diff --git a/CHANGELOG.md b/CHANGELOG.md
index ff6016f8d..80d968378 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -200,6 +200,14 @@
   back to the in-memory cache on database errors or when no Unit of Work
   is wired. Added `ActionRepository.list_all()` for unfiltered action
   listing. (#760)
+- Added TDD bug-capture test for bug #1080 — execution environment resolution
+  ignores project-level override. Three Behave BDD scenarios
+  (`@tdd_bug @tdd_bug_1080 @mock_only`) verify the 6-level precedence chain
+  defined in §Execution Environment Routing. The critical scenario uses
+  `@tdd_expected_fail` to confirm the bug: project-level override (level 2)
+  incorrectly loses to plan-level fallback (level 4). Two regression guard
+  scenarios verify existing correct behaviour (plan override vs project
+  override, project override vs host default). (#1101)
 - Added BuiltinAdapter class and MCP automatic resource slot creation.
   BuiltinAdapter wraps register_file_tools/register_git_tools/register_subplan_tool
   into a unified adapter interface. McpAdapter.infer_resource_slots() analyzes
diff --git a/features/steps/tdd_exec_env_resolution_precedence_steps.py b/features/steps/tdd_exec_env_resolution_precedence_steps.py
new file mode 100644
index 000000000..126086192
--- /dev/null
+++ b/features/steps/tdd_exec_env_resolution_precedence_steps.py
@@ -0,0 +1,174 @@
+"""Step definitions for TDD bug #1080 — execution env resolution precedence.
+
+This file captures bug #1080: the ExecutionEnvironmentResolver does not
+honour the 6-level precedence chain defined in the spec (§Execution
+Environment Routing).  The resolver currently uses a flat chain
+(tool > plan > project > default) without distinguishing "override"
+from "fallback" priority semantics.
+
+Scenario 1 is tagged ``@tdd_expected_fail`` so CI passes while the bug
+is unfixed.  Scenarios 2 and 3 are regression guards (no expected-fail
+tag) that verify existing correct behaviour is not broken.
+
+When the fix for #1080 is implemented:
+
+1. Remove the ``@tdd_expected_fail`` tag from Scenario 1 in the feature file.
+2. Update the resolver to implement the 6-level precedence chain with
+   priority-aware interleaving.
+3. All three scenarios should then pass normally.
+"""
+
+from __future__ import annotations
+
+from behave import given, then, when  # type: ignore[import-untyped]
+from behave.runner import Context  # type: ignore[import-untyped]
+
+from cleveragents.application.services.execution_environment_resolver import (
+    ExecutionEnvironmentResolver,
+)
+
+# Valid priority values accepted by the step definitions.  Guards against
+# typos in feature files and ensures parity with the spec's two-mode model.
+_VALID_PRIORITIES: frozenset[str] = frozenset({"override", "fallback"})
+
+
+@given("a precedence-aware execution environment resolver")
+def step_create_precedence_resolver(context: Context) -> None:
+    """Create an ExecutionEnvironmentResolver for precedence testing."""
+    context.precedence_resolver = ExecutionEnvironmentResolver()
+    context.precedence_resolved_env = None
+
+
+@when(
+    'I resolve precedence with plan env "{plan_env}" at priority '
+    '"{plan_priority}" and project env "{project_env}" at priority '
+    '"{project_priority}"'
+)
+def step_resolve_precedence_with_priorities(
+    context: Context,
+    plan_env: str,
+    plan_priority: str,
+    project_env: str,
+    project_priority: str,
+) -> None:
+    """Resolve execution environment with the 6-level precedence chain.
+
+    Per the spec, the correct precedence is:
+        1. Plan-level with priority: override
+        2. Project-level with priority: override
+        3. Nearest-ancestor devcontainer
+        4. Plan-level with priority: fallback
+        5. Project-level with priority: fallback
+        6. Host (default)
+
+    The current ``resolver.resolve()`` uses a flat chain that ignores
+    priority, so this test will produce incorrect results for Scenario 1
+    until the bug is fixed.  Scenarios 2 and 3 exercise paths that
+    happen to work correctly with the flat chain but still route through
+    this step to ensure the resolver sees both competing environments.
+    """
+    # Validate priority values against the spec's two-mode model.
+    assert plan_priority in _VALID_PRIORITIES, (
+        f"Invalid plan_priority '{plan_priority}'; "
+        f"expected one of {sorted(_VALID_PRIORITIES)}"
+    )
+    assert project_priority in _VALID_PRIORITIES, (
+        f"Invalid project_priority '{project_priority}'; "
+        f"expected one of {sorted(_VALID_PRIORITIES)}"
+    )
+
+    resolver: ExecutionEnvironmentResolver = context.precedence_resolver
+
+    # TODO(#1080): The 6-level precedence chain is built manually here
+    # because the current resolver.resolve() uses a flat priority chain
+    # that ignores priority semantics.  When the bug is fixed, this
+    # manual chain should be replaced by a single call to the resolver's
+    # new priority-aware API.
+
+    # Level 1: plan override
+    if plan_env and plan_priority == "override":
+        # Pass both plan_env AND project_env so the resolver must choose
+        # correctly when competing environments are present.
+        context.precedence_resolved_env = resolver.resolve(
+            plan_env=plan_env,
+            project_env=project_env,
+        )
+        return
+
+    # Level 2: project override
+    if project_env and project_priority == "override":
+        # BUG: the current resolver.resolve() does not support this
+        # precedence level.  Calling resolve(plan_env=..., project_env=...)
+        # always returns plan_env first.  We pass both plan_env and
+        # project_env to the resolver and assert the result matches the
+        # project override.  The current flat chain will return plan_env
+        # instead — triggering the expected assertion failure.
+        context.precedence_resolved_env = resolver.resolve(
+            plan_env=plan_env,
+            project_env=project_env,
+        )
+        return
+
+    # Level 3: devcontainer (not tested in this scenario)
+
+    # Level 4: plan fallback
+    if plan_env and plan_priority == "fallback":
+        context.precedence_resolved_env = resolver.resolve(plan_env=plan_env)
+        return
+
+    # Level 5: project fallback
+    if project_env and project_priority == "fallback":
+        context.precedence_resolved_env = resolver.resolve(project_env=project_env)
+        return
+
+    # Level 6: host default
+    context.precedence_resolved_env = resolver.resolve()
+
+
+@when(
+    'I resolve precedence with no plan env and project env "{project_env}" '
+    'at priority "{project_priority}"'
+)
+def step_resolve_precedence_project_only(
+    context: Context,
+    project_env: str,
+    project_priority: str,
+) -> None:
+    """Resolve with project-level environment and no plan-level environment.
+
+    When no plan-level environment is set, the project-level environment
+    should be returned regardless of priority mode.  This scenario acts
+    as a regression guard: the current resolver handles this case
+    correctly (project_env is returned when plan_env is None), but the
+    resolution path does not check priority — it works only by accident.
+
+    This scenario ensures the fix correctly handles the project-override
+    path and doesn't regress the simple case.
+    """
+    # Validate the priority parameter to catch silent discards.
+    assert project_priority in _VALID_PRIORITIES, (
+        f"Invalid project_priority '{project_priority}'; "
+        f"expected one of {sorted(_VALID_PRIORITIES)}"
+    )
+
+    resolver: ExecutionEnvironmentResolver = context.precedence_resolver
+    context.precedence_resolved_env = resolver.resolve(project_env=project_env)
+
+
+@then('the precedence-resolved environment should be "{expected}"')
+def step_check_precedence_result(context: Context, expected: str) -> None:
+    """Assert the resolved environment matches the expected value.
+
+    For bug #1080, the critical assertion is in the scenario
+    'Project-level override beats plan-level fallback': the resolver
+    is called with plan_env="host" and project_env="container", but
+    the expected result is "container" (project override at level 2
+    beats plan fallback at level 4).  The current resolver returns
+    "host" because it always picks plan_env before project_env.
+    """
+    assert context.precedence_resolved_env is not None, "No environment was resolved"
+    assert context.precedence_resolved_env.value == expected, (
+        f"Expected execution environment '{expected}' but got "
+        f"'{context.precedence_resolved_env.value}'. "
+        f"Bug #1080: resolver does not honour 6-level precedence chain."
+    )
diff --git a/features/tdd_exec_env_resolution_precedence.feature b/features/tdd_exec_env_resolution_precedence.feature
new file mode 100644
index 000000000..11e249451
--- /dev/null
+++ b/features/tdd_exec_env_resolution_precedence.feature
@@ -0,0 +1,47 @@
+@tdd_issue @tdd_issue_1080 @tdd_bug @tdd_bug_1080 @mock_only
+Feature: TDD Bug #1080 — execution environment resolution ignores project-level override
+  As a developer
+  I want execution environment resolution to honour the 6-level precedence chain
+  So that project-level overrides (precedence level 2) take effect correctly
+
+  # Bug #1080: The ExecutionEnvironmentResolver uses a flat priority chain
+  # (tool > plan > project > default) that does not distinguish between
+  # "override" and "fallback" priority semantics.  Per the spec (§Execution
+  # Environment Routing), the correct 6-level precedence is:
+  #
+  #   1. Plan-level with priority: override
+  #   2. Project-level with priority: override
+  #   3. Nearest-ancestor devcontainer (auto-detected)
+  #   4. Plan-level with priority: fallback
+  #   5. Project-level with priority: fallback
+  #   6. Host (default)
+  #
+  # The critical failure: when a project has execution_environment set with
+  # priority "override" and a plan has execution_environment set with
+  # priority "fallback", the plan incorrectly wins because the resolver
+  # treats all plan-level settings as higher priority than project-level.
+  #
+  # This test captures the bug by asserting the CORRECT precedence behavior.
+  # The @tdd_expected_fail tag on the bug-capturing scenario inverts the
+  # result: the assertion fails (proving the bug exists) but CI reports it
+  # as passed.  When bug #1080 is fixed, the @tdd_expected_fail tag must
+  # be removed.
+  #
+  # The @mock_only tag restricts this feature to in-process mock execution;
+  # it does not require any external services or container infrastructure.
+
+  @tdd_expected_fail
+  Scenario: Bug #1080 - Project-level override beats plan-level fallback (precedence level 2 vs 4)
+    Given a precedence-aware execution environment resolver
+    When I resolve precedence with plan env "host" at priority "fallback" and project env "container" at priority "override"
+    Then the precedence-resolved environment should be "container"
+
+  Scenario: Regression guard - Plan-level override still beats project-level override (precedence level 1 vs 2)
+    Given a precedence-aware execution environment resolver
+    When I resolve precedence with plan env "host" at priority "override" and project env "container" at priority "override"
+    Then the precedence-resolved environment should be "host"
+
+  Scenario: Regression guard - Project-level override beats host default (precedence level 2 vs 6)
+    Given a precedence-aware execution environment resolver
+    When I resolve precedence with no plan env and project env "container" at priority "override"
+    Then the precedence-resolved environment should be "container"