fix: [AI-450] address code review — typed StaleFileError, bounded reads, safe stringification

Addresses all findings from multi-model code review:

- CRITICAL: Replace regex-based error detection with typed `StaleFileError` class
  that extends `Error` with a `filePath` property. Use `instanceof` check in
  processor instead of brittle string parsing.

- MAJOR: Add 50KB size limit before auto-reading stale files to prevent OOM and
  token explosion. Files over limit get a message to use the Read tool instead.

- MAJOR: Handle missing files (deleted between error and re-read) gracefully.

- MINOR: Use `String(value.error ?? "Unknown error")` for null-safe stringification.

- MINOR: Use markdown code fences instead of `<file>` XML tags to prevent
  prompt injection via unescaped content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: anandgupta42

packages/opencode/src/file/time.ts

@@ -3,6 +3,17 @@ import { Log } from "../util/log"
 import { Flag } from "../flag/flag"
 import { Filesystem } from "../util/filesystem"
 
+// altimate_change start — typed error for stale file detection (#450)
+export class StaleFileError extends Error {
+  public readonly filePath: string
+  constructor(filePath: string, message: string) {
+    super(message)
+    this.name = "StaleFileError"
+    this.filePath = filePath
+  }
+}
+// altimate_change end
+
 export namespace FileTime {
   const log = Log.create({ service: "file.time" })
   // Per-session read times plus per-file write locks.
@@ -63,9 +74,12 @@ export namespace FileTime {
     const mtime = Filesystem.stat(filepath)?.mtime
     // Allow a 50ms tolerance for Windows NTFS timestamp fuzziness / async flushing
     if (mtime && mtime.getTime() > time.getTime() + 50) {
-      throw new Error(
+      // altimate_change start — use typed StaleFileError (#450)
+      throw new StaleFileError(
+        filepath,
         `File ${filepath} has been modified since it was last read.\nLast modification: ${mtime.toISOString()}\nLast read: ${time.toISOString()}\n\nPlease read the file again before modifying it.`,
       )
+      // altimate_change end
     }
   }
 }

packages/opencode/src/session/processor.ts

@@ -19,7 +19,8 @@ import type { SessionID, MessageID } from "./schema"
 // altimate_change start — import Telemetry for per-generation token tracking
 import { Telemetry } from "@/altimate/telemetry"
 // altimate_change end
-// altimate_change start — import FileTime and Filesystem for stale file recovery (#450)
+// altimate_change start — import StaleFileError and Filesystem for stale file recovery (#450)
+import { StaleFileError } from "@/file/time"
 import { FileTime } from "@/file/time"
 import { Filesystem } from "@/util/filesystem"
 // altimate_change end
@@ -216,17 +217,24 @@ export namespace SessionProcessor {
                   const match = toolcalls[value.toolCallId]
                   if (match && match.state.status === "running") {
                     // altimate_change start — auto-read stale files so model sees current content (#450)
-                    let errorStr = (value.error as any).toString()
-                    const staleFileMatch =
-                      errorStr.match(/File (.+) has been modified since it was last read/) ??
-                      errorStr.match(/You must read file (.+) before overwriting it/)
-                    if (staleFileMatch) {
-                      const staleFilePath = staleFileMatch[1].trim()
+                    let errorStr = String(value.error ?? "Unknown error")
+                    if (value.error instanceof StaleFileError) {
+                      const staleFilePath = value.error.filePath
                       try {
-                        const freshContent = await Filesystem.readText(staleFilePath)
-                        FileTime.read(input.sessionID, staleFilePath)
-                        errorStr += `\n\nThe file has been auto-re-read. Here is the current content:\n<file path="${staleFilePath}">\n${freshContent}\n</file>`
-                        log.info("stale file auto-re-read", { file: staleFilePath, sessionID: input.sessionID })
+                        const stat = Filesystem.stat(staleFilePath)
+                        const MAX_AUTO_READ_BYTES = 50 * 1024
+                        if (!stat) {
+                          errorStr += "\n\nNote: The file no longer exists on disk."
+                        } else if (Number(stat.size) > MAX_AUTO_READ_BYTES) {
+                          FileTime.read(input.sessionID, staleFilePath)
+                          errorStr += `\n\nThe file has been modified (${Math.round(Number(stat.size) / 1024)}KB). It is too large to include here — please use the Read tool to view it.`
+                        } else {
+                          const freshContent = await Filesystem.readText(staleFilePath)
+                          FileTime.read(input.sessionID, staleFilePath)
+                          const fence = "````"
+                          errorStr += `\n\nThe file has been auto-re-read. Here is the current content:\n\n${fence}\n${freshContent}\n${fence}`
+                          log.info("stale file auto-re-read", { file: staleFilePath, sessionID: input.sessionID })
+                        }
                       } catch (readErr) {
                         log.warn("failed to auto-re-read stale file", { file: staleFilePath, error: readErr })
                       }