diff --git a/src/tools/rust-analyzer/crates/proc-macro-api/src/process.rs b/src/tools/rust-analyzer/crates/proc-macro-api/src/process.rs
index e31ab86bdd2d6..d6a8d27bfc42e 100644
--- a/src/tools/rust-analyzer/crates/proc-macro-api/src/process.rs
+++ b/src/tools/rust-analyzer/crates/proc-macro-api/src/process.rs
@@ -58,7 +58,9 @@ impl ProcMacroServerProcess {
             if v.pre.as_str() == "nightly" { *v > VERSION } else { *v >= VERSION }
         });
 
-        let formats: &[_] = if has_working_format_flag {
+        let formats: &[_] = if std::env::var_os("RUST_ANALYZER_USE_POSTCARD").is_some()
+            && has_working_format_flag
+        {
             &[
                 (Some("postcard-legacy"), Protocol::LegacyPostcard { mode: SpanMode::Id }),
                 (Some("json-legacy"), Protocol::LegacyJson { mode: SpanMode::Id }),