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

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