diff --git a/features/steps/tdd_auto_debug_analyze_error_mutation_steps.py b/features/steps/tdd_auto_debug_analyze_error_mutation_steps.py
new file mode 100644
index 000000000..d7dd42012
--- /dev/null
+++ b/features/steps/tdd_auto_debug_analyze_error_mutation_steps.py
@@ -0,0 +1,137 @@
+"""Step definitions for tdd_auto_debug_analyze_error_mutation.feature.
+
+This module captures bug #10494: AutoDebugAgent._analyze_error() mutates the
+input state dict in-place and returns the full state object, violating the
+LangGraph node contract.
+
+LangGraph node functions must return a dict of state updates (only the changed
+keys), not mutate the input state in-place and return the full state object.
+Returning the full mutated state can cause LangGraph to double-apply updates
+when merging state, leading to duplicate messages in the messages list.
+
+Expected fix: return only the changed keys:
+
+    def _analyze_error(self, state: AutoDebugState) -> dict[str, Any]:
+        # ...
+        new_message = {
+            "role": "assistant",
+            "content": analysis,
+            "type": "error_analysis",
+        }
+        return {
+            "messages": state.get("messages", []) + [new_message],
+        }
+
+See CONTRIBUTING.md > Bug Fix Workflow > TDD Issue Test Tags.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from behave import given, then, when
+from langchain_community.llms import FakeListLLM
+
+from cleveragents.agents.graphs.auto_debug import AutoDebugAgent, AutoDebugState
+
+
+@given("an AutoDebugAgent with a FakeListLLM for the mutation test")
+def step_create_agent_with_fake_llm(context: Any) -> None:
+    """Create an AutoDebugAgent backed by a FakeListLLM for deterministic testing."""
+    context.mutation_test_llm = FakeListLLM(
+        responses=["Error analysis: null pointer dereference"]
+    )
+    context.mutation_test_agent = AutoDebugAgent(llm=context.mutation_test_llm)
+
+
+@given("a state with an empty messages list for the mutation test")
+def step_create_state_with_empty_messages(context: Any) -> None:
+    """Create a minimal AutoDebugState with an empty messages list."""
+    context.mutation_test_state: AutoDebugState = {
+        "error_message": "NullPointerException at line 42",
+        "code_context": "x = obj.method()",
+        "messages": [],
+        "context": {},
+        "result": None,
+        "error": None,
+        "metadata": {},
+        "attempted_fixes": [],
+        "current_fix": {},
+        "fix_validated": False,
+    }
+    # Capture a reference to the original messages list before calling the node
+    context.mutation_test_original_messages = context.mutation_test_state["messages"]
+
+
+@when("_analyze_error is called with the state for the mutation test")
+def step_call_analyze_error(context: Any) -> None:
+    """Invoke _analyze_error directly and capture the returned value."""
+    context.mutation_test_result = context.mutation_test_agent._analyze_error(
+        context.mutation_test_state
+    )
+
+
+@then("the analyze_error result should not be the same object as the input state")
+def step_assert_result_is_not_state(context: Any) -> None:
+    """Assert that _analyze_error returns a new dict, not the same state object.
+
+    LangGraph node functions must return a dict of updates, not the full state.
+    This assertion FAILS against the current buggy implementation because
+    _analyze_error returns ``state`` directly (same object identity).
+    """
+    result = context.mutation_test_result
+    state = context.mutation_test_state
+    assert result is not state, (
+        "_analyze_error must not return the same state object. "
+        "LangGraph node functions must return a dict of updates, not the full state."
+    )
+
+
+@then("the original messages list should not have been mutated by analyze_error")
+def step_assert_messages_not_mutated(context: Any) -> None:
+    """Assert that the original state messages list was not mutated in-place.
+
+    This assertion FAILS against the current buggy implementation because
+    _analyze_error calls ``state.setdefault("messages", []).append(...)``
+    which mutates the list in-place.
+    """
+    original_messages = context.mutation_test_original_messages
+    assert original_messages == [], (
+        f"_analyze_error mutated state['messages'] in-place: {original_messages}. "
+        "Node functions must return updates, not mutate state."
+    )
+
+
+@then("the analyze_error result should be a plain dict of updates")
+def step_assert_result_is_plain_dict(context: Any) -> None:
+    """Assert that the returned value is a plain dict (update dict, not full state).
+
+    A correct LangGraph node returns only the keys that changed, e.g.:
+        {"messages": [...new_message...]}
+    The returned dict should NOT be the full AutoDebugState — it should contain
+    only the keys that were modified by this node.
+    """
+    result = context.mutation_test_result
+    assert isinstance(result, dict), (
+        f"Expected _analyze_error to return a dict, got {type(result)}"
+    )
+    # A proper update dict should only contain the keys that changed.
+    # The current buggy implementation returns the full state (all 11 keys).
+    # A correct implementation returns only {"messages": [...]}.
+    all_state_keys = {
+        "messages",
+        "context",
+        "result",
+        "error",
+        "metadata",
+        "error_message",
+        "code_context",
+        "attempted_fixes",
+        "current_fix",
+        "fix_validated",
+    }
+    result_keys = set(result.keys())
+    assert result_keys != all_state_keys, (
+        "_analyze_error returned the full state dict instead of an update dict. "
+        f"Expected only changed keys (e.g. {{'messages'}}), got all keys: {result_keys}"
+    )
diff --git a/features/tdd_auto_debug_analyze_error_mutation.feature b/features/tdd_auto_debug_analyze_error_mutation.feature
new file mode 100644
index 000000000..67f2f0799
--- /dev/null
+++ b/features/tdd_auto_debug_analyze_error_mutation.feature
@@ -0,0 +1,24 @@
+@tdd_issue @tdd_issue_10494 @tdd_expected_fail
+Feature: TDD Issue #10494 — _analyze_error node mutates state in-place instead of returning update dict
+  As a developer using LangGraph
+  I want AutoDebugAgent._analyze_error() to return a dict of state updates
+  So that LangGraph can correctly merge state without double-applying mutations
+
+  The current implementation mutates the input state dict in-place and returns
+  the full state object, violating the LangGraph node contract. This test
+  captures that violation so it can be tracked and fixed.
+
+  LangGraph node functions must return a dict of updates (only the changed keys),
+  not mutate the input state in-place and return the full state object. Returning
+  the full mutated state can cause LangGraph to double-apply updates when merging
+  state, leading to duplicate messages in the messages list.
+
+  See CONTRIBUTING.md > Bug Fix Workflow > TDD Issue Test Tags.
+
+  Scenario: Bug #10494 — _analyze_error returns the same state object instead of an update dict
+    Given an AutoDebugAgent with a FakeListLLM for the mutation test
+    And a state with an empty messages list for the mutation test
+    When _analyze_error is called with the state for the mutation test
+    Then the analyze_error result should not be the same object as the input state
+    And the original messages list should not have been mutated by analyze_error
+    And the analyze_error result should be a plain dict of updates