diff --git a/packages/opencode/src/acp/agent.ts b/packages/opencode/src/acp/agent.ts
index ff71b0453..77aea216b 100644
--- a/packages/opencode/src/acp/agent.ts
+++ b/packages/opencode/src/acp/agent.ts
@@ -475,7 +475,8 @@ export namespace ACP {
           description: agent.description,
         }))
 
-      const currentModeId = availableModes.find((m) => m.name === "build")?.id ?? availableModes[0].id
+      const defaultAgentName = await Agent.getDefault()
+      const currentModeId = availableModes.find((m) => m.name === defaultAgentName)?.id ?? availableModes[0].id
 
       const mcpServers: Record<string, Config.Mcp> = {}
       for (const server of params.mcpServers) {
@@ -577,7 +578,7 @@ export namespace ACP {
       if (!current) {
         this.sessionManager.setModel(session.id, model)
       }
-      const agent = session.modeId ?? "build"
+      const agent = session.modeId ?? (await Agent.getDefault())
 
       const parts: Array<
         { type: "text"; text: string } | { type: "file"; url: string; filename: string; mime: string }
diff --git a/packages/opencode/src/agent/agent.ts b/packages/opencode/src/agent/agent.ts
index 740f67b7e..d387d0954 100644
--- a/packages/opencode/src/agent/agent.ts
+++ b/packages/opencode/src/agent/agent.ts
@@ -183,6 +183,22 @@ export namespace Agent {
     return state().then((x) => x[agent])
   }
 
+  export async function getDefault() {
+    const cfg = await Config.get()
+    const defaultAgent = cfg.default_agent
+
+    if (defaultAgent) {
+      const agents = await state()
+      if (agents[defaultAgent]) {
+        return defaultAgent
+      }
+    }
+
+    // Fallback to build agent if it exists
+    const agents = await state()
+    return agents.build ? "build" : Object.keys(agents)[0]
+  }
+
   export async function list() {
     return state().then((x) => Object.values(x))
   }
diff --git a/packages/opencode/src/cli/cmd/run.ts b/packages/opencode/src/cli/cmd/run.ts
index b646f0b15..b5e94f6ef 100644
--- a/packages/opencode/src/cli/cmd/run.ts
+++ b/packages/opencode/src/cli/cmd/run.ts
@@ -224,7 +224,7 @@ export const RunCommand = cmd({
         await sdk.session.command({
           path: { id: sessionID },
           body: {
-            agent: args.agent || "build",
+            agent: args.agent || (await Agent.getDefault()),
             model: args.model,
             command: args.command,
             arguments: message,
@@ -235,7 +235,7 @@ export const RunCommand = cmd({
         await sdk.session.prompt({
           path: { id: sessionID },
           body: {
-            agent: args.agent || "build",
+            agent: args.agent || (await Agent.getDefault()),
             model: modelParam,
             parts: [...fileParts, { type: "text", text: message }],
           },
diff --git a/packages/opencode/src/config/config.ts b/packages/opencode/src/config/config.ts
index d9c453038..5b6db1ee3 100644
--- a/packages/opencode/src/config/config.ts
+++ b/packages/opencode/src/config/config.ts
@@ -497,6 +497,7 @@ export namespace Config {
         .catchall(Agent)
         .optional()
         .describe("@deprecated Use `agent` field instead."),
+      default_agent: z.string().optional().describe("Default agent to use when no agent is specified"),
       agent: z
         .object({
           plan: Agent.optional(),
diff --git a/packages/opencode/src/session/compaction.ts b/packages/opencode/src/session/compaction.ts
index 0bb949ba9..9b1745e92 100644
--- a/packages/opencode/src/session/compaction.ts
+++ b/packages/opencode/src/session/compaction.ts
@@ -164,7 +164,7 @@ export namespace SessionCompaction {
         time: {
           created: Date.now(),
         },
-        agent: "build",
+        agent: await Agent.getDefault(),
         model: input.model,
       })
       await Session.updatePart({
@@ -197,7 +197,7 @@ export namespace SessionCompaction {
         role: "user",
         model: input.model,
         sessionID: input.sessionID,
-        agent: "build",
+        agent: await Agent.getDefault(),
         time: {
           created: Date.now(),
         },
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 6b77feb37..e7443853b 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -782,7 +782,8 @@ export namespace SessionPrompt {
   }
 
   async function createUserMessage(input: PromptInput) {
-    const agent = await Agent.get(input.agent ?? "build")
+    const agentName = input.agent ?? (await Agent.getDefault())
+    const agent = await Agent.get(agentName)
     const info: MessageV2.Info = {
       id: input.messageID ?? Identifier.ascending("message"),
       role: "user",
@@ -1289,7 +1290,7 @@ export namespace SessionPrompt {
   export async function command(input: CommandInput) {
     log.info("command", input)
     const command = await Command.get(input.command)
-    const agentName = command.agent ?? input.agent ?? "build"
+    const agentName = command.agent ?? input.agent ?? (await Agent.getDefault())
 
     const raw = input.arguments.match(argsRegex) ?? []
     const args = raw.map((arg) => arg.replace(quoteTrimRegex, ""))
@@ -1428,7 +1429,7 @@ export namespace SessionPrompt {
               time: {
                 created: Date.now(),
               },
-              agent: input.message.info.role === "user" ? input.message.info.agent : "build",
+              agent: input.message.info.role === "user" ? input.message.info.agent : await Agent.getDefault(),
               model: {
                 providerID: input.providerID,
                 modelID: input.modelID,