@@ -74,8 +74,9 @@ function splitTypeArray(node: Schema, types: unknown[]): Schema {
     }
   }
   if (nullReachable) branches.push({ type: 'null' });
-  // Nothing satisfies both unions; keep the existing anyOf rather than emit an empty one.
-  if (branches.length === 0) return { ...annotations, ...rest, anyOf: existing };
+  // The two unions are disjoint. Keep both constraints rather than drop the type union,
+  // which would let the anyOf branches admit types the node never allowed.
+  if (branches.length === 0) return { ...annotations, allOf: [splitTypeArray({ ...rest, type: types }, types), { anyOf: existing }] };
   if (branches.length === 1 && isSchema(branches[0])) return { ...annotations, ...branches[0] };
   return { ...annotations, anyOf: branches };
 }
 
@@ -149,6 +149,18 @@ describe("Gemini tool schema type arrays", () => {
     });
   });
 
+  test("a type union disjoint from the existing anyOf keeps both constraints", () => {
+    const out = normalizeDevinToolParameters("gemini-x", {
+      type: ["string", "null"], minLength: 2, anyOf: [{ type: "integer" }],
+    }) as any;
+    expect(out).toEqual({
+      allOf: [
+        { anyOf: [{ minLength: 2, type: "string" }, { type: "null" }] },
+        { anyOf: [{ type: "integer" }] },
+      ],
+    });
+  });
+
   test("an existing anyOf that agrees with the outer keywords is folded in, not nested under allOf", () => {
     const out = normalizeDevinToolParameters("MODEL_GOOGLE_GEMINI_2_5_PRO", {
       type: ["object", "null"], anyOf: [{ required: ["a"] }, { required: ["b"] }],
Original file line number	Diff line number	Diff line change
`@@ -74,8 +74,9 @@ function splitTypeArray(node: Schema, types: unknown[]): Schema {`
`74`	`74`	`}`
`75`	`75`	`}`
`76`	`76`	`if (nullReachable) branches.push({ type: 'null' });`
`77`		`- // Nothing satisfies both unions; keep the existing anyOf rather than emit an empty one.`
`78`		`- if (branches.length === 0) return { ...annotations, ...rest, anyOf: existing };`
	`77`	`+ // The two unions are disjoint. Keep both constraints rather than drop the type union,`
	`78`	`+ // which would let the anyOf branches admit types the node never allowed.`
	`79`	`+ if (branches.length === 0) return { ...annotations, allOf: [splitTypeArray({ ...rest, type: types }, types), { anyOf: existing }] };`
`79`	`80`	`if (branches.length === 1 && isSchema(branches[0])) return { ...annotations, ...branches[0] };`
`80`	`81`	`return { ...annotations, anyOf: branches };`
`81`	`82`	`}`