diff --git a/core/src/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.ts b/core/src/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.ts
index c161adf99..c23943301 100644
--- a/core/src/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.ts
+++ b/core/src/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.ts
@@ -5,6 +5,7 @@
  */
 
 import {OpenAPIV3} from 'openapi-types';
+import {snakeCase} from '../../../utils/case_utils.js';
 import {experimental} from '../../../utils/experimental.js';
 import {ApiParameter, OperationParser} from './operation_parser.js';
 
@@ -251,8 +252,7 @@ function collectOperations(
       operation.parameters = [...opParams, ...pathParams];
 
       if (!operation.operationId) {
-        // Generate operation ID if missing
-        operation.operationId = `${method}_${path.replace(/[^a-zA-Z0-9]/g, '_')}`;
+        operation.operationId = snakeCase(`${path}_${method}`);
       }
 
       const parser = new OperationParser(operation, {
diff --git a/core/src/utils/case_utils.ts b/core/src/utils/case_utils.ts
index 6df137858..ffaf11094 100644
--- a/core/src/utils/case_utils.ts
+++ b/core/src/utils/case_utils.ts
@@ -4,6 +4,27 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/**
+ * Converts a string to snake_case.
+ *
+ * Handles lowerCamelCase, UpperCamelCase, space-separated text, acronyms
+ * (e.g. "REST API") and consecutive uppercase letters.
+ *
+ * This matches the output of `_to_snake_case` in adk-python, which names the
+ * same OpenAPI tools and tool arguments there.
+ *
+ * @param text The string to convert.
+ * @returns The snake_case version of the string.
+ */
+export function snakeCase(text: string): string {
+  return text
+    .replace(/[^a-zA-Z0-9]+/g, '_')
+    .replace(/([a-z0-9])([A-Z])/g, '$1_$2')
+    .replace(/([A-Z]+)([A-Z][a-z])/g, '$1_$2')
+    .toLowerCase()
+    .replace(/^_+|_+$/g, '');
+}
+
 /**
  * Recursively converts snake_case keys of an object to camelCase.
  *
diff --git a/core/test/tools/openapi_tool/openapi_spec_parser_test.ts b/core/test/tools/openapi_tool/openapi_spec_parser_test.ts
index 2840961d8..bed08c94b 100644
--- a/core/test/tools/openapi_tool/openapi_spec_parser_test.ts
+++ b/core/test/tools/openapi_tool/openapi_spec_parser_test.ts
@@ -189,7 +189,7 @@ describe('OpenApiSpecParser', () => {
             },
           ],
           get: {
-            // operationId is missing, should be auto-generated as "get__users__id_"
+            // operationId is missing, so it is synthesized as "users_id_get"
             responses: {},
           },
         },
@@ -201,11 +201,69 @@ describe('OpenApiSpecParser', () => {
 
     expect(parsed.length).toBe(1);
     const op = parsed[0];
-    expect(op.name).toBe('get__users__id_');
+    expect(op.name).toBe('users_id_get');
     expect(op.parameters.length).toBe(1);
     expect(op.parameters[0].name).toBe('id');
   });
 
+  it('should synthesize snake_case operationIds matching adk-python', () => {
+    const spec: OpenAPIV3.Document = {
+      openapi: '3.0.0',
+      info: {title: 'Synthesis API', version: '1.0.0'},
+      paths: {
+        '/userProfiles/{userId}': {get: {responses: {}}},
+        '/pets/{petId}/photos': {delete: {responses: {}}},
+        '/': {get: {responses: {}}},
+      },
+    };
+
+    const parsed = new OpenApiSpecParser().parse(spec);
+
+    expect(parsed.map((op) => op.operation.operationId)).toEqual([
+      'user_profiles_user_id_get',
+      'pets_pet_id_photos_delete',
+      'get',
+    ]);
+    expect(parsed.map((op) => op.name)).toEqual([
+      'user_profiles_user_id_get',
+      'pets_pet_id_photos_delete',
+      'get',
+    ]);
+  });
+
+  it('should synthesize distinct ids for each method on a path', () => {
+    const spec: OpenAPIV3.Document = {
+      openapi: '3.0.0',
+      info: {title: 'Methods API', version: '1.0.0'},
+      paths: {
+        '/test': {
+          get: {responses: {}},
+          post: {responses: {}},
+        },
+      },
+    };
+
+    const parsed = new OpenApiSpecParser().parse(spec);
+
+    expect(parsed.map((op) => op.name)).toEqual(['test_get', 'test_post']);
+  });
+
+  it('should not rewrite a declared operationId', () => {
+    const spec: OpenAPIV3.Document = {
+      openapi: '3.0.0',
+      info: {title: 'Declared API', version: '1.0.0'},
+      paths: {
+        '/test': {get: {operationId: 'testOp', responses: {}}},
+      },
+    };
+
+    const parsed = new OpenApiSpecParser().parse(spec);
+
+    expect(parsed.length).toBe(1);
+    expect(parsed[0].operation.operationId).toBe('testOp');
+    expect(parsed[0].name).toBe('test_op');
+  });
+
   it('should resolve security schemes', () => {
     const spec: OpenAPIV3.Document = {
       openapi: '3.0.0',
diff --git a/core/test/tools/openapi_tool/openapi_toolset_test.ts b/core/test/tools/openapi_tool/openapi_toolset_test.ts
index da2e32380..a8f04130a 100644
--- a/core/test/tools/openapi_tool/openapi_toolset_test.ts
+++ b/core/test/tools/openapi_tool/openapi_toolset_test.ts
@@ -250,7 +250,7 @@ describe('OpenApiSpecParser', () => {
     const operations = parser.parse(specMissingId);
 
     expect(operations.length).toBe(1);
-    expect(operations[0].operation.operationId).toBe('get__test');
+    expect(operations[0].operation.operationId).toBe('test_get');
   });
 
   it('should extract specific security scheme', () => {
diff --git a/core/test/utils/case_utils_test.ts b/core/test/utils/case_utils_test.ts
index e291e3769..e00ab15e2 100644
--- a/core/test/utils/case_utils_test.ts
+++ b/core/test/utils/case_utils_test.ts
@@ -5,9 +5,29 @@
  */
 
 import {describe, expect, it} from 'vitest';
-import {camelCaseKeys} from '../../src/utils/case_utils.js';
+import {camelCaseKeys, snakeCase} from '../../src/utils/case_utils.js';
 
 describe('case_utils', () => {
+  describe('snakeCase', () => {
+    // Every expectation below is the output of `_to_snake_case` in adk-python
+    // (src/google/adk/tools/_gemini_schema_util.py) for the same input.
+    it.each([
+      ['camelCase', 'camel_case'],
+      ['UpperCamelCase', 'upper_camel_case'],
+      ['space separated', 'space_separated'],
+      ['REST API', 'rest_api'],
+      ['HTTPResponseCode', 'http_response_code'],
+      ['already_snake_case', 'already_snake_case'],
+      ['Multiple___Underscores', 'multiple_underscores'],
+      ['  _leading_and_trailing_  ', 'leading_and_trailing'],
+      ['X-API-Key', 'x_api_key'],
+      ['123Start', '123_start'],
+      ['', ''],
+    ])('should convert %j to %j', (input, expected) => {
+      expect(snakeCase(input)).toBe(expected);
+    });
+  });
+
   describe('camelCaseKeys', () => {
     it('should convert simple object keys', () => {
       const input = {