fix(engine): make pi.js imports static to fail fast on partial dist

Replace lazy `await import("./pi.js")` and `require("./pi.js")` calls in
runtime-resolution, agent-session-helpers, agent-heartbeat, and
cron-runner with top-level static imports. These dynamic imports were
documented as plugin-decoupling, but pi.js is already eagerly loaded
through index.ts re-exports and static imports in executor/merger/
reviewer/triage/mission-execution-loop, so the deferral never paid off
in practice.

The deferral did, however, introduce a TOCTOU race: a tsc rebuild that
momentarily emptied dist/pi.js would let the engine load fine and only
fail minutes later when the first session was created (e.g. FN-2860
errored two minutes into execution while pi.js was being rewritten).
With static imports, a missing/half-built dist now fails immediately at
process startup with a clear stack — verified by `mv dist/pi.js
dist/pi.js.bak` reproducing ERR_MODULE_NOT_FOUND on the first import of
runtime-resolution.js.

Also drops the DefaultPiRuntime.describeModelFn cache, which only
existed to paper over the require-on-first-call latency.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/agent-heartbeat.ts

@@ -26,9 +26,7 @@ import { AgentLogger } from "./agent-logger.js";
 import { resolveAgentInstructionsWithRatings, buildSystemPromptWithInstructions } from "./agent-instructions.js";
 import { heartbeatLog, formatError } from "./logger.js";
 import { createRunAuditor, type EngineRunContext } from "./run-audit.js";
-
+import { promptWithFallback } from "./pi.js";
-// Lazy import for pi — avoids pulling the pi SDK into the module graph
-// when heartbeat execution isn't needed.
 
 /** Resolved per-agent heartbeat config after validation and fallback */
 interface ResolvedHeartbeatConfig {
@@ -1099,8 +1097,6 @@ export class HeartbeatMonitor {
           },
         };
 
-        // Lazy-load promptWithFallback
-        const { promptWithFallback } = await import("./pi.js");
         const { createResolvedAgentSession, extractRuntimeHint } = await import("./agent-session-helpers.js");
         const { buildSessionSkillContextSync } = await import("./session-skill-context.js");
 

packages/engine/src/agent-session-helpers.ts

@@ -12,6 +12,7 @@ import type { PluginRunner } from "./plugin-runner.js";
 import type { AgentSession } from "@mariozechner/pi-coding-agent";
 import { resolveRuntime, buildRuntimeResolutionContext, type SessionPurpose } from "./runtime-resolution.js";
 import { createLogger } from "./logger.js";
+import { promptWithFallback, describeModel } from "./pi.js";
 
 /** Logger for agent session helpers */
 const sessionLog = createLogger("agent-session");
@@ -130,10 +131,7 @@ export async function promptWithAutoRetry(
   prompt: string,
   options?: unknown,
 ): Promise<void> {
-  // Dynamic import to get the default runtime's promptWithFallback
+  return promptWithFallback(session, prompt, options);
-  // This works because the default runtime delegates to the existing implementation
-  const { promptWithFallback: pwf } = await import("./pi.js");
-  return pwf(session, prompt, options);
 }
 
 /**
@@ -143,6 +141,5 @@ export async function promptWithAutoRetry(
  * @returns Model description string
  */
 export async function describeAgentModel(session: AgentSession): Promise<string> {
-  const { describeModel: dm } = await import("./pi.js");
+  return describeModel(session);
-  return dm(session);
 }

packages/engine/src/cron-runner.ts

@@ -5,6 +5,7 @@ import type { AutomationStore } from "@fusion/core";
 import type { ScheduledTask, AutomationRunResult, AutomationStep, AutomationStepResult, Column, TaskCreateInput } from "@fusion/core";
 import { createLogger } from "./logger.js";
 import { defaultShell } from "./shell-utils.js";
+import { createFnAgent, promptWithFallback } from "./pi.js";
 
 const execAsync = promisify(exec);
 const log = createLogger("cron-runner");
@@ -628,9 +629,6 @@ const AI_AUTOMATION_SYSTEM_PROMPT = [
  * @returns An AiPromptExecutor function suitable for CronRunnerOptions.
  */
 export async function createAiPromptExecutor(cwd: string): Promise<AiPromptExecutor> {
-  // We import lazily to keep the factory self-contained and to avoid
-  // pulling pi.ts into the module graph when AI execution isn't used.
-  const { createFnAgent, promptWithFallback } = await import("./pi.js");
   const disposeLog = createLogger("cron-runner");
 
   return async (prompt: string, modelProvider?: string, modelId?: string): Promise<string> => {

packages/engine/src/runtime-resolution.ts

@@ -11,6 +11,7 @@ import type { AgentRuntime, AgentRuntimeOptions, AgentSessionResult } from "./ag
 import type { PluginRunner } from "./plugin-runner.js";
 import type { AgentSession } from "@mariozechner/pi-coding-agent";
 import { createLogger } from "./logger.js";
+import { createFnAgent, promptWithFallback, describeModel } from "./pi.js";
 
 /** Logger for the runtime resolution subsystem */
 const runtimeLog = createLogger("runtime-resolver");
@@ -77,36 +78,16 @@ export class DefaultPiRuntime implements AgentRuntime {
   readonly id = "pi";
   readonly name = "Default PI Runtime";
 
-  // Synchronous cached describeModel function
-  private static describeModelFn: ((session: AgentSession) => string) | null = null;
-
-  /**
-   * Create an agent session using the default pi implementation.
-   */
   async createSession(options: AgentRuntimeOptions): Promise<AgentSessionResult> {
-    const { createFnAgent } = await import("./pi.js");
     return createFnAgent(options);
   }
 
-  /**
-   * Prompt with automatic retry and compaction.
-   * Delegates to the existing promptWithFallback implementation.
-   */
   async promptWithFallback(session: AgentSession, prompt: string, options?: unknown): Promise<void> {
-    const { promptWithFallback: pwf } = await import("./pi.js");
+    return promptWithFallback(session, prompt, options);
-    return pwf(session, prompt, options);
   }
 
-  /**
-   * Get model description from session.
-   */
   describeModel(session: AgentSession): string {
-    if (!DefaultPiRuntime.describeModelFn) {
+    return describeModel(session);
-      // eslint-disable-next-line @typescript-eslint/no-require-imports
-      const { describeModel } = require("./pi.js");
-      DefaultPiRuntime.describeModelFn = describeModel;
-    }
-    return DefaultPiRuntime.describeModelFn!(session);
   }
 }
 
@@ -216,18 +197,15 @@ function wrapPluginRuntime(
         return adapter.promptWithFallback(session, prompt, options);
       }
       // Fallback to default pi promptWithFallback
-      const { promptWithFallback: pwf } = await import("./pi.js");
+      return promptWithFallback(session, prompt, options);
-      return pwf(session, prompt, options);
     },
     describeModel: (session: AgentSession) => {
       const adapter = instance as Record<string, unknown>;
       if (typeof adapter.describeModel === "function") {
         return (adapter.describeModel as (s: AgentSession) => string)(session);
       }
-      // Fallback to default pi describeModel - use cached sync function
+      // Fallback to default pi describeModel
-      // eslint-disable-next-line @typescript-eslint/no-require-imports
+      return describeModel(session);
-      const { describeModel: dm } = require("./pi.js");
-      return dm(session);
     },
   };
 }