Expose exp_assignments injection on session create/resume across all SDKs

[ellismg]

What

Adds an internal/trusted-integrator option to inject ExP ("flight") assignment data into session create and resume, forwarded to the wire key expAssignments and omitted entirely when unset. The payload is opaque JSON in the exact shape of CopilotExpAssignmentResponse (matching the runtime contract and each SDK's already-generated SessionOpenOptions.expAssignments field — no typed schema struct introduced).

Implemented consistently across all six SDKs. Per-language surface:

• Rust — #[doc(hidden)] pub fn with_exp_assignments(serde_json::Value) -> Self + #[doc(hidden)] pub exp_assignments: Option<serde_json::Value> on SessionConfig / ResumeSessionConfig; forwarded through the hand-written SessionCreateWire / SessionResumeWire (skip_serializing_if = "Option::is_none").
• Node — @internal expAssignments?: Record<string, unknown> on SessionConfigBase; forwarded in the inline session.create / session.resume payloads in client.ts.
• Python — exp_assignments: dict[str, Any] | None = None kwarg on create_session / resume_session; mapped to payload["expAssignments"] when set.
• Go — ExpAssignments any (documented Internal:) on SessionConfig / ResumeSessionConfig; forwarded into the create/resume wire structs with json:"expAssignments,omitempty".
• .NET — [EditorBrowsable(Never)] JsonElement? ExpAssignments on SessionConfigBase; wired through the internal CreateSessionRequest / ResumeSessionRequest records ([JsonPropertyName("expAssignments")], null auto-omitted).
• Java — JsonNode expAssignments field + fluent setExpAssignments / getExpAssignments on SessionConfig / ResumeSessionConfig; mapped through CreateSessionRequest / ResumeSessionRequest in SessionRequestBuilder (@JsonProperty("expAssignments"), @JsonInclude(NON_NULL)).

Why

Lets out-of-process integrators — the GitHub Copilot desktop app — inject ExP flight data over JSON-RPC, mirroring the runtime contract added in github/copilot-agent-runtime#9955. The runtime already feeds supplied expAssignments through the same FeatureFlagService.setExpAssignments() path as CLI-fetched flights, stamps exp_assignment_context on telemetry, sets the CAPI header X-Copilot-Exp-Assignment-Context, and is non-blocking / fail-open when absent. Injection is supported on both create and resume.

Each SDK's generated SessionOpenOptions already reflects the field, but the create/resume paths serialize hand-written config + wire structs, so an out-of-process consumer had no way to send it. The desktop app is the first out-of-process consumer (starting with the Rust SDK); in-process callers already had the capability. Adding it to every SDK keeps the surface consistent per review feedback. Part of the github/github-app ExP onboarding epic #7452.

Internal posture

The field/builder are marked internal in each language's idiom (Rust #[doc(hidden)], Node @internal, Go Internal: doc, .NET [EditorBrowsable(Never)], Java doc note; Python documents it as a trusted-integrator option) to mirror upstream's .asInternal() posture — a trusted-integrator API, not broadly advertised public surface.

Validation

All six SDKs add create + resume serialization tests asserting expAssignments is emitted on both paths and omitted when unset.

• Rust — cargo +nightly-2026-04-14 fmt --check clean; cargo clippy --all-features --all-targets -- -D warnings clean; cargo test --all-features --lib → 166 passed.
• Node — npm run typecheck clean; new unit tests pass.
• Python — new tests pass; ruff check / ruff format --check clean.
• Go — gofmt clean; go build ./... + go test ./... pass.
• .NET — 37 serialization tests pass (incl. 2 new).
• Java — 82 SessionRequestBuilderTest tests pass (incl. 2 new); mvn spotless:apply applied.

Also verified the github-app consumer compiles against the synced Rust shape (cargo check -p copilot-tauri green); with_exp_assignments is reachable from the inject call site.

Notes

Opening as a draft pending the paired github-app consumer work. The vendored sync into github-app will follow via the auto-sync agent after merge.

cc @criemen — he asked to be kept in the loop on this.

[stephentoub]

Even if the main known consumer at present is the GH Copilot App, I'd like to see us add whatever we need here consistently across all 6 languages. I'd like to avoid having to keep track of differences in exposed functionality by language. And presumably other languages will end up needing this, too.

[Copilot]

Pull request overview

Adds a trusted-integrator/internal option to inject ExP (“flight”) assignment payloads into session.create and session.resume requests, forwarding to the wire key expAssignments and omitting it when unset, implemented consistently across all SDKs.

Changes:

• Adds expAssignments to create/resume wire payload construction in Rust/Node/Python/Go/.NET/Java.
• Introduces internal-facing config surfaces (builder/field/kwarg/property) for supplying opaque ExP assignment JSON.
• Adds/updates serialization tests in each language to assert presence when set and omission when unset.

Show a summary per file

File	Description
rust/src/wire.rs	Adds exp_assignments to SessionCreateWire/SessionResumeWire with skip_serializing_if.
rust/src/types.rs	Adds internal config fields + builder methods and serialization tests for expAssignments.
python/copilot/client.py	Adds exp_assignments kwargs to create_session/resume_session and maps to payload["expAssignments"].
python/test_client.py	Adds tests asserting expAssignments forwarding and omission in create/resume calls.
nodejs/src/types.ts	Adds @internal expAssignments?: Record<string, unknown> to SessionConfigBase.
nodejs/src/client.ts	Forwards config.expAssignments into session.create/session.resume request payloads.
nodejs/test/client.test.ts	Adds tests asserting forwarding and unset behavior for expAssignments.
go/types.go	Adds ExpAssignments any to session configs and wires it into request structs (json:"expAssignments,omitempty").
go/client.go	Copies config.ExpAssignments into create/resume request payloads.
go/client_test.go	Adds JSON marshal tests for expAssignments presence/omission on create/resume wire structs.
dotnet/src/Types.cs	Adds [EditorBrowsable(Never)] JsonElement? ExpAssignments to SessionConfigBase.
dotnet/src/Client.cs	Wires ExpAssignments through internal create/resume request records with JsonPropertyName("expAssignments").
dotnet/test/Unit/SerializationTests.cs	Adds tests asserting expAssignments is serialized when set and omitted when unset.
java/src/main/java/com/github/copilot/SessionRequestBuilder.java	Propagates expAssignments from configs into create/resume request objects.
java/src/main/java/com/github/copilot/rpc/SessionConfig.java	Adds JsonNode expAssignments with getter/setter and clones it.
java/src/main/java/com/github/copilot/rpc/ResumeSessionConfig.java	Adds JsonNode expAssignments setter and clones it.
java/src/main/java/com/github/copilot/rpc/CreateSessionRequest.java	Adds @JsonProperty("expAssignments") JsonNode expAssignments to create request.
java/src/main/java/com/github/copilot/rpc/ResumeSessionRequest.java	Adds @JsonProperty("expAssignments") JsonNode expAssignments to resume request.
java/src/test/java/com/github/copilot/SessionRequestBuilderTest.java	Adds tests for propagation + JSON omission when expAssignments is null.

Copilot's findings

• Files reviewed: 19/19 changed files
• Comments generated: 1

[ellismg]

Good catch — fixed in e3cbdaa. SessionConfigBase's copy constructor now copies ExpAssignments (right after RemoteSession), so both SessionConfig.Clone() and ResumeSessionConfig.Clone() preserve it.

I audited every config copy/clone/normalize path across all 6 SDKs, since the original tests only checked serialization from a freshly-built config and never exercised a copy path:

• .NET — had the gap (this fix). Grepped the other copy sites (SessionConfig/ResumeSessionConfig derived copy ctors only forward their own fields and chain : base(other)), so the base ctor was the one place to fix.
• Java — already safe: SessionConfig.clone() and ResumeSessionConfig.clone() both assign expAssignments.
• Rust — already safe: both configs #[derive(Clone)] with no hand-written Clone impl, so all fields (incl. exp_assignments) are copied.
• Node — already safe: createSession/resumeSession normalize via { ...configDefaultsForMode(), ...config }, and object spread carries all own properties including expAssignments.
• Go — already safe: create/resume assign req.ExpAssignments = config.ExpAssignments directly off the config pointer (no struct-copy in between).
• Python — already safe: exp_assignments is an explicit kwarg mapped straight into payload["expAssignments"]; no intermediate dict that could drop it.

Regression coverage added for the three SDKs with explicit Clone()/clone() methods — each sets expAssignments, clones, and asserts it survives and is forwarded on the resulting create/resume request:

• .NET: SessionConfigClone_PreservesExpAssignments, ResumeSessionConfigClone_PreservesExpAssignments — these fail against the pre-fix copy constructor (verified by reverting the one-line fix: both red, then green again).
• Java: testClonePreservesAndForwardsExpAssignments.
• Rust: session_config_clone_preserves_exp_assignments, resume_session_config_clone_preserves_exp_assignments.
• Node's existing forward test already drives the spread-normalization copy path via createSession.

Re: the two CodeQL findings on this file — they flagged my new tests calling the deprecated single-arg buildCreateRequest(SessionConfig). Switched both to the current buildCreateRequest(SessionConfig, String) overload, so no new CodeQL debt. (The deprecated single-arg form is a pre-existing pattern used ~30 times elsewhere in this test file; I only touched the lines I added.)

Full 6-language validation green post-fix: Rust fmt/clippy + exp tests, Node typecheck + tests, Python tests + ruff, Go gofmt/build/tests, .NET 42 serialization tests, Java 83 SessionRequestBuilderTest + spotless.

[github-actions]

Cross-SDK Consistency Review ✅

Reviewed the expAssignments feature across all six SDKs. The implementation is consistent and well-coordinated.

Coverage

All six SDKs implement the feature on both session.create and session.resume paths:

SDK	Config field/param	Wire key	Omit when absent	Tests
TypeScript	expAssignments?: Record<string, unknown> on SessionConfigBase	expAssignments	✅ (undefined stripped by JSON.stringify)	✅
Python	exp_assignments: dict[str, Any] | None = None param	expAssignments	✅ (explicit if ... is not None guard)	✅
Go	ExpAssignments any on SessionConfig/ResumeSessionConfig	expAssignments	✅ (omitempty)	✅
.NET	JsonElement? ExpAssignments on SessionConfigBase	expAssignments	✅ (serializer null-ignore config)	✅
Java	JsonNode expAssignments on SessionConfig/ResumeSessionConfig	expAssignments	✅ (class-level @JsonInclude(NON_NULL))	✅
Rust	Option<Value> on SessionConfig/ResumeSessionConfig	expAssignments	✅ (skip_serializing_if = "Option::is_none")	✅

Consistency Observations

• Wire key: All SDKs use expAssignments consistently ✅
• Naming conventions: Each SDK follows its language idiom (expAssignments, exp_assignments, ExpAssignments) ✅
• Null omission: All SDKs correctly omit the field from the wire payload when unset ✅
• "Internal" marking: Each SDK uses the appropriate language-level mechanism (@internal, [EditorBrowsable(Never)], #[doc(hidden)], // Internal: doc, doc note) — the Java and Python approaches rely on documentation rather than compiler/IDE enforcement, which is expected given language limitations ✅
• Clone/copy handling: All SDKs with config objects that support cloning (SessionConfig.clone(), ResumeSessionConfig.clone(), .NET copy constructor) properly propagate expAssignments ✅
• Test coverage: Every SDK adds serialization tests asserting the field is emitted when set and omitted when unset, for both create and resume paths ✅

No cross-SDK consistency issues found. The implementation achieves feature parity across all six SDKs as described in the PR.

Generated by SDK Consistency Review Agent for issue #1750 · sonnet46 1.5M · ◷