diff --git a/packages/core/src/utils/schemaValidator.test.ts b/packages/core/src/utils/schemaValidator.test.ts
index 4928832c80a..40fb96483e5 100644
--- a/packages/core/src/utils/schemaValidator.test.ts
+++ b/packages/core/src/utils/schemaValidator.test.ts
@@ -203,6 +203,61 @@ describe('SchemaValidator', () => {
       expect(params.is_active).toBe(true);
     });
 
+    it('should not corrupt string fields whose value is literally "true"/"false"', () => {
+      const mixedSchema = {
+        type: 'object',
+        properties: {
+          old_string: { type: 'string' },
+          new_string: { type: 'string' },
+          is_active: { type: 'boolean' },
+        },
+        required: ['old_string', 'new_string', 'is_active'],
+      };
+      // A self-hosted LLM sends `is_active` as the string "false" (the case this
+      // coercion exists for) which fails initial validation and triggers
+      // fixBooleanValues. The string-typed `old_string`/`new_string` arguments
+      // legitimately hold the text "true"/"false" and must survive untouched —
+      // previously they were rewritten into booleans, corrupting the edit.
+      const params = {
+        old_string: 'true',
+        new_string: 'false',
+        is_active: 'false',
+      };
+      expect(SchemaValidator.validate(mixedSchema, params)).toBeNull();
+      expect(params.old_string).toBe('true');
+      expect(params.new_string).toBe('false');
+      expect(params.is_active).toBe(false);
+    });
+
+    it('should not coerce string booleans for fields that also accept string', () => {
+      const unionSchema = {
+        type: 'object',
+        properties: {
+          value: { anyOf: [{ type: 'string' }, { type: 'boolean' }] },
+          is_active: { type: 'boolean' },
+        },
+        required: ['value', 'is_active'],
+      };
+      const params = { value: 'true', is_active: 'true' };
+      expect(SchemaValidator.validate(unionSchema, params)).toBeNull();
+      // `value` accepts string, so the literal "true" is left as a string.
+      expect(params.value).toBe('true');
+      expect(params.is_active).toBe(true);
+    });
+
+    it('should coerce string booleans inside arrays of booleans', () => {
+      const arraySchema = {
+        type: 'object',
+        properties: {
+          flags: { type: 'array', items: { type: 'boolean' } },
+        },
+        required: ['flags'],
+      };
+      const params = { flags: ['true', 'false', 'true'] };
+      expect(SchemaValidator.validate(arraySchema, params)).toBeNull();
+      expect(params.flags).toEqual([true, false, true]);
+    });
+
     it('should pass through actual boolean values unchanged', () => {
       const params = { is_background: true };
       expect(SchemaValidator.validate(booleanSchema, params)).toBeNull();
diff --git a/packages/core/src/utils/schemaValidator.ts b/packages/core/src/utils/schemaValidator.ts
index 1c858dd2110..794fd10527b 100644
--- a/packages/core/src/utils/schemaValidator.ts
+++ b/packages/core/src/utils/schemaValidator.ts
@@ -159,7 +159,10 @@ export class SchemaValidator {
     let valid = validate(data);
     if (!valid && validate.errors) {
       // Coerce string boolean values ("true"/"false") to actual booleans
-      fixBooleanValues(data as Record<string, unknown>);
+      fixBooleanValues(
+        data as Record<string, unknown>,
+        anySchema as Record<string, unknown>,
+      );
       // Coerce stringified JSON values (arrays/objects) back to their proper types.
       // Some LLMs serialize complex values as strings when the schema uses
       // anyOf/oneOf (e.g., '["url"]' instead of ["url"] for anyOf: [array, null]).
@@ -272,20 +275,45 @@ function fixStringifiedJsonValues(
   }
 }
 
-function fixBooleanValues(data: Record<string, unknown>) {
+function fixBooleanValues(
+  data: Record<string, unknown>,
+  schema?: Record<string, unknown>,
+) {
+  const properties = schema?.['properties'] as
+    | Record<string, Record<string, unknown>>
+    | undefined;
+  const items = schema?.['items'] as Record<string, unknown> | undefined;
+
   for (const key of Object.keys(data)) {
     if (!(key in data)) continue;
     const value = data[key];
+    // Array elements share the `items` schema; object fields use their
+    // per-property schema.
+    const childSchema = Array.isArray(data) ? items : properties?.[key];
 
     if (typeof value === 'object' && value !== null) {
-      fixBooleanValues(value as Record<string, unknown>);
-    } else if (typeof value === 'string') {
-      const lower = value.toLowerCase();
-      if (lower === 'true') {
-        data[key] = true;
-      } else if (lower === 'false') {
-        data[key] = false;
-      }
+      fixBooleanValues(value as Record<string, unknown>, childSchema);
+      continue;
+    }
+
+    if (typeof value !== 'string') continue;
+
+    // Only coerce when the field's schema explicitly types it as boolean (and
+    // does not also accept string). Without this guard a legitimate string
+    // value of "true"/"false" — e.g. an `old_string`/`content` argument that
+    // happens to be the text "true" — would be silently rewritten into a
+    // boolean, corrupting the tool call. Mirrors fixStringifiedJsonValues,
+    // which is already schema-aware for the same reason.
+    const accepted = childSchema ? getAcceptedTypes(childSchema) : null;
+    if (!accepted || accepted.has('string') || !accepted.has('boolean')) {
+      continue;
+    }
+
+    const lower = value.toLowerCase();
+    if (lower === 'true') {
+      data[key] = true;
+    } else if (lower === 'false') {
+      data[key] = false;
     }
   }
 }