Fix Anthropic max_tokens issue causing slow execution (issue #3807)

Make max_tokens optional and compute dynamically when not set by user.

Previously, max_tokens defaulted to 4096 and was always passed to the
Anthropic API, causing the model to generate up to 4096 tokens even for
simple queries that should only need a few tokens. This resulted in
extremely slow execution times.

Changes:
- Changed max_tokens parameter from int (default 4096) to int | None (default None)
- Added dynamic computation in _prepare_completion_params():
  - Default: 1024 tokens (much more reasonable for most queries)
  - Large context models (200k+): 2048 tokens
  - User-specified values are always respected
- Updated docstring to reflect that max_tokens is now optional
- Added comprehensive tests covering:
  - Explicit max_tokens values are passed through unchanged
  - Default behavior computes reasonable max_tokens dynamically
  - max_tokens=None uses dynamic computation
  - Dynamic values are appropriate for model context window size
  - User-provided values are always respected

This fix aligns with the v0.203.1 behavior where max_tokens was optional
and only passed when explicitly set, while maintaining compatibility with
the Anthropic SDK requirement that max_tokens must be provided.

Fixes #3807

Co-Authored-By: João <joao@crewai.com>

lib/crewai/tests/llms/anthropic/test_anthropic.py

@@ -664,3 +664,105 @@ def test_anthropic_token_usage_tracking():
         assert usage["input_tokens"] == 50
         assert usage["output_tokens"] == 25
         assert usage["total_tokens"] == 75
+
+
+def test_anthropic_max_tokens_explicit():
+    """
+    Test that explicit max_tokens is passed through to the API
+    """
+    llm = LLM(model="anthropic/claude-3-5-sonnet-20241022", max_tokens=4096)
+
+    with patch.object(llm.client.messages, 'create') as mock_create:
+        mock_response = MagicMock()
+        mock_response.content = [MagicMock(text="test response")]
+        mock_response.usage = MagicMock(input_tokens=10, output_tokens=20)
+        mock_create.return_value = mock_response
+
+        llm.call("Hello")
+
+        call_kwargs = mock_create.call_args[1]
+        assert call_kwargs["max_tokens"] == 4096
+
+
+def test_anthropic_max_tokens_default_computed():
+    """
+    Test that max_tokens is computed dynamically when not explicitly set
+    """
+    llm = LLM(model="anthropic/claude-3-5-sonnet-20241022")
+
+    with patch.object(llm.client.messages, 'create') as mock_create:
+        mock_response = MagicMock()
+        mock_response.content = [MagicMock(text="test response")]
+        mock_response.usage = MagicMock(input_tokens=10, output_tokens=20)
+        mock_create.return_value = mock_response
+
+        llm.call("Hello")
+
+        call_kwargs = mock_create.call_args[1]
+        assert "max_tokens" in call_kwargs
+        assert call_kwargs["max_tokens"] is not None
+        assert call_kwargs["max_tokens"] < 4096
+
+
+def test_anthropic_max_tokens_none_uses_dynamic_default():
+    """
+    Test that max_tokens=None results in dynamic computation
+    """
+    llm = LLM(model="anthropic/claude-3-5-sonnet-20241022", max_tokens=None)
+
+    assert llm.max_tokens is None
+
+    with patch.object(llm.client.messages, 'create') as mock_create:
+        mock_response = MagicMock()
+        mock_response.content = [MagicMock(text="test response")]
+        mock_response.usage = MagicMock(input_tokens=10, output_tokens=20)
+        mock_create.return_value = mock_response
+
+        llm.call("Hello")
+
+        call_kwargs = mock_create.call_args[1]
+        assert "max_tokens" in call_kwargs
+        assert call_kwargs["max_tokens"] is not None
+        assert call_kwargs["max_tokens"] < 4096
+
+
+def test_anthropic_max_tokens_dynamic_for_large_context():
+    """
+    Test that dynamic max_tokens is larger for models with large context windows
+    """
+    llm = LLM(model="anthropic/claude-3-5-sonnet-20241022")
+
+    with patch.object(llm.client.messages, 'create') as mock_create:
+        mock_response = MagicMock()
+        mock_response.content = [MagicMock(text="test response")]
+        mock_response.usage = MagicMock(input_tokens=10, output_tokens=20)
+        mock_create.return_value = mock_response
+
+        llm.call("Hello")
+
+        call_kwargs = mock_create.call_args[1]
+        computed_max_tokens = call_kwargs["max_tokens"]
+        
+        assert computed_max_tokens >= 1024
+        assert computed_max_tokens <= 2048
+
+
+def test_anthropic_max_tokens_respects_user_value():
+    """
+    Test that user-provided max_tokens is always respected
+    """
+    test_values = [512, 1024, 2048, 4096, 8192]
+    
+    for max_tokens_value in test_values:
+        llm = LLM(model="anthropic/claude-3-5-sonnet-20241022", max_tokens=max_tokens_value)
+
+        with patch.object(llm.client.messages, 'create') as mock_create:
+            mock_response = MagicMock()
+            mock_response.content = [MagicMock(text="test response")]
+            mock_response.usage = MagicMock(input_tokens=10, output_tokens=20)
+            mock_create.return_value = mock_response
+
+            llm.call("Hello")
+
+            call_kwargs = mock_create.call_args[1]
+            assert call_kwargs["max_tokens"] == max_tokens_value