──
 
-function simulateArenaEngine(
+async function simulateArenaEngine(
   arena: ArenaDAEBuilder,
   args: SimulateArgs,
   profiler: Profiler,
@@ -228,16 +240,30 @@ function simulateArenaEngine(
   step: number,
   memProfiles: Record<string, unknown>,
   lastSnap: MemorySnapshot | null,
-): void {
+): Promise<void> {
   profiler.start("simulation");
 
-  const solver = args.solver as "euler" | "rk4" | "dopri5" | "bdf" | "auto";
-  const result = simulateArena(arena, {
+  const outputStringIds: number[] = [];
+  for (let i = 0; i < arena.varCount; i++) {
+    const name = arena.getVarName(i);
+    if (!name.startsWith("$") && !name.startsWith("der(")) {
+      outputStringIds.push(arena.getVarNameId(i));
+    }
+  }
+
+  const solver = args.solver as "euler" | "rk4" | "dopri5" | "bdf" | "auto" | "cvode";
+  const opts: any = {
     startTime,
     stopTime,
     step,
     solver,
-  });
+    outputStringIds,
+  };
+  if (args.tolerance) {
+    opts.atol = Number(args.tolerance);
+    opts.rtol = Number(args.tolerance);
+  }
+  const result = await simulateArenaAsync(arena, opts);
 
   profiler.end("simulation");
 
 
@@ -7,9 +7,9 @@ import path from "node:path";
 import yargs from "yargs/yargs";
 // import { Compile } from "./commands/compile.js";
 // import { Cosim } from "./commands/cosim.js";
-// import { Fmu } from "./commands/fmu.js";
 import { Build } from "./commands/build.js";
 import { BuildCSG } from "./commands/csg.js";
+import { Fmu } from "./commands/fmu.js";
 import { Generate } from "./commands/generate.js";
 // import { I18n } from "./commands/i18n.js";
 // import { Init } from "./commands/init.js";
@@ -23,7 +23,7 @@ import { Playground } from "./commands/playground.js";
 import { Sandbox } from "./commands/sandbox.js";
 // import { Publish } from "./commands/publish.js";
 // import { Render } from "./commands/render.js";
-// import { Simulate } from "./commands/simulate.js";
+import { Simulate } from "./commands/simulate.js";
 // import { Unpublish } from "./commands/unpublish.js";
 
 // import { MC } from "./commands/mc.js";
@@ -38,7 +38,7 @@ yargs(process.argv.slice(2))
   .usage(`CLI for ModelScript ${pkg.content.version}`)
   // .command(Compile)
   // .command(Cosim)
-  // .command(Fmu)
+  .command(Fmu)
   .command(Build)
   .command(Generate)
   // .command(I18n)
@@ -53,7 +53,7 @@ yargs(process.argv.slice(2))
   .command(Sandbox)
   // .command(Publish)
   // .command(Render)
-  // .command(Simulate)
+  .command(Simulate)
   // .command(Surrogate)
   // .command(MC)
   // .command(Verify)