diff --git a/backend/sse.go b/backend/sse.go
index 41ae583..0b08570 100644
--- a/backend/sse.go
+++ b/backend/sse.go
@@ -151,6 +151,16 @@ func (b *SSEBackend) Start(ctx context.Context) error {
 	if resp.StatusCode >= 300 {
 		defer resp.Body.Close()
 		cancel()
+		// 405 on the GET stream is the signature of a streamable-http
+		// endpoint: that transport serves MCP on POST and the spec lets it
+		// refuse the optional GET stream outright. Without naming the fix
+		// the operator sees only the server's "use POST" text relayed
+		// through a 502, which reads like a gateway bug rather than a
+		// one-word connection setting.
+		if resp.StatusCode == http.StatusMethodNotAllowed {
+			return fmt.Errorf("sse backend %s: stream http 405: %s; endpoint speaks streamable-http, set transport: streamable-http",
+				b.name, readExcerpt(resp.Body))
+		}
 		return fmt.Errorf("sse backend %s: stream http %d: %s", b.name, resp.StatusCode, readExcerpt(resp.Body))
 	}
 	if mt := mediaType(resp.Header.Get("Content-Type")); mt != "text/event-stream" {
diff --git a/backend/sse_test.go b/backend/sse_test.go
index 3e0a3e5..d1848b0 100644
--- a/backend/sse_test.go
+++ b/backend/sse_test.go
@@ -263,6 +263,27 @@ func TestSSEWrongContentTypeRejected(t *testing.T) {
 	}
 }
 
+// A streamable-http endpoint refuses the SSE transport's GET stream with 405.
+// Linear does exactly this, and the bare relayed message ("Method not allowed.
+// Use POST for MCP requests.") reaches the operator as a 502 that names no
+// fix. The error must say which setting to change, as the content-type
+// mismatch above already does.
+func TestSSEMethodNotAllowedHintsStreamableHTTP(t *testing.T) {
+	t.Parallel()
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusMethodNotAllowed)
+		fmt.Fprint(w, `{"jsonrpc":"2.0","error":{"code":-32601,"message":"Method not allowed. Use POST for MCP requests."},"id":null}`)
+	}))
+	defer srv.Close()
+
+	b := NewSSE("test", config.Backend{Transport: "sse", URL: srv.URL}, nil)
+	err := b.Start(context.Background())
+	if err == nil || !strings.Contains(err.Error(), "streamable-http") {
+		t.Fatalf("expected 405 to hint at streamable-http, got: %v", err)
+	}
+}
+
 func TestSSEUnauthorizedSurfacesChallenge(t *testing.T) {
 	t.Parallel()
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
diff --git a/gateway/revision.go b/gateway/revision.go
index f7ed5ea..483ae1a 100644
--- a/gateway/revision.go
+++ b/gateway/revision.go
@@ -27,31 +27,42 @@ func classify(r *http.Request, env *jsonrpc.Message) (mcp.Revision, *jsonrpc.Mes
 	meta := mcp.ParseMeta(env.Params)
 	header := r.Header.Get(mcp.HeaderProtocolVersion)
 
-	// Neither the header nor _meta claims a revision: a Legacy client, which
-	// never sent either. Note a Legacy client cannot reach the Modern arm by
-	// accident, so this branch stays the common path.
-	if !meta.HasModernSignal() && header == "" {
+	// MCP-Protocol-Version is not a Modern invention. Since 2025-06-18 the
+	// spec requires a Legacy client to echo the version it negotiated at
+	// `initialize` on every subsequent HTTP request, so the header alone
+	// says nothing about the era — only _meta does. Reading the header as a
+	// Modern signal rejected the very clients that follow the spec most
+	// closely: claude-code negotiates 2025-11-25, echoes it as required, and
+	// its next message (notifications/initialized) came back
+	// "-32022 unsupported protocol version" naming 2025-11-25 as both the
+	// requested and the only supported version.
+	//
+	// So the era is decided by _meta, and the header is only cross-checked
+	// against it.
+	if !meta.HasModernSignal() {
+		if header != "" && header != mcp.VersionLegacy && header != mcp.VersionModern {
+			// A Legacy-shaped request echoing a version this gateway does
+			// not speak. Still worth refusing, but for the right reason.
+			return mcp.RevisionLegacy, unsupportedVersion(env.ID, header)
+		}
 		return mcp.RevisionLegacy, nil
 	}
 
-	// One of the two claims a revision. From here a disagreement is an error
-	// rather than a silent downgrade: a client that mirrors its version into
-	// a header and then contradicts it in the body is exactly the split-brain
-	// the spec's header validation exists to catch.
-	if header != "" && meta.ProtocolVersion != "" && header != meta.ProtocolVersion {
+	// _meta claims a revision. From here a disagreement with the mirrored
+	// header is an error rather than a silent downgrade: a client that
+	// mirrors its version into a header and then contradicts it in the body
+	// is exactly the split-brain the spec's header validation exists to
+	// catch.
+	if header != "" && header != meta.ProtocolVersion {
 		return mcp.RevisionLegacy, headerMismatch(env.ID,
 			fmt.Sprintf("%s header %q does not match _meta protocolVersion %q",
 				mcp.HeaderProtocolVersion, header, meta.ProtocolVersion))
 	}
 
-	claimed := meta.ProtocolVersion
-	if claimed == "" {
-		claimed = header
-	}
-	if claimed != mcp.VersionModern {
+	if meta.ProtocolVersion != mcp.VersionModern {
 		// A version the gateway does not serve. Naming what it does serve is
 		// what lets a dual-era client retry instead of failing outright.
-		return mcp.RevisionLegacy, unsupportedVersion(env.ID, claimed)
+		return mcp.RevisionLegacy, unsupportedVersion(env.ID, meta.ProtocolVersion)
 	}
 
 	if msg := validateModernHeaders(r, env); msg != nil {
diff --git a/gateway/revision_test.go b/gateway/revision_test.go
index a545060..957a94c 100644
--- a/gateway/revision_test.go
+++ b/gateway/revision_test.go
@@ -162,6 +162,55 @@ func TestClassifyUnknownVersionNamesSupported(t *testing.T) {
 	}
 }
 
+// A Legacy client must echo the negotiated version on every request after
+// `initialize` (spec, since 2025-06-18). That header alone is not a Modern
+// signal, and treating it as one rejected the clients that follow the spec
+// most closely: claude-code negotiates 2025-11-25, echoes it as required, and
+// its very next message came back "-32022 unsupported protocol version"
+// naming 2025-11-25 as both requested and the only supported version.
+func TestClassifyLegacyEchoedVersionHeaderIsNotModern(t *testing.T) {
+	// The exact message claude-code sends after a successful handshake.
+	env, _, r := req(t, `{"method":"notifications/initialized","jsonrpc":"2.0"}`,
+		mcp.HeaderProtocolVersion, mcp.VersionLegacy,
+		sessionHeader, "cb6be304f0e1ba50511dddd353de1c5f")
+	rev, errMsg := classify(r, env)
+	if rev != mcp.RevisionLegacy || errMsg != nil {
+		t.Fatalf("got rev=%v err=%v, want legacy/nil", rev, errMsg)
+	}
+
+	// Same for an ordinary request, which additionally carries no Mcp-Method
+	// header — the Modern arm would have demanded one.
+	env, _, r = req(t, `{"jsonrpc":"2.0","id":1,"method":"tools/list"}`,
+		mcp.HeaderProtocolVersion, mcp.VersionLegacy)
+	if rev, errMsg := classify(r, env); rev != mcp.RevisionLegacy || errMsg != nil {
+		t.Fatalf("got rev=%v err=%v, want legacy/nil", rev, errMsg)
+	}
+}
+
+// The header still cannot smuggle in a revision nobody serves. A Legacy-shaped
+// request echoing an unknown version is refused, but for the honest reason.
+func TestClassifyLegacyEchoedUnknownVersionRefused(t *testing.T) {
+	env, _, r := req(t, `{"jsonrpc":"2.0","id":1,"method":"tools/list"}`,
+		mcp.HeaderProtocolVersion, "2099-01-01")
+	_, errMsg := classify(r, env)
+	if errMsg == nil || errMsg.Error.Code != mcp.CodeUnsupportedProtocolVersion {
+		t.Fatalf("got %v, want -32022", errMsg)
+	}
+}
+
+// Only _meta puts a request on the Modern path. A header-only Modern claim is
+// a Legacy client echoing a version it negotiated, not a stateless request:
+// it carries none of the per-request metadata the Modern arm reads.
+func TestClassifyModernRequiresMetaNotJustHeader(t *testing.T) {
+	env, _, r := req(t, `{"jsonrpc":"2.0","id":1,"method":"tools/list"}`,
+		mcp.HeaderProtocolVersion, mcp.VersionModern,
+		mcp.HeaderMethod, "tools/list")
+	rev, errMsg := classify(r, env)
+	if rev != mcp.RevisionLegacy || errMsg != nil {
+		t.Fatalf("got rev=%v err=%v, want legacy/nil", rev, errMsg)
+	}
+}
+
 func TestRefuseModernNamesBlockers(t *testing.T) {
 	// The refusal must be actionable: an operator reading the response needs
 	// to know which controls are keeping the gateway on the Legacy path.