@@ -9,6 +9,8 @@ and versions are tracked in the repo-root `VERSION` file.
 
 ### Added
 
+- Add the public API stability and deprecation policy, migration guide, and
+  `base_cli.deprecated()` warning helper with contract-test guardrails.
 - Add immutable `LifecycleOptions` and `LifecycleOption` policies for enabling,
   disabling, renaming, and configuring each standard option independently, with
   normalized `LifecycleValues` stored in namespaced Click metadata without
 
@@ -41,6 +41,11 @@ Automation-facing JSON output, errors, and logs are opt-in through the
 versioned contracts documented in [`docs/json-contracts.md`](docs/json-contracts.md).
 Human output and Click error behavior remain the default.
 
+The supported public facade, compatibility promises, deprecation warning
+mechanism, and migration requirements are documented in
+[`docs/api-stability.md`](docs/api-stability.md) and
+[`docs/migrations.md`](docs/migrations.md).
+
 Shared record renderers keep machine output stable: CSV and TSV stream
 one-pass iterables without headers or footers, while terminal tables account
 for Unicode display width and safely truncate oversized cells. See
 
@@ -0,0 +1,122 @@
+# API stability and deprecation policy
+
+This document is the compatibility contract for `base-cli`. It applies to the
+current pre-1.0 releases and will be updated before the 1.0 release. The
+repository's contract tests and release checklist are expected to change with
+this document, so a policy change is itself a reviewed, changelogged change.
+
+## Public surface
+
+The supported Python facade is `import base_cli`. The names in
+`base_cli.__all__` are the public facade; documented names in the explicitly
+exported modules (`base_cli.command_protocol`, `base_cli.json_contracts`, and
+the other modules listed in their module-level `__all__`) are public as well.
+Names beginning with `_`, modules not documented here, and implementation
+details are private and may change without notice.
+
+The following machine-facing contracts are public and versioned:
+
+- command framing defaults to `COMMAND_PROTOCOL_V1`; record schemas are owned
+  and registered by the consumer;
+- JSON output, error, and log records use `schema_version: 1` and the schema
+  identifiers documented in [`json-contracts.md`](json-contracts.md); and
+- the exported callable and type signatures exercised by the public API tests.
+
+Human-readable messages, table layout, log wording, temporary directory names,
+and history implementation details are not stable machine interfaces unless a
+separate contract document says otherwise. Consumers that need automation
+should select the JSON or record protocol contracts.
+
+`base_cli.experimental` is reserved for preview APIs. No experimental symbols
+are currently shipped. A future preview must live under that namespace, be
+labelled experimental in its documentation, and must not be re-exported from
+the stable facade until it is promoted.
+
+## Versioning and compatibility
+
+Starting with 1.0, `base-cli` follows Semantic Versioning:
+
+- a **MAJOR** release may remove or change public APIs and versioned contracts;
+- a **MINOR** release adds backwards-compatible functionality and may begin a
+  deprecation; and
+- a **PATCH** release contains compatible fixes, security fixes, and docs.
+
+Until 1.0, the leading zero is meaningful: patch releases remain compatible,
+while a minor release is a compatibility boundary and may contain a breaking
+change. We still prefer additive changes, and any pre-1.0 break must include a
+warning where practical, a migration path, and a changelog entry. Consumers
+that need a frozen API should pin a minor release (for example, `~=0.3.0`).
+
+The core package requires Python `>=3.10` and currently tests CPython 3.10
+through 3.14 on Linux, macOS, and Windows. Core runtime dependencies are
+Click `>=8.1` and PyYAML `>=6.0`. Optional integrations are independently
+versioned and constrained in `pyproject.toml`: Typer `>=0.12,<0.26`, Rich
+`>=13.7,<15`, and OpenTelemetry API `>=1.24,<2`. The lower bounds are the
+minimum supported versions; a dependency major release is supported after it
+passes the compatibility suite. A future minor release may drop an end-of-life
+Python or dependency window with a migration note.
+
+Platform tier details and the operating-system support test matrix are kept in
+[`platform-support.md`](platform-support.md).
+
+## Deprecation process
+
+Use the public `base_cli.deprecated` decorator for callable APIs:
+
+```python
+from base_cli import deprecated
+
+
+@deprecated("0.3", remove="1.0", alternative="new_name")
+def old_name(value: str) -> str:
+    return new_name(value)
+```
+
+The decorator preserves the callable's metadata and behavior, and emits a
+`BaseCliDeprecationWarning` with `stacklevel=2` on every call. Applications can
+show or fail on these warnings with the standard `warnings` filters. The
+warning identifies the release that introduced the deprecation, the planned
+removal release, and (when available) the replacement API.
+
+Every deprecation must:
+
+1. remain supported for at least **two minor releases and 90 calendar days,
+   whichever is longer**;
+2. include a migration note in [`migrations.md`](migrations.md) or the relevant
+   contract document;
+3. appear in `CHANGELOG.md` under the release that introduces the warning and
+   the release that removes the API; and
+4. be removed only in the stated removal release (or a later release), except
+   for an urgent security or legal fix that is explicitly documented.
+
+The removal PR must delete the contract test for the old symbol only after the
+replacement and migration guidance are present. A deprecation is not complete
+until the warning, docs, tests, and changelog agree.
+
+## Contract guardrails
+
+`tests/test_public_api.py` verifies that every facade export is resolvable,
+that module `__all__` surfaces do not silently drift, and that private legacy
+names stay absent. `tests/test_api_stability.py` additionally verifies the
+warning behavior, the default command-protocol header, and the JSON v1 schema
+identifiers and envelope fields. Changes to an exported symbol or a versioned
+schema therefore require an intentional test and documentation update before
+release.
+
+When a schema must evolve incompatibly, add a new version and an adapter rather
+than changing the meaning of an existing field in place. Keep the old version
+available for the same deprecation window and document the migration.
+
+## Release checklist
+
+Before publishing a release, maintainers should confirm:
+
+- the supported Python and dependency windows still match CI and `pyproject.toml`;
+- public exports and schema constants have contract-test coverage;
+- each deprecation has a warning, removal target, migration note, and changelog
+  entry; and
+- the release notes call out any pre-1.0 compatibility boundary or contract
+  version addition.
+
+See [`releasing.md`](releasing.md) for the mechanical package and publishing
+steps.
@@ -0,0 +1,53 @@
+# Migration guide
+
+This page collects the format for changes that affect the public `base-cli`
+contract. The compatibility rules and deprecation timeline are defined in
+[`api-stability.md`](api-stability.md).
+
+## Migrating a deprecated API
+
+1. Upgrade to the first release that emits the warning.
+2. Replace the old symbol with the alternative named in the warning and in the
+   release notes.
+3. Run tests with `BaseCliDeprecationWarning` enabled so no old call sites are
+   missed:
+
+   ```python
+   import warnings
+
+   from base_cli import BaseCliDeprecationWarning
+
+   warnings.simplefilter("error", BaseCliDeprecationWarning)
+   ```
+
+4. Remove temporary compatibility shims before the stated removal release.
+
+Warnings are ordinary Python warnings, so applications can instead record or
+display them with their normal `warnings` configuration.
+
+## Schema migrations
+
+Versioned JSON envelopes and command framing must not change meaning in place.
+Add a new schema or protocol version, keep the old version during its support
+window, and provide an adapter when practical. A schema migration note should
+include:
+
+- old and new version identifiers;
+- field additions, removals, and type changes;
+- producer and consumer rollout order; and
+- the release where the old version will stop being accepted.
+
+## Migration note template
+
+```markdown
+### `old_name` → `new_name`
+
+- **First warning:** 0.x.y
+- **Removal target:** 1.0.0
+- **Why:** explain the problem in one sentence.
+- **Action:** show the smallest before/after example.
+- **Compatibility:** describe the temporary adapter or schema version.
+```
+
+For release mechanics, see [`releasing.md`](releasing.md). For the public
+surface and deprecation requirements, see [`api-stability.md`](api-stability.md).
@@ -30,14 +30,15 @@ def _resolve_version() -> str:
 
 __version__ = _resolve_version()
 
-from . import command_filters, command_protocol, extensions, history, integrations, json_contracts, testing
+from . import command_filters, command_protocol, deprecations, extensions, history, integrations, json_contracts, testing
 from .attachment import (
     AttachmentAdapter,
     AttachmentContextFactory,
     AttachmentContract,
     AttachmentServiceFactory,
 )
 from .config import BatteriesIncludedConfigLoader, ConfigSnapshot, FrameworkConfig
+from .deprecations import BaseCliDeprecationWarning, deprecated
 from .app import (
     App,
     argument,
@@ -143,6 +144,7 @@ def _resolve_version() -> str:
     "AttachmentServiceFactory",
     "TyperAdapter",
     "BatteriesIncludedConfigLoader",
+    "BaseCliDeprecationWarning",
     "BOOLEAN",
     "ApplicationStateT",
     "CliProfile",
@@ -179,6 +181,7 @@ def _resolve_version() -> str:
     "command_filters",
     "command_matches",
     "command_protocol",
+    "deprecations",
     "json_contracts",
     "JSON_CONTRACT_VERSION",
     "JSON_ERROR_SCHEMA",
@@ -202,6 +205,7 @@ def _resolve_version() -> str:
     "command",
     "configure_logger",
     "delegated_display_command",
+    "deprecated",
     "get_command_app",
     "get_current_context",
     "get_typer_command",
 
@@ -0,0 +1,53 @@
+"""Small, consistent deprecation primitives for public base-cli APIs."""
+
+from __future__ import annotations
+
+import warnings
+from collections.abc import Callable
+from functools import wraps
+from typing import ParamSpec, TypeVar
+
+
+P = ParamSpec("P")
+R = TypeVar("R")
+
+
+class BaseCliDeprecationWarning(DeprecationWarning):
+    """Warning emitted when a supported base-cli API is being retired."""
+
+
+def deprecated(
+    since: str,
+    *,
+    remove: str,
+    alternative: str | None = None,
+) -> Callable[[Callable[P, R]], Callable[P, R]]:
+    """Mark a callable as deprecated while preserving its normal behavior.
+
+    ``since`` and ``remove`` are release identifiers, not free-form dates. The
+    decorator emits :class:`BaseCliDeprecationWarning` on every call so an
+    application can choose whether to show, record, or fail on the warning.
+    ``stacklevel=2`` points the warning at the caller rather than this helper.
+    """
+
+    if not since.strip():
+        raise ValueError("since must not be empty")
+    if not remove.strip():
+        raise ValueError("remove must not be empty")
+
+    def decorate(function: Callable[P, R]) -> Callable[P, R]:
+        message = f"{function.__qualname__} is deprecated since {since} and will be removed in {remove}."
+        if alternative:
+            message += f" Use {alternative} instead."
+
+        @wraps(function)
+        def wrapped(*args: P.args, **kwargs: P.kwargs) -> R:
+            warnings.warn(message, BaseCliDeprecationWarning, stacklevel=2)
+            return function(*args, **kwargs)
+
+        return wrapped
+
+    return decorate
+
+
+__all__ = ["BaseCliDeprecationWarning", "deprecated"]