diff --git a/changelog.d/10258-imported-default-ctor-arity.md b/changelog.d/10258-imported-default-ctor-arity.md
new file mode 100644
index 0000000000..1cc6dba602
--- /dev/null
+++ b/changelog.d/10258-imported-default-ctor-arity.md
@@ -0,0 +1 @@
+Fix `new ImportedClass(args)` for an exported class with no own constructor whose `extends` clause is a runtime value (for example effect v4's `class SystemError extends Data.Error {}`). The defining module synthesizes a forwarding constructor with a fixed parameter band because the parent's arity is unknown there, but the imported-class metadata counted only the class's own (absent) constructor and declared it with zero parameters, so the importing module called it with `this` alone and the parent constructor never saw the arguments. Importers now derive the arity from the same rule as the synthesized constructor. This unblocks OpenCode's config loading, where effect's file-not-found `PlatformError` lost its reason and turned a missing optional config file into a fatal error (#10107).
diff --git a/crates/perry-codegen/src/codegen/ctor_arity.rs b/crates/perry-codegen/src/codegen/ctor_arity.rs
index efc3d68469..73917e63e2 100644
--- a/crates/perry-codegen/src/codegen/ctor_arity.rs
+++ b/crates/perry-codegen/src/codegen/ctor_arity.rs
@@ -6,6 +6,33 @@
 
 use std::collections::HashMap;
 
+/// Positional forwarding band for a synthesized default ctor whose parent arity
+/// cannot be resolved while compiling the defining module (see the tail of
+/// [`synthesized_ctor_param_count`]).
+pub const UNRESOLVED_PARENT_FWD_ARITY: usize = 8;
+
+/// The standalone-constructor arity of `class` when it can be decided from the
+/// class definition alone, without the defining module's class table or
+/// imports. Importers use this to describe a class they did not compile, so it
+/// MUST agree with [`synthesized_ctor_param_count`] for every case it answers:
+/// an own constructor, a native parent, no heritage, and a heritage that is only
+/// a runtime value (`extends_expr` with no resolvable `extends_name`), which
+/// always synthesizes the fixed forwarding band. `None` means the arity depends
+/// on the ancestor walk and callers keep their existing behaviour (#10258).
+pub fn context_free_ctor_param_count(class: &perry_hir::Class) -> Option<usize> {
+    if let Some(c) = class.constructor.as_ref() {
+        return Some(c.params.len());
+    }
+    if class.native_extends.is_some() {
+        return Some(0);
+    }
+    match (&class.extends_name, &class.extends_expr) {
+        (None, None) => Some(0),
+        (None, Some(_)) => Some(UNRESOLVED_PARENT_FWD_ARITY),
+        _ => None,
+    }
+}
+
 /// The standalone-constructor arity Perry emits for `class`, accounting for the
 /// JS spec default ctor `constructor(...args) { super(...args) }` that a class
 /// with NO own constructor but WITH heritage inherits. Walks the ancestor chain
@@ -78,6 +105,5 @@ pub(super) fn synthesized_ctor_param_count(
     // positional params: the `new` site pads missing slots with `undefined`,
     // and a parent ctor reading fewer params ignores the trailing `undefined`s,
     // so over-declaring is correct for any (non-native) parent up to this band.
-    const UNRESOLVED_PARENT_FWD_ARITY: usize = 8;
     UNRESOLVED_PARENT_FWD_ARITY
 }
diff --git a/crates/perry-codegen/src/codegen/mod.rs b/crates/perry-codegen/src/codegen/mod.rs
index df28e079e1..8b09ca39a5 100644
--- a/crates/perry-codegen/src/codegen/mod.rs
+++ b/crates/perry-codegen/src/codegen/mod.rs
@@ -188,6 +188,7 @@ mod clone_suffix_tests;
 mod closure;
 mod closure_collect;
 mod ctor_arity;
+pub use ctor_arity::{context_free_ctor_param_count, UNRESOLVED_PARENT_FWD_ARITY};
 #[cfg(test)]
 mod declared_string_add_tests;
 #[cfg(test)]
diff --git a/crates/perry-codegen/src/lib.rs b/crates/perry-codegen/src/lib.rs
index 9eb72e7496..dd085e8618 100644
--- a/crates/perry-codegen/src/lib.rs
+++ b/crates/perry-codegen/src/lib.rs
@@ -75,7 +75,7 @@ pub mod types;
 pub mod unit_cache;
 
 pub use codegen::{
-    compile_module, namespace_member_class_key, namespace_member_func_key,
+    compile_module, context_free_ctor_param_count, namespace_member_class_key, namespace_member_func_key,
     namespace_member_var_key, resolve_target_triple, short_spread_method_capabilities,
     user_function_symbol, AppMetadata, CompileOptions, ExportedObjectLiteralCapability,
     FpContractMode, ImportedClass, ImportedObjectLiteral, ImportedObjectLiteralMethod,
diff --git a/crates/perry/src/commands/compile/run_pipeline.rs b/crates/perry/src/commands/compile/run_pipeline.rs
index a71acbbcd2..1016b348c3 100644
--- a/crates/perry/src/commands/compile/run_pipeline.rs
+++ b/crates/perry/src/commands/compile/run_pipeline.rs
@@ -293,11 +293,20 @@ fn imported_class_from_hir(
         local_alias,
         namespace: None,
         source_prefix,
-        constructor_param_count: class
-            .constructor
-            .as_ref()
-            .map(|ctor| ctor.params.len())
-            .unwrap_or(0),
+        // #10258: must match the arity of the standalone constructor the
+        // defining module emits. A class with no own constructor whose parent
+        // is only a runtime value synthesizes a fixed forwarding band; counting
+        // just the (absent) own constructor declared it as 0 params here, so
+        // `new ImportedClass(args)` passed only `this` and the parent ran
+        // without its arguments.
+        constructor_param_count: perry_codegen::context_free_ctor_param_count(class)
+            .unwrap_or_else(|| {
+                class
+                    .constructor
+                    .as_ref()
+                    .map(|ctor| ctor.params.len())
+                    .unwrap_or(0)
+            }),
         has_own_constructor: class.constructor.is_some(),
         constructor_has_rest: class
             .constructor
diff --git a/crates/perry/tests/source_graph_export_regressions.rs b/crates/perry/tests/source_graph_export_regressions.rs
index ea4df499cb..4797a6c6a9 100644
--- a/crates/perry/tests/source_graph_export_regressions.rs
+++ b/crates/perry/tests/source_graph_export_regressions.rs
@@ -963,3 +963,5 @@ mod issue_10160;
 mod issue_10180;
 #[path = "source_graph_export_regressions/issue_10197.rs"]
 mod issue_10197;
+#[path = "source_graph_export_regressions/issue_10258.rs"]
+mod issue_10258;
diff --git a/crates/perry/tests/source_graph_export_regressions/issue_10258.rs b/crates/perry/tests/source_graph_export_regressions/issue_10258.rs
new file mode 100644
index 0000000000..e2c790d879
--- /dev/null
+++ b/crates/perry/tests/source_graph_export_regressions/issue_10258.rs
@@ -0,0 +1,55 @@
+//! `new ImportedClass(args)` for an exported class with no own constructor whose
+//! parent is a runtime value must forward the arguments (#10258). The defining
+//! module synthesizes a fixed-arity forwarding constructor; the importer used to
+//! declare it with zero parameters and dropped every argument. effect v4's
+//! `PlatformError.SystemError extends Data.Error {}` has this shape.
+
+use super::{compile_and_run, write};
+
+#[test]
+fn imported_default_ctor_with_runtime_parent_forwards_args() {
+    let dir = tempfile::tempdir().unwrap();
+    write(
+        dir.path(),
+        "core.ts",
+        "export const YieldableError = (function () { class YieldableError extends globalThis.Error {}; return YieldableError })()\n\
+         export const Error = (function () {\n\
+         \x20 return class Base extends YieldableError {\n\
+         \x20   constructor(args?: any) { super(args?.message); if (args) Object.assign(this, args) }\n\
+         \x20 }\n\
+         })()\n\
+         export const Plain = (function () { return class P { constructor(a?: any, b?: any, c?: any) { (this as any).sum = [a, b, c] } } })()\n",
+    );
+    write(dir.path(), "data.ts", "import * as core from \"./core\"\nexport const Error = core.Error\nexport const Plain = core.Plain\n");
+    write(
+        dir.path(),
+        "platform.ts",
+        "import * as Data from \"./data\"\n\
+         export class Empty extends (Data.Error as any) {}\n\
+         export class WithGetter extends (Data.Error as any) { get message() { return \"G:\" + (this as any)._tag } }\n\
+         export class WithMethod extends (Data.Error as any) { describe() { return (this as any).module } }\n\
+         export class Explicit extends (Data.Error as any) { constructor(a: any) { super(a) } }\n\
+         export class ThreeArgs extends (Data.Plain as any) {}\n\
+         export const makeLocal = (o: any) => new WithGetter(o)\n",
+    );
+    write(
+        dir.path(),
+        "main.ts",
+        "import { Empty, WithGetter, WithMethod, Explicit, ThreeArgs, makeLocal } from \"./platform\"\n\
+         import * as P from \"./platform\"\n\
+         const o = () => ({ _tag: \"NotFound\", module: \"FS\" })\n\
+         const show = (e: any) => [e._tag, e.module, e instanceof Error].join(\",\")\n\
+         console.log(show(new Empty(o())), show(new WithGetter(o())), new WithGetter(o()).message)\n\
+         console.log(show(new WithMethod(o())), new WithMethod(o()).describe(), show(new Explicit(o())))\n\
+         console.log(show(new P.Empty(o())), show(makeLocal(o())), JSON.stringify((new ThreeArgs(1, 2, 3) as any).sum))\n\
+         class Sub extends Empty {}\n\
+         console.log(show(new Sub(o())))\n",
+    );
+    assert_eq!(
+        compile_and_run(dir.path(), "main.ts"),
+        "NotFound,FS,true NotFound,FS,true G:NotFound\n\
+         NotFound,FS,true FS NotFound,FS,true\n\
+         NotFound,FS,true NotFound,FS,true [1,2,3]\n\
+         NotFound,FS,true\n"
+    );
+}