diff --git a/CLAUDE.md b/CLAUDE.md
index fedbcd1..aac35ff 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -267,17 +267,24 @@ gate a commercially-licensed credential behind, so there was never an
 import-boundary reason to split it out), so read and write share one
 registry directly. The periodic Tantivy commit now commits every tenant
 index that's seen a write, not just the default one
-(`IndexRegistry::commit_all`). **One gap disclosed, not fixed, in this
-same change**: unlike `chwriter.Registry` (built from an
-active-tenants-only snapshot at startup) and unlike the read side (gated
-by `searchclient.TenantChecker`), this consumer's `resolve()` call has
-no active-tenant check — this process has no Postgres access to check
-against, so a still-valid-but-should-be-revoked ingest credential can
-cause an index directory to be created for a tenant that's no longer
-active. Narrow blast radius (an orphan, isolated, empty index — not
-cross-tenant leakage — and only reachable with a real signed
-credential), but real; see `search/src/registry.rs`'s doc comment on
-`resolve`. **The tenant-picker page is now built too**:
+(`IndexRegistry::commit_all`). **The active-tenant gap this same change
+originally disclosed is now closed too**: `search/src/tenants.rs`'s
+`ActiveTenantTracker` polls a new `GET /internal/active-tenants`
+endpoint on `enterprise-auth` (`search` has no Postgres access, unlike
+`chwriter.Registry`'s direct `rbacstore` query or the read side's
+`searchclient.TenantChecker`, so this needed a network call — the same
+"network boundary, not import boundary" shape `ingest`'s
+`TenantResolver` already uses against the same service, authenticated
+with a RoleService credential the same way `alerting` authenticates to
+`api`) and `consumer.rs` refuses any tagged record whose tenant isn't in
+the polled allowlist. Off unless both `ENTERPRISE_AUTH_URL` and
+`ENTERPRISE_AUTH_SERVICE_TOKEN` are set (same "off unless configured"
+default as everything else optional in this codebase); when they are,
+startup blocks on the first fetch succeeding and later refresh failures
+keep serving the last-known-good set rather than clearing it. Verified
+with real HTTP round trips against a hand-rolled TCP test server in this
+environment, no live enterprise-auth needed. **The tenant-picker page is
+now built too**:
 `web/src/routes/select-tenant` calls `GET /auth/memberships`/
 `POST /auth/select-tenant` via `fetch(..., {credentials: 'include'})`
 (new `$lib/api.ts` functions), which needed a second CORS posture
diff --git a/docker-compose.yml b/docker-compose.yml
index 059af88..d626dc1 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -187,6 +187,16 @@ services:
       # -- found by actually checking `docker compose logs search` and
       # seeing nothing, same silent-logging gap the agent had in Phase 0.
       RUST_LOG: "info"
+      # ENTERPRISE_AUTH_URL/ENTERPRISE_AUTH_SERVICE_TOKEN are deliberately
+      # NOT set here -- same reason enterprise-auth's own service below
+      # doesn't wire alerting's API_SERVICE_TOKEN in by default: the
+      # token can't be known ahead of time (mint it with
+      # `enterprise-auth -mint-service-token search` after enterprise-auth
+      # is up), so this is a manual step, not a default. Unset means
+      # search/src/tenants.rs's ActiveTenantTracker never starts and
+      # write-routing has no active-tenant gate, same as every deployment
+      # before this tracker existed -- see search/README.md's "Per-tenant
+      # indices" section for how to turn it on for manual testing.
     volumes:
       - search-index-data:/var/lib/sentry-search
 
diff --git a/docs/architecture.md b/docs/architecture.md
index ade94f4..ac136ca 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -171,17 +171,21 @@ escape hatch is opaque to any compiler-injected filter.
   index. No "second binary" needed here, unlike ClickHouse — Tantivy has
   no grant system to gate a commercially-licensed credential behind, so
   `IndexRegistry` already lived directly in this AGPL-core binary, and
-  read/write just share it. One gap is disclosed rather than closed by
-  this change: unlike `chwriter.Registry` (an active-tenants-only
-  snapshot built at startup) and unlike the read side (gated by
-  `searchclient.TenantChecker`), this consumer's `resolve()` call has no
-  active-tenant check — the process has no Postgres access to check
-  against — so a still-valid-but-should-be-revoked ingest credential can
-  cause an index directory to be created for a tenant that's no longer
-  active. Narrow blast radius (an orphan, isolated, empty index, not
-  cross-tenant leakage, and only reachable with a real signed
-  credential), but real; see `search/src/registry.rs`'s doc comment on
-  `resolve`.
+  read/write just share it. The active-tenant gap this design left open
+  is now closed too: `search/src/tenants.rs`'s `ActiveTenantTracker`
+  polls a new `GET /internal/active-tenants` endpoint on
+  `enterprise-auth` — `search` has no Postgres access, so unlike
+  `chwriter.Registry`'s direct `rbacstore` query or the read side's
+  `searchclient.TenantChecker`, this needed a network call instead (the
+  same "network boundary, not import boundary" shape `ingest`'s
+  `TenantResolver` already uses against the same service, authenticated
+  with a RoleService credential like `alerting`↔`api`) — and
+  `consumer.rs` refuses any tagged record whose tenant isn't in the
+  polled allowlist. Off unless configured, fail-closed on the first
+  fetch, last-known-good on later refresh failures; see
+  `search/src/tenants.rs`'s doc comment for the full design and
+  `search/src/registry.rs`'s for how mechanism (index lifecycle) and
+  policy (the gate) responsibilities split.
 - `deploy/operator`'s `Tenant` CRD and `enterprise-api -provision-tenant`
   are now unified, deliberately lightweight: `-provision-tenant` stays
   the sole real actor (ClickHouse + `rbacstore`), and now also syncs its
diff --git a/docs/phase-4-runbook.md b/docs/phase-4-runbook.md
index f912a78..869423a 100644
--- a/docs/phase-4-runbook.md
+++ b/docs/phase-4-runbook.md
@@ -39,14 +39,18 @@ of what was already run and passed. Two genuine exceptions:
   OIDC: not yet tried against a real external IdP or a running
   `enterprise-auth` container.
 - Tantivy tenant isolation, both directions -- `search/src/registry.rs`'s
-  cross-tenant read isolation (§9) and, since this pass,
-  `search/src/consumer.rs`'s per-tenant write-routing (§14) -- verified
-  live, no disclaimer needed, because Tantivy is an embedded library with
-  no Docker/broker dependency: real indices, real documents, real
-  commits, run in this environment. The one thing about it that's still
-  unverified is not Tantivy itself but the upstream credential/header
-  plumbing feeding it (ingest's `TenantResolver`, `enterprise-auth`'s
-  `/internal/authorize-ingest`) against a real running stack.
+  cross-tenant read isolation (§9), `search/src/consumer.rs`'s per-tenant
+  write-routing (§14), and `search/src/tenants.rs`'s active-tenant gate
+  (§14) -- verified live, no disclaimer needed, because Tantivy is an
+  embedded library with no Docker/broker dependency, and the active-
+  tenant gate's only external dependency (`enterprise-auth`'s HTTP API)
+  was exercised against a real hand-rolled test server, not a live
+  container: real indices, real documents, real commits, real HTTP
+  requests, all run in this environment. The one thing about it that's
+  still unverified is not Tantivy itself but the upstream credential/
+  header plumbing feeding it (ingest's `TenantResolver`, `enterprise-auth`'s
+  `/internal/authorize-ingest` and `/internal/active-tenants`) against a
+  real running stack.
 - The tenant-picker frontend page (§12) -- the first frontend-only piece
   in this phase exercised in a real browser rather than only
   type-checked: `web/src/routes/select-tenant`'s cross-origin
@@ -746,21 +750,54 @@ cargo test --quiet
 # ingest/cmd/ingest's own guard test does on the Go side.
 ```
 
-**What's still not built, for either engine**: a live active-tenant
-recheck at write time. `chwriter.Registry`'s per-tenant writer map is a
-snapshot built once at `enterprise-ingest` startup from
-`rbacstore.ListProvisionedDataSources` (active tenants only) -- an
-unrecognized `tenant_id` is refused outright, but a tenant deprovisioned
-*after* startup keeps writing successfully until the next restart.
-`IndexRegistry.resolve()` has no allowlist at all on either the read or
-write side -- `search` has no Postgres access to check tenant status
-against -- so a still-valid-but-should-be-revoked ingest credential can
-cause an orphan Tantivy index directory to be created for a tenant
-that's no longer active. Narrow blast radius either way (isolated, not
-cross-tenant leakage, reachable only with a real signed credential), but
-real and disclosed, not silently accepted -- see
-`search/src/registry.rs`'s doc comment on `resolve` and
-`/docs/security/threat-model.md`'s "Read this first".
+**Tantivy's write path is now active-tenant-gated too.**
+`search/src/tenants.rs`'s `ActiveTenantTracker` polls a new
+`GET /internal/active-tenants` endpoint on `enterprise-auth` (Go side:
+`rbacstore.ListActiveTenantIDs` + `authhandler.handleActiveTenants`,
+RoleService-credentialed -- mint one with
+`enterprise-auth -mint-service-token search`, the same generic flag
+`alerting` already uses, just a different subject name) and
+`consumer.rs` refuses any tagged record whose tenant isn't in the polled
+set. Off unless `ENTERPRISE_AUTH_URL`/`ENTERPRISE_AUTH_SERVICE_TOKEN` are
+both set (`search/src/config.rs` rejects exactly one being set); startup
+blocks on the first fetch succeeding (fail-closed cold start), and a
+later refresh failure keeps serving the last-known-good set rather than
+clearing it. Genuinely verified in this environment, no live
+enterprise-auth needed:
+
+```sh
+cd search
+cargo test --quiet tenants::
+# start_fetches_and_serves_the_initial_list / start_sends_the_bearer_token
+# run against a real (hand-rolled, dependency-free) TCP server -- actual
+# reqwest request construction (the Authorization: Bearer header, the
+# /internal/active-tenants path) and actual JSON response parsing, not a
+# fake HTTP client. start_fails_closed_when_the_first_fetch_fails and
+# start_fails_closed_when_the_server_is_unreachable prove the cold-start
+# refusal: ActiveTenantTracker::start returns Err rather than falling
+# back to an empty (accept-nothing, silently-safe-looking-but-wrong-for-
+# operators) or permissive (accept-anything, the exact bug being closed)
+# default.
+go test ./internal/authhandler/... -run TestActiveTenants -v
+# GET /internal/active-tenants requires a RoleService credential -- a
+# valid human session (even for a real Owner) is rejected the same way
+# an invalid/missing token is, the regression test for this endpoint's
+# whole reason to distinguish token kinds.
+```
+
+**One asymmetry remains, disclosed rather than fixed**: `chwriter.
+Registry`'s per-tenant writer map is a snapshot built once at
+`enterprise-ingest` startup, never refreshed -- a tenant deprovisioned
+after startup keeps writing successfully to ClickHouse until the next
+restart, a real staleness window `ActiveTenantTracker`'s 60-second
+refresh doesn't have on the Tantivy side. Neither is a live per-write
+check (a database/HTTP round trip on every record would be a real
+throughput cost neither implementation accepts), so both have *some*
+staleness window by design -- the gap between the two windows is what's
+disclosed as inconsistent here, not a claim that either is fully live.
+Closing it would mean adding a periodic refresh to `chwriter.Registry`
+too; not done in this pass. See `/docs/security/threat-model.md`'s "Read
+this first".
 
 **Also not built**: Helm/`docker-compose.yml` do gate *whether*
 `enterprise-ingest` runs at all (`ingest.requireTenantCredential`, same
@@ -811,12 +848,14 @@ Full accounting: `/docs/security/threat-model.md`. Headline items:
   see §14). `search/src/consumer.rs` reads the same header and routes
   each record into its own tenant's Tantivy index -- genuinely verified
   in this environment, unlike the ClickHouse side, since Tantivy needs
-  no Docker to exercise real logic. Both engines share one remaining,
-  disclosed gap: neither write path rechecks tenant-active status live
-  (ClickHouse: a startup-time snapshot, stale until restart; Tantivy: no
-  allowlist at all, since `search` has no Postgres access) -- narrow
-  blast radius, not cross-tenant leakage, but real; see §14 and
-  `/docs/security/threat-model.md`'s "Read this first". A
+  no Docker to exercise real logic. Both engines now also gate writes on
+  an active-tenant check: ClickHouse's is a startup-time snapshot with
+  no refresh (stale until the next `enterprise-ingest` restart), while
+  Tantivy's `ActiveTenantTracker` polls `enterprise-auth` every 60
+  seconds, a tighter staleness bound the ClickHouse side wasn't updated
+  to match -- a real, disclosed asymmetry between the two, not a gap in
+  either alone; see §14 and `/docs/security/threat-model.md`'s "Read
+  this first". A
   newly-provisioned tenant's ClickHouse database and Tantivy index are
   both now real, isolated, and actually populated by write-routed
   traffic (the ClickHouse claim pending live confirmation, the Tantivy
diff --git a/docs/security/threat-model.md b/docs/security/threat-model.md
index db5f164..11b330a 100644
--- a/docs/security/threat-model.md
+++ b/docs/security/threat-model.md
@@ -9,21 +9,27 @@ for the full design rationale behind the controls described here.
 
 ## Read this first: the single most important open finding
 
-**Updated a fourth time.** This section originally read "log data
-queried through `POST /query` is not tenant-isolated at all," then
-"ClickHouse is isolated but Tantivy isn't," then "ingest tags records
-with a tenant identity but nothing routes the write," then "ClickHouse
-write-routing is built but Tantivy's isn't." Both storage engines are
-now isolated on both the read and write paths. What's left is narrower:
-**whether a given deployment actually runs the isolated binaries**
-(deployment-time, not code-level), and two disclosed write-side gaps,
-different in kind: ClickHouse's write registry is a startup-time
-snapshot of active tenants with no live recheck (a *deprovisioned*
-tenant can keep writing successfully until the next `enterprise-ingest`
-restart), while Tantivy's write path has no active-tenant allowlist at
-all — it opens an index for *any* syntactically-valid `tenant_id` a
-record carries, active, deprovisioned, or never real. See below for
-both, in full.
+**Updated a fifth time.** This section originally read "log data queried
+through `POST /query` is not tenant-isolated at all," then "ClickHouse
+is isolated but Tantivy isn't," then "ingest tags records with a tenant
+identity but nothing routes the write," then "ClickHouse write-routing
+is built but Tantivy's isn't," then "both are write-routed but neither
+rechecks tenant-active status live." Both storage engines are now
+isolated on both the read and write paths, **and both now gate writes
+on an active-tenant check** — ClickHouse's (`chwriter.Registry`) and
+Tantivy's (`search/src/tenants.rs`'s `ActiveTenantTracker`, new) differ
+in staleness bound, not in whether the gate exists at all: ClickHouse's
+is a startup-time snapshot with no refresh (a *deprovisioned* tenant can
+keep writing successfully until the next `enterprise-ingest` restart);
+Tantivy's polls `enterprise-auth` every 60 seconds and keeps serving the
+last-known-good set through a transient refresh failure, a materially
+tighter staleness window with no code changed on the ClickHouse side to
+match it — a real, disclosed asymmetry between the two, not a claim
+they're now identical. What's left is narrower still: **whether a given
+deployment actually runs the isolated binaries** (deployment-time, not
+code-level), and closing ClickHouse's staleness gap to match Tantivy's
+if that inconsistency matters for a given deployment. See below for
+both engines' write-routing, in full.
 
 **ClickHouse (the SQL path) is built.** `enterprise/internal/
 tenantprovision` (real `CREATE DATABASE`/`CREATE USER`/`GRANT`) and
@@ -117,25 +123,35 @@ library with no Docker dependency. No "second binary" was needed here,
 unlike ClickHouse: Tantivy has no grant system to gate a
 commercially-licensed credential behind, so `IndexRegistry` already
 lived directly in this AGPL-core `search` binary, and read/write simply
-share it.
+share it. This write path is now also active-tenant-gated:
+`search/src/tenants.rs`'s `ActiveTenantTracker` polls a new
+`GET /internal/active-tenants` endpoint on `enterprise-auth` every 60
+seconds (RoleService-credentialed, the same auth shape `alerting` uses
+against `api`) and `consumer.rs` refuses any tagged record whose tenant
+isn't in the polled allowlist — fail-closed on the first fetch (startup
+blocks until it succeeds), last-known-good on any later refresh failure.
+Off unless `ENTERPRISE_AUTH_URL`/`ENTERPRISE_AUTH_SERVICE_TOKEN` are
+both set. Genuinely verified here too: real HTTP round trips (the Bearer
+header actually sent, JSON parsing, both fail-closed paths) against a
+hand-rolled TCP test server, no live enterprise-auth needed since
+`reqwest` doesn't care that the other end is real.
 
 **Both engines share one open question** — deployment topology, covered
-above — and Tantivy specifically has one gap ClickHouse's design doesn't:
-`chwriter.Registry`'s map is built once at startup from an
-active-tenants-only query, so an unrecognized `tenant_id` is refused
-outright; `IndexRegistry.resolve()` (used for both read and write) has
-no equivalent allowlist at all, because `search` has no Postgres access
-to check tenant status against — a syntactically-valid `tenant_id` on a
-still-valid-but-should-be-revoked ingest credential can cause an orphan
-index directory to be created for a tenant that's no longer active.
-Narrow blast radius (isolated, empty except for that traffic, not
-cross-tenant leakage, and reachable only with a real signed credential),
-but real, and not closed by this change; see
-`search/src/registry.rs`'s doc comment on `resolve`. A newly-provisioned
-tenant's ClickHouse database and Tantivy index are both now real,
-isolated, and actually populated by write-routed agent traffic (the
-ClickHouse claim pending live confirmation, the Tantivy claim already
-verified).
+above — and now differ only in staleness bound, not in whether an
+active-tenant gate exists at all: `chwriter.Registry`'s map is built
+once at `enterprise-ingest` startup from an active-tenants-only query
+and never refreshed, so a tenant deprovisioned after startup keeps
+writing successfully until the next restart; `ActiveTenantTracker`
+refreshes every 60 seconds, a materially tighter window, with no
+corresponding change made to the ClickHouse side to match it. Neither
+is a live per-write check (that would mean a database round trip on
+every record, a real throughput cost neither implementation accepts),
+so both have *some* staleness window by design — the asymmetry between
+the two windows is the one thing disclosed as inconsistent, not fixed,
+here. A newly-provisioned tenant's ClickHouse database and Tantivy index
+are both now real, isolated, and actually populated by write-routed
+agent traffic (the ClickHouse claim pending live confirmation, the
+Tantivy claim already verified).
 
 ## System overview
 
@@ -494,7 +510,7 @@ terms:
 | Tantivy tenant_id resolution (`enterprise/internal/searchclient`) | **Enforced, verified live** — real gRPC wire-level test |
 | Ingest tenant *identity* (credential validation, tagging) | **Built and tested** — fail-closed `TenantResolver`, `tenant_id` Kafka header attached per record |
 | Ingest tenant *write-routing*, ClickHouse | **Built, not yet confirmed against a real ClickHouse** — `enterprise-ingest`/`chwriter.Registry` route each tagged batch to its tenant's own database, fail-closed on an untagged/unprovisioned tenant; Docker-free tests pass, live-database tests are skip-gated. Startup-time active-tenant snapshot, no live recheck — a deprovisioned tenant can keep writing until the next restart |
-| Ingest tenant *write-routing*, Tantivy | **Built and genuinely verified** — `search/src/consumer.rs` routes each record into its own tenant's index via `IndexRegistry`, same registry the (already-verified) read side uses; no Docker needed, real tests pass. No active-tenant allowlist at all on write (`search` has no Postgres access) — a syntactically-valid `tenant_id` on a still-valid credential can create an orphan index for a no-longer-active tenant; narrow, disclosed, not cross-tenant leakage |
+| Ingest tenant *write-routing*, Tantivy | **Built and genuinely verified** — `search/src/consumer.rs` routes each record into its own tenant's index via `IndexRegistry`, same registry the (already-verified) read side uses; no Docker needed, real tests pass. Active-tenant-gated too: `tenants::ActiveTenantTracker` polls `enterprise-auth` every 60s (off unless configured), refusing any tenant not in the polled allowlist — tighter staleness bound than ClickHouse's startup-only snapshot, an asymmetry disclosed above, not a gap on Tantivy's side specifically |
 | Deployment actually routing traffic to `enterprise-api` (Helm) | **Enforced** — `api`/`enterprise-api` are mutually exclusive, same flag as RBAC/audit/SSO |
 | Deployment actually routing traffic to `enterprise-api` (docker-compose) | **Enforced** — `api`/`enterprise-api` are mutually exclusive via `COMPOSE_PROFILES`, same flag choice as Helm's `enterprise.enabled`; verified via `docker compose config`, not an actual `docker compose up` in this environment |
 | Human SSO login — OIDC | **Built, verified with a real fake IdP** (not yet tried against a real external IdP) |
diff --git a/enterprise/README.md b/enterprise/README.md
index 12a447e..15da22a 100644
--- a/enterprise/README.md
+++ b/enterprise/README.md
@@ -284,7 +284,7 @@ this environment.
 ## Package layout
 
 ```
-cmd/enterprise-auth/   config loading, OIDC discovery at startup, health/authorize/features/authorize-ingest endpoints, -mint-service-token, -create-tenant, -grant-membership-*, -revoke-membership-*, -list-memberships-tenant, -transfer-owner-*, -create-ingest-credential-tenant, -list-ingest-credentials-tenant, -revoke-ingest-credential
+cmd/enterprise-auth/   config loading, OIDC discovery at startup, health/authorize/features/authorize-ingest/active-tenants endpoints, -mint-service-token, -create-tenant, -grant-membership-*, -revoke-membership-*, -list-memberships-tenant, -transfer-owner-*, -create-ingest-credential-tenant, -list-ingest-credentials-tenant, -revoke-ingest-credential
 cmd/enterprise-api/     multi-tenant-aware alternative to api/cmd/api -- see its own doc comment
 cmd/enterprise-ingest/   multi-tenant-aware alternative to ingest -mode=consumer -- see its own doc comment
 internal/tenant/        the ID type -- see its package doc comment before touching it
@@ -390,6 +390,21 @@ TOKEN=$(docker compose run --rm enterprise-auth -mint-service-token=alerting)
 # alerting: set API_SERVICE_TOKEN=$TOKEN and restart
 ```
 
+Same shape for `search`'s write-side active-tenant gate
+(`search/src/tenants.rs` -- see `/search/README.md`'s "Per-tenant
+indices" section):
+
+```sh
+docker compose up -d enterprise-auth
+SEARCH_TOKEN=$(docker compose run --rm enterprise-auth -mint-service-token=search)
+# search: set ENTERPRISE_AUTH_URL=http://enterprise-auth:8082 and
+# ENTERPRISE_AUTH_SERVICE_TOKEN=$SEARCH_TOKEN, then restart -- unlike
+# alerting/api above, this doesn't turn on request-level auth
+# enforcement anywhere; it only gates which tenant_ids search will
+# write-route into their own index, refusing anything not in
+# GET /internal/active-tenants' response.
+```
+
 ```sh
 docker build -f Dockerfile -t sentry-enterprise-auth .   # context is enterprise/, not the repo root
 ```
diff --git a/enterprise/cmd/enterprise-auth/main.go b/enterprise/cmd/enterprise-auth/main.go
index 6ac4798..e87cd6c 100644
--- a/enterprise/cmd/enterprise-auth/main.go
+++ b/enterprise/cmd/enterprise-auth/main.go
@@ -205,7 +205,7 @@ func main() {
 		OIDCEnabled: cfg.OIDC.IssuerURL != "",
 		SAMLEnabled: cfg.SAML.IDPMetadataURL != "",
 	}
-	authhandler.New(logger, sessionManager, features, rbac).RegisterRoutes(mux)
+	authhandler.New(logger, sessionManager, features, rbac, rbac).RegisterRoutes(mux)
 	loginhandler.New(logger, oidcProvider, samlProvider, sessionManager, rbac, cfg.PostLoginRedirectURL, cfg.SelectTenantRedirectURL).RegisterRoutes(mux)
 
 	// Credentialed, not plain, CORS: GET /auth/memberships and POST
diff --git a/enterprise/internal/authhandler/authhandler.go b/enterprise/internal/authhandler/authhandler.go
index ea2d7d0..57f54b3 100644
--- a/enterprise/internal/authhandler/authhandler.go
+++ b/enterprise/internal/authhandler/authhandler.go
@@ -13,6 +13,9 @@
 // api/authz) and a different credential type (an ingest bearer token
 // checked against rbacstore's ingest_credentials table, not a
 // session.Manager JWT) -- see ingestCredentialValidator's doc comment.
+//
+// GET /internal/active-tenants is a third sibling, for `search`
+// (core/AGPL, Rust) -- see tenantLister's doc comment.
 package authhandler
 
 import (
@@ -50,21 +53,36 @@ type ingestCredentialValidator interface {
 	ValidateIngestCredential(ctx context.Context, token string) (tenantID string, err error)
 }
 
+// tenantLister is the narrow interface GET /internal/active-tenants
+// needs -- *rbacstore.Store is the production implementation (the same
+// concrete type ingestCredentials above already wires in, just a
+// second narrow interface it happens to also satisfy). Backs
+// search/src/tenants.rs's ActiveTenantTracker: search is AGPL core with
+// no Postgres access and no enterprise/ import allowed, so its write-
+// routing needed a network boundary to learn which tenants are active,
+// the same shape ingest/internal/grpcserver.TenantResolver already uses
+// against this exact service (see that package's doc comment).
+type tenantLister interface {
+	ListActiveTenantIDs(ctx context.Context) ([]string, error)
+}
+
 type Handler struct {
 	logger            *slog.Logger
 	manager           *session.Manager
 	features          Features
 	ingestCredentials ingestCredentialValidator
+	tenants           tenantLister
 }
 
-func New(logger *slog.Logger, manager *session.Manager, features Features, ingestCredentials ingestCredentialValidator) *Handler {
-	return &Handler{logger: logger, manager: manager, features: features, ingestCredentials: ingestCredentials}
+func New(logger *slog.Logger, manager *session.Manager, features Features, ingestCredentials ingestCredentialValidator, tenants tenantLister) *Handler {
+	return &Handler{logger: logger, manager: manager, features: features, ingestCredentials: ingestCredentials, tenants: tenants}
 }
 
 func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("POST /internal/authorize", h.handleAuthorize)
 	mux.HandleFunc("GET /auth/features", h.handleFeatures)
 	mux.HandleFunc("POST /internal/authorize-ingest", h.handleAuthorizeIngest)
+	mux.HandleFunc("GET /internal/active-tenants", h.handleActiveTenants)
 }
 
 type featuresResponse struct {
@@ -146,6 +164,44 @@ func (h *Handler) handleAuthorizeIngest(w http.ResponseWriter, r *http.Request)
 	_ = json.NewEncoder(w).Encode(authorizeIngestResponse{TenantID: tenantID})
 }
 
+type activeTenantsResponse struct {
+	TenantIDs []string `json:"tenant_ids"`
+}
+
+// handleActiveTenants is search/src/tenants.rs's ActiveTenantTracker's
+// server side -- polled periodically, not per-write, to build a local
+// allowlist for its write-routing gate (see that module's doc comment).
+// Requires a RoleService Bearer credential (session.Manager-issued,
+// Role == "service"), not a human session -- server-to-server, the same
+// authentication shape /alerting presents to /api, minted via
+// `enterprise-auth -mint-service-token search` (the flag is already
+// generic over caller name; no change needed there for a new caller).
+// Deliberately does NOT accept the tenant-scoped credential a human
+// session or an ingest credential would carry: this endpoint answers
+// "which tenants exist," a question no single tenant's identity should
+// be able to ask on its own.
+func (h *Handler) handleActiveTenants(w http.ResponseWriter, r *http.Request) {
+	token := bearerToken(r.Header.Get("Authorization"))
+	if token == "" {
+		http.Error(w, "no credentials presented", http.StatusUnauthorized)
+		return
+	}
+	claims, err := h.manager.Validate(token)
+	if err != nil || claims.Role != "service" {
+		http.Error(w, "invalid or expired credentials", http.StatusUnauthorized)
+		return
+	}
+
+	ids, err := h.tenants.ListActiveTenantIDs(r.Context())
+	if err != nil {
+		h.logger.Error("listing active tenant ids", "error", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(activeTenantsResponse{TenantIDs: ids})
+}
+
 func bearerToken(header string) string {
 	const prefix = "Bearer "
 	if !strings.HasPrefix(header, prefix) {
diff --git a/enterprise/internal/authhandler/authhandler_test.go b/enterprise/internal/authhandler/authhandler_test.go
index 2503de3..146f92e 100644
--- a/enterprise/internal/authhandler/authhandler_test.go
+++ b/enterprise/internal/authhandler/authhandler_test.go
@@ -36,13 +36,27 @@ type fakeNotFoundError struct{}
 
 func (*fakeNotFoundError) Error() string { return "not found" }
 
+// fakeTenantLister is an in-memory stand-in for *rbacstore.Store's
+// ListActiveTenantIDs.
+type fakeTenantLister struct {
+	ids []string
+	err error
+}
+
+func (f *fakeTenantLister) ListActiveTenantIDs(_ context.Context) ([]string, error) {
+	if f.err != nil {
+		return nil, f.err
+	}
+	return f.ids, nil
+}
+
 func testHandler(t *testing.T) (*Handler, *session.Manager) {
 	t.Helper()
 	m, err := session.NewManager([]byte("this-is-a-32-byte-test-signing-key!"))
 	if err != nil {
 		t.Fatalf("session.NewManager: %v", err)
 	}
-	return New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{}, newFakeIngestCredentialValidator()), m
+	return New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{}, newFakeIngestCredentialValidator(), &fakeTenantLister{}), m
 }
 
 func doAuthorize(t *testing.T, h *Handler, mutate func(*http.Request)) *httptest.ResponseRecorder {
@@ -146,7 +160,7 @@ func TestFeaturesReflectsConfiguredMechanisms(t *testing.T) {
 	if err != nil {
 		t.Fatalf("session.NewManager: %v", err)
 	}
-	h := New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{OIDCEnabled: true, SAMLEnabled: false}, newFakeIngestCredentialValidator())
+	h := New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{OIDCEnabled: true, SAMLEnabled: false}, newFakeIngestCredentialValidator(), &fakeTenantLister{})
 
 	mux := http.NewServeMux()
 	h.RegisterRoutes(mux)
@@ -219,7 +233,7 @@ func TestAuthorizeIngestResolvesTenant(t *testing.T) {
 	}
 	validator := newFakeIngestCredentialValidator()
 	validator.tenantByToken["real-token"] = "acme"
-	h := New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{}, validator)
+	h := New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{}, validator, &fakeTenantLister{})
 
 	rec := doAuthorizeIngest(t, h, func(r *http.Request) {
 		r.Header.Set("Authorization", "Bearer real-token")
@@ -273,3 +287,96 @@ func TestAuthorizeIngestRejectsSessionToken(t *testing.T) {
 		t.Fatalf("status = %d, want 401 (a session token must not validate as an ingest credential)", rec.Code)
 	}
 }
+
+func doActiveTenants(t *testing.T, h *Handler, mutate func(*http.Request)) *httptest.ResponseRecorder {
+	t.Helper()
+	mux := http.NewServeMux()
+	h.RegisterRoutes(mux)
+	req := httptest.NewRequest(http.MethodGet, "/internal/active-tenants", nil)
+	if mutate != nil {
+		mutate(req)
+	}
+	rec := httptest.NewRecorder()
+	mux.ServeHTTP(rec, req)
+	return rec
+}
+
+func TestActiveTenantsViaServiceToken(t *testing.T) {
+	m, err := session.NewManager([]byte("this-is-a-32-byte-test-signing-key!"))
+	if err != nil {
+		t.Fatalf("session.NewManager: %v", err)
+	}
+	h := New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{}, newFakeIngestCredentialValidator(), &fakeTenantLister{ids: []string{"acme", "globex"}})
+	token, err := m.IssueServiceToken("search")
+	if err != nil {
+		t.Fatalf("IssueServiceToken: %v", err)
+	}
+
+	rec := doActiveTenants(t, h, func(r *http.Request) {
+		r.Header.Set("Authorization", "Bearer "+token)
+	})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, body = %s", rec.Code, rec.Body.String())
+	}
+	var body activeTenantsResponse
+	if err := json.NewDecoder(rec.Body).Decode(&body); err != nil {
+		t.Fatalf("decoding response: %v", err)
+	}
+	if len(body.TenantIDs) != 2 || body.TenantIDs[0] != "acme" || body.TenantIDs[1] != "globex" {
+		t.Fatalf("unexpected response: %+v", body)
+	}
+}
+
+func TestActiveTenantsNoCredentialsIsUnauthorized(t *testing.T) {
+	h, _ := testHandler(t)
+	rec := doActiveTenants(t, h, nil)
+	if rec.Code != http.StatusUnauthorized {
+		t.Fatalf("status = %d, want 401", rec.Code)
+	}
+}
+
+// TestActiveTenantsRejectsHumanSession is the regression test for this
+// endpoint's whole reason to distinguish token kinds: a human session
+// (even a real, validly-signed one) must not be able to list every
+// active tenant in the deployment -- only a RoleService credential can.
+func TestActiveTenantsRejectsHumanSession(t *testing.T) {
+	h, m := testHandler(t)
+	sessionToken, err := m.IssueUserSession("acme", "u1", "owner")
+	if err != nil {
+		t.Fatalf("IssueUserSession: %v", err)
+	}
+	rec := doActiveTenants(t, h, func(r *http.Request) {
+		r.Header.Set("Authorization", "Bearer "+sessionToken)
+	})
+	if rec.Code != http.StatusUnauthorized {
+		t.Fatalf("status = %d, want 401 (a human session must not satisfy the service-only active-tenants endpoint)", rec.Code)
+	}
+}
+
+func TestActiveTenantsInvalidTokenIsUnauthorized(t *testing.T) {
+	h, _ := testHandler(t)
+	rec := doActiveTenants(t, h, func(r *http.Request) {
+		r.Header.Set("Authorization", "Bearer not-a-real-token")
+	})
+	if rec.Code != http.StatusUnauthorized {
+		t.Fatalf("status = %d, want 401", rec.Code)
+	}
+}
+
+func TestActiveTenantsStoreErrorIsInternalError(t *testing.T) {
+	m, err := session.NewManager([]byte("this-is-a-32-byte-test-signing-key!"))
+	if err != nil {
+		t.Fatalf("session.NewManager: %v", err)
+	}
+	h := New(slog.New(slog.NewTextHandler(io.Discard, nil)), m, Features{}, newFakeIngestCredentialValidator(), &fakeTenantLister{err: errNotFound})
+	token, err := m.IssueServiceToken("search")
+	if err != nil {
+		t.Fatalf("IssueServiceToken: %v", err)
+	}
+	rec := doActiveTenants(t, h, func(r *http.Request) {
+		r.Header.Set("Authorization", "Bearer "+token)
+	})
+	if rec.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want 500", rec.Code)
+	}
+}
diff --git a/enterprise/internal/rbacstore/rbacstore.go b/enterprise/internal/rbacstore/rbacstore.go
index 3c96b99..03bad7c 100644
--- a/enterprise/internal/rbacstore/rbacstore.go
+++ b/enterprise/internal/rbacstore/rbacstore.go
@@ -198,6 +198,34 @@ func (s *Store) TenantIsActive(ctx context.Context, tenantID string) (bool, erro
 	return t.Status == "active", nil
 }
 
+// ListActiveTenantIDs backs enterprise-auth's GET /internal/active-tenants
+// -- search (AGPL core) polls this to learn which tenant_ids it may
+// safely write-route into their own Tantivy index, since it has no
+// Postgres access of its own (see search/src/tenants.rs's doc comment
+// for the full design). Deliberately narrower than
+// ListProvisionedDataSources (which also requires ClickHouse
+// credentials to be present, since chwriter.Registry needs those to
+// actually connect): search's write path needs nothing but the id.
+func (s *Store) ListActiveTenantIDs(ctx context.Context) ([]string, error) {
+	rows, err := s.pool.Query(ctx, `SELECT id FROM tenants WHERE status = 'active'`)
+	if err != nil {
+		return nil, fmt.Errorf("rbacstore: listing active tenant ids: %w", err)
+	}
+	defer rows.Close()
+	var ids []string
+	for rows.Next() {
+		var id string
+		if err := rows.Scan(&id); err != nil {
+			return nil, fmt.Errorf("rbacstore: scanning active tenant id: %w", err)
+		}
+		ids = append(ids, id)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("rbacstore: iterating active tenant ids: %w", err)
+	}
+	return ids, nil
+}
+
 // SetTenantStatus is the only way a tenant's status column changes --
 // every tenant-resolution path elsewhere must re-check this via
 // GetTenant, never cache/assume 'active', per
diff --git a/enterprise/internal/rbacstore/rbacstore_test.go b/enterprise/internal/rbacstore/rbacstore_test.go
index 622eea3..be27529 100644
--- a/enterprise/internal/rbacstore/rbacstore_test.go
+++ b/enterprise/internal/rbacstore/rbacstore_test.go
@@ -470,6 +470,41 @@ func createTestDashboard(t *testing.T, s *Store, tenantID, createdBy string) str
 	return id
 }
 
+func TestListActiveTenantIDsExcludesNonActive(t *testing.T) {
+	s := testStore(t)
+	ctx := context.Background()
+
+	active := "test-tenant-" + uniqueSuffix()
+	provisioning := "test-tenant-" + uniqueSuffix()
+
+	for _, id := range []string{active, provisioning} {
+		if _, err := s.CreateTenant(ctx, id, id); err != nil {
+			t.Fatalf("CreateTenant %s: %v", id, err)
+		}
+	}
+	if err := s.SetTenantStatus(ctx, active, "active"); err != nil {
+		t.Fatalf("SetTenantStatus active: %v", err)
+	}
+	// provisioning stays in 'provisioning' (CreateTenant's default).
+
+	ids, err := s.ListActiveTenantIDs(ctx)
+	if err != nil {
+		t.Fatalf("ListActiveTenantIDs: %v", err)
+	}
+	foundActive := false
+	for _, id := range ids {
+		if id == provisioning {
+			t.Fatalf("non-active tenant %q leaked into ListActiveTenantIDs", provisioning)
+		}
+		if id == active {
+			foundActive = true
+		}
+	}
+	if !foundActive {
+		t.Fatal("expected the active tenant to be in the list")
+	}
+}
+
 func TestSetDashboardPermissionThenGet(t *testing.T) {
 	s := testStore(t)
 	ctx := context.Background()
diff --git a/search/Cargo.lock b/search/Cargo.lock
index e932021..13e778e 100644
--- a/search/Cargo.lock
+++ b/search/Cargo.lock
@@ -308,6 +308,17 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "displaydoc"
+version = "0.2.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c6232dd377dcc64799954cbd3a9bb882e9cdc1308ccd87b1c098f1fb2eaf82a8"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 3.0.3",
+]
+
 [[package]]
 name = "downcast-rs"
 version = "1.2.1"
@@ -382,6 +393,15 @@ version = "0.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d9c4f5dac5e15c24eb999c26181a6ca40b39fe946cbe4c263c7209467bc83af2"
 
+[[package]]
+name = "form_urlencoded"
+version = "1.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb4cb245038516f5f85277875cdaa4f7d2c9a0fa0468de06ed190163b1581fcf"
+dependencies = [
+ "percent-encoding",
+]
+
 [[package]]
 name = "fs4"
 version = "0.8.4"
@@ -667,13 +687,16 @@ version = "0.1.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "96547c2556ec9d12fb1578c4eaf448b04993e7fb79cbaad930a656880a6bdfa0"
 dependencies = [
+ "base64",
  "bytes",
  "futures-channel",
  "futures-util",
  "http",
  "http-body",
  "hyper",
+ "ipnet",
  "libc",
+ "percent-encoding",
  "pin-project-lite",
  "socket2 0.6.5",
  "tokio",
@@ -681,6 +704,110 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "icu_collections"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fa68d21081c4a05d5a901a1c62add574c77048b6a1c67be3b50ce0b60d4ca513"
+dependencies = [
+ "displaydoc",
+ "potential_utf",
+ "utf8_iter",
+ "yoke",
+ "zerofrom",
+ "zerovec",
+]
+
+[[package]]
+name = "icu_locale_core"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d56e28588da92eee5c3201a6eff33fabdd49b62269c8938d4ff050ce4d900deb"
+dependencies = [
+ "displaydoc",
+ "litemap",
+ "tinystr",
+ "writeable",
+ "zerovec",
+]
+
+[[package]]
+name = "icu_normalizer"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "12f9cf5f235641ed274641dd81c3f28d870e276763d0797aeeab72317b1c646f"
+dependencies = [
+ "icu_collections",
+ "icu_normalizer_data",
+ "icu_properties",
+ "icu_provider",
+ "smallvec",
+ "zerovec",
+]
+
+[[package]]
+name = "icu_normalizer_data"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1563da1ed3e0b3bf3d74c9b85917ac9c56464d2f57242270c09c9e752f8021a0"
+
+[[package]]
+name = "icu_properties"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7e7ca276ad3145661a65914e6daf131ca5120cd3dcee8f8f3214b8875184a148"
+dependencies = [
+ "displaydoc",
+ "icu_collections",
+ "icu_locale_core",
+ "icu_properties_data",
+ "icu_provider",
+ "zerotrie",
+ "zerovec",
+]
+
+[[package]]
+name = "icu_properties_data"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e590f038c1464a96894fd6d10127e90a8be4509f56ff7ecef851b15cee0b7caa"
+
+[[package]]
+name = "icu_provider"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "92a7ed671a6aad807a8651a2e1782a6598fda9ce5185dd8158549e95a91c6428"
+dependencies = [
+ "displaydoc",
+ "icu_locale_core",
+ "writeable",
+ "yoke",
+ "zerofrom",
+ "zerotrie",
+ "zerovec",
+]
+
+[[package]]
+name = "idna"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b0875f23caa03898994f6ddc501886a45c7d3d62d04d2d90788d47be1b1e4de"
+dependencies = [
+ "idna_adapter",
+ "smallvec",
+ "utf8_iter",
+]
+
+[[package]]
+name = "idna_adapter"
+version = "1.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb68373c0d6620ef8105e855e7745e18b0d00d3bdb07fb532e434244cdb9a714"
+dependencies = [
+ "icu_normalizer",
+ "icu_properties",
+]
+
 [[package]]
 name = "indexmap"
 version = "1.9.3"
@@ -719,6 +846,12 @@ version = "4.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "14c00403deb17c3221a1fe4fb571b9ed0370b3dcd116553c77fa294a3d918699"
 
+[[package]]
+name = "ipnet"
+version = "2.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a756c3fac73139e83f14c2d742155dd2b78d3ee56597b419a0579b7bdd6dd78"
+
 [[package]]
 name = "itertools"
 version = "0.12.1"
@@ -800,6 +933,12 @@ version = "0.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "32a66949e030da00e8c7d4434b251670a91556f4144941d37452769c25d58a53"
 
+[[package]]
+name = "litemap"
+version = "0.8.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "47d9d19d1d6efa0109d2f65ff4c85cddd50bd572e5a00127ab10987290bcefae"
+
 [[package]]
 name = "lock_api"
 version = "0.4.14"
@@ -1080,6 +1219,15 @@ version = "0.3.33"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "19f132c84eca552bf34cab8ec81f1c1dcc229b811638f9d283dceabe58c5569e"
 
+[[package]]
+name = "potential_utf"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d83eb9bc6d8e5cf568e7a1101d60ee05e81ed50ea106026f3d18deeb046d7661"
+dependencies = [
+ "zerovec",
+]
+
 [[package]]
 name = "powerfmt"
 version = "0.2.0"
@@ -1279,6 +1427,38 @@ version = "0.8.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d6f6ff9a378485b298a5286656da665ba74413d36db0979633275d2e708145d4"
 
+[[package]]
+name = "reqwest"
+version = "0.12.28"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eddd3ca559203180a307f12d114c268abf583f59b03cb906fd0b3ff8646c1147"
+dependencies = [
+ "base64",
+ "bytes",
+ "futures-core",
+ "http",
+ "http-body",
+ "http-body-util",
+ "hyper",
+ "hyper-util",
+ "js-sys",
+ "log",
+ "percent-encoding",
+ "pin-project-lite",
+ "serde",
+ "serde_json",
+ "serde_urlencoded",
+ "sync_wrapper",
+ "tokio",
+ "tower 0.5.3",
+ "tower-http",
+ "tower-service",
+ "url",
+ "wasm-bindgen",
+ "wasm-bindgen-futures",
+ "web-sys",
+]
+
 [[package]]
 name = "rsasl"
 version = "2.3.1"
@@ -1377,6 +1557,12 @@ version = "1.0.23"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cf54715a573b99ac80df0bc206da022bcd442c974952c7b9720069370852e21f"
 
+[[package]]
+name = "ryu"
+version = "1.0.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9774ba4a74de5f7b1c1451ed6cd5285a32eddb5cccb8cc655a4e50009e06477f"
+
 [[package]]
 name = "scopeguard"
 version = "1.2.0"
@@ -1395,6 +1581,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "prost",
+ "reqwest",
  "rskafka",
  "serde",
  "serde_json",
@@ -1450,6 +1637,18 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "serde_urlencoded"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3491c14715ca2294c4d6a88f15e84739788c1d030eed8c110436aafdaa2f3fd"
+dependencies = [
+ "form_urlencoded",
+ "itoa",
+ "ryu",
+ "serde",
+]
+
 [[package]]
 name = "sha2"
 version = "0.10.9"
@@ -1589,6 +1788,20 @@ name = "sync_wrapper"
 version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0bf256ce5efdfa370213c1dabab5935a12e49f2c58d15e9eac2870d3b4f27263"
+dependencies = [
+ "futures-core",
+]
+
+[[package]]
+name = "synstructure"
+version = "0.13.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "728a70f3dbaf5bab7f0c4b1ac8d7ae5ea60a4b5549c8a5914361c99147a709d2"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
 
 [[package]]
 name = "tantivy"
@@ -1823,6 +2036,16 @@ dependencies = [
  "time-core",
 ]
 
+[[package]]
+name = "tinystr"
+version = "0.8.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b1e27c91459209c2986af3dcf603a5a74a4368754ce37414f59acc971167f643"
+dependencies = [
+ "displaydoc",
+ "zerovec",
+]
+
 [[package]]
 name = "tinyvec"
 version = "1.12.0"
@@ -1964,10 +2187,29 @@ dependencies = [
  "futures-util",
  "pin-project-lite",
  "sync_wrapper",
+ "tokio",
  "tower-layer",
  "tower-service",
 ]
 
+[[package]]
+name = "tower-http"
+version = "0.6.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4cfcf7e2740e6fc6d4d688b4ef00650406bb94adf4731e43c096c3a19fe40840"
+dependencies = [
+ "bitflags",
+ "bytes",
+ "futures-util",
+ "http",
+ "http-body",
+ "pin-project-lite",
+ "tower 0.5.3",
+ "tower-layer",
+ "tower-service",
+ "url",
+]
+
 [[package]]
 name = "tower-layer"
 version = "0.3.3"
@@ -2080,12 +2322,30 @@ version = "0.1.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7df058c713841ad818f1dc5d3fd88063241cc61f49f5fbea4b951e8cf5a8d71d"
 
+[[package]]
+name = "url"
+version = "2.5.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ff67a8a4397373c3ef660812acab3268222035010ab8680ec4215f38ba3d0eed"
+dependencies = [
+ "form_urlencoded",
+ "idna",
+ "percent-encoding",
+ "serde",
+]
+
 [[package]]
 name = "utf8-ranges"
 version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7fcfc827f90e53a02eaef5e535ee14266c1d569214c6aa70133a624d8a3164ba"
 
+[[package]]
+name = "utf8_iter"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b6c140620e7ffbb22c2dee59cafe6084a59b5ffc27a8859a5f0d494b5d52b6be"
+
 [[package]]
 name = "uuid"
 version = "1.24.0"
@@ -2138,6 +2398,16 @@ dependencies = [
  "wasm-bindgen-shared",
 ]
 
+[[package]]
+name = "wasm-bindgen-futures"
+version = "0.4.77"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6b7777d5cc23d0e91404e53ce2d5e8ec7acae3026b16233dba62cd3246457950"
+dependencies = [
+ "js-sys",
+ "wasm-bindgen",
+]
+
 [[package]]
 name = "wasm-bindgen-macro"
 version = "0.2.127"
@@ -2299,6 +2569,35 @@ version = "0.52.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "589f6da84c646204747d1270a2a5661ea66ed1cced2631d546fdfb155959f9ec"
 
+[[package]]
+name = "writeable"
+version = "0.6.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3ad82d2a33cdc9674dc7465672f271e096168fcdbe0f799d9e6db8c5892679dc"
+
+[[package]]
+name = "yoke"
+version = "0.8.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "709fe23a0424b6a435d82152b1bd3fdfb0833487d5fa90d05d42762a9891fef5"
+dependencies = [
+ "stable_deref_trait",
+ "yoke-derive",
+ "zerofrom",
+]
+
+[[package]]
+name = "yoke-derive"
+version = "0.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "de844c262c8848816172cef550288e7dc6c7b7814b4ee56b3e1553f275f1858e"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+ "synstructure",
+]
+
 [[package]]
 name = "zerocopy"
 version = "0.8.56"
@@ -2319,6 +2618,60 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "zerofrom"
+version = "0.1.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0ec05a11813ea801ff6d75110ad09cd0824ddba17dfe17128ea0d5f68e6c5272"
+dependencies = [
+ "zerofrom-derive",
+]
+
+[[package]]
+name = "zerofrom-derive"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "11532158c46691caf0f2593ea8358fed6bbf68a0315e80aae9bd41fbade684a1"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+ "synstructure",
+]
+
+[[package]]
+name = "zerotrie"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4ea269c3bd32f0a32c321907a2ae912ba6f4649bb0fc764a15627e99a7095a3f"
+dependencies = [
+ "displaydoc",
+ "yoke",
+ "zerofrom",
+]
+
+[[package]]
+name = "zerovec"
+version = "0.11.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "94b5c6b5976d66c1d703c4fd17d3f5e43c8cedaacf604961b171adc7130896d8"
+dependencies = [
+ "yoke",
+ "zerofrom",
+ "zerovec-derive",
+]
+
+[[package]]
+name = "zerovec-derive"
+version = "0.11.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "47402523226a02bfe5230160dc3ccc089aa6f6f19e7fcbb4e6f824bbb1b4aa62"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 3.0.3",
+]
+
 [[package]]
 name = "zmij"
 version = "1.0.23"
diff --git a/search/Cargo.toml b/search/Cargo.toml
index b9a711a..328df5b 100644
--- a/search/Cargo.toml
+++ b/search/Cargo.toml
@@ -20,6 +20,14 @@ rskafka = "0.6"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 
+# tenants.rs's only outbound HTTP call (GET enterprise-auth's
+# /internal/active-tenants) -- default-features = false drops TLS
+# support entirely, matching every other internal service-to-service
+# call in this repo (plain HTTP, network placement is the trust
+# boundary, not TLS -- see ingest/internal/grpcserver.
+# HTTPTenantResolver's identical posture against the same service).
+reqwest = { version = "0.12", default-features = false, features = ["json"] }
+
 anyhow = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
@@ -31,3 +39,8 @@ tonic-build = "0.12"
 # Already in the dependency graph transitively (tantivy/tonic-build both
 # pull it in); promoted to a direct dev-dependency for test use.
 tempfile = "3"
+# tenants.rs's tests spin up a tiny real TCP server to exercise reqwest
+# against, rather than pulling in a mocking crate -- needs tokio
+# features the main binary doesn't (edition 2021's resolver keeps these
+# out of the release build, test-only).
+tokio = { version = "1", features = ["net", "io-util"] }
diff --git a/search/README.md b/search/README.md
index a7630fa..afafd6a 100644
--- a/search/README.md
+++ b/search/README.md
@@ -62,19 +62,28 @@ binary" needed. The periodic Tantivy commit (`COMMIT_INTERVAL_MS`) now
 commits every tenant index that's actually seen a write, plus the
 default index, via `IndexRegistry::commit_all`, not just one index.
 
-**One residual gap, disclosed rather than fixed here**: unlike the read
-side (gated by `enterprise/internal/searchclient`'s `TenantChecker`,
-which refuses to search a tenant that isn't `active` in `rbacstore`) and
-unlike ClickHouse's write side (`chwriter.Registry`, built from an
-active-tenants-only snapshot at startup, so an unrecognized tenant has
-no writer at all), this consumer's `registry.resolve()` call has no
-active-tenant gate — this process has no Postgres access to check
-against, the same reason `IndexRegistry` couldn't do the mid-provisioning
-check itself before `TenantChecker` was added for the read side. A
-still-valid (not yet revoked) ingest credential for a tenant that's no
-longer active can cause an index directory to be created for it here.
-See `src/registry.rs`'s doc comment on `resolve` for the full writeup,
-including why closing it fully isn't scoped yet.
+**The active-tenant gap is closed too, the same way the read side closes
+it**: `src/tenants.rs`'s `ActiveTenantTracker` polls a new
+`GET /internal/active-tenants` endpoint on `enterprise-auth` (this
+process has no Postgres access, so unlike `enterprise/internal/
+searchclient`'s `TenantChecker` — a direct `rbacstore.TenantIsActive`
+call, since that code runs in `enterprise/` — this needed a network
+call instead), and `consumer.rs` refuses (logs and skips, never falls
+back to the default or another tenant's index) any tagged record whose
+`tenant_id` isn't in the polled allowlist. Off unless
+`ENTERPRISE_AUTH_URL`/`ENTERPRISE_AUTH_SERVICE_TOKEN` are both set (see
+Configuration below) — when they aren't, write-routing behaves exactly
+as it did before this tracker existed, trusting any syntactically-valid
+`tenant_id`. When they are, startup blocks on the first fetch succeeding
+(fail-closed cold start — see `tenants.rs`'s doc comment for why a
+partial/degraded startup isn't the safer choice, and for the
+last-known-good behavior periodic refresh failures fall back to).
+Verified with real HTTP round trips against a hand-rolled TCP test
+server (no mocking crate needed for one endpoint) — the Bearer token
+actually sent, the initial-fetch-fails-closed path, and an unreachable
+server also failing closed. See `src/registry.rs`'s doc comment on
+`resolve` for how the mechanism (index lifecycle) and policy (who gets
+gated) responsibilities are split.
 
 ## Offset tracking: why this isn't a Kafka consumer group
 
@@ -118,6 +127,8 @@ Environment variables (see `src/config.rs`):
 | `TENANTS_INDEX_PATH` | `/var/lib/sentry-search/tenants` | Per-tenant index directories live under here, one subdirectory per tenant_id (Phase 4) |
 | `OFFSETS_PATH` | `/var/lib/sentry-search/offsets.json` | Offset tracking file |
 | `COMMIT_INTERVAL_MS` | `2000` | How often buffered writes become searchable |
+| `ENTERPRISE_AUTH_URL` | (empty) | Enables `tenants::ActiveTenantTracker` -- empty means write-routing has no active-tenant gate, same as every deployment before Phase 4. Must be set together with `ENTERPRISE_AUTH_SERVICE_TOKEN` below, or `Config::load` fails |
+| `ENTERPRISE_AUTH_SERVICE_TOKEN` | (empty) | RoleService Bearer credential for `GET /internal/active-tenants`, minted via `enterprise-auth -mint-service-token search` |
 
 ## Building & testing
 
@@ -147,7 +158,11 @@ pure `tenant_id_from_headers` header-extraction helper it uses is
 factored out and unit-tested the same way `ingest/consumer`'s Go
 equivalent is, including a guard test
 (`test_tenant_id_header_key_matches_go`) against the header-key literal
-drifting from the Go side's.
+drifting from the Go side's. `tenants.rs`'s tests genuinely exercise
+`reqwest` against a real (if hand-rolled, dependency-free) TCP server —
+the actual `Authorization: Bearer` header construction, JSON response
+parsing, and both fail-closed paths (a rejected first fetch, an
+unreachable server), not a fake HTTP client substituted in.
 
 ```sh
 # from the repo root, not search/
diff --git a/search/src/config.rs b/search/src/config.rs
index 41de954..bd59360 100644
--- a/search/src/config.rs
+++ b/search/src/config.rs
@@ -13,14 +13,35 @@ pub struct Config {
     pub commit_interval: Duration,
     /// Phase 4: per-tenant index directories live under here, one
     /// subdirectory per tenant_id, opened on demand by
-    /// registry::IndexRegistry -- distinct from `index_path` above,
-    /// which stays the single shared index every ingest-written record
-    /// lands in regardless of tenant (see registry.rs's doc comment and
-    /// /docs/security/threat-model.md's ingest-tenancy caveat). Default
+    /// registry::IndexRegistry for both the read side (SearchRequest.
+    /// tenant_id) and the write side (consumer.rs, routing on each
+    /// record's tenant_id Kafka header) -- `index_path` above stays the
+    /// single shared index an untagged record, or any deployment that
+    /// never turned on ingest's TenantResolver, still lands in. Default
     /// matches the path convention deploy/operator's Tenant controller
     /// and enterprise/internal/rbacstore's seeded default data source
     /// already assume (`/var/lib/sentry-search/tenants/<id>`).
     pub tenants_index_path: PathBuf,
+    /// Base URL of enterprise-auth's HTTP API, e.g.
+    /// `http://enterprise-auth:8082` -- same env var name and "empty
+    /// means off" shape as ingest/internal/config's own
+    /// ENTERPRISE_AUTH_URL. When set (together with
+    /// ENTERPRISE_AUTH_SERVICE_TOKEN below), tenants::ActiveTenantTracker
+    /// gates consumer.rs's write-routing on a polled active-tenant
+    /// allowlist -- see that module's doc comment for why this needed a
+    /// network call instead of direct Postgres access. When unset,
+    /// write-routing behaves exactly as it did before that tracker
+    /// existed: any syntactically-valid tenant_id is trusted.
+    pub enterprise_auth_url: Option<String>,
+    /// RoleService Bearer credential this process presents to
+    /// enterprise-auth's GET /internal/active-tenants -- minted via the
+    /// existing `enterprise-auth -mint-service-token search` (the flag
+    /// is already generic over caller name, no backend change needed to
+    /// mint one for a new caller). Required together with
+    /// enterprise_auth_url above; Config::load fails if exactly one of
+    /// the two is set, rather than silently running with the tracker
+    /// half-configured.
+    pub enterprise_auth_service_token: Option<String>,
 }
 
 impl Config {
@@ -29,6 +50,14 @@ impl Config {
             .parse()
             .context("COMMIT_INTERVAL_MS must be a number")?;
 
+        let enterprise_auth_url = getenv_opt("ENTERPRISE_AUTH_URL");
+        let enterprise_auth_service_token = getenv_opt("ENTERPRISE_AUTH_SERVICE_TOKEN");
+        if enterprise_auth_url.is_some() != enterprise_auth_service_token.is_some() {
+            anyhow::bail!(
+                "ENTERPRISE_AUTH_URL and ENTERPRISE_AUTH_SERVICE_TOKEN must be set together, or neither -- got exactly one"
+            );
+        }
+
         Ok(Self {
             // Rust's SocketAddr parser needs a full address, unlike Go's
             // net package (ingest/api's ":PORT" convention won't parse
@@ -49,6 +78,8 @@ impl Config {
                 "TENANTS_INDEX_PATH",
                 "/var/lib/sentry-search/tenants",
             )),
+            enterprise_auth_url,
+            enterprise_auth_service_token,
         })
     }
 }
@@ -56,3 +87,7 @@ impl Config {
 fn getenv(key: &str, fallback: &str) -> String {
     std::env::var(key).unwrap_or_else(|_| fallback.to_string())
 }
+
+fn getenv_opt(key: &str) -> Option<String> {
+    std::env::var(key).ok().filter(|v| !v.is_empty())
+}
diff --git a/search/src/consumer.rs b/search/src/consumer.rs
index 174b2c8..29fe122 100644
--- a/search/src/consumer.rs
+++ b/search/src/consumer.rs
@@ -10,6 +10,7 @@ use crate::config::Config;
 use crate::logsv1;
 use crate::offsets::OffsetStore;
 use crate::registry::IndexRegistry;
+use crate::tenants::ActiveTenantTracker;
 
 /// Kafka message header a resolved tenant ID rides in, attached by
 /// ingest's gRPC front end. Mirrors `ingest/internal/grpcserver.
@@ -30,7 +31,12 @@ const TENANT_ID_HEADER_KEY: &str = "tenant_id";
 /// discovered dynamically, since it has to match what
 /// /transport/provision-topics.sh actually created anyway (documented
 /// cross-component contract, same as the topic name already is).
-pub async fn run(cfg: Arc<Config>, registry: Arc<IndexRegistry>, partition_count: i32) -> Result<()> {
+pub async fn run(
+    cfg: Arc<Config>,
+    registry: Arc<IndexRegistry>,
+    partition_count: i32,
+    active_tenants: Option<Arc<ActiveTenantTracker>>,
+) -> Result<()> {
     let client = ClientBuilder::new(cfg.redpanda_brokers.clone())
         .build()
         .await
@@ -65,9 +71,10 @@ pub async fn run(cfg: Arc<Config>, registry: Arc<IndexRegistry>, partition_count
         let client = Arc::clone(&client);
         let registry = Arc::clone(&registry);
         let offsets = Arc::clone(&offsets);
+        let active_tenants = active_tenants.clone();
         let topic = cfg.redpanda_topic.clone();
         handles.push(tokio::spawn(async move {
-            consume_partition(client, topic, partition, start_offset, registry, offsets).await
+            consume_partition(client, topic, partition, start_offset, registry, offsets, active_tenants).await
         }));
     }
 
@@ -100,6 +107,7 @@ async fn consume_partition(
     start_offset: i64,
     registry: Arc<IndexRegistry>,
     offsets: Arc<Mutex<OffsetStore>>,
+    active_tenants: Option<Arc<ActiveTenantTracker>>,
 ) -> Result<()> {
     let partition_client = client
         .partition_client(topic.clone(), partition, UnknownTopicHandling::Error)
@@ -143,17 +151,33 @@ async fn consume_partition(
             }
 
             let tenant_id = tenant_id_from_headers(&record_and_offset.record.headers);
+
+            // Fail-closed active-tenant gate (see tenants.rs's doc
+            // comment) -- only applies to tagged records and only when
+            // a tracker is actually configured, matching resolve()'s
+            // own "empty tenant_id always means the default index"
+            // rule and this codebase's "off unless configured" default
+            // everywhere else. This is the check registry.rs's `resolve`
+            // doc comment used to name as missing entirely.
+            if !tenant_id.is_empty() {
+                if let Some(tracker) = &active_tenants {
+                    if !tracker.is_active(&tenant_id).await {
+                        tracing::warn!(record_id = %rec.record_id, tenant_id, "skipping record: tenant is not active");
+                        continue;
+                    }
+                }
+            }
+
             let index = match registry.resolve(&tenant_id).await {
                 Ok(index) => index,
                 Err(e) => {
                     // Shouldn't normally happen -- ingest/grpcserver only
                     // ever attaches a tenant_id it validated against a
-                    // real credential -- but an unsafe/malformed
-                    // tenant_id is a hard skip, never a silent fall-back
-                    // to the default or any other tenant's index. See
-                    // registry.rs's doc comment for the one residual gap
-                    // this consumer doesn't close (no active-tenant
-                    // check, since this process has no Postgres access).
+                    // real credential, and the active-tenant gate above
+                    // already refused anything not currently active when
+                    // configured -- but an unsafe/malformed tenant_id is
+                    // a hard skip regardless, never a silent fall-back to
+                    // the default or any other tenant's index.
                     tracing::error!(error = %e, record_id = %rec.record_id, tenant_id, "skipping record: failed to resolve tenant index");
                     continue;
                 }
diff --git a/search/src/main.rs b/search/src/main.rs
index a3ea7c9..42b6fbb 100644
--- a/search/src/main.rs
+++ b/search/src/main.rs
@@ -4,6 +4,7 @@ mod grpc;
 mod index;
 mod offsets;
 mod registry;
+mod tenants;
 
 pub mod logsv1 {
     tonic::include_proto!("sentry.logs.v1");
@@ -32,6 +33,24 @@ async fn main() -> Result<()> {
 
     let cfg = Arc::new(Config::load().context("loading config")?);
 
+    // Off unless ENTERPRISE_AUTH_URL/ENTERPRISE_AUTH_SERVICE_TOKEN are
+    // both set (Config::load already rejects exactly one being set).
+    // Blocks startup entirely on failure, same "fail hard, let the
+    // orchestrator restart" posture enterprise-ingest's main.go already
+    // uses when its own required startup fetch (rbacstore.
+    // ListProvisionedDataSources) fails -- see tenants.rs's doc comment
+    // for why a partial/degraded startup isn't the safer choice here.
+    let active_tenants = match (&cfg.enterprise_auth_url, &cfg.enterprise_auth_service_token) {
+        (Some(url), Some(token)) => {
+            tracing::info!(url, "active-tenant write-routing gate enabled");
+            Some(tenants::ActiveTenantTracker::start(url, token).await?)
+        }
+        _ => {
+            tracing::info!("ENTERPRISE_AUTH_URL not set -- write-routing has no active-tenant gate");
+            None
+        }
+    };
+
     let index = Arc::new(
         SearchIndex::open_or_create(&cfg.index_path).context("opening tantivy index")?,
     );
@@ -48,8 +67,9 @@ async fn main() -> Result<()> {
 
     let consumer_cfg = Arc::clone(&cfg);
     let consumer_registry = Arc::clone(&registry);
+    let consumer_active_tenants = active_tenants.clone();
     let consumer_handle = tokio::spawn(async move {
-        if let Err(e) = consumer::run(consumer_cfg, consumer_registry, partition_count).await {
+        if let Err(e) = consumer::run(consumer_cfg, consumer_registry, partition_count, consumer_active_tenants).await {
             tracing::error!(error = %e, "redpanda consumer exited with error");
         }
     });
diff --git a/search/src/registry.rs b/search/src/registry.rs
index dad3437..39cb9ea 100644
--- a/search/src/registry.rs
+++ b/search/src/registry.rs
@@ -22,29 +22,27 @@ use crate::index::SearchIndex;
 /// path every Phase 0-3 deployment, and every untagged record, still
 /// uses.
 ///
-/// **Known residual gap, disclosed rather than silently accepted**:
-/// unlike the read side (gated by `enterprise/internal/searchclient`'s
-/// `TenantChecker`, which refuses to even issue a search for a tenant
-/// that isn't `active` in `rbacstore`) and unlike ClickHouse's write
-/// side (`enterprise/internal/chwriter.Registry`, built once at startup
-/// from `rbacstore.ListProvisionedDataSources` -- `active` tenants
-/// only, so an unrecognized `tenant_id` has no writer and the whole
-/// batch is refused), this registry's `resolve` has no equivalent gate
-/// on the write path: `consumer.rs` calls it directly, with no Postgres
-/// access to check tenant status against, the same reason this
-/// module's doc comment used to give for the old read-side gap
-/// `TenantChecker` was built to close. A syntactically-valid `tenant_id`
-/// on an ingest credential that's still valid but should have been
-/// revoked (deprovisioning does not yet revoke `ingest_credentials`
-/// rows -- see `/CLAUDE.md`'s Phase 4 non-goals) can therefore cause an
-/// index directory to be silently created here for a tenant that isn't
-/// really active. The blast radius is narrow -- an orphan, isolated,
-/// empty-except-for-that-tenant's-own-traffic index directory, not
-/// cross-tenant data exposure, and only reachable with a real signed
-/// ingest credential, not by an arbitrary caller -- but it is real, not
-/// hypothetical. Closing it fully would mean giving `search` (AGPL
-/// core, no `enterprise/` import allowed) some way to learn which
-/// tenants are actually active; not designed yet.
+/// `resolve` itself still has no active-tenant gate of its own -- it
+/// will happily open-or-create an index for any syntactically-valid
+/// `tenant_id`, active or not. That's deliberate: this struct's job is
+/// managing index lifecycles, not policy, the same separation
+/// `clickhousewriter.Writer` (mechanism) vs. `chwriter.Registry`
+/// (policy: which tenants get a writer at all) draws on the ClickHouse
+/// side. The gate lives one layer up, at each caller:
+/// `enterprise/internal/searchclient.TenantChecker` for the read side
+/// (refuses to even issue a search for a tenant that isn't `active` in
+/// `rbacstore`, via a direct Postgres-backed query since that code runs
+/// in `enterprise/`), and `consumer.rs`'s `tenants::ActiveTenantTracker`
+/// for the write side (a polled allowlist fetched from a new
+/// `enterprise-auth` endpoint over HTTP -- `search` is AGPL core with no
+/// Postgres access and no `enterprise/` import allowed, so it needed a
+/// network boundary instead of an import one, the same shape
+/// `ingest/internal/grpcserver.TenantResolver` already uses against the
+/// same service). Both gates are optional at this layer -- `resolve`
+/// itself works identically whether or not either caller happens to
+/// gate it -- so a future caller that forgets to gate would silently
+/// reopen this exact class of gap; see `consumer.rs`'s call site for
+/// the write side's enforcement.
 pub struct IndexRegistry {
     default_index: Arc<SearchIndex>,
     tenants_root: PathBuf,
diff --git a/search/src/tenants.rs b/search/src/tenants.rs
new file mode 100644
index 0000000..224b023
--- /dev/null
+++ b/search/src/tenants.rs
@@ -0,0 +1,223 @@
+use anyhow::{Context, Result};
+use std::collections::HashSet;
+use std::sync::Arc;
+use std::time::Duration;
+use tokio::sync::RwLock;
+
+/// How often the tracker re-fetches the active-tenant list after its
+/// first successful fetch. Not configurable -- no deployment has needed
+/// to tune this yet, and a hardcoded value keeps Config's surface
+/// smaller; revisit if that changes.
+const REFRESH_INTERVAL: Duration = Duration::from_secs(60);
+
+/// Tracks which tenant_ids are currently `active` in enterprise-auth's
+/// `tenants` table, polled from a new `GET /internal/active-tenants`
+/// endpoint -- this closes the one gap registry.rs's `resolve` doc
+/// comment used to name: `search` (AGPL core) has no Postgres access,
+/// so unlike `chwriter.Registry` (an active-tenants-only snapshot built
+/// from `rbacstore.ListProvisionedDataSources` at `enterprise-ingest`
+/// startup) or the read side (gated by `enterprise/internal/
+/// searchclient.TenantChecker`, backed by `rbacstore.TenantIsActive`
+/// directly), `consumer.rs`'s write-routing had no allowlist at all --
+/// any syntactically-valid `tenant_id` on a still-valid-but-should-
+/// have-been-revoked ingest credential could get an index directory
+/// created for it.
+///
+/// Network boundary, not import boundary -- same shape
+/// `ingest/internal/grpcserver`'s `TenantResolver` already uses against
+/// this exact service, just Rust calling Go instead of Go calling Go,
+/// and authenticated the same way `/alerting` authenticates to `/api`:
+/// a long-lived RoleService Bearer credential
+/// (`enterprise-auth -mint-service-token search`), not a tenant-scoped
+/// one -- this tracker proves "I am the search service," never "I may
+/// act as tenant X."
+///
+/// Off unless configured: only constructed when both
+/// `ENTERPRISE_AUTH_URL` and `ENTERPRISE_AUTH_SERVICE_TOKEN` are set
+/// (see config.rs). When they aren't, `consumer.rs` holds `None` and
+/// skips the gate entirely -- every tagged write is routed exactly as
+/// it was before this tracker existed, the same "off unless configured"
+/// default every other optional integration point in this codebase
+/// uses.
+pub struct ActiveTenantTracker {
+    tenants: RwLock<HashSet<String>>,
+}
+
+impl ActiveTenantTracker {
+    /// Blocks until the first fetch succeeds. A cold start with
+    /// enterprise-auth unreachable must not silently accept every
+    /// tenant_id it sees -- that's the exact gap this tracker exists to
+    /// close -- so there is deliberately no empty-set-and-keep-going
+    /// fallback here; callers should refuse to start the write-routing
+    /// consumer at all if this returns an error. Once constructed,
+    /// periodic refreshes are best-effort: a transient failure logs and
+    /// keeps serving the last-known-good set rather than clearing it
+    /// (see the spawned task below) -- only the very first fetch is
+    /// fail-closed-to-refusing-startup.
+    pub async fn start(base_url: &str, service_token: &str) -> Result<Arc<Self>> {
+        let client = reqwest::Client::new();
+        let initial = fetch_active_tenants(&client, base_url, service_token)
+            .await
+            .context("fetching initial active-tenant list from enterprise-auth")?;
+        tracing::info!(count = initial.len(), "loaded initial active-tenant list");
+
+        let tracker = Arc::new(Self {
+            tenants: RwLock::new(initial),
+        });
+
+        let refresh_tracker = Arc::clone(&tracker);
+        let base_url = base_url.to_string();
+        let service_token = service_token.to_string();
+        tokio::spawn(async move {
+            let mut ticker = tokio::time::interval(REFRESH_INTERVAL);
+            ticker.tick().await; // fires immediately -- start() already fetched once, skip it
+            loop {
+                ticker.tick().await;
+                match fetch_active_tenants(&client, &base_url, &service_token).await {
+                    Ok(fresh) => {
+                        let count = fresh.len();
+                        *refresh_tracker.tenants.write().await = fresh;
+                        tracing::debug!(count, "refreshed active-tenant list");
+                    }
+                    Err(e) => {
+                        // No staleness ceiling: a prolonged enterprise-auth
+                        // outage means the allowlist just doesn't grow or
+                        // shrink until connectivity resumes, disclosed here
+                        // rather than degrading further (e.g. clearing the
+                        // set, which would stop every tenant's indexing on
+                        // one control-plane blip -- a worse blast radius
+                        // than staleness).
+                        tracing::error!(error = %e, "failed to refresh active-tenant list, keeping last-known-good set");
+                    }
+                }
+            }
+        });
+
+        Ok(tracker)
+    }
+
+    pub async fn is_active(&self, tenant_id: &str) -> bool {
+        self.tenants.read().await.contains(tenant_id)
+    }
+}
+
+#[derive(serde::Deserialize)]
+struct ActiveTenantsResponse {
+    tenant_ids: Vec<String>,
+}
+
+async fn fetch_active_tenants(
+    client: &reqwest::Client,
+    base_url: &str,
+    service_token: &str,
+) -> Result<HashSet<String>> {
+    let resp = client
+        .get(format!("{base_url}/internal/active-tenants"))
+        .bearer_auth(service_token)
+        .send()
+        .await
+        .context("sending request")?
+        .error_for_status()
+        .context("non-2xx response")?
+        .json::<ActiveTenantsResponse>()
+        .await
+        .context("parsing response body")?;
+    Ok(resp.tenant_ids.into_iter().collect())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tokio::io::{AsyncReadExt, AsyncWriteExt};
+    use tokio::net::TcpListener;
+
+    /// Minimal hand-rolled HTTP/1.1 server -- one dependency-free helper
+    /// rather than pulling in a mocking crate for the one endpoint this
+    /// module ever calls. Reads one request, hands it (as raw bytes) to
+    /// `respond`, writes back exactly what `respond` returns, then
+    /// closes -- enough to exercise real reqwest request construction
+    /// (the Bearer header, the URL path) and real response parsing, not
+    /// a fake client substituted in.
+    async fn spawn_fake_server(
+        respond: impl Fn(&str) -> String + Send + Sync + 'static,
+    ) -> String {
+        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        tokio::spawn(async move {
+            loop {
+                let (mut stream, _) = match listener.accept().await {
+                    Ok(v) => v,
+                    Err(_) => return,
+                };
+                let mut buf = vec![0u8; 8192];
+                let n = stream.read(&mut buf).await.unwrap_or(0);
+                let request = String::from_utf8_lossy(&buf[..n]).to_string();
+                let response = respond(&request);
+                let _ = stream.write_all(response.as_bytes()).await;
+            }
+        });
+        format!("http://{addr}")
+    }
+
+    fn json_response(status_line: &str, body: &str) -> String {
+        format!(
+            "{status_line}\r\nContent-Type: application/json\r\nContent-Length: {}\r\nConnection: close\r\n\r\n{body}",
+            body.len()
+        )
+    }
+
+    #[tokio::test]
+    async fn start_fetches_and_serves_the_initial_list() {
+        let base_url = spawn_fake_server(|_req| {
+            json_response("HTTP/1.1 200 OK", r#"{"tenant_ids":["acme","globex"]}"#)
+        })
+        .await;
+
+        let tracker = ActiveTenantTracker::start(&base_url, "test-token")
+            .await
+            .expect("start should succeed against a healthy fake server");
+
+        assert!(tracker.is_active("acme").await);
+        assert!(tracker.is_active("globex").await);
+        assert!(!tracker.is_active("initech").await);
+    }
+
+    #[tokio::test]
+    async fn start_sends_the_bearer_token() {
+        let base_url = spawn_fake_server(|req| {
+            if req.contains("authorization: Bearer secret-token") {
+                json_response("HTTP/1.1 200 OK", r#"{"tenant_ids":["acme"]}"#)
+            } else {
+                json_response("HTTP/1.1 401 Unauthorized", r#"{"error":"no credentials presented"}"#)
+            }
+        })
+        .await;
+
+        let tracker = ActiveTenantTracker::start(&base_url, "secret-token")
+            .await
+            .expect("start should succeed once the fake server sees the right token");
+        assert!(tracker.is_active("acme").await);
+    }
+
+    #[tokio::test]
+    async fn start_fails_closed_when_the_first_fetch_fails() {
+        let base_url = spawn_fake_server(|_req| {
+            json_response("HTTP/1.1 401 Unauthorized", r#"{"error":"invalid or expired credentials"}"#)
+        })
+        .await;
+
+        let result = ActiveTenantTracker::start(&base_url, "wrong-token").await;
+        assert!(
+            result.is_err(),
+            "expected start() to fail (not silently start with an empty/permissive allowlist) when the first fetch fails"
+        );
+    }
+
+    #[tokio::test]
+    async fn start_fails_closed_when_the_server_is_unreachable() {
+        // Port 1 is (almost certainly) not listening -- connection refused,
+        // not a slow timeout, so this test stays fast.
+        let result = ActiveTenantTracker::start("http://127.0.0.1:1", "any-token").await;
+        assert!(result.is_err());
+    }
+}