## Problem
Title generation uses the same model the user selected for chat. This
breaks when:
1. **Thinking/extended thinking models** — `ToolChoice: None` conflicts
with extended thinking on Anthropic. The bare call has no thinking
config, so provider-level defaults can conflict.
2. **Expensive models** — User picks `o3` or `claude-opus-4`, and a
trivial 8-word title generation burns through tokens/cost unnecessarily.
3. **Provider quirks** — Different providers have different constraints
around thinking mode + tool choice combinations.
## Solution
Modeled after how `coder/mux` handles this with
`NAME_GEN_PREFERRED_MODELS` + ordered candidate fallback:
### Phase 1: Candidate model list with fallback
- New `TitleModelFunc` type returns an ordered list of candidate models
- Tries `claude-haiku-4-5` → `gpt-4o-mini` → user's model
- Gracefully skips unavailable candidates (missing API key, provider not
configured)
- Falls back to the user's chat model as last resort
### Phase 2: Provider-safe call options
- Removed `ToolChoice: None` which conflicts with extended thinking on
some providers
- Added `MaxOutputTokens: 256` to cap token usage
- Improved title prompt with verb-noun format guidance (`Fix sidebar
layout`, `Add user authentication`) and explicit
no-markdown/no-code-fences instructions
### Files changed
- `coderd/chatd/title.go` — Candidate loop, improved prompt, safe call
options
- `coderd/chatd/chatd.go` — Build `TitleModelFunc` closure with
lightweight candidates
Kyle Carberry
ec89abd6e5