diff --git a/docker/base-image/agent_server/services/headless_executor.py b/docker/base-image/agent_server/services/headless_executor.py
index 3171d244..90e16300 100644
--- a/docker/base-image/agent_server/services/headless_executor.py
+++ b/docker/base-image/agent_server/services/headless_executor.py
@@ -22,7 +22,7 @@
 import threading
 import time
 import uuid
-from dataclasses import dataclass, field
+from dataclasses import dataclass, field, replace
 from datetime import datetime
 from pathlib import Path
 from typing import Dict, List, Optional, Tuple
@@ -230,6 +230,11 @@ class HeadlessRunContext:
     #   "max_duration"    — the effective_timeout budget was genuinely exhausted
     termination_reason: Optional[str] = None
     stalled_tool: Optional[str] = None
+    # #1025 (salvaged from #980): True when the drain returned anything other
+    # than "completed" (budget_exceeded / errored / leaked) — a reader thread
+    # may still be alive and mutating the shared buffers below, so
+    # _finalize_headless_result snapshots its read fields before reading them.
+    reader_may_be_live: bool = False
 
     # Shared mutable buffers (populated by stream_parser via process_stream_line)
     response_parts: List[str] = field(default_factory=list)
@@ -674,6 +679,10 @@ def _run_stdout() -> None:
             f"[Headless Task] Task {ctx.task_session_id} {reason} — killing process group"
         )
         _terminate_process_group(process, graceful_timeout=5, pgid=ctx.process_pgid, execution_tag=ctx.task_session_id)
+        # Bound the post-kill drain, then re-raise. The orchestrator converts
+        # TimeoutExpired to HTTP 504 WITHOUT calling _finalize_headless_result,
+        # so there is no finalize read to protect here — we intentionally do
+        # not set reader_may_be_live on this path (#1025).
         _drain_bounded(process, stdout_thread, stderr_thread,
                        grace=3, pgid=ctx.process_pgid,
                        execution_tag=ctx.task_session_id)
@@ -682,15 +691,82 @@ def _run_stdout() -> None:
     # Subprocess exited. Drain readers — if a hook grandchild still
     # holds a pipe, the helper will close the pipe FDs so the
     # reader threads can exit.
-    _drain_bounded(process, stdout_thread, stderr_thread,
-                   grace=5, pgid=ctx.process_pgid,
-                   execution_tag=ctx.task_session_id)
+    drain_outcome = _drain_bounded(process, stdout_thread, stderr_thread,
+                                   grace=5, pgid=ctx.process_pgid,
+                                   execution_tag=ctx.task_session_id)
+    # Anything but "completed" means a reader thread may still be alive and
+    # mutating the shared buffers finalize reads (#1025).
+    ctx.reader_may_be_live = drain_outcome != "completed"
 
     # Re-raise permission-mode failure captured by stdout thread
     if ctx.stdout_exc:
         raise ctx.stdout_exc[0]
 
 
+# #1025: number of times _snapshot_for_finalize retries the metadata deep-copy
+# when it loses the race with a still-mutating leaked reader. The mutation
+# window is a single attribute set, so the first retry almost always wins.
+_SNAPSHOT_RETRY_ATTEMPTS = 3
+
+
+def _freeze_event(event: threading.Event) -> threading.Event:
+    """Capture an Event's set/clear state into a fresh, immutable-by-the-reader
+    Event (#1025). The leaked reader holds the ORIGINAL event; the snapshot's
+    copy reflects the verdict as of snapshot time and can't be flipped later."""
+    frozen = threading.Event()
+    if event.is_set():
+        frozen.set()
+    return frozen
+
+
+def _snapshot_for_finalize(ctx: HeadlessRunContext) -> HeadlessRunContext:
+    """Copy the finalize-read fields of ``ctx`` onto a fresh context (#1025/D19).
+
+    Called whenever the drain left a reader thread that may still be alive
+    (``reader_may_be_live``), where that reader can mutate ``ctx`` concurrently.
+    Every field finalize reads is captured: the four buffers (``list(...)`` of a
+    list never raises on a concurrent append), ``metadata`` (deep-copied), and
+    the auth-abort signal (``auth_abort_event`` frozen via :func:`_freeze_event`
+    + ``auth_abort_reason`` list-copied) — the stderr reader is exactly the
+    thread that can leak, and finalize reads ``auth_abort_event.is_set()`` /
+    ``auth_abort_reason[0]``, so leaving them aliased would let a late auth match
+    flip a success to a spurious 503.
+
+    ``metadata.model_copy(deep=True)`` iterates the pydantic model's ``__dict__``
+    / ``__pydantic_fields_set__`` and CAN raise ``RuntimeError("... changed size
+    during iteration")`` if the reader sets a not-yet-set field mid-copy (most
+    relevant on the ``errored`` outcome, where the reader isn't necessarily
+    wedged in ``readline``). The window is a single attribute set, so a bounded
+    retry almost always succeeds on the next attempt; if every attempt loses the
+    race we fall back to the live ``ctx`` (the pre-#1025 behaviour — no worse
+    than not snapshotting).
+    """
+    last_exc: Optional[RuntimeError] = None
+    for _ in range(_SNAPSHOT_RETRY_ATTEMPTS):
+        try:
+            return replace(
+                ctx,
+                raw_messages=list(ctx.raw_messages),
+                response_parts=list(ctx.response_parts),
+                execution_log=list(ctx.execution_log),
+                verbose_output_lines=list(ctx.verbose_output_lines),
+                metadata=ctx.metadata.model_copy(deep=True),
+                auth_abort_event=_freeze_event(ctx.auth_abort_event),
+                auth_abort_reason=list(ctx.auth_abort_reason),
+            )
+        except RuntimeError as exc:
+            # "dictionary/set changed size during iteration" — the leaked
+            # reader mutated metadata mid-deep-copy. Retry.
+            last_exc = exc
+    logger.warning(
+        "[Headless Task] Run-context snapshot lost the race with a leaked "
+        "reader %s times for task %s (%s) — finalizing against the live "
+        "context as a last resort. Issue #1025.",
+        _SNAPSHOT_RETRY_ATTEMPTS, ctx.task_session_id, last_exc,
+    )
+    return ctx
+
+
 def _finalize_headless_result(
     ctx: HeadlessRunContext,
 ) -> Tuple[str, List[Dict], ExecutionMetadata, str]:
@@ -706,6 +782,27 @@ def _finalize_headless_result(
     orchestrator's outer ``except HTTPException: raise`` chain re-surfaces
     them).
     """
+    # #1025 (D19): when the drain left a reader thread that may still be alive
+    # (budget_exceeded / errored / leaked), it can keep mutating the shared
+    # buffers below. Snapshot EVERY field finalize reads onto a fresh context so
+    # iteration can't tear and a late append by the leaked reader can't be
+    # half-read. ``parse_failure_count`` / ``parse_failure_sample`` are an int +
+    # str (atomic reads, no snapshot). The leaked reader keeps mutating the
+    # ORIGINAL ctx (which it captured by closure); rebinding the local ``ctx``
+    # to the snapshot means the rest of finalize — including
+    # _attempt_empty_result_recovery's in-place mutations — operates entirely
+    # on the isolated copy. Clean-drain runs skip this and keep the zero-copy
+    # fast path. The snapshot itself is retry-guarded against the same race
+    # (see _snapshot_for_finalize).
+    if ctx.reader_may_be_live:
+        logger.warning(
+            "[Headless Task] Drain left a possibly-live reader for task %s — "
+            "finalizing against a snapshot of the run context (the reader may "
+            "still be mutating the live buffers). Issue #1025.",
+            ctx.task_session_id,
+        )
+        ctx = _snapshot_for_finalize(ctx)
+
     # Build verbose transcript from stderr (the human-readable execution log)
     # SECURITY: Sanitize stderr output
     sanitized_lines = [sanitize_text(line) for line in ctx.verbose_output_lines]
diff --git a/docker/base-image/agent_server/services/subprocess_lifecycle.py b/docker/base-image/agent_server/services/subprocess_lifecycle.py
index 155c2b27..9285e3d5 100644
--- a/docker/base-image/agent_server/services/subprocess_lifecycle.py
+++ b/docker/base-image/agent_server/services/subprocess_lifecycle.py
@@ -14,7 +14,7 @@
 import logging
 import subprocess
 import threading
-from typing import Optional
+from typing import Literal, Optional
 
 from ..utils.subprocess_pgroup import (
     capture_pgid as _capture_pgid,
@@ -23,8 +23,16 @@
     drain_reader_threads as _drain_reader_threads,
 )
 
+# Outcome of a bounded drain, returned to the caller so the orchestrator can
+# decide whether to treat the run's shared mutable state as trustworthy
+# (#1025 / salvaged from #980). Every non-``completed`` outcome leaves a reader
+# thread that may still be mutating the run context concurrently; the headless
+# path snapshots its finalize-read fields on those outcomes.
+DrainOutcome = Literal["completed", "budget_exceeded", "errored", "leaked"]
+
 __all__ = [
     "_DRAIN_BUDGET_SECONDS",
+    "DrainOutcome",
     "_drain_bounded",
     "_capture_pgid",
     "_terminate_process_group",
@@ -51,7 +59,7 @@ def _drain_bounded(
     grace: int = 5,
     pgid: Optional[int] = None,
     execution_tag: Optional[str] = None,
-) -> None:
+) -> DrainOutcome:
     """Run drain_reader_threads with a hard _DRAIN_BUDGET_SECONDS time cap.
 
     Prevents a TextIOWrapper lock deadlock in safe_close_pipes (Issue #728)
@@ -62,17 +70,52 @@ def _drain_bounded(
 
     Issue #817: ``execution_tag`` is threaded through to the underlying
     ``drain_reader_threads`` so the env-tag sweep runs after every drain.
+
+    Issue #1025 (salvaged from #980): returns the drain outcome instead of
+    ``None`` so the headless orchestrator can detect every case that leaves a
+    reader thread still mutating the shared run context, and snapshot its
+    finalize-read fields before reading them. Only ``"completed"`` guarantees
+    no reader survives:
+
+    - ``"completed"``       — the drain finished within budget AND every
+      reader thread passed in has exited.
+    - ``"budget_exceeded"`` — the daemon thread did not finish within
+      ``_DRAIN_BUDGET_SECONDS`` (the #728 safe_close_pipes deadlock); the
+      reader thread is leaked.
+    - ``"errored"``         — the drain raised. Previously swallowed with
+      ``except Exception: pass``, which masked the failure as a clean
+      ``"completed"``; now captured and logged (honours the project-wide
+      "never swallow exceptions silently" rule).
+    - ``"leaked"``          — the drain returned normally within budget but a
+      reader thread is still alive. ``drain_reader_threads`` force-closes the
+      pipes and *continues* when a grandchild-held reader won't EOF (its own
+      ``outcome=leaked`` METRIC), returning normally — so a within-budget,
+      non-raising drain does NOT by itself prove the readers are dead. This
+      outcome closes that gap (the #586 leaked-reader case).
     """
     done = threading.Event()
+    # Single write-once cell, set only by the daemon thread and read only after
+    # the ``done`` barrier — no second Event / ordering invariant to maintain.
+    outcome: DrainOutcome = "completed"
 
     def _target() -> None:
+        nonlocal outcome
         try:
             asyncio.run(_drain_reader_threads(
                 process, *threads, grace=grace, pgid=pgid,
                 execution_tag=execution_tag,
             ))
         except Exception:
-            pass
+            # #1025: capture + log instead of swallowing. A drain that raises
+            # used to be indistinguishable from a clean completion, hiding a
+            # leaked reader thread from the finalize path.
+            outcome = "errored"
+            logger.exception(
+                "[Subprocess] Drain raised inside the daemon thread "
+                "(pid=%s) — treating as errored; reader thread(s) may be "
+                "leaked. Issue #1025.",
+                process.pid,
+            )
         finally:
             done.set()
 
@@ -84,3 +127,21 @@ def _target() -> None:
             "leaked daemon threads (pid=%s). Issue #728.",
             _DRAIN_BUDGET_SECONDS, process.pid,
         )
+        return "budget_exceeded"
+    # done is set → _target finished, so ``outcome`` is fully settled.
+    if outcome == "errored":
+        return "errored"
+    # The drain returned within budget without raising, but drain_reader_threads
+    # force-closes and continues on the #586 leaked-reader case. If any reader
+    # we were asked to drain is still alive, it can keep mutating ctx — surface
+    # that so the caller snapshots instead of trusting the live buffers (#1025).
+    if any(t is not None and t.is_alive() for t in threads):
+        logger.warning(
+            "[Subprocess] Drain returned within budget but %d reader thread(s) "
+            "are still alive (pid=%s) — reporting 'leaked' so finalize snapshots "
+            "the run context. Issue #1025.",
+            sum(1 for t in threads if t is not None and t.is_alive()),
+            process.pid,
+        )
+        return "leaked"
+    return "completed"
diff --git a/tests/unit/test_drain_bounded.py b/tests/unit/test_drain_bounded.py
index 531ad39d..cdca484b 100644
--- a/tests/unit/test_drain_bounded.py
+++ b/tests/unit/test_drain_bounded.py
@@ -139,3 +139,92 @@ async def _recording_drain(process, *threads, grace=5, pgid=None, **kwargs):
 
     assert received.get("grace") == 3, f"grace not forwarded: {received}"
     assert received.get("pgid") == 42, f"pgid not forwarded: {received}"
+
+
+# ---------------------------------------------------------------------------
+# Issue #1025 (salvaged from #980): _drain_bounded returns a DrainOutcome and
+# no longer swallows daemon-thread exceptions.
+# ---------------------------------------------------------------------------
+
+def test_drain_bounded_returns_completed_on_clean_drain(monkeypatch):
+    """A drain that finishes within budget must report ``completed``."""
+
+    async def _fast_drain(*args, **kwargs):
+        return None
+
+    monkeypatch.setattr(
+        "agent_server.services.subprocess_lifecycle._drain_reader_threads",
+        _fast_drain,
+    )
+
+    assert _drain_bounded(_make_fake_process(), grace=5, pgid=None) == "completed"
+
+
+def test_drain_bounded_returns_budget_exceeded_on_hang(monkeypatch):
+    """A drain that overruns the budget must report ``budget_exceeded`` (a
+    leaked reader thread the finalize path must defend against)."""
+
+    async def _hanging_drain(*args, **kwargs):
+        await asyncio.sleep(600)
+
+    monkeypatch.setattr(
+        "agent_server.services.subprocess_lifecycle._drain_reader_threads",
+        _hanging_drain,
+    )
+    monkeypatch.setattr(
+        "agent_server.services.subprocess_lifecycle._DRAIN_BUDGET_SECONDS",
+        1,
+    )
+
+    assert _drain_bounded(_make_fake_process(), grace=1, pgid=None) == "budget_exceeded"
+
+
+def test_drain_bounded_returns_errored_and_logs_when_drain_raises(monkeypatch, caplog):
+    """A drain that raises must be reported as ``errored`` and logged — not
+    silently swallowed as a clean ``completed`` (the pre-#1025 bug)."""
+
+    async def _raising_drain(*args, **kwargs):
+        raise RuntimeError("boom in drain")
+
+    monkeypatch.setattr(
+        "agent_server.services.subprocess_lifecycle._drain_reader_threads",
+        _raising_drain,
+    )
+
+    with caplog.at_level("ERROR"):
+        outcome = _drain_bounded(_make_fake_process(), grace=5, pgid=None)
+
+    assert outcome == "errored"
+    assert any("Drain raised" in r.message for r in caplog.records), (
+        "errored drain must be logged, not swallowed"
+    )
+
+
+def test_drain_bounded_returns_leaked_when_reader_survives(monkeypatch):
+    """A drain that returns within budget WITHOUT raising but leaves a reader
+    thread alive must report ``leaked`` — drain_reader_threads force-closes and
+    continues on the #586 leaked-reader case, so a within-budget return does not
+    by itself prove the readers are dead (review finding on the #1025 PR)."""
+
+    async def _fast_drain(*args, **kwargs):
+        return None
+
+    monkeypatch.setattr(
+        "agent_server.services.subprocess_lifecycle._drain_reader_threads",
+        _fast_drain,
+    )
+
+    # A real, still-alive reader thread (the leaked case).
+    stop = threading.Event()
+    leaked_reader = threading.Thread(target=stop.wait, daemon=True)
+    leaked_reader.start()
+    try:
+        outcome = _drain_bounded(_make_fake_process(), leaked_reader, grace=5, pgid=None)
+    finally:
+        stop.set()
+        leaked_reader.join(timeout=2)
+
+    assert outcome == "leaked", (
+        "a within-budget drain that leaves a reader alive must surface 'leaked' "
+        "so finalize snapshots instead of trusting the live buffers"
+    )
diff --git a/tests/unit/test_headless_finalize_snapshot.py b/tests/unit/test_headless_finalize_snapshot.py
new file mode 100644
index 00000000..2f025ad3
--- /dev/null
+++ b/tests/unit/test_headless_finalize_snapshot.py
@@ -0,0 +1,118 @@
+"""Issue #1025 (salvaged from #980 / D19): _finalize_headless_result isolates
+itself from a leaked reader thread by snapshotting the run context.
+
+When _drain_bounded reports ``budget_exceeded`` or ``errored``, a reader
+thread is leaked and may still be appending to ctx's shared buffers / setting
+metadata fields. _finalize_headless_result rebinds ``ctx`` to a deep snapshot
+so iteration can't tear and a late append can't be half-read. Clean drains
+keep the zero-copy fast path.
+
+These tests exercise the snapshot helper directly (the concurrency-prone
+part) plus the gating flag — without spawning a real claude subprocess.
+"""
+from __future__ import annotations
+
+from unittest.mock import MagicMock
+
+# conftest.py preloads the real agent_server package; just import.
+from agent_server.services.headless_executor import (  # noqa: E402
+    HeadlessRunContext,
+    _snapshot_for_finalize,
+)
+
+
+def _make_ctx() -> HeadlessRunContext:
+    ctx = HeadlessRunContext(
+        cmd=["claude"],
+        task_session_id="task-1025",
+        task_start_iso="2026-06-05T00:00:00Z",
+        effective_timeout=600,
+        images=None,
+        prompt="hi",
+    )
+    ctx.raw_messages.append({"type": "assistant", "n": 1})
+    ctx.response_parts.append("hello")
+    ctx.metadata.cost_usd = 0.01
+    return ctx
+
+
+def test_snapshot_decouples_lists_from_later_mutation():
+    """A post-snapshot append to the live ctx (simulating a leaked reader)
+    must NOT leak into the snapshot used by finalize."""
+    ctx = _make_ctx()
+    snap = _snapshot_for_finalize(ctx)
+
+    # Snapshot must be a different context object with copied buffers.
+    assert snap is not ctx
+    assert snap.raw_messages is not ctx.raw_messages
+    assert snap.response_parts is not ctx.response_parts
+
+    # Leaked reader keeps mutating the ORIGINAL ctx after the snapshot.
+    ctx.raw_messages.append({"type": "assistant", "n": 2})
+    ctx.response_parts.append(" world")
+
+    # Snapshot is frozen at snapshot time.
+    assert snap.raw_messages == [{"type": "assistant", "n": 1}]
+    assert snap.response_parts == ["hello"]
+
+
+def test_snapshot_deep_copies_metadata():
+    """metadata must be deep-copied so a leaked reader writing a metadata
+    field after the snapshot can't mutate finalize's view."""
+    ctx = _make_ctx()
+    snap = _snapshot_for_finalize(ctx)
+
+    assert snap.metadata is not ctx.metadata
+    assert snap.metadata.cost_usd == 0.01
+
+    # Leaked reader writes more metadata onto the live ctx.
+    ctx.metadata.cost_usd = 0.99
+    ctx.metadata.num_turns = 7
+
+    assert snap.metadata.cost_usd == 0.01
+    assert snap.metadata.num_turns is None
+
+
+def test_snapshot_falls_back_to_live_ctx_when_every_retry_loses():
+    """If the deep-copy loses the race on every attempt, the helper returns the
+    live ctx rather than raising — no worse than the pre-#1025 behaviour."""
+    ctx = _make_ctx()
+
+    # Stand in a metadata whose deep-copy always loses the race (pydantic
+    # forbids patching model_copy on a real instance).
+    fake_metadata = MagicMock()
+    fake_metadata.model_copy.side_effect = RuntimeError(
+        "dictionary changed size during iteration"
+    )
+    ctx.metadata = fake_metadata
+
+    # Must not raise; falls back to the same ctx instance.
+    result = _snapshot_for_finalize(ctx)
+    assert result is ctx
+    assert fake_metadata.model_copy.call_count == 3  # _SNAPSHOT_RETRY_ATTEMPTS
+
+
+def test_reader_may_be_live_defaults_false():
+    """Clean runs must keep the zero-copy fast path (flag off by default)."""
+    assert _make_ctx().reader_may_be_live is False
+
+
+def test_snapshot_freezes_auth_abort_signal():
+    """A leaked stderr reader that sets auth-abort AFTER the snapshot must not
+    flip finalize's view — auth_abort_event/reason are frozen at snapshot time
+    (regression guard for the review finding on spurious 503s)."""
+    ctx = _make_ctx()
+    snap = _snapshot_for_finalize(ctx)
+
+    # Snapshot copies must be distinct objects, frozen at the not-set state.
+    assert snap.auth_abort_event is not ctx.auth_abort_event
+    assert snap.auth_abort_reason is not ctx.auth_abort_reason
+    assert snap.auth_abort_event.is_set() is False
+
+    # Leaked reader fires an auth abort on the live ctx after the snapshot.
+    ctx.auth_abort_reason.append("Not logged in")
+    ctx.auth_abort_event.set()
+
+    # Finalize's view (the snapshot) is unaffected.
+    assert snap.auth_abort_event.is_set() is False
+    assert snap.auth_abort_reason == []