diff --git a/haystack/components/builders/chat_prompt_builder.py b/haystack/components/builders/chat_prompt_builder.py
index 8956d51506..7483df59a9 100644
--- a/haystack/components/builders/chat_prompt_builder.py
+++ b/haystack/components/builders/chat_prompt_builder.py
@@ -6,7 +6,6 @@
 from copy import deepcopy
 from typing import Any, Literal, Optional, Union
 
-from jinja2 import meta
 from jinja2.sandbox import SandboxedEnvironment
 
 from haystack import component, default_from_dict, default_to_dict, logging
@@ -14,6 +13,7 @@
 from haystack.lazy_imports import LazyImport
 from haystack.utils import Jinja2TimeExtension
 from haystack.utils.jinja2_chat_extension import ChatMessageExtension, templatize_part
+from haystack.utils.jinja2_extensions import _extract_template_variables_and_assignments
 
 logger = logging.getLogger(__name__)
 
@@ -179,13 +179,17 @@ def __init__(
                             raise ValueError(NO_TEXT_ERROR_MESSAGE.format(role=message.role.value, message=message))
                         if message.text and "templatize_part" in message.text:
                             raise ValueError(FILTER_NOT_ALLOWED_ERROR_MESSAGE)
-                        ast = self._env.parse(message.text)
-                        template_variables = meta.find_undeclared_variables(ast)
-                        extracted_variables += list(template_variables)
+                        assigned_variables, template_variables = _extract_template_variables_and_assignments(
+                            env=self._env, template=message.text
+                        )
+                        extracted_variables += list(template_variables - assigned_variables)
             elif isinstance(template, str):
-                ast = self._env.parse(template)
-                extracted_variables = list(meta.find_undeclared_variables(ast))
+                assigned_variables, template_variables = _extract_template_variables_and_assignments(
+                    env=self._env, template=template
+                )
+                extracted_variables = list(template_variables - assigned_variables)
 
+        extracted_variables = extracted_variables or []
         self.variables = variables or extracted_variables
         self.required_variables = required_variables or []
 
diff --git a/haystack/components/builders/prompt_builder.py b/haystack/components/builders/prompt_builder.py
index 3e1d3603ca..a7c3f1950c 100644
--- a/haystack/components/builders/prompt_builder.py
+++ b/haystack/components/builders/prompt_builder.py
@@ -4,11 +4,11 @@
 
 from typing import Any, Literal, Optional, Union
 
-from jinja2 import meta
 from jinja2.sandbox import SandboxedEnvironment
 
 from haystack import component, default_to_dict, logging
 from haystack.utils import Jinja2TimeExtension
+from haystack.utils.jinja2_extensions import _extract_template_variables_and_assignments
 
 logger = logging.getLogger(__name__)
 
@@ -174,11 +174,13 @@ def __init__(
             self._env = SandboxedEnvironment()
 
         self.template = self._env.from_string(template)
+
         if not variables:
-            # infer variables from template
-            ast = self._env.parse(template)
-            template_variables = meta.find_undeclared_variables(ast)
-            variables = list(template_variables)
+            assigned_variables, template_variables = _extract_template_variables_and_assignments(
+                env=self._env, template=template
+            )
+            variables = list(template_variables - assigned_variables)
+
         variables = variables or []
         self.variables = variables
 
diff --git a/haystack/components/converters/output_adapter.py b/haystack/components/converters/output_adapter.py
index 24cfdeaa42..bb5fc96ba3 100644
--- a/haystack/components/converters/output_adapter.py
+++ b/haystack/components/converters/output_adapter.py
@@ -7,13 +7,14 @@
 from typing import Any, Callable, Optional
 
 import jinja2.runtime
-from jinja2 import Environment, TemplateSyntaxError, meta
+from jinja2 import TemplateSyntaxError
 from jinja2.nativetypes import NativeEnvironment
 from jinja2.sandbox import SandboxedEnvironment
 from typing_extensions import TypeAlias
 
 from haystack import component, default_from_dict, default_to_dict, logging
 from haystack.utils import deserialize_callable, deserialize_type, serialize_callable, serialize_type
+from haystack.utils.jinja2_extensions import _extract_template_variables_and_assignments
 
 logger = logging.getLogger(__name__)
 
@@ -46,7 +47,7 @@ def __init__(
         output_type: TypeAlias,
         custom_filters: Optional[dict[str, Callable]] = None,
         unsafe: bool = False,
-    ):
+    ) -> None:
         """
         Create an OutputAdapter component.
 
@@ -92,7 +93,10 @@ def __init__(
             self._env.filters[name] = filter_func
 
         # b) extract variables in the template
-        route_input_names = self._extract_variables(self._env)
+        assigned_variables, template_variables = _extract_template_variables_and_assignments(
+            env=self._env, template=self.template
+        )
+        route_input_names = template_variables - assigned_variables
         input_types.update(route_input_names)
 
         # the env is not needed, discarded automatically
@@ -173,13 +177,3 @@ def from_dict(cls, data: dict[str, Any]) -> "OutputAdapter":
                 for name, filter_func in custom_filters.items()
             }
         return default_from_dict(cls, data)
-
-    def _extract_variables(self, env: Environment) -> set[str]:
-        """
-        Extracts all variables from a list of Jinja template strings.
-
-        :param env: A Jinja environment.
-        :return: A set of variable names extracted from the template strings.
-        """
-        ast = env.parse(self.template)
-        return meta.find_undeclared_variables(ast)
diff --git a/haystack/components/routers/conditional_router.py b/haystack/components/routers/conditional_router.py
index c8125b869d..7b310d0419 100644
--- a/haystack/components/routers/conditional_router.py
+++ b/haystack/components/routers/conditional_router.py
@@ -6,12 +6,13 @@
 import contextlib
 from typing import Any, Callable, Mapping, Optional, Sequence, TypedDict, Union, get_args, get_origin
 
-from jinja2 import Environment, TemplateSyntaxError, meta
+from jinja2 import Environment, TemplateSyntaxError
 from jinja2.nativetypes import NativeEnvironment
 from jinja2.sandbox import SandboxedEnvironment
 
 from haystack import component, default_from_dict, default_to_dict, logging
 from haystack.utils import deserialize_callable, deserialize_type, serialize_callable, serialize_type
+from haystack.utils.jinja2_extensions import _extract_template_variables_and_assignments
 
 logger = logging.getLogger(__name__)
 
@@ -403,7 +404,8 @@ def _validate_routes(self, routes: list[Route]):
                 if not self._validate_template(self._env, output):
                     raise ValueError(f"Invalid template for output: {output}")
 
-    def _extract_variables(self, env: Environment, templates: list[str]) -> set[str]:
+    @staticmethod
+    def _extract_variables(env: Environment, templates: list[str]) -> set[str]:
         """
         Extracts all variables from a list of Jinja template strings.
 
@@ -413,7 +415,10 @@ def _extract_variables(self, env: Environment, templates: list[str]) -> set[str]
         """
         variables = set()
         for template in templates:
-            variables.update(meta.find_undeclared_variables(env.parse(template)))
+            assigned_variables, template_variables = _extract_template_variables_and_assignments(
+                env=env, template=template
+            )
+            variables.update(template_variables - assigned_variables)
         return variables
 
     def _validate_template(self, env: Environment, template_text: str):
diff --git a/haystack/utils/jinja2_extensions.py b/haystack/utils/jinja2_extensions.py
index 348b571d60..7d8cb32b47 100644
--- a/haystack/utils/jinja2_extensions.py
+++ b/haystack/utils/jinja2_extensions.py
@@ -4,7 +4,7 @@
 
 from typing import Any, Optional, Union
 
-from jinja2 import Environment, nodes
+from jinja2 import Environment, meta, nodes
 from jinja2.ext import Extension
 
 from haystack.lazy_imports import LazyImport
@@ -94,3 +94,42 @@ def parse(self, parser: Any) -> Union[nodes.Node, list[nodes.Node]]:
         )
 
         return nodes.Output([call_method], lineno=lineno)
+
+
+def _collect_assigned_variables(ast: nodes.Template) -> set[str]:
+    """
+    Extract variables assigned within the Jinja2 template AST.
+
+    :param ast: The Jinja2 Abstract Syntax Tree (AST) of the template.
+
+    :returns:
+        A set of variable names that are assigned within the template.
+    """
+    # Collect all variables assigned inside the template via {% set %}
+    assigned_variables = set()
+
+    for node in ast.find_all(nodes.Assign):
+        if isinstance(node.target, nodes.Name):
+            assigned_variables.add(node.target.name)
+        elif isinstance(node.target, (nodes.List, nodes.Tuple)):
+            for name_node in node.target.items:
+                if isinstance(name_node, nodes.Name):
+                    assigned_variables.add(name_node.name)
+
+    return assigned_variables
+
+
+def _extract_template_variables_and_assignments(env: Environment, template: str) -> tuple[set[str], set[str]]:
+    """
+    Extract variables from a Jinja2 template and variables assigned within it.
+
+    :param env: A Jinja2 environment.
+    :param template: A Jinja2 template string.
+    :returns: A tuple of (assigned_variables, template_variables) where:
+        - assigned_variables: Variables assigned within the template (e.g., via {% set %})
+        - template_variables: All undeclared variables used in the template
+    """
+    jinja2_ast = env.parse(template)
+    template_variables = meta.find_undeclared_variables(jinja2_ast)
+    assigned_variables = _collect_assigned_variables(jinja2_ast)
+    return assigned_variables, template_variables
diff --git a/releasenotes/notes/fix-jinja2-variable-extraction-57f34c6bd249e214.yaml b/releasenotes/notes/fix-jinja2-variable-extraction-57f34c6bd249e214.yaml
new file mode 100644
index 0000000000..5f3b645721
--- /dev/null
+++ b/releasenotes/notes/fix-jinja2-variable-extraction-57f34c6bd249e214.yaml
@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixes jinja2 variable detection in ``ConditionalRouter``, ``ChatPromptBuilder``, ``PromptBuilder`` and ``OutputAdapter`` by properly
+    skipping variables that are assigned within the template.
+    Previously under specific scenarios variables assigned within a template would falsely be picked up as input variables to the component.
+    For more information you can check out the parent issue in the Jinja2 library here: https://github.com/pallets/jinja/issues/2069
diff --git a/test/components/builders/test_chat_prompt_builder.py b/test/components/builders/test_chat_prompt_builder.py
index fa15dcbc56..7d8ec0d987 100644
--- a/test/components/builders/test_chat_prompt_builder.py
+++ b/test/components/builders/test_chat_prompt_builder.py
@@ -957,3 +957,53 @@ def test_from_dict(self):
         assert builder.template == template
         assert builder.variables == ["name", "assistant_name"]
         assert builder.required_variables == ["name"]
+
+    def test_variables_correct_with_assignment(self):
+        template = """{% message role="user" %}
+{% if existing_documents is not none -%}
+{% set x = existing_documents|length -%}
+{% else -%}
+{% set x = 0 -%}
+{% endif -%}
+The number is {{ x }}!
+{% endmessage %}
+"""
+        builder = ChatPromptBuilder(template=template, required_variables="*")
+        assert builder.variables == ["existing_documents"]
+        assert builder.required_variables == "*"
+        res = builder.run(existing_documents=None)
+        assert res["prompt"][0].text == "The number is 0!"
+
+    def test_variables_correct_with_tuple_assignment(self):
+        template = """{% message role="user" %}
+{% if name is not none -%}
+{% set x, y = (0, 1) %}
+{% else -%}
+{% set x, y = (2, 3) %}
+{% endif -%}
+x={{ x }}, y={{ y }}
+Hello, my name is {{name}}!
+{% endmessage %}
+"""
+        builder = ChatPromptBuilder(template=template, required_variables="*")
+        assert builder.variables == ["name"]
+        assert builder.required_variables == "*"
+        res = builder.run(name="John")
+        assert res["prompt"][0].text == "x=0, y=1\nHello, my name is John!"
+
+    def test_variables_correct_with_list_assignment(self):
+        template = """{% message role="user" %}
+{% if name is not none -%}
+{% set x, y = [0, 1] %}
+{% else -%}
+{% set x, y = [2, 3] %}
+{% endif -%}
+x={{ x }}, y={{ y }}
+Hello, my name is {{name}}!
+{% endmessage %}
+"""
+        builder = ChatPromptBuilder(template=template, required_variables="*")
+        assert builder.variables == ["name"]
+        assert builder.required_variables == "*"
+        res = builder.run(name="John")
+        assert res["prompt"][0].text == "x=0, y=1\nHello, my name is John!"
diff --git a/test/components/builders/test_prompt_builder.py b/test/components/builders/test_prompt_builder.py
index 8a5504c5fc..6e8627de3d 100644
--- a/test/components/builders/test_prompt_builder.py
+++ b/test/components/builders/test_prompt_builder.py
@@ -337,3 +337,47 @@ def test_warning_no_required_variables(self, caplog):
         with caplog.at_level(logging.WARNING):
             _ = PromptBuilder(template="This is a {{ variable }}")
             assert "but `required_variables` is not set." in caplog.text
+
+    def test_variables_correct_with_assignment(self) -> None:
+        template = """{% if existing_documents is not none %}
+{% set existing_doc_len = existing_documents|length %}
+{% else %}
+{% set existing_doc_len = 0 %}
+{% endif %}
+{% for doc in docs %}
+<document reference="{{loop.index + existing_doc_len}}">
+{{ doc.content }}
+</document>
+{% endfor %}
+"""
+        builder = PromptBuilder(template=template, required_variables="*")
+        assert set(builder.variables) == {"docs", "existing_documents"}
+        assert builder.required_variables == "*"
+
+    def test_variables_correct_with_tuple_assignment(self):
+        template = """{% if existing_documents is not none -%}
+{% set x, y = (existing_documents|length, 1) -%}
+{% else -%}
+{% set x, y = (0, 1) -%}
+{% endif -%}
+x={{ x }}, y={{ y }}
+"""
+        builder = PromptBuilder(template=template, required_variables="*")
+        assert builder.variables == ["existing_documents"]
+        assert builder.required_variables == "*"
+        res = builder.run(existing_documents=None)
+        assert res["prompt"] == "x=0, y=1"
+
+    def test_variables_correct_with_list_assignment(self):
+        template = """{% if existing_documents is not none -%}
+{% set x, y = [existing_documents|length, 1] -%}
+{% else -%}
+{% set x, y = [0, 1] -%}
+{% endif -%}
+x={{ x }}, y={{ y }}
+"""
+        builder = PromptBuilder(template=template, required_variables="*")
+        assert builder.variables == ["existing_documents"]
+        assert builder.required_variables == "*"
+        res = builder.run(existing_documents=None)
+        assert res["prompt"] == "x=0, y=1"
diff --git a/test/components/converters/test_output_adapter.py b/test/components/converters/test_output_adapter.py
index 2ad2dbb0fd..42e9f04255 100644
--- a/test/components/converters/test_output_adapter.py
+++ b/test/components/converters/test_output_adapter.py
@@ -3,13 +3,14 @@
 # SPDX-License-Identifier: Apache-2.0
 
 import json
-from typing import List
+from typing import Any, List
 
 import pytest
 
 from haystack import Pipeline, component
 from haystack.components.converters import OutputAdapter
 from haystack.components.converters.output_adapter import OutputAdaptationException
+from haystack.core.component.sockets import InputSocket
 from haystack.dataclasses import Document
 
 
@@ -203,3 +204,16 @@ def test_unsafe(self):
         ]
         res = adapter.run(documents=documents)
         assert res["output"] == documents[0]
+
+    def test_variables_correct_with_assignment(self) -> None:
+        template = """{% if control == 'something' %}
+    {% set output = 1 %}
+{% else %}
+    {% set output = 3 %}
+{% endif %}
+{{ output }}
+"""
+        adapter = OutputAdapter(template=template, output_type=int)
+        assert adapter.__haystack_input__._sockets_dict == {"control": InputSocket(name="control", type=Any)}
+        res = adapter.run(control="something")
+        assert res["output"] == 1
diff --git a/test/components/routers/test_conditional_router.py b/test/components/routers/test_conditional_router.py
index 245e451513..17a933d845 100644
--- a/test/components/routers/test_conditional_router.py
+++ b/test/components/routers/test_conditional_router.py
@@ -6,6 +6,7 @@
 from unittest import mock
 
 import pytest
+from jinja2.nativetypes import NativeEnvironment
 
 from haystack import Pipeline
 from haystack.components.routers import ConditionalRouter
@@ -636,3 +637,15 @@ def test_sede_multiple_outputs(self):
         reloaded_router = ConditionalRouter.from_dict(router.to_dict())
         assert reloaded_router.custom_filters == router.custom_filters
         assert reloaded_router.routes == router.routes
+
+    def test_extract_variables_correct_with_assignment(self):
+        condition = """{%- if control == 'something' -%}
+{% set streams = 1 %}
+{%- else -%}
+{% set streams = 2 %}
+{%- endif -%}
+{{streams == 1}}
+"""
+        templates = [condition, "{{query}}"]
+        extracted_variables = ConditionalRouter._extract_variables(env=NativeEnvironment(), templates=templates)
+        assert extracted_variables == {"control", "query"}