# 2026-05-21

## Broker MCP Gateway — Slice 2 (read tools + session cache foundation)

Branch: `feat-tools-broker-mcp-gateway-read-tools`.

Built the read-side of the gateway: lazy-minted, per-email cached world tokens drive `mark_fetch` / `mark_list` / `mark_versions` against the worldPool. The remaining 10 tools still hit `notImplementedHandler`. Plan reference: `/plans/broker-https-gateway.md` v5.

### Decisions made this session that weren't in the plan

- **Session cache is keyed by canonical verified email.** Plan v4 was subject-hash-keyed; investigated the existing Issuer.MintFiltered and found the broker already canonicalizes email and uses it as the ownership key across `/auth/callback`, `/me/install`, `/tokens` list/revoke, and audit logs. Two identity dimensions in one broker would have been confusing. Published v5 of the plan pinning email-keyed session cache as a Non-Negotiable, and updated the slice accordingly. Subject claim stays in id_token for log correlation but is not the cache key.
- **OQ#9 resolution: retry-on-401-after-mint with exponential backoff.** The kubelet → world Secret propagation lag means a freshly-minted token can land at the world before the projected-volume refresh has run; the world then 401s. Considered three options:
  - Sync wait via a world-side `/health/tokens?hash=...` endpoint → rejected (violates "no demarkus-server changes" Non-Negotiable).
  - Prewarm at MCP `initialize` → rejected (front-loads SIGHUPs for worlds the user may never touch).
  - **Retry-on-401 with exponential backoff (chosen)** — broker-only, narrow blast radius, matches `mutateSecret`'s optimistic-concurrency posture.

  Defaults: 6 attempts, 250ms → 8s exponential, ~16s of total waiting across attempts. Configurable via `cfg.Server.MCP.FirstMintMax*`. Distinguishes "fresh mint" 401s (retry with backoff — propagation race) from "cache hit" 401s (invalidate + immediately re-mint without sleeping — operator hand-revoked, sweeper retired, etc.).

- **Singleflight on `(canonicalEmail, worldName)` mints.** Concurrent first-call bursts for the same identity+world coalesce to one Issuer round-trip. Mitigates the "mint storm" risk on broker restart (every active plugin reconnects and re-mints simultaneously). `golang.org/x/sync/singleflight` added as a direct dep on `tools/`.
- **`MCPGatewayWith(version, dispatcher)` test seam.** Production `MCPGateway()` builds a real `*worldPool` (and registers it on `Server.mcpPool` so `CloseMCPGateway()` drains pooled QUIC connections during shutdown). Tests pass a `fakeDispatcher` to drive handlers without standing up a real QUIC server. Adds `Server.CloseMCPGateway()` which `main.go` calls after `mcpSrv.Shutdown` so in-flight tool calls finish before connections close.
- **`formatToolResult` duplicated (not hoisted) from `client/cmd/demarkus-mcp/formatResult`.** 20-line helper, stable. A `formatResultReference` copy in `mcp_tools_read_test.go` is asserted byte-equal to `formatToolResult` for several `fetch.Result` cases — that's the proxy-fidelity gate. If the local helper drifts, the parity test breaks before shipping; that's when hoisting to `client/mcpfmt` becomes the right call.
- **Tool URL shape rejects triple-slash (`mark:///foo`).** `parseToolURL` only accepts the canonical `mark://{worldName}/{path}` form. Considered fallback-parsing `mark:/team-a/foo` (single-slash, world-in-path) but rejected — silently re-interpreting the first path segment as the worldName would mask agent typos. Strict shape, clear error.
- **`tools/` module now `replace`s `client/`.** Pre-Flight 0 already hoisted `client/fetch` and `client/merge` to public, but the broker module didn't have the replace directive in place because Slice 1 didn't yet import client packages. Added in this slice; `go mod tidy` then promoted `client` and `x/sync` to direct deps.

### Bugs found during testing

- **Session-cache clock vs issuer clock.** `newIssuer(t, ...)` pins the issuer's clock to 2026-05-11 (so tokens expire 2026-05-12). The gateway's session cache uses `Server.clock` which defaults to `time.Now`. Result: every minted token's `expiresAt` was already in the past by the broker's clock, so the cache treated every entry as instantly expired. Fix in `newGatewayWithDispatcher`: pin `srv.clock` to the same date as `newIssuer`. Two tests (`TestHandleMarkFetchCacheHitReusesToken`, `TestDispatchReadCacheHit401InvalidatesAndReMints`) caught it; both pass after the clock alignment.
- **Singleflight test deadlock.** Initial design used `select { case ready <- struct{}{}: default: }` to signal the test the mint was in flight, but with select-default the very first sender would lose the signal if the main test wasn't yet reading. Main then blocked forever on `<-ready`. Fix: buffered `ready` channel (size 1). Pinned in the test comment.
- **LRU eviction test using `GetOrMint` as a probe was self-defeating.** Each "is this session still cached?" check via `GetOrMint` either re-minted (mutating LRU) or moved the session to the LRU front, perturbing the very state under inspection. Switched to a direct read of `c.sessions` under the mutex.

### Scope outcome vs plan estimate

- Production code: ~1100 LOC (plan estimate: ~400). Overshoot mostly in the retry loop + mintFuncFor closure + formatter duplication + worldPool dispatcher seam.
- Test code: ~1224 LOC (plan estimate: ~500). Many edge cases — parseToolURL, propagation-race retry exhaustion, cache-hit 401 invalidation, byte-for-byte parity. Worth it.
- `go test -race ./...` green across all modules. `pre-commit.sh` (fmt + vet + golangci-lint across protocol/server/client/tools) green.

### Next session — Slice 3 starting point

- `feat-tools-broker-mcp-gateway-write-tools` branch.
- `mark_publish` / `mark_append` / `mark_archive` handlers in `mcp_tools_write.go`. Reuse `sessionCache.GetOrMint` + `worldDispatcher` (extend interface with Publish/Append/Archive methods on `worldPool`).
- Conflict envelope follows local server's shape (`expected_version`, `on_conflict: "fail"` for Slice 3 — merge-candidate is Slice 6).
- World-side RBAC failure (`not-permitted`) surfaces as MCP tool error without retry (unlike `unauthorized` which triggers the propagation-race retry loop).