Agent Behaviour: a real home for character, replacing three hardcoded style heuristics

[serge-ivo]

Why

An agent's character is configurable nowhere. Three separate failures prove it:

1. Guardrails is unreachable. topicRestrictions, blockedTerms, responseStyle,
   maxResponseLength, requireCitations are stored per-instance and injected into the prompt
   (agent-do-prompt.ts:86) — and there are zero references to guardrails anywhere in
   store/console/src. It has never been editable by anyone.
2. Style is GUESSED, not declared. agent-think.ts:403:

   const technical = repoChatStyle || hasCodingContext;   // "has repos ⇒ you want jargon"

   plus two hardcoded rules with no opt-out: "Never output step-by-step thinking" and
   "Reply in MAX 2 sentences".
3. The agent improvised. Asked to be less technical, the FWS Repo Coder wrote
   preference:response_style into Memory — because there was no proper home. Same failure
   shape as it inventing a pipeline named "coding" when it had no tool to start work.

Memory is for subject-matter knowledge. Agent character is not knowledge.

Scope

lib/agent-behaviour.ts — pure, no Env, the single source of truth for the field set. UI, prompt
and tools all derive from it rather than restating it.

• BEHAVIOUR_FIELDS — declarative schema (id, type, options/range, default, band descriptions).
• sanitizeBehaviour(raw) — clamps + allowlists; unknown keys dropped.
• resolveBehaviour(templateDefault, instanceOverride) — creator seeds a default in
  agents.config.behaviour, subscriber overrides in agent_instances.config.behaviour.
• behaviourPrompt(b) — the load-bearing part. Sliders become language, never numbers.
  technicality: 70 must render as "Assume senior-engineer familiarity; cite real files and
  functions" — not "Technicality: 70/100", which models ignore or overfit. ~4 bands per slider.

Then replace the three heuristics above with the resolved values, keeping today's behaviour as
the default so nothing changes for an agent that has set nothing.

Routes: GET/PUT/DELETE /v1/instances/:id/behaviour. Storage is JSON in existing config columns
— no migration.

Field set

Style — technicality (slider→bands) · verbosity (brief/balanced/thorough) · tone
(casual/neutral/formal) · warmth (matter-of-fact ↔ friendly; orthogonal to formality) · reply language

Reasoning — show its working (overrides the hardcoded no-step-by-step) · hedging (flag
uncertainty ↔ commit) · ask-vs-assume (clarify when ambiguous ↔ assume and proceed)

Formatting — prose/markdown/headings+tables · emoji · code examples (none/on-request/liberal)

Interaction — proactivity (answer only ↔ suggest next steps) · ends-with-a-question · what to
call me · free-text persona (the catch-all)

Guardrails (existing, finally reachable) — stay on topic · never say · cite sources ·
max length

Not in scope

• Temperature/creativity — dropped. High temperature measurably degrades tool-calling, and the
  benefit is speculative. Not worth a knob that breaks Coders.
• Rules & Tips stays in Knowledge. Deliberate: standing rules are about the subject matter
  ("always check the migration head"), behaviour is about the agent. Different things, different
  homes.
• Account-level behaviour defaults. Unlike voice ([docs] Update Coder docs for runner-node picker, engines, and package paths #211), character is legitimately per-agent — a
  Coder should be technical where a Language Buddy should not.

Verification

• Pure unit tests on behaviourPrompt: every slider band emits prose containing no digits from
  the raw value; empty behaviour reproduces today's prompt byte-for-byte (regression guard on
  the three heuristics).
• Mutation-check: break the band mapping and the "no numbers in prompt" test must fail.

[serge-ivo]

Closing as implemented on main. Evidence: commit fb33f89 adds the behaviour model as configuration, and current code has workers/api/src/lib/agent-behaviour.ts, workers/api/src/routes/instances-behaviour.ts, prompt integration in workers/api/src/agent-think.ts, plus tests in workers/api/src/lib/agent-behaviour.test.ts.

[serge-ivo]

Implementation links: no associated PR was found; this landed direct on main in fb33f89.