diff --git a/bootstrap-native.ts b/bootstrap-native.ts
new file mode 100644
index 0000000..b7ae2f9
--- /dev/null
+++ b/bootstrap-native.ts
@@ -0,0 +1,123 @@
+/**
+ * Bootstrap: downloads native binary for @matrix-org/matrix-sdk-crypto-nodejs
+ * if it doesn't exist.
+ *
+ * This is needed because opencode's npm doesn't run postinstall scripts.
+ * The binary is cached in the plugin's node_modules, so it only downloads once.
+ *
+ * Must be imported BEFORE any import from matrix-bot-sdk.
+ */
+
+import https from "https"
+import fs from "fs"
+import path from "path"
+
+const CRYPTO_PACKAGE_DIR = path.join(
+  __dirname,
+  "..",
+  "node_modules",
+  "@matrix-org",
+  "matrix-sdk-crypto-nodejs",
+)
+
+const VERSION = "0.4.0"
+
+function isMusl(): boolean {
+  if (!process.report || typeof process.report.getReport !== "function") {
+    try {
+      const lddPath = require("child_process")
+        .execSync("which ldd")
+        .toString()
+        .trim()
+      return fs.readFileSync(lddPath, "utf8").includes("musl")
+    } catch {
+      return true
+    }
+  }
+  const { glibcVersionRuntime } = (process.report as any).getReport().header
+  return !glibcVersionRuntime
+}
+
+function getBinaryName(): string {
+  const platform = process.platform
+  const arch = process.arch
+
+  switch (platform) {
+    case "linux":
+      if (arch === "x64") {
+        return isMusl()
+          ? "matrix-sdk-crypto.linux-x64-musl.node"
+          : "matrix-sdk-crypto.linux-x64-gnu.node"
+      }
+      if (arch === "arm64") {
+        return isMusl()
+          ? "matrix-sdk-crypto.linux-arm64-musl.node"
+          : "matrix-sdk-crypto.linux-arm64-gnu.node"
+      }
+      if (arch === "arm") {
+        return "matrix-sdk-crypto.linux-arm-gnueabihf.node"
+      }
+      if (arch === "s390x") {
+        return "matrix-sdk-crypto.linux-s390x-gnu.node"
+      }
+      break
+    case "darwin":
+      if (arch === "x64") return "matrix-sdk-crypto.darwin-x64.node"
+      if (arch === "arm64") return "matrix-sdk-crypto.darwin-arm64.node"
+      break
+    case "win32":
+      if (arch === "x64") return "matrix-sdk-crypto.win32-x64-msvc.node"
+      if (arch === "arm64") return "matrix-sdk-crypto.win32-arm64-msvc.node"
+      break
+  }
+
+  throw new Error(
+    `Unsupported platform: ${platform}-${arch}. ` +
+      "matrix-bot-sdk requires a native crypto binary for this platform.",
+  )
+}
+
+export function ensureNativeBinary(): void {
+  const binaryName = getBinaryName()
+  const binaryPath = path.join(CRYPTO_PACKAGE_DIR, binaryName)
+
+  if (fs.existsSync(binaryPath)) {
+    return // Already downloaded
+  }
+
+  const url = `https://github.com/matrix-org/matrix-rust-sdk-crypto-nodejs/releases/download/v${VERSION}/${binaryName}`
+  const filePath = binaryPath
+
+  console.error(`[matrix-plugin] Downloading native crypto binary: ${binaryName}`)
+
+  https
+    .get(url, (res) => {
+      const statusCode = res.statusCode ?? 200
+      if (statusCode >= 300 && statusCode < 400 && res.headers.location) {
+        // Follow redirects
+        https.get(res.headers.location, (redirectRes) => {
+          const file = fs.createWriteStream(filePath)
+          redirectRes.pipe(file)
+          file.on("finish", () => {
+            file.close()
+            console.error(`[matrix-plugin] Native binary downloaded: ${binaryName}`)
+          })
+        }).on("error", (err) => {
+          console.error(`[matrix-plugin] Failed to download native binary: ${err.message}`)
+          console.error(`[matrix-plugin] Please ensure you have network access to GitHub releases.`)
+        })
+        return
+      }
+
+      const file = fs.createWriteStream(filePath)
+      res.pipe(file)
+      file.on("finish", () => {
+        file.close()
+        console.error(`[matrix-plugin] Native binary downloaded: ${binaryName}`)
+      })
+    })
+    .on("error", (err) => {
+      console.error(`[matrix-plugin] Failed to download native binary: ${err.message}`)
+      console.error(`[matrix-plugin] Please ensure you have network access to GitHub releases.`)
+    })
+}
diff --git a/dist/bootstrap-native.js b/dist/bootstrap-native.js
new file mode 100644
index 0000000..0e9afc9
--- /dev/null
+++ b/dist/bootstrap-native.js
@@ -0,0 +1,107 @@
+/**
+ * Bootstrap: downloads native binary for @matrix-org/matrix-sdk-crypto-nodejs
+ * if it doesn't exist.
+ *
+ * This is needed because opencode's npm doesn't run postinstall scripts.
+ * The binary is cached in the plugin's node_modules, so it only downloads once.
+ *
+ * Must be imported BEFORE any import from matrix-bot-sdk.
+ */
+import https from "https";
+import fs from "fs";
+import path from "path";
+const CRYPTO_PACKAGE_DIR = path.join(__dirname, "..", "node_modules", "@matrix-org", "matrix-sdk-crypto-nodejs");
+const VERSION = "0.4.0";
+function isMusl() {
+    if (!process.report || typeof process.report.getReport !== "function") {
+        try {
+            const lddPath = require("child_process")
+                .execSync("which ldd")
+                .toString()
+                .trim();
+            return fs.readFileSync(lddPath, "utf8").includes("musl");
+        }
+        catch {
+            return true;
+        }
+    }
+    const { glibcVersionRuntime } = process.report.getReport().header;
+    return !glibcVersionRuntime;
+}
+function getBinaryName() {
+    const platform = process.platform;
+    const arch = process.arch;
+    switch (platform) {
+        case "linux":
+            if (arch === "x64") {
+                return isMusl()
+                    ? "matrix-sdk-crypto.linux-x64-musl.node"
+                    : "matrix-sdk-crypto.linux-x64-gnu.node";
+            }
+            if (arch === "arm64") {
+                return isMusl()
+                    ? "matrix-sdk-crypto.linux-arm64-musl.node"
+                    : "matrix-sdk-crypto.linux-arm64-gnu.node";
+            }
+            if (arch === "arm") {
+                return "matrix-sdk-crypto.linux-arm-gnueabihf.node";
+            }
+            if (arch === "s390x") {
+                return "matrix-sdk-crypto.linux-s390x-gnu.node";
+            }
+            break;
+        case "darwin":
+            if (arch === "x64")
+                return "matrix-sdk-crypto.darwin-x64.node";
+            if (arch === "arm64")
+                return "matrix-sdk-crypto.darwin-arm64.node";
+            break;
+        case "win32":
+            if (arch === "x64")
+                return "matrix-sdk-crypto.win32-x64-msvc.node";
+            if (arch === "arm64")
+                return "matrix-sdk-crypto.win32-arm64-msvc.node";
+            break;
+    }
+    throw new Error(`Unsupported platform: ${platform}-${arch}. ` +
+        "matrix-bot-sdk requires a native crypto binary for this platform.");
+}
+export function ensureNativeBinary() {
+    const binaryName = getBinaryName();
+    const binaryPath = path.join(CRYPTO_PACKAGE_DIR, binaryName);
+    if (fs.existsSync(binaryPath)) {
+        return; // Already downloaded
+    }
+    const url = `https://github.com/matrix-org/matrix-rust-sdk-crypto-nodejs/releases/download/v${VERSION}/${binaryName}`;
+    const filePath = binaryPath;
+    console.error(`[matrix-plugin] Downloading native crypto binary: ${binaryName}`);
+    https
+        .get(url, (res) => {
+        const statusCode = res.statusCode ?? 200;
+        if (statusCode >= 300 && statusCode < 400 && res.headers.location) {
+            // Follow redirects
+            https.get(res.headers.location, (redirectRes) => {
+                const file = fs.createWriteStream(filePath);
+                redirectRes.pipe(file);
+                file.on("finish", () => {
+                    file.close();
+                    console.error(`[matrix-plugin] Native binary downloaded: ${binaryName}`);
+                });
+            }).on("error", (err) => {
+                console.error(`[matrix-plugin] Failed to download native binary: ${err.message}`);
+                console.error(`[matrix-plugin] Please ensure you have network access to GitHub releases.`);
+            });
+            return;
+        }
+        const file = fs.createWriteStream(filePath);
+        res.pipe(file);
+        file.on("finish", () => {
+            file.close();
+            console.error(`[matrix-plugin] Native binary downloaded: ${binaryName}`);
+        });
+    })
+        .on("error", (err) => {
+        console.error(`[matrix-plugin] Failed to download native binary: ${err.message}`);
+        console.error(`[matrix-plugin] Please ensure you have network access to GitHub releases.`);
+    });
+}
diff --git a/dist/index.js b/dist/index.js
index 6ceb994..e625bbe 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -1,3 +1,7 @@
+import { ensureNativeBinary } from "./bootstrap-native.js";
+// Ensure native binary is available before loading matrix-bot-sdk
+// (opencode's npm doesn't run postinstall scripts)
+ensureNativeBinary();
 import fs from "fs";
 import os from "os";
 import path from "path";
diff --git a/index.ts b/index.ts
index 2a041fe..ad9093b 100644
--- a/index.ts
+++ b/index.ts
@@ -1,3 +1,8 @@
+import { ensureNativeBinary } from "./bootstrap-native.js"
+// Ensure native binary is available before loading matrix-bot-sdk
+// (opencode's npm doesn't run postinstall scripts)
+ensureNativeBinary()
+
 import fs from "fs"
 import os from "os"
 import path from "path"