#20968: fix(ui): enable save button when editing default agent models

## Summary - Problem: When an agent is not explicitly listed in agents.list (relying on agents.defaults), the UI handlers for model and fallback changes returned early, failing to mark the configuration as "dirty." - Why it matters: Users with minimal or single-agent configurations were forced to use the "Config" tab to persist model changes, as the Agents overview UI was effectively read-only for the default agent. - What changed: Updated onModelChange and onModelFallbacksChange in app-render.ts to correctly target agents.defaults.model when the default agent is edited. The fix also ensures that updates to defaults are always wrapped in a { primary: ... } object to comply with the stricter Zod schema for AgentDefaults. - What did NOT change (scope boundary): Handlers for Tools and Skills were not changed because agents.defaults does not support these field overrides; they still require an explicit entry in agents.list. ## Change Type (select all) - [x] Bug fix - [ ] Feature - [ ] Refactor - [ ] Docs - [ ] Security hardening - [ ] Chore/infra ## Scope (select all touched areas) - [ ] Gateway / orchestration - [ ] Skills / tool execution - [ ] Auth / tokens - [ ] Memory / storage - [ ] Integrations - [ ] API / contracts - [x] UI / DX - [ ] CI/CD / infra ## Linked Issue/PR - Closes #20924 ## User-visible / Behavior Changes Users can now edit the Primary model and Fallbacks for the default agent in the /agents sidebar and successfully save those changes to their configuration. ## Security Impact (required) - New permissions/capabilities? (`No`) - Secrets/tokens handling changed? (`No`) - New/changed network calls? (`No`) - Command/tool execution surface changed? (`No`) - Data access scope changed? (`No`) ## Repro + Verification ### Environment - OS: Linux (WSL2) / Windows - Runtime/container: Native Node.js - Model/provider: Any - Integration/channel (if any): N/A - Relevant config: agents.defaults populated, agents.list empty. ### Steps 1. Navigate to /agents in the Control UI. 2. Select the default/main agent from the sidebar. 3. In the Overview panel, change the value of "Primary model (default)" or edit "Fallbacks". 4. Observe the "Save" button. ### Expected - The "Save" button should become enabled immediately. - Clicking "Save" should persist the change to agents.defaults.model in the configuration file. ### Actual - Before this fix, the "Save" button remained disabled and changes were lost on page refresh. ## Evidence Verified via code audit that the update correctly targets agents.defaults.model as an object { primary: modelId } to satisfy AgentDefaultsSchema (found in src/config/zod-schema.agent-defaults.ts). ## Human Verification (required) - Verified scenarios: Editing both Primary Model and Fallbacks individually and together for the default agent. - Edge cases checked: Verified that if the agent is in agents.list, the handler still correctly targets the specific list entry index instead of defaults. - What you did not verify: I did not verify automatic persistence on mobile/TUI environments, as this is a dashboard-specific fix. ## Compatibility / Migration - Backward compatible? (`Yes`) - Config/env changes? (`No`) - Migration needed? (`No`) ## Failure Recovery (if this breaks) - How to disable/revert this change quickly: Revert the changes to ui/src/ui/app-render.ts - Files/config to restore: None. - Known bad symptoms reviewers should watch for: If the object wrapping logic were missing, the backend would reject the save with a Zod validation error. ## Risks and Mitigations - Risk: Potential invalid configuration if the target path for agents.defaults didn't exist in the form state. - Mitigation: The code includes checks for configValue and agentsList.defaultId existence before attempting the update. <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixed Save button in `/agents` UI for default agent model changes. Previously, editing the primary model or fallbacks for the default agent (when not explicitly listed in `agents.list`) would return early without marking the configuration as dirty, leaving the Save button disabled. The handlers now check if the agent matches `state.agentsList?.defaultId` and correctly target `agents.defaults.model` with proper object wrapping (`{ primary: modelId }`) to comply with the AgentDefaults Zod schema. - Both `onModelChange` and `onModelFallbacksChange` handlers now support default agent editing - Wraps model values in `{ primary, fallbacks? }` object when updating defaults - Preserves existing behavior for agents explicitly listed in `agents.list` <h3>Confidence Score: 4/5</h3> - Safe to merge with minor consideration for edge case verification - The fix correctly addresses the described problem by adding fallback logic to target `agents.defaults.model` when the agent isn't in the list. The object wrapping logic properly aligns with the strict Zod schema requirements. However, there's a minor edge case: the code assumes `state.agentsList?.defaultId` exists when falling through to the default path - if `defaultId` is undefined, the code would still return early, which is correct. The fix is well-structured and maintains backward compatibility. - No files require special attention _{Last reviewed commit: 1488ddf} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->