Decision-issue response to #3167 (default wiring for the serve-side MCP server — identity admission first). #3167 makes decision #1 (identity admission) a hard gate: "nothing else in this issue proceeds before it." This PR lands the decision record and that gate, with an end-to-end proof.

Two findings that shaped the response

1. The issue's premise is stale against main. decision: default wiring for the serve-side MCP server (@objectstack/mcp) in dev — identity admission first #3167 says MCPServerPlugin is "wired nowhere by default." Verified against main, MCP is already a default-on core capability: isMcpServerEnabled() defaults ON (packages/types/src/env.ts), packages/cli/src/commands/serve.ts pushes the mcp capability into os serve/os dev unless opted out, and the dispatcher route gate reads the same helper. The HTTP tool surface is also already fail-closed on identity — handleMcp returns 401 for a principal-less caller (no dev-admin fallback, no fall-open), and buildMcpBridge threads the caller ExecutionContext into callData(..., ec) on every verb (OAuth callers run at the ADR-0090 D10 ceiling∩user intersection).

2. What was genuinely missing is exactly the gate. The MCP tool-execution surface had no row in the ADR-0096 D4 authz conformance matrix — so "whose ExecutionContext runs a tools/call" lived only in ADR-0096 prose, not as a checked, red-CI-on-regression property.

What this lands

• docs/adr/0100-serve-side-mcp-default-wiring-identity-admission.md — the four decisions grounded in the verified state:
  • D1 identity admission — caller-principal (user) posture, fail-closed, registered as the gate.
  • D2 the dev/prod boundary is identity + the off-switch (OS_MCP_SERVER_ENABLED=false gates registration and route), not a code-level default flip (an opt-in-for-prod amendment is documented for ratification).
  • D3 HTTP-only default; stdio stays explicit opt-in (os dev owns the process; the stdio resource path carries the ADR-0096 E1 context-less read).
  • D4 bounded default exposure — fixed generic tool set over apiEnabled objects, sys_* fail-closed, actions only when ai.exposed; metadata-declared exposure is the future story.
• authz-conformance.matrix.ts — two rows: enforced mcp-tool-exec-identity (fail-closed 401 gate + buildMcpBridge threading + OAuth scope gate; now carrying a real dogfood proof and marked high-risk) and experimental mcp-stdio-resource-read classifying the ADR-0096 E1 context-less stdio resource read (opt-in, never networked) with its admission requirement — registered honestly, not patched with a discouraged { isSystem: true } literal (ADR-0096 D2 retires those; the systemContext(reason) constructor doesn't exist yet).
• authz-conformance.test.ts — three source probes pinning both halves of the HTTP admission and the stdio fall-open, so dropping the gate/threading or networking the stdio read trips CI (STALE / UNCLASSIFIED) instead of surfacing in an adversarial review later.
• showcase-mcp-identity.dogfood.test.ts — the end-to-end proof: boots the showcase with MCPServerPlugin registered and asserts on the real HTTP stack that anonymous POST /mcp→401 (not 404/501 — the surface is served and the service is registered; the denial is identity), an authenticated member clears the gate, and GET /mcp/skill→200 (served default-on while tool-exec is gated).

Scope / safety

Docs + a private test package (@objectstack/dogfood) only — no runtime behavior change, no changeset. The identity path, OAuth admission (#2698), and ai.exposed + capability action gate are pre-existing and unchanged; this PR registers and proves them. (@objectstack/mcp added to the dogfood dev deps; lockfile updated.)

Validation

Validated locally: pnpm --filter @objectstack/dogfood test for authz-conformance + showcase-mcp-identity → 10/10 pass (7 conformance ratchet + 3 dogfood-boot). The three new probes each match their source anchor exactly once and are a clean bijection with the two rows' covers; the proof:-exists and high-risk assertions pass.

Open for ratification

• D2: keep default-on-everywhere behind the off-switch (recommended), or adopt the NODE_ENV-aware opt-in-for-prod amendment.

Closes #3167 pending ratification of D2.

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