Applying a reasoning/speed default in Desktop Settings -> Model reset an
auxiliary slot a user had pinned via CLI back to provider "auto" / model ""
while leaving reasoning_effort intact (#95460). POST /api/model/set was
never the writer; writeAgentDefault was: it round-tripped the whole
default-expanded config record (loaded when Settings opened) through
PUT /api/config, so every key another surface changed since the snapshot was
echoed back with its stale, default-filled value. The pinned slot's
provider/model existed only as defaults in the snapshot; reasoning_effort was
already in it, hence the asymmetry the report observed.
PUT /api/config deep-merges onto disk, so a writer only needs to send the key
it changed. Every desktop single-key writer now does exactly that (the
config-settings page already diffed against a baseline): Model defaults
(agent.reasoning_effort / service_tier), Appearance resume_last_session,
terminal font, session auto-archive, the two browser.use_real_profile
toggles, and the Capabilities voice fields (diffConfig against a baseline).
The optimistic shared-cache write keeps the full merged record so sibling
surfaces repaint without a refetch. The dashboard's ReasoningPicker had the
same read-modify-write shape and now sends the sparse patch too.
Tests pin the wire contract: only the edited key is sent, a sibling pin that
is not in the snapshot cannot be echoed back.
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