Handle 413 context overflow errors gracefully

When the conversation context exceeds the model's limit, instead of
throwing an opaque error or returning raw JSON, we now:

1. Detect context overflow errors (413, request_too_large, etc.)
2. Return a user-friendly message explaining the issue
3. Suggest using /new or /reset to start fresh

This prevents the assistant from becoming completely unresponsive
when context grows too large (e.g., from many screenshots or long
tool outputs).

Addresses issue #394

src/agents/pi-embedded-helpers.test.ts

@@ -1,6 +1,12 @@
 import { describe, expect, it } from "vitest";
 
-import { buildBootstrapContextFiles } from "./pi-embedded-helpers.js";
+import type { AssistantMessage } from "@mariozechner/pi-ai";
+
+import {
+  buildBootstrapContextFiles,
+  formatAssistantErrorText,
+  isContextOverflowError,
+} from "./pi-embedded-helpers.js";
 import {
   DEFAULT_AGENTS_FILENAME,
   type WorkspaceBootstrapFile,
@@ -46,3 +52,35 @@ describe("buildBootstrapContextFiles", () => {
     expect(result?.content.endsWith(long.slice(-120))).toBe(true);
   });
 });
+
+describe("isContextOverflowError", () => {
+  it("matches known overflow hints", () => {
+    const samples = [
+      "request_too_large",
+      "Request exceeds the maximum size",
+      "context length exceeded",
+      "Maximum context length",
+      "413 Request Entity Too Large",
+    ];
+    for (const sample of samples) {
+      expect(isContextOverflowError(sample)).toBe(true);
+    }
+  });
+
+  it("ignores unrelated errors", () => {
+    expect(isContextOverflowError("rate limit exceeded")).toBe(false);
+  });
+});
+
+describe("formatAssistantErrorText", () => {
+  const makeAssistantError = (errorMessage: string): AssistantMessage =>
+    ({
+      stopReason: "error",
+      errorMessage,
+    }) as AssistantMessage;
+
+  it("returns a friendly message for context overflow", () => {
+    const msg = makeAssistantError("request_too_large");
+    expect(formatAssistantErrorText(msg)).toContain("Context overflow");
+  });
+});

src/agents/pi-embedded-helpers.ts

@@ -126,6 +126,18 @@ export function buildBootstrapContextFiles(
   return result;
 }
 
+export function isContextOverflowError(errorMessage?: string): boolean {
+  if (!errorMessage) return false;
+  const lower = errorMessage.toLowerCase();
+  return (
+    lower.includes("request_too_large") ||
+    lower.includes("request exceeds the maximum size") ||
+    lower.includes("context length exceeded") ||
+    lower.includes("maximum context length") ||
+    (lower.includes("413") && lower.includes("too large"))
+  );
+}
+
 export function formatAssistantErrorText(
   msg: AssistantMessage,
 ): string | undefined {
@@ -133,6 +145,14 @@ export function formatAssistantErrorText(
   const raw = (msg.errorMessage ?? "").trim();
   if (!raw) return "LLM request failed with an unknown error.";
 
+  // Check for context overflow (413) errors
+  if (isContextOverflowError(raw)) {
+    return (
+      "Context overflow: the conversation history is too large. " +
+      "Use /new or /reset to start a fresh session."
+    );
+  }
+
   const invalidRequest = raw.match(
     /"type":"invalid_request_error".*?"message":"([^"]+)"/,
   );

src/agents/pi-embedded-runner.ts

@@ -59,6 +59,7 @@ import {
   formatAssistantErrorText,
   isAuthAssistantError,
   isAuthErrorMessage,
+  isContextOverflowError,
   isRateLimitAssistantError,
   isRateLimitErrorMessage,
   pickFallbackThinkingLevel,
@@ -1153,6 +1154,26 @@ export async function runEmbeddedPiAgent(params: {
           }
           if (promptError && !aborted) {
             const errorText = describeUnknownError(promptError);
+            if (isContextOverflowError(errorText)) {
+              return {
+                payloads: [
+                  {
+                    text:
+                      "Context overflow: the conversation history is too large for the model. " +
+                      "Use /new or /reset to start a fresh session, or try a model with a larger context window.",
+                    isError: true,
+                  },
+                ],
+                meta: {
+                  durationMs: Date.now() - started,
+                  agentMeta: {
+                    sessionId: sessionIdUsed,
+                    provider,
+                    model: model.id,
+                  },
+                },
+              };
+            }
             if (
               (isAuthErrorMessage(errorText) ||
                 isRateLimitErrorMessage(errorText)) &&