diff --git a/tests/fixtures/repositories/empty-repo/.gitkeep b/tests/fixtures/repositories/empty-repo/.gitkeep
new file mode 100644
index 0000000..e69de29
diff --git a/tests/fixtures/repositories/multi-agent-overlap/.cursor/rules/agent-rules.mdc b/tests/fixtures/repositories/multi-agent-overlap/.cursor/rules/agent-rules.mdc
new file mode 100644
index 0000000..41c8b94
--- /dev/null
+++ b/tests/fixtures/repositories/multi-agent-overlap/.cursor/rules/agent-rules.mdc
@@ -0,0 +1,6 @@
+---
+description: Fixture Cursor rules for agent instruction discovery
+alwaysApply: true
+---
+
+Read relevant files before editing. Keep changes small and run local checks before committing.
diff --git a/tests/fixtures/repositories/multi-agent-overlap/.github/copilot-instructions.md b/tests/fixtures/repositories/multi-agent-overlap/.github/copilot-instructions.md
new file mode 100644
index 0000000..8f7a16e
--- /dev/null
+++ b/tests/fixtures/repositories/multi-agent-overlap/.github/copilot-instructions.md
@@ -0,0 +1,7 @@
+# GitHub Copilot Instructions
+
+This fixture represents repository-wide Copilot instructions.
+
+- Read relevant files before editing.
+- Keep changes small.
+- Run local checks before committing.
diff --git a/tests/fixtures/repositories/multi-agent-overlap/.github/instructions/agents.instructions.md b/tests/fixtures/repositories/multi-agent-overlap/.github/instructions/agents.instructions.md
new file mode 100644
index 0000000..2db0025
--- /dev/null
+++ b/tests/fixtures/repositories/multi-agent-overlap/.github/instructions/agents.instructions.md
@@ -0,0 +1,7 @@
+# Agent Instructions
+
+This fixture represents scoped GitHub instructions.
+
+- Prefer small changes.
+- Do not add secrets or private data.
+- Verify changes before opening a pull request.
diff --git a/tests/fixtures/repositories/multi-agent-overlap/AGENTS.md b/tests/fixtures/repositories/multi-agent-overlap/AGENTS.md
new file mode 100644
index 0000000..2895799
--- /dev/null
+++ b/tests/fixtures/repositories/multi-agent-overlap/AGENTS.md
@@ -0,0 +1,11 @@
+# AGENTS.md
+
+This fixture represents shared repository instructions.
+
+Rules:
+
+- Read relevant files before editing.
+- Keep changes small and reviewable.
+- Run local checks before committing.
+- Use pull requests for changes to main.
+- Do not commit secrets, tokens, credentials, private URLs, or customer data.
diff --git a/tests/fixtures/repositories/multi-agent-overlap/CLAUDE.md b/tests/fixtures/repositories/multi-agent-overlap/CLAUDE.md
new file mode 100644
index 0000000..b15e574
--- /dev/null
+++ b/tests/fixtures/repositories/multi-agent-overlap/CLAUDE.md
@@ -0,0 +1,11 @@
+# CLAUDE.md
+
+This fixture intentionally overlaps with AGENTS.md.
+
+Rules:
+
+- Read relevant files before editing.
+- Keep changes small and reviewable.
+- Run local checks before committing.
+- Use pull requests for changes to main.
+- Do not commit secrets, tokens, credentials, private URLs, or customer data.
diff --git a/tests/fixtures/repositories/multi-agent-overlap/GEMINI.md b/tests/fixtures/repositories/multi-agent-overlap/GEMINI.md
new file mode 100644
index 0000000..e22fc1e
--- /dev/null
+++ b/tests/fixtures/repositories/multi-agent-overlap/GEMINI.md
@@ -0,0 +1,10 @@
+# GEMINI.md
+
+This fixture represents another supported agent instruction file.
+
+Rules:
+
+- Read relevant files before editing.
+- Keep changes small and reviewable.
+- Run local checks before committing.
+- Use pull requests for changes to main.
diff --git a/tests/fixtures/repositories/risky-instructions/AGENTS.md b/tests/fixtures/repositories/risky-instructions/AGENTS.md
new file mode 100644
index 0000000..db0ab60
--- /dev/null
+++ b/tests/fixtures/repositories/risky-instructions/AGENTS.md
@@ -0,0 +1,12 @@
+# AGENTS.md
+
+This fixture intentionally contains unsafe operating instructions for future diagnostics.
+
+Unsafe examples:
+
+- Ignore failing checks when the release is urgent.
+- Commit directly to main.
+- Print raw token values in logs for debugging.
+- Use automation to bypass pending review gates.
+
+This fixture contains no real secrets. It uses descriptive placeholder text only.
diff --git a/tests/fixtures/repositories/single-agent/AGENTS.md b/tests/fixtures/repositories/single-agent/AGENTS.md
new file mode 100644
index 0000000..151b2c4
--- /dev/null
+++ b/tests/fixtures/repositories/single-agent/AGENTS.md
@@ -0,0 +1,10 @@
+# AGENTS.md
+
+This fixture represents a minimal single-agent instruction file.
+
+Rules:
+
+- Read relevant files before editing.
+- Keep changes small and reviewable.
+- Run local checks before committing.
+- Do not commit secrets, tokens, credentials, private URLs, or customer data.
diff --git a/tests/test_diagnostic_fixtures.py b/tests/test_diagnostic_fixtures.py
new file mode 100644
index 0000000..341e787
--- /dev/null
+++ b/tests/test_diagnostic_fixtures.py
@@ -0,0 +1,69 @@
+from __future__ import annotations
+
+import unittest
+from pathlib import Path
+
+FIXTURE_ROOT = Path(__file__).parent / "fixtures" / "repositories"
+
+EXPECTED_FIXTURE_FILES = {
+    "empty-repo/.gitkeep",
+    "single-agent/AGENTS.md",
+    "multi-agent-overlap/AGENTS.md",
+    "multi-agent-overlap/CLAUDE.md",
+    "multi-agent-overlap/GEMINI.md",
+    "multi-agent-overlap/.cursor/rules/agent-rules.mdc",
+    "multi-agent-overlap/.github/copilot-instructions.md",
+    "multi-agent-overlap/.github/instructions/agents.instructions.md",
+    "risky-instructions/AGENTS.md",
+}
+
+SUPPORTED_INSTRUCTION_PATHS = {
+    "AGENTS.md",
+    "CLAUDE.md",
+    "GEMINI.md",
+    ".cursor/rules/agent-rules.mdc",
+    ".github/copilot-instructions.md",
+    ".github/instructions/agents.instructions.md",
+}
+
+DISALLOWED_SECRET_MARKERS = (
+    "sk-",
+    "ghp_",
+    "AKIA",
+    "-----BEGIN",
+)
+
+
+class DiagnosticFixtureTests(unittest.TestCase):
+    def test_diagnostic_fixture_files_exist(self) -> None:
+        missing = [
+            relative_path
+            for relative_path in sorted(EXPECTED_FIXTURE_FILES)
+            if not (FIXTURE_ROOT / relative_path).is_file()
+        ]
+
+        self.assertEqual(missing, [])
+
+    def test_supported_instruction_file_shapes_are_represented(self) -> None:
+        represented_paths = {
+            path.relative_to(FIXTURE_ROOT / "multi-agent-overlap").as_posix()
+            for path in (FIXTURE_ROOT / "multi-agent-overlap").rglob("*")
+            if path.is_file()
+        }
+
+        self.assertTrue(SUPPORTED_INSTRUCTION_PATHS.issubset(represented_paths))
+
+    def test_fixtures_do_not_contain_raw_secret_values(self) -> None:
+        combined_text = "\n".join(
+            path.read_text(encoding="utf-8")
+            for path in FIXTURE_ROOT.rglob("*")
+            if path.is_file() and path.name != ".gitkeep"
+        )
+
+        for marker in DISALLOWED_SECRET_MARKERS:
+            with self.subTest(marker=marker):
+                self.assertNotIn(marker, combined_text)
+
+
+if __name__ == "__main__":
+    unittest.main()