diff --git a/src/rpc/methods/eth_estimateUserOperationGas.ts b/src/rpc/methods/eth_estimateUserOperationGas.ts
index 8ffc1091..4581ff58 100644
--- a/src/rpc/methods/eth_estimateUserOperationGas.ts
+++ b/src/rpc/methods/eth_estimateUserOperationGas.ts
@@ -284,6 +284,16 @@ export const ethEstimateUserOperationGasHandler = createMethodHandler({
             chainType
         } = rpcHandler.config
 
+        // Validate userOp fields (sync - fail fast before async checks)
+        const [fieldsValid, fieldsError] = rpcHandler.validateUserOpFields({
+            userOp,
+            entryPoint,
+            isEstimation: true
+        })
+        if (!fieldsValid) {
+            throw new RpcError(fieldsError, ERC7769Errors.InvalidFields)
+        }
+
         // Execute multiple async operations in parallel
         const [
             [validEip7702Auth, validEip7702AuthError],
diff --git a/src/rpc/rpcHandler.ts b/src/rpc/rpcHandler.ts
index 635ca7f5..b02aba12 100644
--- a/src/rpc/rpcHandler.ts
+++ b/src/rpc/rpcHandler.ts
@@ -145,12 +145,32 @@ export class RpcHandler {
     validateUserOpFields({
         userOp,
         entryPoint,
-        isBoosted = false
+        isBoosted = false,
+        isEstimation = false
     }: {
         userOp: UserOperation
         entryPoint: Address
         isBoosted?: boolean
+        isEstimation?: boolean
     }): [boolean, string] {
+        // Check for 0x7702 factory without eip7702Auth
+        if (
+            !userOp.eip7702Auth &&
+            isVersion07(userOp) &&
+            (userOp.factory === "0x7702" ||
+                userOp.factory === "0x7702000000000000000000000000000000000000")
+        ) {
+            return [
+                false,
+                "Invalid EIP-7702 userOperation: factory is 0x7702 but eip7702Auth is not provided."
+            ]
+        }
+
+        // Skip gas/field checks during estimation
+        if (isEstimation) {
+            return [true, ""]
+        }
+
         // Validate paymaster signature for EntryPoint 0.9
         const paymasterSignatureError = validatePaymasterSignature({
             userOp,
@@ -341,11 +361,12 @@ export class RpcHandler {
         if (
             isVersion07(userOp) &&
             userOp.factory !== "0x7702" &&
+            userOp.factory !== "0x7702000000000000000000000000000000000000" &&
             userOp.factory !== null
         ) {
             return [
                 false,
-                "Invalid EIP-7702 authorization: UserOperation cannot contain factory that is neither null or 0x7702."
+                "Invalid EIP-7702 authorization: factory must be null or 0x7702."
             ]
         }
 
diff --git a/test/e2e/tests/eth_estimateUserOperationGas.test.ts b/test/e2e/tests/eth_estimateUserOperationGas.test.ts
index 576e9594..07913280 100644
--- a/test/e2e/tests/eth_estimateUserOperationGas.test.ts
+++ b/test/e2e/tests/eth_estimateUserOperationGas.test.ts
@@ -379,6 +379,71 @@ describe.each([
             )
         })
 
+        test.each([
+            {
+                testName: "short form (0x7702)",
+                factory: "0x7702"
+            },
+            {
+                testName:
+                    "zero-padded form (0x7702000000000000000000000000000000000000)",
+                factory: "0x7702000000000000000000000000000000000000"
+            }
+        ])(
+            "Should reject estimation with factory $testName but no eip7702Auth",
+            async ({ factory }) => {
+                // Skip for v0.6 - doesn't have factory field
+                if (entryPointVersion === "0.6") {
+                    return
+                }
+
+                const bundlerClient = createBundlerClient({
+                    chain: foundry,
+                    transport: http(altoRpc)
+                })
+
+                const smartAccountClient = await getSmartAccountClient({
+                    entryPointVersion,
+                    anvilRpc,
+                    altoRpc
+                })
+
+                const op = (await smartAccountClient.prepareUserOperation({
+                    calls: [
+                        {
+                            to: zeroAddress,
+                            data: "0x"
+                        }
+                    ]
+                })) as UserOperation
+
+                // Set factory to 0x7702 without providing eip7702Auth
+                op.factory = factory
+                op.factoryData = undefined
+
+                try {
+                    await bundlerClient.estimateUserOperationGas({
+                        ...op,
+                        entryPointAddress: entryPoint
+                    })
+                    expect.fail("Must throw")
+                } catch (err) {
+                    expect(err).toBeInstanceOf(BaseError)
+                    const error = err as BaseError
+
+                    expect(error.details).toMatch(
+                        /factory is 0x7702 but eip7702Auth is not provided/i
+                    )
+
+                    const rpcError = error.walk(
+                        (e) => e instanceof RpcRequestError
+                    ) as RpcRequestError
+                    expect(rpcError).toBeDefined()
+                    expect(rpcError.code).toBe(ERC7769Errors.InvalidFields)
+                }
+            }
+        )
+
         test("Should throw AA25 when estimating userOp with nonce + 1", async () => {
             const smartAccountClient = await getSmartAccountClient({
                 entryPointVersion,
diff --git a/test/e2e/tests/eth_sendUserOperation.test.ts b/test/e2e/tests/eth_sendUserOperation.test.ts
index 42f6088f..c24cb69d 100644
--- a/test/e2e/tests/eth_sendUserOperation.test.ts
+++ b/test/e2e/tests/eth_sendUserOperation.test.ts
@@ -533,6 +533,66 @@ describe.each([
             expect(receipt.success)
         })
 
+        test.each([
+            {
+                testName: "short form (0x7702)",
+                factory: "0x7702"
+            },
+            {
+                testName:
+                    "zero-padded form (0x7702000000000000000000000000000000000000)",
+                factory: "0x7702000000000000000000000000000000000000"
+            }
+        ])(
+            "Should reject userOp with factory $testName but no eip7702Auth",
+            async ({ factory }) => {
+                // Skip for v0.6 - doesn't have factory field
+                if (entryPointVersion === "0.6") {
+                    return
+                }
+
+                const client = await getSmartAccountClient({
+                    entryPointVersion,
+                    anvilRpc,
+                    altoRpc
+                })
+
+                const op = (await client.prepareUserOperation({
+                    calls: [
+                        {
+                            to: TO_ADDRESS,
+                            value: VALUE,
+                            data: "0x"
+                        }
+                    ]
+                })) as UserOperation
+
+                // Set factory to 0x7702 without providing eip7702Auth
+                op.factory = factory
+                op.factoryData = undefined
+
+                op.signature = await client.account.signUserOperation(op)
+
+                try {
+                    await client.sendUserOperation(op)
+                    expect.fail("Must throw")
+                } catch (err) {
+                    expect(err).toBeInstanceOf(BaseError)
+                    const error = err as BaseError
+
+                    expect(error.details).toMatch(
+                        /factory is 0x7702 but eip7702Auth is not provided/i
+                    )
+
+                    const rpcError = error.walk(
+                        (e) => e instanceof RpcRequestError
+                    ) as RpcRequestError
+                    expect(rpcError).toBeDefined()
+                    expect(rpcError.code).toBe(ERC7769Errors.InvalidFields)
+                }
+            }
+        )
+
         test.each([
             {
                 testName: "r is zero",