diff --git a/.cursor/rules/pr-after-first-push.mdc b/.cursor/rules/pr-after-first-push.mdc
new file mode 100644
index 0000000..ee32313
--- /dev/null
+++ b/.cursor/rules/pr-after-first-push.mdc
@@ -0,0 +1,58 @@
+---
+description: After first branch push, provide copy-ready PR title and body
+alwaysApply: true
+---
+
+# PR Title and Comment After First Push
+
+The user creates pull requests manually. After the **first push** of a feature branch to remote in a session, always end your response with a copy-ready PR title and PR comment.
+
+## When to provide it
+
+- After the first successful `git push` (or `git push -u origin <branch>`) for a branch in the current task
+- Do **not** create the PR with `gh pr create` unless explicitly asked
+- On later pushes to the same branch, only repeat if the user asks or the change set meaningfully updates what the PR should say
+
+## PR title
+
+- Use a fitting version-bump prefix when changes touch `python_repositories/`:
+  - `[patch]` or `[fix]` — bug fix
+  - `[minor]` or `[feat]` — new feature
+  - `[major]` or `[breaking]` — breaking change
+- Docs-, CI-, or test-only changes: no prefix required
+- Keep the title concise and descriptive
+
+## PR comment format
+
+Provide two separate fenced code blocks the user can copy directly:
+
+1. **PR title** — single line in a code block
+2. **PR comment** — body with `## Summary` and `## Test plan` sections, checklist items reflecting what was actually run
+
+Example structure:
+
+````
+PR title:
+
+```
+[minor] Short description of the change
+```
+
+PR comment:
+
+```
+## Summary
+
+- Bullet points of what changed and why
+
+## Test plan
+
+- [x] Commands or checks that were run
+```
+````
+
+## Notes
+
+- Match the repo PR template ([`.gitea/PULL_REQUEST_TEMPLATE.md`](.gitea/PULL_REQUEST_TEMPLATE.md))
+- Be accurate: only mark test-plan items done if they were run
+- Keep prose clear; the user copies this verbatim into Gitea
diff --git a/.cursor/rules/test-organization.mdc b/.cursor/rules/test-organization.mdc
new file mode 100644
index 0000000..9e2474a
--- /dev/null
+++ b/.cursor/rules/test-organization.mdc
@@ -0,0 +1,31 @@
+---
+description: Unit test file naming and placement conventions
+globs: tests/**/*.py
+alwaysApply: false
+---
+
+# Test Organization
+
+Unit tests use a one-to-one mapping between source modules and test files.
+
+## Naming
+
+- `python_repositories/<path>/<module>.py` → `tests/unit/<module>_test.py`
+- Test files must end with `_test.py` (enforced by pre-commit `name-tests-test`)
+
+## Placement
+
+- Add tests to the existing `*_test.py` for the module under test
+- Do not create cross-cutting test files (e.g. `structural_typing_test.py`); colocate with the relevant interface/adapter/config test file
+- Shared fixtures → `tests/conftest.py`
+- Module-specific helper classes → the matching test file
+
+## Examples
+
+```
+json_repository_interface.py  →  tests/unit/json_repository_interface_test.py
+redis_adapter.py              →  tests/unit/redis_adapter_test.py
+redis_config.py               →  tests/unit/redis_config_test.py
+```
+
+Integration tests live under `tests/integration/<backend>/` with the same `_test.py` suffix.
diff --git a/README.md b/README.md
index 29a8a95..d793889 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,8 @@ Subclass an adapter in your own repository to add domain-specific methods while
 | **Adapters**     | Technology-specific base classes (`RedisAdapter`, `MinioAdapter`) |
 | **Your project** | Subclass an adapter and add domain methods                        |
 
+Each public interface is a `@runtime_checkable` `Protocol` with `@abstractmethod` members. **Subclass an adapter** when you need connection management and shared behavior — explicit subclasses get runtime instantiation guards and inherited default methods (e.g. `scan_keys`). **Type-annotate against an interface** when you want loose coupling — any object with the right methods satisfies the contract for mypy and `isinstance()` checks, without inheriting from this package.
+
 Connection adapters expose `connect()`, `disconnect()`, and `is_connected()`. The latter verifies backend reachability with a cached health probe (default TTL: 1 second). Subclasses may override `health_check_ttl_seconds`. `connect()` is idempotent: calling it while already connected and healthy is a no-op.
 
 ## Future direction
@@ -172,6 +174,15 @@ uv run pytest -v                                      # full suite (requires Doc
 
 Integration tests are marked with `@pytest.mark.integration` and require Docker (testcontainers). Backend-specific markers (`needs_redis`, `needs_minio`) let you run only the containers a test module needs. Run unit tests alone for quick local feedback.
 
+### Test organization
+
+Unit tests live in `tests/unit/` and follow a one-to-one naming convention: `<module>_test.py` tests `python_repositories/<module path>.py`. Examples:
+
+- `json_repository_interface.py` → `tests/unit/json_repository_interface_test.py`
+- `redis_adapter.py` → `tests/unit/redis_adapter_test.py`
+
+Add new tests to the existing file for that module rather than creating cross-cutting test files. Shared fixtures belong in `tests/conftest.py`; module-specific helpers may live in the matching test file.
+
 ### CI base image
 
 Gitea Actions jobs use a pre-built image (`python-repositories-ci`) with Python 3.12,
diff --git a/python_repositories/adapters/minio_adapter.py b/python_repositories/adapters/minio_adapter.py
index 3e71d9f..db4751c 100644
--- a/python_repositories/adapters/minio_adapter.py
+++ b/python_repositories/adapters/minio_adapter.py
@@ -19,7 +19,7 @@ except ImportError as exc:
     ) from exc
 
 
-class MinioAdapter(ObjectRepositoryInterface, ConnectionAwareAdapter):
+class MinioAdapter(ConnectionAwareAdapter, ObjectRepositoryInterface):
     """Minio adapter exposing basic CRUD functionality."""
 
     endpoint_env_var_name: str = "MINIO_ENDPOINT"
diff --git a/python_repositories/adapters/redis_adapter.py b/python_repositories/adapters/redis_adapter.py
index 88bdbfa..5a21c5a 100644
--- a/python_repositories/adapters/redis_adapter.py
+++ b/python_repositories/adapters/redis_adapter.py
@@ -21,7 +21,7 @@ except ImportError as exc:
     ) from exc
 
 
-class RedisAdapter(JsonRepositoryInterface, ConnectionAwareAdapter):
+class RedisAdapter(ConnectionAwareAdapter, JsonRepositoryInterface):
     """Redis adapter exposing basic CRUD functionality."""
 
     uri_env_var_name: str = "REDIS_URI"
diff --git a/python_repositories/interfaces/connection_aware_interface.py b/python_repositories/interfaces/connection_aware_interface.py
index 0cba6a4..e1a6d3a 100644
--- a/python_repositories/interfaces/connection_aware_interface.py
+++ b/python_repositories/interfaces/connection_aware_interface.py
@@ -1,9 +1,11 @@
-"""Definition of ConnectionAwareInterface abstract base class."""
+"""Definition of ConnectionAwareInterface protocol and abstract base class."""
 
-from abc import ABC, abstractmethod
+from abc import abstractmethod
+from typing import Protocol, runtime_checkable
 
 
-class ConnectionAwareInterface(ABC):
+@runtime_checkable
+class ConnectionAwareInterface(Protocol):
     """Interface that defines connection-related methods."""
 
     @abstractmethod
diff --git a/python_repositories/interfaces/context_aware_interface.py b/python_repositories/interfaces/context_aware_interface.py
index 7815304..9f29eae 100644
--- a/python_repositories/interfaces/context_aware_interface.py
+++ b/python_repositories/interfaces/context_aware_interface.py
@@ -1,12 +1,13 @@
-"""Definition of ContextAwareInterface abstract base class."""
+"""Definition of ContextAwareInterface protocol and abstract base class."""
 
 from __future__ import annotations
 
-from abc import ABC, abstractmethod
-from typing import Self
+from abc import abstractmethod
+from typing import Protocol, Self, runtime_checkable
 
 
-class ContextAwareInterface(ABC):
+@runtime_checkable
+class ContextAwareInterface(Protocol):
     """Interface that defines context-related methods."""
 
     @abstractmethod
diff --git a/python_repositories/interfaces/json_repository_interface.py b/python_repositories/interfaces/json_repository_interface.py
index 66ae079..2a13e7a 100644
--- a/python_repositories/interfaces/json_repository_interface.py
+++ b/python_repositories/interfaces/json_repository_interface.py
@@ -1,11 +1,12 @@
-"""Definition of JsonRepositoryInterface abstract base class."""
+"""Definition of JsonRepositoryInterface protocol and abstract base class."""
 
-from abc import ABC, abstractmethod
+from abc import abstractmethod
 from collections.abc import Iterator
-from typing import Any
+from typing import Any, Protocol, runtime_checkable
 
 
-class JsonRepositoryInterface(ABC):
+@runtime_checkable
+class JsonRepositoryInterface(Protocol):
     """Interface that defines JSON document CRUD methods."""
 
     @abstractmethod
diff --git a/python_repositories/interfaces/object_repository_interface.py b/python_repositories/interfaces/object_repository_interface.py
index 85482f9..7f07a9f 100644
--- a/python_repositories/interfaces/object_repository_interface.py
+++ b/python_repositories/interfaces/object_repository_interface.py
@@ -1,10 +1,12 @@
-"""Definition of ObjectRepositoryInterface abstract base class."""
+"""Definition of ObjectRepositoryInterface protocol and abstract base class."""
 
-from abc import ABC, abstractmethod
+from abc import abstractmethod
 from io import BytesIO
+from typing import Protocol, runtime_checkable
 
 
-class ObjectRepositoryInterface(ABC):
+@runtime_checkable
+class ObjectRepositoryInterface(Protocol):
     """Interface that defines binary object CRUD methods."""
 
     @abstractmethod
diff --git a/tests/unit/connection_aware_interface_test.py b/tests/unit/connection_aware_interface_test.py
index 560260c..4a36303 100644
--- a/tests/unit/connection_aware_interface_test.py
+++ b/tests/unit/connection_aware_interface_test.py
@@ -7,6 +7,24 @@ from python_repositories.interfaces.connection_aware_interface import (
 )
 
 
+class FakeConnection:
+    """Plain class that satisfies ConnectionAwareInterface without inheritance."""
+
+    def connect(self) -> None:
+        pass
+
+    def disconnect(self) -> None:
+        pass
+
+    def is_connected(self) -> bool:
+        return True
+
+
+def accepts_connection_aware(connection: ConnectionAwareInterface) -> None:
+    """Type-checking hook for ConnectionAwareInterface structural subtyping."""
+    connection.is_connected()
+
+
 def test_instantiation_fails_when_connect_not_implemented() -> None:
     """Test that instantiation fails if connect is not implemented."""
 
@@ -53,3 +71,10 @@ def test_instantiation_fails_when_is_connected_not_implemented() -> None:
 
     with pytest.raises(TypeError):
         _ = Incomplete()  # type: ignore
+
+
+def test_structural_subtyping() -> None:
+    """Test that a plain class satisfies ConnectionAwareInterface structurally."""
+    connection: ConnectionAwareInterface = FakeConnection()
+    accepts_connection_aware(connection)
+    assert isinstance(connection, ConnectionAwareInterface)
diff --git a/tests/unit/context_aware_interface_test.py b/tests/unit/context_aware_interface_test.py
index 925f322..205dc41 100644
--- a/tests/unit/context_aware_interface_test.py
+++ b/tests/unit/context_aware_interface_test.py
@@ -7,6 +7,24 @@ import pytest
 from python_repositories.interfaces.context_aware_interface import ContextAwareInterface
 
 
+class FakeContextManager:
+    """Plain class that satisfies ContextAwareInterface without inheritance."""
+
+    def __enter__(self) -> FakeContextManager:
+        return self
+
+    def __exit__(
+        self, exc_type: type | None, exc_val: object | None, exc_tb: object | None
+    ) -> None:
+        pass
+
+
+def accepts_context_aware(context: ContextAwareInterface) -> None:
+    """Type-checking hook for ContextAwareInterface structural subtyping."""
+    with context:
+        pass
+
+
 def test_instantiation_fails_when_enter_not_implemented() -> None:
     """Test that instantiation fails if __enter__ is not implemented."""
 
@@ -33,3 +51,10 @@ def test_instantiation_fails_when_exit_not_implemented() -> None:
 
     with pytest.raises(TypeError):
         _ = Incomplete()  # type: ignore
+
+
+def test_structural_subtyping() -> None:
+    """Test that a plain class satisfies ContextAwareInterface structurally."""
+    context: ContextAwareInterface = FakeContextManager()
+    accepts_context_aware(context)
+    assert isinstance(context, ContextAwareInterface)
diff --git a/tests/unit/json_repository_interface_test.py b/tests/unit/json_repository_interface_test.py
index 208b4f9..70ecf1c 100644
--- a/tests/unit/json_repository_interface_test.py
+++ b/tests/unit/json_repository_interface_test.py
@@ -1,5 +1,6 @@
 """Unit tests for JsonRepositoryInterface."""
 
+from collections.abc import Iterator
 from typing import Any
 
 import pytest
@@ -9,6 +10,36 @@ from python_repositories.interfaces.json_repository_interface import (
 )
 
 
+class InMemoryJsonRepo:
+    """Plain class that satisfies JsonRepositoryInterface without inheritance."""
+
+    def get(self, key: str) -> dict[str, Any] | None:
+        return None
+
+    def set(self, key: str, data: dict[str, Any]) -> None:
+        pass
+
+    def delete(self, key: str) -> None:
+        pass
+
+    def list_keys(self, pattern: str) -> list[str]:
+        return []
+
+    def scan_keys(
+        self,
+        pattern: str,
+        *,
+        count: int | None = None,
+    ) -> Iterator[str]:
+        del count
+        yield from self.list_keys(pattern)
+
+
+def accepts_json_repo(repo: JsonRepositoryInterface) -> None:
+    """Type-checking hook for JsonRepositoryInterface structural subtyping."""
+    repo.get("key")
+
+
 def test_instantiation_fails_when_get_not_implemented() -> None:
     """Test that instantiation fails if get is not implemented."""
 
@@ -104,3 +135,10 @@ def test_scan_keys_defaults_to_list_keys() -> None:
     repository = Complete()
 
     assert list(repository.scan_keys("user")) == ["user-1", "user-2"]
+
+
+def test_structural_subtyping() -> None:
+    """Test that a plain class satisfies JsonRepositoryInterface structurally."""
+    repo: JsonRepositoryInterface = InMemoryJsonRepo()
+    accepts_json_repo(repo)
+    assert isinstance(repo, JsonRepositoryInterface)
diff --git a/tests/unit/object_repository_interface_test.py b/tests/unit/object_repository_interface_test.py
index 4cebd2d..3bb1ce3 100644
--- a/tests/unit/object_repository_interface_test.py
+++ b/tests/unit/object_repository_interface_test.py
@@ -9,6 +9,32 @@ from python_repositories.interfaces.object_repository_interface import (
 )
 
 
+class InMemoryObjectRepo:
+    """Plain class that satisfies ObjectRepositoryInterface without inheritance."""
+
+    def get(self, object_name: str) -> BytesIO | None:
+        return None
+
+    def put(
+        self,
+        object_name: str,
+        data: BytesIO,
+        content_type: str = "application/octet-stream",
+    ) -> None:
+        pass
+
+    def delete(self, object_name: str) -> None:
+        pass
+
+    def list_objects(self, prefix: str = "") -> list[str]:
+        return []
+
+
+def accepts_object_repo(repo: ObjectRepositoryInterface) -> None:
+    """Type-checking hook for ObjectRepositoryInterface structural subtyping."""
+    repo.get("object")
+
+
 def test_instantiation_fails_when_get_not_implemented() -> None:
     """Test that instantiation fails if get is not implemented."""
 
@@ -98,3 +124,10 @@ def test_instantiation_fails_when_list_objects_not_implemented() -> None:
 
     with pytest.raises(TypeError):
         _ = Incomplete()  # type: ignore
+
+
+def test_structural_subtyping() -> None:
+    """Test that a plain class satisfies ObjectRepositoryInterface structurally."""
+    repo: ObjectRepositoryInterface = InMemoryObjectRepo()
+    accepts_object_repo(repo)
+    assert isinstance(repo, ObjectRepositoryInterface)