diff --git a/crates/forge_app/src/dto/anthropic/transforms/enforce_schema.rs b/crates/forge_app/src/dto/anthropic/transforms/enforce_schema.rs
index 43c7987e57..18b26ab5e4 100644
--- a/crates/forge_app/src/dto/anthropic/transforms/enforce_schema.rs
+++ b/crates/forge_app/src/dto/anthropic/transforms/enforce_schema.rs
@@ -3,12 +3,20 @@ use forge_domain::Transformer;
 use crate::dto::anthropic::{OutputFormat, Request};
 use crate::utils::enforce_strict_schema;
 
-/// Transformer that normalizes output_format schema to meet Anthropic API
-/// requirements.
+/// Transformer that normalizes JSON schemas in the Anthropic request to meet
+/// API requirements.
 ///
-/// Anthropic requires that all object types in JSON schemas must explicitly set
-/// `additionalProperties: false`. This transformer recursively processes the
-/// schema to add this requirement.
+/// Two categories of schemas are processed:
+///
+/// 1. **`output_format` schema** — Anthropic requires all object types to
+///    explicitly set `additionalProperties: false`.
+///
+/// 2. **Tool `input_schema`** — each tool's parameter schema is sanitized to
+///    remove constructs that produce invalid JSON Schema, such as `null`
+///    values in `enum` arrays that contradict the declared `type`. This is
+///    essential when routing to Anthropic-compatible endpoints backed by
+///    third-party models (e.g. Moonshot/Kimi) whose validators reject such
+///    schemas.
 ///
 /// # Example
 ///
@@ -54,6 +62,13 @@ impl Transformer for EnforceStrictObjectSchema {
             }
         }
 
+        // Normalize each tool's input schema. Non-strict mode keeps the
+        // `nullable` keyword for providers that support it while stripping
+        // invalid constructs (e.g. null in enum) that strict validators reject.
+        for tool in &mut request.tools {
+            enforce_strict_schema(&mut tool.input_schema, false);
+        }
+
         request
     }
 }
@@ -64,6 +79,8 @@ mod tests {
     use schemars::JsonSchema;
     use serde::Deserialize;
 
+    use crate::dto::anthropic::ToolDefinition;
+
     use super::*;
 
     #[derive(Deserialize, JsonSchema)]
@@ -135,4 +152,40 @@ mod tests {
 
         assert_eq!(actual.output_format, None);
     }
+
+    #[test]
+    fn test_normalize_tool_schema_strips_null_from_enum() {
+        // Simulate a tool input schema with a nullable enum that contains
+        // null in the enum array — exactly what Moonshot/Kimi rejects.
+        let fixture = Request::default().max_tokens(1u64).tools(vec![ToolDefinition {
+            name: "fs_search".to_string(),
+            description: Some("Search files".to_string()),
+            cache_control: None,
+            input_schema: serde_json::json!({
+                "type": "object",
+                "properties": {
+                    "output_mode": {
+                        "type": "string",
+                        "enum": ["content", "files_with_matches", "count", null],
+                        "nullable": true
+                    }
+                }
+            }),
+        }]);
+
+        let actual = EnforceStrictObjectSchema.transform(fixture);
+
+        let tool = &actual.tools[0];
+        let enum_values = tool
+            .input_schema
+            .pointer("/properties/output_mode/enum")
+            .and_then(|v| v.as_array())
+            .unwrap();
+
+        assert!(
+            !enum_values.contains(&serde_json::Value::Null),
+            "enum must not contain null after normalization"
+        );
+        assert_eq!(enum_values.len(), 3);
+    }
 }
diff --git a/crates/forge_app/src/utils.rs b/crates/forge_app/src/utils.rs
index 349157d793..36f2b27dde 100644
--- a/crates/forge_app/src/utils.rs
+++ b/crates/forge_app/src/utils.rs
@@ -522,6 +522,25 @@ pub fn enforce_strict_schema(schema: &mut serde_json::Value, strict_mode: bool)
 
             normalize_array_items(map, strict_mode);
 
+            // Always strip null from enum arrays when nullable is set,
+            // regardless of strict mode. A null enum value contradicts any
+            // non-null type declaration, producing invalid JSON Schema that
+            // strict validators (e.g. Moonshot/Kimi) reject. This also guards
+            // against external MCP schemas that may carry the same
+            // inconsistency, which are not processed by NormalizeNullable.
+            if map
+                .get("nullable")
+                .and_then(|v| v.as_bool())
+                .unwrap_or(false)
+            {
+                if let Some(serde_json::Value::Array(enum_values)) = map.get_mut("enum") {
+                    enum_values.retain(|v| !v.is_null());
+                }
+            }
+
+            // In strict mode, convert the nullable keyword into an explicit
+            // anyOf union because OpenAI's Responses API does not support
+            // nullable. The enum array has already been cleaned above.
             if strict_mode
                 && map
                     .get("nullable")
@@ -530,10 +549,6 @@ pub fn enforce_strict_schema(schema: &mut serde_json::Value, strict_mode: bool)
             {
                 map.remove("nullable");
 
-                if let Some(serde_json::Value::Array(enum_values)) = map.get_mut("enum") {
-                    enum_values.retain(|v| !v.is_null());
-                }
-
                 let description = map.remove("description");
                 let non_null_branch = serde_json::Value::Object(std::mem::take(map));
                 let null_branch = serde_json::json!({"type": "null"});
@@ -1327,6 +1342,40 @@ mod tests {
         assert!(schema["properties"]["output_mode"].get("anyOf").is_none());
     }
 
+    #[test]
+    fn test_nullable_enum_null_stripped_in_non_strict_mode() {
+        // Schemas produced by external MCP tools may carry null in enum
+        // alongside nullable: true. Even in non-strict mode, null must be
+        // stripped so the schema stays valid for strict validators like
+        // Moonshot/Kimi.
+        let mut schema = json!({
+            "type": "object",
+            "properties": {
+                "output_mode": {
+                    "nullable": true,
+                    "type": "string",
+                    "enum": ["content", "files_with_matches", "count", null]
+                }
+            }
+        });
+
+        enforce_strict_schema(&mut schema, false);
+
+        let enum_values = schema["properties"]["output_mode"]["enum"]
+            .as_array()
+            .unwrap();
+        assert!(
+            !enum_values.contains(&json!(null)),
+            "enum must not contain null in non-strict mode"
+        );
+        assert_eq!(enum_values.len(), 3);
+        // nullable marker should be preserved
+        assert_eq!(
+            schema["properties"]["output_mode"]["nullable"],
+            json!(true)
+        );
+    }
+
     #[test]
     fn test_schema_valued_additional_properties_is_normalized() {
         let mut schema = json!({
diff --git a/crates/forge_domain/src/tools/catalog.rs b/crates/forge_domain/src/tools/catalog.rs
index ef911357c7..23a06460b2 100644
--- a/crates/forge_domain/src/tools/catalog.rs
+++ b/crates/forge_domain/src/tools/catalog.rs
@@ -860,7 +860,7 @@ fn normalize_tool_name(name: &ToolName) -> ToolName {
 impl ToolCatalog {
     pub fn schema(&self) -> Schema {
         use schemars::generate::SchemaSettings;
-        use schemars::transform::{AddNullable, Transform};
+        use schemars::transform::Transform;
 
         let r#gen = SchemaSettings::default()
             .with(|s| {
@@ -889,8 +889,10 @@ impl ToolCatalog {
             ToolCatalog::TodoRead(_) => r#gen.into_root_schema_for::<TodoRead>(),
         };
 
-        // Apply transform to add nullable property and remove null from type
-        AddNullable::default().transform(&mut schema);
+        // Normalize nullable schemas: add the "nullable": true marker and
+        // ensure enum arrays stay internally consistent (no null values that
+        // contradict the declared type).
+        crate::NormalizeNullable.transform(&mut schema);
 
         schema
     }
diff --git a/crates/forge_domain/src/tools/definition/tool_definition.rs b/crates/forge_domain/src/tools/definition/tool_definition.rs
index e33a3c005f..37f478792f 100644
--- a/crates/forge_domain/src/tools/definition/tool_definition.rs
+++ b/crates/forge_domain/src/tools/definition/tool_definition.rs
@@ -3,6 +3,7 @@ use schemars::Schema;
 use schemars::generate::SchemaGenerator;
 use schemars::transform::{Transform, transform_subschemas};
 use serde::{Deserialize, Serialize};
+use serde_json::Value;
 
 use crate::ToolName;
 
@@ -25,6 +26,60 @@ impl Transform for RemoveSchemaTitles {
     }
 }
 
+/// A [`Transform`] that marks nullable schemas with `"nullable": true` while
+/// keeping `enum` arrays internally consistent.
+///
+/// Schemars' built-in [`schemars::transform::AddNullable`] adds the
+/// `"nullable": true` keyword and removes `"null"` from the `type` keyword,
+/// but it does **not** remove `null` values from `enum` arrays. This leaves
+/// schemas where the `enum` array contains `null` while `type` is e.g.
+/// `"string"` — an invalid combination that strict JSON Schema validators
+/// (such as Moonshot/Kimi) reject.
+///
+/// This transform delegates to [`AddNullable`](schemars::transform::AddNullable)
+/// for the `type` keyword, then recursively strips `null` from every `enum`
+/// array in a schema that carries `"nullable": true`. The `nullable` marker
+/// alone fully conveys nullability in OpenAPI 3.0-style dialects, so `null`
+/// in `enum` is redundant once the marker is present.
+#[derive(Debug, Clone, Default)]
+pub struct NormalizeNullable;
+
+impl Transform for NormalizeNullable {
+    fn transform(&mut self, schema: &mut Schema) {
+        // Delegate type-keyword normalization (including subschema recursion)
+        // to schemars' AddNullable.
+        schemars::transform::AddNullable::default().transform(schema);
+
+        // Recursively clean up enum arrays left inconsistent by AddNullable.
+        remove_null_enum_values(schema);
+        transform_subschemas(&mut remove_null_enum_values, schema);
+    }
+}
+
+/// Removes `null` entries from the `enum` array of a schema that is marked
+/// `nullable: true`. The `nullable` marker already conveys nullability, so
+/// keeping `null` in `enum` is redundant and can make the schema invalid when
+/// the `type` no longer includes `"null"`.
+fn remove_null_enum_values(schema: &mut Schema) {
+    if let Some(map) = schema.as_object_mut() {
+        let is_nullable = map
+            .get("nullable")
+            .and_then(Value::as_bool)
+            .unwrap_or(false);
+
+        if is_nullable {
+            if let Some(Value::Array(enum_values)) = map.get_mut("enum") {
+                enum_values.retain(|v| !v.is_null());
+
+                // Drop the enum key if every value was null.
+                if enum_values.is_empty() {
+                    map.remove("enum");
+                }
+            }
+        }
+    }
+}
+
 /// Returns a [`SchemaGenerator`] whose settings include [`RemoveSchemaTitles`]
 /// as a registered transform.
 ///
@@ -155,4 +210,82 @@ mod tests {
             None
         );
     }
+
+    #[test]
+    fn test_normalize_nullable_strips_null_from_enum() {
+        use schemars::json_schema;
+
+        // Simulate what schemars produces for Option<EnumWithCustomSchema>:
+        // type includes "null", and enum array contains null.
+        let mut schema = json_schema!({
+            "type": ["string", "null"],
+            "enum": ["content", "files_with_matches", "count", null]
+        });
+
+        NormalizeNullable.transform(&mut schema);
+
+        let actual = serde_json::to_value(&schema).unwrap();
+
+        // nullable marker should be present
+        assert_eq!(actual["nullable"], serde_json::Value::Bool(true));
+
+        // type should no longer contain "null"
+        assert_eq!(actual["type"], serde_json::json!("string"));
+
+        // enum must NOT contain null — this is the bug fix
+        let enum_values = actual["enum"].as_array().unwrap();
+        assert!(
+            !enum_values.contains(&serde_json::Value::Null),
+            "enum must not contain null after NormalizeNullable"
+        );
+        assert_eq!(enum_values.len(), 3);
+    }
+
+    #[test]
+    fn test_normalize_nullable_preserves_non_nullable_enum() {
+        use schemars::json_schema;
+
+        // A non-nullable enum should be left untouched.
+        let mut schema = json_schema!({
+            "type": "string",
+            "enum": ["a", "b", "c"]
+        });
+
+        let expected = serde_json::to_value(&schema).unwrap();
+
+        NormalizeNullable.transform(&mut schema);
+
+        let actual = serde_json::to_value(&schema).unwrap();
+        assert_eq!(actual, expected);
+    }
+
+    #[test]
+    fn test_normalize_nullable_recurses_into_properties() {
+        use schemars::json_schema;
+
+        // Nullable enum nested inside a properties object.
+        let mut schema = json_schema!({
+            "type": "object",
+            "properties": {
+                "mode": {
+                    "type": ["string", "null"],
+                    "enum": ["fast", "slow", null]
+                }
+            }
+        });
+
+        NormalizeNullable.transform(&mut schema);
+
+        let actual = serde_json::to_value(&schema).unwrap();
+        let mode_enum = actual
+            .pointer("/properties/mode/enum")
+            .and_then(|v| v.as_array())
+            .unwrap();
+
+        assert!(
+            !mode_enum.contains(&serde_json::Value::Null),
+            "nested enum must not contain null"
+        );
+        assert_eq!(mode_enum.len(), 2);
+    }
 }
diff --git a/crates/forge_domain/src/tools/snapshots/forge_domain__tools__catalog__tests__tool_definition_json.snap b/crates/forge_domain/src/tools/snapshots/forge_domain__tools__catalog__tests__tool_definition_json.snap
index c12f16e7f9..768b32d86a 100644
--- a/crates/forge_domain/src/tools/snapshots/forge_domain__tools__catalog__tests__tool_definition_json.snap
+++ b/crates/forge_domain/src/tools/snapshots/forge_domain__tools__catalog__tests__tool_definition_json.snap
@@ -125,8 +125,7 @@ expression: tools
       "enum": [
         "content",
         "files_with_matches",
-        "count",
-        null
+        "count"
       ],
       "nullable": true
     },