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

.changeset/calltoolresult-content-default.md

@@ -0,0 +1,14 @@
+---
+'@modelcontextprotocol/core-internal': patch
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/core': patch
+'@modelcontextprotocol/server': patch
+---
+
+Restore the v1 parse tolerance for `CallToolResult.content`: an inbound legacy-era `tools/call` result without `content` defaults to `[]` instead of failing validation. Deployed servers — accepted by SDK v1 for years — return `structuredContent`-only (or otherwise content-less) results, and the strict parse turned every such call into an `INVALID_RESULT` error before application code could run.
+
+The silent-empty-success hazard the strictness guarded is preserved where it matters: the 2025 era's wire-seam schema refuses to default `content` for a body carrying another result family's vocabulary (`task`, `inputRequests`, `requestState` — the era is frozen, so the list is complete), and the 2026-era wire schemas stay strict — modern-revision servers have no legacy excuse. Task interop through an explicit result schema is untouched (including bodies that also stamp a foreign `resultType`), and the server-side authoring normalization refuses the same foreign-family vocabulary.
+
+Server-side authoring is era-independent: a handler result without `content` (dynamic/JS callers — the TypeScript surface requires it) is normalized to `content: []` before era validation on every leg, reaching the wire spec-valid.
+
+Conscious call: the nested sampling `ToolResultContentSchema` stays spec-strict — v1 had defaulted its `content` too, but it is params-side (tool results a caller authors into a sampling message), deliberately not restored.

docs/migration/support-2026-07-28.md

@@ -340,9 +340,11 @@ If you were on a v2 alpha and consumed wire schemas directly:
 
 The `resultType` / `EmptyResultSchema` / `specTypeSchemas` rules above have **no v1.x
 impact** — these members did not exist before 2026-07-28. The neutral-model wire
-tightening that **does** affect v1 code (`content` required, custom-handler `_meta`
-passthrough, `specTypeSchemas` narrowing) is in
-[upgrade-to-v2.md › Wire tightening](./upgrade-to-v2.md#wire-tightening-every-era).
+tightening that **does** affect v1 code (custom-handler `_meta` passthrough,
+`specTypeSchemas` narrowing) is in
+[upgrade-to-v2.md › Wire tightening](./upgrade-to-v2.md#wire-tightening-every-era);
+`CallToolResult.content` keeps its v1 default on the legacy era (2026-07-28
+connections require it explicitly).
 
 > **If you were on a v2 alpha:** the 2026-07-28 draft error codes were renumbered:
 > `HeaderMismatch` `-32001`→`-32020`, `MissingRequiredClientCapability` `-32003`→`-32021`,

docs/migration/upgrade-to-v2.md

@@ -1673,10 +1673,16 @@ requests, the per-request `_meta.logLevel` envelope key is the filter — see
 
 #### Wire tightening (every era)
 
-- **`CallToolResult.content` is required at the wire boundary.** The `content.default([])`
-  affordance was removed. Tool handlers MUST include `content` (the TypeScript surface
-  always required it; `content: []` is fine). A handler result without it is rejected
-  with `-32602`.
+- **`CallToolResult.content` keeps the v1 parse tolerance on the legacy era.** An
+  inbound result without `content` defaults to `[]` (deployed servers omit it
+  alongside `structuredContent`); 2026-07-28 connections stay strict. Authoring is
+  unchanged and era-independent: the TypeScript surface requires `content` on handler
+  results, and a content-less handler result is normalized to `content: []` before it
+  reaches the wire. One sharpening remains: a content-less body carrying another
+  result family's vocabulary (a task handle or an `input_required` round) is still
+  rejected loudly — tolerance never turns a different result kind into a silent empty
+  success. A body whose only foreign key was `resultType` strips to an empty object
+  and defaults, exactly as v1 parsed a payload-free body.
 - **`ElicitResult.content` values are typed and validated as
   `string | number | boolean | string[]`.** v1's TypeScript surface accepted
   `Record<string, unknown>` content values; an elicitation handler returning arbitrary

packages/core-internal/eslint.config.mjs

@@ -7,7 +7,9 @@ export default [
     {
         // Wire-layer isolation, outbound direction: nothing outside src/wire/ may
         // reach into a wire revision module. The wire layer's only public surface
-        // is src/wire/codec.ts (the WireCodec interface) and src/wire/bootstrap.ts.
+        // is src/wire/codec.ts (the WireCodec interface), src/wire/bootstrap.ts,
+        // and the leaf result-family module src/wire/resultFamilies.ts (the shared
+        // tools/call-result ruling, re-exported on the barrel).
         // test/wire/layeringInvariants.test.ts re-derives the same invariant with
         // zero exceptions. Type-only imports are exempted at the lint layer (a
         // type-only crossing is erased at runtime), but the test allows none.

packages/core-internal/src/index.ts

@@ -28,11 +28,15 @@ export * from './util/inMemory';
 // 2026-only seam runs in. NOTHING per-revision (registries, codec objects,
 // per-revision schemas) is ever exported on this barrel — sibling packages
 // reach the wire layer ONLY through `codecForVersion`'s function-only
-// `WireCodec` surface.
+// `WireCodec` surface. Sole exemption: the shared result-family ruling
+// (`wire/resultFamilies.ts`), era-independent by design — the server's
+// authoring normalization and the e2e wire sniffer apply the same ruling
+// (and name the same vocabulary) as the 2025 wire seam.
 export * from './util/schema';
 export * from './util/standardSchema';
 export * from './util/zodCompat';
 export { codecForVersion, MODERN_WIRE_REVISION } from './wire/codec';
+export { normalizeContentlessToolResult, TOOL_RESULT_FOREIGN_FAMILY_KEYS } from './wire/resultFamilies';
 
 // Validator provider classes stay subpath-only. Re-exporting them here, even as
 // `type`, can make generated client/server root declarations advertise

packages/core-internal/src/types/guards.ts

@@ -84,7 +84,9 @@ export const isJSONRPCResponse = (value: unknown): value is JSONRPCResponse => J
  * @returns True if the value is a valid {@linkcode CallToolResult}, false otherwise.
  */
 export const isCallToolResult = (value: unknown): value is CallToolResult => {
-    if (typeof value !== 'object' || value === null || !('content' in value)) return false;
+    // content === undefined covers an explicit undefined key too: the schema's
+    // .default([]) would parse it, but the narrowed type requires the array.
+    if (typeof value !== 'object' || value === null || (value as { content?: unknown }).content === undefined) return false;
     return CallToolResultSchema.safeParse(value).success;
 };
 

packages/core-internal/src/types/schemas.ts

@@ -1381,8 +1381,11 @@ export const CallToolResultSchema = ResultSchema.extend({
      *
      * If the `Tool` does not define an outputSchema, this field MUST be present in the result.
      * Required on the wire per the specification (it may be an empty array).
+     *
+     * Parse-tolerant: absent `content` defaults to `[]` (v1 parity — deployed
+     * servers omit it alongside `structuredContent`).
      */
-    content: z.array(ContentBlockSchema),
+    content: z.array(ContentBlockSchema).default([]),
 
     /**
      * Structured tool output.

packages/core-internal/src/wire/resultFamilies.ts

@@ -0,0 +1,24 @@
+/**
+ * Result-family keys that must never default into a `{content: []}` tools/call
+ * success. Shared by the 2025 wire-seam schema and server normalization.
+ * Leaf module (like `textFallback.ts`): imported by registry/server paths, so
+ * it must NOT import from `./codec.js` — that would close a runtime cycle.
+ */
+export const TOOL_RESULT_FOREIGN_FAMILY_KEYS = ['task', 'inputRequests', 'requestState'] as const;
+
+/**
+ * Single owner of the v1-parity ruling: a plain-object tool result without `content` (and
+ * without foreign-family keys) gains `content: []`. Shared by the 2025 wire seam and server-side handler normalization.
+ */
+export function normalizeContentlessToolResult(value: unknown): unknown {
+    if (
+        value === null ||
+        typeof value !== 'object' ||
+        Array.isArray(value) ||
+        (value as { content?: unknown }).content !== undefined ||
+        TOOL_RESULT_FOREIGN_FAMILY_KEYS.some(key => key in value)
+    ) {
+        return value;
+    }
+    return { ...value, content: [] };
+}

packages/core-internal/src/wire/rev2025-11-25/codec.ts

@@ -105,8 +105,9 @@ export const rev2025Codec: WireCodec = {
 
     decodeResult(_method: string, raw: unknown): DecodedResult {
         // Strip-on-lift (Q1-SD3 ii): a foreign `resultType` on the 2025 leg is
-        // dropped before validation, whatever its value. There is no
-        // discrimination on this era — `resultType` carries no meaning here.
+        // dropped before validation, whatever its value. Validation judges the
+        // husk — the registry wire-seam schema on the plain path, the caller's
+        // schema on the explicit path (task interop).
         if (isPlainObject(raw) && 'resultType' in raw) {
             const stripped = { ...raw };
             delete stripped['resultType'];

packages/core-internal/src/wire/rev2025-11-25/registry.ts

@@ -21,9 +21,10 @@
  * shells, `resultType`, the `_meta` envelope) has NO entry and NO code path
  * here — the inverse-leak guarantee is physical absence, not discipline.
  */
-import type * as z from 'zod/v4';
+import * as z from 'zod/v4';
 
 import type { NotificationMethod, NotificationTypeMap, RequestMethod, RequestTypeMap, ResultTypeMap } from '../../types/types';
+import { normalizeContentlessToolResult, TOOL_RESULT_FOREIGN_FAMILY_KEYS } from '../resultFamilies';
 import type { ClientNotificationSchema, ClientRequestSchema, ServerNotificationSchema, ServerRequestSchema } from './schemas';
 import {
     CallToolRequestSchema,
@@ -106,6 +107,31 @@ type Rev2025TypedRequestMethod = Extract<RequestMethod, Rev2025RequestMethod>;
 // no key may fall outside it (no `tasks/*` entries — the task methods are
 // 2025-11-25 wire vocabulary with no SDK runtime; callers needing task
 // interop pass an explicit schema).
+/**
+ * Wire seam: owns both halves of the v1-parity ruling — the guard (a content-less body
+ * carrying another result family's keys fails loudly; the era is frozen so the key list is
+ * complete) and the tolerance (`content` defaults to `[]`). The era file stays twin-conformant.
+ */
+export const CallToolResultWireSchema = z
+    .unknown()
+    .superRefine((value, ctx) => {
+        // content === undefined covers both an absent key and an explicit
+        // undefined from server-side authoring objects.
+        if (typeof value !== 'object' || value === null || Array.isArray(value) || (value as Record<string, unknown>).content !== undefined)
+            return;
+        for (const key of TOOL_RESULT_FOREIGN_FAMILY_KEYS) {
+            if (key in value) {
+                ctx.addIssue({
+                    code: 'custom',
+                    message: `content is required when the body carries '${key}' — another result family cannot default into an empty tools/call success`
+                });
+                return;
+            }
+        }
+    })
+    .transform(normalizeContentlessToolResult)
+    .pipe(CallToolResultSchema);
+
 const resultSchemas: { readonly [M in Rev2025TypedRequestMethod]: z.ZodType<ResultTypeMap[M]> } = {
     ping: EmptyResultSchema,
     initialize: InitializeResultSchema,
@@ -118,7 +144,7 @@ const resultSchemas: { readonly [M in Rev2025TypedRequestMethod]: z.ZodType<Resu
     'resources/read': ReadResourceResultSchema,
     'resources/subscribe': EmptyResultSchema,
     'resources/unsubscribe': EmptyResultSchema,
-    'tools/call': CallToolResultSchema,
+    'tools/call': CallToolResultWireSchema,
     'tools/list': ListToolsResultSchema,
     'sampling/createMessage': CreateMessageResultWithToolsSchema,
     'elicitation/create': ElicitResultSchema,

packages/core-internal/test/shared/contentDefaultParity.test.ts

@@ -0,0 +1,109 @@
+import { describe, expect, it } from 'vitest';
+
+import { SdkError } from '../../src/errors/sdkErrors';
+import type { BaseContext } from '../../src/shared/protocol';
+import { Protocol } from '../../src/shared/protocol';
+import type { JSONRPCRequest } from '../../src/types/index';
+import { isJSONRPCRequest } from '../../src/types/index';
+import { InMemoryTransport } from '../../src/util/inMemory';
+
+class TestProtocolImpl extends Protocol<BaseContext> {
+    protected assertCapabilityForMethod(): void {}
+    protected assertNotificationCapability(): void {}
+    protected assertRequestHandlerCapability(): void {}
+    protected buildContext(ctx: BaseContext): BaseContext {
+        return ctx;
+    }
+}
+
+/**
+ * v1 parse-parity for `CallToolResult.content` on the legacy era: absent
+ * content defaults to []; another result family's content-less body still
+ * fails loudly via the registry wire-seam schema.
+ */
+describe('CallToolResult content default (v1 parity)', () => {
+    async function respondWith(body: Record<string, unknown>, resultSchema?: Parameters<Protocol<BaseContext>['request']>[1]) {
+        const protocol = new TestProtocolImpl();
+        const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+        serverTransport.onmessage = message => {
+            if (isJSONRPCRequest(message)) {
+                void serverTransport.send({
+                    jsonrpc: '2.0',
+                    id: (message as JSONRPCRequest).id,
+                    result: body
+                });
+            }
+        };
+        await serverTransport.start();
+        await protocol.connect(clientTransport);
+        try {
+            return resultSchema === undefined
+                ? await protocol.request({ method: 'tools/call', params: { name: 'echo', arguments: {} } })
+                : await protocol.request({ method: 'tools/call', params: { name: 'echo', arguments: {} } }, resultSchema);
+        } finally {
+            await protocol.close().catch(() => {});
+        }
+    }
+
+    it('a structured-only result resolves with content: []', async () => {
+        const result = (await respondWith({ structuredContent: { ok: true } })) as {
+            content: unknown;
+            structuredContent: unknown;
+        };
+        expect(result.content).toEqual([]);
+        expect(result.structuredContent).toEqual({ ok: true });
+    });
+
+    it('an entirely empty result resolves with content: []', async () => {
+        const result = (await respondWith({})) as { content: unknown };
+        expect(result.content).toEqual([]);
+    });
+
+    it('a task-shaped body without content still fails loudly (wire-seam guard)', async () => {
+        await expect(respondWith({ task: { taskId: 't-1', status: 'working' } })).rejects.toBeInstanceOf(SdkError);
+    });
+
+    it('task interop via an explicit result schema still works — the guard never touches that overload', async () => {
+        const { CreateTaskResultSchema } = await import('../../src/wire/rev2025-11-25/schemas');
+        const body = {
+            task: {
+                taskId: '786af6b0-2779-48ed-9cc1-b8a8a25b8a86',
+                status: 'working',
+                createdAt: '2025-11-25T10:30:00Z',
+                lastUpdatedAt: '2025-11-25T10:30:05Z',
+                ttl: 60000,
+                pollInterval: 5000
+            }
+        };
+        const result = (await respondWith(body, CreateTaskResultSchema)) as { task: { taskId: string } };
+        expect(result.task.taskId).toBe('786af6b0-2779-48ed-9cc1-b8a8a25b8a86');
+    });
+
+    it('explicit-schema task interop resolves even when the body also stamps a foreign resultType', async () => {
+        const { CreateTaskResultSchema } = await import('../../src/wire/rev2025-11-25/schemas');
+        const body = {
+            resultType: 'complete',
+            task: {
+                taskId: '786af6b0-2779-48ed-9cc1-b8a8a25b8a86',
+                status: 'working',
+                createdAt: '2025-11-25T10:30:00Z',
+                lastUpdatedAt: '2025-11-25T10:30:05Z',
+                ttl: 60000,
+                pollInterval: 5000
+            }
+        };
+        const result = (await respondWith(body, CreateTaskResultSchema)) as { task: { taskId: string } };
+        expect(result.task.taskId).toBe('786af6b0-2779-48ed-9cc1-b8a8a25b8a86');
+    });
+
+    it('the wire-seam guard treats an explicit content: undefined like an absent key', async () => {
+        const { getResultSchema } = await import('../../src/wire/rev2025-11-25/registry');
+        const wireSeam = getResultSchema('tools/call')!;
+        expect(wireSeam.safeParse({ task: { taskId: 't-1', status: 'working' }, content: undefined }).success).toBe(false);
+    });
+
+    it('an input_required-shaped body without content still fails loudly (wire-seam guard)', async () => {
+        await expect(respondWith({ inputRequests: { r1: { method: 'elicitation/create' } } })).rejects.toBeInstanceOf(SdkError);
+        await expect(respondWith({ requestState: 'opaque-token' })).rejects.toBeInstanceOf(SdkError);
+    });
+});

packages/core-internal/test/shared/rawResultTypeFirst.test.ts

@@ -137,12 +137,9 @@ describe('raw-first resultType discrimination — 2026 era (codec decode step 1)
 
 describe('raw-first resultType handling — 2025 era (strip-on-lift, Q1-SD3 ii)', () => {
     test('a foreign input_required body is stripped, then validation judges the content — never a silent success', async () => {
-        // BEHAVIOR MIGRATION (ledgered): pre-split, the era-blind funnel arm
-        // rejected this with UnsupportedResultType on every leg. On the 2025
-        // era resultType carries no meaning — the ruled posture strips the
-        // foreign key and lets validation decide. The body has no content,
-        // so it fails the (default-free) tools/call result schema LOUDLY —
-        // the V-1 invariant (never a hollow success) holds.
+        // BEHAVIOR MIGRATION (ledgered): the strip drops the foreign key and
+        // the wire-seam schema refuses to default a husk carrying
+        // input_required keys — V-1 (never a hollow success) holds at the seam.
         const protocol = await wireWithRawResult(INPUT_REQUIRED_BODY);
         const outcome = await settle(protocol);
 

packages/core-internal/test/shared/typedMapAlignment.test.ts

@@ -93,18 +93,11 @@ describe('task-shaped result bodies against the narrowed runtime map', () => {
         await protocol.close();
     });
 
-    test('tools/call: a CreateTaskResult body is now a typed invalid-result error too (content-default removal flip)', async () => {
-        // FLIPPED PIN (Q1 increment 2, ledgered with the content-default
-        // removal — changeset: codec-split-wire-break). The previous "Honest
-        // pin, not an endorsement" recorded that CallToolResultSchema's
-        // content.default([]) swallowed ANY object — including a task body —
-        // as a content-empty success, which made the old union member
-        // unreachable and the map narrowing observationally invisible for
-        // tools/call. With `content` now REQUIRED at the wire boundary the
-        // masking surface is gone: a task body has no `content`, fails the
-        // plain schema, and surfaces as the same typed invalid-result error
-        // as sampling/elicit. The result-schema-strictness question the old
-        // pin deferred is hereby resolved: loud rejection.
+    test('tools/call: a CreateTaskResult body on the plain path is a typed invalid-result error (wire-seam guard)', async () => {
+        // FLIPPED PIN, twice-ledgered (changesets: codec-split-wire-break,
+        // calltoolresult-content-default). The wire-seam schema restores the
+        // v1 default for plain results but refuses to default a body carrying
+        // another result family's keys — a task body fails it loudly.
         const protocol = await wireWithRawResult(CREATE_TASK_RESULT_BODY);
 
         const rejection = await protocol

packages/core-internal/test/types.test.ts

@@ -296,12 +296,14 @@ describe('Types', () => {
             }
         });
 
-        test('requires content: the empty-object result no longer parses (deliberate flip)', () => {
-            // BEHAVIOR MIGRATION (Q1 increment 2, ledgered): content.default([])
-            // was removed from the wire schema (the T6 silent-empty-success
-            // masking root). Content is spec-required in every revision.
-            // Changeset: codec-split-wire-break.
-            expect(CallToolResultSchema.safeParse({}).success).toBe(false);
+        test('tolerates absent content: the empty-object result parses with content [] (v1 parity restored)', () => {
+            // BEHAVIOR MIGRATION (reversal, ledgered): content.default([]) is
+            // back on the neutral layer + 2025 era; T6 closed at the wire seam.
+            const empty = CallToolResultSchema.safeParse({});
+            expect(empty.success).toBe(true);
+            if (empty.success) {
+                expect(empty.data.content).toEqual([]);
+            }
             const result = CallToolResultSchema.safeParse({ content: [] });
             expect(result.success).toBe(true);
             if (result.success) {

packages/core-internal/test/types/guards.test.ts

@@ -81,6 +81,10 @@ describe('isCallToolResult', () => {
         expect(isCallToolResult({})).toBe(false);
     });
 
+    it('returns false for an explicit content: undefined (the schema default parses it, but the narrowed type requires the array)', () => {
+        expect(isCallToolResult({ content: undefined, structuredContent: { ok: true } })).toBe(false);
+    });
+
     it('returns true for a result with content', () => {
         expect(
             isCallToolResult({

packages/core-internal/test/types/registryPins.test.ts

@@ -22,7 +22,7 @@ import { describe, expect, it } from 'vitest';
 
 // Post-relocation home (Q1 increment-2 step 1): the pinned contents are
 // unchanged — only the module housing the registries moved.
-import { getNotificationSchema, getRequestSchema, getResultSchema } from '../../src/wire/rev2025-11-25/registry';
+import { getNotificationSchema, getRequestSchema, CallToolResultWireSchema, getResultSchema } from '../../src/wire/rev2025-11-25/registry';
 // The 2025 wire schemas are fully self-contained in the era's schema module:
 // every per-method schema the registry serves is a FROZEN 2025-11-25 copy so
 // the public/neutral layer can evolve (e.g. SEP-2106 widening) without
@@ -130,7 +130,9 @@ const EXPECTED_RESULT_SCHEMAS = {
     'resources/read': ReadResourceResultSchema,
     'resources/subscribe': EmptyResultSchema,
     'resources/unsubscribe': EmptyResultSchema,
-    'tools/call': CallToolResultSchema,
+    // The wire-seam wrapper (content-default guard) IS the pinned entry —
+    // identity to the wrapper, which pipes into the plain schema.
+    'tools/call': CallToolResultWireSchema,
     'tools/list': ListToolsResultSchema,
     'sampling/createMessage': CreateMessageResultWithToolsSchema,
     'elicitation/create': ElicitResultSchema,

packages/core-internal/test/types/schemaBoundaryPins.test.ts

@@ -34,6 +34,8 @@ import {
     ListToolsResultSchema as Wire2026ListToolsResultSchema,
     RequestMetaEnvelopeSchema
 } from '../../src/wire/rev2026-07-28/schemas';
+import { getResultSchema as getRev2025ResultSchema } from '../../src/wire/rev2025-11-25/registry';
+import { CallToolResultSchema as Wire2025CallToolResultSchema } from '../../src/wire/rev2025-11-25/schemas';
 import type {
     CallToolResult,
     CompleteResult,
@@ -139,18 +141,17 @@ describe('typed result schemas are loose', () => {
         expect((parsed as Record<string, unknown>).ttlMs).toBe(5);
     });
 
-    test('CallToolResult requires content on the wire (the silent-empty-success default is gone)', () => {
-        // BEHAVIOR MIGRATION (Q1 increment 2, ledgered): `content.default([])`
-        // was removed from the wire schema. The default was the T6 width-leak
-        // root: a task-shaped (or otherwise content-less) body parsed as a
-        // silent `{content: []}` success. Content is required by the spec in
-        // every revision; a content-less body now fails the parse LOUDLY.
-        // Changeset: codec-split-wire-break; docs/migration/upgrade-to-v2.md
-        // "Wire tightening (every era)".
-        expect(CallToolResultSchema.safeParse({ structuredContent: { ok: true } }).success).toBe(false);
-        const parsed = CallToolResultSchema.parse({ content: [], structuredContent: { ok: true } });
+    test('CallToolResult tolerates absent content on the wire (defaults to [], v1 parity)', () => {
+        // BEHAVIOR MIGRATION (reversal, ledgered): `content.default([])` was
+        // removed in the codec split (T6 width-leak root: a task-shaped body
+        // parsed as a silent success), then RESTORED for ecosystem parity —
+        // real deployments omit `content` alongside `structuredContent`. The
+        // T6 leak stays closed at the 2025 wire-seam schema; 2026 stays strict.
+        const parsed = CallToolResultSchema.parse({ structuredContent: { ok: true } });
         expect(parsed.content).toEqual([]);
         expect(parsed.structuredContent).toEqual({ ok: true });
+        const explicit = CallToolResultSchema.parse({ content: [], structuredContent: { ok: true } });
+        expect(explicit.content).toEqual([]);
     });
 
     test('CallToolResult preserves isError and sibling members through the parse', () => {
@@ -167,6 +168,15 @@ describe('typed result schemas are loose', () => {
     });
 });
 
+describe('2025 wire layering: era file spec-strict, era seam tolerant', () => {
+    test('the era-schema file rejects a content-less CallToolResult body; the registry seam defaults it', () => {
+        // Layering rule: era-schema files stay spec-verbatim (the CallToolResult twin requires content); the seam's v1-parity tolerance defaults CallToolResult-family bodies only — foreign-family results (task/inputRequests/requestState) are never defaulted.
+        expect(Wire2025CallToolResultSchema.safeParse({ structuredContent: {} }).success).toBe(false);
+        const seamParsed = getRev2025ResultSchema('tools/call')!.parse({ structuredContent: {} }) as { content: unknown };
+        expect(seamParsed.content).toEqual([]);
+    });
+});
+
 describe('completion result boundary', () => {
     test('the completion object is loose: unknown sibling fields are preserved', () => {
         const parsed = CompleteResultSchema.parse({ completion: { values: ['alpha'], extraField: 'kept' } });

packages/core-internal/test/types/specTypeSchema.test.ts

@@ -90,20 +90,20 @@ describe('isSpecType', () => {
         }
     });
 
-    it('CallToolResult requires content at the boundary (the wire default was removed)', () => {
-        // BEHAVIOR MIGRATION (Q1 increment 2, ledgered): CallToolResultSchema
-        // lost `content.default([])` — the silent-empty-success masking root.
-        // The guard's input shape now requires content, matching the spec in
-        // every revision. Changeset: codec-split-wire-break.
+    it('CallToolResult tolerates absent content at the boundary (default restored, v1 parity)', () => {
+        // BEHAVIOR MIGRATION (reversal, ledgered): the guard accepts a
+        // content-less body as v1's did; the task-husk leak is closed at the
+        // 2025 wire-seam schema instead.
         const empty: unknown = {};
-        expect(isSpecType.CallToolResult(empty)).toBe(false);
+        expect(isSpecType.CallToolResult(empty)).toBe(true);
         const v: unknown = { content: [] };
         expect(isSpecType.CallToolResult(v)).toBe(true);
         if (isSpecType.CallToolResult(v)) {
-            expectTypeOf(v.content).toEqualTypeOf<ContentBlock[]>();
-            expectTypeOf(v.content).not.toEqualTypeOf<ContentBlock[] | undefined>();
+            // The guard narrows to the INPUT type: content optional pre-parse.
+            expectTypeOf(v.content).toEqualTypeOf<ContentBlock[] | undefined>();
         }
-        void (0 as unknown as CallToolResult);
+        // The parsed/public type keeps content required (z.output).
+        expectTypeOf<CallToolResult['content']>().toEqualTypeOf<ContentBlock[]>();
     });
 
     it('JSONValue / JSONObject — narrows to the JSON type, not unknown', () => {

packages/core-internal/test/wire/eraGates.test.ts

@@ -574,27 +574,19 @@ describe('T6 width-leak killed at both roots', () => {
         expect(decoded.kind).toBe('invalid');
     });
 
-    test('2025 era: with the content default gone, a bare task-shaped body fails the plain schema loudly', async () => {
+    test('2025 era: a bare task-shaped body fails the wire-seam schema — the content default cannot mask it', async () => {
         const { rev2025Codec } = await import('../../src/wire/rev2025-11-25/codec');
-        const { CallToolResultSchema } = await import('../../src/wire/rev2025-11-25/schemas');
         const decoded = rev2025Codec.decodeResult('tools/call', { task: { taskId: 't-1', status: 'working' } });
+        // Decode passes bodies without resultType through (explicit-schema
+        // task interop must work); the wire-seam schema does the refusing.
         expect(decoded.kind).toBe('complete');
-        if (decoded.kind === 'complete') {
-            // The plain schema (which IS the registry entry — the result map
-            // is aligned to the typed map, no task-widened union): no
-            // default([]) means no silent {content: []} masking.
-            expect(CallToolResultSchema.safeParse(decoded.result).success).toBe(false);
-        }
-        // The GENERIC path agrees: the registry serves the same plain schema,
-        // so even a fully conforming CreateTaskResult body is a loud schema
-        // failure (surfaced as a typed INVALID_RESULT — see
-        // test/shared/typedMapAlignment.test.ts). Task interop is the
-        // explicit-schema overload, never a silent union member.
+        // The wire-seam schema rejects even a fully conforming
+        // CreateTaskResult on the plain path (typed INVALID_RESULT); task
+        // interop is the explicit-schema overload only.
         const { getResultSchema } = await import('../../src/wire/rev2025-11-25/registry');
-        const plain = getResultSchema('tools/call');
-        expect(plain).toBe(CallToolResultSchema);
+        const wireSeam = getResultSchema('tools/call');
         expect(
-            plain!.safeParse({
+            wireSeam!.safeParse({
                 task: {
                     taskId: '786af6b0-2779-48ed-9cc1-b8a8a25b8a86',
                     status: 'working',
@@ -605,5 +597,7 @@ describe('T6 width-leak killed at both roots', () => {
                 }
             }).success
         ).toBe(false);
+        // While a plain content-less tool result still defaults:
+        expect(wireSeam!.safeParse({ structuredContent: { ok: true } }).success).toBe(true);
     });
 });

packages/server/src/server/server.ts

@@ -52,6 +52,7 @@ import {
     missingClientCapabilities,
     MissingRequiredClientCapabilityError,
     modernProtocolVersions,
+    normalizeContentlessToolResult,
     parseSchema,
     Protocol,
     ProtocolError,
@@ -524,7 +525,12 @@ export class Server extends Protocol<ServerContext> {
                 return result;
             }
 
-            const validationResult = codec.validateResult('tools/call', result);
+            // v1-parity authoring affordance, era-independent: a content-less
+            // handler result normalizes to content: [] before era validation.
+            // Other result families' bodies stay un-normalized and fail loudly.
+            const normalizedResult = normalizeContentlessToolResult(result);
+
+            const validationResult = codec.validateResult('tools/call', normalizedResult);
             if (!validationResult.ok) {
                 throw new ProtocolError(
                     validationResult.reason === 'not-in-era' ? ProtocolErrorCode.InternalError : ProtocolErrorCode.InvalidParams,