diff --git a/crates/arc-cli/tests/cli.rs b/crates/arc-cli/tests/cli.rs
index 2ed055546..43dc1f477 100644
--- a/crates/arc-cli/tests/cli.rs
+++ b/crates/arc-cli/tests/cli.rs
@@ -543,7 +543,7 @@ fn dry_run_writes_jsonl_and_live_json() {
     // Events should contain WorkflowRunStarted (may not be first due to exec env events)
     let has_run_started = lines.iter().any(|line| {
         let parsed: serde_json::Value = serde_json::from_str(line).unwrap();
-        parsed["event"].get("WorkflowRunStarted").is_some()
+        parsed["event"].as_str() == Some("WorkflowRunStarted")
     });
     assert!(
         has_run_started,
diff --git a/crates/arc-workflows/src/cli/run.rs b/crates/arc-workflows/src/cli/run.rs
index 0731e2875..65a889bb3 100644
--- a/crates/arc-workflows/src/cli/run.rs
+++ b/crates/arc-workflows/src/cli/run.rs
@@ -205,11 +205,25 @@ pub async fn run_command(args: RunArgs, styles: &'static Styles) -> anyhow::Resu
             if let crate::event::WorkflowRunEvent::WorkflowRunStarted { run_id, .. } = event {
                 *run_id_clone.lock().unwrap() = run_id.clone();
             }
-            let envelope = serde_json::json!({
-                "timestamp": Utc::now().to_rfc3339_opts(chrono::SecondsFormat::Millis, true),
-                "run_id": *run_id_clone.lock().unwrap(),
-                "event": event,
-            });
+            let (event_name, event_fields) = crate::event::flatten_event(event);
+            let mut envelope = serde_json::Map::new();
+            envelope.insert(
+                "timestamp".to_string(),
+                serde_json::Value::String(
+                    Utc::now()
+                        .to_rfc3339_opts(chrono::SecondsFormat::Millis, true),
+                ),
+            );
+            envelope.insert(
+                "run_id".to_string(),
+                serde_json::Value::String(run_id_clone.lock().unwrap().clone()),
+            );
+            envelope.insert(
+                "event".to_string(),
+                serde_json::Value::String(event_name),
+            );
+            envelope.extend(event_fields);
+            let envelope = serde_json::Value::Object(envelope);
             // Append to progress.jsonl
             if let Ok(line) = serde_json::to_string(&envelope) {
                 let line = arc_util::redact::redact_jsonl_line(&line);
diff --git a/crates/arc-workflows/src/event.rs b/crates/arc-workflows/src/event.rs
index 65db0a9a1..968b20942 100644
--- a/crates/arc-workflows/src/event.rs
+++ b/crates/arc-workflows/src/event.rs
@@ -438,6 +438,161 @@ impl WorkflowRunEvent {
     }
 }
 
+/// Flatten a `WorkflowRunEvent` into its event name and a map of top-level fields.
+///
+/// Simple variants like `StageStarted` return `("StageStarted", {fields})`.
+/// Wrapper variants use dot notation:
+/// - `Agent { stage, event: ToolCallStarted { .. } }` → `"Agent.ToolCallStarted"`
+/// - `Sandbox { event: Initializing { .. } }` → `"Sandbox.Initializing"`
+/// - `Agent { stage, event: SubAgentEvent { event: inner, .. } }` → `"Agent.SubAgentEvent.{Inner}"`
+///   with one level of flattening; deeper nesting stays as JSON.
+pub fn flatten_event(
+    event: &WorkflowRunEvent,
+) -> (String, serde_json::Map<String, serde_json::Value>) {
+    let value = serde_json::to_value(event).expect("WorkflowRunEvent must serialize");
+    match value {
+        serde_json::Value::Object(map) => {
+            // Externally-tagged enum: { "VariantName": { fields } }
+            let (variant_name, inner) = map.into_iter().next().expect("enum must have one key");
+            match variant_name.as_str() {
+                "Agent" => flatten_agent(inner),
+                "Sandbox" => flatten_sandbox(inner),
+                _ => {
+                    let fields = match inner {
+                        serde_json::Value::Object(m) => m,
+                        _ => serde_json::Map::new(),
+                    };
+                    (variant_name, fields)
+                }
+            }
+        }
+        // Unit variants serialize as strings
+        serde_json::Value::String(name) => (name, serde_json::Map::new()),
+        _ => ("Unknown".to_string(), serde_json::Map::new()),
+    }
+}
+
+fn flatten_agent(inner: serde_json::Value) -> (String, serde_json::Map<String, serde_json::Value>) {
+    let serde_json::Value::Object(mut agent_fields) = inner else {
+        return ("Agent".to_string(), serde_json::Map::new());
+    };
+    let stage = agent_fields.remove("stage");
+    let agent_event = agent_fields.remove("event").unwrap_or(serde_json::Value::Null);
+
+    match agent_event {
+        serde_json::Value::Object(event_map) => {
+            let (inner_name, inner_value) = event_map
+                .into_iter()
+                .next()
+                .expect("agent event must have one key");
+            if inner_name == "SubAgentEvent" {
+                flatten_sub_agent_event(stage, inner_value)
+            } else {
+                let mut fields = match inner_value {
+                    serde_json::Value::Object(m) => m,
+                    _ => serde_json::Map::new(),
+                };
+                if let Some(s) = stage {
+                    fields.insert("stage".to_string(), s);
+                }
+                (format!("Agent.{inner_name}"), fields)
+            }
+        }
+        // Unit variant inside Agent (e.g. SessionStarted)
+        serde_json::Value::String(name) => {
+            let mut fields = serde_json::Map::new();
+            if let Some(s) = stage {
+                fields.insert("stage".to_string(), s);
+            }
+            (format!("Agent.{name}"), fields)
+        }
+        _ => {
+            let mut fields = serde_json::Map::new();
+            if let Some(s) = stage {
+                fields.insert("stage".to_string(), s);
+            }
+            ("Agent".to_string(), fields)
+        }
+    }
+}
+
+fn flatten_sandbox(
+    inner: serde_json::Value,
+) -> (String, serde_json::Map<String, serde_json::Value>) {
+    let serde_json::Value::Object(mut sandbox_fields) = inner else {
+        return ("Sandbox".to_string(), serde_json::Map::new());
+    };
+    let sandbox_event = sandbox_fields
+        .remove("event")
+        .unwrap_or(serde_json::Value::Null);
+
+    match sandbox_event {
+        serde_json::Value::Object(event_map) => {
+            let (inner_name, inner_value) = event_map
+                .into_iter()
+                .next()
+                .expect("sandbox event must have one key");
+            let fields = match inner_value {
+                serde_json::Value::Object(m) => m,
+                _ => serde_json::Map::new(),
+            };
+            (format!("Sandbox.{inner_name}"), fields)
+        }
+        serde_json::Value::String(name) => (format!("Sandbox.{name}"), serde_json::Map::new()),
+        _ => ("Sandbox".to_string(), serde_json::Map::new()),
+    }
+}
+
+fn flatten_sub_agent_event(
+    stage: Option<serde_json::Value>,
+    inner_value: serde_json::Value,
+) -> (String, serde_json::Map<String, serde_json::Value>) {
+    let serde_json::Value::Object(mut sub_fields) = inner_value else {
+        let mut fields = serde_json::Map::new();
+        if let Some(s) = stage {
+            fields.insert("stage".to_string(), s);
+        }
+        return ("Agent.SubAgentEvent".to_string(), fields);
+    };
+    let agent_id = sub_fields.remove("agent_id");
+    let depth = sub_fields.remove("depth");
+    let nested_event = sub_fields.remove("event").unwrap_or(serde_json::Value::Null);
+
+    let (nested_name, nested_fields) = match &nested_event {
+        serde_json::Value::Object(event_map) => {
+            let (name, value) = event_map
+                .iter()
+                .next()
+                .expect("nested event must have one key");
+            let fields = match value {
+                serde_json::Value::Object(m) => m.clone(),
+                _ => serde_json::Map::new(),
+            };
+            (Some(name.clone()), fields)
+        }
+        serde_json::Value::String(name) => (Some(name.clone()), serde_json::Map::new()),
+        _ => (None, serde_json::Map::new()),
+    };
+
+    let event_name = match &nested_name {
+        Some(name) => format!("Agent.SubAgentEvent.{name}"),
+        None => "Agent.SubAgentEvent".to_string(),
+    };
+
+    let mut fields = nested_fields;
+    if let Some(s) = stage {
+        fields.insert("stage".to_string(), s);
+    }
+    if let Some(id) = agent_id {
+        fields.insert("agent_id".to_string(), id);
+    }
+    if let Some(d) = depth {
+        fields.insert("depth".to_string(), d);
+    }
+
+    (event_name, fields)
+}
+
 /// Current time as epoch milliseconds.
 fn epoch_millis() -> i64 {
     std::time::SystemTime::now()
@@ -1032,6 +1187,86 @@ mod tests {
         );
     }
 
+    #[test]
+    fn flatten_event_simple_variant() {
+        let event = WorkflowRunEvent::StageStarted {
+            name: "plan".to_string(),
+            index: 0,
+            handler_type: Some("codergen".to_string()),
+            attempt: 1,
+            max_attempts: 3,
+        };
+        let (name, fields) = flatten_event(&event);
+        assert_eq!(name, "StageStarted");
+        assert_eq!(fields["name"], "plan");
+        assert_eq!(fields["index"], 0);
+        assert_eq!(fields["handler_type"], "codergen");
+        assert_eq!(fields["attempt"], 1);
+        assert_eq!(fields["max_attempts"], 3);
+    }
+
+    #[test]
+    fn flatten_event_agent_tool_call_started() {
+        let event = WorkflowRunEvent::Agent {
+            stage: "code".to_string(),
+            event: AgentEvent::ToolCallStarted {
+                tool_name: "read_file".to_string(),
+                tool_call_id: "call_1".to_string(),
+                arguments: serde_json::json!({"path": "/tmp/test.txt"}),
+            },
+        };
+        let (name, fields) = flatten_event(&event);
+        assert_eq!(name, "Agent.ToolCallStarted");
+        assert_eq!(fields["stage"], "code");
+        assert_eq!(fields["tool_name"], "read_file");
+        assert_eq!(fields["tool_call_id"], "call_1");
+    }
+
+    #[test]
+    fn flatten_event_sandbox_initializing() {
+        let event = WorkflowRunEvent::Sandbox {
+            event: SandboxEvent::Initializing {
+                provider: "docker".into(),
+            },
+        };
+        let (name, fields) = flatten_event(&event);
+        assert_eq!(name, "Sandbox.Initializing");
+        assert_eq!(fields["provider"], "docker");
+    }
+
+    #[test]
+    fn flatten_event_agent_sub_agent_event() {
+        let event = WorkflowRunEvent::Agent {
+            stage: "code".to_string(),
+            event: AgentEvent::SubAgentEvent {
+                agent_id: "sub_1".to_string(),
+                depth: 1,
+                event: Box::new(AgentEvent::ToolCallStarted {
+                    tool_name: "write_file".to_string(),
+                    tool_call_id: "call_2".to_string(),
+                    arguments: serde_json::json!({}),
+                }),
+            },
+        };
+        let (name, fields) = flatten_event(&event);
+        assert_eq!(name, "Agent.SubAgentEvent.ToolCallStarted");
+        assert_eq!(fields["stage"], "code");
+        assert_eq!(fields["agent_id"], "sub_1");
+        assert_eq!(fields["depth"], 1);
+        assert_eq!(fields["tool_name"], "write_file");
+    }
+
+    #[test]
+    fn flatten_event_agent_session_started() {
+        let event = WorkflowRunEvent::Agent {
+            stage: "plan".to_string(),
+            event: AgentEvent::SessionStarted,
+        };
+        let (name, fields) = flatten_event(&event);
+        assert_eq!(name, "Agent.SessionStarted");
+        assert_eq!(fields["stage"], "plan");
+    }
+
     #[test]
     fn setup_events_serialization() {
         let events = vec![
diff --git a/crates/arc-workflows/src/retro.rs b/crates/arc-workflows/src/retro.rs
index 458b786d9..bde8a50dd 100644
--- a/crates/arc-workflows/src/retro.rs
+++ b/crates/arc-workflows/src/retro.rs
@@ -6,7 +6,6 @@ use serde::{Deserialize, Serialize};
 
 use crate::checkpoint::Checkpoint;
 use crate::error::{ArcError, Result};
-use crate::event::WorkflowRunEvent;
 use crate::outcome::StageStatus;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
@@ -183,18 +182,16 @@ pub fn extract_stage_durations(logs_root: &Path) -> HashMap<String, u64> {
         let Ok(envelope) = serde_json::from_str::<serde_json::Value>(line) else {
             continue;
         };
-        let Some(event_value) = envelope.get("event") else {
+        if envelope.get("event").and_then(|v| v.as_str()) != Some("StageCompleted") {
             continue;
-        };
-        let Ok(event) = serde_json::from_value::<WorkflowRunEvent>(event_value.clone()) else {
-            continue;
-        };
-        if let WorkflowRunEvent::StageCompleted {
-            name, duration_ms, ..
-        } = event
-        {
-            durations.insert(name, duration_ms);
         }
+        let Some(name) = envelope.get("name").and_then(|v| v.as_str()) else {
+            continue;
+        };
+        let Some(duration_ms) = envelope.get("duration_ms").and_then(|v| v.as_u64()) else {
+            continue;
+        };
+        durations.insert(name.to_string(), duration_ms);
     }
     durations
 }
@@ -524,44 +521,38 @@ mod tests {
         let event1 = serde_json::json!({
             "timestamp": "2025-01-01T00:00:00.000Z",
             "run_id": "r1",
-            "event": {
-                "StageCompleted": {
-                    "name": "plan",
-                    "index": 0,
-                    "duration_ms": 5000,
-                    "status": "success",
-                    "preferred_label": null,
-                    "suggested_next_ids": [],
-                    "usage": null,
-                    "failure_reason": null,
-                    "notes": null,
-                    "files_touched": [],
-                    "attempt": 1,
-                    "max_attempts": 1,
-                    "failure_class": null
-                }
-            }
+            "event": "StageCompleted",
+            "name": "plan",
+            "index": 0,
+            "duration_ms": 5000,
+            "status": "success",
+            "preferred_label": null,
+            "suggested_next_ids": [],
+            "usage": null,
+            "failure_reason": null,
+            "notes": null,
+            "files_touched": [],
+            "attempt": 1,
+            "max_attempts": 1,
+            "failure_class": null
         });
         let event2 = serde_json::json!({
             "timestamp": "2025-01-01T00:00:05.000Z",
             "run_id": "r1",
-            "event": {
-                "StageCompleted": {
-                    "name": "code",
-                    "index": 1,
-                    "duration_ms": 15000,
-                    "status": "success",
-                    "preferred_label": null,
-                    "suggested_next_ids": [],
-                    "usage": null,
-                    "failure_reason": null,
-                    "notes": null,
-                    "files_touched": [],
-                    "attempt": 1,
-                    "max_attempts": 1,
-                    "failure_class": null
-                }
-            }
+            "event": "StageCompleted",
+            "name": "code",
+            "index": 1,
+            "duration_ms": 15000,
+            "status": "success",
+            "preferred_label": null,
+            "suggested_next_ids": [],
+            "usage": null,
+            "failure_reason": null,
+            "notes": null,
+            "files_touched": [],
+            "attempt": 1,
+            "max_attempts": 1,
+            "failure_class": null
         });
         let content = format!(
             "{}\n{}\n",