Migrate the MCP plan-path sandbox off the deprecated Roots feature (SEP-2577)

[erikdarlingdata]

The MCP specification deprecated the Roots feature as of spec version 2026-07-28 (SEP-2577), and the ModelContextProtocol C# SDK now marks its Roots types obsolete (MCP9005 warnings).

Roots is still the contract our stdio MCP server's file sandbox enforces: McpPlanPathPolicy (src/PlanViewer.Cli/ReplSurface/McpPlanPathPolicy.cs) resolves the client's advertised roots via Repl.Mcp's IMcpClientRoots and denies plan_open outside them (including symlink escapes and empty-roots clients). McpSmokeTests.cs is the regression coverage for that guard, and its client-side Roots setup is what trips MCP9005 — suppressed there with a comment referencing this issue as of the 2026-09 maintenance pass.

What eventually needs to happen:

• Decide the replacement mechanism for scoping which plan files the server may open once Roots is removed from the spec/SDK (whatever SEP-2577 points to, or an explicit allowlist argument at serve time).
• This also depends on Repl.Mcp: the IMcpClientRoots abstraction comes from the Repl packages, so their migration path matters.
• When migrated, remove the pragma warning disable MCP9005 from tests/PlanViewer.Core.Tests/McpSmokeTests.cs and re-point the smoke tests at the new mechanism.

No urgency until the SDK actually removes the feature, but a future ModelContextProtocol bump may force the decision. Found during the 2026-09 quarterly maintenance pass.

[erikdarlingdata]

Workability check (2026-09-12): the replacement now exists in the spec. MCP 2026-07-28 replaces server-to-client RPCs (roots/list included) with Multi-Round-Trip Requests - the ListRootsRequest payload survives, embedded in InputRequiredResult.input_requests, and deprecated Roots keeps working for at least twelve months from 2026-07-28.

Gates for the protocol-faithful migration here:

• ModelContextProtocol: we ship 2.2.0 (current stable; it marks Roots obsolete). Confirm which 2.x minor exposes a usable MRTR surface server-side.
• Repl.Mcp: our IMcpClientRoots abstraction comes from Repl.Mcp 0.11.0. The 0.12 line exists only as dev prereleases (0.12.0-dev.59 as of today) - no stable release with a Roots replacement yet. This is the practical trigger: when Repl.Mcp 0.12 ships stable, schedule this migration.

Alternative that needs nothing external: make serve-time scoping primary (explicit --root flag; McpPlanPathPolicy already falls back to CWD when a client has no roots) and drop the protocol negotiation. Downside: loses client-advertised scoping (e.g. Claude Code advertising its workspace) while every current client still speaks Roots. Given the 12-month runway, not worth doing preemptively.

Verdict: stays parked. Trigger to pick it up: Repl.Mcp 0.12 stable, or an SDK bump that actually removes the Roots types.