fix: address Copilot review round 34 — structured constants, non-required const controls

- A `const` may be an object or an array, with or without a `type`, and
  `String({...})` is "[object Object]" — which no CLI argument can equal, so
  the only value such a schema accepts never matched. Supplied text is parsed
  and compared structurally through the resolver's own comparison (exported as
  `sameJsonValue` so the two cannot disagree), for both branch selection and
  the substitution.
- The TUI's one-option const control is never marked required: its option may
  legitimately be the empty string, which ink-form's required gate can never
  accept, so submission would not even reach `decodeFormValues`. The value is
  fixed by the schema, and `missingRequiredFields` still validates the call.

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

clients/tui/__tests__/schemaToForm.test.ts

@@ -503,6 +503,24 @@ describe("schemaToForm", () => {
       });
     });
 
+    it("never marks a const control required", () => {
+      // Its one option may be the empty string, which ink-form's required gate
+      // can never accept — the call would not even reach `decodeFormValues`.
+      const form = schemaToForm(
+        {
+          type: "object",
+          properties: { kind: { type: "string", const: "" } },
+          required: ["kind"],
+        },
+        "empty_const",
+      );
+      expect(form.sections[0]!.fields[0]).toMatchObject({
+        name: "kind",
+        type: "select",
+        required: false,
+      });
+    });
+
     it("renders a const outside a union the same way", () => {
       const form = schemaToForm(
         {

clients/tui/src/utils/schemaToForm.ts

@@ -357,6 +357,12 @@ function buildFields(schema: JsonSchemaObject): FormField[] {
       fields.push({
         type: "select",
         ...baseField,
+        // Never required: the one option may legitimately be the empty string,
+        // which ink-form's required gate can never accept — submission would
+        // not even reach `decodeFormValues`, which reapplies the constant. The
+        // value is fixed by the schema, and `missingRequiredFields` still
+        // validates the decoded call.
+        required: false,
         initialValue: String(pinned),
         options: [{ label: String(pinned), value: String(pinned) }],
       } as FormField);

clients/web/src/test/core/jsonUtils.test.ts

@@ -312,6 +312,28 @@ describe("JSON Utils", () => {
       ).toEqual({ value: "true" });
     });
 
+    it("matches a structured const by parsing the supplied text (#2123)", () => {
+      const structured: Tool = {
+        name: "structured-const",
+        inputSchema: {
+          type: "object",
+          properties: { tag: { const: { kind: "x", n: 1 } } },
+        },
+      };
+      // `String({...})` is "[object Object]", which no argument can equal — so
+      // the only value the schema accepts would never have matched.
+      expect(
+        convertToolParameters(structured, { tag: '{"n":1,"kind":"x"}' }),
+      ).toEqual({ tag: { kind: "x", n: 1 } });
+      // Text that is not that value, or not JSON at all, is left alone.
+      expect(convertToolParameters(structured, { tag: "{}" })).toEqual({
+        tag: "{}",
+      });
+      expect(convertToolParameters(structured, { tag: "nope" })).toEqual({
+        tag: "nope",
+      });
+    });
+
     it("leaves an ambiguously typed argument uncoerced (#2123)", () => {
       const ambiguous: Tool = {
         name: "ambiguous",

core/json/jsonUtils.ts

@@ -2,6 +2,7 @@ import type { Tool } from "@modelcontextprotocol/client";
 import { normalizeNullableUnion } from "./nullableUnion.js";
 import {
   narrowBySuppliedNames,
+  sameJsonValue,
   resolveRootUnion,
   type RootUnionBranch,
   type RootUnionSchema,
@@ -253,10 +254,29 @@ function matchesConstants(
     // read the inherited one and rule out every branch that pins that name.
     if (!Object.hasOwn(params, name)) return true;
     const supplied = params[name];
-    return supplied === undefined || supplied === String(constValue);
+    return supplied === undefined || suppliedMatchesConst(supplied, constValue);
   });
 }
 
+/**
+ * Whether the text a CLI argument carries is the value a `const` fixes.
+ *
+ * A primitive constant is compared as text, which is all a command line has. A
+ * structured one — a `const` may be an object or an array, with or without a
+ * `type` — is parsed first: `String({...})` is `"[object Object]"`, which no
+ * argument can equal, so the only value the schema accepts would never match.
+ */
+function suppliedMatchesConst(value: string, constValue: unknown): boolean {
+  if (constValue === null || typeof constValue !== "object") {
+    return value === String(constValue);
+  }
+  try {
+    return sameJsonValue(JSON.parse(value), constValue);
+  } catch {
+    return false;
+  }
+}
+
 /**
  * Convert string parameters to JSON values based on tool schema
  */
@@ -288,7 +308,7 @@ export function convertToolParameters(
     // user's input and is left alone.
     const pinned = (paramSchema as { const?: unknown } | undefined)?.const;
     const converted =
-      pinned !== undefined && String(pinned) === value
+      pinned !== undefined && suppliedMatchesConst(value, pinned)
         ? (pinned as JsonValue)
         : paramSchema
           ? convertParameterValue(value, paramSchema)

core/json/rootUnion.ts

@@ -240,7 +240,17 @@ function canonicalJson(value: unknown): string {
   return JSON.stringify(value) ?? "undefined";
 }
 
-/** Structural equality, via canonical JSON — enough for schema keyword values. */
+/**
+ * Structural equality, via canonical JSON — enough for schema keyword values.
+ *
+ * Exported because a caller comparing a *supplied* value against a schema's
+ * `const` has to reach the same answer this module does; two implementations
+ * would disagree the moment one of them met an object.
+ */
+export function sameJsonValue(a: unknown, b: unknown): boolean {
+  return sameValue(a, b);
+}
+
 function sameValue(a: unknown, b: unknown): boolean {
   return a === b || canonicalJson(a) === canonicalJson(b);
 }