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docs: cover the remaining Tier 1 audit items (#3325)

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docs/advanced/low-level-server.md
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@ -111,6 +111,17 @@ The `_meta` block is the server's identity stamp: the SDK adds it to every 2026-
The server never compares the two fields. This SDK's `Client` does: return `structured_content` that doesn't satisfy the `output_schema` you declared and `call_tool` raises a `RuntimeError` that starts with `Invalid structured content returned by tool search_books` and goes on to quote the `jsonschema` failure. Promising a schema is cheap; keeping it is on you. The whole ladder of return types and schemas is in **[Structured Output](../servers/structured-output.md)**.
## The dialect is JSON Schema 2020-12
`input_schema` and `output_schema` are JSON Schema, and the [MCP specification](https://modelcontextprotocol.io/specification/latest/basic#json-schema-usage) fixes the dialect: a schema with no `$schema` key is **JSON Schema 2020-12**. The schemas `MCPServer` generates rely on that default (Pydantic writes 2020-12 and omits the key), and a hand-written dict is held to it too, so the full 2020-12 vocabulary is available:
```python title="server.py" hl_lines="8 14-15"
--8<-- "docs_src/lowlevel/tutorial007.py"
```
* The root of `input_schema` must be `"type": "object"`. Beside it, `oneOf`, `additionalProperties`, `anyOf`, `if`/`then`/`else`, `prefixItems`, `$defs` with local `$ref`s and the rest of the 2020-12 keywords reach the client exactly as written.
* No `$schema` key is needed. Add one only to opt into an older draft: this SDK's `Client`, which validates `structured_content` against a tool's `output_schema`, picks its validator from `$schema` and uses 2020-12 when there is none.
## `_meta`: for the application, not the model
`content` is the part of the answer the model reads. `structured_content` is the same answer as typed data. `_meta` is the third channel: data that rides along with the result for the **client application**, without being part of the answer at all.