diff --git a/packages/core/src/apis/google/mapGeminiResponseToToolInvocations.ts b/packages/core/src/apis/google/mapGeminiResponseToToolInvocations.ts
index 350ac7d..707b697 100644
--- a/packages/core/src/apis/google/mapGeminiResponseToToolInvocations.ts
+++ b/packages/core/src/apis/google/mapGeminiResponseToToolInvocations.ts
@@ -1,4 +1,4 @@
-import { ConvertParamMapToArgs, Tool, ToolParamMap } from "../shared";
+import { ConvertParamMapToArgs, Tool, ToolParamMap } from "../../utils/Tool";
 
 import type { GoogleGeminiResponse } from "./gemini";
 
diff --git a/packages/core/src/apis/shared/ToolUseRequestOptions.ts b/packages/core/src/apis/shared/ToolUseRequestOptions.ts
index 95e02af..7c6ad5b 100644
--- a/packages/core/src/apis/shared/ToolUseRequestOptions.ts
+++ b/packages/core/src/apis/shared/ToolUseRequestOptions.ts
@@ -1,94 +1,4 @@
-type ToolParameterTypes = "STR" | "NUM" | "BOOL";
-
-export type ToolParamMap = {
-  [key: string]: {
-    description: string;
-    type: ToolParameterTypes;
-    required: boolean;
-  };
-};
-
-type ExtractArgumentType<T extends ToolParameterTypes> = T extends "STR"
-  ? string
-  : T extends "NUM"
-    ? number
-    : T extends "BOOL"
-      ? boolean
-      : never;
-
-export type ConvertParamMapToArgs<TParamMap extends ToolParamMap> = {
-  [K in keyof TParamMap as TParamMap[K]["required"] extends true
-    ? K
-    : never]: ExtractArgumentType<TParamMap[K]["type"]>;
-} & {
-  [K in keyof TParamMap as TParamMap[K]["required"] extends false | undefined
-    ? K
-    : never]?: ExtractArgumentType<TParamMap[K]["type"]>;
-};
-
-/*
- *  Ifaces
- */
-interface ToolParam {
-  name: string;
-  description: string;
-  type: ToolParameterTypes;
-  required: boolean;
-}
-
-interface ToolInvocation<TArgs> {
-  arguments: TArgs;
-  returned?: unknown;
-}
-
-interface ToolDescriptor {
-  name: string;
-  description: string;
-  parameters: ToolParam[];
-  invocations: ToolInvocation<any>[];
-}
-
-export class Tool<TParamMap extends ToolParamMap, TReturns = unknown> {
-  private invokeFn: (args: ConvertParamMapToArgs<TParamMap>) => TReturns;
-
-  public descriptor: ToolDescriptor;
-
-  constructor(
-    name: string,
-    description: string,
-    paramMap: TParamMap,
-    invokeFn: (args: ConvertParamMapToArgs<TParamMap>) => TReturns,
-  ) {
-    this.invokeFn = invokeFn;
-    this.descriptor = {
-      name,
-      description,
-      parameters: this.createParameters(paramMap),
-      invocations: [],
-    };
-  }
-
-  // eslint-disable-next-line class-methods-use-this
-  private createParameters(paramMap: TParamMap): ToolParam[] {
-    return Object.entries(paramMap).map(([name, paramInfo]) => ({
-      name,
-      description: paramInfo.description,
-      type: paramInfo.type,
-      required: paramInfo.required,
-    }));
-  }
-
-  public invoke(args: ConvertParamMapToArgs<TParamMap>): TReturns {
-    const returned = this.invokeFn(args);
-
-    this.descriptor.invocations.push({
-      arguments: args,
-      returned,
-    });
-
-    return returned;
-  }
-}
+import { ToolDescriptor } from "@typeDefs";
 
 /**
  * @category Core Interfaces
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 89c60a3..da336ca 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -91,6 +91,7 @@ export {
  * Public API - Utils
  */
 export { FnTemplate } from "./utils";
+// TODO export Tool
 
 /*
  * Public API - TypeDefs
diff --git a/packages/core/src/typeDefs.ts b/packages/core/src/typeDefs.ts
index 4c35b13..7890491 100644
--- a/packages/core/src/typeDefs.ts
+++ b/packages/core/src/typeDefs.ts
@@ -71,3 +71,24 @@ export type InferRequestOptions<T> =
   T extends ModelApi<infer U, unknown> ? U : never;
 export type InferResponse<T> =
   T extends ModelApi<ModelRequestOptions, infer V> ? V : never;
+
+export type ToolParameterTypes = "STR" | "NUM" | "BOOL";
+
+export interface ToolParam {
+  name: string;
+  description: string;
+  type: ToolParameterTypes;
+  required: boolean;
+}
+
+export interface ToolInvocation<TArgs> {
+  arguments: TArgs;
+  returned?: unknown;
+}
+
+export interface ToolDescriptor {
+  name: string;
+  description: string;
+  parameters: ToolParam[];
+  invocations: ToolInvocation<any>[];
+}
diff --git a/packages/core/src/utils/Tool.ts b/packages/core/src/utils/Tool.ts
new file mode 100644
index 0000000..37bf65e
--- /dev/null
+++ b/packages/core/src/utils/Tool.ts
@@ -0,0 +1,144 @@
+import { ToolDescriptor, ToolParam, ToolParameterTypes } from "@typeDefs";
+
+export type ToolParamMap = {
+  [key: string]: {
+    description: string;
+    type: ToolParameterTypes;
+    required: boolean;
+  };
+};
+
+type ExtractArgumentType<T extends ToolParameterTypes> = T extends "STR"
+  ? string
+  : T extends "NUM"
+    ? number
+    : T extends "BOOL"
+      ? boolean
+      : never;
+
+export type ConvertParamMapToArgs<TParamMap extends ToolParamMap> = {
+  [K in keyof TParamMap as TParamMap[K]["required"] extends true
+    ? K
+    : never]: ExtractArgumentType<TParamMap[K]["type"]>;
+} & {
+  [K in keyof TParamMap as TParamMap[K]["required"] extends false | undefined
+    ? K
+    : never]?: ExtractArgumentType<TParamMap[K]["type"]>;
+};
+
+export class Tool<TParamMap extends ToolParamMap, TReturns = unknown> {
+  private invokeFn: (args: ConvertParamMapToArgs<TParamMap>) => TReturns;
+
+  public descriptor: ToolDescriptor;
+
+  constructor(
+    name: string,
+    description: string,
+    paramMap: TParamMap,
+    invokeFn: (args: ConvertParamMapToArgs<TParamMap>) => TReturns,
+  ) {
+    this.invokeFn = invokeFn;
+    this.descriptor = {
+      name,
+      description,
+      parameters: this.createParameters(paramMap),
+      invocations: [],
+    };
+  }
+
+  // eslint-disable-next-line class-methods-use-this
+  private createParameters(paramMap: TParamMap): ToolParam[] {
+    return Object.entries(paramMap).map(([name, paramInfo]) => ({
+      name,
+      description: paramInfo.description,
+      type: paramInfo.type,
+      required: paramInfo.required,
+    }));
+  }
+
+  public invoke(args: ConvertParamMapToArgs<TParamMap>): TReturns {
+    const returned = this.invokeFn(args);
+
+    this.descriptor.invocations.push({
+      arguments: args,
+      returned,
+    });
+
+    return returned;
+  }
+}
+
+export const a = new Tool(
+  "get_current_weather",
+  "Get the current weather for a given location",
+  {
+    city: {
+      description: "The city name",
+      type: "STR",
+      required: true,
+    },
+    state: {
+      description: "The state name",
+      type: "STR",
+      required: true,
+    },
+    zipcode: {
+      description: "An optional zipcode",
+      type: "NUM",
+      required: false,
+    },
+  },
+  // should work:
+  ({ city, state, zipcode }) => {
+    console.log("Invoking get_current_weather tool...", {
+      city,
+      state,
+      zipcode,
+    });
+    return {
+      temperature: "70",
+    };
+  },
+);
+
+a.invoke({ city: "San Francisco", state: "CA" });
+a.invoke({ city: "San Francisco", state: "CA", zipcode: 94105 });
+
+// bad:
+// a.invoke({ city: "San Francisco" });
+// a.invoke({ city: "San Francisco", state: "CA", zipcode: 94105, xxx: "yyy" });
+// a.invoke({ city: "San Francisco", state: 123 });
+
+// export const x1 = new Tool(
+//   "get_current_weather",
+//   "Get the current weather for a given location",
+//   {
+//     city: {
+//       description: "The city name",
+//       type: "STR",
+//       required: true,
+//     },
+//     state: {
+//       description: "The state name",
+//       type: "STR",
+//       required: true,
+//     },
+//     zipcode: {
+//       description: "An optional zipcode",
+//       type: "NUM",
+//       required: false,
+//     },
+//   },
+//   // should break, prop not defined:
+//   ({ city, state, zipcode, xxx }) => {
+//     console.log("Invoking get_current_weather tool...", {
+//       city,
+//       state,
+//       zipcode,
+//       xxx,
+//     });
+//     return {
+//       temperature: "70",
+//     };
+//   },
+// );
diff --git a/packages/core/src/utils/index.ts b/packages/core/src/utils/index.ts
index c676e80..404cf23 100644
--- a/packages/core/src/utils/index.ts
+++ b/packages/core/src/utils/index.ts
@@ -1 +1,2 @@
 export * from "./Template";
+export * from "./Tool";
diff --git a/tests/integration/vertexai-tools.test.ts b/tests/integration/vertexai-tools.test.ts
index bd7b879..6efc159 100644
--- a/tests/integration/vertexai-tools.test.ts
+++ b/tests/integration/vertexai-tools.test.ts
@@ -1,7 +1,8 @@
 /* eslint-disable import/no-relative-packages */
 import { createVertexAiModelProvider } from "@packages/gcloud-vertex-ai";
+// TODO decide on these import/exports as part of public API
 import { mapGeminiResponseToToolInvocations } from "../../packages/core/src/apis/google/mapGeminiResponseToToolInvocations";
-import { Tool } from "../../packages/core/src/apis/shared/ToolUseRequestOptions";
+import { Tool } from "../../packages/core/src/utils/Tool";
 
 xtest("VertexAI - Google Gemini (Tools)", async () => {
   // arrange