diff --git a/src/internals/helpers/object.ts b/src/internals/helpers/object.ts
index a389158b..834ec8eb 100644
--- a/src/internals/helpers/object.ts
+++ b/src/internals/helpers/object.ts
@@ -65,7 +65,7 @@ export function getPropStrict(target: NonNullable<unknown>, path: string): any {
 
 export function getProp(
   target: unknown,
-  paths: readonly (string | symbol)[],
+  paths: readonly (keyof any)[],
   defaultValue: any = undefined,
 ) {
   let value: any = target;
diff --git a/src/tools/base.ts b/src/tools/base.ts
index d8b4c984..61bde9c0 100644
--- a/src/tools/base.ts
+++ b/src/tools/base.ts
@@ -203,6 +203,8 @@ export abstract class Tool<
   }
 
   run(input: ToolInputRaw<this>, options?: TRunOptions): Promise<TOutput> {
+    input = shallowCopy(input);
+
     return RunContext.enter(
       this,
       { signal: options?.signal, params: [input, options] as const },
@@ -211,6 +213,7 @@ export abstract class Tool<
         let errorPropagated = false;
 
         try {
+          this.preprocessInput(input);
           await this.assertInput(input);
 
           const output = await new Retryable({
@@ -340,6 +343,9 @@ export abstract class Tool<
     this.validateInput(schema, input);
   }
 
+  // eslint-disable-next-line unused-imports/no-unused-vars
+  protected preprocessInput(rawInput: unknown): void {}
+
   protected validateInput(
     schema: AnyToolSchemaLike,
     rawInput: unknown,
diff --git a/src/tools/weather/openMeteo.ts b/src/tools/weather/openMeteo.ts
index c8b9d0d7..54ff0d5f 100644
--- a/src/tools/weather/openMeteo.ts
+++ b/src/tools/weather/openMeteo.ts
@@ -27,6 +27,7 @@ import { createURLParams } from "@/internals/fetcher.js";
 import { isNullish, pick, pickBy } from "remeda";
 import { Cache } from "@/cache/decoratorCache.js";
 import { RunContext } from "@/context.js";
+import { getProp, setProp } from "@/internals/helpers/object.js";
 
 type ToolOptions = { apiKey?: string } & BaseToolOptions;
 type ToolRunOptions = BaseToolRunOptions;
@@ -102,6 +103,20 @@ export class OpenMeteoTool extends Tool<
     this.register();
   }
 
+  protected preprocessInput(rawInput: unknown) {
+    super.preprocessInput(rawInput);
+
+    const fixDate = (key: keyof ToolInput<this>) => {
+      const value = getProp(rawInput, [key]);
+      if (value) {
+        setProp(rawInput, [key], value.substring(0, 10));
+      }
+    };
+
+    fixDate("start_date");
+    fixDate("end_date");
+  }
+
   protected async _run(
     { location, start_date: startDate, end_date: endDate, ...input }: ToolInput<this>,
     _options: BaseToolRunOptions | undefined,