diff --git a/.changeset/wicked-starfishes-bow.md b/.changeset/wicked-starfishes-bow.md
new file mode 100644
index 0000000000..4afd4e9156
--- /dev/null
+++ b/.changeset/wicked-starfishes-bow.md
@@ -0,0 +1,5 @@
+---
+"@effect/ai": patch
+---
+
+Avoid inclusion of `_tag` in tool call JSONSchema
diff --git a/packages/ai/ai/src/Completions.ts b/packages/ai/ai/src/Completions.ts
index 144e11a8f5..885f5412d0 100644
--- a/packages/ai/ai/src/Completions.ts
+++ b/packages/ai/ai/src/Completions.ts
@@ -315,7 +315,7 @@ const convertTool = <A, I, R>(
 ) => ({
   name,
   description: getDescription(schema.ast),
-  parameters: makeJsonSchema(schema.ast),
+  parameters: makeJsonSchema(AST.omit(schema.ast, ["_tag"])),
   structured
 })