Add session.extensions.sendAttachmentsToMessage SDK wrapper

[jmoseley]

Regenerates the SDK clients to expose the new session.extensions.sendAttachmentsToMessage RPC endpoint added in copilot-agent-runtime PR https://github.com/github/copilot-agent-runtime/pull/8971.

This lets canvas extensions push attachments — including a new extension_context attachment variant — into the next user message's composer.

What's in this PR

The extension-facing RPC block now exposes:

sendAttachmentsToMessage(params: {
    instanceId?: string;
    attachments: Array<
        | { type: "extension_context"; title: string; payload: unknown }
        | Attachment // any standard variant
    >;
}): Promise<void>;

Plus the wire types for the new extension_context attachment variant on both directions (params + SessionExtensionsAttachmentsPushedEvent event) so the SDK exports typed shapes consumers can rely on.

Call path heads-up (JS): the wrapper lives at session.rpc.extensions.sendAttachmentsToMessage, not session.extensions.sendAttachmentsToMessage. Sibling methods like session.send and session.getEvents are on the session class directly, so this is an easy mistake — worth flagging for anyone writing docs or examples.

Languages regenerated

Language	Status
TypeScript	✅ regenerated, built, tests + lint pass
.NET	✅ regenerated, dotnet build passes
Python	✅ regenerated, unit tests pass
Rust	✅ regenerated, cargo build passes
Java	✅ regenerated (mvn not run locally — please verify in CI)
Go	⚠️ intentionally not regenerated — see below

Go follow-up

The Go codegen at scripts/codegen/go.ts (~line 1840–1855, emitGoFlatDiscriminatedUnion) has a duplicate-method bug that surfaces with this schema: when a leaf type is a variant of two discriminated unions with the same discriminator property (here, SendAttachment and the new PushAttachment both use "type"), the generator emits func (X) Type() FirstUnionEnum and then func (X) Type() SecondUnionEnum, which fails to compile.

Possible fixes (all non-trivial, out of scope here):

• Rename the discriminator method per-union (e.g. PushAttachmentType())
• Share the discriminator enum between unions when values coincide
• Skip per-variant method emission on the second union

I'd suggest landing this PR to unblock the github-app side and tracking the Go regen as a follow-up.

Approach note

To keep the diff minimal and reviewable, I patched the new method/types into the installed @github/copilot schemas directly rather than bumping the dependency — the runtime branch carries ~500 lines of other in-flight changes that aren't part of this work. The patch is additive only and uses the SDK's existing naming conventions (SendAttachment*, UserMessageAttachment*) rather than runtime's Attachment*. When the next @github/copilot release ships, a normal dep bump + regen will absorb this cleanly.

Coordination

Paired session in github-app has verified the new method end-to-end against a hand-patched local runtime build; this PR is the proper replacement for that patch.

[Copilot]

Copilot wasn't able to review any files in this pull request.

[github-actions]

Cross-SDK Consistency Review ✅

All updated SDKs (TypeScript, .NET, Python, Rust, Java) consistently expose:

• SendAttachmentExtensionContext type with extension_context discriminator
• SendAttachmentsToMessageParams / SessionExtensionsSendAttachmentsToMessageRequest wire types
• session.extensions.sendAttachmentsToMessage RPC method

Go SDK gap: The PR explicitly documents the intentional omission of Go due to a codegen bug (emitGoFlatDiscriminatedUnion emitting duplicate Type() methods when a leaf type participates in two discriminated unions). This is a pre-existing limitation in the Go code generator, not an oversight in this PR. The existing Go SDK already has SendAttachment and related types — it just lacks the new extension_context variant and the sendAttachmentsToMessage method.

Suggested follow-up issue for the Go gap (not blocking this PR):

• Fix scripts/codegen/go.ts emitGoFlatDiscriminatedUnion to handle shared leaf types across multiple discriminated unions, then regenerate Go.

Generated by SDK Consistency Review Agent for issue #1502 · ● 2.9M · ◷

[github-actions]

Commit pushed: ca30377

Generated by Java Codegen Agentic Fix · ● 78.5M

[github-actions]

Agentic Fix Applied

The java-codegen-check workflow failed because SessionExtensionsAttachmentsPushedEvent.java was manually added to java/src/generated/ but is not present in the current @github/copilot schema (1.0.55-5). When the codegen regenerated SessionEvent.java from the schema, it omitted this class from the sealed permits clause — but the file still extended SessionEvent, causing a sealed-class compilation error.

Fix

Added cleanGeneratedFilesInDirectory() to java/scripts/codegen/java.ts and called it at the start of generateSessionEvents(). This removes all auto-generated files directly in the session-events package directory before regenerating, ensuring no orphaned event classes can linger and violate the sealed SessionEvent permits clause.

Also removed 4 other stale files (McpServerStatusChangedStatus, McpServersLoadedServerStatus, PermissionCompletedKind, PermissionCompletedResult) that were no longer emitted by the current schema but had accumulated in the repo.

Note: The mvn verify step in java-codegen-check uses JDK 17, but pom.xml now requires JDK 25+ for the virtual-thread overlay. This will need to be addressed separately (either updating the workflow to use JDK 25, or the enforcer check), but is a pre-existing issue unrelated to this PR's code changes.

Warning

Firewall blocked 1 domain

The following domain was blocked by the firewall during workflow execution:

• repo.maven.apache.org

To allow these domains, add them to the network.allowed list in your workflow frontmatter:

network:
  allowed:
    - defaults
    - "repo.maven.apache.org"

See Network Configuration for more information.

Generated by Java Codegen Agentic Fix · ● 78.5M · ◷