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feat!: enforce required MCP spec fields in McpSchema; lenient wire deserialization (#928)
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>
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‎CONTRIBUTING.md‎

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## Evolving wire-serialized records
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Records in `McpSchema` are serialized directly to the MCP JSON wire format. Follow these rules whenever you add a field to an existing record to keep the protocol forward- and backward-compatible.
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Records in `McpSchema` are serialized directly to the MCP JSON wire format. The rules differ depending on whether the field you are adding (or maintaining) is *optional* — Java code may legitimately leave it `null` and the wire may omit it — or *spec-required* by MCP. Follow **Case A** for optional fields and **Case B** for spec-required fields.
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### Rules
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### Case A — Optional fields
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1. **Add new components only at the end** of the record's component list. Never reorder or rename existing components.
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2. **Annotate every component** with `@JsonProperty("fieldName")` even when the Java name already matches. This survives local renames via refactoring tools.
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3. **Use boxed types** (`Boolean`, `Integer`, `Long`, `Double`) so the field can be absent on the wire without a special sentinel.
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4. **Default to `null`**, not an empty collection or neutral value, so the `@JsonInclude(NON_NULL)` rule omits the field for clients that don't know about it yet.
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4. **Default to `null`**, not an empty collection or neutral value, so the `@JsonInclude(NON_ABSENT)` rule omits the field for clients that don't know about it yet.
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5. **Keep existing constructors as source-compatible overloads** that delegate to the new canonical constructor and pass `null` for the new component. Do not remove them in the same release that adds the field.
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6. **Do not put `@JsonCreator` on the canonical constructor** unless strictly necessary. Jackson auto-detects record canonical constructors; adding `@JsonCreator` pins deserialization to that exact parameter order forever.
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7. **Do not convert `null` to a default value in the canonical constructor.** Null carries "absent" semantics and must be preserved through the serialization round-trip.
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6. **Do not put `@JsonCreator` on the canonical constructor** unless strictly necessary. Jackson auto-detects record canonical constructors; adding `@JsonCreator` pins deserialization to that exact parameter order forever. *(For records that also have spec-required fields, the `@JsonCreator` belongs on a separate static `fromJson` factory — see Case B, Rule 2.)*
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7. **Do not convert `null` to a default value in the canonical constructor.** Null carries "absent" semantics and must be preserved through the serialization round-trip. *(Spec-required fields are the exception — see Case B, Rule 1.)*
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8. **Add three tests per new field** (put them in the relevant test class in `mcp-test`):
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- Deserialize JSON *without* the field → succeeds, field is `null`.
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- Serialize an instance with the field unset (`null`) → the key is absent from output.
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- Deserialize JSON with an extra *unknown* field → succeeds.
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9. **An inner `Builder` subclass can be used.** This improves the developer experience since frequently not all fields are required.
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9. **An inner `Builder` subclass can be used.** This improves the developer experience since frequently not all fields are required.
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### Case B — Spec-required fields
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When the MCP specification marks a field as required, callers must not be able to construct a structurally invalid record, but the wire parser must still tolerate peers that fail to send it. Follow these rules in addition to the relevant Case A rules (annotation, naming, append-only).
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1. **Reject `null` in the compact constructor.** Use `Assert.notNull` for required objects or `Assert.hasText` for required `String` identifiers (`name`, `uri`, `uriTemplate`, `version`). This throws `IllegalArgumentException` at construction time instead of producing a record that fails later in serialization or protocol handling. Overrides Case A Rule 7 for this field.
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2. **Add a `@JsonCreator` static `fromJson` factory** alongside the canonical constructor. When a required field is absent from the wire, substitute a documented safe default (`""` for strings, `[]` for collections, `{}` for maps, `0` / `0.0` for numerics, `INFO` for `LoggingLevel`, etc.) and log at `WARN` naming the field and the value used. The SDK must not halt the conversation because of a missing field. Place `@JsonCreator` on this `fromJson` factory, never on the canonical constructor (Case A Rule 6 still applies to the canonical constructor itself).
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- Exception: `JSONRPCResponse.JSONRPCError` fails fast on missing `code` / `message` because a malformed JSON-RPC error envelope is unrecoverable.
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3. **Provide a required-first builder factory** `builder(req1, req2, …)` and remove the corresponding setters from the `Builder`. A no-arg `builder()` factory must not exist on a record that has required fields. If one already exists for source compatibility, mark it `@Deprecated`.
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4. **Add tests per required field**:
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- Constructing the record with `null` for the field throws `IllegalArgumentException`.
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- Deserializing JSON *without* the field succeeds and yields the documented default.
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- Deserializing JSON with an extra *unknown* field still succeeds (Case A Rule 8 also applies).
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### Example
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‎MIGRATION-2.0.md‎

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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.

‎conformance-tests/client-jdk-http-client/src/main/java/io/modelcontextprotocol/conformance/client/ConformanceJdkClientMcpClient.java‎

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HttpClientStreamableHttpTransport transport = HttpClientStreamableHttpTransport.builder(serverUrl).build();
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return McpClient.sync(transport)
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.clientInfo(new McpSchema.Implementation("test-client", "1.0.0"))
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.clientInfo(McpSchema.Implementation.builder("test-client", "1.0.0").build())
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.requestTimeout(Duration.ofSeconds(30))
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}
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var capabilities = McpSchema.ClientCapabilities.builder().elicitation().build();
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return McpClient.sync(transport)
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.clientInfo(new McpSchema.Implementation("test-client", "1.0.0"))
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.clientInfo(McpSchema.Implementation.builder("test-client", "1.0.0").build())
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.capabilities(capabilities)
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.elicitation(request -> {
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arguments.put("b", 3);
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‎conformance-tests/client-spring-http-client/src/main/java/io/modelcontextprotocol/conformance/client/scenario/DefaultScenario.java‎

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.requestTimeout(Duration.ofSeconds(30))
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‎conformance-tests/client-spring-http-client/src/main/java/io/modelcontextprotocol/conformance/client/scenario/PreRegistrationScenario.java‎

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.clientInfo(McpSchema.Implementation.builder("test-client", "1.0.0").build())
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.requestTimeout(Duration.ofSeconds(30))
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.build();
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