Go codegen: per-event-type data structs

[stephentoub]

Fixes #1031.

Problem

The Go codegen for session events used quicktype, which represented the discriminated union of all session event Data payloads as a single flat struct with every property from every event type merged together — all as optional pointers. Adding a new event type with a same-named-but-different-typed property silently changed the shared field's type, breaking all existing consumers.

Solution

Replace quicktype-based session events generation with custom codegen that produces per-event-type data structs, matching the C# codegen approach. Each event type gets its own data struct (e.g., SessionStartData, AssistantMessageData), and SessionEvent.Data is now a SessionEventData interface accessed via type switches:

switch d := event.Data.(type) {
case *copilot.AssistantMessageData:
    fmt.Println(d.Content)
case *copilot.SessionErrorData:
    fmt.Println(d.Message)
}

Adding event type B never changes the struct shape of event type A.

Changes

• scripts/codegen/go.ts — Replaced generateSessionEvents() with custom codegen producing 74 per-event data structs, SessionEventData interface, custom UnmarshalJSON/MarshalJSON on SessionEvent, and RawSessionEventData for forward-compatible handling of unknown event types.
• go/generated_session_events.go — Regenerated output with per-event structs, shared nested types, enums, and type/const aliases for backward compatibility (PermissionRequest, Attachment, AttachmentType*, PermissionRequestKind*).
• go/session.go — Updated handleBroadcastEvent() and SendAndWait() to use idiomatic Go type switches. Updated doc examples.
• go/samples/chat.go — Updated to use type switches.
• go/session_test.go — Updated to construct events with per-event data structs.
• E2E tests (10 files) — Updated all event.Data.* access patterns to use type assertions.
• go/rpc/generated_rpc.go — Minor renames from adding ui, uri, mime to Go initialisms (Ui→UI, Uri→URI — correct Go convention).
• .github/workflows/codegen-check.yml — Removed the git checkout -- go/generated_session_events.go workaround.
• .gitattributes — Added eol=lf for generated files to prevent cross-platform line ending diffs.

Breaking changes

• event.Data is now a SessionEventData interface instead of a flat Data struct. Consumers must use type switches or type assertions to access per-event fields.
• RPC field renames: SessionRpc.Ui → SessionRpc.UI, UiApi → UIApi, FormatUri → FormatURI (correct Go initialism convention).

Testing

• go build ./... — clean
• go vet ./... — clean
• Unit tests — all pass
• E2E tests — compile clean (runtime requires test harness)

[Copilot]

Pull request overview

This PR fixes Go SDK breakages caused by quicktype’s “flat union” Data struct by switching Go session-event codegen to a custom generator that emits per-event-type *XxxData structs behind a SessionEventData interface, aligning Go with the C# approach and making additions of new event types additive-only.

Changes:

• Replaced Go session-events quicktype generation with custom codegen producing per-event data structs plus custom SessionEvent (un)marshaling and forward-compatible RawSessionEventData.
• Updated Go SDK runtime code, samples, README docs, and tests/E2E to use type assertions / type switches on event.Data.
• Updated Go RPC generated types to follow Go initialism casing (UI, URI, MIME) and re-enabled codegen-check determinism (plus .gitattributes LF enforcement).

Show a summary per file

File	Description
scripts/codegen/go.ts	Implements custom Go session-events codegen (per-event structs, interface payloads, custom JSON (un)marshal).
go/generated_session_events.go	Regenerated Go session-events types with per-event payload structs and SessionEventData interface.
go/session.go	Updates event handling and examples to use type switches/assertions; updates RPC UI casing usage.
go/session_test.go	Updates unit tests to construct events with new per-event data structs.
go/samples/chat.go	Updates sample to read event payloads via type assertions.
go/rpc/generated_rpc.go	Renames RPC API/types for Go initialisms (UIApi, FormatURI, SessionRpc.UI).
go/README.md	Updates documentation examples to use type switches on event.Data.
go/client.go	Updates inline usage example to use type assertion on event.Data.
go/internal/e2e/testharness/helper.go	Updates harness helpers to interpret event payloads via type assertions.
go/internal/e2e/tools_test.go	Updates E2E tool tests to use *AssistantMessageData payloads.
go/internal/e2e/tool_results_test.go	Updates tool-results E2E checks to use typed payloads.
go/internal/e2e/streaming_fidelity_test.go	Updates streaming delta/message checks to use typed payloads.
go/internal/e2e/skills_test.go	Updates skill-marker assertions to use typed payloads.
go/internal/e2e/session_test.go	Updates session start/message assertions and helper to extract message from typed payloads.
go/internal/e2e/permissions_test.go	Updates permission/tool-complete checks to use typed payloads.
go/internal/e2e/multi_client_test.go	Updates multi-client assertions for assistant/perms to use typed payloads.
go/internal/e2e/mcp_and_agents_test.go	Updates MCP/agent E2E assertions to use typed payloads.
go/internal/e2e/compaction_test.go	Updates compaction-success/message assertions to use typed payloads.
go/internal/e2e/commands_and_elicitation_test.go	Updates commands/capabilities checks to use typed payloads.
.github/workflows/codegen-check.yml	Removes the prior workaround that excluded go/generated_session_events.go from codegen checks.
.gitattributes	Forces LF for generated files to reduce cross-platform diffs and marks them as generated.

Copilot's findings

• Files reviewed: 20/21 changed files
• Comments generated: 3

[github-actions]

Cross-SDK Consistency Review

This PR is Go-specific, addressing a genuine codegen bug by replacing the quicktype-generated flat Data struct with per-event-type data structs. Here's a cross-SDK consistency assessment:

✅ Aligned with .NET

The new Go approach (SessionEventData interface + per-event structs accessed via type switches) aligns well with the .NET SDK's design, which uses [JsonPolymorphic]-decorated derived types per event. Both SDKs now give consumers type-safe access to event-specific fields without a merged flat struct.

✅ Node.js/TypeScript already type-safe

Node.js uses TypeScript discriminated union types for SessionEvent, so event.data is already typed per-event-type. No consistency gap there.

⚠️ Python has the same underlying quicktype issue

Python's generated/session_events.py still uses a quicktype-generated flat Data class with all event properties merged together — the same pattern this PR fixes in Go. This means:

• SessionEvent.data in Python is a single Data class with every field from every event type (all optional)
• Adding a new event type with a same-named but differently-typed property would silently break Python consumers in the same way it broke Go

This is a pre-existing inconsistency (not introduced by this PR), but worth tracking as a follow-up. The Python SDK would benefit from a similar codegen improvement — either per-event-type dataclasses or a Union discriminated on type.

Summary

The PR improves Go consistency with .NET and is well-scoped. The Python gap is pre-existing and worth addressing in a follow-up issue.

Generated by SDK Consistency Review Agent for issue #1037 · ◷