A method implemented by one or more processors, comprising: receiving natural language input generated based on user interface input provided by a user during a human-to-automated assistant dialog, wherein the natural language input fails to explicitly indicate an agent to be invoked based on the natural language input; prior to invoking any agent in response to the natural language input: applying, as input to a trained machine learning model, input features that are generated based on the natural language input, the input features including values for intent parameters; generating, over the machine learning model based on the input, output that includes a plurality of values, wherein each of the values corresponds to a respective one of a plurality of agents, and wherein each of the values indicates an ability of a respective one of the plurality of agents to generate responsive content for the natural language input; selecting a particular agent, from the plurality of agents, based on the generated values, wherein selecting the particular agent occurs without providing any user interface output that explicitly solicits the user to select between the particular agent and one or more other of the plurality of agents; responsive to receiving the natural language input and based on selecting the particular agent: transmitting, via an application programming interface, an invocation request to the selected particular agent, wherein the invocation request invokes the particular agent and causes the particular agent to generate particular responsive content for presentation to the user via one or more user interface output devices, wherein only the selected particular agent is invoked in response to receiving the natural language input. receiving natural language input generated based on user interface input provided by a user during a human-to-automated assistant dialog, wherein the natural language input fails to explicitly indicate an agent to be invoked based on the natural language input; prior to invoking any agent in response to the natural language input: applying, as input to a trained machine learning model, input features that are generated based on the natural language input, the input features including values for intent parameters; generating, over the machine learning model based on the input, output that includes a plurality of values, wherein each of the values corresponds to a respective one of a plurality of agents, and wherein each of the values indicates an ability of a respective one of the plurality of agents to generate responsive content for the natural language input; selecting a particular agent, from the plurality of agents, based on the generated values, wherein selecting the particular agent occurs without providing any user interface output that explicitly solicits the user to select between the particular agent and one or more other of the plurality of agents; applying, as input to a trained machine learning model, input features that are generated based on the natural language input, the input features including values for intent parameters; generating, over the machine learning model based on the input, output that includes a plurality of values, wherein each of the values corresponds to a respective one of a plurality of agents, and wherein each of the values indicates an ability of a respective one of the plurality of agents to generate responsive content for the natural language input; selecting a particular agent, from the plurality of agents, based on the generated values, wherein selecting the particular agent occurs without providing any user interface output that explicitly solicits the user to select between the particular agent and one or more other of the plurality of agents; responsive to receiving the natural language input and based on selecting the particular agent: transmitting, via an application programming interface, an invocation request to the selected particular agent, wherein the invocation request invokes the particular agent and causes the particular agent to generate particular responsive content for presentation to the user via one or more user interface output devices, wherein only the selected particular agent is invoked in response to receiving the natural language input. transmitting, via an application programming interface, an invocation request to the selected particular agent, wherein the invocation request invokes the particular agent and causes the particular agent to generate particular responsive content for presentation to the user via one or more user interface output devices, wherein only the selected particular agent is invoked in response to receiving the natural language input.