diff --git a/cmd/symbols/upload.go b/cmd/symbols/upload.go
index 980b95e3..f5e8fd24 100644
--- a/cmd/symbols/upload.go
+++ b/cmd/symbols/upload.go
@@ -52,9 +52,19 @@ const (
 	// no sources.
 	sourcePathFlag = "source-path"
 
-	defaultPath       = "."
+	defaultPath = "."
+
+	// defaultBackendUrl is the observability API for LaunchDarkly production, which
+	// is what defaultBackendURLFor derives for the default base URI.
 	defaultBackendUrl = "https://pri.observability.app.launchdarkly.com"
 
+	// Every LaunchDarkly instance publishes the observability API under its own
+	// host, named for the instance the app is served from: staging's app at
+	// ld-stg.launchdarkly.com has its API at pri.observability.ld-stg.launchdarkly.com.
+	// "pri" is the authenticated graph, which is the one that hands out upload URLs.
+	launchDarklyDomain     = "launchdarkly.com"
+	observabilityAPIPrefix = "pri.observability."
+
 	// reactNativeSymbolsIDPrefix is the storage "version" segment for symbols-id
 	// addressed JS maps (Symbols Id Lane). Keys become _sym/js/id/<symbolsID>/<file>,
 	// matching what the symbolication backend derives from the reported symbols id.
@@ -218,7 +228,7 @@ func runE(client resources.Client) func(cmd *cobra.Command, args []string) error
 		skipExisting := !viper.GetBool(noSkipExistingFlag)
 
 		if backendUrl == "" {
-			backendUrl = defaultBackendUrl
+			backendUrl = defaultBackendURLFor(viper.GetString(cliflags.BaseURIFlag))
 		}
 
 		// Apple dSYMs take a dedicated path: they are compiled to per-arch .dsymmap
@@ -518,6 +528,26 @@ func readSymbolsIDFile(filePath string) string {
 	return strings.TrimSpace(string(content))
 }
 
+// defaultBackendURLFor derives the observability API endpoint from the LaunchDarkly
+// base URI, so aiming the CLI at another instance takes the one flag that names the
+// instance rather than two flags that have to agree.
+//
+// Only LaunchDarkly's own hosts are derived from. A base URI pointing at a local or
+// proxied stack says nothing about where its observability API listens, so those keep
+// the production default and --backend-url stays the way to say otherwise.
+func defaultBackendURLFor(baseURI string) string {
+	parsed, err := url.Parse(strings.TrimSpace(baseURI))
+	if err != nil {
+		return defaultBackendUrl
+	}
+
+	host := parsed.Hostname()
+	if host != launchDarklyDomain && !strings.HasSuffix(host, "."+launchDarklyDomain) {
+		return defaultBackendUrl
+	}
+	return "https://" + observabilityAPIPrefix + host
+}
+
 // getSymbolUploadUrls returns one upload URL per requested key, in order.
 //
 // With skipExisting, a key whose bytes the backend already stores comes back empty
@@ -712,7 +742,7 @@ func initFlags(cmd *cobra.Command) {
 	cmd.Flags().String(basePathFlag, "", "An optional base path for the uploaded symbol files")
 	_ = viper.BindPFlag(basePathFlag, cmd.Flags().Lookup(basePathFlag))
 
-	cmd.Flags().String(backendUrlFlag, defaultBackendUrl, "An optional backend url for self-hosted deployments")
+	cmd.Flags().String(backendUrlFlag, "", fmt.Sprintf("An optional backend url for self-hosted deployments. Defaults to the observability API of whichever instance --%s names (%s for the default)", cliflags.BaseURIFlag, defaultBackendUrl))
 	_ = viper.BindPFlag(backendUrlFlag, cmd.Flags().Lookup(backendUrlFlag))
 
 	cmd.Flags().Bool(includeSourcesFlag, false, fmt.Sprintf("Also upload your source files so the errors page can show source context around native frames (%s and %s). Your source is stored in LaunchDarkly", typeAppleDSYM, typeAndroid))
diff --git a/cmd/symbols/upload_test.go b/cmd/symbols/upload_test.go
index 04cc87be..6b0cf468 100644
--- a/cmd/symbols/upload_test.go
+++ b/cmd/symbols/upload_test.go
@@ -34,6 +34,28 @@ func TestNewUploadCmd(t *testing.T) {
 	assert.Equal(t, []string{"true"}, cmd.Flags().Lookup("project").Annotations["required"])
 }
 
+// Naming the instance once, with --base-uri, is what should aim an upload at it: the
+// observability API of every LaunchDarkly instance is named for that instance, so
+// asking for both is asking for two flags that can disagree.
+func TestDefaultBackendURLFor(t *testing.T) {
+	for name, tc := range map[string]struct{ baseURI, want string }{
+		"production":             {"https://app.launchdarkly.com", defaultBackendUrl},
+		"staging":                {"https://ld-stg.launchdarkly.com", "https://pri.observability.ld-stg.launchdarkly.com"},
+		"trailing slash":         {"https://ld-stg.launchdarkly.com/", "https://pri.observability.ld-stg.launchdarkly.com"},
+		"surrounding whitespace": {" https://ld-stg.launchdarkly.com ", "https://pri.observability.ld-stg.launchdarkly.com"},
+		"regional instance":      {"https://app.eu.launchdarkly.com", "https://pri.observability.app.eu.launchdarkly.com"},
+
+		// Nothing about one of these says where an observability API listens, so the
+		// production default stands and --backend-url remains how to say otherwise.
+		"local stack": {"http://localhost:3000", defaultBackendUrl},
+		"host that merely ends in the domain name": {"https://notlaunchdarkly.com", defaultBackendUrl},
+		"unset":   {"", defaultBackendUrl},
+		"garbage": {"://", defaultBackendUrl},
+	} {
+		assert.Equal(t, tc.want, defaultBackendURLFor(tc.baseURI), name)
+	}
+}
+
 func TestIsReactNativeUploadFile(t *testing.T) {
 	// React Native iOS bundle + map.
 	assert.True(t, isReactNativeUploadFile("main.jsbundle"))