diff --git a/Cargo.lock b/Cargo.lock
index 047ae48..65347bf 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -912,7 +912,6 @@ version = "0.3.0"
 dependencies = [
  "arrow-array",
  "arrow-buffer",
- "arrow-cast",
  "arrow-schema",
  "arrow-select",
  "bytemuck",
diff --git a/crates/qroissant-arrow/Cargo.toml b/crates/qroissant-arrow/Cargo.toml
index 84c368d..395c0d8 100644
--- a/crates/qroissant-arrow/Cargo.toml
+++ b/crates/qroissant-arrow/Cargo.toml
@@ -12,7 +12,6 @@ path = "src/lib.rs"
 [dependencies]
 arrow-array = "58.0.0"
 arrow-buffer = "58.0.0"
-arrow-cast = "58.0.0"
 arrow-schema = "58.0.0"
 arrow-select = "58.0.0"
 bytemuck = { version = "1", features = ["derive", "extern_crate_alloc"] }
diff --git a/crates/qroissant-arrow/src/lib.rs b/crates/qroissant-arrow/src/lib.rs
index 807df23..d4e617a 100644
--- a/crates/qroissant-arrow/src/lib.rs
+++ b/crates/qroissant-arrow/src/lib.rs
@@ -14,7 +14,6 @@ pub use error::IngestionError;
 pub use ingestion::ingest_array;
 pub use ingestion::ingest_record_batch;
 pub use ingestion::ingest_record_batch_reader;
-pub use options::HeterogeneousListMode;
 pub use options::ListProjection;
 pub use options::ProjectionOptions;
 pub use options::StringProjection;
diff --git a/crates/qroissant-arrow/src/options.rs b/crates/qroissant-arrow/src/options.rs
index e16443c..8938919 100644
--- a/crates/qroissant-arrow/src/options.rs
+++ b/crates/qroissant-arrow/src/options.rs
@@ -52,28 +52,6 @@ pub enum UnionMode {
     Sparse,
 }
 
-/// How heterogeneous q general lists are projected when child Arrow types differ.
-///
-/// KDB returns mixed-typed columns (most commonly: temporal nulls of varying
-/// precision interleaved in a single column) as q general lists. The
-/// faithful representation is an Arrow Union, but consumers such as Polars
-/// reject Union types outright.
-#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
-pub enum HeterogeneousListMode {
-    /// Faithful: emit Arrow Union (Dense or Sparse per `union_mode`).
-    /// Lossless. Default.
-    #[default]
-    Union,
-    /// Lossy: when every arm of the union is a temporal type (Timestamp,
-    /// Date32, Date64, Time32, Time64, Duration), promote all values to
-    /// `Timestamp(Nanosecond, None)` and emit a flat array. Falls back to
-    /// Union if any arm is non-temporal or the cast fails.
-    ///
-    /// Use this when the downstream consumer cannot handle Arrow Union
-    /// (Polars) and the column is known to be temporal-only.
-    CoalesceTemporals,
-}
-
 /// Combined projection options threaded through `project()` / `project_table()`.
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct ProjectionOptions {
@@ -81,9 +59,6 @@ pub struct ProjectionOptions {
     pub string: StringProjection,
     pub list: ListProjection,
     pub union_mode: UnionMode,
-    /// How heterogeneous general lists are projected when child Arrow types
-    /// differ. Default: `Union` (faithful, lossless, may be rejected by Polars).
-    pub heterogeneous_list_mode: HeterogeneousListMode,
     /// When `true`, q infinity sentinels (e.g. `0Wi`, `0Wj`, `0w`) are mapped
     /// to Arrow nulls alongside the standard null sentinels. Default: `false`.
     pub treat_infinity_as_null: bool,
@@ -102,7 +77,6 @@ impl Default for ProjectionOptions {
             string: StringProjection::default(),
             list: ListProjection::default(),
             union_mode: UnionMode::default(),
-            heterogeneous_list_mode: HeterogeneousListMode::default(),
             treat_infinity_as_null: false,
             parallel: true,
             assume_symbol_utf8: true,
diff --git a/crates/qroissant-arrow/src/projection.rs b/crates/qroissant-arrow/src/projection.rs
index 0f97f10..6219b9e 100644
--- a/crates/qroissant-arrow/src/projection.rs
+++ b/crates/qroissant-arrow/src/projection.rs
@@ -99,7 +99,6 @@ use rayon::prelude::*;
 use crate::error::ProjectionError;
 use crate::error::ProjectionResult;
 use crate::metadata::q_field;
-use crate::options::HeterogeneousListMode;
 use crate::options::ListProjection;
 use crate::options::ProjectionOptions;
 use crate::options::StringProjection;
@@ -965,55 +964,11 @@ fn project_list(list: &List, opts: &ProjectionOptions) -> ProjectionResult<Array
 // Heterogeneous list → Union projection
 // ---------------------------------------------------------------------------
 
-fn is_temporal_dt(dt: &DataType) -> bool {
-    matches!(
-        dt,
-        DataType::Timestamp(_, _)
-            | DataType::Date32
-            | DataType::Date64
-            | DataType::Time32(_)
-            | DataType::Time64(_)
-            | DataType::Duration(_)
-    )
-}
-
-/// Attempts to coalesce a heterogeneous list of temporal Arrow arrays into a
-/// flat `Timestamp(Nanosecond, None)` array. Returns `None` if any arm is
-/// non-temporal or `arrow_cast::cast` rejects an arm. Callers fall back to
-/// Union encoding in that case.
-fn try_coalesce_temporals_to_ns(child_exports: &[ArrayExport]) -> Option<ArrayRef> {
-    let target = DataType::Timestamp(TimeUnit::Nanosecond, None);
-    let mut cast_arrays: Vec<ArrayRef> = Vec::with_capacity(child_exports.len());
-    for export in child_exports {
-        let dt = export.array.data_type();
-        if !is_temporal_dt(dt) {
-            return None;
-        }
-        match arrow_cast::cast(export.array.as_ref(), &target) {
-            Ok(arr) => cast_arrays.push(arr),
-            Err(_) => return None,
-        }
-    }
-    let refs: Vec<&dyn arrow_array::Array> = cast_arrays.iter().map(|a| a.as_ref()).collect();
-    arrow_select::concat::concat(&refs).ok()
-}
-
 fn project_heterogeneous_list(
     values: &[Value],
     child_exports: Vec<ArrayExport>,
     opts: &ProjectionOptions,
 ) -> ProjectionResult<ArrayExport> {
-    // Opt-in: collapse all-temporal heterogeneous lists into Timestamp(ns).
-    // Polars and similar consumers cannot ingest Arrow Union; this lets users
-    // accept the lossy promotion in exchange for a flat, ingestible column.
-    if opts.heterogeneous_list_mode == HeterogeneousListMode::CoalesceTemporals
-        && let Some(flat) = try_coalesce_temporals_to_ns(&child_exports)
-    {
-        let dt = DataType::Timestamp(TimeUnit::Nanosecond, None);
-        let field = q_field(dt, false, "list", None, None, None);
-        return Ok(ArrayExport { array: flat, field });
-    }
-
     // Assign a type_id (i8) to each unique Arrow DataType in insertion order.
     let mut type_id_map: Vec<(DataType, i8)> = Vec::new();
     let mut type_ids: Vec<i8> = Vec::with_capacity(values.len());
@@ -1203,70 +1158,3 @@ fn attribute_meta_str(a: Attribute) -> &'static str {
         Attribute::Grouped => "grouped",
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use qroissant_core::Atom;
-    use qroissant_core::List;
-
-    fn mixed_temporal_list() -> Value {
-        Value::List(List::new(
-            Attribute::None,
-            vec![
-                Value::Atom(Atom::Timestamp(0)),
-                Value::Atom(Atom::Datetime(0.0)),
-            ],
-        ))
-    }
-
-    #[test]
-    fn heterogeneous_temporals_emit_union_by_default() {
-        let value = mixed_temporal_list();
-        let opts = ProjectionOptions::default();
-        let export = project(&value, &opts).expect("projection");
-        assert!(
-            matches!(export.array.data_type(), DataType::Union(_, _)),
-            "expected Union under default mode, got {:?}",
-            export.array.data_type()
-        );
-    }
-
-    #[test]
-    fn heterogeneous_temporals_coalesce_to_timestamp_ns() {
-        let value = mixed_temporal_list();
-        let opts = ProjectionOptions {
-            heterogeneous_list_mode: HeterogeneousListMode::CoalesceTemporals,
-            ..ProjectionOptions::default()
-        };
-        let export = project(&value, &opts).expect("projection");
-        assert_eq!(
-            export.array.data_type(),
-            &DataType::Timestamp(TimeUnit::Nanosecond, None),
-            "expected flat Timestamp(ns) under CoalesceTemporals"
-        );
-        assert_eq!(export.array.len(), 2, "expected 2 rows, one per arm");
-    }
-
-    #[test]
-    fn coalesce_falls_back_to_union_for_non_temporal_arm() {
-        // Mixed temporal + int: not all-temporal, should fall through to Union.
-        let value = Value::List(List::new(
-            Attribute::None,
-            vec![
-                Value::Atom(Atom::Timestamp(0)),
-                Value::Atom(Atom::Int(42)),
-            ],
-        ));
-        let opts = ProjectionOptions {
-            heterogeneous_list_mode: HeterogeneousListMode::CoalesceTemporals,
-            ..ProjectionOptions::default()
-        };
-        let export = project(&value, &opts).expect("projection");
-        assert!(
-            matches!(export.array.data_type(), DataType::Union(_, _)),
-            "non-temporal arm must force Union fallback, got {:?}",
-            export.array.data_type()
-        );
-    }
-}
diff --git a/crates/qroissant-kernels/src/temporal.rs b/crates/qroissant-kernels/src/temporal.rs
index fbc0af8..85c90a9 100644
--- a/crates/qroissant-kernels/src/temporal.rs
+++ b/crates/qroissant-kernels/src/temporal.rs
@@ -8,6 +8,7 @@
 //! Each transform function uses `portable_simd` for the aligned middle of the
 //! slice and falls back to a scalar loop for the head and tail.
 
+use std::simd::Select;
 use std::simd::prelude::*;
 
 use crate::nulls::Q_NULL_DATE;
diff --git a/crates/qroissant-python/src/serde.rs b/crates/qroissant-python/src/serde.rs
index 469eb06..397faae 100644
--- a/crates/qroissant-python/src/serde.rs
+++ b/crates/qroissant-python/src/serde.rs
@@ -3,7 +3,6 @@ use std::sync::Arc;
 use pyo3::prelude::*;
 use pyo3::types::PyAny;
 use pyo3::types::PyBytes;
-use qroissant_arrow::HeterogeneousListMode;
 use qroissant_arrow::ListProjection;
 use qroissant_arrow::ProjectionOptions;
 use qroissant_arrow::StringProjection;
@@ -57,11 +56,6 @@ pub fn decode_options_to_proj_opts(opts: Option<&DecodeOptions>) -> Arc<Projecti
             crate::types::UnionMode::Dense => qroissant_arrow::UnionMode::Dense,
             crate::types::UnionMode::Sparse => qroissant_arrow::UnionMode::Sparse,
         },
-        heterogeneous_list_mode: if opts.coalesce_temporals_value() {
-            HeterogeneousListMode::CoalesceTemporals
-        } else {
-            HeterogeneousListMode::Union
-        },
         treat_infinity_as_null: opts.treat_infinity_as_null(),
         parallel: opts.parallel_value(),
         assume_symbol_utf8: opts.assume_symbol_utf8_value(),
diff --git a/crates/qroissant-python/src/types.rs b/crates/qroissant-python/src/types.rs
index 1f54cdf..4f2eabd 100644
--- a/crates/qroissant-python/src/types.rs
+++ b/crates/qroissant-python/src/types.rs
@@ -817,7 +817,6 @@ pub struct DecodeOptions {
     validate_compressed_trailing_bytes: bool,
     temporal_nulls: bool,
     treat_infinity_as_null: bool,
-    coalesce_temporals: bool,
 }
 
 #[pymethods]
@@ -876,11 +875,6 @@ impl DecodeOptions {
     fn get_treat_infinity_as_null(&self) -> bool {
         self.treat_infinity_as_null
     }
-
-    #[getter]
-    fn coalesce_temporals(&self) -> bool {
-        self.coalesce_temporals
-    }
 }
 
 impl Default for DecodeOptions {
@@ -896,7 +890,6 @@ impl Default for DecodeOptions {
             validate_compressed_trailing_bytes: true,
             temporal_nulls: true,
             treat_infinity_as_null: false,
-            coalesce_temporals: false,
         }
     }
 }
@@ -933,10 +926,6 @@ impl DecodeOptions {
     pub(crate) fn assume_symbol_utf8_value(&self) -> bool {
         self.assume_symbol_utf8
     }
-
-    pub(crate) fn coalesce_temporals_value(&self) -> bool {
-        self.coalesce_temporals
-    }
 }
 
 #[pyclass(module = "qroissant", frozen, eq)]
@@ -1007,12 +996,6 @@ impl DecodeOptionsBuilder {
         next
     }
 
-    fn with_coalesce_temporals(&self, value: bool) -> Self {
-        let mut next = self.clone();
-        next.options.coalesce_temporals = value;
-        next
-    }
-
     fn build(&self) -> DecodeOptions {
         self.options.clone()
     }
@@ -1066,11 +1049,6 @@ impl DecodeOptionsBuilder {
     fn treat_infinity_as_null(&self) -> bool {
         self.options.treat_infinity_as_null
     }
-
-    #[getter]
-    fn coalesce_temporals(&self) -> bool {
-        self.options.coalesce_temporals
-    }
 }
 
 #[pyclass(module = "qroissant", frozen, eq)]
diff --git a/python/qroissant/_config.pyi b/python/qroissant/_config.pyi
index 22669a2..304f569 100644
--- a/python/qroissant/_config.pyi
+++ b/python/qroissant/_config.pyi
@@ -260,14 +260,6 @@ class DecodeOptions:
     def treat_infinity_as_null(self) -> bool:
         """Whether ±∞ sentinels are mapped to ``None`` in Arrow arrays."""
         ...
-    @property
-    def coalesce_temporals(self) -> bool:
-        """Whether heterogeneous all-temporal lists are flattened to ``Timestamp(ns)``
-        instead of being emitted as Arrow ``Union``.  Use this when the
-        downstream consumer (e.g. Polars) cannot ingest ``Union`` types.
-        Lossy: mixed precisions are promoted to nanoseconds.
-        """
-        ...
 
 
 class DecodeOptionsBuilder:
@@ -311,15 +303,6 @@ class DecodeOptionsBuilder:
     def with_treat_infinity_as_null(self, value: bool, /) -> DecodeOptionsBuilder:
         """Set whether ±∞ sentinels are mapped to ``None`` in Arrow arrays."""
         ...
-    def with_coalesce_temporals(self, value: bool, /) -> DecodeOptionsBuilder:
-        """Set whether heterogeneous all-temporal lists are flattened to
-        ``Timestamp(ns)`` instead of being emitted as Arrow ``Union``.
-
-        Set to ``True`` for consumers (e.g. Polars) that reject Arrow union
-        types.  Lossy: mixed precisions are promoted to nanoseconds.  Default
-        ``False`` (faithful ``Union`` representation).
-        """
-        ...
     def build(self) -> DecodeOptions:
         """Finalize the builder into an immutable :class:`DecodeOptions` instance."""
         ...