Expose session feature flag overrides across SDKs

[jmoseley]

Summary

• expose host-resolved feature flag overrides on create and resume across Node.js, Python, Go, .NET, Java, and Rust
• forward configured values as featureFlags across the JSON-RPC boundary while omitting unset values
• preserve explicit false values and the distinction between an unset map and an explicitly empty map
• add focused create/resume serialization or request-forwarding coverage in every SDK

Testing

• Node.js focused client test and typecheck
• Python focused client test and Ruff checks
• Go focused request serialization test
• .NET serialization tests and formatting check
• Java SessionRequestBuilderTest
• Rust focused serialization test and pinned nightly formatting check

[Copilot]

Copilot review overview

🟢 Approval recommended

The public API, wire serialization, defaults, documentation, and tests consistently implement the stated behavior.

Review tier: Balanced
Findings: None

What changed in this PR

Exposes host-resolved feature flags through Rust session creation and resume configuration.

Changes:

• Adds feature_flags fields and fluent builders.
• Serializes configured flags as featureFlags while omitting unset values.
• Adds focused serialization tests.

File	Description
rust/​src/​wire.rs	Adds optional feature flags to create and resume payloads.
rust/​src/​types.rs	Exposes, forwards, documents, and tests feature flag configuration.

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[github-actions]

Cross-SDK Consistency Review ✅

This PR adds featureFlags support consistently across all six SDK implementations. The feature is implemented in parallel across:

SDK	Config field	Wire key	Builder/fluent method
Node.js/TS	featureFlags?: Record<string, boolean> on SessionConfigBase	featureFlags	direct property
Python	feature_flags: dict[str, bool] | None param on create_session / resume_session	featureFlags	function parameter
Go	FeatureFlags map[string]bool on SessionConfig / ResumeSessionConfig	featureFlags	direct field
.NET	IDictionary<string, bool>? FeatureFlags on SessionConfigBase	featureFlags	direct property
Java	Map<String, Boolean> featureFlags on SessionConfig / ResumeSessionConfig	featureFlags	setFeatureFlags() fluent setter
Rust	feature_flags: Option<HashMap<String, bool>> on SessionConfig / ResumeSessionConfig	featureFlags	with_feature_flags() builder method

Observations:

• All SDKs omit the field from the wire payload when unset (null/None/nil), preserving the distinction between "not set" and "explicitly empty"
• All SDKs use the same JSON wire key featureFlags
• API naming follows each language's conventions (camelCase for TS/Java, snake_case for Python/Rust, PascalCase for Go exported fields)
• Clone/copy semantics are implemented correctly in .NET and Java to avoid aliasing
• All SDKs include focused serialization/forwarding tests

No consistency gaps found. The PR maintains strong cross-SDK parity. 👍

Generated by SDK Consistency Review Agent for #2451 · sonnet46 25.4 AIC · ⌖ 5.46 AIC · ⊞ 6.6K · ◷

[SteveSandersonMS]

Reviewed the expanded six-language implementation. Create/resume forwarding, unset and explicit false semantics, Go empty-map handling, clone behavior, and language-native tests are covered. The full CI matrix passes.