diff --git a/packages/ai/src/protocols/open-responses.ts b/packages/ai/src/protocols/open-responses.ts
index c1ff98ba2de1..00e0f5213a17 100644
--- a/packages/ai/src/protocols/open-responses.ts
+++ b/packages/ai/src/protocols/open-responses.ts
@@ -522,7 +522,7 @@ const lowerToolCall = (part: ToolCallPart, providerMetadataKey: string): OpenRes
     call_id: part.id,
     name: part.name,
     namespace: part.namespace,
-    arguments: ProviderShared.encodeJson(part.input),
+    arguments: ProviderShared.encodeJson(part.input === undefined ? {} : part.input),
   }
 }
 
diff --git a/packages/ai/src/protocols/openai-chat.ts b/packages/ai/src/protocols/openai-chat.ts
index 7f16b542836a..0649d28fba28 100644
--- a/packages/ai/src/protocols/openai-chat.ts
+++ b/packages/ai/src/protocols/openai-chat.ts
@@ -315,7 +315,7 @@ const lowerToolCall = (part: ToolCallPart, options: LoweringOptions): OpenAIChat
   type: "function",
   function: {
     name: part.name,
-    arguments: ProviderShared.encodeJson(part.input),
+    arguments: ProviderShared.encodeJson(part.input === undefined ? {} : part.input),
   },
 })
 
diff --git a/packages/ai/test/provider/missing-tool-input.test.ts b/packages/ai/test/provider/missing-tool-input.test.ts
new file mode 100644
index 000000000000..ace51b374a0a
--- /dev/null
+++ b/packages/ai/test/provider/missing-tool-input.test.ts
@@ -0,0 +1,73 @@
+import { expect } from "bun:test"
+import { Effect, Schema } from "effect"
+import { LLM, Message, ToolCallPart } from "../../src/index.js"
+import { OpenAIChat } from "../../src/protocols/openai-chat.js"
+import { OpenAIResponses } from "../../src/protocols/openai-responses.js"
+import { Auth } from "../../src/route/auth.js"
+import { compileRequest } from "../../src/route/client.js"
+import { it } from "../lib/effect.js"
+
+for (const route of [OpenAIChat.route, OpenAIResponses.route]) {
+  const model = route
+    .with({ endpoint: { baseURL: "https://api.openai.test/v1" }, auth: Auth.bearer("test") })
+    .model({ id: "gpt-4.1-mini" })
+  const chat = route.id === "openai-chat"
+
+  it.effect(`${route.id} serializes schema-valid undefined historical tool input as an empty object`, () =>
+    Effect.gen(function* () {
+      const message = Schema.decodeUnknownSync(Message)({
+        role: "assistant",
+        content: [{ type: "tool-call", id: "call_1", name: "lookup", input: undefined }],
+      })
+      expect(Schema.is(Message)(message)).toBe(true)
+      const prepared = yield* compileRequest(
+        LLM.request({
+          model,
+          messages: [
+            Message.user("Look up the value."),
+            message,
+            Message.tool({ id: "call_1", name: "lookup", result: "Missing input", resultType: "error" }),
+          ],
+        }),
+      )
+      if (chat)
+        expect(prepared.body.messages).toContainEqual({
+          role: "assistant",
+          content: null,
+          tool_calls: [{ id: "call_1", type: "function", function: { name: "lookup", arguments: "{}" } }],
+        })
+      if (!chat)
+        expect(prepared.body.input).toContainEqual({
+          type: "function_call",
+          call_id: "call_1",
+          name: "lookup",
+          arguments: "{}",
+        })
+      expect(message.content[0]).toEqual({ type: "tool-call", id: "call_1", name: "lookup", input: undefined })
+    }),
+  )
+
+  it.effect(`${route.id} preserves defined historical tool inputs`, () =>
+    Effect.gen(function* () {
+      for (const [input, encoded] of [
+        [null, "null"],
+        [[], "[]"],
+        [42, "42"],
+        ["invalid", '"invalid"'],
+        [{ value: "original" }, '{"value":"original"}'],
+      ] as const) {
+        const prepared = yield* compileRequest(
+          LLM.request({
+            model,
+            messages: [
+              Message.assistant([ToolCallPart.make({ id: "call_1", name: "lookup", input })]),
+              Message.tool({ id: "call_1", name: "lookup", result: "Invalid input", resultType: "error" }),
+            ],
+          }),
+        )
+        if (chat) expect(prepared.body.messages[0].tool_calls[0].function.arguments).toBe(encoded)
+        if (!chat) expect(prepared.body.input[0].arguments).toBe(encoded)
+      }
+    }),
+  )
+}