diff --git a/majit/majit-gc/src/collector.rs b/majit/majit-gc/src/collector.rs
index 1bac85a5ce4..8adbb5ab9b0 100644
--- a/majit/majit-gc/src/collector.rs
+++ b/majit/majit-gc/src/collector.rs
@@ -9,6 +9,7 @@
 use indexmap::IndexSet;
 use majit_ir::GcRef;
 use std::collections::VecDeque;
+use std::sync::RwLock;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 use crate::address_dict::AddressMap;
@@ -83,6 +84,52 @@ pub struct GcConfig {
     pub taggedpointers: bool,
 }
 
+/// The variables [`GcConfig`] and [`MiniMarkGC::with_config`] resolve against,
+/// for an embedder that has to hand its environment over rather than share it.
+///
+/// Published here so such a host does not keep its own copy of the list in step
+/// with the collector; `PYPY_GC_DEBUG` and the tracing knobs are absent because
+/// nothing in this file reads them.
+pub const GC_ENV_NAMES: &[&str] = &[
+    "PYPY_GC_NURSERY",
+    "PYPY_GC_INCREMENT_STEP",
+    "PYPY_GC_MAJOR_COLLECT",
+    "PYPY_GC_GROWTH",
+    "PYPY_GC_MIN",
+    "PYPY_GC_MAX",
+    "PYPY_GC_MAX_DELTA",
+];
+
+/// Environment an embedder supplies because the platform gives the process
+/// none. Read only where `std::env` misses, so a host that has a real
+/// environment resolves against it exactly as before.
+///
+/// `wasm32-unknown-unknown` is the case that needs it: `std::env::var` there
+/// always fails, so every name in [`GC_ENV_NAMES`] reads as unset and a guest
+/// runs the built-in defaults no matter what its host was configured with. The
+/// interpreter's launcher options have the same problem and the same answer
+/// (`pyre-wasm`'s `LAUNCH_ENV`).
+static SUPPLIED_ENV: RwLock<Vec<(String, String)>> = RwLock::new(Vec::new());
+
+/// Install the environment [`GC_ENV_NAMES`] resolves against when the process
+/// has none. Call before the first allocation: the values are read once, when
+/// the collector is built.
+pub fn set_supplied_env(entries: Vec<(String, String)>) {
+    *SUPPLIED_ENV.write().unwrap() = entries;
+}
+
+/// `std::env::var`, falling back to what the embedder supplied.
+fn env_var(varname: &str) -> Option<String> {
+    if let Ok(value) = std::env::var(varname) {
+        return Some(value);
+    }
+    let supplied = SUPPLIED_ENV.read().unwrap();
+    supplied
+        .iter()
+        .find(|(name, _)| name == varname)
+        .map(|(_, value)| value.clone())
+}
+
 /// env.py:17-36 `_read_float_and_factor_from_env`. Parse `varname` as a float
 /// with an optional `k`/`m`/`g` size suffix (optionally followed by `b`/`B`),
 /// returning `(value, factor)`. `None` mirrors PyPy's `(0.0, 0)` absent /
@@ -91,7 +138,7 @@ pub struct GcConfig {
 /// non-finite handling happens at the `int`/`r_uint` conversion sites, as
 /// upstream where `int(inf)`/`r_uint(inf)` raise.
 fn read_float_and_factor_from_env(varname: &str) -> Option<(f64, f64)> {
-    let raw = std::env::var(varname).ok()?;
+    let raw = env_var(varname)?;
     let mut value = raw.trim();
     if value.is_empty() {
         return None;
@@ -5338,6 +5385,29 @@ mod tests {
 
     static SHADOW_STACK_TEST_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
 
+    /// A supplied environment answers a name the process does not define, and
+    /// yields to one it does. The name is not in [`GC_ENV_NAMES`], so a
+    /// concurrently built collector cannot see this table.
+    #[test]
+    fn supplied_env_fills_in_only_what_the_process_lacks() {
+        let absent = "PYRE_TEST_SUPPLIED_ENV_ABSENT";
+        let present = "PYRE_TEST_SUPPLIED_ENV_PRESENT";
+        // SAFETY: single-threaded within this test; the names are unique to it.
+        unsafe { std::env::set_var(present, "2m") };
+
+        assert_eq!(read_uint_from_env(absent), None);
+        set_supplied_env(vec![
+            (absent.to_string(), "1m".to_string()),
+            (present.to_string(), "4m".to_string()),
+        ]);
+        assert_eq!(read_uint_from_env(absent), Some(1024 * 1024));
+        assert_eq!(read_uint_from_env(present), Some(2 * 1024 * 1024));
+
+        set_supplied_env(Vec::new());
+        assert_eq!(read_uint_from_env(absent), None);
+        unsafe { std::env::remove_var(present) };
+    }
+
     /// Helper: create a GC with a small nursery for testing.
     fn test_gc(nursery_size: usize) -> MiniMarkGC {
         MiniMarkGC::with_config(GcConfig {
diff --git a/pyre/bench/synth/arith_int_bool.wasm.jitstats b/pyre/bench/synth/arith_int_bool.wasm.jitstats
index 63ad9a4924b..36ef78d98f9 100644
--- a/pyre/bench/synth/arith_int_bool.wasm.jitstats
+++ b/pyre/bench/synth/arith_int_bool.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2214
+guard_failures=2211
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=7
diff --git a/pyre/bench/synth/binary_slice_index.wasm.jitstats b/pyre/bench/synth/binary_slice_index.wasm.jitstats
index 63ea4fe852c..9c16916b038 100644
--- a/pyre/bench/synth/binary_slice_index.wasm.jitstats
+++ b/pyre/bench/synth/binary_slice_index.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=4
+guard_failures=2
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=3
diff --git a/pyre/bench/synth/bool_dunder_error_no_leak.wasm.jitstats b/pyre/bench/synth/bool_dunder_error_no_leak.wasm.jitstats
index 26fead6b346..1cc731febcf 100644
--- a/pyre/bench/synth/bool_dunder_error_no_leak.wasm.jitstats
+++ b/pyre/bench/synth/bool_dunder_error_no_leak.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/build_set_hashability.wasm.jitstats b/pyre/bench/synth/build_set_hashability.wasm.jitstats
index 26fead6b346..1cc731febcf 100644
--- a/pyre/bench/synth/build_set_hashability.wasm.jitstats
+++ b/pyre/bench/synth/build_set_hashability.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/closure_per_call.wasm.jitstats b/pyre/bench/synth/closure_per_call.wasm.jitstats
index 04e0b5011b3..a8c5e65dc64 100644
--- a/pyre/bench/synth/closure_per_call.wasm.jitstats
+++ b/pyre/bench/synth/closure_per_call.wasm.jitstats
@@ -5,7 +5,7 @@ descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
 field_pos_attached_misplaced=0
 field_pos_spec_misplaced=0
-guard_failures=468
+guard_failures=420
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=4
diff --git a/pyre/bench/synth/dict_set.wasm.jitstats b/pyre/bench/synth/dict_set.wasm.jitstats
index 908b18b78d5..ab7acb61f13 100644
--- a/pyre/bench/synth/dict_set.wasm.jitstats
+++ b/pyre/bench/synth/dict_set.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=609
+guard_failures=606
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=5
diff --git a/pyre/bench/synth/divmod_long_int_pair.wasm.jitstats b/pyre/bench/synth/divmod_long_int_pair.wasm.jitstats
index ef51a080b1c..3787785f8c2 100644
--- a/pyre/bench/synth/divmod_long_int_pair.wasm.jitstats
+++ b/pyre/bench/synth/divmod_long_int_pair.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=9
+guard_failures=4
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=4
diff --git a/pyre/bench/synth/exc_caught_in_callee_return_loop.wasm.jitstats b/pyre/bench/synth/exc_caught_in_callee_return_loop.wasm.jitstats
index b58f2529486..f1958f4d488 100644
--- a/pyre/bench/synth/exc_caught_in_callee_return_loop.wasm.jitstats
+++ b/pyre/bench/synth/exc_caught_in_callee_return_loop.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=613
+guard_failures=611
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/exception_escape_hot_callee_tb_node_once.wasm.jitstats b/pyre/bench/synth/exception_escape_hot_callee_tb_node_once.wasm.jitstats
index 62e3f6d3e6a..bf7bfa5ac55 100644
--- a/pyre/bench/synth/exception_escape_hot_callee_tb_node_once.wasm.jitstats
+++ b/pyre/bench/synth/exception_escape_hot_callee_tb_node_once.wasm.jitstats
@@ -1,9 +1,9 @@
-bridges_compiled=5
+bridges_compiled=4
 descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=1016
+guard_failures=814
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=15
diff --git a/pyre/bench/synth/exception_inline_callee_tb_frames.wasm.jitstats b/pyre/bench/synth/exception_inline_callee_tb_frames.wasm.jitstats
index 8aeebdb89ea..385dea9c0be 100644
--- a/pyre/bench/synth/exception_inline_callee_tb_frames.wasm.jitstats
+++ b/pyre/bench/synth/exception_inline_callee_tb_frames.wasm.jitstats
@@ -1,9 +1,9 @@
-bridges_compiled=5
+bridges_compiled=4
 descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=1008
+guard_failures=807
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=6
diff --git a/pyre/bench/synth/exception_inlined_callee_caught.wasm.jitstats b/pyre/bench/synth/exception_inlined_callee_caught.wasm.jitstats
index 9dc67f88e2a..8beed56f050 100644
--- a/pyre/bench/synth/exception_inlined_callee_caught.wasm.jitstats
+++ b/pyre/bench/synth/exception_inlined_callee_caught.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=1
diff --git a/pyre/bench/synth/exception_oserror_fields.wasm.jitstats b/pyre/bench/synth/exception_oserror_fields.wasm.jitstats
index cbfba7e5e41..114b48b9fd0 100644
--- a/pyre/bench/synth/exception_oserror_fields.wasm.jitstats
+++ b/pyre/bench/synth/exception_oserror_fields.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=202
+guard_failures=201
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=1
diff --git a/pyre/bench/synth/exception_subclass_attrs.wasm.jitstats b/pyre/bench/synth/exception_subclass_attrs.wasm.jitstats
index 6575161c7ec..8beed56f050 100644
--- a/pyre/bench/synth/exception_subclass_attrs.wasm.jitstats
+++ b/pyre/bench/synth/exception_subclass_attrs.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=3
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=1
diff --git a/pyre/bench/synth/exception_traceback_frame_lineno.wasm.jitstats b/pyre/bench/synth/exception_traceback_frame_lineno.wasm.jitstats
index 95b7ea20405..6a29fca26ca 100644
--- a/pyre/bench/synth/exception_traceback_frame_lineno.wasm.jitstats
+++ b/pyre/bench/synth/exception_traceback_frame_lineno.wasm.jitstats
@@ -5,7 +5,7 @@ descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
 field_pos_attached_misplaced=0
 field_pos_spec_misplaced=0
-guard_failures=819
+guard_failures=817
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=17
diff --git a/pyre/bench/synth/exception_traceback_loop_forms.wasm.jitstats b/pyre/bench/synth/exception_traceback_loop_forms.wasm.jitstats
index 742556ba522..197a4ce61e6 100644
--- a/pyre/bench/synth/exception_traceback_loop_forms.wasm.jitstats
+++ b/pyre/bench/synth/exception_traceback_loop_forms.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=822
+guard_failures=811
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=8
diff --git a/pyre/bench/synth/exception_value_op_caught.wasm.jitstats b/pyre/bench/synth/exception_value_op_caught.wasm.jitstats
index 9dc67f88e2a..8beed56f050 100644
--- a/pyre/bench/synth/exception_value_op_caught.wasm.jitstats
+++ b/pyre/bench/synth/exception_value_op_caught.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=1
diff --git a/pyre/bench/synth/float_div_zero_caught_loop.wasm.jitstats b/pyre/bench/synth/float_div_zero_caught_loop.wasm.jitstats
index b58f2529486..f1958f4d488 100644
--- a/pyre/bench/synth/float_div_zero_caught_loop.wasm.jitstats
+++ b/pyre/bench/synth/float_div_zero_caught_loop.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=613
+guard_failures=611
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/foriter_body_return.wasm.jitstats b/pyre/bench/synth/foriter_body_return.wasm.jitstats
index 4c0ca1a180a..99a92443498 100644
--- a/pyre/bench/synth/foriter_body_return.wasm.jitstats
+++ b/pyre/bench/synth/foriter_body_return.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=612
+guard_failures=604
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=3
diff --git a/pyre/bench/synth/foriter_exempt_nested_foriter.wasm.jitstats b/pyre/bench/synth/foriter_exempt_nested_foriter.wasm.jitstats
index 4f25297e805..8f9aaeb4a9c 100644
--- a/pyre/bench/synth/foriter_exempt_nested_foriter.wasm.jitstats
+++ b/pyre/bench/synth/foriter_exempt_nested_foriter.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=1
 loops_compiled=3
diff --git a/pyre/bench/synth/foriter_exempt_shared_generator.wasm.jitstats b/pyre/bench/synth/foriter_exempt_shared_generator.wasm.jitstats
index 4f25297e805..8f9aaeb4a9c 100644
--- a/pyre/bench/synth/foriter_exempt_shared_generator.wasm.jitstats
+++ b/pyre/bench/synth/foriter_exempt_shared_generator.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=1
 loops_compiled=3
diff --git a/pyre/bench/synth/gc_bug_bridge_flavor_traceback_names.wasm.jitstats b/pyre/bench/synth/gc_bug_bridge_flavor_traceback_names.wasm.jitstats
index c229b7a7832..9c9e7bc0d9c 100644
--- a/pyre/bench/synth/gc_bug_bridge_flavor_traceback_names.wasm.jitstats
+++ b/pyre/bench/synth/gc_bug_bridge_flavor_traceback_names.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2037
+guard_failures=2027
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=4
diff --git a/pyre/bench/synth/gc_deque_backing_list.wasm.jitstats b/pyre/bench/synth/gc_deque_backing_list.wasm.jitstats
index c2c345bc6ae..6ba592f1135 100644
--- a/pyre/bench/synth/gc_deque_backing_list.wasm.jitstats
+++ b/pyre/bench/synth/gc_deque_backing_list.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=204
+guard_failures=203
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=5
diff --git a/pyre/bench/synth/gc_iterator_source_drop.wasm.jitstats b/pyre/bench/synth/gc_iterator_source_drop.wasm.jitstats
index 6e3b5767b12..40f0d87bd34 100644
--- a/pyre/bench/synth/gc_iterator_source_drop.wasm.jitstats
+++ b/pyre/bench/synth/gc_iterator_source_drop.wasm.jitstats
@@ -5,7 +5,7 @@ descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
 field_pos_attached_misplaced=0
 field_pos_spec_misplaced=0
-guard_failures=614
+guard_failures=613
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=4
diff --git a/pyre/bench/synth/getattribute_override_no_bind.wasm.jitstats b/pyre/bench/synth/getattribute_override_no_bind.wasm.jitstats
index 7d39b749f20..26fead6b346 100644
--- a/pyre/bench/synth/getattribute_override_no_bind.wasm.jitstats
+++ b/pyre/bench/synth/getattribute_override_no_bind.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=14
+guard_failures=2
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/global_cell_shortpreamble_hot.wasm.jitstats b/pyre/bench/synth/global_cell_shortpreamble_hot.wasm.jitstats
index 2b9c55c75d0..205058aeee4 100644
--- a/pyre/bench/synth/global_cell_shortpreamble_hot.wasm.jitstats
+++ b/pyre/bench/synth/global_cell_shortpreamble_hot.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=409
+guard_failures=408
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/inline_gate_operand_provenance.wasm.jitstats b/pyre/bench/synth/inline_gate_operand_provenance.wasm.jitstats
index 18b28dd8f70..b4fa09e329c 100644
--- a/pyre/bench/synth/inline_gate_operand_provenance.wasm.jitstats
+++ b/pyre/bench/synth/inline_gate_operand_provenance.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=10
+guard_failures=4
 internal_compile_panics=0
 loops_aborted=3
 loops_compiled=7
diff --git a/pyre/bench/synth/inline_subwalk_mutating_residual.wasm.jitstats b/pyre/bench/synth/inline_subwalk_mutating_residual.wasm.jitstats
index a2529fd23d5..26207624a12 100644
--- a/pyre/bench/synth/inline_subwalk_mutating_residual.wasm.jitstats
+++ b/pyre/bench/synth/inline_subwalk_mutating_residual.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=815
+guard_failures=813
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=4
diff --git a/pyre/bench/synth/inline_subwalk_property_mutates.wasm.jitstats b/pyre/bench/synth/inline_subwalk_property_mutates.wasm.jitstats
index b58f2529486..f1958f4d488 100644
--- a/pyre/bench/synth/inline_subwalk_property_mutates.wasm.jitstats
+++ b/pyre/bench/synth/inline_subwalk_property_mutates.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=613
+guard_failures=611
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/inline_subwalk_user_iterator.wasm.jitstats b/pyre/bench/synth/inline_subwalk_user_iterator.wasm.jitstats
index 4f05176c78a..93e39b6768e 100644
--- a/pyre/bench/synth/inline_subwalk_user_iterator.wasm.jitstats
+++ b/pyre/bench/synth/inline_subwalk_user_iterator.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=402
+guard_failures=401
 internal_compile_panics=0
 loops_aborted=1
 loops_compiled=3
diff --git a/pyre/bench/synth/kept_stack_deep_var_shortcircuit.wasm.jitstats b/pyre/bench/synth/kept_stack_deep_var_shortcircuit.wasm.jitstats
index 2321d60dcb6..d68b413d6f3 100644
--- a/pyre/bench/synth/kept_stack_deep_var_shortcircuit.wasm.jitstats
+++ b/pyre/bench/synth/kept_stack_deep_var_shortcircuit.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=824
+guard_failures=819
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=6
diff --git a/pyre/bench/synth/list_ops.wasm.jitstats b/pyre/bench/synth/list_ops.wasm.jitstats
index 349d754e74b..b6327714294 100644
--- a/pyre/bench/synth/list_ops.wasm.jitstats
+++ b/pyre/bench/synth/list_ops.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=408
+guard_failures=407
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=7
diff --git a/pyre/bench/synth/loops_comprehension.wasm.jitstats b/pyre/bench/synth/loops_comprehension.wasm.jitstats
index 928aaec299b..719c9a944ce 100644
--- a/pyre/bench/synth/loops_comprehension.wasm.jitstats
+++ b/pyre/bench/synth/loops_comprehension.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=2613
+guard_failures=2611
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=7
diff --git a/pyre/bench/synth/match_sequence_of_class_patterns.wasm.jitstats b/pyre/bench/synth/match_sequence_of_class_patterns.wasm.jitstats
index 454ae31ec45..f4509ffc3bb 100644
--- a/pyre/bench/synth/match_sequence_of_class_patterns.wasm.jitstats
+++ b/pyre/bench/synth/match_sequence_of_class_patterns.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=402
+guard_failures=401
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/math_isqrt_compare_bridge_resume.wasm.jitstats b/pyre/bench/synth/math_isqrt_compare_bridge_resume.wasm.jitstats
index d0ac585f531..e2698431419 100644
--- a/pyre/bench/synth/math_isqrt_compare_bridge_resume.wasm.jitstats
+++ b/pyre/bench/synth/math_isqrt_compare_bridge_resume.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=409
+guard_failures=402
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=4
diff --git a/pyre/bench/synth/mutate_then_raise_caught.wasm.jitstats b/pyre/bench/synth/mutate_then_raise_caught.wasm.jitstats
index fc2889ef19d..f1958f4d488 100644
--- a/pyre/bench/synth/mutate_then_raise_caught.wasm.jitstats
+++ b/pyre/bench/synth/mutate_then_raise_caught.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=612
+guard_failures=611
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/newslice_step_hot.wasm.jitstats b/pyre/bench/synth/newslice_step_hot.wasm.jitstats
index 023ac9e6f5e..e1158a4643b 100644
--- a/pyre/bench/synth/newslice_step_hot.wasm.jitstats
+++ b/pyre/bench/synth/newslice_step_hot.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=24
+guard_failures=4
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/recursion_memo_branch.wasm.jitstats b/pyre/bench/synth/recursion_memo_branch.wasm.jitstats
index 4600bdb39c2..2fc0df5d183 100644
--- a/pyre/bench/synth/recursion_memo_branch.wasm.jitstats
+++ b/pyre/bench/synth/recursion_memo_branch.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=4731
+guard_failures=4724
 internal_compile_panics=0
 loops_aborted=2
 loops_compiled=3
diff --git a/pyre/bench/synth/recursive_call_frame_relocation.wasm.jitstats b/pyre/bench/synth/recursive_call_frame_relocation.wasm.jitstats
index 4bc1b8e3595..54792ece17e 100644
--- a/pyre/bench/synth/recursive_call_frame_relocation.wasm.jitstats
+++ b/pyre/bench/synth/recursive_call_frame_relocation.wasm.jitstats
@@ -5,7 +5,7 @@ descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
 field_pos_attached_misplaced=0
 field_pos_spec_misplaced=0
-guard_failures=648
+guard_failures=638
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=3
diff --git a/pyre/bench/synth/sre_pattern_methods.wasm.jitstats b/pyre/bench/synth/sre_pattern_methods.wasm.jitstats
index bedde7b54df..235d297fa8f 100644
--- a/pyre/bench/synth/sre_pattern_methods.wasm.jitstats
+++ b/pyre/bench/synth/sre_pattern_methods.wasm.jitstats
@@ -5,7 +5,7 @@ descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
 field_pos_attached_misplaced=0
 field_pos_spec_misplaced=0
-guard_failures=1013
+guard_failures=1012
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=7
diff --git a/pyre/bench/synth/str_fstring.wasm.jitstats b/pyre/bench/synth/str_fstring.wasm.jitstats
index 3fb932f4dc9..046924bea32 100644
--- a/pyre/bench/synth/str_fstring.wasm.jitstats
+++ b/pyre/bench/synth/str_fstring.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=666
+guard_failures=658
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=6
diff --git a/pyre/bench/synth/type_name_setter.wasm.jitstats b/pyre/bench/synth/type_name_setter.wasm.jitstats
index 6575161c7ec..8beed56f050 100644
--- a/pyre/bench/synth/type_name_setter.wasm.jitstats
+++ b/pyre/bench/synth/type_name_setter.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=3
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=1
diff --git a/pyre/bench/synth/type_name_surrogate_reject.wasm.jitstats b/pyre/bench/synth/type_name_surrogate_reject.wasm.jitstats
index b6d47c17adc..283898ec6a3 100644
--- a/pyre/bench/synth/type_name_surrogate_reject.wasm.jitstats
+++ b/pyre/bench/synth/type_name_surrogate_reject.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=202
+guard_failures=201
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/unary_negative.wasm.jitstats b/pyre/bench/synth/unary_negative.wasm.jitstats
index f19add84d62..26fead6b346 100644
--- a/pyre/bench/synth/unary_negative.wasm.jitstats
+++ b/pyre/bench/synth/unary_negative.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=5
+guard_failures=2
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=2
diff --git a/pyre/bench/synth/unary_positive_resume.wasm.jitstats b/pyre/bench/synth/unary_positive_resume.wasm.jitstats
index 6575161c7ec..8beed56f050 100644
--- a/pyre/bench/synth/unary_positive_resume.wasm.jitstats
+++ b/pyre/bench/synth/unary_positive_resume.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=3
+guard_failures=1
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=1
diff --git a/pyre/bench/synth/unpack_ex_hot.wasm.jitstats b/pyre/bench/synth/unpack_ex_hot.wasm.jitstats
index 40f1f7e21cb..9dc67f88e2a 100644
--- a/pyre/bench/synth/unpack_ex_hot.wasm.jitstats
+++ b/pyre/bench/synth/unpack_ex_hot.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=15
+guard_failures=2
 internal_compile_panics=0
 loops_aborted=0
 loops_compiled=1
diff --git a/pyre/bench/synth/wasm_ca_trampoline_decline.wasm.jitstats b/pyre/bench/synth/wasm_ca_trampoline_decline.wasm.jitstats
index f98f2ca9404..bf08a9bf682 100644
--- a/pyre/bench/synth/wasm_ca_trampoline_decline.wasm.jitstats
+++ b/pyre/bench/synth/wasm_ca_trampoline_decline.wasm.jitstats
@@ -3,7 +3,7 @@ descr_set_absent=0
 descr_set_ambiguous=0
 descr_set_stale_absent=0
 fbw_rolled_back_with_effects=0
-guard_failures=420
+guard_failures=404
 internal_compile_panics=0
 loops_aborted=1
 loops_compiled=2
diff --git a/pyre/check.py b/pyre/check.py
index b1b9bfdafc0..f16a2197188 100644
--- a/pyre/check.py
+++ b/pyre/check.py
@@ -465,6 +465,11 @@ def pyre_env():
     # (0.125 * total memory) enters only as an upper bound at collector.rs:3570
     # and the `min_heap_size` floor is applied after it (:2452), so the floor
     # wins on every host.
+    #
+    # Both pins reach the wasm backend only because `pyre-wasm-runner` hands
+    # them to the guest explicitly (`pyre_set_gc_env`): a wasm32-unknown-unknown
+    # module's `std::env` is permanently empty, so setting them here does
+    # nothing on its own there.
     env.setdefault("PYPY_GC_MIN", str(256 * 1024 * 1024))
     # Keep the bench directory off `sys.path` (`-P`), so the jit-stats counters
     # describe the fixture rather than the directory it happens to sit in.
diff --git a/pyre/extra_tests/parity_tests/memoryio_seek_whence_range.py b/pyre/extra_tests/parity_tests/memoryio_seek_whence_range.py
new file mode 100644
index 00000000000..b717ac086a0
--- /dev/null
+++ b/pyre/extra_tests/parity_tests/memoryio_seek_whence_range.py
@@ -0,0 +1,75 @@
+"""The memory streams take `seek`'s whence as a C int, its position wider."""
+
+import io
+
+
+def raises(exc_type, operation):
+    try:
+        result = operation()
+    except exc_type:
+        return
+    except BaseException as exc:
+        raise AssertionError(f"expected {exc_type.__name__}, got {exc!r}") from exc
+    raise AssertionError(f"expected {exc_type.__name__}, got {result!r}")
+
+
+class Index:
+    """Only `__index__` — what the whence is read through."""
+
+    def __init__(self, value):
+        self.value = value
+
+    def __index__(self):
+        return self.value
+
+
+class Both:
+    """Both dunders, disagreeing, so which one is consulted is observable."""
+
+    def __index__(self):
+        return 0  # SEEK_SET
+
+    def __int__(self):
+        return 2  # SEEK_END
+
+
+class IntOnly:
+    def __int__(self):
+        return 0
+
+
+for factory, data in ((io.BytesIO, b"abcdefgh"), (io.StringIO, "abcdefgh")):
+    size = len(data)
+    stream = factory(data)
+
+    # In range: rejected for what it is, not for how wide it is. The message is
+    # the converter's on one side and the range check's on the other, so only
+    # the type is compared.
+    raises(ValueError, lambda: stream.seek(0, 3))
+    raises(ValueError, lambda: stream.seek(0, -1))
+
+    # Out of a C int's range. A whence of 2**32 narrowed to a C int instead
+    # would truncate to 0 and seek to the start; taken as a machine int it
+    # reaches the range check and comes back as the wrong exception type.
+    for whence in (2**32, -(2**32), 2**63, -(2**63) - 1):
+        raises(OverflowError, lambda whence=whence: stream.seek(0, whence))
+
+    # The whence is taken through the index protocol. Asserted by value against
+    # a non-empty buffer, so SEEK_SET and SEEK_END are distinguishable: reading
+    # `Both` through `__int__` would answer `size` here rather than 0.
+    assert stream.seek(0, Index(0)) == 0
+    assert stream.seek(0, Index(2)) == size
+    assert stream.seek(0, Both()) == 0
+    raises(OverflowError, lambda: stream.seek(0, Index(2**32)))
+    raises(ValueError, lambda: stream.seek(0, Index(3)))
+
+    # `__int__` alone is not the index protocol, so it is not a whence.
+    raises(TypeError, lambda: stream.seek(0, IntOnly()))
+    raises(TypeError, lambda: stream.seek(0, "0"))
+    raises(TypeError, lambda: stream.seek(0, 0.0))
+
+    # The position is a C ssize_t, so one a C int could not hold is a position.
+    assert stream.seek(2**32) == 2**32
+    raises(OverflowError, lambda: stream.seek(2**63))
+
+print("OK")
diff --git a/pyre/extra_tests/parity_tests/run.py b/pyre/extra_tests/parity_tests/run.py
index 50d89156389..64664abf0db 100644
--- a/pyre/extra_tests/parity_tests/run.py
+++ b/pyre/extra_tests/parity_tests/run.py
@@ -18,6 +18,15 @@
 applies to in its header (`# pyre-check: platforms=linux,darwin`) and is
 skipped elsewhere.
 
+A script whose defect is only reachable under a particular runtime
+configuration declares it with one or more
+
+    # parity-env: NAME=VALUE
+
+lines, which are added to the environment of every runner for that script
+only. Without this a script can keep passing after the shape it exercises
+stops being reachable, i.e. cover nothing while still looking green.
+
 Usage:
     python3 pyre/extra_tests/parity_tests/run.py
         [--dynasm-only|--cranelift-only] [--gc-poison]
@@ -34,6 +43,7 @@
 
 import argparse
 import os
+import re
 import subprocess
 import sys
 from pathlib import Path
@@ -82,6 +92,15 @@ def _scripts() -> tuple[list[Path], list[Path]]:
     return out, skipped
 
 
+_ENV_DIRECTIVE = re.compile(r"^#\s*parity-env:\s*(\w+)=(\S*)\s*$", re.MULTILINE)
+
+
+def _script_env(script: Path) -> dict[str, str]:
+    """Environment a script pins for itself with `# parity-env: NAME=VALUE`."""
+    text = script.read_text(encoding="utf-8", errors="replace")
+    return {m.group(1): m.group(2) for m in _ENV_DIRECTIVE.finditer(text)}
+
+
 def _run(cmd: list[str], script: Path, env: dict[str, str] | None) -> tuple[bool, str]:
     try:
         proc = subprocess.run(
@@ -151,9 +170,11 @@ def main() -> int:
     fail = 0
     for script in scripts:
         name = script.name
+        pinned = _script_env(script)
         row: list[str] = [f"  {name:<36s}"]
         for backend, cmd, env in runners:
-            ok, detail = _run(cmd, script, env)
+            merged = {**(env or {}), **pinned}
+            ok, detail = _run(cmd, script, merged or None)
             mark = "OK" if ok else "FAIL"
             row.append(f"{backend}={mark}")
             if not ok:
diff --git a/pyre/extra_tests/parity_tests/thread_start_walk_abort_no_replay.py b/pyre/extra_tests/parity_tests/thread_start_walk_abort_no_replay.py
index 0c9dd5ad4ae..f6a3a27ab30 100644
--- a/pyre/extra_tests/parity_tests/thread_start_walk_abort_no_replay.py
+++ b/pyre/extra_tests/parity_tests/thread_start_walk_abort_no_replay.py
@@ -29,8 +29,18 @@
 
 Every acquire on the main thread is bounded so a lost release reports instead of
 hanging, and `die` uses `os._exit` so a failure cannot block on shutdown.
+
+⚠️ The `parity-env` line below is what keeps this file covering anything.  All
+three kept-stack decline hazards are scoped to `!ctx.vstack_valid`, so once the
+callee operand-stack mirror became the default an inline sub-walk describes its
+own stack and the aborting guard is never reached.  Measured on a binary WITHOUT
+the fix: 4/4 pass at the default setting, 4/4 fail with the mirror off (the same
+deterministic round 97 either way).  Drop the pin and this test goes green
+against the very defect it exists to catch.
 """
 
+# parity-env: PYRE_FBW_CALLEE_VSTACK=0
+
 import contextvars
 import os
 import threading
diff --git a/pyre/pyre-interpreter/src/baseobjspace.rs b/pyre/pyre-interpreter/src/baseobjspace.rs
index bf455c379ab..e9997af60c6 100644
--- a/pyre/pyre-interpreter/src/baseobjspace.rs
+++ b/pyre/pyre-interpreter/src/baseobjspace.rs
@@ -13017,6 +13017,19 @@ pub fn index_int_w_preserve_negative(obj: PyObjectRef) -> Result<i64, PyError> {
     }
 }
 
+/// The index-protocol counterpart of [`c_int_w`], for a 3.14 Argument Clinic
+/// `int` parameter: the value arrives through `__index__` alone, and one
+/// outside a C int's range is an OverflowError rather than a truncation.
+///
+/// [`c_int_w`] cannot serve here because `gateway_int_w` is `int_w`
+/// (baseobjspace.py:2043), which converts through `__int__` first — so an
+/// object carrying both dunders would be read from the wrong one, and one
+/// carrying only `__int__` would be accepted where 3.14 raises TypeError.
+pub fn index_c_int_w(obj: PyObjectRef) -> Result<i32, PyError> {
+    let value = int_w(space_index(obj)?)?;
+    i32::try_from(value).map_err(|_| PyError::overflow_error("expected a 32-bit integer"))
+}
+
 /// `objspace.honor__builtins__` default is False — the frame builtin is
 /// `space.builtin`, ignoring a custom `__builtins__` in globals.  The
 /// `pick_builtin*` family below is the `honor__builtins__=True` path,
diff --git a/pyre/pyre-interpreter/src/call.rs b/pyre/pyre-interpreter/src/call.rs
index 7490d526f6c..92a520adad7 100644
--- a/pyre/pyre-interpreter/src/call.rs
+++ b/pyre/pyre-interpreter/src/call.rs
@@ -3649,25 +3649,18 @@ fn call_metaclass_with_kwargs(
     kwargs: PyObjectRef,
 ) -> PyObjectRef {
     if unsafe { !pyre_object::is_type(w_metaclass) } {
-        // compiling.py:213-219 — `space.call_args(w_meta, Arguments(name,
+        // compiling.py:215-221 — `space.call_args(w_meta, Arguments(name,
         // bases, ns, **kwds))`; a non-type metaclass receives the
-        // class-definition keywords too.
+        // class-definition keywords too, and `call_args`
+        // (descroperation.py:189) takes no frame.
         let kwds: Vec<(Wtf8Buf, PyObjectRef)> = if unsafe { pyre_object::is_dict(kwargs) } {
             unsafe { pyre_object::w_dict_str_entries_wtf8(kwargs) }
         } else {
             Vec::new()
         };
-        let frame = {
-            let stored = take_last_exec_ctx();
-            if stored.is_null() {
-                std::ptr::null_mut()
-            } else {
-                unsafe { (*stored).gettopframe_raw() }
-            }
-        };
-        if !kwds.is_empty() && !frame.is_null() {
-            return match call_with_kwargs(
-                unsafe { &mut *frame },
+        if !kwds.is_empty() {
+            return match call_with_kwargs_in_ctx(
+                take_last_exec_ctx(),
                 w_metaclass,
                 &[name, bases, w_namespace_dict],
                 &kwds,
@@ -4075,26 +4068,18 @@ fn build_class_inner(
         // namespace.
         match crate::baseobjspace::getattr_str(w_metaclass, "__prepare__") {
             Ok(prepare) => {
-                // compiling.py:190-196 — call __prepare__ with the
+                // compiling.py:194-199 — call __prepare__ with the
                 // class-definition keywords ('metaclass' already popped by
-                // the caller).
+                // the caller), through the frameless `space.call_args`.
                 let prepare_kwds: Vec<(Wtf8Buf, PyObjectRef)> = match current_kwds() {
                     Some(kw) if unsafe { pyre_object::is_dict(kw) } => unsafe {
                         pyre_object::w_dict_str_entries_wtf8(kw)
                     },
                     _ => Vec::new(),
                 };
-                let prepare_frame = {
-                    let stored = take_last_exec_ctx();
-                    if stored.is_null() {
-                        std::ptr::null_mut()
-                    } else {
-                        unsafe { (*stored).gettopframe_raw() }
-                    }
-                };
-                let ns_obj = if !prepare_kwds.is_empty() && !prepare_frame.is_null() {
-                    call_with_kwargs(
-                        unsafe { &mut *prepare_frame },
+                let ns_obj = if !prepare_kwds.is_empty() {
+                    call_with_kwargs_in_ctx(
+                        take_last_exec_ctx(),
                         prepare,
                         &[pyre_object::w_str_new(name), bases],
                         &prepare_kwds,
@@ -4865,37 +4850,15 @@ pub(crate) fn call_init_subclass_on_bases(
     let w_objtype = crate::builtins::super_check(w_type, w_type)?;
     let w_super = pyre_object::descriptor::w_super_new(w_type, w_objtype, w_type);
     let w_func = crate::baseobjspace::getattr_str(w_super, "__init_subclass__")?;
-    // `__args__.replace_arguments([])` — keywords only, no positionals.
+    // typeobject.py:1025-1026 — `args = __args__.replace_arguments([])` then
+    // `space.call_args(w_func, args)`: keywords only, no positionals, and no
+    // frame, because `call_args` (descroperation.py:189) never takes one.
     let kwds: Vec<(Wtf8Buf, PyObjectRef)> = init_subclass_kwargs
         .iter()
         .filter(|(k, _)| unsafe { pyre_object::is_str(*k) })
         .map(|(k, v)| (unsafe { pyre_object::w_str_get_wtf8(*k) }.to_owned(), *v))
         .collect();
-    let frame = {
-        let stored = take_last_exec_ctx();
-        if stored.is_null() {
-            std::ptr::null_mut()
-        } else {
-            unsafe { (*stored).gettopframe_raw() }
-        }
-    };
-    if !frame.is_null() {
-        call_with_kwargs(unsafe { &mut *frame }, w_func, &[], &kwds)?;
-    } else if kwds.is_empty() {
-        // No live frame to thread through call_with_kwargs (direct
-        // embedding entry); the bound method carries the receiver.
-        clear_call_error();
-        let res = crate::call_function(w_func, &[]);
-        if res.is_null() {
-            if let Some(err) = take_call_error() {
-                return Err(err);
-            }
-        }
-    } else {
-        return Err(crate::PyError::type_error(
-            "__init_subclass__() takes no keyword arguments",
-        ));
-    }
+    call_with_kwargs_in_ctx(take_last_exec_ctx(), w_func, &[], &kwds)?;
     Ok(())
 }
 
diff --git a/pyre/pyre-interpreter/src/module/_io/bytesio.rs b/pyre/pyre-interpreter/src/module/_io/bytesio.rs
index 48b5eeb9869..940625a0549 100644
--- a/pyre/pyre-interpreter/src/module/_io/bytesio.rs
+++ b/pyre/pyre-interpreter/src/module/_io/bytesio.rs
@@ -389,9 +389,14 @@ impl W_BytesIO {
     fn seek(
         &mut self,
         pos: PyIndexInt,
-        #[default(0)] whence: PyIndexInt,
+        #[default(0)] whence: PyIndexCInt,
     ) -> Result<i64, crate::PyError> {
         // interp_bytesio.py:162-180 validation followed by RStringIO.seek.
+        // `@unwrap_spec(pos=r_longlong, whence=int)` (:162): the position is a
+        // long long but the whence is a C int, so one that does not fit is an
+        // OverflowError from the converter rather than a value the arms below
+        // ever see. Both come through `__index__` alone — 3.14 rejects a whence
+        // carrying only `__int__`, and reads one carrying both from `__index__`.
         self.check_closed()?;
         match whence {
             0 if pos < 0 => {
@@ -416,7 +421,7 @@ impl W_BytesIO {
                 )));
             }
         }
-        self.seek_pos(pos, whence);
+        self.seek_pos(pos, whence as i64);
         Ok(self.tell_pos())
     }
 
diff --git a/pyre/pyre-interpreter/src/module/_io/stringio.rs b/pyre/pyre-interpreter/src/module/_io/stringio.rs
index fbfbfcaa0c4..0f3064407af 100644
--- a/pyre/pyre-interpreter/src/module/_io/stringio.rs
+++ b/pyre/pyre-interpreter/src/module/_io/stringio.rs
@@ -387,8 +387,13 @@ impl W_StringIO {
         self.check_closed()?;
         let _roots = pyre_object::gc_roots::push_roots();
         let slot = self.pin_self();
+        // `@unwrap_spec(pos=int, mode=int)` (:403). The whence is a C int, so
+        // one that does not fit is an OverflowError from the converter rather
+        // than a value the range check below ever sees. The position is not:
+        // 3.14 takes it as a `Py_ssize_t`, and `seek(2**32)` is a position it
+        // accepts. Both stay on the index protocol.
         let pos = crate::baseobjspace::index_int_w_preserve_negative(w_pos)?;
-        let whence = crate::baseobjspace::index_int_w_preserve_negative(w_whence)?;
+        let whence = crate::baseobjspace::index_c_int_w(w_whence)?;
         let this = Self::from_slot(slot);
         this.check_closed()?;
         if !(0..=2).contains(&whence) {
diff --git a/pyre/pyre-jit/src/lib.rs b/pyre/pyre-jit/src/lib.rs
index 888a679272d..4d49d53519b 100644
--- a/pyre/pyre-jit/src/lib.rs
+++ b/pyre/pyre-jit/src/lib.rs
@@ -67,6 +67,12 @@ pub fn fbw_diag_counter(i: usize) -> u64 {
     pyre_jit_trace::trace::fbw_diag::get(i)
 }
 
+/// The `PYPY_GC_*` variables the collector sizes itself from, and the setter
+/// for an embedder whose platform hands the process no environment to read them
+/// out of. Re-exported so such a host reaches them through the crate it already
+/// builds the interpreter with.
+pub use majit_gc::collector::{GC_ENV_NAMES, set_supplied_env as set_gc_supplied_env};
+
 /// Diagnostic only: `(oldgen_total_bytes, nursery_used_bytes)` of the wasm
 /// backend's GC on this thread. Lets the wasm runner attribute guest
 /// linear-memory growth to GC-retained objects vs. host-heap allocations.
diff --git a/pyre/pyre-macros/src/lib.rs b/pyre/pyre-macros/src/lib.rs
index c2ee30fcbc5..07afdd187b0 100644
--- a/pyre/pyre-macros/src/lib.rs
+++ b/pyre/pyre-macros/src/lib.rs
@@ -538,6 +538,14 @@ fn typed_alias(
                 crate::baseobjspace::index_int_w_preserve_negative(args[#idx])?
             },
         ),
+        "PyIndexCInt" => (
+            // The same parameters narrowed to a C int, as an `int` parameter
+            // whose callee compares against a small fixed set is.  `PyCInt`
+            // does not serve: its converter reaches the value through
+            // `__int__` first, which the index protocol does not.
+            quote! { i32 },
+            quote! { crate::baseobjspace::index_c_int_w(args[#idx])? },
+        ),
         // Integer aliases — route through the `space.gateway_nonnegint_w`
         // / `space.c_*_w` converters in baseobjspace.rs so the range / sign
         // checks and their exception messages live in one place.
diff --git a/pyre/pyre-wasm-runner/src/main.rs b/pyre/pyre-wasm-runner/src/main.rs
index 6602f7f3dad..b06eb5e5c2c 100644
--- a/pyre/pyre-wasm-runner/src/main.rs
+++ b/pyre/pyre-wasm-runner/src/main.rs
@@ -450,6 +450,50 @@ fn run(module_path: &PathBuf, source: &str, script: &Path) -> Result<i32> {
         }
     }
 
+    // The environment the collector sizes itself from, forwarded the same way
+    // and for the same reason. It is not a diagnostic knob: `PYPY_GC_MIN` and
+    // `PYPY_GC_NURSERY` fix where the major-collection threshold falls, the
+    // major step arms the eval-breaker word, and every compiled loop's back edge
+    // polls that word through a real guard — so a guest that cannot read them
+    // counts a different number of guard failures than the native backends run
+    // beside it, from the same settings. Absent on a module predating the
+    // export, which then keeps its built-in defaults.
+    let gc_env_names = instance
+        .get_typed_func::<(), u64>(&mut store, "pyre_gc_env_names")
+        .ok();
+    let set_gc_env = instance
+        .get_typed_func::<(u32, u32), ()>(&mut store, "pyre_set_gc_env")
+        .ok();
+    if let (Some(names), Some(set_gc_env)) = (gc_env_names, set_gc_env) {
+        let packed = names.call(&mut store, ())?;
+        let (nptr, nlen) = ((packed >> 32) as u32, packed as u32);
+        let mut buf = vec![0u8; nlen as usize];
+        memory.read(&store, nptr as usize, &mut buf)?;
+        dealloc.call(&mut store, (nptr, nlen))?;
+
+        // `env::var`, not `var_os`: the guest parses these as numbers, so a
+        // value that does not decode could not have been one and is left unset
+        // exactly as it would be natively.
+        let blob = String::from_utf8_lossy(&buf)
+            .split('\0')
+            .filter(|name| !name.is_empty())
+            .filter_map(|name| {
+                std::env::var(name)
+                    .ok()
+                    .map(|value| format!("{name}={value}"))
+            })
+            .collect::<Vec<_>>()
+            .join("\0");
+
+        let blen = blob.len() as u32;
+        if blen != 0 {
+            let p = alloc.call(&mut store, blen)?;
+            memory.write(&mut store, p as usize, blob.as_bytes())?;
+            set_gc_env.call(&mut store, (p, blen))?;
+            dealloc.call(&mut store, (p, blen))?;
+        }
+    }
+
     // Name the script so the guest compiles it under its real path: that is
     // what a traceback prints, what its source-line lookup reads back through
     // `pyre_host.host_read`, and the directory that heads `sys.path`. Absent
diff --git a/pyre/pyre-wasm/src/lib.rs b/pyre/pyre-wasm/src/lib.rs
index 810421f137e..fa1c9bf1a64 100644
--- a/pyre/pyre-wasm/src/lib.rs
+++ b/pyre/pyre-wasm/src/lib.rs
@@ -793,9 +793,9 @@ pub fn run_python(source: &str) -> String {
 ///      and `pyre_exit_code()` → the status to exit with.
 ///
 /// `pyre_set_script_path(ptr, len)` may precede step 2 to name the file the
-/// source came from, and `pyre_set_launch_env(ptr, len)` to supply the
-/// environment the launcher options resolve against — the guest has none of
-/// its own.
+/// source came from, `pyre_set_launch_env(ptr, len)` to supply the environment
+/// the launcher options resolve against, and `pyre_set_gc_env(ptr, len)` the
+/// one the collector sizes itself from — the guest has none of its own.
 #[cfg(feature = "wasm-host")]
 mod host_abi {
     use super::run_python_impl;
@@ -926,6 +926,41 @@ mod host_abi {
         )
     }
 
+    /// Supply the `PYPY_GC_*` variables the collector sizes itself from, in the
+    /// same NUL-separated `NAME=VALUE` form as [`pyre_set_launch_env`]. The
+    /// guest has no environment, so without this the nursery, the major-collection
+    /// threshold and the growth rates all take their built-in defaults however
+    /// the host was configured — and the threshold decides when the collector
+    /// arms the eval-breaker word, which every compiled loop's back edge polls
+    /// through a real guard. `pyre_gc_env_names` lists the names that are read.
+    ///
+    /// The collector reads them once, when it is built, which the first
+    /// allocation does — so this must precede `pyre_run_python`, not merely the
+    /// script's own first line.
+    #[unsafe(no_mangle)]
+    pub extern "C" fn pyre_set_gc_env(ptr: *const u8, len: usize) {
+        let Some(blob) = guest_str(ptr, len) else {
+            return;
+        };
+        let entries = blob
+            .split('\0')
+            .filter_map(|record| {
+                let (name, value) = record.split_once('=')?;
+                (!name.is_empty()).then(|| (name.to_string(), value.to_string()))
+            })
+            .collect();
+        pyre_jit::set_gc_supplied_env(entries);
+    }
+
+    /// The names [`pyre_set_gc_env`] is worth being given, as NUL-separated
+    /// records in a buffer the host must free with `pyre_dealloc`. Returned for
+    /// the same reason as [`pyre_launch_env_names`]: so a host does not keep its
+    /// own copy of the list in step with the collector.
+    #[unsafe(no_mangle)]
+    pub extern "C" fn pyre_gc_env_names() -> u64 {
+        pack_into_guest(pyre_jit::GC_ENV_NAMES.join("\0").into_bytes())
+    }
+
     /// Set `-P` / PYTHONSAFEPATH for the next `pyre_run_python`, suppressing the
     /// `sys.path[0]` entry `pyre_set_script_path` would otherwise seed. Kept for
     /// a host predating [`pyre_set_launch_env`], which carries the same flag