fix(agent): support tool calls in fromChatMessages and fix input typing

.changeset/from-chat-messages-tool-calls.md

@@ -0,0 +1,5 @@
+---
+'@openrouter/agent': patch
+---
+
+Fix `fromChatMessages` to convert assistant `toolCalls` to `function_call` items, and widen `callModel`'s `input` type to accept `models.InputsUnion`.

packages/agent/src/lib/async-params.ts

@@ -82,7 +82,7 @@ type BaseCallModelInput<
     models.ResponsesRequest[K]
   >;
 } & {
-  input: FieldOrAsyncFunction<Item[]> | string;
+  input: FieldOrAsyncFunction<models.InputsUnion | Item[]>;
   tools?: TTools;
   /**
    * Optional filter restricting which tools are exposed to the model for this

packages/agent/src/lib/chat-compat.test.ts

@@ -1,6 +1,7 @@
+import type { OpenRouterCore } from '@openrouter/sdk/core';
 import type * as models from '@openrouter/sdk/models';
-
 import { describe, expect, it } from 'vitest';
+import { callModel } from '../inner-loop/call-model.js';
 import { fromChatMessages, toChatMessage } from './chat-compat.js';
 
 /**
@@ -203,6 +204,195 @@ describe('fromChatMessages', () => {
     });
   });
 
+  describe('assistant message tool calls conversion', () => {
+    it('converts assistant message with toolCalls and null content to function_call item', () => {
+      const messages: models.ChatMessages[] = [
+        {
+          role: 'user',
+          content: 'What is the weather?',
+        },
+        {
+          role: 'assistant',
+          content: null,
+          toolCalls: [
+            {
+              id: 'call_123',
+              type: 'function',
+              function: {
+                name: 'get_weather',
+                arguments: '{"location":"Paris"}',
+              },
+            },
+          ],
+        },
+        {
+          role: 'tool',
+          toolCallId: 'call_123',
+          content: '{"temperature": 20}',
+        },
+      ];
+
+      const result = fromChatMessages(messages);
+
+      expect(result).toEqual([
+        {
+          role: 'user',
+          content: 'What is the weather?',
+        },
+        {
+          type: 'function_call',
+          callId: 'call_123',
+          name: 'get_weather',
+          arguments: '{"location":"Paris"}',
+        },
+        {
+          type: 'function_call_output',
+          callId: 'call_123',
+          output: '{"temperature": 20}',
+        },
+      ]);
+    });
+
+    it('converts assistant message with both content and toolCalls', () => {
+      const messages: models.ChatMessages[] = [
+        {
+          role: 'assistant',
+          content: 'Let me check that for you.',
+          toolCalls: [
+            {
+              id: 'call_123',
+              type: 'function',
+              function: {
+                name: 'get_weather',
+                arguments: '{"location":"Paris"}',
+              },
+            },
+          ],
+        },
+      ];
+
+      const result = fromChatMessages(messages);
+
+      expect(result).toEqual([
+        {
+          role: 'assistant',
+          content: 'Let me check that for you.',
+        },
+        {
+          type: 'function_call',
+          callId: 'call_123',
+          name: 'get_weather',
+          arguments: '{"location":"Paris"}',
+        },
+      ]);
+    });
+
+    it('supports snake_case tool_calls and tool_call_id', () => {
+      const messages = [
+        {
+          role: 'assistant' as const,
+          content: null,
+          tool_calls: [
+            {
+              id: 'call_456',
+              type: 'function' as const,
+              function: {
+                name: 'search',
+                arguments: '{"query":"vitest"}',
+              },
+            },
+          ],
+        },
+        {
+          role: 'tool' as const,
+          tool_call_id: 'call_456',
+          content: 'Found results',
+        },
+      ];
+
+      const result = fromChatMessages(messages as unknown as models.ChatMessages[]);
+
+      expect(result).toEqual([
+        {
+          type: 'function_call',
+          callId: 'call_456',
+          name: 'search',
+          arguments: '{"query":"vitest"}',
+        },
+        {
+          type: 'function_call_output',
+          callId: 'call_456',
+          output: 'Found results',
+        },
+      ]);
+    });
+
+    it('stringifies object arguments on tool calls', () => {
+      const messages = [
+        {
+          role: 'assistant' as const,
+          content: null,
+          toolCalls: [
+            {
+              id: 'call_789',
+              type: 'function' as const,
+              function: {
+                name: 'calculate',
+                arguments: {
+                  a: 1,
+                  b: 2,
+                } as unknown as string,
+              },
+            },
+          ],
+        },
+      ];
+
+      const result = fromChatMessages(messages as unknown as models.ChatMessages[]);
+
+      expect(result).toEqual([
+        {
+          type: 'function_call',
+          callId: 'call_789',
+          name: 'calculate',
+          arguments: JSON.stringify({
+            a: 1,
+            b: 2,
+          }),
+        },
+      ]);
+    });
+
+    it('produces input accepted by callModel without type errors', () => {
+      const chatMessages: models.ChatMessages[] = [
+        {
+          role: 'system',
+          content: 'You are a helpful assistant.',
+        },
+        {
+          role: 'user',
+          content: 'Hello!',
+        },
+        {
+          role: 'assistant',
+          content: 'Hi there! How can I help you?',
+        },
+        {
+          role: 'user',
+          content: 'What is the weather like?',
+        },
+      ];
+
+      const fakeClient = {} as OpenRouterCore;
+      const result = callModel(fakeClient, {
+        model: 'openai/gpt-5-nano',
+        input: fromChatMessages(chatMessages),
+      });
+
+      expect(result).toBeDefined();
+    });
+  });
+
   describe('content array handling', () => {
     it('stringifies array content for user messages', () => {
       const messages: models.ChatMessages[] = [

packages/agent/src/lib/chat-compat.ts

@@ -80,28 +80,70 @@ function contentToString(content: unknown): string {
  * ```
  */
 export function fromChatMessages(messages: models.ChatMessages[]): models.InputsUnion {
-  return messages.map((msg): models.EasyInputMessage | models.FunctionCallOutputItem => {
+  const result: (
+    | models.EasyInputMessage
+    | models.FunctionCallOutputItem
+    | models.OutputFunctionCallItem
+  )[] = [];
+
+  for (const msg of messages) {
     if (isToolResponseMessage(msg)) {
-      return {
+      result.push({
         type: 'function_call_output' as const,
-        callId: msg.toolCallId,
+        callId:
+          msg.toolCallId ??
+          (
+            msg as {
+              tool_call_id?: string;
+            }
+          ).tool_call_id ??
+          '',
         output: contentToString(msg.content),
-      };
+      });
+      continue;
     }
 
     if (isAssistantMessage(msg)) {
-      return {
-        role: mapChatRole('assistant'),
-        content: contentToString(msg.content),
-      };
+      const toolCalls =
+        msg.toolCalls ??
+        (
+          msg as {
+            tool_calls?: models.ChatToolCall[];
+          }
+        ).tool_calls;
+      const content = contentToString(msg.content);
+
+      if (content.length > 0 || !toolCalls?.length) {
+        result.push({
+          role: mapChatRole('assistant'),
+          content,
+        });
+      }
+
+      if (toolCalls?.length) {
+        for (const tc of toolCalls) {
+          result.push({
+            type: 'function_call' as const,
+            callId: tc.id,
+            name: tc.function.name,
+            arguments:
+              typeof tc.function.arguments === 'string'
+                ? tc.function.arguments
+                : JSON.stringify(tc.function.arguments),
+          });
+        }
+      }
+      continue;
     }
 
     // System, user, developer messages
-    return {
+    result.push({
       role: mapChatRole(msg.role),
       content: contentToString(msg.content),
-    };
-  });
+    });
+  }
+
+  return result;
 }
 
 /**