diff --git a/.github/workflows/requirements-evidence.yml b/.github/workflows/requirements-evidence.yml
index 5c490e06..b3e8f1c4 100644
--- a/.github/workflows/requirements-evidence.yml
+++ b/.github/workflows/requirements-evidence.yml
@@ -656,7 +656,7 @@ jobs:
               selected_change="${selected_change%/requirements-proof/review-evidence.json}"
             fi
           fi
-          clean_environment=(env -i "HOME=${HOME:-}" "LANG=${LANG:-}" "LC_ALL=${LC_ALL:-}" "LC_CTYPE=${LC_CTYPE:-}" "PATH=${PATH:-}" "RUNNER_TEMP=${RUNNER_TEMP}" "SPECFACT_MODULES_REPO=${SPECFACT_MODULES_REPO}" "SPECFACT_MODULES_ROOTS=${SPECFACT_MODULES_ROOTS}")
+          clean_environment=(env -i "HOME=${HOME:-}" "LANG=${LANG:-}" "LC_ALL=${LC_ALL:-}" "LC_CTYPE=${LC_CTYPE:-}" "PATH=${PATH:-}" "RUNNER_TEMP=${RUNNER_TEMP}" "SPECFACT_MODULES_REPO=${SPECFACT_MODULES_REPO}" "SPECFACT_MODULES_ROOTS=${SPECFACT_MODULES_ROOTS}" "SPECFACT_MODULES_EXCLUSIVE=1")
           evidence_arguments=(
             "${clean_environment[@]}" "${isolated_specfact[@]}" requirements evidence
             --repo-root "$GITHUB_WORKSPACE" \
diff --git a/openspec/changes/security-requirements-evidence-exclusive-discovery/TDD_EVIDENCE.md b/openspec/changes/security-requirements-evidence-exclusive-discovery/TDD_EVIDENCE.md
new file mode 100644
index 00000000..a43783ee
--- /dev/null
+++ b/openspec/changes/security-requirements-evidence-exclusive-discovery/TDD_EVIDENCE.md
@@ -0,0 +1,26 @@
+# TDD Evidence: security-requirements-evidence-exclusive-discovery
+
+## Failing-before
+
+- **Command**: `hatch run pytest tests/unit/registry/test_module_discovery.py::test_exclusive_module_roots_reject_project_and_user_shadowing tests/unit/scripts/test_requirements_evidence_delivery_gate.py::test_failed_command_writes_missing_diagnostic_reports_and_exports_fixture_roots tests/unit/workflows/test_requirements_evidence_delivery_workflow.py::test_required_requirements_context_is_pull_request_only -q`
+- **Result**: 2 failed, 1 passed.
+- **Evidence**: discovery selected the repository-local same-identity module instead of the explicit fixture, and the adapter did not set an exclusive-discovery control.
+
+## Passing-after
+
+- **Command**: `hatch run pytest tests/unit/registry/test_module_discovery.py tests/unit/scripts/test_requirements_evidence_delivery_gate.py tests/unit/workflows/test_requirements_evidence_delivery_workflow.py -q`
+- **Result**: 49 passed, 4 skipped because the optional pinned fixture checkout is unavailable.
+- **Evidence**: exclusive discovery retained the bundled root and verified explicit root while excluding project, user, marketplace, custom, and legacy roots; adapter and workflow contracts require the control.
+
+## Quality gates
+
+- `hatch run format`, `hatch run type-check`, and `hatch run lint` passed. BasedPyright reported the repository's existing warning baseline with 0 errors; Ruff reported no findings.
+- `hatch run yaml-lint` completed with pre-existing errors in archived Requirements evidence and the active Requirements 07 evidence ledger; no changed YAML file produced a finding.
+- `hatch run openspec validate security-requirements-evidence-exclusive-discovery --strict`, `hatch run python scripts/check_reproducible_delivery.py`, and `uv lock --check` passed.
+- Semgrep SAST and its baseline gate passed with 0 findings; Bandit reported no medium/high findings.
+- Changed-scope `specfact code review run` passed with no findings after installing its declared BasedPyright and Pylint review tools.
+- `hatch run contract-test` used the valid cached baseline. `hatch run smart-test` ran the full suite but reported two unrelated missing fixture-package imports (`specfact_backlog` and `specfact_spec`); the 49-test focused affected-scope suite remained green.
+
+## Internal wiki follow-up
+
+The sibling `specfact-cli-internal` checkout is unavailable in this environment. After it is available, add or update `wiki/sources/security-requirements-evidence-exclusive-discovery.md` and run `python3 scripts/wiki_rebuild_graph.py` from that repository root.
diff --git a/openspec/changes/security-requirements-evidence-exclusive-discovery/proposal.md b/openspec/changes/security-requirements-evidence-exclusive-discovery/proposal.md
new file mode 100644
index 00000000..1fc2912b
--- /dev/null
+++ b/openspec/changes/security-requirements-evidence-exclusive-discovery/proposal.md
@@ -0,0 +1,24 @@
+# Proposal: security-requirements-evidence-exclusive-discovery
+
+## Why
+
+The Requirements evidence delivery gate verifies a pinned external module fixture, but normal module discovery still examines repository-local and user-controlled roots before or alongside that fixture. A pull request can therefore shadow the trusted Requirements package with an unsigned project module and execute attacker-controlled Python while fabricating a passing report.
+
+## What Changes
+
+- Add an explicit module-discovery mode that limits dynamic package discovery to `SPECFACT_MODULES_ROOTS` (while retaining bundled core modules).
+- Require both the local adapter and pull-request evidence workflow to enable that mode when invoking the pinned Requirements fixture.
+- Add regressions proving project, user, marketplace, custom, and legacy roots cannot shadow an exclusive fixture invocation.
+
+## Capabilities
+
+### New Capabilities
+
+- `trusted-module-fixture-discovery`
+
+## Impact
+
+- Affected code: module discovery and the Requirements evidence adapter.
+- Affected workflow: `.github/workflows/requirements-evidence.yml`.
+- Affected tests: focused discovery, adapter, and workflow contract tests.
+- Compatibility: ordinary CLI discovery remains unchanged; exclusivity is opt-in for trusted fixture execution.
diff --git a/openspec/changes/security-requirements-evidence-exclusive-discovery/specs/trusted-module-fixture-discovery/spec.md b/openspec/changes/security-requirements-evidence-exclusive-discovery/specs/trusted-module-fixture-discovery/spec.md
new file mode 100644
index 00000000..c6a4f1dd
--- /dev/null
+++ b/openspec/changes/security-requirements-evidence-exclusive-discovery/specs/trusted-module-fixture-discovery/spec.md
@@ -0,0 +1,19 @@
+## ADDED Requirements
+
+### Requirement: Trusted fixture discovery is exclusive
+
+Security-sensitive delivery gates that execute a verified external module fixture SHALL restrict dynamic module discovery to the explicitly configured fixture roots while retaining bundled core commands.
+
+#### Scenario: Prevent untrusted module shadowing
+
+- **GIVEN** a verified Requirements fixture and an untrusted module with the same identity in a project, user, marketplace, custom, or legacy discovery root
+- **WHEN** local or CI requirements evidence enforcement invokes the released command
+- **THEN** dynamic module discovery uses only the explicit verified fixture root
+- **AND** bundled core commands remain available
+- **AND** no untrusted same-identity module is imported or executed.
+
+#### Scenario: Preserve ordinary module discovery
+
+- **GIVEN** an ordinary CLI invocation that does not request exclusive discovery
+- **WHEN** SpecFact discovers dynamic modules
+- **THEN** existing project, explicit, user, marketplace, custom, and legacy discovery behavior remains unchanged.
diff --git a/openspec/changes/security-requirements-evidence-exclusive-discovery/tasks.md b/openspec/changes/security-requirements-evidence-exclusive-discovery/tasks.md
new file mode 100644
index 00000000..b51dc969
--- /dev/null
+++ b/openspec/changes/security-requirements-evidence-exclusive-discovery/tasks.md
@@ -0,0 +1,19 @@
+# Tasks: security-requirements-evidence-exclusive-discovery
+
+## 1. Specification and failing evidence
+
+- [x] 1.1 Add the exclusive-discovery security delta.
+- [x] 1.2 Add focused regressions for project-module shadowing and gate environment isolation.
+- [x] 1.3 Run the focused regressions against HEAD and record failing-before evidence.
+
+## 2. Implementation
+
+- [x] 2.1 Make exclusive discovery retain bundled modules and explicit fixture roots only.
+- [x] 2.2 Enable exclusive discovery in the local evidence adapter and pull-request workflow.
+
+## 3. Verification
+
+- [x] 3.1 Run focused tests and record passing-after evidence.
+- [x] 3.2 Run required formatting, typing, linting, YAML, security, and changed-scope review gates.
+- [x] 3.3 Validate the OpenSpec change strictly.
+- [x] 3.4 Update the internal wiki mirror and graph when the sibling checkout is available; otherwise record the follow-up.
diff --git a/scripts/requirements_evidence_delivery_gate.py b/scripts/requirements_evidence_delivery_gate.py
index 75073f39..f52c6b6c 100644
--- a/scripts/requirements_evidence_delivery_gate.py
+++ b/scripts/requirements_evidence_delivery_gate.py
@@ -167,6 +167,7 @@ def run_evidence_command(
     environment = {key: value for key, value in os.environ.items() if key in ALLOWED_ENVIRONMENT_KEYS}
     environment["SPECFACT_MODULES_REPO"] = str(fixture_root.resolve())
     environment["SPECFACT_MODULES_ROOTS"] = str((fixture_root / "packages").resolve())
+    environment["SPECFACT_MODULES_EXCLUSIVE"] = "1"
     environment.pop("SPECFACT_CLI_MODULES_REPO", None)
     try:
         exit_code = command_runner(arguments, environment)
diff --git a/src/specfact_cli/registry/module_discovery.py b/src/specfact_cli/registry/module_discovery.py
index 31a80538..6aebe94f 100644
--- a/src/specfact_cli/registry/module_discovery.py
+++ b/src/specfact_cli/registry/module_discovery.py
@@ -18,6 +18,7 @@
 USER_MODULES_ROOT = Path.home() / ".specfact" / "modules"
 MARKETPLACE_MODULES_ROOT = Path.home() / ".specfact" / "marketplace-modules"
 CUSTOM_MODULES_ROOT = Path.home() / ".specfact" / "custom-modules"
+EXCLUSIVE_MODULE_ROOTS_ENV = "SPECFACT_MODULES_EXCLUSIVE"
 _SHADOW_HINT_KEYS: set[tuple[str, str, str, str]] = set()
 
 
@@ -116,6 +117,10 @@ def _discovery_root_list(options: _DiscoveryRootOptions) -> list[tuple[str, Path
     effective_custom_root = options.custom_root or CUSTOM_MODULES_ROOT
 
     roots: list[tuple[str, Path]] = [("builtin", effective_builtin_root)]
+    if os.environ.get(EXCLUSIVE_MODULE_ROOTS_ENV) == "1":
+        _append_explicit_module_roots(roots)
+        return roots
+
     project_matches_user_root = False
     if effective_project_root is not None:
         try:
diff --git a/tests/unit/registry/test_module_discovery.py b/tests/unit/registry/test_module_discovery.py
index bb5e500a..6590b869 100644
--- a/tests/unit/registry/test_module_discovery.py
+++ b/tests/unit/registry/test_module_discovery.py
@@ -143,6 +143,39 @@ def test_explicit_module_roots_take_priority_over_user_installs(
     assert sources["code-review"] == "custom"
 
 
+def test_exclusive_module_roots_reject_project_and_user_shadowing(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Trusted fixture discovery must ignore every non-bundled implicit root."""
+    repo_root = tmp_path / "repo"
+    project_root = repo_root / ".specfact" / "modules"
+    builtin_root = tmp_path / "builtin"
+    explicit_root = tmp_path / "verified-fixture" / "packages"
+    user_root = tmp_path / "user-modules"
+    marketplace_root = tmp_path / "marketplace-modules"
+    custom_root = tmp_path / "custom-modules"
+    _write_manifest(builtin_root, "init")
+    for untrusted_root in (project_root, user_root, marketplace_root, custom_root):
+        _write_manifest(untrusted_root, "nold-ai-specfact-requirements")
+    _write_manifest(explicit_root, "nold-ai-specfact-requirements")
+    monkeypatch.chdir(repo_root)
+    monkeypatch.setenv("SPECFACT_MODULES_ROOTS", str(explicit_root))
+    monkeypatch.setenv("SPECFACT_MODULES_EXCLUSIVE", "1")
+
+    discovered = discover_all_modules(
+        builtin_root=builtin_root,
+        user_root=user_root,
+        marketplace_root=marketplace_root,
+        custom_root=custom_root,
+        include_legacy_roots=True,
+    )
+
+    assert [(entry.metadata.name, entry.source) for entry in discovered] == [
+        ("init", "builtin"),
+        ("nold-ai-specfact-requirements", "custom"),
+    ]
+
+
 def test_project_shadow_warning_is_actionable_and_emitted_once(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
     """Project-over-user shadow guidance should be user-facing but deduplicated per process."""
     repo_root = tmp_path / "repo"
diff --git a/tests/unit/scripts/test_requirements_evidence_delivery_gate.py b/tests/unit/scripts/test_requirements_evidence_delivery_gate.py
index 523b9b45..eb664cf1 100644
--- a/tests/unit/scripts/test_requirements_evidence_delivery_gate.py
+++ b/tests/unit/scripts/test_requirements_evidence_delivery_gate.py
@@ -287,6 +287,7 @@ def test_failed_command_writes_missing_diagnostic_reports_and_exports_fixture_ro
     assert observed == 1
     assert observed_environment["SPECFACT_MODULES_REPO"] == str(fixture.resolve())
     assert observed_environment["SPECFACT_MODULES_ROOTS"] == str((fixture / "packages").resolve())
+    assert observed_environment["SPECFACT_MODULES_EXCLUSIVE"] == "1"
     assert observed_environment["PATH"] == os.environ["PATH"]
     assert "SPECFACT_TEST_SECRET" not in observed_environment
     assert "PYTHONPATH" not in observed_environment
diff --git a/tests/unit/workflows/test_requirements_evidence_delivery_workflow.py b/tests/unit/workflows/test_requirements_evidence_delivery_workflow.py
index 226444c4..4ce8823f 100644
--- a/tests/unit/workflows/test_requirements_evidence_delivery_workflow.py
+++ b/tests/unit/workflows/test_requirements_evidence_delivery_workflow.py
@@ -425,6 +425,7 @@ def _assert_command_contract(workflow: dict[str, object]) -> None:
     assert run_evidence["env"]["EVIDENCE_BASE_BRANCH"]  # type: ignore[index]
     assert "workflow_dispatch" not in workflow["on"]  # type: ignore[operator]
     assert run_evidence["run"].count("clean_environment=(env -i") == 1  # type: ignore[union-attr]
+    assert '"SPECFACT_MODULES_EXCLUSIVE=1"' in run_evidence["run"]  # type: ignore[operator]
     assert run_evidence["run"].count('"${clean_environment[@]}" "${isolated_specfact[@]}"') == 2  # type: ignore[union-attr]