diff --git a/go/internal/auth/token_test.go b/go/internal/auth/token_test.go
index 056434e7..adaa9714 100644
--- a/go/internal/auth/token_test.go
+++ b/go/internal/auth/token_test.go
@@ -154,3 +154,97 @@ func TestCrossDoorTokenIsWrongKind(t *testing.T) {
 		}
 	})
 }
+
+// TestServiceTokenCrossDoorMatrix extends the cross-door rejection above to the
+// third subject kind (SubjectService, the first-party supervised compute tier).
+// The matrix proves the ONE shared resolver gates on `want` against all three
+// kinds, not a hard-coded pair: a service token authenticates only at
+// want=SubjectService, and is ErrWrongKind at each of the other two wants;
+// symmetrically, an account token and a Runner token are each ErrWrongKind at
+// want=SubjectService. Never a success, never ErrTokenNotFound — the hash
+// resolves in every case, so only the kind gate can be what fails.
+func TestServiceTokenCrossDoorMatrix(t *testing.T) {
+	ctx := context.Background()
+	st, admin, _ := openTestStore(t)
+
+	// A service-kind row put directly into the store: SubjectService issuance is
+	// a later slice, and this test needs only a live service-subject row to
+	// present at each door. hashToken agrees with IssueAccountToken on the key.
+	const serviceID = "llm-gateway"
+	serviceToken := "c2VydmljZS10b2tlbg" // base64url-shaped, never account-issued
+	if err := st.PutTokenHash(ctx, hashToken(serviceToken), store.Subject{Kind: store.SubjectService, ID: serviceID}); err != nil {
+		t.Fatalf("PutTokenHash(service): %v", err)
+	}
+
+	t.Run("service token wanted as service resolves", func(t *testing.T) {
+		subj, err := ResolveToken(ctx, st, serviceToken, store.SubjectService)
+		if err != nil {
+			t.Fatalf("a live service token must resolve at the service door: %v", err)
+		}
+		if subj.Kind != store.SubjectService {
+			t.Fatalf("resolved kind = %d, want SubjectService", subj.Kind)
+		}
+		if subj.ID != serviceID {
+			t.Fatalf("resolve must return the service the token was stored for: got %q, want %q", subj.ID, serviceID)
+		}
+	})
+
+	t.Run("service token wanted as account is WrongKind", func(t *testing.T) {
+		_, err := ResolveToken(ctx, st, serviceToken, store.SubjectAccount)
+		if !errors.Is(err, ErrWrongKind) {
+			t.Fatalf("a service token at the account door must be ErrWrongKind, got %v", err)
+		}
+	})
+
+	t.Run("service token wanted as runner is WrongKind", func(t *testing.T) {
+		_, err := ResolveToken(ctx, st, serviceToken, store.SubjectRunner)
+		if !errors.Is(err, ErrWrongKind) {
+			t.Fatalf("a service token at the Runner door must be ErrWrongKind, got %v", err)
+		}
+	})
+
+	t.Run("account token wanted as service is WrongKind", func(t *testing.T) {
+		accountToken, err := IssueAccountToken(ctx, st, admin)
+		if err != nil {
+			t.Fatalf("IssueAccountToken: %v", err)
+		}
+		_, err = ResolveToken(ctx, st, accountToken, store.SubjectService)
+		if !errors.Is(err, ErrWrongKind) {
+			t.Fatalf("an account token at the service door must be ErrWrongKind, got %v", err)
+		}
+	})
+
+	t.Run("runner token wanted as service is WrongKind", func(t *testing.T) {
+		runnerToken := "cnVubmVyLWZvci1zZXJ2aWNl" // base64url-shaped, never account-issued
+		if err := st.PutTokenHash(ctx, hashToken(runnerToken), store.Subject{Kind: store.SubjectRunner, ID: "some-runner"}); err != nil {
+			t.Fatalf("PutTokenHash(runner): %v", err)
+		}
+		_, err := ResolveToken(ctx, st, runnerToken, store.SubjectService)
+		if !errors.Is(err, ErrWrongKind) {
+			t.Fatalf("a Runner token at the service door must be ErrWrongKind, got %v", err)
+		}
+	})
+
+	t.Run("runner token wanted as runner resolves", func(t *testing.T) {
+		// The runner positive diagonal: the account diagonal is covered by
+		// TestIssueThenResolveRoundTripsToTheIssuedAccount and the service one
+		// above, so this closes the 3x3 — a resolvable token of each kind
+		// succeeds at its own door, pinning the `want` comparison against all
+		// three values rather than only the two rejection axes.
+		runnerToken := "cnVubmVyLWRpYWdvbmFs" // base64url-shaped, never account-issued
+		const runnerID = "diagonal-runner"
+		if err := st.PutTokenHash(ctx, hashToken(runnerToken), store.Subject{Kind: store.SubjectRunner, ID: runnerID}); err != nil {
+			t.Fatalf("PutTokenHash(runner): %v", err)
+		}
+		subj, err := ResolveToken(ctx, st, runnerToken, store.SubjectRunner)
+		if err != nil {
+			t.Fatalf("a live Runner token must resolve at the Runner door: %v", err)
+		}
+		if subj.Kind != store.SubjectRunner {
+			t.Fatalf("resolved kind = %d, want SubjectRunner", subj.Kind)
+		}
+		if subj.ID != runnerID {
+			t.Fatalf("resolve must return the runner the token was stored for: got %q, want %q", subj.ID, runnerID)
+		}
+	})
+}
diff --git a/go/internal/store/migrations/0001_init.sql b/go/internal/store/migrations/0001_init.sql
index 1c107cbb..30b540d8 100644
--- a/go/internal/store/migrations/0001_init.sql
+++ b/go/internal/store/migrations/0001_init.sql
@@ -369,12 +369,12 @@ CREATE TABLE channel_pins (
 
 -- ── Token hashes ────────────────────────────────────────────────────────────
 -- Subject-typed token store (design.md:1175-1183): the SHA-256 hash is the PK
--- (the plaintext token is returned once and never stored). subject_kind is
--- 0 account / 1 runner; subject_id spans both id spaces. revoked_at is set on
--- RevokeToken so ResolveTokenHash can distinguish revoked from never-issued.
+-- (the plaintext token is returned once and never stored). subject_kind is 0
+-- account / 1 runner / 2 service; subject_id spans those id spaces. revoked_at
+-- is set on RevokeToken so ResolveTokenHash tells revoked from never-issued.
 CREATE TABLE tokens (
     hash         BYTEA PRIMARY KEY,
-    subject_kind SMALLINT NOT NULL CHECK (subject_kind IN (0, 1)),
+    subject_kind SMALLINT NOT NULL CHECK (subject_kind IN (0, 1, 2)),
     subject_id   TEXT NOT NULL,
     created_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
     revoked_at   TIMESTAMPTZ
diff --git a/go/internal/store/tokens.go b/go/internal/store/tokens.go
index 031b9098..7cc791a8 100644
--- a/go/internal/store/tokens.go
+++ b/go/internal/store/tokens.go
@@ -17,7 +17,7 @@ func (s *Store) PutTokenHash(ctx context.Context, hash [32]byte, subj Subject) e
 	}
 	if err := s.q.InsertTokenHash(ctx, db.InsertTokenHashParams{
 		Hash:        hash[:],
-		SubjectKind: int16(subj.Kind), //nolint:gosec // G115: SubjectKind is a CHECK-constrained 0/1 enum (tokens.subject_kind), always within int16
+		SubjectKind: int16(subj.Kind), //nolint:gosec // G115: SubjectKind is a CHECK-constrained 0/1/2 enum (tokens.subject_kind), always within int16
 		SubjectID:   subj.ID,
 	}); err != nil {
 		if pgErrIs(err, pgUniqueViolation) {
diff --git a/go/internal/store/tokens_test.go b/go/internal/store/tokens_test.go
index 53c68ba7..ae86cdb9 100644
--- a/go/internal/store/tokens_test.go
+++ b/go/internal/store/tokens_test.go
@@ -38,6 +38,40 @@ func TestPutResolveRoundTripCarriesKind(t *testing.T) {
 	}
 }
 
+// A SubjectService row persists and its kind round-trips: the store-level proof
+// that the widened tokens.subject_kind CHECK admits 2.
+func TestPutResolveRoundTripCarriesServiceKind(t *testing.T) {
+	ctx := context.Background()
+	s := newTestStore(t)
+
+	hash := tokenHash("service-token")
+	want := Subject{Kind: SubjectService, ID: "llm-gateway"}
+	if err := s.PutTokenHash(ctx, hash, want); err != nil {
+		t.Fatalf("PutTokenHash(service): %v", err)
+	}
+	got, err := s.ResolveTokenHash(ctx, hash)
+	if err != nil {
+		t.Fatalf("ResolveTokenHash: %v", err)
+	}
+	if got != want {
+		t.Fatalf("resolved = %+v, want %+v (service kind must round-trip)", got, want)
+	}
+}
+
+// The widened CHECK is still a CLOSED set of exactly {0, 1, 2}: an out-of-range
+// kind is rejected by the database, not silently stored. The subject id is
+// non-empty and the hash fresh, so the CHECK violation is the sole possible
+// failure source — hence the assertion is only that it failed. A 23514 has no
+// typed store sentinel by design; it falls through to the bare wrap.
+func TestPutUnknownSubjectKindViolatesCheck(t *testing.T) {
+	ctx := context.Background()
+	s := newTestStore(t)
+
+	if err := s.PutTokenHash(ctx, tokenHash("kind-3"), Subject{Kind: SubjectKind(3), ID: "nope"}); err == nil {
+		t.Fatal("PutTokenHash with an out-of-range subject kind must fail the CHECK, got nil")
+	}
+}
+
 func TestTokenKindsDoNotCollideOnSameID(t *testing.T) {
 	ctx := context.Background()
 	s := newTestStore(t)
diff --git a/go/internal/store/types.go b/go/internal/store/types.go
index fd736808..3e3f9137 100644
--- a/go/internal/store/types.go
+++ b/go/internal/store/types.go
@@ -87,8 +87,8 @@ const (
 // subject share the token store but never collide — the prefix-separation the
 // auth layer (T4) depends on. A resolved token carries its kind, so a door can
 // reject a cross-kind token (a Runner token on CompassService/CommsService, an
-// account token on RunnerService). Sealed to exactly these two (design.md:
-// 1175-1183).
+// account token on RunnerService). Sealed to exactly these three
+// (docs/designs/server/compass-service-subject-principal.md).
 type SubjectKind int32
 
 const (
@@ -96,15 +96,21 @@ const (
 	SubjectAccount SubjectKind = 0
 	// SubjectRunner is a provisioned-Runner token subject.
 	SubjectRunner SubjectKind = 1
+	// SubjectService is a first-party supervised compute tier (LLM gateway,
+	// future MCP gateway) authenticating back to the Server. One class for all
+	// tiers; instances are distinguished by Subject.ID, isolated per-surface.
+	SubjectService SubjectKind = 2
 )
 
 // Subject is a token's authenticated principal: its kind plus the id of the
-// account or Runner it authenticates. ResolveTokenHash returns it with the kind
-// set so a cross-kind token is rejected at the door, not silently accepted.
+// account, Runner, or service it authenticates. ResolveTokenHash returns it
+// with the kind set so a cross-kind token is rejected at the door, not
+// silently accepted.
 type Subject struct {
 	Kind SubjectKind
-	// ID is the AccountID (SubjectAccount) or the Runner id (SubjectRunner), as
-	// a bare string because it spans two id spaces.
+	// ID is the AccountID (SubjectAccount), the Runner id (SubjectRunner), or a
+	// stable service name such as "llm-gateway" (SubjectService), as a bare
+	// string because it spans those id spaces.
 	ID string
 }