fix: address Copilot review rounds 7 and 8

- rootUnion: a `oneOf` demands that EXACTLY one alternative match, which
  flattening cannot preserve, so it is offered only when a discriminator makes
  the alternatives mutually exclusive; `anyOf` makes no such claim and is
  unaffected. An empty `additionalProperties` schema is the equivalent of
  `true` and no longer declines a union.
- selectBranchIndex: falls back to the branch whose own required fields the
  values supply, so an undiscriminated union's values open the picker on the
  shape they describe rather than on the first branch.
- admitsNull: honors a sibling `nullable: true` beside `const: null`.
- SchemaForm: a value is carried only where the incoming branch leaves the
  root's declaration as it found it; anything it declares itself is reset,
  since it may type the name differently from where the value was typed.
  `hasOwn`/`fromEntries` throughout, so `constructor` is not read as a
  supplied value and `__proto__` is not dropped into the prototype setter.
- TUI: the same prototype-safe construction on the way out of the form, and
  the chosen branch's `required` list is checked at submit — rendering a
  branch's fields optional makes the FORM satisfiable, not the call, so the
  missing names are reported rather than a known-invalid call being sent.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/tui/src/components/ToolTestModal.tsx

@@ -5,7 +5,11 @@ import { InspectorClient } from "@inspector/core/mcp/index.js";
 import { AuthRecoveryRequiredError } from "@inspector/core/auth/challenge.js";
 import type { Tool, CallToolResult } from "@modelcontextprotocol/client";
 import type { JsonValue } from "@inspector/core/mcp/index.js";
-import { decodeFormValues, schemaToForm } from "../utils/schemaToForm.js";
+import {
+  decodeFormValues,
+  missingRequiredFields,
+  schemaToForm,
+} from "../utils/schemaToForm.js";
 import { ScrollView, type ScrollViewRef } from "ink-scroll-view";
 
 interface ToolTestModalProps {
@@ -123,6 +127,23 @@ export function ToolTestModal({
     // the base fields plus the chosen branch's, and nothing from the others.
     const values = decodeFormValues(tool.inputSchema, rawValues);
 
+    // A branch's fields are rendered optional — only one alternative applies to
+    // a call, and requiring every branch's would deadlock a static form — so
+    // the chosen shape's own requirements are checked here instead. Reported
+    // rather than sent: a call known to violate the schema teaches the user
+    // nothing about the server (#2123).
+    const missing = missingRequiredFields(tool.inputSchema, values, rawValues);
+    if (missing.length > 0) {
+      setResult({
+        input: values,
+        output: null,
+        error: `Missing required argument${missing.length > 1 ? "s" : ""}: ${missing.join(", ")}`,
+        duration: 0,
+      });
+      setState("results");
+      return;
+    }
+
     setState("loading");
     const startTime = Date.now();
 

clients/tui/src/utils/schemaToForm.ts

@@ -138,7 +138,7 @@ function branchFields(
   branches: { declaredFields: string[] }[],
   index: number,
 ): string[] {
-  const own = branches[index]?.declaredFields ?? [];
+  const own = branches[index]!.declaredFields;
   return [...new Set([...own, ...sharedFieldNames(base, branches)])];
 }
 
@@ -256,11 +256,7 @@ export function decodeFormValues<T>(
   }
 
   const { variant, prefix } = generatedNames(base, branches);
-  const selected = Number(values[variant]);
-  const branchIndex =
-    Number.isInteger(selected) && selected >= 0 && selected < branches.length
-      ? selected
-      : 0;
+  const branchIndex = selectedBranchIndex(base, branches, values);
   const branch = branches[branchIndex]!;
 
   const generated = new Set<string>([variant]);
@@ -270,21 +266,42 @@ export function decodeFormValues<T>(
     }
   });
 
-  const decoded: Record<string, T> = {};
-  for (const [name, value] of Object.entries(values)) {
-    if (!generated.has(name)) {
-      decoded[name] = value;
-    }
-  }
-  for (const name of branchFields(base, branches, branchIndex)) {
-    const value = values[branchFieldName(prefix, branchIndex, name)];
-    if (value !== undefined) {
-      decoded[name] = value;
-    }
-  }
+  // Built through `fromEntries` rather than by assignment: `__proto__` is a
+  // legal argument name, and assigning it would invoke the legacy prototype
+  // setter — the field would be prefixed safely in the form and then vanish on
+  // the way to the call.
+  const decoded: Record<string, T> = Object.fromEntries([
+    ...Object.entries(values).filter(([name]) => !generated.has(name)),
+    ...branchFields(base, branches, branchIndex)
+      .map(
+        (name) =>
+          [name, values[branchFieldName(prefix, branchIndex, name)]] as const,
+      )
+      .filter(([, value]) => value !== undefined),
+  ]);
+  /* v8 ignore next -- an offerable branch always carries properties */
   return applyConstants(branch.schema.properties ?? {}, decoded);
 }
 
+/**
+ * Which branch the variant select names, clamped to one that exists — a form
+ * value is whatever the user's terminal produced, and the fallback is the first
+ * branch, which is what the select opens on.
+ */
+function selectedBranchIndex(
+  base: { properties?: Record<string, unknown> },
+  branches: { declaredFields: string[] }[],
+  values: Record<string, unknown>,
+): number {
+  const { variant } = generatedNames(base, branches);
+  const selected = Number(values[variant]);
+  return Number.isInteger(selected) &&
+    selected >= 0 &&
+    selected < branches.length
+    ? selected
+    : 0;
+}
+
 /** Overwrite every `const`-pinned field with the value its schema fixes. */
 function applyConstants<T>(
   properties: Record<string, unknown>,
@@ -294,11 +311,10 @@ function applyConstants<T>(
     ([, schema]) => constOf(schema) !== undefined,
   );
   if (pinned.length === 0) return values;
-  const result = { ...values };
-  for (const [name, schema] of pinned) {
-    result[name] = constOf(schema) as T;
-  }
-  return result;
+  return Object.fromEntries([
+    ...Object.entries(values),
+    ...pinned.map(([name, schema]) => [name, constOf(schema) as T]),
+  ]);
 }
 
 /** Build the ink-form fields for one already-flattened object schema. */
@@ -365,6 +381,7 @@ function buildFields(schema: JsonSchemaObject): FormField[] {
       field = {
         type: "select",
         ...baseField,
+        /* v8 ignore next -- guarded by `isStringEnum(property.enum)` above */
         options: toSelectOptions(property.enum ?? [], property.enumNames),
       } as FormField;
     } else {
@@ -420,3 +437,32 @@ function buildFields(schema: JsonSchemaObject): FormField[] {
 
   return fields;
 }
+
+/**
+ * The required fields the chosen shape does not supply — what a static form
+ * cannot enforce for itself.
+ *
+ * A branch's fields are rendered optional because only one alternative applies
+ * to a call and ink-form would otherwise demand every branch's, deadlocking the
+ * form. That makes the *form* satisfiable, not the call: selecting `email` and
+ * leaving `address` empty still violates the schema. So the requirement is
+ * checked at submit instead, against the branch the variant select names, and
+ * the caller reports it rather than sending a call known to be invalid.
+ *
+ * Takes the **decoded** values — what would actually be sent.
+ */
+export function missingRequiredFields(
+  schema: JsonSchemaObject | null | undefined,
+  decoded: Record<string, unknown>,
+  rawValues: Record<string, unknown> = {},
+): string[] {
+  const { base, branches } = resolveRootUnion(schema ?? {});
+  const effective =
+    branches.length === 0
+      ? base
+      : branches[selectedBranchIndex(base, branches, rawValues)]!.schema;
+  return (effective.required ?? []).filter((name) => {
+    const value = decoded[name];
+    return value === undefined || value === "";
+  });
+}