fix: Set model_max_length in the Tokenizer of DefaultPromptHandler (#5596)

* Set model_max_length in tokenizer in prompt handler

* Add release note

Author: bogdankostic

haystack/nodes/prompt/invocation_layer/handlers.py

@@ -63,6 +63,7 @@ class DefaultPromptHandler:
 
     def __init__(self, model_name_or_path: str, model_max_length: int, max_length: int = 100):
         self.tokenizer = AutoTokenizer.from_pretrained(model_name_or_path)
+        self.tokenizer.model_max_length = model_max_length
         self.model_max_length = model_max_length
         self.max_length = max_length
 

releasenotes/notes/fix-model_max_length-prompt_handler-7f34c40c62a8c55b.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fix model_max_length not being set in the Tokenizer in DefaultPromptHandler.

test/prompt/test_handlers.py

@@ -57,6 +57,13 @@ def test_prompt_handler_negative():
     }
 
 
+@pytest.mark.unit
+@patch("haystack.nodes.prompt.invocation_layer.handlers.AutoTokenizer.from_pretrained")
+def test_prompt_handler_model_max_length_set_in_tokenizer(mock_tokenizer):
+    prompt_handler = DefaultPromptHandler(model_name_or_path="model_path", model_max_length=10, max_length=3)
+    assert prompt_handler.tokenizer.model_max_length == 10
+
+
 @pytest.mark.integration
 def test_prompt_handler_basics():
     handler = DefaultPromptHandler(model_name_or_path="gpt2", model_max_length=20, max_length=10)
@@ -65,6 +72,9 @@ def test_prompt_handler_basics():
     handler = DefaultPromptHandler(model_name_or_path="gpt2", model_max_length=20)
     assert handler.max_length == 100
 
+    # test model_max_length is set in tokenizer
+    assert handler.tokenizer.model_max_length == 20
+
 
 @pytest.mark.integration
 def test_gpt2_prompt_handler():