feat(handoff): scale transcript budget to the destination context window

src/renderer/actions/handoffTranscript.test.ts

@@ -2,7 +2,11 @@ import { beforeEach, describe, expect, it } from "vitest";
 import type { Thread } from "@/shared/contracts";
 import { useAppStore } from "../state/appStore";
 import type { RuntimeChatItem } from "../state/slices/runtimeEventSlice";
-import { buildTranscriptContext, MAX_TRANSCRIPT_CONTEXT_CHARS } from "./handoffTranscript";
+import { buildTranscriptContext, handoffTranscriptBudget } from "./handoffTranscript";
+
+// The budget-filling tests below build fixtures sized against this value, so
+// they assert selection behaviour rather than whatever the default happens to be.
+const TEST_BUDGET = 50_000;
 import { MAX_HANDOFF_MESSAGE_CHARS } from "./handoffTranscriptRows";
 
 const thread: Thread = {
@@ -145,7 +149,7 @@ describe("buildTranscriptContext", () => {
       userMessage("u2", "Latest follow-up"),
     ]);
 
-    const summary = buildTranscriptContext(thread, "Claude")?.summary ?? "";
+    const summary = buildTranscriptContext(thread, "Claude", TEST_BUDGET)?.summary ?? "";
 
     expect(summary).toContain("User:\nOriginal ask: migrate the auth module");
     expect(summary).toContain("User:\nLatest follow-up");
@@ -156,7 +160,7 @@ describe("buildTranscriptContext", () => {
   });
 
   it("spends the budget on conversation before tool activity", () => {
-    // Eight near-cap assistant rows take ~48k of the 50k budget; twenty
+    // Eight near-cap assistant rows take ~48k of the 50k test budget; twenty
     // 480-char command rows cannot all fit in what remains.
     const long = "z".repeat(MAX_HANDOFF_MESSAGE_CHARS - 10);
     const commands: RuntimeChatItem[] = Array.from({ length: 20 }, (_, index) => ({
@@ -171,7 +175,7 @@ describe("buildTranscriptContext", () => {
       ...Array.from({ length: 8 }, (_, index) => assistantMessage(`a${index}`, `${index}:${long}`)),
     ]);
 
-    const summary = buildTranscriptContext(thread, "Claude")?.summary ?? "";
+    const summary = buildTranscriptContext(thread, "Claude", TEST_BUDGET)?.summary ?? "";
 
     expect(summary).toContain("0:zzz");
     expect(summary).toContain("7:zzz");
@@ -183,7 +187,7 @@ describe("buildTranscriptContext", () => {
   it("truncates a single oversized user message from the tail, keeping its start", () => {
     seed([userMessage("u1", `ASK ${"w".repeat(MAX_HANDOFF_MESSAGE_CHARS * 2)}`)]);
 
-    const summary = buildTranscriptContext(thread, "Claude")?.summary ?? "";
+    const summary = buildTranscriptContext(thread, "Claude", TEST_BUDGET)?.summary ?? "";
 
     expect(summary).toContain("User:\nASK ");
     expect(summary).toContain("[message truncated]");
@@ -205,10 +209,30 @@ describe("buildTranscriptContext", () => {
     ]).flat();
     seed(interleaved);
 
-    const summary = buildTranscriptContext(thread, "Claude")?.summary ?? "";
+    const summary = buildTranscriptContext(thread, "Claude", TEST_BUDGET)?.summary ?? "";
 
     expect(summary).toContain("[turns omitted]");
     // The header line rides outside the row budget, hence the small slack.
-    expect(summary.length).toBeLessThanOrEqual(MAX_TRANSCRIPT_CONTEXT_CHARS + 500);
+    expect(summary.length).toBeLessThanOrEqual(TEST_BUDGET + 500);
+  });
+});
+
+describe("handoffTranscriptBudget", () => {
+  it("scales the budget with the destination model's context window", () => {
+    // A handoff is truncated from the front, so a fixed small budget silently
+    // drops the beginning of the conversation on large-context models.
+    expect(handoffTranscriptBudget("1m")).toBeGreaterThan(handoffTranscriptBudget("200k"));
+    expect(handoffTranscriptBudget("1m")).toBeGreaterThan(1_000_000);
+  });
+
+  it("falls back to the default when the context size is missing or unparsable", () => {
+    const fallback = handoffTranscriptBudget(undefined);
+    expect(fallback).toBe(400_000);
+    expect(handoffTranscriptBudget("unlimited")).toBe(fallback);
+    expect(handoffTranscriptBudget("")).toBe(fallback);
+  });
+
+  it("never returns less than the default", () => {
+    expect(handoffTranscriptBudget("1k")).toBeGreaterThanOrEqual(400_000);
   });
 });

src/renderer/actions/handoffTranscript.ts

@@ -7,7 +7,46 @@ import { formatHandoffRow, type HandoffRow } from "./handoffTranscriptRows";
  * window, but the file rides in the new provider's first message for the rest
  * of its session, so it is filled by priority rather than recency alone.
  */
-export const MAX_TRANSCRIPT_CONTEXT_CHARS = 50_000;
+export /**
+ * Handoff transcripts are truncated from the front, so a small budget silently
+ * throws away the beginning of the conversation — the part that usually carries
+ * the goal and the decisions. 50k chars is roughly 12k tokens, a sliver of any
+ * current model's window, so the receiving agent used to start half-blind.
+ *
+ * The budget now scales with the destination model's context window and only
+ * claims a share of it, leaving the rest for the actual work.
+ */
+const HANDOFF_CONTEXT_SHARE = 0.35;
+const CHARS_PER_TOKEN = 4;
+/** Used when the destination declares no context size. */
+const DEFAULT_MAX_TRANSCRIPT_CONTEXT_CHARS = 400_000;
+
+/** `"1m"` / `"200k"` / `"272000"` -> token count, or undefined when unparsable. */
+function tokensFromContextSize(contextSize: string | undefined): number | undefined {
+  if (!contextSize) return undefined;
+  const match = /^(\d+(?:\.\d+)?)\s*([mk])?$/i.exec(contextSize.trim());
+  if (!match) return undefined;
+  const value = Number(match[1]);
+  if (!Number.isFinite(value) || value <= 0) return undefined;
+  const unit = match[2]?.toLowerCase();
+  if (unit === "m") return value * 1_000_000;
+  if (unit === "k") return value * 1_000;
+  return value;
+}
+
+/**
+ * Character budget for a handoff transcript aimed at a model with `contextSize`.
+ * Exported so the caller can size the transcript to the provider it is handing
+ * off to rather than to a fixed constant.
+ */
+export function handoffTranscriptBudget(contextSize?: string): number {
+  const tokens = tokensFromContextSize(contextSize);
+  if (tokens === undefined) return DEFAULT_MAX_TRANSCRIPT_CONTEXT_CHARS;
+  return Math.max(
+    DEFAULT_MAX_TRANSCRIPT_CONTEXT_CHARS,
+    Math.floor(tokens * CHARS_PER_TOKEN * HANDOFF_CONTEXT_SHARE),
+  );
+}
 const ROW_SEPARATOR = "\n\n";
 const LEADING_GAP_MARKER = "[earlier turns omitted]";
 const INNER_GAP_MARKER = "[turns omitted]";
@@ -30,13 +69,13 @@ const GAP_MARKER_ALLOWANCE = ROW_SEPARATOR.length + LEADING_GAP_MARKER.length;
  * Each tier stops at the first row that does not fit, so the kept set is a
  * recent contiguous run per tier rather than a scatter of small rows.
  */
-function selectRows(rows: readonly HandoffRow[]): ReadonlySet<HandoffRow> {
+function selectRows(rows: readonly HandoffRow[], maxChars: number): ReadonlySet<HandoffRow> {
   const kept = new Set<HandoffRow>();
   let used = 0;
   const tryKeep = (candidate: HandoffRow): boolean => {
     const cost =
       candidate.text.length + (kept.size > 0 ? ROW_SEPARATOR.length + GAP_MARKER_ALLOWANCE : 0);
-    if (used + cost > MAX_TRANSCRIPT_CONTEXT_CHARS) return false;
+    if (used + cost > maxChars) return false;
     kept.add(candidate);
     used += cost;
     return true;
@@ -80,6 +119,7 @@ function joinRows(rows: readonly HandoffRow[], kept: ReadonlySet<HandoffRow>): s
 export function buildTranscriptContext(
   thread: Thread,
   sourceLabel: string,
+  maxChars: number = DEFAULT_MAX_TRANSCRIPT_CONTEXT_CHARS,
 ): ExtractContextResult | null {
   const state = useAppStore.getState();
   const itemIds = state.runtimeItemIdsByThread[thread.id] ?? [];
@@ -95,7 +135,7 @@ export function buildTranscriptContext(
   });
   if (rows.length === 0) return null;
 
-  const transcript = joinRows(rows, selectRows(rows));
+  const transcript = joinRows(rows, selectRows(rows, maxChars));
   if (!transcript.trim()) return null;
 
   return {

src/renderer/components/thread/ContinueInProviderDialog.tsx

@@ -90,7 +90,10 @@ import {
 import { resolveSavedProviderDraftConfig, supportsUsableFastMode } from "./threadDraftViewHelpers";
 import { useAppStore } from "@/renderer/state/appStore";
 import { useSharedSettings } from "@/renderer/state/sharedSettingsStore";
-import { buildTranscriptContext } from "@/renderer/actions/handoffTranscript";
+import {
+  buildTranscriptContext,
+  handoffTranscriptBudget,
+} from "@/renderer/actions/handoffTranscript";
 import {
   defaultHandoffPrompt,
   type ProviderHandoffContext,
@@ -857,7 +860,11 @@ export function ContinueInProviderDialog(props: {
     // before the new provider can. This holds whether or not the user typed a
     // prompt: a typed prompt narrows the next step, not the history behind it.
     // A terminal source has no stored rows and still goes through extraction.
-    const history = buildTranscriptContext(thread, sourceAgent?.label ?? thread.agentKind);
+    const history = buildTranscriptContext(
+      thread,
+      sourceAgent?.label ?? thread.agentKind,
+      handoffTranscriptBudget(targetConfig.contextSize),
+    );
     if (history) {
       onContinue(
         selectedKind,