feat: support configuring a default_agent across all API/user surfaces (#5843)

Co-authored-by: observerw <observerw@users.noreply.github.com>

github/action.yml

@@ -9,6 +9,10 @@ inputs:
     description: "Model to use"
     required: true
 
+  agent:
+    description: "Agent to use. Must be a primary agent. Falls back to default_agent from config or 'build' if not found."
+    required: false
+
   share:
     description: "Share the opencode session (defaults to true for public repos)"
     required: false
@@ -62,6 +66,7 @@ runs:
       run: opencode github run
       env:
         MODEL: ${{ inputs.model }}
+        AGENT: ${{ inputs.agent }}
         SHARE: ${{ inputs.share }}
         PROMPT: ${{ inputs.prompt }}
         USE_GITHUB_TOKEN: ${{ inputs.use_github_token }}

github/index.ts

@@ -318,6 +318,10 @@ function useEnvRunUrl() {
   return `/${repo.owner}/${repo.repo}/actions/runs/${runId}`
 }
 
+function useEnvAgent() {
+  return process.env["AGENT"] || undefined
+}
+
 function useEnvShare() {
   const value = process.env["SHARE"]
   if (!value) return undefined
@@ -578,16 +582,38 @@ async function summarize(response: string) {
   }
 }
 
+async function resolveAgent(): Promise<string | undefined> {
+  const envAgent = useEnvAgent()
+  if (!envAgent) return undefined
+
+  // Validate the agent exists and is a primary agent
+  const agents = await client.agent.list<true>()
+  const agent = agents.data?.find((a) => a.name === envAgent)
+
+  if (!agent) {
+    console.warn(`agent "${envAgent}" not found. Falling back to default agent`)
+    return undefined
+  }
+
+  if (agent.mode === "subagent") {
+    console.warn(`agent "${envAgent}" is a subagent, not a primary agent. Falling back to default agent`)
+    return undefined
+  }
+
+  return envAgent
+}
+
 async function chat(text: string, files: PromptFiles = []) {
   console.log("Sending message to opencode...")
   const { providerID, modelID } = useEnvModel()
+  const agent = await resolveAgent()
 
   const chat = await client.session.chat<true>({
     path: session,
     body: {
       providerID,
       modelID,
-      agent: "build",
+      agent,
       parts: [
         {
           type: "text",