diff --git a/protos/local/activity_result.proto b/protos/local/activity_result.proto
new file mode 100644
index 000000000..8e99a1e89
--- /dev/null
+++ b/protos/local/activity_result.proto
@@ -0,0 +1,29 @@
+syntax = "proto3";
+
+package coresdk.activity_result;
+
+import "temporal/api/failure/v1/message.proto";
+import "temporal/api/common/v1/message.proto";
+
+/// Used in ActivityResult to report cancellation
+message Cancellation {
+}
+
+/// Used in ActivityResult to report successful completion
+message Success {
+    temporal.api.common.v1.Payload result = 1;
+}
+
+/// Used in ActivityResult to report failure
+message Failure {
+    temporal.api.failure.v1.Failure failure = 1;
+}
+
+/// Used to report activity completion to core and to resolve the activity in a workflow activation
+message ActivityResult {
+    oneof status {
+        Success successful = 1;
+        Cancellation cancelled = 2;
+        Failure failed = 3;
+    }
+}
diff --git a/protos/local/activity_task.proto b/protos/local/activity_task.proto
new file mode 100644
index 000000000..c2f8fad61
--- /dev/null
+++ b/protos/local/activity_task.proto
@@ -0,0 +1,51 @@
+/**
+ * Definitions of the different activity tasks returned from [crate::Core::poll_task].
+ */
+
+syntax = "proto3";
+
+package coresdk.activity_task;
+
+import "google/protobuf/timestamp.proto";
+import "google/protobuf/duration.proto";
+import "dependencies/gogoproto/gogo.proto";
+import "temporal/api/common/v1/message.proto";
+import "activity_result.proto";
+import "timer_result.proto";
+
+// TODO: docstrings everywhere
+message StartActivity {
+    string workflow_namespace = 1;
+    temporal.api.common.v1.WorkflowType workflow_type = 2;
+    temporal.api.common.v1.WorkflowExecution workflow_execution = 3;
+    temporal.api.common.v1.ActivityType activity_type = 4;
+    temporal.api.common.v1.Header header = 5;
+    temporal.api.common.v1.Payloads input = 6;
+    temporal.api.common.v1.Payloads heartbeat_details = 7;
+    google.protobuf.Timestamp scheduled_time = 8 [(gogoproto.stdtime) = true];
+    google.protobuf.Timestamp current_attempt_scheduled_time = 9 [(gogoproto.stdtime) = true];
+    google.protobuf.Timestamp started_time = 10 [(gogoproto.stdtime) = true];
+    int32 attempt = 11;
+    google.protobuf.Duration schedule_to_close_timeout = 12 [(gogoproto.stdduration) = true];
+    google.protobuf.Duration start_to_close_timeout = 13 [(gogoproto.stdduration) = true];
+    google.protobuf.Duration heartbeat_timeout = 14 [(gogoproto.stdduration) = true];
+    // This is an actual retry policy the service uses.
+    // It can be different from the one provided (or not) during activity scheduling
+    // as the service can override the provided one in case its values are not specified
+    // or exceed configured system limits.
+    temporal.api.common.v1.RetryPolicy retry_policy = 15;
+}
+
+/// Request to cancel an activity from a workflow
+message CancelActivity {
+}
+
+message ActivityTask {
+    string activity_id = 1;
+    oneof variant {
+        // Start activity execution.
+        StartActivity start = 2;
+        // Attempt to cancel activity execution.
+        CancelActivity cancel = 3;
+    }
+}
diff --git a/protos/local/core_interface.proto b/protos/local/core_interface.proto
index 0880ad626..403211ca8 100644
--- a/protos/local/core_interface.proto
+++ b/protos/local/core_interface.proto
@@ -1,3 +1,7 @@
+/**
+ * Core<->Lang request / response definitions
+ */
+
 syntax = "proto3";
 
 package coresdk;
@@ -6,245 +10,46 @@ package coresdk;
 // the include paths. You can make it work by going to the "Protobuf Support" settings section
 // and adding the "api_upstream" subdir as an include path.
 
-import "google/protobuf/timestamp.proto";
-import "google/protobuf/duration.proto";
-import "google/protobuf/empty.proto";
-import "dependencies/gogoproto/gogo.proto";
-import "temporal/api/workflowservice/v1/request_response.proto";
-import "temporal/api/taskqueue/v1/message.proto";
-import "temporal/api/enums/v1/failed_cause.proto";
-import "temporal/api/failure/v1/message.proto";
-import "temporal/api/history/v1/message.proto";
 import "temporal/api/common/v1/message.proto";
-import "temporal/api/command/v1/message.proto";
-import "temporal/api/query/v1/message.proto";
+import "workflow_activation.proto";
+import "workflow_activation_completion.proto";
+import "activity_task.proto";
+import "activity_result.proto";
 
-// A request as given to [crate::Core::poll_task]
+/// A request as given to [crate::Core::poll_task]
 message PollTaskReq {
-    // What type of task to poll for
-    enum TaskType {
-        // Poll for workflows
-        WORKFLOWS = 0;
-        // Poll for activities
-        ACTIVITIES = 1;
-    }
-
-    // A list of task types to poll for
-    repeated TaskType types = 1;
+    bool workflows = 1;
+    bool activities = 2;
 }
 
-// An instruction to perform work from core->lang sdk
+/// An instruction to perform work from core->lang SDK, returned from [crate::Core::poll_task]
 message Task {
     // A unique identifier for this task
     bytes task_token = 1;
     // The type of task to be performed
     oneof variant {
         // Wake up a workflow
-        WFActivation workflow = 2;
-        // Run an activity
-        ActivityTask activity = 3;
-    }
-}
-
-// An instruction to the lang sdk to run some workflow code, whether for the first time or from
-// a cached state.
-message WFActivation {
-    // The current time as understood by the workflow, which is set by workflow task started events
-    google.protobuf.Timestamp timestamp = 1 [(gogoproto.stdtime) = true];
-    // The id of the currently active run of the workflow
-    string run_id = 2;
-    // The things to do upon activating the workflow
-    repeated WFActivationJob jobs = 3;
-}
-
-message WFActivationJob {
-    oneof variant {
-        // Begin a workflow for the first time
-        StartWorkflow start_workflow = 1;
-        // A timer has fired, allowing whatever was waiting on it (if anything) to proceed
-        FireTimer fire_timer = 2;
-        // A timer was canceled, and needs to be unblocked on the lang side.
-        CancelTimer cancel_timer = 3;
-        // Workflow was reset. The randomness seed must be updated.
-        UpdateRandomSeed update_random_seed = 4;
-        // A request to query the workflow was received.
-        QueryWorkflow query_workflow = 5;
-        // A request to cancel the workflow was received.
-        CancelWorkflow cancel_workflow = 6;
-        // A request to signal the workflow was received.
-        SignalWorkflow signal_workflow = 7;
-        // An activity was resolved with, result could be completed, failed or cancelled
-        ResolveActivity resolve_activity = 8;
-    }
-}
-
-message StartWorkflow {
-    // The identifier the lang-specific sdk uses to execute workflow code
-    string workflow_type = 1;
-    // The workflow id used on the temporal server
-    string workflow_id = 2;
-    // Input to the workflow code
-    temporal.api.common.v1.Payloads arguments = 3;
-    // The seed must be used to initialize the random generator used by SDK.
-    // RandomSeedUpdatedAttributes are used to deliver seed updates.
-    uint64 randomness_seed = 4;
-
-    // TODO: Do we need namespace here, or should that just be fetchable easily?
-    //   will be others - workflow exe started attrs, etc
-}
-
-message FireTimer {
-    string timer_id = 1;
-}
-
-message ResolveActivity {
-    string activity_id = 1;
-    ActivityResult result = 2;
-}
-
-message CancelTimer {
-    string timer_id = 1;
-}
-
-message UpdateRandomSeed {
-    uint64 randomness_seed = 1;
-}
-
-message QueryWorkflow {
-    temporal.api.query.v1.WorkflowQuery query = 1;
-}
-
-message CancelWorkflow {
-    // TODO: add attributes here
-}
-
-message SignalWorkflow {
-    // The signal information from the workflow's history
-    temporal.api.history.v1.WorkflowExecutionSignaledEventAttributes signal = 1;
-}
-
-message StartActivity {
-    string workflow_namespace = 1;
-    temporal.api.common.v1.WorkflowType workflow_type = 2;
-    temporal.api.common.v1.WorkflowExecution workflow_execution = 3;
-    temporal.api.common.v1.ActivityType activity_type = 4;
-    temporal.api.common.v1.Header header = 5;
-    temporal.api.common.v1.Payloads input = 6;
-    temporal.api.common.v1.Payloads heartbeat_details = 7;
-    google.protobuf.Timestamp scheduled_time = 8 [(gogoproto.stdtime) = true];
-    google.protobuf.Timestamp current_attempt_scheduled_time = 9 [(gogoproto.stdtime) = true];
-    google.protobuf.Timestamp started_time = 10 [(gogoproto.stdtime) = true];
-    int32 attempt = 11;
-    // (-- api-linter: core::0140::prepositions=disabled
-    //     aip.dev/not-precedent: "to" is used to indicate interval. --)
-    google.protobuf.Duration schedule_to_close_timeout = 12 [(gogoproto.stdduration) = true];
-    // (-- api-linter: core::0140::prepositions=disabled
-    //     aip.dev/not-precedent: "to" is used to indicate interval. --)
-    google.protobuf.Duration start_to_close_timeout = 13 [(gogoproto.stdduration) = true];
-    google.protobuf.Duration heartbeat_timeout = 14 [(gogoproto.stdduration) = true];
-    // This is an actual retry policy the service uses.
-    // It can be different from the one provided (or not) during activity scheduling
-    // as the service can override the provided one in case its values are not specified
-    // or exceed configured system limits.
-    temporal.api.common.v1.RetryPolicy retry_policy = 15;
-}
-
-message CancelActivity {
-    // TODO: add attributes
-}
-
-message ActivityTask {
-    string activity_id = 1;
-    oneof job {
-        // Start activity execution.
-        StartActivity start = 2;
-        // Attempt to cancel activity execution.
-        CancelActivity cancel = 3;
+        coresdk.workflow_activation.WFActivation workflow = 2;
+        // Start or cancel an activity
+        coresdk.activity_task.ActivityTask activity = 3;
     }
 }
 
 
-// Sent from lang side to core when calling [crate::Core::complete_task]
+/// Sent from lang side to core when calling [crate::Core::complete_task]
 message TaskCompletion {
     // The id from the [Task] being completed
     bytes task_token = 1;
     oneof variant {
-        // Complete a workflow task
-        WFActivationCompletion workflow = 2;
-        // Complete an activity task
-        ActivityResult activity = 3;
-    }
-}
-
-message WFActivationCompletion {
-    oneof status {
-        WFActivationSuccess successful = 1;
-        WFActivationFailure failed = 2;
-    }
-}
-
-/// Used to report activity completion to core and to resolve the activity in a workflow activtion
-message ActivityResult {
-    oneof status {
-        ActivityTaskSuccess completed = 1;
-        ActivityTaskCancelation canceled = 2;
-        ActivityTaskFailure failed = 3;
-    }
-}
-
-/// Request cancellation of an activity from a workflow
-message RequestActivityCancellation {
-    string activity_id = 1;
-    string reason = 2;
-}
-
-message CoreCommand {
-    oneof variant {
-        temporal.api.query.v1.WorkflowQueryResult respond_to_query = 1;
-        RequestActivityCancellation request_activity_cancellation = 2;
+        /// Complete a workflow task
+        coresdk.workflow_activation_completion.WFActivationCompletion workflow = 2;
+        /// Complete an activity task
+        coresdk.activity_result.ActivityResult activity = 3;
     }
 }
 
-// Included in successful [WfActivationCompletion]s, indicates what the workflow wishes to do next
-message Command {
-    oneof variant {
-        temporal.api.command.v1.Command api = 1;
-        CoreCommand core = 2;
-    }
-}
-
-message WFActivationSuccess {
-    // A list of commands to send back to the temporal server
-    repeated Command commands = 1;
-
-    // Other bits from RespondWorkflowTaskCompletedRequest as needed
-}
-
-message WFActivationFailure {
-    temporal.api.enums.v1.WorkflowTaskFailedCause cause = 1;
-    temporal.api.failure.v1.Failure failure = 2;
-
-    // Other bits from RespondWorkflowTaskFailedRequest as needed
-}
-
-/// Used in ActivityResult to report cancellation
-message ActivityTaskCancelation {
-}
-
-/// Used in ActivityResult to report successful completion
-message ActivityTaskSuccess {
-    temporal.api.common.v1.Payloads result = 1;
-    // Other bits from RespondActivityTaskCompletedRequest as needed
-}
-
-/// Used in ActivityResult to report failure
-message ActivityTaskFailure {
-    temporal.api.failure.v1.Failure failure = 1;
-    // Other bits from RespondActivityTaskFailedRequest as needed
-}
-
-// A request as given to [crate::Core::send_activity_heartbeat]
+/// A request as given to [crate::Core::send_activity_heartbeat]
 message ActivityHeartbeat {
     string activity_id = 1;
-    temporal.api.common.v1.Payloads details = 2;
+    repeated temporal.api.common.v1.Payload details = 2;
 }
diff --git a/protos/local/timer_result.proto b/protos/local/timer_result.proto
new file mode 100644
index 000000000..509051fc5
--- /dev/null
+++ b/protos/local/timer_result.proto
@@ -0,0 +1,19 @@
+syntax = "proto3";
+
+package coresdk.timer_result;
+
+/// Used in TimerResult to report cancellation
+message Cancellation {
+}
+
+/// Used in TimerResult to report successful completion
+message Success {
+}
+
+/// Used to resolve a timer in a workflow activation
+message TimerResult {
+    oneof status {
+        Success successful = 1;
+        Cancellation cancelled = 2;
+    }
+}
diff --git a/protos/local/workflow_activation.proto b/protos/local/workflow_activation.proto
new file mode 100644
index 000000000..442f96177
--- /dev/null
+++ b/protos/local/workflow_activation.proto
@@ -0,0 +1,95 @@
+/**
+ * Definitions of the different workflow activation jobs returned from [crate::Core::poll_task].
+ * The lang SDK applies these activation jobs to drives workflows.
+ */
+
+syntax = "proto3";
+
+package coresdk.workflow_activation;
+
+import "google/protobuf/timestamp.proto";
+import "dependencies/gogoproto/gogo.proto";
+import "temporal/api/common/v1/message.proto";
+import "activity_result.proto";
+import "timer_result.proto";
+
+/// Start a new workflow
+message StartWorkflow {
+    /// The identifier the lang-specific sdk uses to locate which workflow to execute
+    string workflow_type = 1;
+    /// The workflow id used on the temporal server
+    string workflow_id = 2;
+    /// Input to the workflow code - apply to the workflow method
+    repeated temporal.api.common.v1.Payload arguments = 3;
+    /// The seed must be used to initialize the random generator used by SDK.
+    /// UpdateRandomSeed is used to deliver seed updates.
+    uint64 randomness_seed = 4;
+
+    // TODO: Do we need namespace here, or should that just be fetchable easily?
+    //   will be others - workflow exe started attrs, etc
+}
+
+/// Cancel a running workflow
+message CancelWorkflow {
+}
+
+/// Send a signal to a workflow
+message SignalWorkflow {
+    string name = 1;
+    repeated temporal.api.common.v1.Payload arguments = 2;
+}
+
+/// Query a workflow
+message QueryWorkflow {
+    string name = 1;
+    repeated temporal.api.common.v1.Payload arguments = 2;
+}
+
+
+/// Notify a workflow that a timer has been resolved
+message ResolveTimer {
+    string timer_id = 1;
+    coresdk.timer_result.TimerResult result = 2;
+}
+
+/// Notify a workflow that an activity has been resolved
+message ResolveActivity {
+    string activity_id = 1;
+    coresdk.activity_result.ActivityResult result = 2;
+}
+
+/// Update the workflow's random seed
+message UpdateRandomSeed {
+    uint64 randomness_seed = 1;
+}
+
+/// An instruction to the lang sdk to run some workflow code, whether for the first time or from
+/// a cached state.
+message WFActivation {
+    /// The current time as understood by the workflow, which is set by workflow task started events
+    google.protobuf.Timestamp timestamp = 1 [(gogoproto.stdtime) = true];
+    /// The id of the currently active run of the workflow
+    string run_id = 2;
+    /// The things to do upon activating the workflow
+    repeated WFActivationJob jobs = 3;
+}
+
+message WFActivationJob {
+    oneof variant {
+        /// Begin a workflow for the first time
+        StartWorkflow start_workflow = 1;
+        /// A timer has fired or was cancelled, allowing whatever was waiting on it (if anything) to proceed
+        ResolveTimer resolve_timer = 2;
+        /// An activity was resolved with, result could be completed, failed or cancelled
+        ResolveActivity resolve_activity = 3;
+        /// Workflow was reset. The randomness seed must be updated
+        UpdateRandomSeed update_random_seed = 4;
+        /// A request to cancel the workflow was received
+        CancelWorkflow cancel_workflow = 5;
+        /// A request to query the workflow was received
+        QueryWorkflow query_workflow = 6;
+        /// A request to signal the workflow was received
+        SignalWorkflow signal_workflow = 7;
+    }
+}
+
diff --git a/protos/local/workflow_activation_completion.proto b/protos/local/workflow_activation_completion.proto
new file mode 100644
index 000000000..eb4c0f66f
--- /dev/null
+++ b/protos/local/workflow_activation_completion.proto
@@ -0,0 +1,32 @@
+syntax = "proto3";
+
+package coresdk.workflow_activation_completion;
+
+import "temporal/api/enums/v1/failed_cause.proto";
+import "temporal/api/failure/v1/message.proto";
+import "workflow_command.proto";
+
+/// Result of a single workflow activation, reported from lang to core
+message WFActivationCompletion {
+    oneof status {
+        Success successful = 1;
+        Failure failed = 2;
+    }
+}
+
+/// Successful workflow activation with a list of commands generated by the workflow execution
+message Success {
+    // A list of commands to send back to the temporal server
+    repeated coresdk.workflow_command.WorkflowCommand commands = 1;
+
+    // Other bits from RespondWorkflowTaskCompletedRequest as needed
+}
+
+/// Failure to activate a workflow
+message Failure {
+    // TODO: port these protos
+    temporal.api.enums.v1.WorkflowTaskFailedCause cause = 1;
+    temporal.api.failure.v1.Failure failure = 2;
+
+    // Other bits from RespondWorkflowTaskFailedRequest as needed
+}
diff --git a/protos/local/workflow_command.proto b/protos/local/workflow_command.proto
new file mode 100644
index 000000000..24536cbf9
--- /dev/null
+++ b/protos/local/workflow_command.proto
@@ -0,0 +1,31 @@
+/**
+ * Definitions for commands from a workflow in lang SDK to core.
+ * While a workflow processes a batch of activiation jobs,
+ * it accumulates these commands to be sent back to core to conclude an activation.
+ */
+
+syntax = "proto3";
+
+package coresdk.workflow_command;
+
+import "google/protobuf/timestamp.proto";
+import "google/protobuf/duration.proto";
+import "temporal/api/common/v1/message.proto";
+import "temporal/api/query/v1/message.proto";
+import "activity_result.proto";
+import "timer_result.proto";
+
+/// Request cancellation of an activity from a workflow
+message RequestActivityCancellation {
+    string activity_id = 1;
+    string reason = 2;
+}
+
+// Included in successful [WfActivationCompletion]s, indicates what the workflow wishes to do next
+message WorkflowCommand {
+    oneof variant {
+        temporal.api.query.v1.WorkflowQueryResult respond_to_query = 1;
+        RequestActivityCancellation request_activity_cancellation = 2;
+        // TODO: add more, port API commands
+    }
+}