[anukuljha]

What changes

A Mac still on a Desktop host older than #1010 no longer has its Agent Host link closed when a command prints a lot between two reads. Its processes are started with at most 256 KiB of output kept, so no process.read answer can pass the API's 1 MiB websocket ceiling. A burst past that keeps its newest output and reports the rest as truncated, as any full output ring does. Current hosts are unchanged.

Why

The review of #1010 found the gap: an old host ignores process.read's max_bytes and answers with up to 1 MiB of buffered output (1.33 MiB once base64'd). The socket closed with 1009 mid-run, and the run ended without an answer. That host's output ring is bounded by the output_limit_bytes it is given at process.start (desktop/agent-host/src/host_exec/ring.rs: newest kept, truncated_before_sequence reported). So the server can bound every read without a Desktop update.

• old_host_params in agent_host_link_ops.py caps output_limit_bytes at LINK_INLINE_DATA_BYTES on process.start for a host that doesn't advertise transfers.
• Docs: desktop-host-execution.md §4 and agent-host.md.

The review's other finding (0058 edited in place) needs no change: no database has applied 0058, per the owner.

How to verify

cd lemma-backend && uv run pytest app/modules/agent/tests/e2e/test_agent_host_transfers_e2e.py
make quality
make format-check

test_an_old_hosts_process_keeps_no_more_output_than_one_message_holds drives a pre-transfers host and a current host through the real link and op client. It fails with the cap removed. Transfers e2e: 7 passed. Agent unit tests: 3,172 passed. make quality: 32/32.

Checklist

• Tests cover the behavioral change
• Lint, typecheck, and architecture gates pass locally

Compatibility and rollback notes

No API, schema or wire change. It only lowers the output buffer requested from hosts that predate transfers. A plain revert is safe.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved process output handling on hosts without transfer support by limiting inline output, retaining the newest content, and reporting truncation instead of disconnecting.
  • Legacy hosts now enforce a safe inline output limit while preserving lower requested limits.
  • Added clearer indicators when earlier process output has been dropped.

• Documentation

  • Clarified legacy desktop transfer behavior, including inline image limits and reconnect behavior.
  • Updated guidance to reflect that process output limits are applied when a process starts.

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• docs/architecture/desktop-host-execution.md
• lemma-backend/app/modules/agent/services/agent_host_link_ops.py
• lemma-backend/app/modules/agent/tests/e2e/test_agent_host_transfers_e2e.py
• lemma-backend/app/modules/agent/tests/unit/test_agent_host_old_host_params.py

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• lemma-backend/app/modules/agent/tests/e2e/test_agent_host_transfers_e2e.py
• lemma-backend/app/modules/agent/services/agent_host_link_ops.py

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📝 Walkthrough

The relay caps process.start output for legacy hosts. Process output collection now reports dropped data. Architecture documentation and tests describe and verify these behaviors.

Changes: Legacy host output limits and truncation

Layer / File(s)	Summary
Apply legacy host output limits lemma-backend/app/modules/agent/services/agent_host_link_ops.py, docs/architecture/agent-host.md, docs/architecture/desktop-host-execution.md	The relay applies old_host_params to legacy-host process.start operations. The helper caps valid limits at the calculated safe ceiling and replaces invalid or missing limits with that bound. The documentation describes inline reads, bounded output, and dropped-data reporting.
Report dropped process output lemma-backend/app/modules/workspace/process_output.py, lemma-backend/app/modules/workspace/tests/unit/test_process_output_dropped.py	The collector detects unread output removed from the runtime buffer. It adds DROPPED_OUTPUT_MARKER to stdout and returns output_truncated while preserving cursor behavior.
Validate output-limit behavior lemma-backend/app/modules/agent/tests/unit/test_agent_host_old_host_params.py, lemma-backend/app/modules/agent/tests/e2e/test_agent_host_transfers_e2e.py	Unit tests cover limit normalization, parameter preservation, and frame-size boundaries. The end-to-end test checks different limits for released and current hosts.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Suggested reviewers: deepak-jha-kgp

Merge Risk: ⚪ Minimal · up to 3cb44

Processes started on older Desktop hosts now receive an output limit of about 14 KiB. A single read therefore fits within the 1 MiB websocket message limit even in the worst case. Dropped output is now reported to callers instead of being lost silently, and current hosts are unchanged. No open merge-blocking issues remain.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 29.03% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 31 functions across 5 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: keeping process reads from old Desktop hosts within the 1 MiB socket limit.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 29.03% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 31 functions across 5 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

✨ Finishing Touches: 💡 1

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Actionable comments posted: 1

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Inline comments:
Review comments at
@lemma-backend/app/modules/agent/services/agent_host_link_ops.py:
- Around line 311-325: Update old_host_params to reject bool values when
validating output_limit_bytes, since Python bool is a subclass of int but must
not be forwarded as a numeric limit. Preserve the existing positive-integer
handling and LINK_INLINE_DATA_BYTES fallback for invalid values.

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CodeQL

No new findings on lines this pull request changed (as of 3cb44b8).

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[github-actions]

Module	Unit	E2E only	Combined
Overall	73.0%	82.3%	91.6%
agent	78.6%	83.3%	93.0%
agent_surfaces	72.5%	83.3%	90.8%
apps	75.7%	87.6%	92.4%
connectors	78.6%	73.7%	90.2%
datastore	69.1%	86.4%	90.9%
function	70.2%	87.7%	92.8%
icon	88.8%	77.1%	89.6%
identity	64.4%	86.3%	92.5%
pod	62.3%	89.7%	92.2%
pod_bundle	75.8%	69.5%	88.2%
schedule	63.5%	88.9%	91.7%
usage	74.4%	84.5%	91.9%
workflow	70.0%	82.9%	89.9%
workspace	79.4%	59.4%	86.8%

[deepak-jha-kgp]

Factory review: Ready to merge

round 2 · reviewed at 9bc45bf · updated 20 Oct, 12:54 UTC · waiting on @anukuljha · low risk

Old Desktop hosts stop killing the link on a chatty command

• Before: On a computer whose Desktop predates transfers, Lemma asked a process for at most 256 KiB per read, but that host ignored the request and answered with its whole output ring -- up to 1 MiB, 1.33 MiB once base64'd. A command printing more than about 750 KB between two reads pushed the frame past the 1 MiB socket ceiling, and the link closed with 1009 mid-run.
• After: Lemma now starts a process on such a host with an output ring of at most 256 KiB, so a read cannot exceed the ceiling. A command that prints more between two reads keeps its newest output and loses the middle, instead of ending the run.

What could break

• If a released Desktop build clamps or ignores a smaller output_limit_bytes, then the fix does nothing and the 1009 keeps happening for that build. (low likelihood, high impact; guard: none -- no test runs a pre-transfers Desktop binary)
• If a command on a pre-transfers Desktop prints more than 256 KiB between two reads, then its middle output is dropped and the agent gets a gap in stdout with no marker, so it reasons on partial output. _(high likelihood, medium impact; guard: none -- truncated_before_sequence is set on ProcessOutputSnapshot (providers/agent_host_ops.py:262) but process_output.drain (process_output.py:107) never reads it)
• If a process was already running when this backend shipped, then it keeps the old ring and one read of it can still close the link with 1009, failing that run. (medium likelihood, medium impact; guard: none)

33 of 134 changed lines worth reading. The rest is mechanical, generated or lockfiles.

Decisions this PR makes

1. A pre-transfers host's process is started with an output ring of at most 256 KiB — The ring is the only thing that bounds a read on such a host; too small loses a command's output, too large keeps closing the link (lemma-backend/app/modules/agent/services/agent_host_link_ops.py)
2. The cap is applied only where the relay already knows the host moves no file data itself — The same guard decides both halves; if it were wrong a current host would silently be given an eighth of the output it asked for (lemma-backend/app/modules/agent/services/agent_host_link_ops.py)
3. Anything that is not a positive byte count falls back to the bound — A caller sending True, a string or None would otherwise get an unbounded ring and the link would close again (lemma-backend/app/modules/agent/services/agent_host_link_ops.py)
4. The docs state the ceiling as absolute for such a host — A reader takes 'a read can then never pass the ceiling' as a guarantee; a process started before this ships does not have it (docs/architecture/desktop-host-execution.md)
5. The cap is pinned by unit cases in the fast lane and one e2e case on the wire — Without the unit cases the arithmetic is only exercised where Docker runs (lemma-backend/app/modules/agent/tests/unit/test_agent_host_old_host_params.py)

Findings · 3 open · 1 settled

• Note · The doc says a read can never pass the ceiling, but a process already running when this ships keeps its old ring — docs/architecture/desktop-host-execution.md:191

• Note · The doc says the dropped output is reported as truncated, but nothing on the agent's path reads that marker — docs/architecture/desktop-host-execution.md:192 · New this round

• Note · The whole fix rests on a released Desktop honouring a smaller output ring, and nothing here proves that build does — lemma-backend/app/modules/agent/services/agent_host_link_ops.py:311 · New this round

• The new cap is pinned only by an e2e test; the helper itself has no unit test — fixed

Rounds

#	When	Commit	What	Verdict
1	20 Oct, 12:14 UTC	8f42b9b	full read	Ready to merge
2	20 Oct, 12:54 UTC	9bc45bf	full read	Ready to merge

One comment, edited after every round. Last updated 20 Oct, 12:54 UTC.

[coderabbitai]

🧹 Nitpick comments: 1

lemma-backend/app/modules/agent/services/agent_host_link_ops.py: 1

323-325: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add a unit test for old_host_params.

The e2e test checks one requested limit on each host type. Add a unit test for a missing limit, a non-positive limit, a limit below the cap, a limit above the cap, and a method other than process.start. As per coding guidelines, “Behaviour changes need a test, in the right suite” and “One function gets a unit test.”

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Review comment at
@lemma-backend/app/modules/agent/services/agent_host_link_ops.py around lines
323 - 325:
In the unit-test suite for old_host_params, add coverage for a missing
output_limit_bytes value, non-positive values, values below and above
LINK_INLINE_DATA_BYTES, and a method other than process.start. Verify each
case’s normalized output_limit_bytes and unchanged parameter behavior, while
keeping the tests focused on the old_host_params function.

Source: Coding guidelines

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Nitpick comments:
Review comments at
@lemma-backend/app/modules/agent/services/agent_host_link_ops.py:
- Around line 323-325: In the unit-test suite for old_host_params, add coverage
for a missing output_limit_bytes value, non-positive values, values below and
above LINK_INLINE_DATA_BYTES, and a method other than process.start. Verify each
case’s normalized output_limit_bytes and unchanged parameter behavior, while
keeping the tests focused on the old_host_params function.

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lemma-backend/app/modules/agent/tests/e2e/test_agent_host_transfers_e2e.py: 1

219-245: 🎯 Functional Correctness | 🔵 Trivial | ⚡ Quick win

Exercise process.read after process.start.

The test only checks the forwarded output_limit_bytes. It does not issue process.read, so it cannot detect an oversized response, incorrect newest-output retention, or missing truncated_before_sequence. Extend this case to send process.read over the same workspace and link, return a response with known chunks and truncation metadata, and assert the returned chunk size, newest sequence, and truncation marker. Use an old-host execution fixture if the test must validate actual retention rather than only the relay path.

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Review comment at
@lemma-backend/app/modules/agent/tests/e2e/test_agent_host_transfers_e2e.py
around lines 219 - 245:
Extend the test around AgentHostOpClient and the existing workspace/link to
issue a process.read request after process.start, returning known output chunks
with truncation metadata through the link. Assert the client receives the
expected chunk size, newest sequence, and truncated_before_sequence marker,
using an old-host execution fixture when needed to exercise actual retention
rather than only relay forwarding.

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Inline comments:
Review comments at
@lemma-backend/app/modules/agent/services/agent_host_link_ops.py:
- Around line 323-328: Update collect_process_output() to inspect
snapshot.truncated_before_sequence while draining legacy-host output and ensure
the caller is informed when earlier chunks were evicted; alternatively, return
an explicit failure instead of treating incomplete output as normal. Preserve
normal output handling when the snapshot is not truncated.

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Review comments at
@lemma-backend/app/modules/agent/tests/e2e/test_agent_host_transfers_e2e.py:
- Around line 219-245: Extend the test around AgentHostOpClient and the existing
workspace/link to issue a process.read request after process.start, returning
known output chunks with truncation metadata through the link. Assert the client
receives the expected chunk size, newest sequence, and truncated_before_sequence
marker, using an old-host execution fixture when needed to exercise actual
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@lemma-backend/app/modules/agent/services/agent_host_link_ops.py:
- Line 328: Update the process.read limit calculation in the surrounding
host-link operation to bound the serialized reply, accounting for per-chunk
metadata and base64 expansion rather than only raw output bytes. Preserve the
existing too_large handling for oversized responses, and add a regression case
covering many short chunks whose raw data fits the ring but whose serialized
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