From 4f2947152e79cb657fb6c27f5d02b0d51da6359d Mon Sep 17 00:00:00 2001
From: Giulio Canti <giulio.canti@gmail.com>
Date: Thu, 4 Jun 2026 19:00:30 +0200
Subject: [PATCH] Emit `additionalProperties: false` for records with string
 keys and `Schema.Never` values

---
 .../json-schema-never-additional-properties.md       |  5 +++++
 packages/effect/src/JSONSchema.ts                    |  5 ++++-
 packages/effect/test/Schema/JSONSchema.test.ts       | 12 ++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/json-schema-never-additional-properties.md

diff --git a/.changeset/json-schema-never-additional-properties.md b/.changeset/json-schema-never-additional-properties.md
new file mode 100644
index 00000000000..e95f825d68d
--- /dev/null
+++ b/.changeset/json-schema-never-additional-properties.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Emit `additionalProperties: false` for records with string keys and `Schema.Never` values.
diff --git a/packages/effect/src/JSONSchema.ts b/packages/effect/src/JSONSchema.ts
index 4b5220eb434..c4ec30cf49b 100644
--- a/packages/effect/src/JSONSchema.ts
+++ b/packages/effect/src/JSONSchema.ts
@@ -843,7 +843,7 @@ function go(
         const parameter = is.parameter
         switch (parameter._tag) {
           case "StringKeyword": {
-            output.additionalProperties = go(
+            const additionalProperties = go(
               pruned,
               $defs,
               "handle-identifier",
@@ -852,6 +852,9 @@ function go(
               "handle-annotation",
               errors
             )
+            output.additionalProperties = isNeverWithoutCustomAnnotations(additionalProperties)
+              ? false
+              : additionalProperties
             break
           }
           case "TemplateLiteral": {
diff --git a/packages/effect/test/Schema/JSONSchema.test.ts b/packages/effect/test/Schema/JSONSchema.test.ts
index e3eaec514b3..be580edbe53 100644
--- a/packages/effect/test/Schema/JSONSchema.test.ts
+++ b/packages/effect/test/Schema/JSONSchema.test.ts
@@ -2149,6 +2149,18 @@ details: Cannot encode Symbol(effect/Schema/test/a) key to JSON Schema`
       })
     })
 
+    it("Record(string, never)", () => {
+      const jsonSchema = expectJSONSchema(Schema.Record({ key: Schema.String, value: Schema.Never }), {
+        "type": "object",
+        "properties": {},
+        "required": [],
+        "additionalProperties": false
+      })
+      const validate = getAjvValidate(jsonSchema)
+      assertTrue(validate({}))
+      assertFalse(validate({ a: null }))
+    })
+
     it("Record('a' | 'b', number)", () => {
       expectJSONSchemaAnnotations(
         Schema.Record(