## Why
Selected plugin metadata is stable, but MCP processes are live runtime
state. They need different lifetimes:
- the MCP extension caches manifest, MCP, and connector declarations for
each stable selected root;
- each model step projects that cached metadata through the roots that
resolved as ready for that exact step;
- the MCP manager is rebuilt only when that availability projection
changes.
This matches executor skills: both features consume the same resolved
step roots instead of inferring readiness from the turn's selected
environments.
## Behavior
```text
E1 not ready for this step
-> no E1 MCP servers or connectors
-> cached plugin metadata stays in ext/mcp
E1 becomes ready
-> reuse cached metadata
-> publish one MCP runtime containing E1 capabilities
same ready roots on the next step
-> reuse the exact runtime; no rediscovery and no MCP restart
resume
-> create new extension thread state and a new MCP runtime
```
All model-facing consumers use the same step snapshot:
```text
resolved selected roots
|
v
extension MCP/connector projection
|
v
{ MCP config, connector snapshot, MCP manager }
|
+-> advertise model tools
+-> build app/connector tools
+-> execute MCP calls
```
## Cache contract
The existing MCP extension owns a cache keyed by the full
`SelectedCapabilityRoot`:
```rust
let state = thread_store.get_or_init(SelectedExecutorPluginMcpState::default);
```
The cache lives with extension thread state. Environment availability
filters projection but does not invalidate metadata. Resume creates new
thread state. There is no file watcher or executor generation because
contents behind a stable environment/root are assumed stable.
## What changes
- Keeps executor plugin discovery and cached metadata in `ext/mcp`.
- Caches MCP and connector declarations together per selected root.
- Uses the step's already-resolved capability roots, including lazy
environments that are not turn environments.
- Reuses the current MCP runtime when the ready-root projection is
unchanged.
- Uses the same step MCP manager and connector snapshot for
model-visible tools and execution.
- Resolves direct thread-scoped MCP requests from the current
selected-root projection.
## Deliberately out of scope
- `app/list` remains based on the latest global host-plugin state; this
PR does not make its response or notifications thread-specific.
- `required = true` startup semantics do not apply to delayed executor
MCP activation.
- No filesystem/content invalidation.
- No transport-disconnect watcher.
- No executor generations or environment replacement semantics.
- No client sharing across complete manager replacements.
## Stack
1. Extension-owned World State sections.
2. Project executor skills through World State.
3. Pin one MCP runtime to each model step.
4. **This PR:** project selected MCP and connector state from
extension-owned metadata.
5. Integration coverage for selected capability availability and resume.
## Verification
-
`selected_plugin_servers_use_managed_requirements_for_the_selected_root_id`
- The stacked integration PR covers unavailable to ready activation,
unchanged-runtime reuse, skills, MCP tools, connector attribution, and
cold resume.
## Why
An MCP refresh can replace the session's current manager while a model
step is still running. The step must execute calls through the same
manager whose tools it advertised.
## Boundary
```text
current session MCP runtime
|
| capture once for this model step
v
StepContext.mcp
- exact MCP config
- exact connection manager
- exact runtime environment context
```
```rust
pub struct McpRuntimeSnapshot {
config: Arc<McpConfig>,
manager: Arc<McpConnectionManager>,
runtime_context: McpRuntimeContext,
}
```
## Example
```text
step A captures runtime A and advertises A's tools
refresh publishes runtime B
step A tool call -> runtime A
next step -> runtime B
```
Capturing the snapshot is only an `Arc` clone. It does not restart MCPs
or make an RPC.
## What changes
- Captures one MCP runtime in `StepContext`.
- Uses it for tool planning, tool calls, resources, approvals, connector
attribution, and elicitation.
- Publishes replacement runtimes atomically.
- Lets an old runtime live only while an in-flight step or request still
holds its `Arc`.
Most of this diff is mechanical routing from the session-global manager
to `step_context.mcp`; it does not introduce selected-plugin discovery
yet.
## What does not change
- No plugin or extension migration.
- No new MCP cache policy.
- No environment file watching.
- No client sharing between separate managers.
## Stack
1. Extension-owned World State sections.
2. Project executor skills through World State.
3. **This PR:** pin one MCP runtime to each model step.
4. Project selected MCP/app/connector metadata by environment
availability.
5. One end-to-end integration scenario.
jif