diff --git a/packages/opencode/src/acp/agent.ts b/packages/opencode/src/acp/agent.ts
index 867b830cf2..a8eacf835c 100644
--- a/packages/opencode/src/acp/agent.ts
+++ b/packages/opencode/src/acp/agent.ts
@@ -1094,8 +1094,8 @@ export class Agent implements ACPAgent {
 
     const currentModeId = await (async () => {
       if (!availableModes.length) return undefined
-      const defaultAgentName = await AppRuntime.runPromise(AgentModule.Service.use((svc) => svc.defaultAgent()))
-      const resolvedModeId = availableModes.find((mode) => mode.name === defaultAgentName)?.id ?? availableModes[0].id
+      const defaultAgent = await AppRuntime.runPromise(AgentModule.Service.use((svc) => svc.defaultInfo()))
+      const resolvedModeId = availableModes.find((mode) => mode.name === defaultAgent.name)?.id ?? availableModes[0].id
       this.sessionManager.setMode(sessionId, resolvedModeId)
       return resolvedModeId
     })()
@@ -1328,7 +1328,7 @@ export class Agent implements ACPAgent {
     if (!current) {
       this.sessionManager.setModel(session.id, model)
     }
-    const agent = session.modeId ?? (await AppRuntime.runPromise(AgentModule.Service.use((svc) => svc.defaultAgent())))
+    const agent = session.modeId ?? (await AppRuntime.runPromise(AgentModule.Service.use((svc) => svc.defaultInfo()))).name
 
     const parts: Array<
       | { type: "text"; text: string; synthetic?: boolean; ignored?: boolean }
diff --git a/packages/opencode/src/agent/agent.ts b/packages/opencode/src/agent/agent.ts
index c1a644282b..423a513180 100644
--- a/packages/opencode/src/agent/agent.ts
+++ b/packages/opencode/src/agent/agent.ts
@@ -57,6 +57,7 @@ const GeneratedAgent = Schema.Struct({
 export interface Interface {
   readonly get: (agent: string) => Effect.Effect<Info>
   readonly list: () => Effect.Effect<Info[]>
+  readonly defaultInfo: () => Effect.Effect<Info>
   readonly defaultAgent: () => Effect.Effect<string>
   readonly generate: (input: {
     description: string
@@ -333,23 +334,28 @@ export const layer = Layer.effect(
           )
         })
 
-        const defaultAgent = Effect.fnUntraced(function* () {
+        const defaultInfo = Effect.fnUntraced(function* () {
           const c = yield* config.get()
           if (c.default_agent) {
             const agent = agents[c.default_agent]
             if (!agent) throw new Error(`default agent "${c.default_agent}" not found`)
             if (agent.mode === "subagent") throw new Error(`default agent "${c.default_agent}" is a subagent`)
             if (agent.hidden === true) throw new Error(`default agent "${c.default_agent}" is hidden`)
-            return agent.name
+            return agent
           }
           const visible = Object.values(agents).find((a) => a.mode !== "subagent" && a.hidden !== true)
           if (!visible) throw new Error("no primary visible agent found")
-          return visible.name
+          return visible
+        })
+
+        const defaultAgent = Effect.fnUntraced(function* () {
+          return (yield* defaultInfo()).name
         })
 
         return {
           get,
           list,
+          defaultInfo,
           defaultAgent,
         } satisfies State
       }),
@@ -362,6 +368,9 @@ export const layer = Layer.effect(
       list: Effect.fn("Agent.list")(function* () {
         return yield* InstanceState.useEffect(state, (s) => s.list())
       }),
+      defaultInfo: Effect.fn("Agent.defaultInfo")(function* () {
+        return yield* InstanceState.useEffect(state, (s) => s.defaultInfo())
+      }),
       defaultAgent: Effect.fn("Agent.defaultAgent")(function* () {
         return yield* InstanceState.useEffect(state, (s) => s.defaultAgent())
       }),
diff --git a/packages/opencode/src/server/routes/instance/httpapi/handlers/experimental.ts b/packages/opencode/src/server/routes/instance/httpapi/handlers/experimental.ts
index 360daf54a5..9cf668cebb 100644
--- a/packages/opencode/src/server/routes/instance/httpapi/handlers/experimental.ts
+++ b/packages/opencode/src/server/routes/instance/httpapi/handlers/experimental.ts
@@ -79,7 +79,7 @@ export const experimentalHandlers = HttpApiBuilder.group(InstanceHttpApi, "exper
       const list = yield* registry.tools({
         providerID: ctx.query.provider,
         modelID: ctx.query.model,
-        agent: yield* agents.get(yield* agents.defaultAgent()),
+        agent: yield* agents.defaultInfo(),
       })
       return list.map((item) => ({
         id: item.id,
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 15246dac39..b89561d5d1 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1083,8 +1083,8 @@ NOTE: At any point in time through this workflow you should feel free to ask the
     })
 
     const createUserMessage = Effect.fn("SessionPrompt.createUserMessage")(function* (input: PromptInput) {
-      const agentName = input.agent || (yield* agents.defaultAgent())
-      const ag = yield* agents.get(agentName)
+      const agentName = input.agent
+      const ag = agentName ? yield* agents.get(agentName) : yield* agents.defaultInfo()
       if (!ag) {
         const available = (yield* agents.list()).filter((a) => !a.hidden).map((a) => a.name)
         const hint = available.length ? ` Available agents: ${available.join(", ")}` : ""
@@ -1875,7 +1875,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
         yield* bus.publish(Session.Event.Error, { sessionID: input.sessionID, error: error.toObject() })
         throw error
       }
-      const agentName = cmd.agent ?? input.agent ?? (yield* agents.defaultAgent())
+      const agentName = cmd.agent ?? input.agent
 
       const raw = input.arguments.match(argsRegex) ?? []
       const args = raw.map((arg) => arg.replace(quoteTrimRegex, ""))
@@ -1928,7 +1928,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
 
       yield* getModel(taskModel.providerID, taskModel.modelID, input.sessionID)
 
-      const agent = yield* agents.get(agentName)
+      const agent = agentName ? yield* agents.get(agentName) : yield* agents.defaultInfo()
       if (!agent) {
         const available = (yield* agents.list()).filter((a) => !a.hidden).map((a) => a.name)
         const hint = available.length ? ` Available agents: ${available.join(", ")}` : ""
@@ -1952,7 +1952,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
           ]
         : [...templateParts, ...(input.parts ?? [])]
 
-      const userAgent = isSubtask ? (input.agent ?? (yield* agents.defaultAgent())) : agentName
+      const userAgent = isSubtask ? (input.agent ?? (yield* agents.defaultInfo()).name) : agent.name
       const userModel = isSubtask
         ? input.model
           ? Provider.parseModel(input.model)
diff --git a/packages/opencode/test/agent/agent.test.ts b/packages/opencode/test/agent/agent.test.ts
index 9893ee8223..a781a855c3 100644
--- a/packages/opencode/test/agent/agent.test.ts
+++ b/packages/opencode/test/agent/agent.test.ts
@@ -638,6 +638,14 @@ it.instance("defaultAgent returns build when no default_agent config", () =>
   }),
 )
 
+it.instance("defaultInfo returns resolved build agent when no default_agent config", () =>
+  Effect.gen(function* () {
+    const agent = yield* load((svc) => svc.defaultInfo())
+    expect(agent.name).toBe("build")
+    expect(agent.mode).toBe("primary")
+  }),
+)
+
 it.instance(
   "defaultAgent respects default_agent config set to plan",
   () =>
diff --git a/packages/opencode/test/tool/registry.test.ts b/packages/opencode/test/tool/registry.test.ts
index 5ee56300c4..fb4dd31a5f 100644
--- a/packages/opencode/test/tool/registry.test.ts
+++ b/packages/opencode/test/tool/registry.test.ts
@@ -180,7 +180,7 @@ describe("tool.registry", () => {
       const promptTools = yield* registry.tools({
         providerID: ProviderID.opencode,
         modelID: ModelID.make("test"),
-        agent: yield* agents.get(yield* agents.defaultAgent()),
+        agent: yield* agents.defaultInfo(),
       })
       const promptTool = promptTools.find((tool) => tool.id === "sql")
       if (!promptTool) throw new Error("custom sql tool was not returned for prompts")