diff --git a/crates/aether-cli/src/headless/run.rs b/crates/aether-cli/src/headless/run.rs
index 5b55b4103..c2ac71922 100644
--- a/crates/aether-cli/src/headless/run.rs
+++ b/crates/aether-cli/src/headless/run.rs
@@ -254,31 +254,34 @@ fn setup_tracing(verbose: bool) {
 
 #[cfg(test)]
 mod tests {
-    use aether_core::events::ContextUsage;
+    use aether_core::events::{ContextUsage, StreamState};
 
     use super::*;
 
     #[test]
     fn format_text_formats_complete_text() {
-        assert_eq!(format_text(&AgentEvent::text("id", "hello world", true)), Some("hello world".to_string()));
+        assert_eq!(
+            format_text(&AgentEvent::text("id", "hello world", StreamState::Complete)),
+            Some("hello world".to_string())
+        );
     }
 
     #[test]
     fn format_text_skips_incomplete_text() {
-        assert_eq!(format_text(&AgentEvent::text("id", "partial", false)), None);
+        assert_eq!(format_text(&AgentEvent::text("id", "partial", StreamState::Partial)), None);
     }
 
     #[test]
     fn format_text_formats_complete_thought() {
         assert_eq!(
-            format_text(&AgentEvent::thought("id", "reasoning here", true)),
+            format_text(&AgentEvent::thought("id", "reasoning here", StreamState::Complete)),
             Some("Thought: reasoning here".to_string())
         );
     }
 
     #[test]
     fn format_text_skips_incomplete_thought() {
-        assert_eq!(format_text(&AgentEvent::thought("id", "partial", false)), None);
+        assert_eq!(format_text(&AgentEvent::thought("id", "partial", StreamState::Partial)), None);
     }
 
     #[test]
@@ -419,8 +422,8 @@ mod tests {
 
     #[test]
     fn event_kind_none_for_non_output_fragments() {
-        assert_eq!(event_kind(&AgentEvent::text("id", "x", false)), None);
-        assert_eq!(event_kind(&AgentEvent::thought("id", "x", false)), None);
+        assert_eq!(event_kind(&AgentEvent::text("id", "x", StreamState::Partial)), None);
+        assert_eq!(event_kind(&AgentEvent::thought("id", "x", StreamState::Partial)), None);
         assert_eq!(
             event_kind(&AgentEvent::Tool(ToolEvent::CallUpdate {
                 tool_call_id: "tc1".to_string(),
@@ -443,7 +446,7 @@ mod tests {
             &[]
         ));
         assert!(should_emit(&AgentEvent::turn_ended(TurnOutcome::Completed), &[]));
-        assert!(!should_emit(&AgentEvent::text("id", "x", false), &[]));
+        assert!(!should_emit(&AgentEvent::text("id", "x", StreamState::Partial), &[]));
         assert!(!should_emit(
             &AgentEvent::Tool(ToolEvent::CallUpdate { tool_call_id: "tc1".to_string(), chunk: "x".to_string() }),
             &[],
@@ -478,8 +481,8 @@ mod tests {
         use clap::ValueEnum;
 
         let samples = vec![
-            (AgentEvent::text("id", "x", true), CliEventKind::Text),
-            (AgentEvent::thought("id", "x", true), CliEventKind::Thought),
+            (AgentEvent::text("id", "x", StreamState::Complete), CliEventKind::Text),
+            (AgentEvent::thought("id", "x", StreamState::Complete), CliEventKind::Thought),
             (tool_call_msg(), CliEventKind::ToolCall),
             (tool_result_msg(), CliEventKind::ToolResult),
             (
diff --git a/crates/aether-core/src/context/ext.rs b/crates/aether-core/src/context/ext.rs
index 8dd7ebf3c..2d9600d4f 100644
--- a/crates/aether-core/src/context/ext.rs
+++ b/crates/aether-core/src/context/ext.rs
@@ -84,7 +84,7 @@ fn apply_agent_event(ctx: &mut Context, event: &AgentEvent, acc: &mut TurnAccumu
 
 #[cfg(test)]
 mod tests {
-    use crate::events::TurnOutcome;
+    use crate::events::{StreamState, TurnOutcome};
     use crate::session::{SessionControlEvent, last_agent_from_events};
 
     use super::*;
@@ -103,7 +103,7 @@ mod tests {
     }
 
     fn text_complete(chunk: &str) -> AgentEvent {
-        AgentEvent::text("msg_1", chunk, true)
+        AgentEvent::text("msg_1", chunk, StreamState::Complete)
     }
 
     fn tool_result(id: &str, name: &str, result: &str) -> AgentEvent {
@@ -253,7 +253,7 @@ mod tests {
 
     #[test]
     fn streaming_chunks_are_ignored() {
-        let ctx = run_agent_events(&[AgentEvent::text("msg_1", "partial", false)]);
+        let ctx = run_agent_events(&[AgentEvent::text("msg_1", "partial", StreamState::Partial)]);
         assert_eq!(ctx.message_count(), 1);
     }
 
@@ -262,7 +262,7 @@ mod tests {
         let ctx = run_agent_events(&[
             text_complete("Turn 1"),
             AgentEvent::turn_ended(TurnOutcome::Completed),
-            AgentEvent::text("msg_2", "Turn 2", true),
+            AgentEvent::text("msg_2", "Turn 2", StreamState::Complete),
             AgentEvent::turn_ended(TurnOutcome::Completed),
         ]);
         assert_eq!(ctx.message_count(), 3);
diff --git a/crates/aether-core/src/core/agent.rs b/crates/aether-core/src/core/agent.rs
index 646f5f823..7f57169e5 100644
--- a/crates/aether-core/src/core/agent.rs
+++ b/crates/aether-core/src/core/agent.rs
@@ -4,7 +4,7 @@ use crate::core::prompt_cache_key::derive_prompt_cache_key;
 pub use crate::core::retry_config::RetryConfig;
 use crate::events::{
     AgentCommand, AgentEvent, AgentObserver, Command, CompactionOutcome, ContextEvent, ContextUsage, LlmCallOutcome,
-    LlmCallPurpose, ModelEvent, ToolEvent, TurnEvent, TurnOutcome, UserCommand,
+    LlmCallPurpose, ModelEvent, StreamState, ToolEvent, TurnEvent, TurnOutcome, UserCommand,
 };
 use crate::mcp::run_mcp_task::{McpCommand, ToolExecutionEvent};
 use futures::Stream;
@@ -211,10 +211,10 @@ impl Agent {
             let reasoning = AssistantReasoning::from_parts(reasoning_summary_text.clone(), encrypted_reasoning);
             self.update_context(&message_content, reasoning, completed_tool_calls);
 
-            self.emit(AgentEvent::text(&id, &message_content, true)).await;
+            self.emit(AgentEvent::text(&id, &message_content, StreamState::Complete)).await;
 
             if !reasoning_summary_text.is_empty() {
-                self.emit(AgentEvent::thought(&id, &reasoning_summary_text, true)).await;
+                self.emit(AgentEvent::thought(&id, &reasoning_summary_text, StreamState::Complete)).await;
             }
         }
 
@@ -451,7 +451,7 @@ impl Agent {
             Reasoning { chunk } => {
                 state.reasoning_summary_text.push_str(&chunk);
                 if let Some(id) = state.current_message_id.clone() {
-                    self.emit(AgentEvent::thought(&id, &chunk, false)).await;
+                    self.emit(AgentEvent::thought(&id, &chunk, StreamState::Partial)).await;
                 }
             }
 
@@ -498,7 +498,7 @@ impl Agent {
         state.message_content.push_str(&chunk);
 
         if let Some(id) = state.current_message_id.clone() {
-            self.emit(AgentEvent::text(&id, &chunk, false)).await;
+            self.emit(AgentEvent::text(&id, &chunk, StreamState::Partial)).await;
         }
     }
 
diff --git a/crates/aether-core/src/events/agent_event.rs b/crates/aether-core/src/events/agent_event.rs
index 3dcca614e..5dcf52787 100644
--- a/crates/aether-core/src/events/agent_event.rs
+++ b/crates/aether-core/src/events/agent_event.rs
@@ -1,7 +1,7 @@
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 
-use super::{ContextEvent, MessageEvent, ModelEvent, ToolEvent, TurnEvent, TurnOutcome};
+use super::{ContextEvent, MessageEvent, ModelEvent, StreamState, ToolEvent, TurnEvent, TurnOutcome};
 
 /// A canonical event on the agent's output stream.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
@@ -15,12 +15,20 @@ pub enum AgentEvent {
 }
 
 impl AgentEvent {
-    pub fn text(message_id: &str, chunk: &str, is_complete: bool) -> Self {
-        Self::Message(MessageEvent::Text { message_id: message_id.into(), chunk: chunk.into(), is_complete })
+    pub fn text(message_id: &str, chunk: &str, state: StreamState) -> Self {
+        Self::Message(MessageEvent::Text {
+            message_id: message_id.into(),
+            chunk: chunk.into(),
+            is_complete: state.is_complete(),
+        })
     }
 
-    pub fn thought(message_id: &str, chunk: &str, is_complete: bool) -> Self {
-        Self::Message(MessageEvent::Thought { message_id: message_id.into(), chunk: chunk.into(), is_complete })
+    pub fn thought(message_id: &str, chunk: &str, state: StreamState) -> Self {
+        Self::Message(MessageEvent::Thought {
+            message_id: message_id.into(),
+            chunk: chunk.into(),
+            is_complete: state.is_complete(),
+        })
     }
 
     pub fn turn_ended(outcome: TurnOutcome) -> Self {
@@ -56,7 +64,7 @@ mod tests {
 
     #[test]
     fn serializes_nested_event_contract() {
-        let event = AgentEvent::text("m1", "hello", true);
+        let event = AgentEvent::text("m1", "hello", StreamState::Complete);
         assert_eq!(
             serde_json::to_value(event).unwrap(),
             serde_json::json!({"category":"message","event":{"type":"text","message_id":"m1","chunk":"hello","is_complete":true}})
@@ -66,7 +74,7 @@ mod tests {
     #[test]
     fn nested_events_roundtrip() {
         let events = [
-            AgentEvent::text("m", "text", true),
+            AgentEvent::text("m", "text", StreamState::Complete),
             AgentEvent::Tool(ToolEvent::DefinitionsUpdated { tools: vec![] }),
             AgentEvent::Turn(TurnEvent::LlmCallEnded {
                 purpose: LlmCallPurpose::Chat,
@@ -85,7 +93,7 @@ mod tests {
     #[test]
     fn turn_outcome_returns_outcome_only_for_turn_end() {
         assert_eq!(AgentEvent::turn_ended(TurnOutcome::Completed).turn_outcome(), Some(&TurnOutcome::Completed));
-        assert_eq!(AgentEvent::text("m", "text", true).turn_outcome(), None);
+        assert_eq!(AgentEvent::text("m", "text", StreamState::Complete).turn_outcome(), None);
         assert_eq!(AgentEvent::Turn(TurnEvent::Started { content: vec![] }).turn_outcome(), None);
     }
 }
diff --git a/crates/aether-core/src/events/message_event.rs b/crates/aether-core/src/events/message_event.rs
index ad4d44ba2..455fff721 100644
--- a/crates/aether-core/src/events/message_event.rs
+++ b/crates/aether-core/src/events/message_event.rs
@@ -1,6 +1,29 @@
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 
+/// Whether a streamed text or thought chunk is the final one for its message.
+///
+/// `Partial` chunks stream as they arrive; a single `Complete` event carries the
+/// full accumulated text and is emitted when the turn wraps up (which may be after
+/// the originating LLM call's
+/// [`TurnEvent::LlmCallEnded`](crate::events::TurnEvent::LlmCallEnded)).
+///
+/// This stands in for the raw `is_complete: bool` on event constructors so that
+/// call sites read as `StreamState::Complete` instead of an opaque `true` literal.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum StreamState {
+    /// More chunks may follow for this message.
+    Partial,
+    /// This is the final chunk for the message.
+    Complete,
+}
+
+impl StreamState {
+    pub fn is_complete(self) -> bool {
+        matches!(self, Self::Complete)
+    }
+}
+
 /// Streaming message content from the agent.
 ///
 /// Chunks stream with `is_complete: false`; a final event with `is_complete: true`
diff --git a/crates/aether-core/src/events/mod.rs b/crates/aether-core/src/events/mod.rs
index 4bf5321f7..6c96399a9 100644
--- a/crates/aether-core/src/events/mod.rs
+++ b/crates/aether-core/src/events/mod.rs
@@ -19,7 +19,7 @@ mod user_message;
 pub use acp::{aether_tool_name_meta, humanize_tool_name, mcp_tool_name, parse_tool_call_chunk};
 pub use agent_event::AgentEvent;
 pub use context_event::{CompactionOutcome, ContextEvent, ContextUsage};
-pub use message_event::MessageEvent;
+pub use message_event::{MessageEvent, StreamState};
 pub use model_event::ModelEvent;
 pub use observer::{AgentObserver, DynObserverFactory, McpRequestInstrumentation, ObserverFactory};
 pub use sub_agent_progress::SubAgentProgressPayload;
diff --git a/crates/aether-core/src/session.rs b/crates/aether-core/src/session.rs
index 4becca35d..afedb2743 100644
--- a/crates/aether-core/src/session.rs
+++ b/crates/aether-core/src/session.rs
@@ -202,7 +202,7 @@ pub fn last_agent_from_events(initial: Option<String>, events: &[SessionEvent])
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::events::{LlmCallPurpose, TurnOutcome};
+    use crate::events::{LlmCallPurpose, StreamState, TurnOutcome};
 
     fn agent(event: AgentEvent) -> SessionEvent {
         SessionEvent::Agent(event)
@@ -217,7 +217,7 @@ mod tests {
             delay_ms: 10,
         }));
         let cancelled = agent(AgentEvent::Turn(TurnEvent::Ended { outcome: TurnOutcome::Cancelled }));
-        let partial = agent(AgentEvent::text("m", "partial", false));
+        let partial = agent(AgentEvent::text("m", "partial", StreamState::Partial));
         let compaction_ended = agent(AgentEvent::Context(ContextEvent::CompactionEnded {
             outcome: crate::events::CompactionOutcome::Completed,
         }));
diff --git a/crates/aether-core/src/testing/agent_event_builder.rs b/crates/aether-core/src/testing/agent_event_builder.rs
index d64f97f8b..06b2437dc 100644
--- a/crates/aether-core/src/testing/agent_event_builder.rs
+++ b/crates/aether-core/src/testing/agent_event_builder.rs
@@ -1,5 +1,4 @@
-use crate::events::AgentEvent;
-use crate::events::ToolEvent;
+use crate::events::{AgentEvent, StreamState, ToolEvent};
 use llm::{ToolCallError, ToolCallRequest, ToolCallResult};
 use serde::Serialize;
 
@@ -20,7 +19,7 @@ impl AgentEventBuilder {
 
     pub fn text(mut self, chunks: &[&str]) -> Self {
         for chunk in chunks {
-            self.chunks.push(AgentEvent::text(&self.message_id, chunk, false));
+            self.chunks.push(AgentEvent::text(&self.message_id, chunk, StreamState::Partial));
             self.full_text.push_str(chunk);
         }
         self
@@ -80,7 +79,7 @@ impl AgentEventBuilder {
     }
 
     pub fn build(mut self) -> Vec<AgentEvent> {
-        self.chunks.push(AgentEvent::text(&self.message_id, &self.full_text, true));
+        self.chunks.push(AgentEvent::text(&self.message_id, &self.full_text, StreamState::Complete));
 
         self.chunks
     }
diff --git a/crates/aether-evals/src/agents/fake_agent.rs b/crates/aether-evals/src/agents/fake_agent.rs
index d4981c206..bbe361ca6 100644
--- a/crates/aether-evals/src/agents/fake_agent.rs
+++ b/crates/aether-evals/src/agents/fake_agent.rs
@@ -1,6 +1,6 @@
 use super::agent::{Agent, AgentRunResult, RunError};
 use crate::Task;
-use aether_core::events::{AgentEvent, ToolEvent, TurnOutcome};
+use aether_core::events::{AgentEvent, StreamState, ToolEvent, TurnOutcome};
 use async_stream::try_stream;
 use futures::Stream;
 use llm::ToolCallResult;
@@ -21,7 +21,7 @@ impl FakeAgent {
 
     pub fn success() -> Self {
         Self::new(vec![
-            AgentEvent::text("fake_1", "Task completed successfully", true),
+            AgentEvent::text("fake_1", "Task completed successfully", StreamState::Complete),
             AgentEvent::turn_ended(TurnOutcome::Completed),
         ])
     }
@@ -38,7 +38,7 @@ impl FakeAgent {
                 },
                 result_meta: None,
             }),
-            AgentEvent::text("fake_2", "Task completed using tools", true),
+            AgentEvent::text("fake_2", "Task completed using tools", StreamState::Complete),
             AgentEvent::turn_ended(TurnOutcome::Completed),
         ])
     }
diff --git a/crates/aether-evals/src/judge.rs b/crates/aether-evals/src/judge.rs
index 8a540449a..5a3489a6d 100644
--- a/crates/aether-evals/src/judge.rs
+++ b/crates/aether-evals/src/judge.rs
@@ -465,7 +465,7 @@ fn truncate_chars(value: &str, max_chars: usize) -> String {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use aether_core::events::{AgentEvent, TurnOutcome};
+    use aether_core::events::{AgentEvent, StreamState, TurnOutcome};
     use llm::testing::FakeLlmProvider;
     use llm::{LlmError, ToolCallRequest, ToolCallResult};
 
@@ -481,14 +481,17 @@ mod tests {
             },
         });
 
-        assert_eq!(get_transcript_line(&AgentEvent::text("msg_1", "hi", true), 100).unwrap(), "[agent] hi");
+        assert_eq!(
+            get_transcript_line(&AgentEvent::text("msg_1", "hi", StreamState::Complete), 100).unwrap(),
+            "[agent] hi"
+        );
         assert_eq!(get_transcript_line(&call, 100).unwrap(), "[tool-call] bash arguments={}");
         assert_eq!(get_transcript_line(&AgentEvent::turn_ended(TurnOutcome::Completed), 100).unwrap(), "[done]");
     }
 
     #[test]
     fn transcript_lines_truncate_long_payloads() {
-        let line = get_transcript_line(&AgentEvent::text("msg_1", &"a".repeat(50), true), 10).unwrap();
+        let line = get_transcript_line(&AgentEvent::text("msg_1", &"a".repeat(50), StreamState::Complete), 10).unwrap();
 
         assert_eq!(line, format!("[agent] {}... [truncated]", "a".repeat(10)));
     }
@@ -545,7 +548,7 @@ mod tests {
                     arguments: "{}".to_string(),
                 },
             }),
-            AgentEvent::text("msg_1", "all done", true),
+            AgentEvent::text("msg_1", "all done", StreamState::Complete),
         ];
 
         let judge = judge()
diff --git a/crates/aether-telemetry/tests/observer_tests.rs b/crates/aether-telemetry/tests/observer_tests.rs
index 1cb5236f6..77998e724 100644
--- a/crates/aether-telemetry/tests/observer_tests.rs
+++ b/crates/aether-telemetry/tests/observer_tests.rs
@@ -4,7 +4,7 @@ use std::time::Duration;
 
 use aether_core::core::RetryConfig;
 use aether_core::events::{
-    AgentEvent, AgentObserver, LlmCallOutcome, LlmCallPurpose, ToolEvent, TurnEvent, TurnOutcome,
+    AgentEvent, AgentObserver, LlmCallOutcome, LlmCallPurpose, StreamState, ToolEvent, TurnEvent, TurnOutcome,
 };
 use aether_core::testing::{AddNumbersRequest, AgentTrace, DivideNumbersRequest, TestScenario, test_agent};
 use aether_telemetry::{
@@ -285,7 +285,7 @@ async fn completed_llm_calls_capture_token_usage_breakdown() -> Result<(), Box<d
 async fn completed_message_sets_turn_output_without_streamed_chunks() -> Result<(), Box<dyn Error>> {
     let trace = AgentTrace::from_events(vec![
         AgentEvent::Turn(TurnEvent::Started { content: vec![] }),
-        AgentEvent::text("m1", "complete response", true),
+        AgentEvent::text("m1", "complete response", StreamState::Complete),
         AgentEvent::turn_ended(TurnOutcome::Completed),
     ]);