Configuration Deep-Merge Failure Causes Loss of Default Model Aliases

[aniruddhaadak80]

Description

The core settings hydration logic currently fails to recursively deep-merge user overrides with the application's default configurations. If a user defines any custom generation setting (e.g., just overrides) in their settings.json, it entirely obliterates the default model aliases. A brittle, manual hack was introduced to forcibly re-inject DEFAULT_MODEL_CONFIGS as a fallback.

Location: packages/core/src/config/config.ts#L1141-L1155

Why it's important

While the hack patches the current symptom, it leaves the configuration system vulnerable to future regressions as more nested configuration options are added. The initialization logic should be refactored to use a robust deep-merge utility (like lodash/merge or a custom recursive implementation) rather than relying on shallow destruction and manual re-stitching.

[gemini-cli]

Effort Analysis:
In packages/core/src/config/config.ts (specifically the hydration logic around lines 1141-1155), the implementation currently employs shallow spread operators to merge DEFAULT_MODEL_CONFIGS with user-defined settings. This architectural pattern fails for nested configuration structures such as modelDefinitions and overrides, causing user overrides to obliterate the application's default aliases. The root cause is the absence of a recursive merging mechanism in the core config hydration path. The fix involves refactoring this specific block to use a robust deep-merge utility, which is a localized, single-file change fitting the 'effort/small' guideline for trivial logic and configuration fixes.

[gemini-cli]

This issue describes a bug in the configuration hydration logic that prevents recursive deep-merging of settings, leading to the loss of default model aliases. Please provide your CLI version.

[AriaZhao-coder]

I've opened #28364 with a fix for this: it replaces the shallow-merge // HACK block in Config (the one guarded by the now-closed TODO(12593)) with a proper recursive deep-merge, reusing the existing genericDeepMerge via a new ModelConfigService.mergeConfigs. Partial user configs now augment the defaults instead of obliterating the default aliases; arrays like overrides are still replaced wholesale. Includes unit tests plus updated hydration tests.

Re: the CLI-version question — this is a code-level bug evident in config.ts on main (the // HACK comment explicitly documents the obliteration), so it's independent of CLI version.

Since this issue isn't labeled help wanted, the PR is at risk of being auto-closed. Given it's already a triaged kind/bug / priority/p2 / effort/small, could a maintainer add help wanted so the fix can be reviewed? Thanks!