diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 8963c65153..fde0d4343a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -428,7 +428,7 @@ $ node
 > eval(result.js_code);
 ```
 
-### Lambda debug outputs (API v9)
+### Lambda debug outputs
 
 `comp.rescript.compileWithDebug(source)` returns `lambda` before the backend's
 Lambda optimization passes and `lambda_optimized` after the whole-term passes,
@@ -437,8 +437,8 @@ so their text can be compared directly. JavaScript lowering performs further
 transformations; this snapshot is not the final JavaScript IR.
 
 Regular `comp.rescript.compile(source)` does not capture these debug outputs.
-Older bundles do not provide `lambda_optimized`; consumers should treat it as
-optional when supporting multiple API versions.
+Older bundles do not provide `lambda_optimized`; detect this optional field's
+presence rather than inferring support from the API version.
 
 ### Testing the Playground bundle
 
diff --git a/changes/developer-playground-lambda-diff.internal.md b/changes/developer-playground-lambda-diff.internal.md
index e20c50b6b7..c8d02b90b0 100644
--- a/changes/developer-playground-lambda-diff.internal.md
+++ b/changes/developer-playground-lambda-diff.internal.md
@@ -1 +1 @@
-- Add before/after optimization views and inline diffs to the developer playground's Lambda tab, with optimized Lambda output in playground API v9, deferred hidden-panel updates, and debounced comparisons while editing. https://github.com/rescript-lang/rescript/pull/8636
+- Add before/after optimization views and inline diffs to the developer playground's Lambda tab, with optional optimized Lambda output, deferred hidden-panel updates, and debounced comparisons while editing. https://github.com/rescript-lang/rescript/pull/8636
diff --git a/changes/playground-api-v8-compatibility.fix.md b/changes/playground-api-v8-compatibility.fix.md
new file mode 100644
index 0000000000..27506114df
--- /dev/null
+++ b/changes/playground-api-v8-compatibility.fix.md
@@ -0,0 +1 @@
+- Keep playground compiler API v8 when adding optional optimized Lambda output, preserving compatibility with existing playground clients. https://github.com/rescript-lang/rescript/pull/8930
diff --git a/compiler/jsoo/jsoo_playground_main.ml b/compiler/jsoo/jsoo_playground_main.ml
index b66c568ffb..e47c92fbe8 100644
--- a/compiler/jsoo/jsoo_playground_main.ml
+++ b/compiler/jsoo/jsoo_playground_main.ml
@@ -53,9 +53,8 @@
  * v6: Added `config.experimental_features` and `config.jsx_preserve_mode` to the BundleConfig.
  * v7: Added debug dump output APIs for developer playground tooling.
  * v8: Added genType and source map configuration and compilation outputs.
- * v9: Added optimized Lambda output to debug compilation.
  * *)
-let api_version = "9"
+let api_version = "8"
 
 module Js = Js_of_ocaml.Js
 
diff --git a/packages/dev-playground/src/LambdaView.res b/packages/dev-playground/src/LambdaView.res
index 4f67e6dc77..67701b4742 100644
--- a/packages/dev-playground/src/LambdaView.res
+++ b/packages/dev-playground/src/LambdaView.res
@@ -112,7 +112,7 @@ let make = (~input: unit => option<input>, ~visible, ~mode: Signal.t<mode>) => {
   let unavailable = () =>
     <p class="lambda-notice">
       {View.text(
-        "This compiler bundle does not provide optimized Lambda. Select a compiler with playground API v9 or newer.",
+        "This compiler bundle does not provide optimized Lambda. Select a newer compiler bundle.",
       )}
     </p>
   let panelInput = Signal.make(None)
diff --git a/packages/playground/playground_test.cjs b/packages/playground/playground_test.cjs
index 82dd9dcd9a..2fa5fbd25d 100644
--- a/packages/playground/playground_test.cjs
+++ b/packages/playground/playground_test.cjs
@@ -174,7 +174,7 @@ const syntaxErrorResult = compiler.rescript.compile("let = 1");
 assert.equal(syntaxErrorResult.type, "syntax_error");
 assert.match(syntaxErrorResult.errors[0].fullMsg, /\u001b\[/);
 
-assert.equal(rescript_compiler.api_version, "9");
+assert.equal(rescript_compiler.api_version, "8");
 const lambdaSource = "let compute = x => { let unused = x + 1; x }";
 const lambdaResult = compiler.rescript.compileWithDebug(lambdaSource);
 assert.equal(lambdaResult.type, "success");