fix(core): deep-merge user model config over defaults

[AriaZhao-coder]

Problem

The Config constructor merges the user's modelConfigServiceConfig into DEFAULT_MODEL_CONFIGS using shallow spreads plus a ?? DEFAULT fallback for aliases/overrides. Because DEFAULT_MODEL_CONFIGS is deeply nested (aliases → modelConfig → generateContentConfig → …), any partial user override obliterates the default model aliases.

This was a self-admitted // HACK in config.ts, guarded by TODO(12593) — whose tracking issue is now closed as stale.

Fixes #28264.

Fix

Replace the hack with a proper recursive merge by reusing the module's existing genericDeepMerge, exposed via a new typed ModelConfigService.mergeConfigs(base, override):

• Object maps (aliases, modelDefinitions, modelIdResolutions, classifierIdResolutions, modelChains) are merged recursively, so partial user overrides augment the defaults instead of dropping them.
• Arrays (overrides, customOverrides) are replaced wholesale, so a user can still fully override them (matching the documented genericDeepMerge behavior).
• A missing user config returns the defaults unchanged.

The config.ts hydration block shrinks from ~40 lines of manual re-stitching to a single mergeConfigs call, and the stale TODO(12593) is removed.

Testing

• New unit tests for ModelConfigService.mergeConfigs (partial merge preserves defaults, nested deep-merge, key-add, array replace/keep, undefined override, no input mutation).
• Updated the existing config-hydration tests to assert the fixed (merge, not obliterate) behavior.
• Lint and tsc clean for the changed files.

[github-actions]

📊 PR Size: size/M

• Lines changed: 218
• Additions: +164
• Deletions: -54
• Files changed: 4

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a long-standing issue where partial user model configurations would inadvertently overwrite default model aliases and definitions. By implementing a proper recursive merge strategy, the system now correctly augments default settings with user-provided overrides, improving the flexibility and reliability of the configuration hydration process. This change simplifies the core configuration logic and removes technical debt associated with previous workarounds.

Highlights

• Deep Merge Implementation: Replaced the manual shallow-merge hack in the Config constructor with a robust recursive merge using a new ModelConfigService.mergeConfigs method.
• Preservation of Defaults: Ensured that partial user configurations now augment default model aliases and definitions instead of obliterating them.
• Array Handling: Maintained the existing behavior where array-based configurations (like overrides) are replaced wholesale, allowing users to fully override default lists.
• Testing: Added comprehensive unit tests for mergeConfigs to verify deep-merging, key preservation, and immutability of the base configuration.

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[gemini-code-assist]

Code Review

This pull request replaces a manual configuration merging hack with a robust ModelConfigService.mergeConfigs method that deep-merges user-provided model configurations over built-in defaults, preserving default aliases and nested structures. Comprehensive unit tests have been added to verify this behavior. The reviewer identified a critical issue where nested properties of the global DEFAULT_MODEL_CONFIGS could be mutated by reference because genericDeepMerge does not deep-clone the base object. It is recommended to use structuredClone(base) before merging to prevent accidental mutation of global defaults.

_{Note: Security Review did not run due to the size of the PR.}

[gemini-code-assist]

Issue: Potential Shared Reference Mutation of Global Defaults

Because genericDeepMerge only recursively merges keys that are present in both objects, any nested properties in base (such as default aliases) that are not overridden by override are copied directly by reference.

Since base is passed as DEFAULT_MODEL_CONFIGS (which is a module-level global constant), any subsequent mutation of the merged configuration (e.g., by the Gemini SDK or runtime overrides) will propagate back and mutate the global DEFAULT_MODEL_CONFIGS object. This can lead to extremely subtle bugs, memory leaks, or flaky tests in concurrent environments.

Solution

Deep-clone the base configuration using structuredClone before merging to completely decouple the merged result from the global defaults.

Suggested change

	static mergeConfigs(
	base: ModelConfigServiceConfig,
	override: ModelConfigServiceConfig | undefined,
	): ModelConfigServiceConfig {
	return ModelConfigService.genericDeepMerge(
	// eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
	base as Record<string, unknown>,
	// eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
	override as Record<string, unknown> | undefined,
	) as ModelConfigServiceConfig;
	}
	static mergeConfigs(
	base: ModelConfigServiceConfig,
	override: ModelConfigServiceConfig | undefined,
	): ModelConfigServiceConfig {
	return ModelConfigService.genericDeepMerge(
	// eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
	structuredClone(base) as Record<string, unknown>,
	// eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
	override as Record<string, unknown> | undefined,
	) as ModelConfigServiceConfig;
	}