diff --git a/codex-rs/tools/src/json_schema.rs b/codex-rs/tools/src/json_schema.rs
index 29740a02b8..559a5a0044 100644
--- a/codex-rs/tools/src/json_schema.rs
+++ b/codex-rs/tools/src/json_schema.rs
@@ -673,6 +673,13 @@ fn singleton_null_schema_error() -> serde_json::Error {
     ))
 }
 
+#[cfg(test)]
+pub(crate) use crate::json_schema_structured_outputs::validate_structured_outputs_schema;
+
 #[cfg(test)]
 #[path = "json_schema_tests.rs"]
 mod tests;
+
+#[cfg(test)]
+#[path = "json_schema_structured_outputs_tests.rs"]
+mod structured_outputs_tests;
diff --git a/codex-rs/tools/src/json_schema_structured_outputs.rs b/codex-rs/tools/src/json_schema_structured_outputs.rs
new file mode 100644
index 0000000000..62a22f6c38
--- /dev/null
+++ b/codex-rs/tools/src/json_schema_structured_outputs.rs
@@ -0,0 +1,103 @@
+use crate::AdditionalProperties;
+use crate::JsonSchema;
+use crate::JsonSchemaPrimitiveType;
+use crate::JsonSchemaType;
+use std::collections::BTreeSet;
+
+/// Validates the subset of JSON Schema invariants required by OpenAI
+/// Structured Outputs for the object-heavy MCP regression tests in this crate.
+///
+/// Source of truth:
+/// https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas
+///
+/// This validator currently focuses on the object constraints that matter for
+/// the `start` / `end` nested-object regression:
+/// - the root schema must be an object and must not use root-level `anyOf`
+/// - every object must set `additionalProperties: false`
+/// - every object property must appear in `required`
+/// - nested `anyOf` branches and array items must themselves satisfy the same
+///   subset whenever they contain objects
+///
+/// It intentionally does not yet enforce the documented global size limits
+/// (property count, nesting depth, enum count, total string budget). Those are
+/// broader policy checks and can be layered on later without changing the
+/// regression tests that pin the object-shape bug fixed in PR #18159.
+pub(crate) fn validate_structured_outputs_schema(schema: &JsonSchema) -> Result<(), String> {
+    validate_structured_outputs_schema_at_path(schema, "root", /*is_root*/ true)
+}
+
+fn validate_structured_outputs_schema_at_path(
+    schema: &JsonSchema,
+    path: &str,
+    is_root: bool,
+) -> Result<(), String> {
+    if is_root {
+        if schema.any_of.is_some() {
+            return Err(format!(
+                "{path}: root schema must not use `anyOf`; see https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas"
+            ));
+        }
+
+        if schema.schema_type != Some(JsonSchemaType::Single(JsonSchemaPrimitiveType::Object)) {
+            return Err(format!(
+                "{path}: root schema must be an object; see https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas"
+            ));
+        }
+    }
+
+    if let Some(any_of) = &schema.any_of {
+        for (index, variant) in any_of.iter().enumerate() {
+            validate_structured_outputs_schema_at_path(
+                variant,
+                &format!("{path}.anyOf[{index}]"),
+                /*is_root*/ false,
+            )?;
+        }
+    }
+
+    if let Some(items) = &schema.items {
+        validate_structured_outputs_schema_at_path(
+            items,
+            &format!("{path}.items"),
+            /*is_root*/ false,
+        )?;
+    }
+
+    if let Some(JsonSchemaType::Single(JsonSchemaPrimitiveType::Object)) = &schema.schema_type {
+        let Some(properties) = schema.properties.as_ref() else {
+            return Err(format!(
+                "{path}: object schemas must carry a properties map"
+            ));
+        };
+
+        if schema.additional_properties != Some(AdditionalProperties::Boolean(false)) {
+            return Err(format!(
+                "{path}: object schemas must set `additionalProperties: false`; see https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas"
+            ));
+        }
+
+        let property_names = properties.keys().cloned().collect::<BTreeSet<_>>();
+        let Some(required) = schema.required.as_ref() else {
+            return Err(format!(
+                "{path}: object schemas must list every field in `required`; see https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas"
+            ));
+        };
+        let required_names = required.iter().cloned().collect::<BTreeSet<_>>();
+
+        if required_names != property_names {
+            return Err(format!(
+                "{path}: object schema `required` entries must exactly match declared properties; see https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas"
+            ));
+        }
+
+        for (property_name, property_schema) in properties {
+            validate_structured_outputs_schema_at_path(
+                property_schema,
+                &format!("{path}.{property_name}"),
+                /*is_root*/ false,
+            )?;
+        }
+    }
+
+    Ok(())
+}
diff --git a/codex-rs/tools/src/json_schema_structured_outputs_tests.rs b/codex-rs/tools/src/json_schema_structured_outputs_tests.rs
new file mode 100644
index 0000000000..5b8a3e427c
--- /dev/null
+++ b/codex-rs/tools/src/json_schema_structured_outputs_tests.rs
@@ -0,0 +1,144 @@
+use super::JsonSchema;
+use super::parse_tool_input_schema;
+use super::validate_structured_outputs_schema;
+use pretty_assertions::assert_eq;
+use std::collections::BTreeMap;
+
+// This file codifies the Structured Outputs invariants that matter for the MCP
+// schema regression in PR #18159.
+//
+// Source of truth:
+// https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas
+//
+// The doc page calls out several constraints that are relevant to the object
+// schemas we expose through the Responses API:
+// - the root schema must be an object and must not be `anyOf`
+// - all object fields / function parameters must appear in `required`
+// - every object must set `additionalProperties: false`
+//
+// The flattening bug in this PR was specifically about `$ref` and
+// single-variant combiner wrappers causing nested object parameters like
+// `start` / `end` to collapse to `string`. The tests below intentionally use
+// Structured Outputs-compliant inputs so we can assert not only that the object
+// shape survives, but also that the surviving shape still lives inside the
+// Responses API subset documented above.
+
+#[test]
+fn parse_tool_input_schema_keeps_local_ref_objects_inside_structured_outputs_subset() {
+    // This mirrors the Outlook Calendar `create_event.start` / `end` shape we
+    // care about, but does so with the exact Structured Outputs object
+    // invariants from:
+    // https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas
+    //
+    // We want to prove that resolving a local `$ref` no longer collapses the
+    // nested object to `string`, while also preserving:
+    // - root object shape
+    // - `additionalProperties: false`
+    // - `required` coverage for every field
+    let schema = parse_tool_input_schema(&serde_json::json!({
+        "type": "object",
+        "properties": {
+            "start": { "$ref": "#/$defs/date_time_zone" }
+        },
+        "required": ["start"],
+        "additionalProperties": false,
+        "$defs": {
+            "date_time_zone": {
+                "type": "object",
+                "properties": {
+                    "dateTime": { "type": "string" },
+                    "timeZone": { "type": "string" }
+                },
+                "required": ["dateTime", "timeZone"],
+                "additionalProperties": false
+            }
+        }
+    }))
+    .expect("parse schema");
+
+    validate_structured_outputs_schema(&schema).expect("schema should stay in supported subset");
+
+    assert_eq!(
+        schema,
+        JsonSchema::object(
+            BTreeMap::from([(
+                "start".to_string(),
+                JsonSchema::object(
+                    BTreeMap::from([
+                        (
+                            "dateTime".to_string(),
+                            JsonSchema::string(/*description*/ None),
+                        ),
+                        (
+                            "timeZone".to_string(),
+                            JsonSchema::string(/*description*/ None),
+                        ),
+                    ]),
+                    Some(vec!["dateTime".to_string(), "timeZone".to_string()]),
+                    Some(false.into()),
+                ),
+            )]),
+            Some(vec!["start".to_string()]),
+            Some(false.into()),
+        )
+    );
+}
+
+#[test]
+fn parse_tool_input_schema_keeps_single_variant_combiner_objects_inside_structured_outputs_subset()
+{
+    // The docs allow nested objects, but those nested objects still need the
+    // same strict object invariants:
+    // https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas
+    //
+    // This test covers the second half of the regression: a single-variant
+    // `allOf` wrapper around the `DateTimeTimeZone` object must unwrap back to
+    // an object without losing the Structured Outputs constraints that make the
+    // schema acceptable to the Responses API.
+    let schema = parse_tool_input_schema(&serde_json::json!({
+        "type": "object",
+        "properties": {
+            "end": {
+                "allOf": [{
+                    "type": "object",
+                    "properties": {
+                        "dateTime": { "type": "string" },
+                        "timeZone": { "type": "string" }
+                    },
+                    "required": ["dateTime", "timeZone"],
+                    "additionalProperties": false
+                }]
+            }
+        },
+        "required": ["end"],
+        "additionalProperties": false
+    }))
+    .expect("parse schema");
+
+    validate_structured_outputs_schema(&schema).expect("schema should stay in supported subset");
+
+    assert_eq!(
+        schema,
+        JsonSchema::object(
+            BTreeMap::from([(
+                "end".to_string(),
+                JsonSchema::object(
+                    BTreeMap::from([
+                        (
+                            "dateTime".to_string(),
+                            JsonSchema::string(/*description*/ None),
+                        ),
+                        (
+                            "timeZone".to_string(),
+                            JsonSchema::string(/*description*/ None),
+                        ),
+                    ]),
+                    Some(vec!["dateTime".to_string(), "timeZone".to_string()]),
+                    Some(false.into()),
+                ),
+            )]),
+            Some(vec!["end".to_string()]),
+            Some(false.into()),
+        )
+    );
+}
diff --git a/codex-rs/tools/src/lib.rs b/codex-rs/tools/src/lib.rs
index b7c6a57d2b..bf57902930 100644
--- a/codex-rs/tools/src/lib.rs
+++ b/codex-rs/tools/src/lib.rs
@@ -25,6 +25,9 @@ mod tool_suggest;
 mod utility_tool;
 mod view_image;
 
+#[cfg(test)]
+mod json_schema_structured_outputs;
+
 pub use agent_job_tool::create_report_agent_job_result_tool;
 pub use agent_job_tool::create_spawn_agents_on_csv_tool;
 pub use agent_tool::SpawnAgentToolOptions;