diff --git a/index.ts b/index.ts
index 3fef764..56b0a02 100644
--- a/index.ts
+++ b/index.ts
@@ -4,7 +4,7 @@ import { fileURLToPath } from "node:url";
 import { createRunArtifacts } from "./src/artifacts.ts";
 import { monitorRun } from "./src/monitor.ts";
 import { listAvailableModelReferences } from "./src/models.ts";
-import { createTmuxSingleRunner } from "./src/runner.ts";
+import { createTmuxSingleRunner } from "./src/tmux-runner.ts";
 import {
   buildCurrentWindowArgs,
   buildKillPaneArgs,
diff --git a/src/runner.ts b/src/runner.ts
index 2c384a8..f33741f 100644
--- a/src/runner.ts
+++ b/src/runner.ts
@@ -1,44 +1,9 @@
-export function createTmuxSingleRunner(deps: {
-  assertInsideTmux(): void;
-  getCurrentWindowId: () => Promise<string>;
-  createArtifacts: (cwd: string, meta: Record<string, unknown>) => Promise<any>;
-  buildWrapperCommand: (metaPath: string) => string;
-  createPane: (input: { windowId: string; cwd: string; command: string }) => Promise<string>;
-  monitorRun: (input: { eventsPath: string; resultPath: string; onEvent?: (event: any) => void }) => Promise<any>;
-  killPane: (paneId: string) => Promise<void>;
-}) {
-  return async function runSingleTask(input: {
-    cwd: string;
-    meta: Record<string, unknown>;
-    onEvent?: (event: any) => void;
-  }) {
-    deps.assertInsideTmux();
+import type { SubagentRunResult } from "./schema.ts";
 
-    const artifacts = await deps.createArtifacts(input.cwd, input.meta);
-    const windowId = await deps.getCurrentWindowId();
-    const command = deps.buildWrapperCommand(artifacts.metaPath);
-    const paneId = await deps.createPane({ windowId, cwd: input.cwd, command });
-
-    try {
-      const result = await deps.monitorRun({
-        eventsPath: artifacts.eventsPath,
-        resultPath: artifacts.resultPath,
-        onEvent: input.onEvent,
-      });
-
-      return {
-        ...result,
-        runId: result.runId ?? artifacts.runId,
-        paneId,
-        windowId,
-        sessionPath: result.sessionPath ?? artifacts.sessionPath,
-        stdoutPath: result.stdoutPath ?? artifacts.stdoutPath,
-        stderrPath: result.stderrPath ?? artifacts.stderrPath,
-        resultPath: artifacts.resultPath,
-        eventsPath: artifacts.eventsPath,
-      };
-    } finally {
-      await deps.killPane(paneId);
-    }
-  };
+export interface RunSingleTaskInput {
+  cwd: string;
+  meta: Record<string, unknown>;
+  onEvent?: (event: any) => void;
 }
+
+export type RunSingleTask = (input: RunSingleTaskInput) => Promise<SubagentRunResult>;
diff --git a/src/runner.test.ts b/src/tmux-runner.test.ts
similarity index 50%
rename from src/runner.test.ts
rename to src/tmux-runner.test.ts
index 2b92351..f090922 100644
--- a/src/runner.test.ts
+++ b/src/tmux-runner.test.ts
@@ -1,6 +1,6 @@
 import test from "node:test";
 import assert from "node:assert/strict";
-import { createTmuxSingleRunner } from "./runner.ts";
+import { createTmuxSingleRunner } from "./tmux-runner.ts";
 
 test("createTmuxSingleRunner always kills the pane after monitor completion", async () => {
   const killed: string[] = [];
@@ -31,3 +31,30 @@ test("createTmuxSingleRunner always kills the pane after monitor completion", as
   assert.equal(result.finalText, "done");
   assert.deepEqual(killed, ["%9"]);
 });
+
+test("createTmuxSingleRunner surfaces explicit tmux precondition errors", async () => {
+  const runSingleTask = createTmuxSingleRunner({
+    assertInsideTmux() {
+      throw new Error("tmux-backed subagents require pi to be running inside tmux.");
+    },
+    getCurrentWindowId: async () => "@1",
+    createArtifacts: async () => ({
+      metaPath: "/tmp/meta.json",
+      runId: "run-1",
+      eventsPath: "/tmp/events.jsonl",
+      resultPath: "/tmp/result.json",
+      sessionPath: "/tmp/child-session.jsonl",
+      stdoutPath: "/tmp/stdout.log",
+      stderrPath: "/tmp/stderr.log",
+    }),
+    buildWrapperCommand: () => "'node' '/wrapper.mjs' '/tmp/meta.json'",
+    createPane: async () => "%9",
+    monitorRun: async () => ({ finalText: "done", exitCode: 0 }),
+    killPane: async () => {},
+  });
+
+  await assert.rejects(
+    () => runSingleTask({ cwd: "/repo", meta: { task: "inspect auth" } as any }),
+    /tmux-backed subagents require pi to be running inside tmux/,
+  );
+});
diff --git a/src/tmux-runner.ts b/src/tmux-runner.ts
new file mode 100644
index 0000000..2c384a8
--- /dev/null
+++ b/src/tmux-runner.ts
@@ -0,0 +1,44 @@
+export function createTmuxSingleRunner(deps: {
+  assertInsideTmux(): void;
+  getCurrentWindowId: () => Promise<string>;
+  createArtifacts: (cwd: string, meta: Record<string, unknown>) => Promise<any>;
+  buildWrapperCommand: (metaPath: string) => string;
+  createPane: (input: { windowId: string; cwd: string; command: string }) => Promise<string>;
+  monitorRun: (input: { eventsPath: string; resultPath: string; onEvent?: (event: any) => void }) => Promise<any>;
+  killPane: (paneId: string) => Promise<void>;
+}) {
+  return async function runSingleTask(input: {
+    cwd: string;
+    meta: Record<string, unknown>;
+    onEvent?: (event: any) => void;
+  }) {
+    deps.assertInsideTmux();
+
+    const artifacts = await deps.createArtifacts(input.cwd, input.meta);
+    const windowId = await deps.getCurrentWindowId();
+    const command = deps.buildWrapperCommand(artifacts.metaPath);
+    const paneId = await deps.createPane({ windowId, cwd: input.cwd, command });
+
+    try {
+      const result = await deps.monitorRun({
+        eventsPath: artifacts.eventsPath,
+        resultPath: artifacts.resultPath,
+        onEvent: input.onEvent,
+      });
+
+      return {
+        ...result,
+        runId: result.runId ?? artifacts.runId,
+        paneId,
+        windowId,
+        sessionPath: result.sessionPath ?? artifacts.sessionPath,
+        stdoutPath: result.stdoutPath ?? artifacts.stdoutPath,
+        stderrPath: result.stderrPath ?? artifacts.stderrPath,
+        resultPath: artifacts.resultPath,
+        eventsPath: artifacts.eventsPath,
+      };
+    } finally {
+      await deps.killPane(paneId);
+    }
+  };
+}