Ports 41 new commits from the official github/copilot-sdk reference implementation into the Java SDK, updating the API to match the latest feature set.

Before the change?

• SessionConfig/ResumeSessionConfig had no way to override model capabilities, control sub-agent streaming events, or enable config discovery
• McpServers typed as Map — no type safety
• CustomAgentConfig had no skills field for per-agent skill preloading
• ProviderConfig had no headers field; MessageOptions had no requestHeaders for per-turn headers
• CopilotSession.setModel() had no overload accepting capability overrides
• .lastmerge pointed to c3fa6cbfb83d4a20b7912b1a17013d48f5a277a1

After the change?

New types (hand-authored in json package — not candidates for java.ts codegen):

• McpServerConfig — abstract base with typed subclasses McpStdioServerConfig (local/stdio) and McpHttpServerConfig (remote HTTP/SSE); McpServers is now Map. The mcpServers field is absent from api.schema.json, so these cannot be generated.
• ModelCapabilitiesOverride — per-property overrides for model capabilities (vision, reasoning effort, token limits). While modelCapabilities appears in api.schema.json under session.model.switchTo.params, the codegen already generates it as the verbose inner record SessionModelSwitchToParamsModelCapabilities. A hand-authored class is required because session.create and session.resume are not in the schema, and the codegen has no mechanism to emit standalone named types shared across hand-authored request classes.

API additions on SessionConfig / ResumeSessionConfig:

• setModelCapabilities(ModelCapabilitiesOverride) — deep-merged over runtime defaults
• setIncludeSubAgentStreamingEvents(Boolean) — suppress/expose streaming deltas from sub-agents
• setEnableConfigDiscovery(Boolean) — auto-discover MCP configs and skill dirs from working dir

Other additions:

• CustomAgentConfig.setSkills(List) — preload named skill content into the agent's context
• ProviderConfig.setHeaders(Map) — custom headers on outbound provider requests; imports reordered to place java.* before com.* per codebase convention
• MessageOptions.setRequestHeaders(Map) — per-turn headers on model requests
• CopilotSession.setModel(String, String, ModelCapabilitiesOverride) — new overload; now correctly maps both supports and limits from ModelCapabilitiesOverride into the generated params (including Integer→Double conversion for limit fields), so token limit overrides are properly honored when switching models

Example — disable vision for a session then re-enable on model switch:

var session = client.createSession(new SessionConfig()
    .setModel("claude-sonnet-4.5")
    .setModelCapabilities(new ModelCapabilitiesOverride()
        .setSupports(new ModelCapabilitiesOverride.Supports().setVision(false)))).get();

// Later, re-enable vision
session.setModel("claude-sonnet-4.5", null,
    new ModelCapabilitiesOverride().setSupports(new ModelCapabilitiesOverride.Supports().setVision(true))).get();

Example — per-agent skills with typed MCP server:

var agent = new CustomAgentConfig()
    .setName("my-agent")
    .setSkills(List.of("my-skill"));

var server = new McpStdioServerConfig()
    .setCommand("npx").setArgs(List.of("-y", "@mcp/server")).setTools(List.of("*"));

var config = new SessionConfig()
    .setSkillDirectories(List.of("/path/to/skills"))
    .setCustomAgents(List.of(agent))
    .setMcpServers(Map.of("my-server", server));

Test updates:

• McpAndAgentsTest migrated to typed McpStdioServerConfig API
• SkillsTest — added tests for per-agent skill injection (skills field)
• Removed setModel("fake-test-model") from CopilotSessionTest / ClosedSessionGuardTest — the new CLI validates model availability against the /models list
• .lastmerge updated to 922959f4a7b83509c3620d4881733c6c5677f00c

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• mvn spotless:apply has been run to format the code
• mvn clean verify passes locally

Does this introduce a breaking change?

• Yes
• No

SessionConfig.getMcpServers() / setMcpServers() return type changed from Map to Map. Similarly for ResumeSessionConfig and CustomAgentConfig. Callers using raw maps must migrate to McpStdioServerConfig or McpHttpServerConfig.