diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index 878a2cd917e..297ca90c170 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -120,6 +120,10 @@ import {
 } from "../prompt-cache-observability.js";
 import { resolveCacheRetention } from "../prompt-cache-retention.js";
 import { sanitizeSessionHistory, validateReplayTurns } from "../replay-history.js";
+import {
+  PREEMPTIVE_OVERFLOW_ERROR_TEXT,
+  shouldPreemptivelyCompactBeforePrompt,
+} from "./preemptive-compaction.js";
 import {
   clearActiveEmbeddedRun,
   type EmbeddedPiQueueHandle,
@@ -1521,7 +1525,7 @@ export async function runEmbeddedAttempt(
       const hookAgentId = sessionAgentId;
 
       let promptError: unknown = null;
-      let promptErrorSource: "prompt" | "compaction" | null = null;
+      let promptErrorSource: "prompt" | "compaction" | "precheck" | null = null;
       let prePromptMessageCount = activeSession.messages.length;
       try {
         const promptStartedAt = Date.now();
@@ -1761,6 +1765,27 @@ export async function runEmbeddedAttempt(
               });
           }
 
+          const reserveTokens = settingsManager.getCompactionReserveTokens();
+          const preemptiveCompaction = shouldPreemptivelyCompactBeforePrompt({
+            messages: activeSession.messages,
+            systemPrompt: systemPromptText,
+            prompt: effectivePrompt,
+            contextTokenBudget: params.contextTokenBudget,
+            reserveTokens,
+          });
+          if (preemptiveCompaction.shouldCompact) {
+            promptError = new Error(PREEMPTIVE_OVERFLOW_ERROR_TEXT);
+            promptErrorSource = "precheck";
+            log.warn(
+              `[context-overflow-precheck] sessionKey=${params.sessionKey ?? params.sessionId} ` +
+                `provider=${params.provider}/${params.modelId} ` +
+                `estimatedPromptTokens=${preemptiveCompaction.estimatedPromptTokens} ` +
+                `promptBudgetBeforeReserve=${preemptiveCompaction.promptBudgetBeforeReserve} ` +
+                `reserveTokens=${reserveTokens} sessionFile=${params.sessionFile}`,
+            );
+            return;
+          }
+
           const btwSnapshotMessages = activeSession.messages.slice(-MAX_BTW_SNAPSHOT_MESSAGES);
           updateActiveEmbeddedRunSnapshot(params.sessionId, {
             transcriptLeafId,
diff --git a/src/agents/pi-embedded-runner/run/preemptive-compaction.test.ts b/src/agents/pi-embedded-runner/run/preemptive-compaction.test.ts
new file mode 100644
index 00000000000..81919a477f9
--- /dev/null
+++ b/src/agents/pi-embedded-runner/run/preemptive-compaction.test.ts
@@ -0,0 +1,61 @@
+import { describe, expect, it } from "vitest";
+import {
+  PREEMPTIVE_OVERFLOW_ERROR_TEXT,
+  estimatePrePromptTokens,
+  shouldPreemptivelyCompactBeforePrompt,
+} from "./preemptive-compaction.js";
+
+describe("preemptive-compaction", () => {
+  const verboseHistory =
+    "alpha beta gamma delta epsilon zeta eta theta iota kappa lambda mu ".repeat(40);
+  const verboseSystem =
+    "system guidance with multiple distinct words to avoid tokenizer overcompression ".repeat(25);
+  const verbosePrompt =
+    "user request with distinct content asking for a detailed answer and more context ".repeat(25);
+
+  it("exports a context-overflow-compatible precheck error text", () => {
+    expect(PREEMPTIVE_OVERFLOW_ERROR_TEXT).toContain("Context overflow:");
+    expect(PREEMPTIVE_OVERFLOW_ERROR_TEXT).toContain("(precheck)");
+  });
+
+  it("raises the estimate as prompt-side content grows", () => {
+    const smaller = estimatePrePromptTokens({
+      messages: [{ role: "assistant", content: verboseHistory }],
+      systemPrompt: "sys",
+      prompt: "hello",
+    });
+    const larger = estimatePrePromptTokens({
+      messages: [{ role: "assistant", content: verboseHistory }],
+      systemPrompt: verboseSystem,
+      prompt: verbosePrompt,
+    });
+
+    expect(larger).toBeGreaterThan(smaller);
+  });
+
+  it("requests preemptive compaction when the reserve-based prompt budget would be exceeded", () => {
+    const result = shouldPreemptivelyCompactBeforePrompt({
+      messages: [{ role: "assistant", content: verboseHistory }],
+      systemPrompt: verboseSystem,
+      prompt: verbosePrompt,
+      contextTokenBudget: 500,
+      reserveTokens: 50,
+    });
+
+    expect(result.shouldCompact).toBe(true);
+    expect(result.estimatedPromptTokens).toBeGreaterThan(result.promptBudgetBeforeReserve);
+  });
+
+  it("does not request preemptive compaction when the reserve-based prompt budget still fits", () => {
+    const result = shouldPreemptivelyCompactBeforePrompt({
+      messages: [{ role: "assistant", content: "short history" }],
+      systemPrompt: "sys",
+      prompt: "hello",
+      contextTokenBudget: 10_000,
+      reserveTokens: 1_000,
+    });
+
+    expect(result.shouldCompact).toBe(false);
+    expect(result.estimatedPromptTokens).toBeLessThan(result.promptBudgetBeforeReserve);
+  });
+});
diff --git a/src/agents/pi-embedded-runner/run/preemptive-compaction.ts b/src/agents/pi-embedded-runner/run/preemptive-compaction.ts
new file mode 100644
index 00000000000..baf153c0256
--- /dev/null
+++ b/src/agents/pi-embedded-runner/run/preemptive-compaction.ts
@@ -0,0 +1,47 @@
+import type { AgentMessage } from "@mariozechner/pi-agent-core";
+import { estimateTokens } from "@mariozechner/pi-coding-agent";
+import { SAFETY_MARGIN, estimateMessagesTokens } from "../../compaction.js";
+
+export const PREEMPTIVE_OVERFLOW_ERROR_TEXT =
+  "Context overflow: prompt too large for the model (precheck).";
+
+export function estimatePrePromptTokens(params: {
+  messages: AgentMessage[];
+  systemPrompt?: string;
+  prompt: string;
+}): number {
+  const { messages, systemPrompt, prompt } = params;
+  const syntheticMessages: AgentMessage[] = [];
+  if (typeof systemPrompt === "string" && systemPrompt.trim().length > 0) {
+    syntheticMessages.push({ role: "system", content: systemPrompt } as AgentMessage);
+  }
+  syntheticMessages.push({ role: "user", content: prompt } as AgentMessage);
+
+  const estimated =
+    estimateMessagesTokens(messages) +
+    syntheticMessages.reduce((sum, message) => sum + estimateTokens(message), 0);
+  return Math.max(0, Math.ceil(estimated * SAFETY_MARGIN));
+}
+
+export function shouldPreemptivelyCompactBeforePrompt(params: {
+  messages: AgentMessage[];
+  systemPrompt?: string;
+  prompt: string;
+  contextTokenBudget: number;
+  reserveTokens: number;
+}): {
+  shouldCompact: boolean;
+  estimatedPromptTokens: number;
+  promptBudgetBeforeReserve: number;
+} {
+  const estimatedPromptTokens = estimatePrePromptTokens(params);
+  const promptBudgetBeforeReserve = Math.max(
+    1,
+    Math.floor(params.contextTokenBudget) - Math.max(0, Math.floor(params.reserveTokens)),
+  );
+  return {
+    shouldCompact: estimatedPromptTokens > promptBudgetBeforeReserve,
+    estimatedPromptTokens,
+    promptBudgetBeforeReserve,
+  };
+}