diff --git a/src/transactions.ts b/src/transactions.ts
index 0924f86..166d9cb 100644
--- a/src/transactions.ts
+++ b/src/transactions.ts
@@ -237,6 +237,31 @@ export function buildTransaction(options: BuildTransactionOptions): ShellTransac
   if (options.maxFeePerBlobGas !== undefined && options.maxFeePerBlobGas !== null) {
     validateNonNegativeInteger(options.maxFeePerBlobGas, "maxFeePerBlobGas");
   }
+  const txType = options.txType ?? DEFAULT_TX_TYPE;
+  const hasBlobFee = options.maxFeePerBlobGas !== undefined && options.maxFeePerBlobGas !== null;
+  const hasBlobHashes =
+    options.blobVersionedHashes !== undefined && options.blobVersionedHashes !== null;
+  if (txType === 3) {
+    if (!hasBlobFee) {
+      throw new RangeError("type-3 transactions require maxFeePerBlobGas");
+    }
+    if (!hasBlobHashes || options.blobVersionedHashes?.length === 0) {
+      throw new RangeError("type-3 transactions require at least one blobVersionedHash");
+    }
+    if ((options.blobVersionedHashes?.length ?? 0) > 6) {
+      throw new RangeError("type-3 transactions support at most 6 blobVersionedHashes");
+    }
+    if (options.to === null) {
+      throw new RangeError("type-3 transactions cannot create contracts");
+    }
+  } else {
+    if (hasBlobFee) {
+      throw new RangeError("non-blob transactions cannot set maxFeePerBlobGas");
+    }
+    if (hasBlobHashes) {
+      throw new RangeError("non-blob transactions cannot set blobVersionedHashes");
+    }
+  }
 
   return {
     chain_id: options.chainId,
@@ -248,7 +273,7 @@ export function buildTransaction(options: BuildTransactionOptions): ShellTransac
     max_fee_per_gas: maxFeePerGas,
     max_priority_fee_per_gas: maxPriorityFeePerGas,
     access_list: options.accessList ?? null,
-    tx_type: options.txType ?? DEFAULT_TX_TYPE,
+    tx_type: txType,
     max_fee_per_blob_gas: options.maxFeePerBlobGas ?? null,
     blob_versioned_hashes: options.blobVersionedHashes ?? null,
   };
diff --git a/tests/transactions.test.mjs b/tests/transactions.test.mjs
new file mode 100644
index 0000000..ec66988
--- /dev/null
+++ b/tests/transactions.test.mjs
@@ -0,0 +1,75 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { buildTransaction } from '../dist/transactions.js';
+
+const recipient = `0x${'11'.repeat(32)}`;
+const blobHash = `0x${'22'.repeat(32)}`;
+
+function baseOptions(overrides = {}) {
+  return {
+    chainId: 1337,
+    nonce: 0,
+    to: recipient,
+    ...overrides,
+  };
+}
+
+test('buildTransaction rejects blob-only fields on non-blob transactions', () => {
+  assert.throws(
+    () => buildTransaction(baseOptions({ maxFeePerBlobGas: 1 })),
+    /non-blob transactions cannot set maxFeePerBlobGas/,
+  );
+  assert.throws(
+    () => buildTransaction(baseOptions({ blobVersionedHashes: [blobHash] })),
+    /non-blob transactions cannot set blobVersionedHashes/,
+  );
+});
+
+test('buildTransaction requires complete type-3 blob fields', () => {
+  assert.throws(
+    () => buildTransaction(baseOptions({ txType: 3, blobVersionedHashes: [blobHash] })),
+    /require maxFeePerBlobGas/,
+  );
+  assert.throws(
+    () => buildTransaction(baseOptions({ txType: 3, maxFeePerBlobGas: 1 })),
+    /require at least one blobVersionedHash/,
+  );
+  assert.throws(
+    () =>
+      buildTransaction(
+        baseOptions({
+          txType: 3,
+          maxFeePerBlobGas: 1,
+          blobVersionedHashes: Array(7).fill(blobHash),
+        }),
+      ),
+    /at most 6 blobVersionedHashes/,
+  );
+  assert.throws(
+    () =>
+      buildTransaction(
+        baseOptions({
+          txType: 3,
+          to: null,
+          maxFeePerBlobGas: 1,
+          blobVersionedHashes: [blobHash],
+        }),
+      ),
+    /cannot create contracts/,
+  );
+});
+
+test('buildTransaction accepts valid type-3 blob fields', () => {
+  const tx = buildTransaction(
+    baseOptions({
+      txType: 3,
+      maxFeePerBlobGas: 1,
+      blobVersionedHashes: [blobHash],
+    }),
+  );
+
+  assert.equal(tx.tx_type, 3);
+  assert.equal(tx.max_fee_per_blob_gas, 1);
+  assert.deepEqual(tx.blob_versioned_hashes, [blobHash]);
+});