diff --git a/CHANGELOG.md b/CHANGELOG.md
index fc623cc..96d6b3b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- **`resetRadiusThreshold()` workaround for the radius threshold leak** (#200)
+  - A radius-filtered Flat/IVF query poisons the thread-local search context: subsequent radius-less queries (refiner, plain, any field with the same index type) return only the radius-filtered subset, with no error. Upstream zvec caches the threshold on a `thread_local` context (`zvec/src/core/interface/index.cc`) and its Flat/IVF `reset()` is a no-op; radius is gated by `if (radius > 0.0f)` in `flat_index.cc`/`ivf_index.cc`.
+  - **`ZVec::resetRadiusThreshold(string $fieldName, array $vector)`** runs a throwaway topk-1 query with `radius = FLT_MAX` (`ZVec::RADIUS_THRESHOLD_RESET`) on the same field, which overwrites the leaked threshold and restores unfiltered behavior for later queries on that thread.
+  - Works for all metrics: for normalizing metrics (IP, sign-flip denormalization) radius filters docs with similarity below `-radius` (e.g. opposite vectors), so the leak can occur there too — verified: `resetRadiusThreshold()` restores the missing negative-similarity docs. For corpora where every similarity is ≥ `-radius`, the call is a harmless no-op.
+  - Regression test `test_radius_threshold_leak.phpt` documents the leak and the recovery.
+
 - **`fetch()` with `outputFields` parameter** (#176)
   - `ZVecCollection::fetch()` now accepts an optional output-fields array to select which scalar columns are returned: `fetch(['pk1', 'pk2'], ['name', 'score'])`.
   - BC-compatible: the legacy variadic form `fetch('pk1', 'pk2')` (all fields) still works unchanged.
diff --git a/docs/helpers/decisions.md b/docs/helpers/decisions.md
index b7e82a7..4363dcc 100644
--- a/docs/helpers/decisions.md
+++ b/docs/helpers/decisions.md
@@ -73,3 +73,29 @@ present-null). Note: the Python SDK (pybind `Fetch`) does **not** apply
 this normalization — PHP intentionally follows the C API here.
 
 **Reference:** issue #192.
+
+---
+
+### resetRadiusThreshold(): caller-driven purge instead of automatic mitigation
+
+**Decision:** issue #200 (radius threshold leak) is mitigated with an
+explicit `ZVecCollection::resetRadiusThreshold(string $fieldName, array
+$vector)` + `ZVec::RADIUS_THRESHOLD_RESET` (3.4028235e38) constant, not by
+transparently fixing every radius-less query in the bindings.
+
+**Rationale:** the leak lives in the upstream zvec core (thread-local
+context, no-op Flat/IVF reset); hiding it in `queryVector()` would need
+per-call purge queries (cost) and could change result semantics. The
+explicit call documents the upstream bug at the API surface, costs one
+topk-1 query. Empirically verified against the built library for L2, IVF,
+COSINE, MIPSL2 and IP (incl. negative-similarity docs): the purge restores
+unfiltered results on every metric; it is a no-op only when no doc's
+similarity is below `-radius`. For IP, radius filters docs with similarity
+< `-radius` (sign-flip denormalization: engine converts radius to internal
+distance, `set_threshold()` denormalizes it back — `index_context.h:235-
+241`), so with opposite vectors the leak occurs there too.
+`RADIUS_THRESHOLD_RESET` survives every metric's denormalization (sign
+flip → -FLT_MAX, cosine `-= 1` → FLT_MAX in float32) and mirrors
+`reset_threshold()` (FLT_MAX), which is unreachable from the C API.
+
+**Reference:** issue #200.
diff --git a/docs/helpers/faq.md b/docs/helpers/faq.md
index 9fee8b0..5b629a3 100644
--- a/docs/helpers/faq.md
+++ b/docs/helpers/faq.md
@@ -109,3 +109,28 @@ output (rocksdb_context.cc, inverted_indexer.cc, id_map.cc) — failing
 block, before the `finally` cleanup removes the directory.
 
 **Reference:** issue #192 (new test initially failed on shutdown noise).
+
+---
+
+### Radius query poisons later queries on the same thread (Flat/IVF)
+
+**Problem:** one `setRadius(>0)` query makes every later radius-less query
+on the same thread (refiner, plain, any field with the same index type,
+even other collections) return only the radius-filtered subset — silently.
+Upstream caches the threshold on a `thread_local` context shared per index
+type (`zvec/src/core/interface/index.cc:33-35`) and Flat/IVF `reset()` is
+a no-op; the `if (radius > 0.0f)` gate in `flat_index.cc:63` blocks
+resetting it to 0.
+
+**Solution:** call `$c->resetRadiusThreshold($field, $queryVector)` after
+radius-filtered queries. It runs a throwaway topk-1 query with
+`radius = FLT_MAX`, which overwrites the stale threshold. Use the
+`ZVec::RADIUS_THRESHOLD_RESET` constant, not `PHP_FLOAT_MAX` — the latter
+overflows to `inf` in float32 (untested upstream semantics); FLT_MAX is
+what upstream's own `reset_threshold()` uses. For IP metrics radius
+filters docs with similarity below `-radius` (e.g. opposite vectors), so
+the leak CAN occur there too — verified: `resetRadiusThreshold()` restores
+the missing negative-similarity docs. With corpora where every similarity
+is ≥ `-radius`, the call is a harmless no-op.
+
+**Reference:** issue #200, `tests/test_radius_threshold_leak.phpt`.
diff --git a/src/ZVec.php b/src/ZVec.php
index 2a5fd26..f6ac940 100644
--- a/src/ZVec.php
+++ b/src/ZVec.php
@@ -1076,6 +1076,19 @@ public function fetch(array|string|bool ...$args): array
      */
     public const BYTES_PER_MB = 1048576;
 
+    /**
+     * Radius used by resetRadiusThreshold() to overwrite a stale threshold
+     * leaked into the thread-local Flat/IVF search context (issue #200).
+     *
+     * Maximum finite float (what upstream's reset_threshold() itself uses),
+     * chosen over PHP_FLOAT_MAX: the latter overflows to inf in float32,
+     * and it must survive every metric's denormalization (sign flip for
+     * IP → -FLT_MAX, cosine `-= 1` → FLT_MAX) and filter nothing.
+     *
+     * Value: 3.4028235e38 (FLT_MAX)
+     */
+    public const RADIUS_THRESHOLD_RESET = 3.4028235e38;
+
     /**
      * Default HNSW parameter: M (max connections per node).
      *
@@ -1653,6 +1666,39 @@ public function queryVector(ZVecVectorQuery $query): array
         return self::parseQueryResult($result);
     }
 
+    /**
+     * Clear a stale radius threshold leaked into the thread-local search
+     * context by an earlier radius-filtered Flat/IVF query (issue #200).
+     *
+     * Upstream zvec caches the threshold on a thread-local context shared by
+     * every index of the same type in the process, and its Flat/IVF context
+     * reset() is a no-op — so one setRadius() > 0 query makes all later
+     * radius-less queries on that thread (refiner, plain, any field with the
+     * same index type) return only the radius-filtered subset.
+     *
+     * Pass the field and query vector of the radius query that caused the
+     * leak; a throwaway topk-1 query with a maximum radius overwrites the
+     * leaked threshold with the neutral FLT_MAX. Its result is meaningless
+     * and is discarded. For metrics that denormalize with a sign flip (IP)
+     * radius filters docs with similarity below -radius (e.g. opposite
+     * vectors), so the leak can occur there too — verified: this call
+     * restores the missing negative-similarity docs.
+     *
+     * Dense vector fields only: the purge query is always issued as a dense
+     * fp32 vector; sparse fields cannot be purged through this method.
+     *
+     * @param float[]|int[] $vector The query vector of the leaking radius query
+     * @throws ZVecException On FFI error
+     */
+    public function resetRadiusThreshold(string $fieldName, array $vector): void
+    {
+        $this->checkClosed();
+
+        $query = new ZVecVectorQuery($fieldName, $vector);
+        $query->setTopk(1)->setRadius(self::RADIUS_THRESHOLD_RESET);
+        $this->queryVector($query);
+    }
+
     /**
      * GroupBy query using a native ZVecGroupByVectorQuery object.
      *
diff --git a/tests/test_radius_threshold_leak.phpt b/tests/test_radius_threshold_leak.phpt
new file mode 100644
index 0000000..a0f6d94
--- /dev/null
+++ b/tests/test_radius_threshold_leak.phpt
@@ -0,0 +1,92 @@
+--TEST--
+Radius threshold leak: radius query poisons subsequent refiner/plain queries, resetRadiusThreshold() restores them (#200)
+--SKIPIF--
+<?php
+if (extension_loaded('zvec')) die('skip Native zvec extension loaded (use FFI)');
+if (!extension_loaded('ffi')) die('skip FFI extension not available');
+?>
+--FILE--
+<?php
+require_once __DIR__ . '/../src/ZVec.php';
+ZVec::init(logType: ZVec::LOG_CONSOLE, logLevel: ZVec::LOG_WARN);
+
+$path = __DIR__ . '/../test_dbs/radius_leak_' . uniqid();
+
+/** @return string */
+function pks(array $results): string
+{
+    return implode(',', array_map(fn($d) => $d->getPk(), $results));
+}
+
+try {
+    $schema = new ZVecSchema('radius_leak');
+    $schema->addVectorFp32('vf', dimension: 4, metricType: ZVecSchema::METRIC_L2);
+
+    $c = ZVec::create($path, $schema);
+    $c->createIndex('vf', ZVecIndexParams::forFlat(ZVecSchema::METRIC_L2));
+
+    // Squared L2 distances from query [1,0,0,0]: doc1=0, doc2=0.25, doc3=4
+    $vecs = [
+        'doc1' => [1.0, 0.0, 0.0, 0.0],
+        'doc2' => [1.5, 0.0, 0.0, 0.0],
+        'doc3' => [3.0, 0.0, 0.0, 0.0],
+    ];
+    $docs = [];
+    foreach ($vecs as $pk => $v) {
+        $docs[] = (new ZVecDoc($pk))->setVectorFp32('vf', $v);
+    }
+    $c->insert(...$docs);
+    $c->flush();
+    $c->optimize();
+
+    $qv = [1.0, 0.0, 0.0, 0.0];
+
+    // Sanity: radius query filters as expected
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams()->setRadius(0.3);
+    echo 'radius 0.3:            ', pks($c->queryVector($q)), "\n";
+
+    // The leak (#200): after a radius query, refiner and plain queries on the
+    // same thread return only the radius-filtered subset.
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams()->setUsingRefiner(true);
+    echo 'refiner after radius:  ', pks($c->queryVector($q)), "\n";
+
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams();
+    echo 'plain after radius:    ', pks($c->queryVector($q)), "\n";
+
+    // resetRadiusThreshold() purges the stale threshold
+    $c->resetRadiusThreshold('vf', $qv);
+
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams()->setUsingRefiner(true);
+    echo 'refiner after reset:   ', pks($c->queryVector($q)), "\n";
+
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams();
+    echo 'plain after reset:     ', pks($c->queryVector($q)), "\n";
+
+    // Radius filtering still works after the reset
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams()->setRadius(0.3);
+    echo 'radius after reset:    ', pks($c->queryVector($q)), "\n";
+
+    // ... and the reset clears a fresh leak again
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams();
+    echo 'plain after 2nd leak:  ', pks($c->queryVector($q)), "\n";
+
+    $c->resetRadiusThreshold('vf', $qv);
+    $q = (new ZVecVectorQuery('vf', $qv))->setTopk(10)->setFlatParams();
+    echo 'plain after 2nd reset: ', pks($c->queryVector($q)), "\n";
+
+    echo "ALL TESTS PASSED\n";
+} finally {
+    if (isset($c)) { try { $c->destroy(); } catch (Exception $e) {} }
+    exec("rm -rf " . escapeshellarg($path));
+}
+?>
+--EXPECT--
+radius 0.3:            doc1,doc2
+refiner after radius:  doc1,doc2
+plain after radius:    doc1,doc2
+refiner after reset:   doc1,doc2,doc3
+plain after reset:     doc1,doc2,doc3
+radius after reset:    doc1,doc2
+plain after 2nd leak:  doc1,doc2
+plain after 2nd reset: doc1,doc2,doc3
+ALL TESTS PASSED