diff --git a/packages/opencode/src/acp/agent.ts b/packages/opencode/src/acp/agent.ts
index d4d556485da..2155eef1d35 100644
--- a/packages/opencode/src/acp/agent.ts
+++ b/packages/opencode/src/acp/agent.ts
@@ -1007,7 +1007,7 @@ export namespace ACP {
         })
 
       const availableModes = agents
-        .filter((agent) => agent.mode !== "subagent" && !agent.hidden)
+        .filter((agent) => AgentModule.isPrimaryVisible(agent))
         .map((agent) => ({
           id: agent.name,
           name: agent.name,
diff --git a/packages/opencode/src/agent/agent.ts b/packages/opencode/src/agent/agent.ts
index 2b44308f130..8dfc5a6c77f 100644
--- a/packages/opencode/src/agent/agent.ts
+++ b/packages/opencode/src/agent/agent.ts
@@ -247,6 +247,19 @@ export namespace Agent {
     return state().then((x) => x[agent])
   }
 
+  export function isPrimaryVisible(agent: Info) {
+    return agent.mode !== "subagent" && agent.hidden !== true
+  }
+
+  export async function getOrDefault(name?: string): Promise<Info> {
+    const agent = name ? await get(name) : undefined
+    if (agent) return agent
+    const fallback = await defaultAgent()
+    const resolved = await get(fallback)
+    if (!resolved) throw new Error(`Agent not found: ${name}, fallback ${fallback} also missing`)
+    return resolved
+  }
+
   export async function list() {
     const cfg = await Config.get()
     return pipe(
@@ -268,7 +281,7 @@ export namespace Agent {
       return agent.name
     }
 
-    const primaryVisible = Object.values(agents).find((a) => a.mode !== "subagent" && a.hidden !== true)
+    const primaryVisible = Object.values(agents).find((a) => isPrimaryVisible(a))
     if (!primaryVisible) throw new Error("no primary visible agent found")
     return primaryVisible.name
   }
diff --git a/packages/opencode/src/cli/cmd/github.ts b/packages/opencode/src/cli/cmd/github.ts
index 927c964c9d8..856df42dedb 100644
--- a/packages/opencode/src/cli/cmd/github.ts
+++ b/packages/opencode/src/cli/cmd/github.ts
@@ -887,7 +887,8 @@ export const GithubRunCommand = cmd({
             providerID,
             modelID,
           },
-          // agent is omitted - server will use default_agent from config or fall back to "build"
+          // agent is omitted - server will use default_agent from config (must be valid)
+          // or fall back to the first visible primary agent (usually "build")
           parts: [
             {
               id: Identifier.ascending("part"),
diff --git a/packages/opencode/src/cli/cmd/tui/context/local.tsx b/packages/opencode/src/cli/cmd/tui/context/local.tsx
index d058ce54fb3..6ef9e7caaf3 100644
--- a/packages/opencode/src/cli/cmd/tui/context/local.tsx
+++ b/packages/opencode/src/cli/cmd/tui/context/local.tsx
@@ -9,6 +9,7 @@ import { iife } from "@/util/iife"
 import { createSimpleContext } from "./helper"
 import { useToast } from "../ui/toast"
 import { Provider } from "@/provider/provider"
+import { Agent } from "@/agent/agent"
 import { useArgs } from "./args"
 import { useSDK } from "./sdk"
 import { RGBA } from "@opentui/core"
@@ -34,7 +35,7 @@ export const { use: useLocal, provider: LocalProvider } = createSimpleContext({
     }
 
     const agent = iife(() => {
-      const agents = createMemo(() => sync.data.agent.filter((x) => x.mode !== "subagent" && !x.hidden))
+      const agents = createMemo(() => sync.data.agent.filter((x) => Agent.isPrimaryVisible(x)))
       const [agentStore, setAgentStore] = createStore<{
         current: string
       }>({
diff --git a/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx b/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
index 1294ab849e9..ef2f41d076e 100644
--- a/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
+++ b/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
@@ -205,10 +205,12 @@ export function Session() {
     if (part.id === lastSwitch) return
 
     if (part.tool === "plan_exit") {
-      local.agent.set("build")
+      const target = (part.state.metadata as { agent?: string })?.agent ?? "build"
+      local.agent.set(target)
       lastSwitch = part.id
     } else if (part.tool === "plan_enter") {
-      local.agent.set("plan")
+      const target = (part.state.metadata as { agent?: string })?.agent ?? "plan"
+      local.agent.set(target)
       lastSwitch = part.id
     }
   })
diff --git a/packages/opencode/src/config/config.ts b/packages/opencode/src/config/config.ts
index 020e626cba8..380047df59a 100644
--- a/packages/opencode/src/config/config.ts
+++ b/packages/opencode/src/config/config.ts
@@ -931,7 +931,7 @@ export namespace Config {
         .string()
         .optional()
         .describe(
-          "Default agent to use when none is specified. Must be a primary agent. Falls back to 'build' if not set or if the specified agent is invalid.",
+          "Default agent to use when none is specified. Must be a primary agent. If not set, falls back to the first visible primary agent (usually 'build'). If set to an invalid/hidden/subagent, opencode will error.",
         ),
       username: z
         .string()
diff --git a/packages/opencode/src/session/processor.ts b/packages/opencode/src/session/processor.ts
index 27071056180..16deac20eaf 100644
--- a/packages/opencode/src/session/processor.ts
+++ b/packages/opencode/src/session/processor.ts
@@ -153,7 +153,7 @@ export namespace SessionProcessor {
                           JSON.stringify(p.state.input) === JSON.stringify(value.input),
                       )
                     ) {
-                      const agent = await Agent.get(input.assistantMessage.agent)
+                      const agent = await Agent.getOrDefault(input.assistantMessage.agent)
                       await PermissionNext.ask({
                         permission: "doom_loop",
                         patterns: [value.toolName],
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index de62788200b..1d1456cb10e 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -511,7 +511,7 @@ export namespace SessionPrompt {
       }
 
       // normal processing
-      const agent = await Agent.get(lastUser.agent)
+      const agent = await Agent.getOrDefault(lastUser.agent)
       const maxSteps = agent.steps ?? Infinity
       const isLastStep = step >= maxSteps
       msgs = await insertReminders({
@@ -820,7 +820,7 @@ export namespace SessionPrompt {
   }
 
   async function createUserMessage(input: PromptInput) {
-    const agent = await Agent.get(input.agent ?? (await Agent.defaultAgent()))
+    const agent = await Agent.getOrDefault(input.agent)
     const info: MessageV2.Info = {
       id: input.messageID ?? Identifier.ascending("message"),
       role: "user",
@@ -1351,7 +1351,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
     if (session.revert) {
       SessionRevert.cleanup(session)
     }
-    const agent = await Agent.get(input.agent)
+    const agent = await Agent.getOrDefault(input.agent)
     const model = input.model ?? agent.model ?? (await lastModel(input.sessionID))
     const userMsg: MessageV2.User = {
       id: Identifier.ascending("message"),
diff --git a/packages/opencode/src/tool/plan.ts b/packages/opencode/src/tool/plan.ts
index 6cb7a691c88..0321c59c8bf 100644
--- a/packages/opencode/src/tool/plan.ts
+++ b/packages/opencode/src/tool/plan.ts
@@ -7,9 +7,36 @@ import { MessageV2 } from "../session/message-v2"
 import { Identifier } from "../id/id"
 import { Provider } from "../provider/provider"
 import { Instance } from "../project/instance"
+import { Agent } from "../agent/agent"
 import EXIT_DESCRIPTION from "./plan-exit.txt"
 import ENTER_DESCRIPTION from "./plan-enter.txt"
 
+/**
+ * Resolve target agent for plan_exit.
+ * Prefers "build" if available, otherwise falls back to default_agent or first visible primary.
+ * Exported for testing.
+ */
+export async function resolveImplementationAgent(): Promise<string> {
+  const build = await Agent.get("build")
+  if (build && Agent.isPrimaryVisible(build)) return "build"
+
+  const agents = await Agent.list()
+  const defaultName = await Agent.defaultAgent().catch(() => undefined)
+
+  // Prefer default_agent if it's a usable implementation agent (not plan, not subagent, not hidden)
+  if (defaultName && defaultName !== "plan") {
+    const agent = agents.find((a) => a.name === defaultName)
+    if (agent && Agent.isPrimaryVisible(agent)) return defaultName
+  }
+
+  // Fallback: first visible primary agent that is not "plan"
+  const fallback = agents.find((a) => Agent.isPrimaryVisible(a) && a.name !== "plan")
+  if (fallback) return fallback.name
+
+  // Last resort: use defaultAgent result even if it's "plan"
+  return defaultName ?? "plan"
+}
+
 async function getLastModel(sessionID: string) {
   for await (const item of MessageV2.stream(sessionID)) {
     if (item.info.role === "user" && item.info.model) return item.info.model
@@ -23,15 +50,17 @@ export const PlanExitTool = Tool.define("plan_exit", {
   async execute(_params, ctx) {
     const session = await Session.get(ctx.sessionID)
     const plan = path.relative(Instance.worktree, Session.plan(session))
+    const target = await resolveImplementationAgent()
+
     const answers = await Question.ask({
       sessionID: ctx.sessionID,
       questions: [
         {
-          question: `Plan at ${plan} is complete. Would you like to switch to the build agent and start implementing?`,
-          header: "Build Agent",
+          question: `Plan at ${plan} is complete. Would you like to switch to the ${target} agent and start implementing?`,
+          header: "Switch Agent",
           custom: false,
           options: [
-            { label: "Yes", description: "Switch to build agent and start implementing the plan" },
+            { label: "Yes", description: `Switch to ${target} agent and start implementing the plan` },
             { label: "No", description: "Stay with plan agent to continue refining the plan" },
           ],
         },
@@ -51,7 +80,7 @@ export const PlanExitTool = Tool.define("plan_exit", {
       time: {
         created: Date.now(),
       },
-      agent: "build",
+      agent: target,
       model,
     }
     await Session.updateMessage(userMsg)
@@ -65,9 +94,9 @@ export const PlanExitTool = Tool.define("plan_exit", {
     } satisfies MessageV2.TextPart)
 
     return {
-      title: "Switching to build agent",
-      output: "User approved switching to build agent. Wait for further instructions.",
-      metadata: {},
+      title: `Switching to ${target} agent`,
+      output: `User approved switching to ${target} agent. Wait for further instructions.`,
+      metadata: { agent: target },
     }
   },
 })
@@ -124,7 +153,7 @@ export const PlanEnterTool = Tool.define("plan_enter", {
     return {
       title: "Switching to plan agent",
       output: `User confirmed to switch to plan mode. A new message has been created to switch you to plan mode. The plan file will be at ${plan}. Begin planning.`,
-      metadata: {},
+      metadata: { agent: "plan" },
     }
   },
 })
diff --git a/packages/opencode/test/agent/agent.test.ts b/packages/opencode/test/agent/agent.test.ts
index 1ff303b7662..592507ef010 100644
--- a/packages/opencode/test/agent/agent.test.ts
+++ b/packages/opencode/test/agent/agent.test.ts
@@ -636,3 +636,72 @@ test("defaultAgent throws when all primary agents are disabled", async () => {
     },
   })
 })
+
+test("getOrDefault returns agent when name exists", async () => {
+  await using tmp = await tmpdir()
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const agent = await Agent.getOrDefault("build")
+      expect(agent).toBeDefined()
+      expect(agent.name).toBe("build")
+    },
+  })
+})
+
+test("getOrDefault returns default agent when name is undefined", async () => {
+  await using tmp = await tmpdir()
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const agent = await Agent.getOrDefault()
+      expect(agent).toBeDefined()
+      expect(agent.name).toBe("build") // default agent
+    },
+  })
+})
+
+test("getOrDefault returns default agent when name does not exist", async () => {
+  await using tmp = await tmpdir()
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const agent = await Agent.getOrDefault("nonexistent_agent")
+      expect(agent).toBeDefined()
+      expect(agent.name).toBe("build") // falls back to default
+    },
+  })
+})
+
+test("getOrDefault respects custom default_agent config", async () => {
+  await using tmp = await tmpdir({
+    config: {
+      default_agent: "plan",
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const agent = await Agent.getOrDefault()
+      expect(agent).toBeDefined()
+      expect(agent.name).toBe("plan")
+    },
+  })
+})
+
+test("getOrDefault throws when both name and fallback do not exist", async () => {
+  await using tmp = await tmpdir({
+    config: {
+      agent: {
+        build: { disable: true },
+        plan: { disable: true },
+      },
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      await expect(Agent.getOrDefault("nonexistent")).rejects.toThrow()
+    },
+  })
+})
diff --git a/packages/opencode/test/tool/plan.test.ts b/packages/opencode/test/tool/plan.test.ts
new file mode 100644
index 00000000000..d5fb267c32f
--- /dev/null
+++ b/packages/opencode/test/tool/plan.test.ts
@@ -0,0 +1,99 @@
+import { test, expect } from "bun:test"
+import { tmpdir } from "../fixture/fixture"
+import { Instance } from "../../src/project/instance"
+import { Agent } from "../../src/agent/agent"
+import { resolveImplementationAgent } from "../../src/tool/plan"
+
+test("resolveImplementationAgent returns build when build exists", async () => {
+  await using tmp = await tmpdir()
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const target = await resolveImplementationAgent()
+      expect(target).toBe("build")
+    },
+  })
+})
+
+test("resolveImplementationAgent returns default_agent when build disabled", async () => {
+  await using tmp = await tmpdir({
+    config: {
+      default_agent: "myagent",
+      agent: {
+        build: { disable: true },
+        myagent: { mode: "primary" },
+      },
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const build = await Agent.get("build")
+      expect(build).toBeUndefined()
+      const target = await resolveImplementationAgent()
+      expect(target).toBe("myagent")
+    },
+  })
+})
+
+test("resolveImplementationAgent falls back to first visible primary when build disabled and no default_agent", async () => {
+  await using tmp = await tmpdir({
+    config: {
+      agent: {
+        build: { disable: true },
+      },
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const build = await Agent.get("build")
+      expect(build).toBeUndefined()
+      const target = await resolveImplementationAgent()
+      // plan is the next native primary, but we prefer non-plan if possible
+      // Since only plan is left as primary visible, it should return plan
+      expect(target).toBe("plan")
+    },
+  })
+})
+
+test("resolveImplementationAgent prefers non-plan custom agent over plan", async () => {
+  await using tmp = await tmpdir({
+    config: {
+      default_agent: "custom",
+      agent: {
+        build: { disable: true },
+        custom: { mode: "primary" },
+      },
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const build = await Agent.get("build")
+      expect(build).toBeUndefined()
+      const target = await resolveImplementationAgent()
+      expect(target).toBe("custom")
+    },
+  })
+})
+
+test("resolveImplementationAgent skips subagent when selecting fallback", async () => {
+  await using tmp = await tmpdir({
+    config: {
+      default_agent: "explore", // subagent - should be skipped
+      agent: {
+        build: { disable: true },
+        myimpl: { mode: "primary" },
+      },
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const target = await resolveImplementationAgent()
+      // explore is subagent, so it should fallback to myimpl or plan
+      expect(["myimpl", "plan"]).toContain(target)
+    },
+  })
+})
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index fabb16e8a2b..9eca77c70da 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -1663,7 +1663,7 @@ export type Config = {
    */
   small_model?: string
   /**
-   * Default agent to use when none is specified. Must be a primary agent. Falls back to 'build' if not set or if the specified agent is invalid.
+   * Default agent to use when none is specified. Must be a primary agent. If not set, falls back to the first visible primary agent (usually 'build'). If set to an invalid/hidden/subagent, opencode will error.
    */
   default_agent?: string
   /**