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+# PRD 0052: User-defined agent provider plugins
+
+- **Status:** Draft
+- **Author:** claude
+- **Created:** 2026-06-04
+
+## Summary
+
+The `get_provider()` registry in `bot_bottle/agent_provider.py` is a closed list —
+only `"claude"` and `"codex"` are valid templates, validated at manifest-load time and
+again at launch. Users who want to run a different agent (Gemini, Aider, a custom
+local model wrapper) cannot add a provider without forking the package.
+
+This PRD opens the registry to user-defined plugins. A plugin placed at
+`~/.bot-bottle/contrib/<name>/agent_provider.py` is discovered and loaded at launch
+time. The manifest accepts any non-empty template string that names a built-in or
+resolves to a user plugin at that path. No changes to the built-in providers or the
+internal `bot_bottle/contrib/` layout.
+
+The preceding commit on this PR moves `codex_auth.py` from `bot_bottle/` into
+`bot_bottle/contrib/codex/` — a clean-up that fits naturally here since this PR
+also clarifies that `contrib/` is the per-provider home.
+
+## Problem
+
+Users building unconventional setups hit a hard wall: the template validation in
+`manifest_agent.AgentProvider.from_dict` rejects any string not in `PROVIDER_TEMPLATES`.
+There is no escape hatch short of editing bot-bottle's source.
+
+PRD 0050 moved provider logic into `contrib/` specifically so a third provider would
+be "cheap to add" — but "cheap" today still means a pull request against the bot-bottle
+repo, not a drop-in file in the user's home directory. The filesystem layout is already
+the right shape; the discovery step is missing.
+
+## Goals / Success Criteria
+
+1. A user places `~/.bot-bottle/contrib/<name>/agent_provider.py` — a file that exports
+   a class inheriting `AgentProvider` — sets `agent_provider.template: <name>` in a
+   bottle's frontmatter, and launches a bottle using that provider with no changes to
+   the bot-bottle source.
+2. The manifest validator accepts any non-empty template string. Unknown templates that
+   resolve to no user plugin still raise a clear error, but at launch (via `get_provider`)
+   rather than at manifest-load time.
+3. Built-in provider knobs (`auth_token` → claude only; `forward_host_credentials` →
+   codex only) are guarded to built-in template names. Bottles using a user provider
+   may set neither knob.
+4. `get_provider(template)` checks `~/.bot-bottle/contrib/<template>/agent_provider.py`
+   before the built-ins, so a user can shadow a built-in for local testing.
+5. A clear `ValueError` is raised if the user plugin file exists but contains no
+   `AgentProvider` subclass.
+
+## Non-goals
+
+- Packaging or distributing user plugins as installable Python packages.
+- A plugin registry, index, or discovery beyond the filesystem path convention.
+- Adding a third built-in provider.
+- Changing the `AgentProvider` ABC contract — user plugins implement the same abstract
+  methods as `ClaudeAgentProvider` and `CodexAgentProvider`.
+- Validating that user plugin images, Dockerfiles, or commands exist before launch
+  (same policy as built-ins).
+- Sandboxing user plugin code — plugins run with full Python interpreter access.
+
+## Scope
+
+### In scope
+
+- `get_provider(template: str) -> AgentProvider` gains a `_load_user_plugin(template)`
+  step that checks `~/.bot-bottle/contrib/<template>/agent_provider.py` first, then
+  falls through to the built-in look-ups.
+- `_load_user_plugin` uses `importlib.util.spec_from_file_location` to load the module
+  and returns the first `AgentProvider` subclass found in its `__dict__`. Raises
+  `ValueError` if the file exists but exports no subclass.
+- `manifest_agent.AgentProvider.from_dict`: the `template not in PROVIDER_TEMPLATES`
+  check is removed; the two built-in-specific knob guards (`auth_token` → claude,
+  `forward_host_credentials` → codex) are tightened to `template in PROVIDER_TEMPLATES`
+  so they are skipped for user-defined names.
+- `PROVIDER_TEMPLATES` remains in `agent_provider.py` as the set of built-in names for
+  use by tests and any enumeration callers.
+- Unit tests for the discovery path:
+  - Plugin found and loaded → correct `AgentProvider` instance returned.
+  - Plugin file exists but exports no subclass → `ValueError`.
+  - Unknown template with no user plugin → `ValueError` from `get_provider`.
+  - Built-in template name still works normally even when no user plugin exists.
+- One paragraph added to `README.md` under a new "Custom providers" section describing
+  the `~/.bot-bottle/contrib/<name>/agent_provider.py` convention and pointing at the
+  existing contrib providers as reference implementations.
+
+### Out of scope
+
+- Hot-reloading plugins during a running session.
+- Plugin versioning or dependency declaration.
+- Changes to smolmachines or Docker backend provisioning paths.
+
+## Proposed design
+
+### Discovery in `get_provider`
+
+```python
+import importlib.util
+
+def get_provider(template: str) -> AgentProvider:
+    user_plugin = _load_user_plugin(template)
+    if user_plugin is not None:
+        return user_plugin
+    if template == PROVIDER_CLAUDE:
+        from .contrib.claude.agent_provider import ClaudeAgentProvider
+        return ClaudeAgentProvider()
+    if template == PROVIDER_CODEX:
+        from .contrib.codex.agent_provider import CodexAgentProvider
+        return CodexAgentProvider()
+    raise ValueError(f"unknown agent provider template: {template!r}")
+
+
+def _load_user_plugin(template: str) -> AgentProvider | None:
+    plugin_path = (
+        Path.home() / ".bot-bottle" / "contrib" / template / "agent_provider.py"
+    )
+    if not plugin_path.exists():
+        return None
+    spec = importlib.util.spec_from_file_location(
+        f"_user_contrib_{template}.agent_provider", plugin_path
+    )
+    if spec is None or spec.loader is None:
+        raise ValueError(f"user plugin at {plugin_path} could not be loaded")
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)  # type: ignore[union-attr]
+    for obj in vars(mod).values():
+        if (
+            isinstance(obj, type)
+            and issubclass(obj, AgentProvider)
+            and obj is not AgentProvider
+        ):
+            return obj()
+    raise ValueError(
+        f"user plugin at {plugin_path} defines no AgentProvider subclass"
+    )
+```
+
+### Manifest validation change
+
+In `manifest_agent.AgentProvider.from_dict`, remove the hard rejection:
+
+```python
+# Before
+if template not in PROVIDER_TEMPLATES:
+    raise ManifestError(
+        f"bottle '{bottle_name}' agent_provider.template {template!r} "
+        f"is not one of {', '.join(sorted(PROVIDER_TEMPLATES))}"
+    )
+
+# After — removed entirely; get_provider() raises at launch for unknown names
+```
+
+Guard the built-in knob checks with `template in PROVIDER_TEMPLATES`:
+
+```python
+if auth_token and template == "claude":   # unchanged
+    ...
+if auth_token and template not in PROVIDER_TEMPLATES:
+    raise ManifestError(
+        f"bottle '{bottle_name}' agent_provider.auth_token is only "
+        f"supported for built-in templates ({', '.join(sorted(PROVIDER_TEMPLATES))})"
+    )
+if forward_host_credentials and template == "codex":  # unchanged
+    ...
+if forward_host_credentials and template not in PROVIDER_TEMPLATES:
+    raise ManifestError(
+        f"bottle '{bottle_name}' agent_provider.forward_host_credentials "
+        f"is only supported for built-in templates"
+    )
+```
+
+## Open questions
+
+1. **Shadow order.** This PRD puts user plugins before built-ins, allowing local
+   overrides. If the preference is built-ins-first (to prevent accidental shadowing),
+   swap the order and document accordingly.
+2. **`BOT_BOTTLE_CONTRIB_DIR` env var.** Omitted for now — `~/.bot-bottle/contrib/`
+   is consistent with the rest of the user config layout. Revisit if the need surfaces.
+
+## References
+
+- PRD 0050 — agent provider contrib (established `contrib/` as the per-provider home)
+- PRD 0048 — SSH deploy key provisioning (the `contrib/` convention)
+- `bot_bottle/agent_provider.py` — `get_provider`, `PROVIDER_TEMPLATES`, `AgentProvider` ABC
+- `bot_bottle/manifest_agent.py` — template validation that this PRD relaxes