fix(chat): remember a letta seen outside its start folder by its stable start

changelog.d/letta-moved-process-identity.md

@@ -0,0 +1,3 @@
+### Fixed
+- A Letta Code pane whose letta was seen outside the folder it started in keeps showing Terminal
+  after the PC's clock is corrected, instead of Chat showing a conversation it may have left.

server/letta-store.ts

@@ -22,7 +22,7 @@ import { basename, isAbsolute, join, normalize } from "node:path";
 import type { ConversationMetadata, HerdrPane } from "../shared/protocol.ts";
 import { darwinProcess } from "./claude-store.ts";
 import { paneProcessInfo, type ProcessInfo } from "./herdr/client.ts";
-import { processStartLowerBound } from "./process-start.ts";
+import { processStartIdentity, processStartLowerBound } from "./process-start.ts";
 
 /** Letta's ids: `conv-<uuid>`, `local-conv-<n>`, `agent-<uuid>`, `agent-local-<uuid>`, and `default`. */
 const ID = /^[A-Za-z0-9][A-Za-z0-9_-]{0,199}$/;
@@ -150,10 +150,13 @@ export function isLettaProcess(argv: readonly string[]): boolean {
   return ["node", "bun"].includes(name(argv[0])) && ["letta", "letta.js"].includes(name(argv[1]));
 }
 
-/** `startedAt`: no later than the process started (processStartLowerBound) */
-export interface LettaProcess { pid: number; cwd: string; startedAt: number; env: Env }
+/**
+ * `startedAt`: no later than the process started (processStartLowerBound), which may read differently each time;
+ * `identity`: processStartIdentity, the same on every read, so with the pid it names the process
+ */
+export interface LettaProcess { pid: number; cwd: string; startedAt: number; identity: number; env: Env }
 
-/** Letta processes, by pid and start, seen in a folder other than the one they started in. */
+/** Letta processes, by pid and start identity, seen in a folder other than the one they started in. */
 const moved = new Set<string>();
 
 /**
@@ -164,7 +167,7 @@ const moved = new Set<string>();
  * once seen elsewhere, it is never bound again.
  */
 export function lettaInStartFolder(owner: LettaProcess): boolean {
-  const key = `${owner.pid}:${owner.startedAt}`;
+  const key = `${owner.pid}:${owner.identity}`;
   if (moved.has(key)) return false;
   const start = owner.env["PWD"];
   const here = canonical(owner.cwd);
@@ -184,8 +187,9 @@ export async function paneLettaProcess(paneId: string, known?: ProcessInfo | nul
   const { pid, cwd } = found[0]!;
   if (typeof cwd !== "string" || !isAbsolute(cwd)) return null;
   const startedAt = processStartLowerBound(pid);
+  const identity = processStartIdentity(pid);
   const env = processEnvironment(pid);
-  return startedAt === null || env === null ? null : { pid, cwd, startedAt, env };
+  return startedAt === null || identity === null || env === null ? null : { pid, cwd, startedAt, identity, env };
 }
 
 export type LettaTranscript = { path: string; session: string; agent: string; conversation: string };

server/letta.test.ts

@@ -108,7 +108,7 @@ describe("Letta conversation binding", () => {
       writeFileSync(join(cwd, ".letta", "settings.local.json"), settings);
       utimesSync(join(cwd, ".letta", "settings.local.json"), at / 1000, at / 1000);
     }
-    return { pid: 4242, cwd, startedAt: started, env: { HOME: root, ...env } };
+    return { pid: 4242, cwd, startedAt: started, identity: started, env: { HOME: root, ...env } };
   };
   const now = Date.now();
 
@@ -179,7 +179,7 @@ describe("Letta conversation binding", () => {
     const other = join(root, "project2");
     mkdirSync(start); mkdirSync(other);
     symlinkSync(start, join(root, "link1"));
-    const at = (pid: number, cwd: string, env: Record<string, string>): LettaProcess => ({ pid, cwd, startedAt: now, env });
+    const at = (pid: number, cwd: string, env: Record<string, string>, startedAt = now): LettaProcess => ({ pid, cwd, startedAt, identity: now, env });
     expect(lettaInStartFolder(at(101, start, { PWD: start }))).toBe(true);
     expect(lettaInStartFolder(at(102, start, { PWD: join(root, "link1") }))).toBe(true);
     // no start folder to compare: its shell set no PWD
@@ -188,6 +188,9 @@ describe("Letta conversation binding", () => {
     // SetWorkingDirectory moved it into project2; a later move back does not make project1's record its own again
     expect(lettaInStartFolder(at(105, other, { PWD: start }))).toBe(false);
     expect(lettaInStartFolder(at(105, start, { PWD: start }))).toBe(false);
+    // the lower bound read differently on its return (a stepped wall clock moves it on Linux): still the same process
+    expect(lettaInStartFolder(at(106, other, { PWD: start }))).toBe(false);
+    expect(lettaInStartFolder(at(106, start, { PWD: start }, now - 1_000))).toBe(false);
   });
 
   test("a closed peer of the same agent that reopened the pane's old conversation binds nothing", () => {

server/process-start.ts

@@ -39,6 +39,22 @@ function linuxBirth(pid: number): number | null {
   }
 }
 
+/**
+ * What tells one process from another on every read, however the wall clock moves: Linux's start in
+ * ticks after boot (`btime`, and so processStartedAt, follows a clock that is set later), macOS's
+ * kernel start record. null where neither answers.
+ */
+export function processStartIdentity(pid: number): number | null {
+  if (process.platform === "darwin") return darwinStart(pid);
+  if (process.platform !== "linux") return null;
+  try {
+    const ticks = Number(readFileSync(`/proc/${pid}/stat`, "utf8").split(") ").pop()!.split(" ")[19]);
+    return Number.isFinite(ticks) ? ticks : null;
+  } catch {
+    return null;
+  }
+}
+
 /** Whether `pid` names a running process: signal 0 checks without sending anything, and a refusal means it runs. */
 export function processAlive(pid: number): boolean {
   try {