diff --git a/pineforge_codegen/codegen/emit_top.py b/pineforge_codegen/codegen/emit_top.py
index 58be635..89a7476 100644
--- a/pineforge_codegen/codegen/emit_top.py
+++ b/pineforge_codegen/codegen/emit_top.py
@@ -89,6 +89,7 @@
 from ..symbols import PineType, method_receiver_cpp_token
 from .tables import (
     BAR_SERIES_PUSH,
+    TA_CHART_PREV_CLOSE_ARG,
     DRAWING_TYPE_TO_CPP,
     PINE_TYPE_TO_CPP,
     RUNTIME_REGISTER_SECURITY_EVAL_FN,
@@ -1982,6 +1983,14 @@ def _emit_precalculate_and_run(self, lines: list[str]) -> None:
                 if self._ta_site_uses_precalc(site):
                     compute_args = self._ta_compute_args_for_site(site)
                     compute_args_bars = compute_args.replace("current_bar_.", "bars[i].")
+                    # issue #178: the precalc pre-pass walks the bar array
+                    # itself, so the previous CHART close is bars[i - 1]
+                    # (the engine's prev_chart_close() tracker only advances
+                    # at on_bar dispatch).
+                    compute_args_bars = compute_args_bars.replace(
+                        TA_CHART_PREV_CLOSE_ARG,
+                        "(i > 0 ? bars[i - 1].close : na<double>())",
+                    )
                     lines.append(f"            _precalc_{site.member_name}[i] = {site.member_name}.compute({compute_args_bars});")
         finally:
             self._precalc_loop_active = False
diff --git a/pineforge_codegen/codegen/ta.py b/pineforge_codegen/codegen/ta.py
index d43f714..63b3bc3 100644
--- a/pineforge_codegen/codegen/ta.py
+++ b/pineforge_codegen/codegen/ta.py
@@ -31,7 +31,12 @@
     Ternary, TupleAssign, TupleLiteral, TypeDecl, TypeField, UnaryOp, VarDecl,
     WhileStmt,
 )
-from .tables import TA_IMPLICIT_APPEND, TA_IMPLICIT_COMPUTE_FULL
+from .tables import (
+    TA_CHART_PREV_CLOSE,
+    TA_CHART_PREV_CLOSE_ARG,
+    TA_IMPLICIT_APPEND,
+    TA_IMPLICIT_COMPUTE_FULL,
+)
 
 if TYPE_CHECKING:
     from ..analyzer import TACallSite
@@ -113,6 +118,10 @@ def _ta_compute_args_for_site(self, site: "TACallSite") -> str:
 
         if ta_name in TA_IMPLICIT_COMPUTE_FULL:
             implicit = TA_IMPLICIT_COMPUTE_FULL[ta_name]
+            # issue #178: chart-context atr / tr take the previous CHART
+            # bar's close as a 4th argument (see TA_CHART_PREV_CLOSE).
+            if ta_name in TA_CHART_PREV_CLOSE:
+                implicit = f"{implicit}, {TA_CHART_PREV_CLOSE_ARG}"
             if site.compute_args:
                 explicit = ", ".join(self._visit_expr(a) for a in site.compute_args)
                 if ta_name in self._TA_IMPLICIT_REPLACE:
diff --git a/pineforge_codegen/codegen/tables.py b/pineforge_codegen/codegen/tables.py
index 645f07f..7419e30 100644
--- a/pineforge_codegen/codegen/tables.py
+++ b/pineforge_codegen/codegen/tables.py
@@ -222,6 +222,18 @@ def tz_time_field_lambda(field_expr: str, ts_arg: str, tz_arg: str) -> str:
     "max": [0], "min": [0], "rci": [0],
 }
 
+# issue #178: ta.atr / ta.tr read their true range against the previous
+# CHART bar's close (close[1]) even when the call site executes sparsely —
+# the RMA advances on the executions only (TradingView pin 2026-09-06,
+# lab tv i178-sparse-atr-sense, BINANCE:BTCUSDT 60: 398/398 executions equal
+# the chart-close model, 0/398 the previous-execution model). The chart
+# context therefore passes ``BacktestEngine::prev_chart_close()`` as the
+# 4th compute() argument (the engine's always-on tracker); the per-object
+# prev_close (3-arg form) remains for request.security contexts, whose
+# ``bar.`` rewrite has no chart-close tracker.
+TA_CHART_PREV_CLOSE = {"atr", "tr"}
+TA_CHART_PREV_CLOSE_ARG = "prev_chart_close()"
+
 # TA functions whose ``.compute()`` always receives bar OHLC implicitly.
 TA_IMPLICIT_COMPUTE_FULL = {
     "atr": "current_bar_.high, current_bar_.low, current_bar_.close",
diff --git a/tests/test_codegen_new.py b/tests/test_codegen_new.py
index fc6fdee..5708d68 100644
--- a/tests/test_codegen_new.py
+++ b/tests/test_codegen_new.py
@@ -168,7 +168,7 @@ def test_ta_tr_handle_na_false_routes_through_tr_class():
     assert re.search(r"_ta_tr_\d+\(false\)", cpp), cpp
     # Compute is invoked with the bar OHLC implicitly threaded in.
     assert (
-        "_ta_tr_1.compute(current_bar_.high, current_bar_.low, current_bar_.close)"
+        "_ta_tr_1.compute(current_bar_.high, current_bar_.low, current_bar_.close, prev_chart_close())"
         in cpp
     )
 
diff --git a/tests/test_sparse_atr_prev_close.py b/tests/test_sparse_atr_prev_close.py
new file mode 100644
index 0000000..b5e0bbd
--- /dev/null
+++ b/tests/test_sparse_atr_prev_close.py
@@ -0,0 +1,65 @@
+"""issue #178: ta.atr / ta.tr take the previous CHART bar's close.
+
+TradingView pin (2026-09-06, lab tv i178-sparse-atr-sense, BINANCE:BTCUSDT 60,
+398/398 sparse executions): ``ta.atr(n)`` called inside a block that does not
+execute every bar advances its RMA on the executions only, but its true range
+always reads ``close[1]`` of the CHART, never the close of the site's previous
+execution. The engine's per-object ``prev_close`` is the refuted model, so the
+chart-context compute() passes ``BacktestEngine::prev_chart_close()`` as a 4th
+argument; the precalc pre-pass (which walks ``bars[i]`` itself) passes
+``bars[i - 1].close``; a request.security context keeps the 3-arg form.
+"""
+
+from __future__ import annotations
+
+import re
+
+import pineforge_codegen as pc
+
+PRELUDE = '//@version=6\nstrategy("t", overlay=true)\n'
+
+
+def _gen(src: str) -> str:
+    return pc.transpile(PRELUDE + src, check_support=False)
+
+
+def test_sparse_atr_chart_context_passes_prev_chart_close():
+    cpp = _gen(
+        "reach = close[1] > open[1] and close < open\n"
+        "var float a = na\n"
+        "if reach\n"
+        "    a := ta.atr(3)\n"
+        "plot(a)\n"
+    )
+    assert (
+        "_ta_atr_1.compute(current_bar_.high, current_bar_.low, current_bar_.close, prev_chart_close())"
+        in cpp
+    ), cpp
+    assert (
+        "_ta_atr_1.recompute(current_bar_.high, current_bar_.low, current_bar_.close, prev_chart_close())"
+        in cpp
+    ), cpp
+
+
+def test_every_bar_tr_passes_prev_chart_close_and_precalc_uses_bars_i_minus_1():
+    cpp = _gen("t = ta.tr(true)\nplot(t)\n")
+    assert (
+        "_ta_tr_1.compute(current_bar_.high, current_bar_.low, current_bar_.close, prev_chart_close())"
+        in cpp
+    ), cpp
+    # The precalc pre-pass, when emitted, walks the bar array itself.
+    for m in re.finditer(r"_precalc__ta_tr_1\[i\] = _ta_tr_1\.compute\(([^;]*)\);", cpp):
+        assert m.group(1) == "bars[i].high, bars[i].low, bars[i].close, (i > 0 ? bars[i - 1].close : na<double>())", m.group(0)
+    assert "prev_chart_close()" not in "".join(
+        l for l in cpp.splitlines() if "_precalc_" in l
+    ), cpp
+
+
+def test_security_context_atr_keeps_three_arg_form():
+    cpp = _gen(
+        'a = request.security(syminfo.tickerid, "D", ta.atr(14))\n'
+        "plot(a)\n"
+    )
+    sec_lines = [l for l in cpp.splitlines() if "compute(bar.high, bar.low, bar.close" in l]
+    assert sec_lines, cpp
+    assert all("prev_chart_close()" not in l for l in sec_lines), sec_lines
diff --git a/tests/test_transpile_tr_handle_na.py b/tests/test_transpile_tr_handle_na.py
index aba76f5..63bf232 100644
--- a/tests/test_transpile_tr_handle_na.py
+++ b/tests/test_transpile_tr_handle_na.py
@@ -92,7 +92,7 @@ def test_ta_tr_compute_uses_implicit_bar_ohlc():
     # implicit bar OHLC (high, low, close) — it never sees ``handle_na``.
     cpp = _generate(_wrap("x = ta.tr()"))
     assert (
-        "_ta_tr_1.compute(current_bar_.high, current_bar_.low, current_bar_.close)"
+        "_ta_tr_1.compute(current_bar_.high, current_bar_.low, current_bar_.close, prev_chart_close())"
         in cpp
     ), cpp
     # Defensive: the bool literal must NOT appear inside the compute call.