22a0efa577
Every wire-serialized record in McpSchema now validates spec-required
fields at construction time. Wire deserialization is intentionally
lenient: a missing required field is replaced with a documented default
and a WARN is logged, instead of failing the parse. A required-first
builder convention is introduced across the schema so it is no longer
possible to obtain a builder that is missing required state.
Required-field guards
---------------------
Every wire record's compact constructor asserts non-null (and non-empty
for String identifiers like name, uri, uriTemplate, version) on its
spec-required components. Passing null now throws
IllegalArgumentException at construction time instead of silently
producing invalid JSON via @JsonInclude(NON_ABSENT). This applies to
the JSON-RPC envelopes, lifecycle types, resource/prompt/tool requests
and results, sampling and elicitation, content records, root, complete,
logging and progress notifications, and the two CompleteReference
implementations. See MIGRATION-2.0.md for the full list.
Lenient wire deserialization
----------------------------
For each of those records (except JSONRPCResponse.JSONRPCError, which
still fails fast) a @JsonCreator static `fromJson` factory substitutes
a documented default for any absent required field — "" for strings,
[] for collections, {} for maps, 0 / 0.0 for numerics, INFO for
LoggingLevel, USER for SamplingMessage.role, ASSISTANT for
CreateMessageResult.role, CANCEL for ElicitResult.action, {values=[]}
for CompleteResult.completion — and logs a WARN naming the field and
the value used. Application code can still observe a malformed message
and react, but the SDK no longer halts the conversation.
Builder convention
------------------
Records that have a builder gain a required-first factory method
`builder(req1, req2, …)`; setters for required fields are removed from
the builder so it cannot be left in an invalid state. Existing no-arg
`builder()` factories and required-field setters are kept where source
compatibility demands it but are marked @Deprecated. New builders are
also added for several records that previously had none (ProgressNotification,
JSONRPCError, CompleteRequest, list/result types, content records, ...).
JSON-RPC envelope ergonomics
----------------------------
Previously every JSON-RPC envelope had to be constructed via the
canonical record constructor and the literal "2.0" string had to be
threaded through every call site, e.g.
new JSONRPCRequest("2.0", "tools/call", id, params)
new JSONRPCResponse("2.0", id, result, null)
new JSONRPCResponse("2.0", id, null, new JSONRPCError(code, message, null))
Now:
new JSONRPCRequest("tools/call", id, params) // jsonrpc defaulted
new JSONRPCNotification("notifications/initialized") // params optional
JSONRPCResponse.result(id, result) // factory
JSONRPCResponse.error(id, new JSONRPCError(code, message)) // 2-arg error
JSONRPCResponse's compact constructor additionally enforces the
JSON-RPC invariant that exactly one of `result` / `error` is set —
previously the SDK could build envelopes that violated the protocol.
CompleteReference changes
-------------------------
- PromptReference.equals/hashCode now key on `name` only (previously
derived from identifier()+type()). Two refs with the same name but
different titles now compare equal — code using PromptReference as a
map/set key should be audited.
- PromptReference's compact constructor pins `type` to "ref/prompt"
and logs a WARN if the caller supplies a different non-null value.
The legacy two-arg `PromptReference(String type, String name)`
constructor remains @Deprecated.
- ResourceReference's record components are reduced from (type, uri)
to (uri) — positional construction breaks. The legacy
`ResourceReference(String type, String uri)` constructor stays
@Deprecated and ignores `type`.
- CompleteReference.identifier() is @Deprecated and now returns null
via a default method on the interface.
Tests / refactor
----------------
Conformance harness, integration tests, sample apps, and internal
callers were migrated to the new builder factories and convenience
constructors. No semantic changes to client/server runtime behaviour
beyond the McpSchema changes above.
Docs
----
- MIGRATION-2.0.md: required-field section now covers the broader
record set and the lenient-deserialization behaviour; PromptReference
WARN behaviour and ResourceReference component reduction are
documented; the JSON-RPC envelope section is rewritten to compare the
pre-2.0 surface with the new one.
- CONTRIBUTING.md: the "Evolving wire-serialized records" recipe is
split into two cases — adding a new optional field (existing rules)
and adding/maintaining a spec-required field (new rules covering
Assert in compact constructor, @JsonCreator fromJson with defaults
and WARN, required-first builder factory).
Signed-off-by: Dariusz Jędrzejczyk <2554306+chemicL@users.noreply.github.com>
175 lines
14 KiB
Markdown
175 lines
14 KiB
Markdown
# Migration Guide — 2.0
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This document covers breaking and behavioural changes introduced in the 2.0 release of the MCP Java SDK.
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---
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## Jackson / JSON serialization changes
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### Sealed interfaces removed
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The following interfaces were `sealed` in 1.x and are now plain interfaces in 2.0:
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- `McpSchema.JSONRPCMessage`
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- `McpSchema.Request`
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- `McpSchema.Result`
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- `McpSchema.Notification`
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- `McpSchema.ResourceContents`
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- `McpSchema.CompleteReference`
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- `McpSchema.Content`
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**Impact:** Exhaustive `switch` expressions or `switch` statements that relied on the sealed hierarchy for completeness checking must add a `default` branch. The compiler will no longer reject switches that omit one of the known subtypes.
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### `CompleteReference` now carries `@JsonTypeInfo`
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`CompleteReference` (and its implementations `PromptReference` and `ResourceReference`) is now annotated with `@JsonTypeInfo(use = NAME, include = EXISTING_PROPERTY, property = "type", visible = true)`. Jackson will automatically dispatch to the correct subtype based on the `"type"` field in the JSON without any hand-written map-walking code.
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**Action:** Remove any custom code that manually inspected the `"type"` field of a completion reference map and instantiated `PromptReference` / `ResourceReference` by hand. A plain `mapper.readValue(json, CompleteRequest.class)` or `mapper.convertValue(paramsMap, CompleteRequest.class)` is sufficient.
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### `Prompt` canonical constructor no longer coerces `null` arguments
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In 1.x, `new Prompt(name, description, null)` silently stored an empty list for `arguments`. In 2.0 it stores `null`.
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**Action:**
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- Code that expected `prompt.arguments()` to return an empty list when not provided will now receive `null`. Add a null-check or use the new `Prompt.withDefaults(name, description, arguments)` factory, which preserves the old behaviour by coercing `null` to `[]`.
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- On the wire, a prompt without an `arguments` field deserializes with `arguments == null` (it is not coerced to an empty list).
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### `CompleteCompletion` optional fields omitted when null
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`CompleteResult.CompleteCompletion.total` and `CompleteCompletion.hasMore` are now omitted from serialized JSON when they are `null` (previously they were always emitted). Deserializers that required these fields to be present in every response must be updated to treat their absence as `null`.
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### `CompleteCompletion.values` is mandatory in the Java API
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The compact constructor for `CompleteCompletion` asserts that `values` is not `null`. Code that constructed a completion result with a null `values` list will now fail at runtime.
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**Action:** Always pass a non-null list (for example `List.of()` when there are no suggestions).
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### `LoggingLevel` deserialization is lenient
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`LoggingLevel` now uses a `@JsonCreator` factory (`fromValue`) so that JSON string values deserialize in a case-insensitive way. **Unrecognized level strings deserialize to `null`** instead of causing deserialization to fail.
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**Impact:** `SetLevelRequest`, `LoggingMessageNotification`, and any other type that embeds `LoggingLevel` can observe a `null` level when the wire value is unknown or misspelled. Downstream code must null-check or validate before use.
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### `Content.type()` is ignored for Jackson serialization
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The `Content` interface still exposes `type()` as a convenience for Java callers, but the method is annotated with `@JsonIgnore` so Jackson does not treat it as a duplicate `"type"` property alongside `@JsonTypeInfo` on the interface.
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**Impact:** Custom serializers or `ObjectMapper` configuration that relied on serializing `Content` through the default `type()` accessor alone should use the concrete content records (each of which carries a real `"type"` property) or the polymorphic setup on `Content`.
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### `ServerParameters` no longer carries Jackson annotations
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`ServerParameters` (in `client/transport`) has had its `@JsonProperty` and `@JsonInclude` annotations removed. It was never a wire type and is not serialized to JSON in normal SDK usage. If your code serialized or deserialized `ServerParameters` using Jackson, switch to a plain map or a dedicated DTO.
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### Record annotation sweep
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Wire-oriented `public record` types in `McpSchema` consistently use `@JsonInclude(JsonInclude.Include.NON_ABSENT)` (or equivalent per-type configuration) and `@JsonIgnoreProperties(ignoreUnknown = true)`. Nested capability objects under `ClientCapabilities` / `ServerCapabilities` (for example `Sampling`, `Elicitation`, `CompletionCapabilities`, `LoggingCapabilities`, prompt/resource/tool capability records) also ignore unknown JSON properties. This means:
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- **Unknown fields** in incoming JSON are silently ignored, improving forward compatibility with newer server or client versions.
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- **Absent optional properties** are omitted from outgoing JSON where `NON_ABSENT` applies, and optional Java components deserialize as `null` when missing on the wire.
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### `Tool.inputSchema` is `Map<String, Object>`, not `JsonSchema`
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The `Tool` record now models `inputSchema` (and `outputSchema`) as arbitrary JSON Schema objects as `Map<String, Object>`, so dialect-specific keywords (`$ref`, `unevaluatedProperties`, vendor extensions, and so on) round-trip without being trimmed by a narrow `JsonSchema` record.
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**Impact:**
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- Java code that used `Tool.inputSchema()` as a `JsonSchema` must switch to `Map<String, Object>` (or copy into your own schema wrapper).
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- `Tool.Builder.inputSchema(JsonSchema)` remains as a **deprecated** helper that maps the old record into a map; prefer `inputSchema(Map)` or `inputSchema(McpJsonMapper, String)`.
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### Required MCP spec fields are enforced at construction time
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Every wire record in `McpSchema` whose fields are marked required by the MCP spec now asserts non-null (and non-empty for `String` identifiers like `name`, `uri`, `uriTemplate`, `version`) in its compact constructor. Passing `null` throws `IllegalArgumentException` immediately, instead of producing a structurally invalid object that fails later in serialization or protocol handling.
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This applies to (non-exhaustive):
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- JSON-RPC envelopes: `JSONRPCRequest`, `JSONRPCNotification`, `JSONRPCResponse`, `JSONRPCResponse.JSONRPCError`
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- Lifecycle: `InitializeRequest`, `InitializeResult`, `Implementation`
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- Resources: `Resource`, `ResourceTemplate`, `ListResourcesResult`, `ListResourceTemplatesResult`, `ReadResourceRequest`, `ReadResourceResult`, `SubscribeRequest`, `UnsubscribeRequest`, `ResourcesUpdatedNotification`, `TextResourceContents`, `BlobResourceContents`
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- Prompts: `Prompt`, `PromptArgument`, `PromptMessage`, `ListPromptsResult`, `GetPromptRequest`, `GetPromptResult`
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- Tools: `Tool`, `ListToolsResult`, `CallToolRequest`, `CallToolResult`
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- Sampling / elicitation: `SamplingMessage`, `CreateMessageRequest`, `CreateMessageResult`, `ElicitRequest`, `ElicitResult`
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- Misc: `ProgressNotification`, `SetLevelRequest`, `LoggingMessageNotification`, `CompleteRequest`, `CompleteResult`, `CompleteRequest.CompleteArgument`, content records (`TextContent`, `ImageContent`, `AudioContent`, `EmbeddedResource`), `Root`, `ListRootsResult`, `PromptReference`, `ResourceReference`
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**Action:** Audit any code that constructs these records with potentially-null values and provide valid, non-null arguments.
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**Wire deserialization is lenient.** Records expose a `@JsonCreator fromJson` factory that substitutes safe defaults (e.g. `[]`, `""`, `0`, `INFO`, `Action.CANCEL`) for any absent required field and logs a `WARN` naming the field and the substituted value. `JSONRPCResponse.JSONRPCError` is excluded — malformed JSON-RPC error envelopes still fail immediately.
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**Note:** `LoggingMessageNotification`/`SetLevelRequest` default a *missing* `level` to `INFO`, but an *unrecognized* level string still deserializes to `null` (see the `LoggingLevel` section above) and will then fail the canonical constructor. Ensure clients and servers send only recognized level strings.
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### `PromptReference` discriminator pinning and equality
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`PromptReference` keeps its `(type, name, title)` record components, so positional construction from 1.x still compiles. Two behavioural changes:
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- The compact constructor pins `type` to `ref/prompt`. Any non-null value other than `ref/prompt` is replaced with `ref/prompt` and a `WARN` is logged. The legacy two-arg `PromptReference(String type, String name)` constructor remains `@Deprecated` and routes through the canonical constructor, so it triggers the same WARN.
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- `equals`/`hashCode` now consider `name` only (title and type are ignored). Two refs with the same name but different titles compare equal.
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**Action:** Audit any code that used `PromptReference` as a map key or in a `Set` — equality semantics changed. If your code constructed instances with a custom `type` string for testing, switch to `PromptReference.builder(name)` (or `new PromptReference(name)`); the WARN tells you which call sites still pass the discriminator.
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`CompleteReference.identifier()` is `@Deprecated` and now returns `null` via a default method on the interface.
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### `ResourceReference` record component reduced
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Components changed from `(type, uri)` to `(uri)`. Positional construction with two arguments breaks. The legacy `ResourceReference(String type, String uri)` constructor stays `@Deprecated`; it ignores `type` and logs a `WARN`. Use `new ResourceReference(uri)` or `ResourceReference.builder(uri)`. The `type()` accessor still returns `ref/resource` and Jackson serializes it via `@JsonProperty("type")` on the accessor.
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### Builder API: required-first factories; old setters/no-arg builders deprecated
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Most records that have a builder have gained a required-first factory method (`builder(req1, req2, …)`) and the corresponding setters for required fields are removed from the builder. The old no-arg `builder()` factory and public no-arg `Builder()` constructor are kept but `@Deprecated` where they would allow constructing a builder without required state.
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Examples:
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| Type | Old (deprecated) | New |
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|------|-----------------|-----|
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| `Resource` | `Resource.builder().uri(u).name(n)…` | `Resource.builder(uri, name)…` |
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| `ResourceTemplate` | `ResourceTemplate.builder().uriTemplate(u).name(n)…` | `ResourceTemplate.builder(uriTemplate, name)…` |
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| `Implementation` | `new Implementation(name, version)` | `Implementation.builder(name, version)…` |
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| `InitializeRequest` / `InitializeResult` | `… .builder()…` | `… .builder(protocolVersion, capabilities, clientInfo/serverInfo)` |
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| `Tool` | `Tool.builder().name(n)…` | `Tool.builder(name)…` |
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| `Prompt` / `PromptArgument` / `GetPromptRequest` | `… .builder().name(n)…` | `… .builder(name)…` |
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| `PromptMessage` / `SamplingMessage` | `… .builder().role(r).content(c)…` | `… .builder(role, content)…` |
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| `CreateMessageRequest` | `… .builder().messages(m).maxTokens(n)…` | `… .builder(messages, maxTokens)…` |
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| `ElicitRequest` | `… .builder().message(m).requestedSchema(s)…` | `… .builder(message, requestedSchema)…` |
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| `LoggingMessageNotification` | `… .builder().level(l).data(d)…` | `… .builder(level, data)…` |
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| `ListResourcesResult` / `ListResourceTemplatesResult` / `ListPromptsResult` / `ListToolsResult` / `ListRootsResult` | `… .builder()…` | `… .builder(items)…` |
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| `ReadResourceRequest` / `SubscribeRequest` / `UnsubscribeRequest` / `ResourcesUpdatedNotification` / `Root` | n/a | `… .builder(uri)…` |
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| `ReadResourceResult` | n/a | `ReadResourceResult.builder(contents)…` |
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| `TextResourceContents` / `BlobResourceContents` | n/a | `… .builder(uri, text|blob)…` |
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| `TextContent` / `ImageContent` / `AudioContent` / `EmbeddedResource` | n/a | `… .builder(text \| data, mimeType \| resource)…` |
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| `CallToolResult` | unchanged | also: required-first content set via builder constructor remains optional |
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| `ProgressNotification` | n/a | `ProgressNotification.builder(progressToken, progress)` |
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| `JSONRPCResponse.JSONRPCError` | n/a | `JSONRPCError.builder(code, message)` |
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| `CompleteRequest` | n/a | `CompleteRequest.builder(ref, argument)` |
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| `Annotations` | n/a | `Annotations.builder()` |
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| Capabilities (`Sampling`, `Elicitation`, `Roots`, `LoggingCapabilities`, `CompletionCapabilities`, prompt/resource/tool capabilities) | n/a | `… .builder()…` |
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### JSON-RPC envelope ergonomics
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In 1.x, every envelope was constructed via the canonical record constructor and the literal `"2.0"` `jsonrpc` string had to be threaded through every call site:
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```java
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new JSONRPCRequest("2.0", "tools/call", id, params);
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new JSONRPCNotification("2.0", "notifications/initialized", null);
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new JSONRPCResponse("2.0", id, result, null);
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new JSONRPCResponse("2.0", id, null, new JSONRPCError(code, message, null));
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```
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2.0 adds defaulting constructors and static factories so the `"2.0"` constant and the unused `result`/`error` slot disappear from caller code:
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```java
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new JSONRPCRequest("tools/call", id); // params optional
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new JSONRPCRequest("tools/call", id, params);
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new JSONRPCNotification("notifications/initialized"); // params optional
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new JSONRPCNotification("notifications/initialized", params);
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JSONRPCResponse.result(id, result);
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JSONRPCResponse.error(id, new JSONRPCError(code, message)); // 2-arg error
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```
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`JSONRPCResponse`'s compact constructor additionally enforces the JSON-RPC invariant that exactly one of `result` / `error` is set — previously the SDK could build envelopes that violated the protocol.
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The 1.x canonical 4-arg constructors continue to compile.
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### Optional JSON Schema validation on `tools/call` (server)
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When a `JsonSchemaValidator` is available (including the default from `McpJsonDefaults.getSchemaValidator()` when you do not configure one explicitly) and `validateToolInputs` is left at its default of `true`, the server validates incoming tool arguments against `tool.inputSchema()` before invoking the tool. Failed validation produces a `CallToolResult` with `isError` set and a textual error in the content.
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**Action:** Ensure `inputSchema` maps are valid for your validator, tighten client arguments, or disable validation with `validateToolInputs(false)` on the server builder if you must preserve pre-2.0 behaviour.
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