fix: round-trip serialization of Callable types with an explicit parameter list

[vidigoat]

Problem

serialize_type / deserialize_type corrupt Callable types that declare an explicit parameter list, so components that serialize such a type can't round-trip through to_dict/from_dict.

from typing import Callable
from haystack.utils.type_serialization import serialize_type, deserialize_type

serialize_type(Callable[[int, str], bool])
# 'typing.Callable[, bool]'   <- parameter list dropped
deserialize_type('typing.Callable[, bool]')
# DeserializationError: Could not deserialize type:

typing.get_args(Callable[[int, str], bool]) returns ([int, str], bool) — the first arg is a Python list, not a type. The arg serializer passed it to serialize_type, whose name handling turns a value starting with [ into an empty string, so the whole parameter list vanished. The result then fails to deserialize.

This affects any component whose serialized schema uses a Callable with a parameter list — e.g. ConditionalRouter / OutputAdapter with a Callable[[...], ...] output type: silent corruption on save, hard failure on load. (A previous fix, #12062, only handled the Callable[..., R] Ellipsis form.)

Fix

Handle the parameter-list argument explicitly on both sides — render it as [X, Y] on serialize and read it back into a Python list on deserialize — so these Callable types round-trip. Everything else delegates to the existing functions, so behaviour for all other types is unchanged.

Added tests for the explicit-parameter-list case (serialize, deserialize, round-trip); they fail before this change and pass after. Full test_type_serialization.py is green (192 passed), and ruff is clean. Release note included.

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Disclosure: I used an AI assistant while working on this; I reproduced the bug, verified the fix, and reviewed every line myself.

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/utils
type_serialization.py
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[rkfshakti]

This is a well-scoped fix. The Callable[[X, Y], R] parameter list is indeed a special case in Python's typing system — typing.get_args returns it as a plain list, not a type, so the existing serialize_type path can't handle it.

The approach of adding _serialize_type_arg / _deserialize_type_arg helper functions is clean and minimal — it isolates the special case without touching the main serialization logic. The bracket-delimited format [X, Y] round-trips correctly because _parse_generic_args already splits on commas at the top level, so nested brackets are preserved.

The test coverage is thorough: all common Callable shapes are tested (with params, empty params, nested generics, nested Callables, Optional Callables, and Dict/List of Callables). The release note is well-written.

One suggestion: consider adding a test for Callable[..., int] (ellipsis) to confirm it still works after the change, since _parse_generic_args would see ... as a single argument. But this is not a blocker — the existing test_output_type_deserialization_legacy_ellipsis_literal test covers that path on the deserialization side.