fix(subagent): count mailbox spill remaining as omitted body

src/subagent/fleet-dry-drive.ts

@@ -211,10 +211,13 @@ function clipDigestSection(text: string): string {
   return `${text.slice(0, MAILBOX_DIGEST_SECTION_CHARS - 1).trimEnd()}…`;
 }
 
-function mailboxReportSpillNotice(text: string, uri: string): string {
+function mailboxSpillNotice(text: string, uri: string): string {
+  // Digest inlines the notice only, so remaining is the omitted body — not
+  // length minus a 2048-char prefix that was never kept. A short spill that
+  // claimed "0 more chars omitted" looks complete and the parent skips read_file.
   return truncationNotice({
     maxChars: MAILBOX_DIGEST_SECTION_CHARS,
-    remaining: Math.max(0, text.length - MAILBOX_DIGEST_SECTION_CHARS),
+    remaining: text.length,
     fullLength: text.length,
     contentType: "text/plain",
     uri,
@@ -287,8 +290,14 @@ export async function digestCollectedReport(
     report.report === undefined
       ? undefined
       : reportUri !== undefined
-        ? mailboxReportSpillNotice(report.report, reportUri)
+        ? mailboxSpillNotice(report.report, reportUri)
         : await clipField(report.report, report.agent_id, "report");
+  const errorInline =
+    report.error === undefined
+      ? undefined
+      : errorUri !== undefined
+        ? mailboxSpillNotice(report.error, errorUri)
+        : clipDigestSection(report.error);
   return {
     agent_id: report.agent_id,
     status: report.status,
@@ -300,9 +309,7 @@ export async function digestCollectedReport(
     ...(blockers !== undefined ? { blockers } : {}),
     ...(reportInline !== undefined ? { report: reportInline } : {}),
     ...(reportUri !== undefined ? { report_uri: reportUri } : {}),
-    ...(report.error !== undefined
-      ? { error: clipDigestSection(report.error) }
-      : {}),
+    ...(errorInline !== undefined ? { error: errorInline } : {}),
     ...(errorUri !== undefined ? { error_uri: errorUri } : {}),
     ...(report.hint !== undefined ? { hint: report.hint } : {}),
     ...(report.provider_failure === true ? { provider_failure: true } : {}),

src/subagent/mailbox-mail-drive.test.ts

@@ -5,6 +5,7 @@ import {
   driveMailboxMail,
   latchMailboxMailDrive,
   MAILBOX_MAIL_WAKE_PREFIX,
+  mailboxMailErrorUriHint,
   mailboxMailReportUriHint,
   mailboxMailWakeLine,
   occupancyShouldYieldWait,
@@ -14,6 +15,7 @@ import {
   digestCollectedReports,
   fleetDrySpillKey,
   FLEET_DRY_REPORT_CHARS,
+  MAILBOX_DIGEST_SECTION_CHARS,
   type FleetDryMailboxRecord,
 } from "./fleet-dry-drive.js";
 import {
@@ -125,6 +127,22 @@ describe("buildMailboxMailPrompt", () => {
     expect(prompt).toContain(uri);
   });
 
+  test("names error_uri with a read_file truncation notice", () => {
+    const uri = `tool-output:///${fleetDrySpillKey("worker-1", "error")}`;
+    const prompt = buildMailboxMailPrompt([
+      {
+        agent_id: "worker-1",
+        status: "failed",
+        error_uri: uri,
+      },
+    ]);
+    expect(prompt).toContain(mailboxMailErrorUriHint());
+    expect(prompt).toContain(
+      "use read_file with that URI (offset/limit supported)",
+    );
+    expect(prompt).toContain(uri);
+  });
+
   test("does not paste a duplicate agent_id", () => {
     const prompt = buildMailboxMailPrompt([
       { agent_id: "w1", status: "done" },
@@ -163,6 +181,10 @@ describe("digestCollectedReports", () => {
       "use read_file with that URI (offset/limit supported)",
     );
     expect(digest?.report).not.toContain("SECRET_FINDINGS_BODY");
+    expect(digest?.report).toContain(
+      `${ENVELOPE_REPORT.length.toLocaleString()} more chars omitted here`,
+    );
+    expect(digest?.report).not.toMatch(/— 0 more chars omitted/);
     expect(
       new TextDecoder().decode(
         store.blobs.get(fleetDrySpillKey("worker-1", "report"))?.bytes ??
@@ -222,6 +244,63 @@ describe("digestCollectedReports", () => {
     expect(digest?.report).toContain("SECRET_FINDINGS_BODY");
     expect(digest?.summary).toBe("Shipped the digest.");
   });
+
+  test("spill notice remaining is the omitted body when no prefix is kept", async () => {
+    const store = fakeBlobStore();
+    const short = "short spilled body";
+    const [digest] = await digestCollectedReports(
+      [{ agent_id: "short", status: "done", report: short }],
+      store.writeBlob,
+    );
+    expect(short.length).toBeLessThan(MAILBOX_DIGEST_SECTION_CHARS);
+    expect(digest?.report).toContain(
+      `${short.length.toLocaleString()} more chars omitted here`,
+    );
+    expect(digest?.report).not.toMatch(/— 0 more chars omitted/);
+    expect(digest?.report).not.toContain(short);
+    expect(digest?.report_uri).toBe(
+      `tool-output:///${fleetDrySpillKey("short", "report")}`,
+    );
+  });
+
+  test("oversized spilled report remaining is the full body, not length minus the digest cap", async () => {
+    const store = fakeBlobStore();
+    const original = "x".repeat(MAILBOX_DIGEST_SECTION_CHARS + 80);
+    const [digest] = await digestCollectedReports(
+      [{ agent_id: "long", status: "done", report: original }],
+      store.writeBlob,
+    );
+    expect(digest?.report).toContain(
+      `${original.length.toLocaleString()} more chars omitted here`,
+    );
+    expect(digest?.report).not.toContain(
+      `${(original.length - MAILBOX_DIGEST_SECTION_CHARS).toLocaleString()} more chars omitted here`,
+    );
+  });
+
+  test("spilled error names error_uri with truncation-notice language", async () => {
+    const store = fakeBlobStore();
+    const error = "provider boom";
+    const [digest] = await digestCollectedReports(
+      [{ agent_id: "fail", status: "failed", error }],
+      store.writeBlob,
+    );
+    const errorUri = `tool-output:///${fleetDrySpillKey("fail", "error")}`;
+    expect(digest?.error_uri).toBe(errorUri);
+    expect(digest?.error).toContain("[output truncated");
+    expect(digest?.error).toContain(errorUri);
+    expect(digest?.error).toContain(
+      `${error.length.toLocaleString()} more chars omitted here`,
+    );
+    expect(digest?.error).not.toMatch(/— 0 more chars omitted/);
+    expect(digest?.error).not.toContain("provider boom");
+    expect(
+      new TextDecoder().decode(
+        store.blobs.get(fleetDrySpillKey("fail", "error"))?.bytes ??
+          new Uint8Array(),
+      ),
+    ).toBe(error);
+  });
 });
 
 describe("driveMailboxMail", () => {

src/subagent/mailbox-mail-drive.ts

@@ -35,6 +35,11 @@ export function mailboxMailReportUriHint(): string {
   return "report_uri names a spilled full report — use read_file with that URI (offset/limit supported) to see the rest.";
 }
 
+/** Names error_uri with truncation-notice language so the parent may fetch it. */
+export function mailboxMailErrorUriHint(): string {
+  return "error_uri names a spilled full error — use read_file with that URI (offset/limit supported) to see the rest.";
+}
+
 /**
  * Whether inbound text is occupancy's mailbox mail. Internal runtime→agent
  * traffic — the fleet board already owns worker status and the payload is
@@ -85,13 +90,16 @@ export function occupancyShouldYieldWait(
 }
 
 export function buildMailboxMailPrompt<
-  T extends { agent_id: string; report_uri?: string },
+  T extends { agent_id: string; report_uri?: string; error_uri?: string },
 >(reports: readonly T[]): string {
   const unique = dedupeByAgentId(reports);
   const lines = [mailboxMailWakeLine()];
   if (unique.some((report) => report.report_uri !== undefined)) {
     lines.push(mailboxMailReportUriHint());
   }
+  if (unique.some((report) => report.error_uri !== undefined)) {
+    lines.push(mailboxMailErrorUriHint());
+  }
   lines.push(JSON.stringify(unique));
   return lines.join("\n");
 }