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docs: document v1 ZodObject-vs-raw-shape failure modes #2845

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7 changes: 7 additions & 0 deletions docs/migration/upgrade-to-v2.md
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Expand Up @@ -659,6 +659,13 @@ are still **accepted via `@deprecated` overloads** on `registerTool`/`registerPr
(auto-wrapped with `z.object()`), and `completable()` accepts any `StandardSchemaV1`;
prefer wrapping explicitly. Zod v4, ArkType, and Valibot all implement the spec.

> **Do not apply the v2 style backwards on v1:** on SDK v1, `server.tool()`
> (and `registerTool` before 1.22) expects the **raw shape**, not `z.object(...)`.
> A `ZodObject` there crashes `tools/list` on v1 ≤ 1.21 and silently publishes
> an empty schema (arguments stripped) via `server.tool()` on v1 ≤ 1.26.
> If you backport v2 examples to a v1 server, unwrap to the raw shape — see
> [Troubleshooting](../troubleshooting.md#mcp-error--32603-cannot-read-properties-of-null-reading-_def).

For **optional completable arguments**, apply `.optional()` to the _result_ of
`completable()` — `completable(z.string(), cb).optional()`, not
`completable(z.string().optional(), cb)`. v2 resolves completion metadata on the schema
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17 changes: 17 additions & 0 deletions docs/troubleshooting.md
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Expand Up @@ -167,6 +167,23 @@ The Resource Server helpers did not move there: `requireBearerAuth`, `mcpAuthMet

HTTP SSE streams emit a `: keepalive` comment every 15 seconds by default so client body-idle timeouts and intermediaries do not terminate an otherwise idle connection. Configure the interval with `keepAliveMs` on the transport or `createMcpHandler`; set it to `0` to disable heartbeats.

## `MCP error -32603: Cannot read properties of null (reading '_def')`

On SDK v1, `server.tool()` (and `registerTool` before 1.22) expects a **Zod raw shape** — `{ name: z.string() }` — not a `ZodObject`. Passing `z.object({ name: z.string() })` fails without ever naming the mistake:

- v1 ≤ 1.21: `tools/list` crashes with this error.
- v1 ≤ 1.26 via `server.tool()`: no error anywhere, but the published schema is an empty `{"type":"object"}` — clients strip every argument and the handler receives none.
- v1 ≥ 1.28: registration throws a clear error, and `registerTool` has normalized `z.object()` since 1.22.

Fix the call site — unwrap to the raw shape:

```diff
- server.tool('greet', 'desc', z.object({ name: z.string() }), handler);
+ server.tool('greet', 'desc', { name: z.string() }, handler);
```

On v2 the direction reverses: `registerTool` takes schema objects (`z.object(...)`), with raw shapes only on deprecated overloads — see the [v1→v2 migration guide](./migration/upgrade-to-v2.md#standard-schema-objects-raw-shapes-deprecated).

## Recap

- Every heading on this page is the exact message you searched for.
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