diff --git a/package.json b/package.json
index e5df6c69b4a8..09848d739f99 100644
--- a/package.json
+++ b/package.json
@@ -2,7 +2,7 @@
   "private": true,
   "scripts": {
     "build": "node ./scripts/verify-packages-versions.js && yarn run-p build:rollup build:types build:bundle && yarn build:extras",
-    "build:bundle": "lerna run --parallel build:bundle",
+    "build:bundle": "yarn ts-node scripts/ensure-bundle-deps.ts && yarn lerna run --parallel build:bundle",
     "build:dev": "run-p build:types build:rollup",
     "build:extras": "lerna run --parallel build:extras",
     "build:rollup": "lerna run --parallel build:rollup",
diff --git a/packages/browser/package.json b/packages/browser/package.json
index 05c3cd7b30e4..487294870c5b 100644
--- a/packages/browser/package.json
+++ b/packages/browser/package.json
@@ -44,7 +44,7 @@
   },
   "scripts": {
     "build": "run-p build:rollup build:bundle build:types",
-    "build:bundle": "rollup --config rollup.bundle.config.js",
+    "build:bundle": "yarn ts-node ../../scripts/ensure-bundle-deps.ts && yarn rollup --config rollup.bundle.config.js",
     "build:dev": "run-p build:rollup build:types",
     "build:rollup": "rollup -c rollup.npm.config.js",
     "build:types": "tsc -p tsconfig.types.json",
diff --git a/packages/hub/src/session.ts b/packages/hub/src/session.ts
index fed5cfa004ad..c8cba3e30a04 100644
--- a/packages/hub/src/session.ts
+++ b/packages/hub/src/session.ts
@@ -1,5 +1,4 @@
-import { Session, SessionContext, SessionStatus } from '@sentry/types';
-import { SerializedSession } from '@sentry/types/build/types/session';
+import { SerializedSession, Session, SessionContext, SessionStatus } from '@sentry/types';
 import { dropUndefinedKeys, timestampInSeconds, uuid4 } from '@sentry/utils';
 
 /**
diff --git a/packages/integrations/package.json b/packages/integrations/package.json
index dc64f179c932..ea1e050dad5a 100644
--- a/packages/integrations/package.json
+++ b/packages/integrations/package.json
@@ -26,7 +26,7 @@
   },
   "scripts": {
     "build": "run-p build:rollup build:types build:bundle",
-    "build:bundle": "bash scripts/buildBundles.sh",
+    "build:bundle": "yarn ts-node ../../scripts/ensure-bundle-deps.ts && bash scripts/buildBundles.sh",
     "build:dev": "run-p build:rollup build:types",
     "build:rollup": "rollup -c rollup.npm.config.js",
     "build:types": "tsc -p tsconfig.types.json",
diff --git a/packages/tracing/package.json b/packages/tracing/package.json
index 8344bbdec582..25510c2a1376 100644
--- a/packages/tracing/package.json
+++ b/packages/tracing/package.json
@@ -27,7 +27,7 @@
   },
   "scripts": {
     "build": "run-p build:rollup build:types build:bundle && yarn build:extras #necessary for integration tests",
-    "build:bundle": "rollup --config rollup.bundle.config.js",
+    "build:bundle": "yarn ts-node ../../scripts/ensure-bundle-deps.ts && yarn rollup --config rollup.bundle.config.js",
     "build:dev": "run-p build:rollup build:types",
     "build:extras": "yarn build:prepack",
     "build:prepack": "ts-node ../../scripts/prepack.ts --bundles",
diff --git a/packages/types/src/index.ts b/packages/types/src/index.ts
index 8fe14cd3ac16..91470886bb2c 100644
--- a/packages/types/src/index.ts
+++ b/packages/types/src/index.ts
@@ -46,6 +46,7 @@ export type {
   RequestSession,
   RequestSessionStatus,
   SessionFlusherLike,
+  SerializedSession,
 } from './session';
 
 // eslint-disable-next-line deprecation/deprecation
diff --git a/packages/vue/package.json b/packages/vue/package.json
index 430d46cbac0f..e1f37eade04a 100644
--- a/packages/vue/package.json
+++ b/packages/vue/package.json
@@ -27,7 +27,7 @@
   },
   "scripts": {
     "build": "run-p build:rollup build:types",
-    "build:bundle": "rollup --config rollup.bundle.config.js",
+    "build:bundle": "yarn ts-node ../../scripts/ensure-bundle-deps.ts && yarn rollup --config rollup.bundle.config.js",
     "build:dev": "run-p build:rollup build:types",
     "build:rollup": "rollup -c rollup.npm.config.js",
     "build:types": "tsc -p tsconfig.types.json",
diff --git a/packages/wasm/package.json b/packages/wasm/package.json
index baf0f28e3bc5..2bc4a612132b 100644
--- a/packages/wasm/package.json
+++ b/packages/wasm/package.json
@@ -31,7 +31,7 @@
   },
   "scripts": {
     "build": "run-p build:rollup build:bundle build:types",
-    "build:bundle": "rollup --config rollup.bundle.config.js",
+    "build:bundle": "yarn ts-node ../../scripts/ensure-bundle-deps.ts && yarn rollup --config rollup.bundle.config.js",
     "build:dev": "run-p build:rollup build:types",
     "build:rollup": "rollup -c rollup.npm.config.js",
     "build:types": "tsc -p tsconfig.types.json",
diff --git a/scripts/ensure-bundle-deps.ts b/scripts/ensure-bundle-deps.ts
new file mode 100644
index 000000000000..61e724fc0029
--- /dev/null
+++ b/scripts/ensure-bundle-deps.ts
@@ -0,0 +1,155 @@
+/* eslint-disable no-console */
+import * as childProcess from 'child_process';
+import * as fs from 'fs';
+import * as path from 'path';
+import * as util from 'util';
+
+/**
+ * Ensure that `build:bundle` has all of the dependencies it needs to run. Works at both the repo and package level.
+ */
+async function ensureBundleBuildPrereqs(options: { dependencies: string[]; maxRetries?: number }): Promise<void> {
+  const { maxRetries = 12, dependencies } = options;
+
+  const {
+    // The directory in which the yarn command was originally invoked (which won't necessarily be the same as
+    // `process.cwd()`)
+    INIT_CWD: yarnInitialDir,
+    // JSON containing the args passed to `yarn`
+    npm_config_argv: yarnArgJSON,
+  } = process.env;
+
+  if (!yarnInitialDir || !yarnArgJSON) {
+    const received = { INIT_CWD: yarnInitialDir, npm_config_argv: yarnArgJSON };
+    throw new Error(
+      `Missing environment variables needed for ensuring bundle dependencies. Received:\n${util.inspect(received)}\n`,
+    );
+  }
+
+  // Did this build get invoked by a repo-level script, or a package-level script, and which script was it?
+  const isTopLevelBuild = path.basename(yarnInitialDir) === 'sentry-javascript';
+  const yarnScript = (JSON.parse(yarnArgJSON) as { original: string[] }).original[0];
+
+  // convert '@sentry/xyz` to `xyz`
+  const dependencyDirs = dependencies.map(npmPackageName => npmPackageName.split('/')[1]);
+
+  // The second half of the conditional tests if this script is being run by the original top-level command or a
+  // package-level command spawned by it.
+  const packagesDir = isTopLevelBuild && yarnInitialDir === process.cwd() ? 'packages' : '..';
+
+  if (checkForBundleDeps(packagesDir, dependencyDirs)) {
+    // We're good, nothing to do, the files we need are there
+    return;
+  }
+
+  // If we get here, the at least some of the dependencies are missing, but how we handle that depends on how we got
+  // here. There are six possibilities:
+  // - We ran `build` or `build:bundle` at the repo level
+  // - We ran `build` or `build:bundle` at the package level
+  // - We ran `build` or `build:bundle` at the repo level and lerna then ran `build:bundle` at the package level. (We
+  //   shouldn't ever land here under this scenario - the top-level build should already have handled any missing
+  //   dependencies - but it's helpful to consider all the possibilities.)
+  //
+  // In the first version of the first scenario (repo-level `build` -> repo-level `build:bundle`), all we have to do is
+  // wait, because other parts of `build` are creating them as this check is being done. (Waiting 5 or 10 or even 15
+  // seconds to start running `build:bundle` in parallel is better than pushing it to the second half of `build`,
+  // because `build:bundle` is the slowest part of the build and therefore the one we most want to parallelize with
+  // other slow parts, like `build:types`.)
+  //
+  // In all other scenarios, if the dependencies are missing, we have to build them ourselves - with `build:bundle` at
+  // either level, we're the only thing happening (so no one's going to do it for us), and with package-level `build`,
+  // types and npm assets are being built simultaneously, but only for the package being bundled, not for its
+  // dependencies. Either way, it's on us to fix the problem.
+  //
+  // TODO: This actually *doesn't* work for package-level `build`, not because of a flaw in this logic, but because
+  // `build:rollup` has similar dependency needs (it needs types rather than npm builds). We should do something like
+  // this for that at some point.
+
+  if (isTopLevelBuild && yarnScript === 'build') {
+    let retries = 0;
+
+    console.log('\nSearching for bundle dependencies...');
+
+    while (retries < maxRetries && !checkForBundleDeps(packagesDir, dependencyDirs)) {
+      console.log('Bundle dependencies not found. Trying again in 5 seconds.');
+      retries += 1;
+      await sleep(5000);
+    }
+
+    if (retries === maxRetries) {
+      throw new Error(
+        `\nERROR: \`yarn build:bundle\` (triggered by \`yarn build\`) cannot find its depdendencies, despite waiting ${
+          5 * maxRetries
+        } seconds for the rest of \`yarn build\` to create them. Something is wrong - it shouldn't take that long. Exiting.`,
+      );
+    }
+
+    console.log(`\nFound all bundle dependencies after ${retries} retries. Beginning bundle build...`);
+  }
+
+  // top-level `build:bundle`, package-level `build` and `build:bundle`
+  else {
+    console.warn('\nWARNING: Missing dependencies for bundle build. They will be built before continuing.');
+
+    for (const dependencyDir of dependencyDirs) {
+      console.log(`\nBuilding \`${dependencyDir}\` package...`);
+      run('yarn build:rollup', { cwd: `${packagesDir}/${dependencyDir}` });
+    }
+
+    console.log('\nAll dependencies built successfully. Beginning bundle build...');
+  }
+}
+
+/**
+ * See if all of the necessary dependencies exist
+ */
+function checkForBundleDeps(packagesDir: string, dependencyDirs: string[]): boolean {
+  for (const dependencyDir of dependencyDirs) {
+    const depBuildDir = `${packagesDir}/${dependencyDir}/build`;
+
+    // Checking that the directories exist isn't 100% the same as checking that the files themselves exist, of course,
+    // but it's a decent proxy, and much simpler to do than checking  for individual files.
+    if (
+      !(
+        (fs.existsSync(`${depBuildDir}/cjs`) && fs.existsSync(`${depBuildDir}/esm`)) ||
+        (fs.existsSync(`${depBuildDir}/npm/cjs`) && fs.existsSync(`${depBuildDir}/npm/esm`))
+      )
+    ) {
+      // Fail fast
+      return false;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * Wait the given number of milliseconds before continuing.
+ */
+async function sleep(ms: number): Promise<void> {
+  await new Promise(resolve =>
+    setTimeout(() => {
+      resolve();
+    }, ms),
+  );
+}
+
+/**
+ * Run the given shell command, piping the shell process's `stdin`, `stdout`, and `stderr` to that of the current
+ * process. Returns contents of `stdout`.
+ */
+function run(cmd: string, options?: childProcess.ExecSyncOptions): string {
+  return String(childProcess.execSync(cmd, { stdio: 'inherit', ...options }));
+}
+
+// TODO: Not ideal that we're hard-coding this, and it's easy to get when we're in a package directory, but would take
+// more work to get from the repo level. Fortunately this list is unlikely to change very often, and we're the only ones
+// we'll break if it gets out of date.
+const dependencies = ['@sentry/utils', '@sentry/hub', '@sentry/core'];
+
+if (['sentry-javascript', 'tracing', 'wasm'].includes(path.basename(process.cwd()))) {
+  dependencies.push('@sentry/browser');
+}
+
+void ensureBundleBuildPrereqs({
+  dependencies,
+});