fix(vscode): allow clearing agent model override by sending null

Clearing the Model Override field for native agents (e.g. Explore,
General) sent the patch as `{ model: undefined }`. JSON.stringify drops
undefined, so the backend received an empty patch, the override stayed
in kilo.json, and the next configUpdated push repopulated the field.

Send `null` instead — the backend's patchJsonc already treats null as
a delete sentinel (matches the pattern already used in ModelsTab and
AutoApproveTab). Fixes #9527.

.changeset/clear-agent-model-override.md

@@ -0,0 +1,5 @@
+---
+"kilo-code": patch
+---
+
+Fix clearing an agent's Model Override in Agent Behaviour settings. Previously, clearing the field and saving would repopulate the old value because the empty input was sent as `undefined` and dropped by `JSON.stringify`, so the backend never received a delete instruction. The field now reverts to the global default model as expected.

packages/kilo-vscode/tests/unit/config-utils.test.ts

@@ -155,4 +155,53 @@ describe("ConfigState", () => {
     expect(s.config.snapshot).toBe(true)
     expect(s.dirty).toBe(false)
   })
+
+  // -------------------------------------------------------------------------
+  // Issue #9527: clearing an agent model override must unset it, not repopulate
+  // -------------------------------------------------------------------------
+  describe("clearing an agent model override (issue #9527)", () => {
+    it("keeps null in the draft so the backend receives a delete sentinel", () => {
+      const s = new ConfigState()
+      s.handleConfigLoaded({ agent: { explore: { model: "anthropic/claude-sonnet-4-20250514" } } })
+
+      // User clears the Model Override field. ModeEditView now sends `null`
+      // instead of `undefined` (the fix). null is the delete sentinel that
+      // patchJsonc maps to jsonc-parser's remove operation.
+      s.updateConfig({ agent: { explore: { model: null } } })
+
+      // Optimistic UI: stripNulls removes the key so the field renders empty.
+      expect(s.config.agent?.explore?.model).toBeUndefined()
+      expect(s.dirty).toBe(true)
+
+      // Draft must retain the null so it survives JSON.stringify on the wire
+      // and reaches patchJsonc as an explicit delete.
+      expect(s.draft.agent?.explore?.model).toBeNull()
+      expect(JSON.parse(JSON.stringify(s.draft))).toEqual({
+        agent: { explore: { model: null } },
+      })
+    })
+
+    it("undefined (the old buggy behavior) is dropped by JSON.stringify", () => {
+      // Reproduction of the pre-fix bug: sending `undefined` results in an
+      // empty patch on the wire, so the backend never deletes the override
+      // and the next configUpdated pushes the stale model back into the UI.
+      const draft = { agent: { explore: { model: undefined } } }
+      expect(JSON.parse(JSON.stringify(draft))).toEqual({ agent: { explore: {} } })
+    })
+
+    it("confirms the save and drops the draft once the backend acks", () => {
+      const s = new ConfigState()
+      s.handleConfigLoaded({ agent: { explore: { model: "anthropic/claude-sonnet-4-20250514" } } })
+      s.updateConfig({ agent: { explore: { model: null } } })
+      s.saveConfig()
+
+      // Backend removed the override and pushes the stripped config back.
+      s.handleConfigUpdated({ agent: { explore: {} } })
+
+      expect(s.config.agent?.explore?.model).toBeUndefined()
+      expect(s.dirty).toBe(false)
+      expect(s.saving).toBe(false)
+      expect(Object.keys(s.draft).length).toBe(0)
+    })
+  })
 })

packages/kilo-vscode/webview-ui/src/components/settings/ModeEditView.tsx

@@ -145,7 +145,7 @@ const ModeEditView: Component<Props> = (props) => {
           <TextField
             value={cfg().model ?? ""}
             placeholder="e.g. anthropic/claude-sonnet-4-20250514"
-            onChange={(val) => update({ model: val || undefined })}
+            onChange={(val) => update({ model: val || null })}
           />
         </SettingsRow>