Plugins/Hooks: avoid duplicate before_agent_start executions

src/agents/pi-embedded-runner/run/attempt.e2e.test.ts

@@ -1,7 +1,7 @@
 import type { AgentMessage } from "@mariozechner/pi-agent-core";
 import type { ImageContent } from "@mariozechner/pi-ai";
-import { describe, expect, it } from "vitest";
-import { injectHistoryImagesIntoMessages } from "./attempt.js";
+import { describe, expect, it, vi } from "vitest";
+import { injectHistoryImagesIntoMessages, resolvePromptBuildHookResult } from "./attempt.js";
 
 describe("injectHistoryImagesIntoMessages", () => {
   const image: ImageContent = { type: "image", data: "abc", mimeType: "image/png" };
@@ -58,3 +58,51 @@ describe("injectHistoryImagesIntoMessages", () => {
     expect(firstAssistant?.content).toBe("noop");
   });
 });
+
+describe("resolvePromptBuildHookResult", () => {
+  it("reuses precomputed legacy before_agent_start result without invoking hook again", async () => {
+    const hookRunner = {
+      hasHooks: vi.fn(
+        (hookName: "before_prompt_build" | "before_agent_start") =>
+          hookName === "before_agent_start",
+      ),
+      runBeforePromptBuild: vi.fn(async () => undefined),
+      runBeforeAgentStart: vi.fn(async () => ({ prependContext: "from-hook" })),
+    };
+    const result = await resolvePromptBuildHookResult({
+      prompt: "hello",
+      messages: [],
+      hookCtx: {},
+      hookRunner,
+      legacyBeforeAgentStartResult: { prependContext: "from-cache", systemPrompt: "legacy-system" },
+    });
+
+    expect(hookRunner.runBeforeAgentStart).not.toHaveBeenCalled();
+    expect(result).toEqual({
+      prependContext: "from-cache",
+      systemPrompt: "legacy-system",
+    });
+  });
+
+  it("calls legacy hook when precomputed result is absent", async () => {
+    const hookRunner = {
+      hasHooks: vi.fn(
+        (hookName: "before_prompt_build" | "before_agent_start") =>
+          hookName === "before_agent_start",
+      ),
+      runBeforePromptBuild: vi.fn(async () => undefined),
+      runBeforeAgentStart: vi.fn(async () => ({ prependContext: "from-hook" })),
+    };
+    const messages = [{ role: "user", content: "ctx" }];
+    const result = await resolvePromptBuildHookResult({
+      prompt: "hello",
+      messages,
+      hookCtx: {},
+      hookRunner,
+    });
+
+    expect(hookRunner.runBeforeAgentStart).toHaveBeenCalledTimes(1);
+    expect(hookRunner.runBeforeAgentStart).toHaveBeenCalledWith({ prompt: "hello", messages }, {});
+    expect(result.prependContext).toBe("from-hook");
+  });
+});

src/agents/pi-embedded-runner/run/attempt.ts

@@ -14,6 +14,11 @@ import { resolveChannelCapabilities } from "../../../config/channel-capabilities
 import { getMachineDisplayName } from "../../../infra/machine-name.js";
 import { MAX_IMAGE_BYTES } from "../../../media/constants.js";
 import { getGlobalHookRunner } from "../../../plugins/hook-runner-global.js";
+import type {
+  PluginHookAgentContext,
+  PluginHookBeforeAgentStartResult,
+  PluginHookBeforePromptBuildResult,
+} from "../../../plugins/types.js";
 import {
   isCronSessionKey,
   isSubagentSessionKey,
@@ -111,6 +116,18 @@ import {
 import { detectAndLoadPromptImages } from "./images.js";
 import type { EmbeddedRunAttemptParams, EmbeddedRunAttemptResult } from "./types.js";
 
+type PromptBuildHookRunner = {
+  hasHooks: (hookName: "before_prompt_build" | "before_agent_start") => boolean;
+  runBeforePromptBuild: (
+    event: { prompt: string; messages: unknown[] },
+    ctx: PluginHookAgentContext,
+  ) => Promise<PluginHookBeforePromptBuildResult | undefined>;
+  runBeforeAgentStart: (
+    event: { prompt: string; messages: unknown[] },
+    ctx: PluginHookAgentContext,
+  ) => Promise<PluginHookBeforeAgentStartResult | undefined>;
+};
+
 export function injectHistoryImagesIntoMessages(
   messages: AgentMessage[],
   historyImagesByIndex: Map<number, ImageContent[]>,
@@ -159,6 +176,53 @@ export function injectHistoryImagesIntoMessages(
   return didMutate;
 }
 
+export async function resolvePromptBuildHookResult(params: {
+  prompt: string;
+  messages: unknown[];
+  hookCtx: PluginHookAgentContext;
+  hookRunner?: PromptBuildHookRunner | null;
+  legacyBeforeAgentStartResult?: PluginHookBeforeAgentStartResult;
+}): Promise<PluginHookBeforePromptBuildResult> {
+  const promptBuildResult = params.hookRunner?.hasHooks("before_prompt_build")
+    ? await params.hookRunner
+        .runBeforePromptBuild(
+          {
+            prompt: params.prompt,
+            messages: params.messages,
+          },
+          params.hookCtx,
+        )
+        .catch((hookErr: unknown) => {
+          log.warn(`before_prompt_build hook failed: ${String(hookErr)}`);
+          return undefined;
+        })
+    : undefined;
+  const legacyResult =
+    params.legacyBeforeAgentStartResult ??
+    (params.hookRunner?.hasHooks("before_agent_start")
+      ? await params.hookRunner
+          .runBeforeAgentStart(
+            {
+              prompt: params.prompt,
+              messages: params.messages,
+            },
+            params.hookCtx,
+          )
+          .catch((hookErr: unknown) => {
+            log.warn(
+              `before_agent_start hook (legacy prompt build path) failed: ${String(hookErr)}`,
+            );
+            return undefined;
+          })
+      : undefined);
+  return {
+    systemPrompt: promptBuildResult?.systemPrompt ?? legacyResult?.systemPrompt,
+    prependContext: [promptBuildResult?.prependContext, legacyResult?.prependContext]
+      .filter((value): value is string => Boolean(value))
+      .join("\n\n"),
+  };
+}
+
 function summarizeMessagePayload(msg: AgentMessage): { textChars: number; imageBlocks: number } {
   const content = (msg as { content?: unknown }).content;
   if (typeof content === "string") {
@@ -934,42 +998,13 @@ export async function runEmbeddedAttempt(
           workspaceDir: params.workspaceDir,
           messageProvider: params.messageProvider ?? undefined,
         };
-        const promptBuildResult = hookRunner?.hasHooks("before_prompt_build")
-          ? await hookRunner
-              .runBeforePromptBuild(
-                {
-                  prompt: params.prompt,
-                  messages: activeSession.messages,
-                },
-                hookCtx,
-              )
-              .catch((hookErr: unknown) => {
-                log.warn(`before_prompt_build hook failed: ${String(hookErr)}`);
-                return undefined;
-              })
-          : undefined;
-        const legacyResult = hookRunner?.hasHooks("before_agent_start")
-          ? await hookRunner
-              .runBeforeAgentStart(
-                {
-                  prompt: params.prompt,
-                  messages: activeSession.messages,
-                },
-                hookCtx,
-              )
-              .catch((hookErr: unknown) => {
-                log.warn(
-                  `before_agent_start hook (legacy prompt build path) failed: ${String(hookErr)}`,
-                );
-                return undefined;
-              })
-          : undefined;
-        const hookResult = {
-          systemPrompt: promptBuildResult?.systemPrompt ?? legacyResult?.systemPrompt,
-          prependContext: [promptBuildResult?.prependContext, legacyResult?.prependContext]
-            .filter((value): value is string => Boolean(value))
-            .join("\n\n"),
-        };
+        const hookResult = await resolvePromptBuildHookResult({
+          prompt: params.prompt,
+          messages: activeSession.messages,
+          hookCtx,
+          hookRunner,
+          legacyBeforeAgentStartResult: params.legacyBeforeAgentStartResult,
+        });
         {
           if (hookResult?.prependContext) {
             effectivePrompt = `${hookResult.prependContext}\n\n${params.prompt}`;

src/agents/pi-embedded-runner/run/types.ts

@@ -2,6 +2,7 @@ import type { AgentMessage } from "@mariozechner/pi-agent-core";
 import type { Api, AssistantMessage, Model } from "@mariozechner/pi-ai";
 import type { ThinkLevel } from "../../../auto-reply/thinking.js";
 import type { SessionSystemPromptReport } from "../../../config/sessions/types.js";
+import type { PluginHookBeforeAgentStartResult } from "../../../plugins/types.js";
 import type { MessagingToolSend } from "../../pi-embedded-messaging.js";
 import type { AuthStorage, ModelRegistry } from "../../pi-model-discovery.js";
 import type { NormalizedUsage } from "../../usage.js";
@@ -19,6 +20,7 @@ export type EmbeddedRunAttemptParams = EmbeddedRunAttemptBase & {
   authStorage: AuthStorage;
   modelRegistry: ModelRegistry;
   thinkLevel: ThinkLevel;
+  legacyBeforeAgentStartResult?: PluginHookBeforeAgentStartResult;
 };
 
 export type EmbeddedRunAttemptResult = {