diff --git a/src/libraries/rust/stargate/crates/pylon-lib/src/bringup.rs b/src/libraries/rust/stargate/crates/pylon-lib/src/bringup.rs
index 03e0634d30..be8330e459 100644
--- a/src/libraries/rust/stargate/crates/pylon-lib/src/bringup.rs
+++ b/src/libraries/rust/stargate/crates/pylon-lib/src/bringup.rs
@@ -1245,6 +1245,273 @@ mod tests {
         progress.await.unwrap();
     }
 
+    /// Character text whose estimate is one token per character.
+    fn estimated_tokens(count: usize) -> String {
+        "\u{e9}".repeat(count)
+    }
+
+    fn reasoning_event(text: &str) -> Value {
+        serde_json::json!({
+            "object": "chat.completion.chunk",
+            "choices": [{"index": 0, "delta": {"reasoning_content": text}}],
+        })
+    }
+
+    fn usage_event(text: &str, completion_tokens: u32) -> Value {
+        serde_json::json!({
+            "object": "chat.completion.chunk",
+            "choices": [{"index": 0, "delta": {"reasoning_content": text}}],
+            "usage": {"completion_tokens": completion_tokens},
+        })
+    }
+
+    #[derive(Clone, Copy, PartialEq, Eq)]
+    enum CanaryEnd {
+        Done,
+        Eof,
+        Stall,
+    }
+
+    /// Serves the given canary events, then ends the stream as `end` says.
+    async fn spawn_canary_event_server(events: Vec<Value>, end: CanaryEnd) -> TestHttpServer {
+        use futures::StreamExt as _;
+
+        TestHttpServer::spawn(Router::new().route(
+            "/v1/chat/completions",
+            post(move || async move {
+                let mut body = events
+                    .iter()
+                    .map(|event| format!("data: {event}\n\n"))
+                    .collect::<String>();
+                if end == CanaryEnd::Done {
+                    body.push_str("data: [DONE]\n\n");
+                }
+                let first = futures::stream::once(async move {
+                    Ok::<_, std::io::Error>(bytes::Bytes::from(body))
+                });
+                let body = if end == CanaryEnd::Stall {
+                    axum::body::Body::from_stream(first.chain(futures::stream::pending()))
+                } else {
+                    axum::body::Body::from_stream(first)
+                };
+                ([("content-type", "text/event-stream")], body)
+            }),
+        ))
+        .await
+    }
+
+    async fn send_test_canary(events: Vec<Value>, end: CanaryEnd) -> Result<(), BringupError> {
+        let server = spawn_canary_event_server(events, end).await;
+        let result = send_canary_request(
+            &reqwest::Client::new(),
+            server.as_str(),
+            &test_generation(),
+            Duration::from_millis(200),
+            7,
+        )
+        .await;
+        server.shutdown().await;
+        result
+    }
+
+    #[tokio::test]
+    async fn canary_exact_usage_at_threshold_settles_a_split_estimate_overshoot() {
+        send_test_canary(
+            vec![reasoning_event(&estimated_tokens(15)), usage_event("", 7)],
+            CanaryEnd::Done,
+        )
+        .await
+        .expect("exact usage at the cap should override a higher estimate");
+    }
+
+    #[tokio::test]
+    async fn canary_exact_usage_at_threshold_settles_an_overshoot_in_the_same_event() {
+        send_test_canary(vec![usage_event(&estimated_tokens(15), 7)], CanaryEnd::Done)
+            .await
+            .expect("exact usage at the cap should override a higher estimate");
+    }
+
+    #[tokio::test]
+    async fn canary_mid_stream_usage_then_final_usage_at_threshold_passes() {
+        send_test_canary(
+            vec![
+                usage_event(&estimated_tokens(5), 5),
+                reasoning_event(&estimated_tokens(4)),
+                usage_event("", 7),
+            ],
+            CanaryEnd::Done,
+        )
+        .await
+        .expect("final exact usage at the cap should settle the estimated tail");
+    }
+
+    #[tokio::test]
+    async fn canary_judges_a_completed_stream_by_exact_usage_despite_a_later_estimated_tail() {
+        send_test_canary(
+            vec![
+                usage_event(&estimated_tokens(7), 7),
+                reasoning_event(&estimated_tokens(9)),
+            ],
+            CanaryEnd::Done,
+        )
+        .await
+        .expect("accepted exact usage should decide the verdict");
+    }
+
+    #[tokio::test]
+    async fn canary_ignores_regressed_exact_usage() {
+        send_test_canary(
+            vec![usage_event(&estimated_tokens(5), 5), usage_event("", 0)],
+            CanaryEnd::Done,
+        )
+        .await
+        .expect("regressed usage should keep the prior exact count");
+    }
+
+    #[tokio::test]
+    async fn canary_failure_event_with_usage_above_threshold_is_not_runaway() {
+        let error = send_test_canary(
+            vec![serde_json::json!({
+                "type": "error",
+                "usage": {"completion_tokens": 8},
+            })],
+            CanaryEnd::Done,
+        )
+        .await
+        .expect_err("a failed stream should fail");
+
+        assert!(
+            matches!(error, BringupError::InvalidResponse(_)),
+            "unexpected canary error: {error}"
+        );
+    }
+
+    #[tokio::test]
+    async fn canary_exact_usage_above_threshold_is_runaway() {
+        let error = send_test_canary(
+            vec![reasoning_event(&estimated_tokens(3)), usage_event("", 8)],
+            CanaryEnd::Done,
+        )
+        .await
+        .expect_err("exact usage above the cap should fail");
+
+        assert!(
+            matches!(error, BringupError::RunawayGeneration { tokens: 8 }),
+            "unexpected canary error: {error}"
+        );
+    }
+
+    #[tokio::test]
+    async fn canary_exact_usage_above_threshold_fails_before_the_stream_ends() {
+        let error = send_test_canary(vec![usage_event("", 8)], CanaryEnd::Stall)
+            .await
+            .expect_err("exact usage above the cap should fail immediately");
+
+        assert!(
+            matches!(error, BringupError::RunawayGeneration { tokens: 8 }),
+            "unexpected canary error: {error}"
+        );
+    }
+
+    #[tokio::test]
+    async fn canary_without_usage_uses_the_estimate_at_completion() {
+        send_test_canary(vec![reasoning_event(&estimated_tokens(7))], CanaryEnd::Done)
+            .await
+            .expect("an estimate at the cap should pass");
+
+        let error = send_test_canary(vec![reasoning_event(&estimated_tokens(8))], CanaryEnd::Done)
+            .await
+            .expect_err("an unsettled estimate above the cap should fail");
+
+        assert!(
+            matches!(error, BringupError::RunawayGeneration { tokens: 8 }),
+            "unexpected canary error: {error}"
+        );
+    }
+
+    #[tokio::test]
+    async fn canary_stall_after_estimate_overshoot_is_a_timeout() {
+        let error = send_test_canary(
+            vec![reasoning_event(&estimated_tokens(9))],
+            CanaryEnd::Stall,
+        )
+        .await
+        .expect_err("a stalled canary should fail");
+
+        assert!(error.is_timeout(), "unexpected canary error: {error}");
+    }
+
+    #[tokio::test]
+    async fn canary_eof_after_estimate_overshoot_is_an_invalid_response() {
+        let error = send_test_canary(vec![reasoning_event(&estimated_tokens(9))], CanaryEnd::Eof)
+            .await
+            .expect_err("a stream without [DONE] should fail");
+
+        assert!(
+            matches!(error, BringupError::InvalidResponse(_)),
+            "unexpected canary error: {error}"
+        );
+    }
+
+    #[tokio::test]
+    async fn canary_completion_event_without_done_is_an_invalid_response() {
+        let error = send_test_canary(
+            vec![
+                reasoning_event("2"),
+                serde_json::json!({"type": "response.completed"}),
+            ],
+            CanaryEnd::Eof,
+        )
+        .await
+        .expect_err("a canary stream must end with [DONE]");
+
+        assert!(
+            matches!(error, BringupError::InvalidResponse(_)),
+            "unexpected canary error: {error}"
+        );
+    }
+
+    #[tokio::test]
+    async fn canary_read_error_after_estimate_overshoot_is_not_runaway() {
+        use futures::StreamExt as _;
+
+        let server = TestHttpServer::spawn(Router::new().route(
+            "/v1/chat/completions",
+            post(|| async {
+                let event = format!("data: {}\n\n", reasoning_event(&estimated_tokens(9)));
+                let first = futures::stream::once(async move {
+                    Ok::<_, std::io::Error>(bytes::Bytes::from(event))
+                });
+                let reset = futures::stream::once(async {
+                    tokio::time::sleep(Duration::from_millis(50)).await;
+                    Err(std::io::Error::other("connection reset"))
+                });
+                let body = first.chain(reset);
+                (
+                    [("content-type", "text/event-stream")],
+                    axum::body::Body::from_stream(body),
+                )
+            }),
+        ))
+        .await;
+
+        let error = send_canary_request(
+            &reqwest::Client::new(),
+            server.as_str(),
+            &test_generation(),
+            Duration::from_secs(1),
+            7,
+        )
+        .await
+        .expect_err("a broken canary stream should fail");
+
+        assert!(
+            matches!(error, BringupError::InvalidResponse(_)),
+            "unexpected canary error: {error}"
+        );
+        server.shutdown().await;
+    }
+
     #[tokio::test]
     async fn canary_stream_stall_is_classified_as_a_timeout() {
         use futures::StreamExt as _;
diff --git a/src/libraries/rust/stargate/crates/pylon-lib/src/bringup/upstream.rs b/src/libraries/rust/stargate/crates/pylon-lib/src/bringup/upstream.rs
index d73ef22a84..87a0bdda56 100644
--- a/src/libraries/rust/stargate/crates/pylon-lib/src/bringup/upstream.rs
+++ b/src/libraries/rust/stargate/crates/pylon-lib/src/bringup/upstream.rs
@@ -165,13 +165,18 @@ pub(super) async fn send_canary_request(
         DEFAULT_MAX_SSE_BUFFER_BYTES,
         false,
     );
+    let threshold = u64::from(canary_max_generation_threshold);
     let mut output_tokens = OutputTokenParser::new();
-    let mut observed_tokens = 0_u64;
+    let mut estimated_tokens = 0_u64;
+    let mut exact_tokens = None;
+    let runaway = |tokens: u64| BringupError::RunawayGeneration {
+        tokens: u32::try_from(tokens).unwrap_or(u32::MAX),
+    };
     let mut completed = false;
     while let Some(message) = messages.next().await {
         let message = message.map_err(canary_stream_error)?;
         if let Some(generated_output) = message.facts.generated_output {
-            observed_tokens = output_tokens
+            estimated_tokens = output_tokens
                 .observe_generated_characters(generated_output.characters)
                 .displayed_tokens;
         }
@@ -181,27 +186,33 @@ pub(super) async fn send_canary_request(
             .and_then(|usage| usage.output_tokens)
             && output_tokens.observe_exact_output_tokens(tokens) == ExactOutputUpdate::Applied
         {
-            observed_tokens = tokens;
-        }
-        if observed_tokens > u64::from(canary_max_generation_threshold) {
-            return Err(BringupError::RunawayGeneration {
-                tokens: u32::try_from(observed_tokens).unwrap_or(u32::MAX),
-            });
+            exact_tokens = Some(tokens);
         }
         match message.facts.terminal {
-            Some(RelayOutcome::Complete) => {
+            Some(RelayOutcome::Complete) if message.facts.done_sentinel => {
                 completed = true;
                 break;
             }
             Some(RelayOutcome::Failed) => break,
-            None => {}
+            Some(RelayOutcome::Complete) | None => {}
+        }
+        if let Some(tokens) = exact_tokens
+            && tokens > threshold
+        {
+            return Err(runaway(tokens));
         }
     }
+    // A character estimate can overshoot a response that stopped at the cap,
+    // so it is judged only when a completed stream reports no exact usage.
+    let observed_tokens = exact_tokens.unwrap_or(estimated_tokens);
     if !completed || observed_tokens == 0 {
         return Err(BringupError::InvalidResponse(
             "canary stream must contain output and end with [DONE]".to_string(),
         ));
     }
+    if observed_tokens > threshold {
+        return Err(runaway(observed_tokens));
+    }
     Ok(())
 }
 
diff --git a/src/libraries/rust/stargate/crates/pylon-lib/src/sse_message_stream.rs b/src/libraries/rust/stargate/crates/pylon-lib/src/sse_message_stream.rs
index 84a8b97c32..8cea901169 100644
--- a/src/libraries/rust/stargate/crates/pylon-lib/src/sse_message_stream.rs
+++ b/src/libraries/rust/stargate/crates/pylon-lib/src/sse_message_stream.rs
@@ -67,6 +67,8 @@ pub(crate) struct SseEventFacts {
     pub(crate) reasoning_output_observed: bool,
     pub(crate) calibration_ineligible: bool,
     pub(crate) terminal: Option<RelayOutcome>,
+    /// The event's data is the Chat Completions `[DONE]` sentinel.
+    pub(crate) done_sentinel: bool,
 }
 
 #[derive(Debug, PartialEq)]
@@ -346,6 +348,7 @@ fn classify_sse_event(
                 calibration_ineligible: output_token_calibration_enabled
                     && event_name.is_some_and(|event_name| event_name.starts_with("response.")),
                 terminal: Some(terminal),
+                done_sentinel: true,
                 ..SseEventFacts::default()
             },
         );
@@ -394,6 +397,7 @@ fn classify_sse_event(
             terminal_outcome(json_event_type),
             terminal_outcome(event_name),
         ),
+        done_sentinel: false,
     };
     (parsed, facts)
 }
diff --git a/src/libraries/rust/stargate/crates/pylon/src/main.rs b/src/libraries/rust/stargate/crates/pylon/src/main.rs
index 8d1d9f3322..d27057fb83 100644
--- a/src/libraries/rust/stargate/crates/pylon/src/main.rs
+++ b/src/libraries/rust/stargate/crates/pylon/src/main.rs
@@ -107,7 +107,7 @@ struct Args {
     /// Interval between active canary requests in milliseconds. Models with request progress within the interval skip the canary. `0` disables active canaries
     #[arg(long, default_value_t = 5000, value_name = "MS")]
     active_canary_interval_ms: u64,
-    /// Treat canary responses that generate this many tokens as runaway generation
+    /// Canary `max_tokens`. Generating more tokens than this is runaway generation; exact usage overrides the output estimate
     #[arg(long, default_value_t = 237, value_name = "TOKENS")]
     canary_max_generation_threshold: u32,
     /// Initial calibration request count; doubles after each completed load step