fix(mcp): per-project registries, registry-independent rendering, per-server ${VAR} handling - #193
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McpToolOutput is self-describing, so a recorded MCP execution can be deserialized without a live server or registry entry. Expose that as deserialize_mcp_output() (McpTool::deserialize_output delegates to it) plus an is_mcp_tool_name() predicate for the reserved mcp__ prefix.
Session rendering and agent session state resolved recorded tool executions through whichever registry the instance happened to hold — so a parallel session in another project (or a second code-assistant instance viewing the session) silently dropped MCP tool calls whose server the current registry had not connected. MCP executions now always deserialize from their self-describing JSON via deserialize_tool_execution(); only native tools still require a registry entry. A session renders identically wherever it is viewed.
The registry cache held a single slot, so sessions running in different projects at the same time invalidated each other on every run start: each switch rebuilt the registry and the eviction pass shut down the other project's local MCP servers. The cache is now a map keyed by the included local-MCP directory (None = global-only; requests without trusted local MCP normalize to it), so each project's registry stays valid while others build. Cache and connection pool hold Weak references: a registry lives exactly as long as a session instance or in-flight run holds it, connections live as long as some registry references them (dropping the last Arc terminates the server child). This replaces the explicit building_keys/evict_unreferenced machinery with ownership-driven lifetimes and answers the previously open eviction question: a project's servers run while a session that uses them is open.
refresh_tool_registry broadcast every fresh registry to all active session instances, so starting a run in one project rewired every other session's rendering to that project's registry — a session viewed while another project's agent ran turns lost its own MCP tool executions. The refresh now updates the manager-level registry (this run's working registry and the default for not-yet-run instances) and the requesting session's instance alone. Other sessions keep the registry of their own last run; MCP executions render registry-independently anyway.
Rework the design note into feature documentation: .mcp.json format and merge semantics, the trust flow, per-session server toggles, the per-project registry cache with ownership-driven connection lifetimes, the registry-independent rendering of recorded MCP executions, and the known limitations. Cross-reference from mcp-client-mode.md and AGENTS.md.
Variable substitution was all-or-nothing: one unset variable failed the whole configuration — the global mcp-servers.json registered nothing, and a project's .mcp.json was dropped entirely. A server missing its secret cannot connect anyway, but it must not take the other servers down with it. McpServersConfig::substitute_env_values now substitutes per server, drops the failing ones, and returns them as (name, error) for the loaders to log.
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Follow-up to #189. With sessions running in parallel across projects, the single-slot registry cache and the registry broadcast caused real misbehavior; this PR makes registries per-project and makes session rendering independent of connection state.
Problems
refresh_tool_registry), so a session viewed while another project's agent was running silently dropped its MCP tool calls. The same failure hit a session viewed in a second code-assistant instance that never launched (or never trusted) the producing servers.${VAR}substitution was all-or-nothing. One unset variable failed the wholemcp-servers.json(nothing registered) or dropped the whole.mcp.json.Changes
McpToolOutputis self-describing; recordedmcp__*executions now always deserialize viamcp_client::deserialize_mcp_output(tools::mcp::deserialize_tool_execution), never through a registry lookup. A session renders identically wherever it is viewed. Only native tools still require a registry entry.ConfigToolRegistrykeys registries by the included local-MCP directory (None= global-only). Cache and connection pool holdWeakreferences: a registry lives while a session instance or in-flight run holds it; connections live while some registry references them (dropping the lastArcterminates the server child). This removes thebuilding_keys/evict_unreferencedmachinery and answers the eviction question deterministically: a project's servers run while a session that uses them is open. Parallel sessions in different projects no longer invalidate each other or relaunch each other's servers.refresh_tool_registryassigns the fresh registry to the requesting session's instance only (and catches an instance up when the provider returns an already-cachedArcfor a second session in the same project).${VAR}degradation.McpServersConfig::substitute_env_valuesdrops only the servers whose variables cannot be resolved and returns them for the loaders to log; the other servers keep working. Applies to bothmcp-servers.jsonand.mcp.json.docs/project-scoped-mcp-servers.mdreworked from a design note into feature documentation (format, merge, trust, toggles, lifecycle, rendering guarantee, known limitations); cross-referenced frommcp-client-mode.mdandAGENTS.md.Notes
SessionServiceneeded no changes — provider signature andRegistryRequestare unchanged.session_mode_returns_session_id_while_running(PTY execute_command) is flaky under full-suite load onmainas well — unrelated to this branch.