diff --git a/src/harness/agent_loop/test.rs b/src/harness/agent_loop/test.rs
index 1177491..a179486 100644
--- a/src/harness/agent_loop/test.rs
+++ b/src/harness/agent_loop/test.rs
@@ -2165,3 +2165,76 @@ async fn mock_streaming_script_invoke_folds_items_to_response() {
         .expect("fold succeeds");
     assert_eq!(response.text(), "abcd");
 }
+
+#[tokio::test]
+async fn tool_completed_event_carries_outcome() {
+    use crate::harness::events::RecordingListener;
+
+    // A tool that fails: its ToolCompleted event must carry the failure message,
+    // a real duration, and the output size — from the event itself, not a
+    // side-channel — so journal-backed exporters can render the outcome.
+    struct FailTool;
+    #[async_trait]
+    impl Tool<()> for FailTool {
+        fn name(&self) -> &str {
+            "boom"
+        }
+        fn description(&self) -> &str {
+            "always fails"
+        }
+        fn schema(&self) -> ToolSchema {
+            ToolSchema::new("boom", "always fails", json!({ "type": "object" }))
+        }
+        async fn call(&self, _state: &(), call: ToolCall) -> Result<ToolResult> {
+            Ok(ToolResult {
+                call_id: call.id,
+                name: "boom".to_string(),
+                content: "nope".to_string(),
+                raw: None,
+                error: Some("kaboom".to_string()),
+                elapsed_ms: 0,
+            })
+        }
+    }
+
+    let mut harness: AgentHarness<()> = AgentHarness::new();
+    harness.register_model(
+        "mock",
+        Arc::new(MockModel::with_responses(vec![
+            tool_call_response("c1", "boom", json!({})),
+            text_response("done", 1, 1),
+        ])),
+    );
+    harness.register_tool(Arc::new(FailTool));
+
+    let recorder = Arc::new(RecordingListener::new());
+    let ctx: RunContext<()> = RunContext::new(RunConfig::new("run-tc"), ());
+    ctx.events.subscribe(recorder.clone());
+    harness
+        .invoke_in_context(&(), ctx, vec![Message::user("go")])
+        .await
+        .expect("run succeeds");
+
+    let (error, duration_ms, output_bytes) = recorder
+        .events()
+        .into_iter()
+        .find_map(|record| match record.event {
+            AgentEvent::ToolCompleted {
+                tool_name,
+                error,
+                duration_ms,
+                output_bytes,
+                ..
+            } if tool_name == "boom" => Some((error, duration_ms, output_bytes)),
+            _ => None,
+        })
+        .expect("a ToolCompleted event for `boom`");
+
+    assert_eq!(
+        error.as_deref(),
+        Some("kaboom"),
+        "failure message on the event"
+    );
+    assert!(duration_ms.is_some(), "wall-clock duration present");
+    assert_eq!(output_bytes, Some(4), "\"nope\".len() == 4");
+}
diff --git a/src/harness/agent_loop/tools.rs b/src/harness/agent_loop/tools.rs
index b35c9b5..cf7e563 100644
--- a/src/harness/agent_loop/tools.rs
+++ b/src/harness/agent_loop/tools.rs
@@ -260,12 +260,22 @@ impl<State: Send + Sync, Ctx: Send + Sync> AgentHarness<State, Ctx> {
             .capture
             .tool_io
             .then(|| Value::String(result.content.clone()));
+        // Outcome fields carried on the event itself (not a side-channel) so
+        // journal-backed exporters render duration/size/success without the
+        // live run's state. Duration is wall-clock (completion minus start);
+        // `error` mirrors `ToolResult::error` (`None` == success).
+        let duration_ms = crate::harness::ids::now_ms().saturating_sub(prepared.started_at_ms);
+        let output_bytes = result.content.len() as u64;
+        let error = result.error.clone();
         let record = ctx.emit(AgentEvent::ToolCompleted {
             call_id: prepared.call_id,
             tool_name: prepared.tool_name,
             started_at_ms: Some(prepared.started_at_ms),
             input: prepared.captured_input,
             output: captured_output,
+            duration_ms: Some(duration_ms),
+            output_bytes: Some(output_bytes),
+            error,
         });
         status.set_last_event(record.id);
 
diff --git a/src/harness/events/test.rs b/src/harness/events/test.rs
index 0ad048f..ee69e17 100644
--- a/src/harness/events/test.rs
+++ b/src/harness/events/test.rs
@@ -147,6 +147,9 @@ fn completed_events_deserialize_without_started_at_ms() {
         started_at_ms: Some(1_704_067_199_000),
         input: None,
         output: None,
+        duration_ms: Some(12),
+        output_bytes: Some(5),
+        error: None,
     };
     let json = serde_json::to_value(&event).unwrap();
     assert_eq!(json["started_at_ms"], 1_704_067_199_000u64);
diff --git a/src/harness/events/types.rs b/src/harness/events/types.rs
index 030e0b9..27d9c47 100644
--- a/src/harness/events/types.rs
+++ b/src/harness/events/types.rs
@@ -143,6 +143,22 @@ pub enum AgentEvent {
         /// exporter can render the result in a tool observation's Output panel.
         #[serde(default, skip_serializing_if = "Option::is_none")]
         output: Option<serde_json::Value>,
+        /// Wall-clock duration of the call in milliseconds (completion minus
+        /// [`started_at_ms`]). Present regardless of payload capture, so an
+        /// exporter renders a real duration without a side-channel. `None` for
+        /// events serialized before this field existed.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        duration_ms: Option<u64>,
+        /// Size, in bytes, of the tool's textual result content. Present even in
+        /// payload-free mode (unlike [`output`]), so an exporter can show result
+        /// size without capturing the body. `None` for older events.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        output_bytes: Option<u64>,
+        /// Failure message when the tool call failed; `None` on success. Lets an
+        /// exporter render success/failure and a reason from the journalled
+        /// event itself rather than a live outcome side-channel.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        error: Option<String>,
     },
 
     /// The model called a tool that is not registered, and the run's
diff --git a/src/harness/observability/langfuse/mod.rs b/src/harness/observability/langfuse/mod.rs
index 4453ca8..527bcc0 100644
--- a/src/harness/observability/langfuse/mod.rs
+++ b/src/harness/observability/langfuse/mod.rs
@@ -316,26 +316,43 @@ fn observation_event(trace_id: &str, obs: &AgentObservation) -> Value {
             started_at_ms,
             input,
             output,
-        } => json!({
-            "id": obs.event_id.as_str(),
-            "timestamp": timestamp,
-            // A tool call is modelled as a span. `tool-create` is not a valid
-            // Langfuse ingestion observation type — older/self-hosted Langfuse
-            // rejects it, silently dropping every tool observation.
-            "type": "span-create",
-            "body": clean_nulls(json!({
-                "id": call_id.as_str(),
-                "traceId": trace_id,
-                "name": tool_name,
-                // Loop-captured start time when available (see the
-                // generation branch above).
-                "startTime": started_at_ms.map(iso_ms).unwrap_or_else(|| timestamp.clone()),
-                "endTime": timestamp,
-                "input": input,
-                "output": output,
-                "metadata": metadata,
-            })),
-        }),
+            duration_ms,
+            output_bytes,
+            error,
+        } => {
+            // Prefer the loop-captured start + real duration for the end time;
+            // fall back to the journal timestamp. A failed call is marked ERROR
+            // with the failure reason, and result size rides metadata so the
+            // span is informative even in payload-free mode.
+            let end_time = match (started_at_ms, duration_ms) {
+                (Some(start), Some(dur)) => iso_ms(start.saturating_add(*dur)),
+                _ => timestamp.clone(),
+            };
+            let mut tool_metadata = metadata.clone();
+            if let (Some(map), Some(bytes)) = (tool_metadata.as_object_mut(), output_bytes) {
+                map.insert("output_bytes".into(), json!(bytes));
+            }
+            json!({
+                "id": obs.event_id.as_str(),
+                "timestamp": timestamp,
+                // A tool call is modelled as a span. `tool-create` is not a
+                // valid Langfuse ingestion observation type — older/self-hosted
+                // Langfuse rejects it, silently dropping every tool observation.
+                "type": "span-create",
+                "body": clean_nulls(json!({
+                    "id": call_id.as_str(),
+                    "traceId": trace_id,
+                    "name": tool_name,
+                    "startTime": started_at_ms.map(iso_ms).unwrap_or_else(|| timestamp.clone()),
+                    "endTime": end_time,
+                    "input": input,
+                    "output": output,
+                    "level": error.as_ref().map(|_| "ERROR"),
+                    "statusMessage": error,
+                    "metadata": tool_metadata,
+                })),
+            })
+        }
         AgentEvent::RunFailed { error, .. } => json!({
             "id": obs.event_id.as_str(),
             "timestamp": timestamp,
diff --git a/src/harness/observability/langfuse/test.rs b/src/harness/observability/langfuse/test.rs
index c3a3de5..c062215 100644
--- a/src/harness/observability/langfuse/test.rs
+++ b/src/harness/observability/langfuse/test.rs
@@ -116,6 +116,9 @@ fn populates_generation_and_tool_io_when_captured() {
                         started_at_ms: Some(1_704_067_199_500),
                         input: Some(json!({ "query": "weather" })),
                         output: Some(json!("sunny")),
+                        duration_ms: Some(250),
+                        output_bytes: Some(5),
+                        error: None,
                     },
                 ),
             ],
@@ -135,7 +138,12 @@ fn populates_generation_and_tool_io_when_captured() {
     assert_eq!(events[1]["body"]["startTime"], iso_ms(1_704_067_199_000));
     assert_eq!(events[1]["body"]["endTime"], iso_ms(1_704_067_200_000));
     assert_eq!(events[2]["body"]["startTime"], iso_ms(1_704_067_199_500));
-    assert_eq!(events[2]["body"]["endTime"], iso_ms(1_704_067_200_001));
+    // The tool span's end time is now start + the event's own duration_ms
+    // (1_704_067_199_500 + 250), a real execution window rather than the
+    // journal-append timestamp.
+    assert_eq!(events[2]["body"]["endTime"], iso_ms(1_704_067_199_750));
+    // Result size rides metadata even though this fixture also captured output.
+    assert_eq!(events[2]["body"]["metadata"]["output_bytes"], 5);
 
     // Observation metadata carries only lineage + event kind, not the whole
     // event payload (which would duplicate input/output already in `body`).
diff --git a/src/harness/observability/test.rs b/src/harness/observability/test.rs
index 40b6129..3d2ed68 100644
--- a/src/harness/observability/test.rs
+++ b/src/harness/observability/test.rs
@@ -140,6 +140,9 @@ fn agent_latency_metrics_include_model_tool_and_run_elapsed() {
                 started_at_ms: None,
                 input: None,
                 output: None,
+                duration_ms: None,
+                output_bytes: None,
+                error: None,
             },
         ),
         obs(
diff --git a/src/harness/testkit/test.rs b/src/harness/testkit/test.rs
index 0ebeaf6..875a58a 100644
--- a/src/harness/testkit/test.rs
+++ b/src/harness/testkit/test.rs
@@ -341,6 +341,9 @@ fn make_trajectory() -> Vec<AgentEvent> {
             started_at_ms: None,
             input: None,
             output: None,
+            duration_ms: None,
+            output_bytes: None,
+            error: None,
         },
         AgentEvent::ModelStarted {
             call_id: CallId::new("c2"),
diff --git a/tests/e2e_orchestrator_subagents.rs b/tests/e2e_orchestrator_subagents.rs
index d29559a..4d13d0b 100644
--- a/tests/e2e_orchestrator_subagents.rs
+++ b/tests/e2e_orchestrator_subagents.rs
@@ -174,6 +174,9 @@ async fn orchestrator_resolves_and_runs_only_the_chosen_subagents() -> Result<()
                 started_at_ms: None,
                 input: None,
                 output: None,
+                duration_ms: None,
+                output_bytes: None,
+                error: None,
             });
             Ok::<(String, String), TinyAgentsError>((name, result.content))
         }
diff --git a/tests/e2e_registry_observability_contracts.rs b/tests/e2e_registry_observability_contracts.rs
index fde5b14..97d403f 100644
--- a/tests/e2e_registry_observability_contracts.rs
+++ b/tests/e2e_registry_observability_contracts.rs
@@ -186,6 +186,9 @@ fn component_metadata_and_event_kinds_are_stable_serializable_contracts() {
             started_at_ms: None,
             input: None,
             output: None,
+            duration_ms: None,
+            output_bytes: None,
+            error: None,
         },
         AgentEvent::StateUpdate,
         AgentEvent::MiddlewareStarted { name: "mw".into() },
@@ -310,6 +313,9 @@ async fn event_sinks_journals_and_status_stores_preserve_run_lineage() {
         started_at_ms: None,
         input: None,
         output: None,
+        duration_ms: None,
+        output_bytes: None,
+        error: None,
     });
     assert_eq!(journal.len(), 2);
     assert_eq!(journal.replay_from(1)[0].event.kind(), "tool.completed");