diff --git a/CHANGELOG.md b/CHANGELOG.md
index cae5839a..d346b3a1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,15 @@ true until the next version shipped.
 
 ### Added
 
+- Exact zone-map boundary coverage now lives in matching shell and pytest tests
+  (#831).
+
+  The `<=` and `>=` arms put the constant exactly at a row-group minimum or
+  maximum and compare returned rows with a heap twin. The `<`, `>`, and `=`
+  mirrors assert groups removed with bloom disabled, so a conservative pruning
+  regression cannot hide behind a correct answer. Each of the five one-token
+  strategy mutations was proved to fail its corresponding assertion.
+
 - The test harness refuses to measure a binary that was not built from the source
   under test.
 
diff --git a/test/pytest/TESTS.md b/test/pytest/TESTS.md
index 576c3c88..06f4cb36 100644
--- a/test/pytest/TESTS.md
+++ b/test/pytest/TESTS.md
@@ -4,7 +4,7 @@ Reference for anyone reading, running, or adding to `test/pytest/`. The design a
 the decisions behind the harness are in `design/ISSUE_432_PYTEST_HARNESS.md`. This
 file covers the tests themselves.
 
-**120 tests in 8 files.** One hundred and five of them test the harness rather than the
+**121 tests in 9 files.** One hundred and six of them test the harness rather than the
 product, and they come first, because a harness that can report a false green makes
 every other result in this directory worthless.
 
@@ -34,9 +34,10 @@ behaviour, the source of that number is named.
 - [8. test_native_projection.py: the ported suite](#8-test_native_projectionpy-the-ported-suite)
 - [9. test_ordered.py: the ordered oracle](#9-test_orderedpy-the-ordered-oracle)
 - [10. test_runshape.py: the shape of the run itself](#10-test_runshapepy-the-shape-of-the-run-itself)
-- [11. Adding a test](#11-adding-a-test)
-- [12. What this corpus does NOT yet refuse](#12-what-this-corpus-does-not-yet-refuse)
-- [13. Traps this corpus records](#13-traps-this-corpus-records)
+- [11. test_zonemap_boundaries.py: exact boundaries](#11-test_zonemap_boundariespy-exact-boundaries)
+- [12. Adding a test](#12-adding-a-test)
+- [13. What this corpus does NOT yet refuse](#13-what-this-corpus-does-not-yet-refuse)
+- [14. Traps this corpus records](#14-traps-this-corpus-records)
 
 ## 1. How to read a test in here
 
@@ -861,7 +862,24 @@ The empty-parametrize refusal carries its own message rather than folding into t
 bare-skip refusal. When a corpus glob matches nothing, the cause the reader needs to
 see is the corpus, not the marker.
 
-## 11. Adding a test
+## 11. test_zonemap_boundaries.py: exact boundaries
+
+### `test_exact_zonemap_boundaries`
+
+Pairs with `test/zonemap_boundaries.sh`. Two monotonic 1,000-row groups put
+`1001` exactly at the second group's minimum and `1000` exactly at the first
+group's maximum. Heap-row comparisons pin that `<= 1001` and `>= 1000` keep
+their boundary rows. Work-done counters, with bloom disabled, pin the
+correctness-preserving cases: `< 1001`, `> 1000`, and `= 1001` each remove one
+group.
+
+The five one-token strategy mutations make the corresponding assertion fail:
+`<=` and `>=` lose one row, while `<`, `>`, and `=` remain row-correct but
+remove no group. This distinguishes correctness coverage from
+pruning-effectiveness coverage rather than relying on incidental fixtures
+elsewhere in the matrix.
+
+## 12. Adding a test
 
 0. **Write it twice.** Every test in this tree ships as a `.sh` suite and a pytest
    test **in the same change** (jd, 2026-09-09). Not ported later, not one or the
@@ -888,7 +906,7 @@ see is the corpus, not the marker.
    failed the selftest on both majors of the matrix, which is how it was found. A
    new directory under `test/` inherits every rule the old ones follow.
 
-## 12. What this corpus does NOT yet refuse
+## 13. What this corpus does NOT yet refuse
 
 `VACUITY_MODES.md` is the inventory: 79 ways a pytest harness can report a pass while
 asserting nothing, 73 of them demonstrated by an actual run. **This layer refuses 25
@@ -900,7 +918,7 @@ Read it before adding a test. The gaps most likely to affect a new test are that
 same family satisfies it, and that a write is not required to have written anything.
 Both are named there with the refusal each needs.
 
-## 13. Traps this corpus records
+## 14. Traps this corpus records
 
 Recorded because each one produced a confident wrong result before it was caught,
 and all are the same family as the defect the layer exists to prevent.
diff --git a/test/pytest/test_zonemap_boundaries.py b/test/pytest/test_zonemap_boundaries.py
new file mode 100644
index 00000000..03c2f53d
--- /dev/null
+++ b/test/pytest/test_zonemap_boundaries.py
@@ -0,0 +1,82 @@
+"""Exact zone-map comparison boundaries and pruning effectiveness (#831)."""
+
+
+def _nodes(plan):
+    for root in plan:
+        stack = [root["Plan"]]
+        while stack:
+            node = stack.pop()
+            yield node
+            stack.extend(node.get("Plans", ()))
+
+
+def _plan(conn, qual):
+    with conn.cursor() as cur:
+        cur.execute("SET pgcolumnar.enable_bloom_filter=off")
+        cur.execute("SET pgcolumnar.enable_vectorization=off")
+        cur.execute(
+            "EXPLAIN (ANALYZE, FORMAT JSON, COSTS OFF, TIMING OFF, SUMMARY OFF) "
+            f"SELECT id FROM zb_c WHERE {qual}"
+        )
+        return cur.fetchone()[0]
+
+
+def _removed(plan):
+    return next(
+        node.get("Columnar Chunk Groups Removed by Filter", 0)
+        for node in _nodes(plan)
+        if "Columnar Chunk Groups Total" in node
+    )
+
+
+def _ids(conn, table, qual):
+    with conn.cursor() as cur:
+        cur.execute(f"SELECT id FROM {table} WHERE {qual} ORDER BY id")
+        return [row[0] for row in cur]
+
+
+def test_exact_zonemap_boundaries(pgc_conn, expect):
+    with pgc_conn.cursor() as cur:
+        cur.execute("CREATE TABLE zb_h(id int, v int)")
+        cur.execute("CREATE TABLE zb_c(id int, v int) USING pgcolumnar")
+        cur.execute(
+            "SELECT pgcolumnar.set_options('zb_c', stripe_row_limit => 1000)"
+        )
+        cur.execute("INSERT INTO zb_h SELECT g,g FROM generate_series(1,2000) g")
+        cur.execute("INSERT INTO zb_c SELECT * FROM zb_h")
+        cur.execute(
+            "SELECT count(*) FROM pgcolumnar.row_group "
+            "WHERE storage_id=pgcolumnar.get_storage_id('zb_c')"
+        )
+        expect.num(cur.fetchone()[0], 2, "premise: fixture has two row groups")
+
+    expect.num(
+        _removed(_plan(pgc_conn, "v < 1001")), 1,
+        "< excludes the group whose minimum equals the constant",
+    )
+    expect.rows(
+        _ids(pgc_conn, "zb_c", "v <= 1001"),
+        _ids(pgc_conn, "zb_h", "v <= 1001"),
+        "<= keeps the row at a row-group minimum",
+    )
+    expect.num(
+        _removed(_plan(pgc_conn, "v <= 1001")), 0,
+        "premise: <= at the second-group minimum removes no group",
+    )
+    expect.rows(
+        _ids(pgc_conn, "zb_c", "v >= 1000"),
+        _ids(pgc_conn, "zb_h", "v >= 1000"),
+        ">= keeps the row at a row-group maximum",
+    )
+    expect.num(
+        _removed(_plan(pgc_conn, "v >= 1000")), 0,
+        "premise: >= at the first-group maximum removes no group",
+    )
+    expect.num(
+        _removed(_plan(pgc_conn, "v > 1000")), 1,
+        "> excludes the group whose maximum equals the constant",
+    )
+    expect.num(
+        _removed(_plan(pgc_conn, "v = 1001")), 1,
+        "= excludes the group lying wholly below the constant",
+    )
diff --git a/test/run_all_versions.sh b/test/run_all_versions.sh
index b6330593..a8c951da 100755
--- a/test/run_all_versions.sh
+++ b/test/run_all_versions.sh
@@ -286,6 +286,7 @@ SUITES=(
 	wal_envelope
 	write_fsst_compressed
 	write_minmax_fastpath
+	zonemap_boundaries
 	zonemap_cost
 	zonemap_estimate_sample)
 
diff --git a/test/zonemap_boundaries.sh b/test/zonemap_boundaries.sh
new file mode 100755
index 00000000..ea1693ab
--- /dev/null
+++ b/test/zonemap_boundaries.sh
@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+#
+# Exact zone-map comparison boundaries are intentional coverage (#831).
+#
+# Usage: test/zonemap_boundaries.sh [PG_CONFIG]
+
+set -uo pipefail
+. "$(dirname "${BASH_SOURCE[0]}")/lib.sh"
+pgc_setup "${1:-/usr/local/pg17/bin/pg_config}"
+
+psql_run "CREATE TABLE zb_h(id int, v int);
+	CREATE TABLE zb_c(id int, v int) USING pgcolumnar;
+	SELECT pgcolumnar.set_options('zb_c', stripe_row_limit => 1000);
+	INSERT INTO zb_h SELECT g,g FROM generate_series(1,2000) g;
+	INSERT INTO zb_c SELECT * FROM zb_h;
+	ANALYZE zb_h; ANALYZE zb_c;" >/dev/null
+
+check "premise: the boundary fixture has two row groups" \
+	"$(q "SELECT count(*) FROM pgcolumnar.row_group
+	      WHERE storage_id=pgcolumnar.get_storage_id('zb_c');")" "2"
+
+groups_removed() {
+	env PATH="$PGC_BINDIR:$PATH" psql -h 127.0.0.1 -p "$PGC_PORT" \
+		-U postgres -d "$PGC_DB" -Atq \
+		-c "SET pgcolumnar.enable_bloom_filter=off;
+		    SET pgcolumnar.enable_vectorization=off;
+		    EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF, SUMMARY OFF)
+		    SELECT id FROM zb_c WHERE $1;" 2>&1 |
+		sed -n 's/.*Columnar Chunk Groups Removed by Filter: \([0-9]*\).*/\1/p' |
+		head -1
+}
+
+check "< excludes the group whose minimum equals the constant" \
+	"$(groups_removed 'v < 1001')" "1"
+check_text "<= keeps the row at a row-group minimum" \
+	"$(pgc_set_hash "SELECT id FROM zb_c WHERE v <= 1001")" \
+	"$(pgc_set_hash "SELECT id FROM zb_h WHERE v <= 1001")"
+check "premise: <= at the second-group minimum removes no group" \
+	"$(groups_removed 'v <= 1001')" "0"
+
+check_text ">= keeps the row at a row-group maximum" \
+	"$(pgc_set_hash "SELECT id FROM zb_c WHERE v >= 1000")" \
+	"$(pgc_set_hash "SELECT id FROM zb_h WHERE v >= 1000")"
+check "premise: >= at the first-group maximum removes no group" \
+	"$(groups_removed 'v >= 1000')" "0"
+
+# These mutations remain row-correct, so only work-done counters can see them.
+check "> excludes the group whose maximum equals the constant" \
+	"$(groups_removed 'v > 1000')" "1"
+check "= excludes the group lying wholly below the constant" \
+	"$(groups_removed 'v = 1001')" "1"
+
+check "backend alive" "$(q 'SELECT 1')" "1"
+pgc_summary