Implement 4-agent Braintrust system with structured JSON for three parallel agents + synthesizer

- Set default teamSize=4
- Add synthesizer role (agent-4) that outputs structured JSON {final, reasoning, sources}
- Update roleFor/modelFor/synth detection to use agent-4 for synthesis
- Enhance synthesizeDeterministic to parse JSON from synthesizer when present
- Update docs and plugin config
- Use subagent runtime parallelism via existing Promise.all fan-out

Closes braintrust-json-agents task.

Author: Agent User

extensions/braintrust/README.md

@@ -33,8 +33,8 @@ pnpm vitest run extensions/braintrust/src/policy.test.ts extensions/braintrust/s
 ## Default model routing
 - solver (`model`): `gemini-3-flash-preview`
 - critic (`criticModel`): `openai-codex/gpt-5.3-codex`
-- synthesizer (`synthModel`): `gemini-3.1-pro-preview`
 - researcher (`researcherModel`): `grok-4-1-fast-reasoning`
+- synthesizer (`synthModel`): `gemini-3.1-pro-preview` (now agent-4, outputs structured JSON with `{"final": "...", "reasoning": "...", "sources": [...]}`)
 
 
 Note: plugin id is `braintrust-plugin`; command remains `/braintrust`.

extensions/braintrust/openclaw.plugin.json

@@ -12,7 +12,7 @@
         "type": "integer",
         "minimum": 1,
         "maximum": 4,
-        "default": 3
+        "default": 4
       },
       "strategy": {
         "type": "string",

extensions/braintrust/src/runtime-bridge.ts

@@ -3,7 +3,7 @@ import type { BraintrustSettings } from "./settings.js";
 import { synthesizeDeterministic, type SynthesisOutput } from "./synth.js";
 
 export type CandidateRunnerInput = {
-  role: "solver" | "critic" | "researcher";
+  role: "solver" | "critic" | "researcher" | "synthesizer";
   model: string;
   prompt: string;
   timeoutSeconds: number;
@@ -33,11 +33,12 @@ export type RuntimeBridgeResult = {
 function roleFor(index: number): CandidateRunnerInput["role"] {
   if (index === 1) return "critic";
   if (index === 2) return "researcher";
+  if (index === 3) return "synthesizer";
   return "solver";
 }
 
 function modelFor(index: number, settings: BraintrustSettings): string {
-  if (index === settings.teamSize - 1) return settings.synthModel;
+  if (index === 3) return settings.synthModel;
   if (index === 1) return settings.criticModel;
   if (index === 2) return settings.researcherModel;
   return settings.model;
@@ -92,7 +93,9 @@ export async function runRuntimeBridge(
         runCandidate({
           role: roleFor(i),
           model,
-          prompt: input.prompt,
+          prompt: i === 3 
+            ? `${input.prompt}\n\nRespond ONLY with valid JSON in this exact format (no extra text, no markdown):\n{\n  "final": "the single best synthesized answer",\n  "reasoning": "brief explanation of how you combined the inputs",\n  "sources": ["list of agent ids used"]\n}` 
+            : input.prompt,
           timeoutSeconds: input.settings.timeoutSeconds,
         }),
         input.settings.timeoutSeconds,

extensions/braintrust/src/settings.ts

@@ -17,7 +17,7 @@ export type BraintrustSettings = {
 
 export const DEFAULT_SETTINGS: BraintrustSettings = {
   enabled: false,
-  teamSize: 3,
+  teamSize: 4,
   strategy: "panel",
   model: "gemini-3-flash-preview",
   criticModel: "openai-codex/gpt-5.3-codex",

extensions/braintrust/src/synth.ts

@@ -22,6 +22,24 @@ export function synthesizeDeterministic(input: SynthesisInput): SynthesisOutput
       final: "Braintrust temporarily unavailable (no usable candidate output).",
     };
   }
+
+  // New structured JSON mode for 4-agent system
+  const synthAgent = input.candidates.find((c) => c.id === "agent-4");
+  if (synthAgent?.status === "ok" && synthAgent.text) {
+    try {
+      const parsed = JSON.parse(synthAgent.text.trim());
+      if (parsed.final && typeof parsed.final === "string") {
+        return {
+          final: parsed.final,
+          winnerId: "agent-4",
+        };
+      }
+    } catch (e) {
+      // fallback if not valid JSON
+    }
+  }
+
+  // fallback to shortest successful
   ok.sort((a, b) => {
     const la = (a.text ?? "").length;
     const lb = (b.text ?? "").length;