feat: add model field to CustomAgentConfig across all SDKs

[patniko]

Summary

Add an optional model property to CustomAgentConfig in all five SDKs (Node/TS, Python, Go, .NET, Rust). This closes the gap between the runtime's SweCustomAgent.model support and the SDK's public API.

Motivation

The runtime already supports per-subagent model selection via YAML frontmatter in .github/agents/*.md files, but the programmatic SDK API (customAgents array in session config) did not expose this capability. Users who define agents programmatically could not specify which model an agent should use.

Changes

Each SDK gets its own commit for easy review:

• Node/TS (nodejs/src/types.ts): model?: string on CustomAgentConfig interface — passed through directly to wire format
• Python (python/copilot/session.py + client.py): model: NotRequired[str] on CustomAgentConfig TypedDict + wire format conversion
• Go (go/types.go): Model string field with json:"model,omitempty" tag
• .NET (dotnet/src/Types.cs): string? Model property with [JsonPropertyName("model")]
• Rust (rust/src/types.rs): model: Option<String> field + with_model() builder method

Behavior

When set, the runtime will attempt to use the specified model for the agent, falling back to the parent session model if unavailable. When omitted/null, the agent inherits the session's model (existing behavior, no breaking change).

Validation

All five SDKs compile successfully: npx tsc --noEmit, python3 -c "import copilot", go build ./..., dotnet build, cargo check --all-features.

[Copilot]

Pull request overview

Adds support for per-custom-agent model selection across all SDKs by exposing an optional model field on CustomAgentConfig and ensuring Python maps it into the wire payload.

Changes:

• Added model to custom agent config types in Node/TS, Python, Go, .NET, and Rust.
• Added Python wire-format forwarding for model.
• Added a Rust with_model() builder method.

Show a summary per file

File	Description
rust/src/types.rs	Adds optional serialized model field and builder method.
python/copilot/session.py	Adds model to the CustomAgentConfig TypedDict.
python/copilot/client.py	Forwards model into custom agent wire format.
nodejs/src/types.ts	Adds model to the TypeScript custom agent interface.
go/types.go	Adds JSON-serialized Model field to Go custom agent config.
dotnet/src/Types.cs	Adds JSON-serialized nullable Model property to .NET custom agent config.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 0

[github-actions]

✅ Cross-SDK Consistency Review

This PR adds the model field to CustomAgentConfig across all five SDK implementations. The changes are consistent and well-aligned:

SDK	Type	JSON key	Optional behavior
Node.js/TS	model?: string	"model"	Omitted when undefined
Python	model: NotRequired[str]	"model"	Omitted from wire format when absent
Go	Model string + omitempty	"model"	Omitted when zero value — consistent with other optional string fields (DisplayName, Description) in the same struct
.NET	string? Model	"model"	Null-safe; Model = other.Model is included in the SessionConfig copy constructor
Rust	pub model: Option<String> + with_model() builder	"model"	skip_serializing_if = "Option::is_none" — consistent with existing builder pattern (with_display_name, with_skills, etc.)

Documentation is uniform across all SDKs describing the runtime fallback behavior.

Tests cover both "field present" and "field absent/omitted" cases in every SDK.

No inconsistencies found. 🎉

Generated by SDK Consistency Review Agent for issue #1309 · ● 440.9K · ◷