diff --git a/src/study.rs b/src/study.rs
index 6009246..5705200 100644
--- a/src/study.rs
+++ b/src/study.rs
@@ -398,6 +398,58 @@ where
         // They could be stored in a separate list for debugging if needed
     }
 
+    /// Request a new trial with suggested parameters.
+    ///
+    /// This is the first half of the ask-and-tell interface. After calling
+    /// `ask()`, use parameter types to suggest values on the returned trial,
+    /// evaluate your objective externally, then pass the trial back to
+    /// [`tell()`](Self::tell) with the result.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use optimizer::parameter::{FloatParam, Parameter};
+    /// use optimizer::{Direction, Study};
+    ///
+    /// let study: Study<f64> = Study::new(Direction::Minimize);
+    /// let x = FloatParam::new(0.0, 10.0);
+    ///
+    /// let mut trial = study.ask();
+    /// let x_val = x.suggest(&mut trial).unwrap();
+    /// let value = x_val * x_val;
+    /// study.tell(trial, Ok::<_, &str>(value));
+    /// ```
+    pub fn ask(&self) -> Trial {
+        self.create_trial()
+    }
+
+    /// Report the result of a trial obtained from [`ask()`](Self::ask).
+    ///
+    /// Pass `Ok(value)` for a successful evaluation or `Err(reason)` for a
+    /// failure. Failed trials are not stored in the study's history.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use optimizer::{Direction, Study};
+    ///
+    /// let study: Study<f64> = Study::new(Direction::Minimize);
+    ///
+    /// let trial = study.ask();
+    /// study.tell(trial, Ok::<_, &str>(42.0));
+    /// assert_eq!(study.n_trials(), 1);
+    ///
+    /// let trial = study.ask();
+    /// study.tell(trial, Err::<f64, _>("evaluation failed"));
+    /// assert_eq!(study.n_trials(), 1); // failed trials not counted
+    /// ```
+    pub fn tell(&self, trial: Trial, value: core::result::Result<V, impl ToString>) {
+        match value {
+            Ok(v) => self.complete_trial(trial, v),
+            Err(e) => self.fail_trial(trial, e),
+        }
+    }
+
     /// Records a pruned trial, preserving its intermediate values.
     ///
     /// Pruned trials are stored alongside completed trials so that samplers
diff --git a/tests/integration.rs b/tests/integration.rs
index 0e484bc..8cfd84d 100644
--- a/tests/integration.rs
+++ b/tests/integration.rs
@@ -1601,3 +1601,102 @@ fn test_top_trials_excludes_pruned() {
     assert_eq!(top.len(), 3, "pruned trial should be excluded");
     assert_eq!(top[0].value, 1.0);
 }
+
+// =============================================================================
+// Test: ask-and-tell interface
+// =============================================================================
+
+#[test]
+fn test_ask_and_tell_basic() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+    let x_param = FloatParam::new(0.0, 10.0);
+
+    for _ in 0..10 {
+        let mut trial = study.ask();
+        let x = x_param.suggest(&mut trial).unwrap();
+        let value = x * x;
+        study.tell(trial, Ok::<_, &str>(value));
+    }
+
+    assert_eq!(study.n_trials(), 10);
+    assert!(study.best_value().unwrap() >= 0.0);
+}
+
+#[test]
+fn test_ask_and_tell_with_failures() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+    let x_param = FloatParam::new(-5.0, 5.0);
+
+    // Alternate success and failure
+    for i in 0..10 {
+        let mut trial = study.ask();
+        let x = x_param.suggest(&mut trial).unwrap();
+        if i % 2 == 0 {
+            study.tell(trial, Ok::<_, &str>(x * x));
+        } else {
+            study.tell(trial, Err::<f64, _>("simulated failure"));
+        }
+    }
+
+    // Only successful trials are counted
+    assert_eq!(study.n_trials(), 5);
+}
+
+#[test]
+fn test_ask_and_tell_with_tpe_sampler() {
+    let sampler = TpeSampler::builder()
+        .seed(42)
+        .n_startup_trials(5)
+        .build()
+        .unwrap();
+    let study: Study<f64> = Study::minimize(sampler);
+    let x_param = FloatParam::new(-10.0, 10.0);
+
+    for _ in 0..30 {
+        let mut trial = study.ask();
+        let x = x_param.suggest(&mut trial).unwrap();
+        study.tell(trial, Ok::<_, &str>((x - 3.0).powi(2)));
+    }
+
+    assert_eq!(study.n_trials(), 30);
+    assert!(
+        study.best_value().unwrap() < 5.0,
+        "TPE ask-and-tell should find a reasonable value"
+    );
+}
+
+#[test]
+fn test_ask_and_tell_batch() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+    let x_param = FloatParam::new(0.0, 10.0);
+
+    // Ask a batch of trials
+    let batch: Vec<_> = (0..5)
+        .map(|_| {
+            let mut t = study.ask();
+            let x = x_param.suggest(&mut t).unwrap();
+            (t, x)
+        })
+        .collect();
+
+    // Tell results for the batch
+    for (trial, x) in batch {
+        study.tell(trial, Ok::<_, &str>(x * x));
+    }
+
+    assert_eq!(study.n_trials(), 5);
+}
+
+#[test]
+fn test_ask_and_tell_with_custom_value_type() {
+    // Ask-and-tell works with non-f64 value types too
+    let study: Study<i32> = Study::new(Direction::Maximize);
+
+    for i in 0..5 {
+        let trial = study.ask();
+        study.tell(trial, Ok::<_, &str>(i * 10));
+    }
+
+    assert_eq!(study.n_trials(), 5);
+    assert_eq!(study.best_value().unwrap(), 40);
+}