fix(core): restore the v1 content default for tools/call results on the legacy era (#2456)

test/e2e/helpers/wire-sniffer.ts

@@ -4,7 +4,8 @@ import {
     ClientResultSchema,
     ServerNotificationSchema,
     ServerRequestSchema,
-    ServerResultSchema
+    ServerResultSchema,
+    TOOL_RESULT_FOREIGN_FAMILY_KEYS
 } from '@modelcontextprotocol/core-internal';
 import type { Transport } from '@modelcontextprotocol/server';
 import {
@@ -16,6 +17,10 @@ import {
     isSpecType
 } from '@modelcontextprotocol/server';
 
+function isPlainRecord(v: unknown): v is Record<string, unknown> {
+    return typeof v === 'object' && v !== null && !Array.isArray(v);
+}
+
 export type WireParty = 'client' | 'server';
 
 export interface SnifferOptions {
@@ -100,7 +105,28 @@ export function assertWireMessage(msg: unknown, party: WireParty, opts: SnifferO
         // (InputRequiredResultSchema lives alongside, never widening it).
         // Era-gated: only cells wired for the modern era opt in, so an
         // input_required on a 2025-era cell's wire is still flagged.
-        if (party === 'server' && opts.allowInputRequiredResults === true && isInputRequiredResult(result)) return;
+        if (party === 'server' && isInputRequiredResult(result)) {
+            if (opts.allowInputRequiredResults === true) return;
+            fail(party, `input_required result on a cell not opted into multi-round-trip`, msg);
+        }
+        // With content.default([]) restored, the neutral union accepts any
+        // object via the CallToolResult member — so union conformance below
+        // is a weak check, and the other result families are asserted here
+        // explicitly, by vocabulary. Cells that opt into vendor-extension
+        // methods skip this check (the sniffer is method-blind for results;
+        // before the default was restored these bodies failed the union parse
+        // and were excused by the same flag).
+        if (party === 'server' && !opts.allowCustomMethods && isPlainRecord(result) && result.content === undefined) {
+            for (const key of TOOL_RESULT_FOREIGN_FAMILY_KEYS) {
+                if (key in result) {
+                    fail(
+                        party,
+                        `content-less result carrying '${key}' — another result family must not read as an empty tools/call success`,
+                        msg
+                    );
+                }
+            }
+        }
         const r = schemas.result.safeParse(result);
         if (!r.success) {
             // A result for a vendor-extension request legitimately won't match the spec union.

test/e2e/scenarios/raw-result-type.test.ts

@@ -14,10 +14,10 @@
  *    ABSENT `resultType` is a spec violation surfaced as a typed error
  *    naming it.
  *  - Negotiated legacy (2025 era): `resultType` is FOREIGN vocabulary —
- *    strip-on-lift (Q1-SD3 ii; a deliberate, ledgered change from the
- *    pre-split era-blind rejection — changeset: codec-split-wire-break). The
- *    stripped body then fails the (default-free) result schema loudly
- *    because it has no content.
+ *    strip-on-lift (Q1-SD3 ii). The stripped husk still carries the
+ *    input_required family keys, so the wire-seam schema rejects it loudly
+ *    (a content-less body without family keys would default to content: []
+ *    — changeset: calltoolresult-content-default).
  *
  * Either way the V-1 invariant holds: never an empty-content success.
  */
@@ -115,8 +115,8 @@ verifies('typescript:client:raw-result-type-first', async ({ transport }: TestAr
 
         try {
             const outcome = await callToolOutcome(client);
-            // Strip-on-lift (Q1-SD3 ii, ledgered): the foreign resultType is
-            // dropped; the body has no content, so validation fails LOUDLY.
+            // Strip-on-lift: the foreign resultType is dropped; the husk's
+            // input_required family keys fail the wire-seam schema LOUDLY.
             // Never an empty-content success.
             expect('resolved' in outcome, `must not resolve: ${JSON.stringify(outcome)}`).toBe(false);
             const rejection = (outcome as { rejected: unknown }).rejected;

test/integration/test/client/client.test.ts

@@ -1828,8 +1828,6 @@ describe('outputSchema validation', () => {
         server.setRequestHandler('tools/call', async request => {
             if (request.params.name === 'test-tool') {
                 return {
-                    // content is spec-required (the wire default([]) was removed
-                    // - ledgered; changeset codec-split-wire-break)
                     content: [],
                     structuredContent: { result: 'success', count: 42 }
                 };