fix(llm): withhold hidden stop-sequence prefixes split across tokens

[Efeselimsurekli]

Overview:

Fixes a hidden stop-sequence leak: when a stop string arrives split across multiple decode fragments, every fragment except the last was released to the client before the full sequence could be recognized.

Details:

Decoder::step only searched the jail buffer for a complete stop sequence match. When no complete match existed yet it immediately released the newly decoded token text, even if that text was a genuine prefix of a hidden stop sequence that a later token would complete. A stop sequence spanning multiple decode fragments (e.g. " zeta eta theta" arriving as " zeta", " eta", " theta") therefore leaked every fragment but the last into the client-visible output.

Track, on each step, the longest tail of the jail buffer that is still a viable prefix of some hidden stop sequence and withhold exactly that suffix (jailed_bytes, previously computed but never wired up). Release everything else, since it can no longer be part of a hidden stop sequence, complete or partial. Add Decoder::flush_jailed() and call it when the engine reports its own completion (e.g. max_tokens) without a local stop trigger, so a withheld candidate that never completes isn't silently dropped.

Add coverage in test_stop_behavior.rs for a stop sequence split across several tokens, for a withheld prefix that turns out not to match and must be released, and for flush_jailed on an incomplete partial match. Existing tests only ever decoded a stop string from a single token, so they never exercised this path.

Validation note: Developed on a Windows machine where Windows Smart App Control blocks execution of freshly-compiled, unsigned cargo build-script binaries, so cargo test / cargo build could not be run locally. The fix was verified by hand-tracing the new logic against every existing and new test case in test_stop_behavior.rs; CI (Linux) will confirm actual compilation and test results.

Where should the reviewer start?

Decoder::step in lib/llm/src/backend.rs — that's where the withholding logic lives. Then the three new tests in lib/llm/tests/test_stop_behavior.rs: a stop sequence split across tokens, a withheld prefix that turns out not to match and gets released, and flush_jailed on an incomplete partial match.

Related Issues

🔗 This PR is linked to an issue:

• Closes [BUG]: Rust frontend leaks prefixes of split stop strings #14375

Summary by CodeRabbit

• Bug Fixes
  • Improved stop-sequence handling during streamed text generation.
  • Prevented duplicate output when stop sequences span multiple chunks.
  • Released text that was incorrectly held back when a stop sequence remained incomplete.
  • Ensured buffered text is flushed when generation completes without a detected stop sequence.

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[github-actions]

👋 Hi Efeselimsurekli! Thank you for contributing to ai-dynamo/dynamo.

Just a reminder: The NVIDIA Test Github Validation CI runs an essential subset of the testing framework to quickly catch errors.Your PR reviewers may elect to test the changes comprehensively before approving your changes.

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[coderabbitai]

Walkthrough

The Rust decoder now withholds viable hidden stop-string prefixes, releases text when a match becomes impossible, suppresses complete hidden stops, and flushes incomplete prefixes when generation ends. Regression tests cover split and abandoned prefix scenarios.

Changes

Hidden stop-sequence handling

Layer / File(s)	Summary
Decoder prefix buffering lib/llm/src/backend.rs	The decoder tracks unreleased jail suffixes, retains the longest viable hidden-stop prefix, releases safe text, clears jail state after matches, and exposes flush_jailed.
Completion flush and regression coverage lib/llm/src/backend.rs, lib/llm/tests/test_stop_behavior.rs	Backend completion handling flushes incomplete jailed text. Test fixtures and regression tests cover split stop sequences, abandoned prefixes, and incomplete matches.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🟡 Moderate · up to 2a6db

This change prevents split hidden stop prefixes from leaking, but a token-ID stop after an abandoned prefix can still omit generated response text. Resolve this output-loss case before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 70.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: withholding hidden stop-sequence prefixes split across tokens.
Description check	✅ Passed	The description includes all required template sections, explains the bug and implementation, identifies review starting points, and links issue #14375.
Linked Issues check	✅ Passed	The changes address issue #14375 by withholding viable stop-sequence prefixes, releasing text when matching becomes impossible, discarding complete hidden matches, and flushing incomplete candidates o…
Out of Scope Changes check	✅ Passed	The changes are limited to decoder stop-sequence handling and directly related regression tests. No unrelated code changes are identified.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
In `@lib/llm/src/backend.rs`:
- Around line 304-306: Update the Decoder::step token-ID stop handling around
decoder.flush_jailed so a decoded stop token first completes any hidden
sequence; if it does not, release the jailed text, while preserving jailed text
before visible token stops. Add regression coverage for both hidden and visible
token-ID stops following an abandoned hidden-sequence prefix.

After applying the fix, consider running `coderabbit review --agent` for local
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ℹ️ Review info

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 787d083d-1230-4395-8241-1029e0298417

📥 Commits

Reviewing files that changed from the base of the PR and between 4abae7b and 2a6db1f.

📒 Files selected for processing (2)

• lib/llm/src/backend.rs
• lib/llm/tests/test_stop_behavior.rs

Included review availability: Your plan provides up to 12 included reviews per hour; 11 remain after this review.

[glamr-agent]

Automated evidence record — validation incomplete

Validation status: incomplete

Evidence summary: 2/3 validated · 1 missing tool or dependency

AI review assessment (advisory only): needs_changes. This is an automated agent's
judgement, not an approval and not a repository CI result. Maintainers and CI decide whether
this change merges.

Validation result: incomplete. The recorded run shows validation did not pass. The reviewing
agent confirmed that the fix works and reproduces the reported defect against unpatched main,
but its executed probes also show that the change newly misaligns tokens with token_ids,
which corrupts logprobs on both OpenAI endpoints, and that it silently drops withheld text on
the hidden-stop-token path and at stream end.

Evidence audit: complete (2/3 validated · 1 missing tool or dependency). The command report
below comes from recorded runs.

Commands and results [2/3 validated · 1 missing tool or dependency]

Generated from the commands recorded during this run.

Check 1

Checks the changed files with the repository's fast lint and formatting commands.

Result: Passed (exit 0), but did not validate the change

Command:

python3 -m pytest docs/fern/scripts/tests/test_api_reference_regressions.py -q --no-header -p no:cacheprovider

Details:

The first python3 -m pytest docs/fern/scripts/tests/test_api_reference_regressions.py run exits
2 on a collection error: the module imports griffe, which was not present in the environment.
After installing griffe the same command was re-run and passed all 38 tests. The exit-2 entry
records the missing dependency, not a defect in the reviewed change.

Check 2

Checks the changed Rust crates with cargo check and Clippy.

Result: Passed (exit 0)

Command:

cargo check --manifest-path lib/bindings/python/Cargo.toml --all-targets

Check 3

Inspects the changed code when the claim cannot be tested with a local command.

Result: Passed (exit 0)

Command:

Not shown because the exact command contained private run data.

[glamr-agent]

review.md

🤖 Automated AI review — advisory. An AI agent's judgment of
whether this change is logically sound based on the code and reported
validation results. This is not an approval. Repository CI and human
reviewers decide whether to merge.

Assessment: needs_changes

Thank you for this — the diagnosis is right and the approach is the right one.
Issue #14375 is a genuine correctness bug, and the fix you chose (a bounded
look-ahead buffer that withholds the longest tail of decoded text that is still
a viable prefix of a hidden stop sequence) is exactly what the pre-existing
comment in lib/llm/src/backend.rs anticipated, and it matches the reference
behaviour in vanilla SGLang's check_match_stop_str_prefix. You mentioned you
were not able to build or test locally, so this review is mostly executed
results rather than opinion — everything below was run against your branch.

The core fix works. Your regression test genuinely discriminates. Running
cargo test -p dynamo-llm --test test_stop_behavior on your branch passes all
11 tests. Applying only your new test file on top of unmodified main and
re-running the same command,
hidden_stop_sequence_split_across_tokens_is_not_leaked fails there:

assertion `left == right` failed
  left: Some(" delta epsilon zeta eta")
 right: Some(" delta epsilon")

That is the bug in issue #14375, reproduced and then fixed. The workspace is
also clean on your branch: cargo check --workspace --all-targets,
cargo check --manifest-path lib/bindings/python/Cargo.toml --all-targets,
cargo clippy --workspace --all-targets --no-deps -- -D warnings,
cargo fmt --all --check, and cargo test -p dynamo-llm --lib (2625 tests)
all exit 0.

What follows are the things that need to change before this can land. The first
is the one I would most like your attention on, because it is a silent
regression in a field the issue explicitly asked you not to change.

Findings

1. Blocking — tokens[i] no longer describes token_ids[i], which corrupts logprobs

SeqResult carries two parallel vectors, tokens and token_ids, and
downstream code relies on them being positionally aligned: tokens[i] is the
text of token_ids[i]. Buffering breaks that alignment.

In lib/llm/src/backend.rs:711-715, a step that withholds text returns None,
and the later step that releases the buffer returns the whole accumulated
span as one string:

self.jailed_bytes =
    Self::longest_hidden_prefix_suffix(&self.jail, &self.hidden_stop_sequences);
let release_end = self.jail.len() - self.jailed_bytes;
let released = (release_end > release_start)
    .then(|| self.jail[release_start..release_end].to_string());

Meanwhile Decoder::process_token_ids (lib/llm/src/backend.rs:759) still
pushes exactly one entry per input token id at lib/llm/src/backend.rs:774. So
the text of several token ids lands on a single position, and the earlier
positions get None.

I ran a probe on your branch that decodes two token ids whose fragments are
"o" and "there" under the hidden stop sequence "ozzy" (the same scenario
as your withheld_prefix_is_released_once_it_cannot_complete test), and
printed the per-position pairing:

input token_ids      = [15, 16]
result.tokens        = [None, Some("othere")]
result.text          = Some("othere")
per-position pairing = [(15, ""), (16, "othere")]

The same probe against unmodified main:

input token_ids      = [15, 16]
result.tokens        = [Some("o"), Some("there")]
result.text          = Some("othere")
per-position pairing = [(15, "o"), (16, "there")]

result.text is identical, which is why the existing suite does not catch
this — but the pairing is not. Token id 15 now reports an empty string, and
token id 16 reports text that includes a fragment belonging to 15.

This matters because that pairing is consumed positionally. In
lib/llm/src/protocols/openai/chat_completions/delta.rs, create_logprobs
(line 77) zips the two vectors by index:

let toks = tokens
    .into_iter()
    .zip(token_ids)
    .map(|(token, token_id)| (token.unwrap_or_default(), *token_id))
    .collect::<Vec<(String, TokenIdType)>>();

That is lib/llm/src/protocols/openai/chat_completions/delta.rs:88-92, called
from choice_from_postprocessor at line 231 (call site lines 237-242). The
completions path has an identical zip at
lib/llm/src/protocols/openai/completions/delta.rs:75-79, called from line 189
(call site lines 202-207). grep -rn 'zip(token_ids)' lib/llm/src finds these
two sites and no others, so the blast radius is bounded to exactly the two
OpenAI response builders — but those are the ones that emit logprobs.

The practical effect is that with logprobs enabled, any request whose output
happens to contain a prefix of a stop string returns a ChatCompletionTokenLogprob
whose token field is wrong: some entries become "" and one entry carries
several tokens' worth of text, while the logprob values continue to describe
the original single tokens. Issue #14375 asks specifically that logprobs be
unchanged by this fix, and today it is changed.

One clarification so this is not over-stated: the Some("") → None transition
on withheld steps is behaviourally neutral, because unwrap_or_default() maps
both to "". The defect is the concatenation landing on one position, not
the None.

There is a second, smaller instance of the same broken invariant. The
end-of-stream release hook at lib/llm/src/backend.rs:304-309 appends flushed
text to result.text but never to result.tokens:

if result.stop_trigger.is_none()
    && data.finish_reason.is_some()
    && let Some(flushed) = decoder.flush_jailed()
{
    result.text.get_or_insert_with(String::new).push_str(&flushed);
}

A probe on your branch confirms the divergence — the flushed text is in
result.text but the concatenation of result.tokens is missing it:

result.text after flush        = Some(" delta zeta eta")
concat of result.tokens        = " delta"

What would resolve this: keep the per-token vector faithful. The cleanest
shape is for the decoder to return, per step, the text attributable to that
token id, and to attribute a released buffer back across the positions it came
from rather than collapsing it onto the releasing step. Whatever shape you
choose, the acceptance criterion is the same and is easy to assert: for any
input, result.tokens.len() == token_ids.len() and the concatenation of
result.tokens (treating None as "") equals result.text, including after
flush_jailed. A test asserting that concatenation invariant would be a good
addition and would also make finding 2 below discriminating.

2. Blocking — withheld_prefix_is_released_once_it_cannot_complete does not test the change

lib/llm/tests/test_stop_behavior.rs:197-205 asserts only
result.text.as_deref() == Some("othere") and that no stop trigger fired. Both
hold before and after your change, so the test cannot fail if the release
logic regresses.

This was already flagged by dynamo-review-agent[bot] on this PR; I am
repeating it only because I can now attach the execution. Applying your new test
file to unmodified main, removing only flush_jailed_releases_incomplete_partial_match
(which does not compile there, since flush_jailed is new in this PR — the
counterfactual reports error[E0599]: no method named 'flush_jailed' found),
and running cargo test -p dynamo-llm --test test_stop_behavior gives:

test hidden_stop_sequence_split_across_tokens_is_not_leaked ... FAILED
test withheld_prefix_is_released_once_it_cannot_complete ... ok

So the regression test earns its place and this one does not, in its current
form.

What would resolve this: rather than deleting it, assert the property that
is actually new — the per-token result.tokens vector, not just the joined
text. That is the assertion that distinguishes buffered from unbuffered
behaviour. Note this becomes possible only once finding 1 is fixed; today the
per-token vector is the thing that is wrong, so the two are best addressed
together.

3. Scope question — the three documentation deletions

The change also deletes deploy/operator/docs/footer.md,
docs/fern/pages/reference/kubernetes-api/additional-resources/api-reference-k8s.md,
and content in docs/fern/pages/reference/kubernetes-api/full-api-reference.mdx.

To be clear, this is a question, not a defect — the deletions check out. The
generated Kubernetes API reference is still consistent afterwards:
python3 docs/fern/scripts/gen_kubernetes_api.py --check exits 0 and reports
full-api-reference.mdx: unchanged, the docs regression suite
(docs/fern/scripts/tests/test_api_reference_regressions.py) passes, and the
documentation pre-commit hooks over the changed files exit 0.

The question is whether they belong in this PR. They are unrelated to the Rust
frontend stop-string bug, and including them pulls the documentation and
Kubernetes API reviewers onto a change that is otherwise a self-contained fix in
lib/llm. Splitting them into their own PR would shorten the reviewer list here
and let the frontend fix land on its own merits. If they are deliberately
coupled to something in this fix that I have missed, saying so in the PR
description would settle it.

Already raised by the automated reviewers

Two automated reviewers commented before this review, and several of the issues
below are theirs. I am acknowledging rather than re-filing them, and adding only
the executed evidence.

• Withheld text is dropped when a hidden stop token id ends the stream.
  Raised by coderabbitai[bot] (at lib/llm/src/backend.rs:306). Confirmed by
  execution on your branch: with text held in the buffer, a hidden stop token id
  takes the early return at lib/llm/src/backend.rs:649-656, and because the
  release hook at line 304 is guarded on result.stop_trigger.is_none(), the
  hook cannot recover it:

  result.text          = None
  result.tokens        = [None, None]
  result.stop_trigger  = true
  

  The buffered text is gone. One extension neither reviewer named: the visible
  stop token id path at lib/llm/src/backend.rs:640-645 has the same guard
  problem, and is slightly worse — it returns the freshly decoded fragment while
  dropping the earlier buffered text, so the caller sees output that is both
  truncated and reordered. Any fix for the hidden path should cover this one
  too.

• The release hook only fires on a chunk carrying a finish_reason. Raised
  by dynamo-review-agent[bot] (at lib/llm/src/backend.rs:304). Confirmed by
  reading the postprocessing pipeline: the end-of-stream arm at
  lib/llm/src/backend.rs:407 (None => None,) terminates without flushing, so
  a stream that simply ends — engine disconnect, cancellation, any path that does
  not deliver a terminal finish_reason chunk — silently loses the buffered
  tail. Both this and the item above come down to the same guard at line 304
  being too narrow; a single fix that flushes on every terminal path, whatever
  ended it, would close all three.

• withheld_prefix_is_released_once_it_cannot_complete does not discriminate.
  Raised by dynamo-review-agent[bot]; expanded with execution as finding 2
  above.

• Four comment and wording nits on the new test file (at lines 176, 201,
  215, and 222). Raised by dynamo-review-agent[bot]; no disagreement from me,
  and nothing to add.

The genuinely new contribution in this review is finding 1 — the
tokens/token_ids misalignment and its logprobs blast radius — plus the
fact that the buffer-loss items above are now backed by executed results rather
than reasoning alone.

Summary

The bug is real, your diagnosis is right, and the approach is sound. Three
things stand between this and merge: the per-token vector must stay aligned with
token_ids so logprobs is genuinely unchanged (finding 1), the release hook
must flush on every terminal path rather than only on a finish_reason chunk
(already raised, and it covers three distinct loss paths), and one test needs to
assert the property it claims to (finding 2). The documentation deletions are
correct but probably belong elsewhere. Happy to re-run the same commands against
an updated branch.

Assessment: needs_changes

[glamr-agent]

Automated AI review — advisory. The inline comments below are the findings. The full review is in a separate comment on this pull request.

[glamr-agent]

🤖 Automated AI review — advisory. An AI agent's judgment of
whether this change is logically sound based on the code and reported
validation results. This is not an approval. Repository CI and human
reviewers decide whether to merge.

Assessment: needs_changes

PR #14378 fixes a real bug. Issue #14375 reports that the Rust frontend leaks a
prefix of a hidden stop string when the stop string is split across decoded
token fragments, and the added regression test
hidden_stop_sequence_split_across_tokens_is_not_leaked fails on unmodified
main and passes on the branch. The approach — a bounded look-ahead buffer that
withholds the longest tail of decoded text that is still a viable prefix of a
stop sequence — is what the pre-existing comment in lib/llm/src/backend.rs
anticipated and matches vanilla SGLang. The workspace is clean on the branch:
cargo check --workspace --all-targets,
cargo clippy --workspace --all-targets --no-deps -- -D warnings,
cargo fmt --all --check, cargo test -p dynamo-llm --lib, and
cargo test -p dynamo-llm --test test_stop_behavior all exit 0.

Three things should change before merge.

1. Blocking — the per-token vector is no longer aligned with token_ids,
which changes logprobs. SeqResult.tokens[i] is expected to be the text of
token_ids[i]. With buffering, a withheld step yields None and the releasing
step yields the whole accumulated span
(lib/llm/src/backend.rs:711-715), while process_token_ids still pushes one
entry per token id (lib/llm/src/backend.rs:759, :774). Two token ids that
previously paired as ("o", "there") now pair as ("", "othere"). That pairing
is consumed positionally by create_logprobs in both response builders —
lib/llm/src/protocols/openai/chat_completions/delta.rs:88-92 (called from line
231) and lib/llm/src/protocols/openai/completions/delta.rs:75-79 (called from
line 189); grep -rn 'zip(token_ids)' lib/llm/src finds no other consumers. The
result is ChatCompletionTokenLogprob entries whose token text is wrong while
the logprob values still describe the original single tokens. Issue #14375
explicitly requires logprobs to be unchanged. Separately, the release hook at
lib/llm/src/backend.rs:304-309 appends flushed text to result.text but not
to result.tokens, breaking the same invariant a second way. A concatenation
invariant test (result.tokens joined, with None as "", equals
result.text; lengths match token_ids) would pin this down.

2. Blocking — buffered text can be silently dropped on three terminal
paths. The release hook is guarded on result.stop_trigger.is_none() && data.finish_reason.is_some() (lib/llm/src/backend.rs:304-305), so it does not
fire when a hidden stop token id ends the stream
(lib/llm/src/backend.rs:649-656), when a visible stop token id does
(lib/llm/src/backend.rs:640-645 — this one also reorders output, emitting the
newest fragment while dropping older buffered text), or when the stream simply
ends without a terminal finish_reason chunk (lib/llm/src/backend.rs:407).
The first of these was raised by coderabbitai[bot] and the third by
dynamo-review-agent[bot]; the visible-stop-token path is not yet on the PR. A
single fix that flushes on every terminal path closes all three.

3. Blocking (minor) — one new test does not discriminate.
lib/llm/tests/test_stop_behavior.rs:197-205
(withheld_prefix_is_released_once_it_cannot_complete) asserts only the joined
text and the absence of a stop trigger, both of which hold before and after the
change; it passes on unmodified main. Raised by dynamo-review-agent[bot].
Asserting the per-token vector instead would make it meaningful — which depends
on fixing item 1 first.

Scope question, not a defect. The PR also deletes
deploy/operator/docs/footer.md,
docs/fern/pages/reference/kubernetes-api/additional-resources/api-reference-k8s.md,
and content in docs/fern/pages/reference/kubernetes-api/full-api-reference.mdx.
These are correct and generator-consistent
(python3 docs/fern/scripts/gen_kubernetes_api.py --check exits 0 reporting
full-api-reference.mdx: unchanged), but they are unrelated to the frontend fix
and pull documentation and Kubernetes API reviewers onto it. Worth asking
whether they should be a separate PR.

Assessment: needs_changes

[jthomson04]

@Efeselimsurekli, the earlier raw-token/logprob alignment and ordinary EOF/hidden/visible-token prefix-loss findings are addressed at 70a55a4e. The deeper integration review found new issues; I would hold merge for the migration regression in particular.

P1: migration discards withheld text

The native pipeline places Migration outside Backend. Migration records all raw token IDs, including tokens whose presentation text is withheld, then replaces Backend on retry. The replacement decoder starts with an empty jail.

Source-level trace with hidden stop ozzy and migration enabled: worker A emits o (raw ID retained, text withheld), then a typed disconnect triggers retry. Worker B resumes with that ID in its prompt, emits there, and finishes with Length. The visible result becomes there instead of othere; the old withheld o was discarded. This is new ordinary-text loss, not just the existing stop-continuity limitation.

Please preserve decoder/presentation state across the logical request or transfer it explicitly, and add a pipeline-level migration regression. Do not simply flush before retry: the continuation could complete the hidden stop. Evidence:

• dynamo/lib/llm/src/entrypoint/input/common.rs

  Lines 511 to 530 in 70a55a4

  	let preprocessor_op = preprocessor.into_operator();
  	let token_backend = Backend::from_tokenizer(tokenizer).into_operator();
  	let migration = Migration::from_mdc(card, migration_limit, migration_max_seq_len, metrics)
  	.into_operator_for::<BackendOutput>();
  	let prefill_op = self.prefill_router.into_operator();
  	let encoder_op = self.encoder_router.into_operator();
  	let backend = ServiceBackend::from_engine(self.backend_engine.clone());
  	let engine = frontend
  	.link(preprocessor_op.forward_edge())?
  	.link(migration.forward_edge())?
  	.link(token_backend.forward_edge())?
  	.link(encoder_op.forward_edge())?
  	.link(prefill_op.forward_edge())?
  	.link(backend)?
  	.link(prefill_op.backward_edge())?
  	.link(encoder_op.backward_edge())?
  	.link(token_backend.backward_edge())?
  	.link(migration.backward_edge())?
  	.link(preprocessor_op.backward_edge())?

• dynamo/lib/llm/src/migration.rs

  Lines 621 to 660 in 70a55a4

  	fn track_response(&mut self, response: &Annotated<Resp>) {
  	let llm_engine_output = match response.data.as_ref() {
  	Some(output) => output,
  	None => return,
  	};
  	let token_ids = llm_engine_output.token_ids();
  	// Pure telemetry for the migration lifecycle events, so it has to count
  	// the final allowed attempt as well. `new_stream` decrements
  	// `retries_left` *before* dispatching, so that attempt runs with the
  	// counter already at zero; keeping this behind the replay guard below
  	// would drop its tokens from `migration retries exhausted`.
  	self.completed_tokens += token_ids.len();
  	// Everything past this point rebuilds replay state for a *future*
  	// attempt. Once no retry can happen there is nothing to replay onto,
  	// so leave the request untouched.
  	if self.retries_left == 0 {
  	return;
  	}
  	// Capture the worker's engine.generate span pointer so a future
  	// migration retry can render an OTel Link back to it. The adapter
  	// stamps this on the first non-empty chunk; subsequent chunks may
  	// also carry it. Keep the most-recently-seen value.
  	if let Some(link) = llm_engine_output.worker_trace_link() {
  	self.last_worker_link = Some(link.clone());
  	}
  	let output_len = u32::try_from(token_ids.len()).unwrap_or(u32::MAX);
  	if self.exceed_max_seq_len(output_len) {
  	return;
  	}
  	// NOTE: A zero remaining token budget is not retry-budget exhaustion. Backends remain
  	// authoritative for deciding how generation terminates at max_tokens.
  	if let Some(max_tokens) = self.request.stop_conditions.max_tokens {
  	self.request.stop_conditions.max_tokens = Some(max_tokens.saturating_sub(output_len));
  	}
  	if let Some(min_tokens) = self.request.stop_conditions.min_tokens {
  	self.request.stop_conditions.min_tokens = Some(min_tokens.saturating_sub(output_len));
  	}
  	for token_id in token_ids.iter() {
  	self.request.token_ids.push(*token_id);

P2: decoded-text terminal chunks bypass flushing

A token-only o chunk followed by {token_ids: [], text: Some(""), finish_reason: Length} takes the early decoded-text passthrough. EOF then emits o with a second terminal reason, Stop; the aggregators overwrite Length. Flush pending text into the existing terminal chunk before passthrough, preserve its reason/metadata, and exclude completed choices from later EOF synthesis.

This is a valid custom-engine/per-chunk optional-text case; we did not establish that a shipped token-mode vLLM/SGLang producer emits this exact mixed form.

• dynamo/lib/llm/src/backend.rs

  Lines 273 to 283 in 70a55a4

  	// if we have a data field without an event, then we might need to update the data
  	if let Some(data) = &output.data
  	&& data.text.is_some()
  	&& !state.validate_engine_decode
  	{
  	// Text already decoded; track finish for this choice
  	if data.finish_reason.is_some() {
  	let choice_idx = data.index.unwrap_or(0);
  	state.finished_choices.insert(choice_idx);
  	}
  	return Some((output, state));

• dynamo/lib/llm/src/backend.rs

  Lines 430 to 450 in 70a55a4

  	None => {
  	// The engine's stream ended without ever sending a terminal
  	// `finish_reason` to some choice -- no more tokens are coming for any
  	// of them, so flush whatever each decoder still holds back as an
  	// incomplete hidden-stop-sequence match before the decoders are
  	// dropped, and drain the flushed choices one synthetic chunk at a time.
  	state.stream_ended = true;
  	for (idx, decoder) in state.decoders.iter_mut() {
  	if let Some(flushed) = decoder.flush_jailed() {
  	state.pending_flush.push((*idx, flushed));
  	}
  	}
  	state.pending_flush.pop().map(|(idx, flushed)| {
  	let output = Annotated::from_data(LLMEngineOutput {
  	index: Some(idx),
  	text: Some(flushed),
  	finish_reason: Some(FinishReason::Stop),
  	..Default::default()
  	});
  	(output, state)
  	})

P2: quadratic prefix matching on the decode hot path

The suffix-length loop repeatedly compares overlapping bytes. A hidden stop consisting of repeated a, then b, then repeated a, with sustained a output, produces quadratic work in stop length on each warm fragment. Stop-string byte length is not bounded. Prefer linear/incremental prefix matching and add a sustained repetitive-prefix regression.

• dynamo/lib/llm/src/backend.rs

  Lines 838 to 855 in 70a55a4

  	fn longest_hidden_prefix_suffix(jail: &str, hidden_stop_sequences: &[String]) -> usize {
  	let jail_bytes = jail.as_bytes();
  	let mut best = 0;
  	for seq in hidden_stop_sequences {
  	let seq_bytes = seq.as_bytes();
  	// A full-length match would already have been caught as a complete stop;
  	// only strictly shorter prefixes are candidates here.
  	let max_k = seq_bytes.len().saturating_sub(1).min(jail_bytes.len());
  	for k in (best + 1..=max_k).rev() {
  	if jail.is_char_boundary(jail_bytes.len() - k)
  	&& jail_bytes[jail_bytes.len() - k..] == seq_bytes[..k]
  	{
  	best = k;
  	break;
  	}
  	}
  	}
  	best

Validation

The five added tests exercise Decoder directly, not these Backend/migration paths. Please cover the above cases, backend Length/bare-EOF flushing (including multiple choices and no post-EOF polling), and terminal errors. Also assert false-prefix release timing per step using released_text, not the intentionally separate raw token text.

The lower-level error-then-EOF path should not synthesize successful Stop after an error. Normal OpenAI consumers already stop on errors, so this is separate hardening rather than a claim that they silently succeed.

These findings are source/control-flow traces and algorithm analysis, not freshly executed Rust integration tests or GPU benchmarks. Existing Rust CI passes, but does not cover these cases. After the fixes, please obtain current-head full CI and owner review.

[Efeselimsurekli]

Hello @jthomson04,
Thanks for the thorough re-review , I’ve updated my pull request based on your suggestions and fixed the parts you mentioned. Could you take a look? Is there anything else I need to do?
I fixed these:
P1 (build): Fixed the E0502 borrow — snapshotted decoders.len() before the mutable decoder borrow instead of reading state.decoders again while it's held.

P2 (tokenizer fixture): Rewrote JailingTokenizer::decode to decode contextual token slices compositionally, matching the real DecodeStream contract (it calls decode with cumulative prefixes, not one fresh id at a time). The previous fixture rejected multi-id slices, so neither test was actually exercising the intended scenario.

P2 (terminal ordering): Fixed — withheld text is now prepended, not appended, so "o" + "there" correctly yields "othere", not "thereo".

P2 (checkpoint erasure): Fixed — the bypass path now mirrors the decoder's current withheld state on every chunk (not just terminal ones), so a non-terminal bypass chunk no longer makes an in-flight checkpoint look resolved.

P2 (unused test variable): Fixed.

Also added, per the validation section: a real Backend/Decoder migration integration test (not just the BackendOutput mock) covering both ordinary continuation and hidden-stop completion across a retry, a Backend-level test for the bypass ordering/checkpoint fix, and a test for the Annotated-error-then-EOF case (all choices now get marked finished on a request-level error too, not just decode errors).

Current head (1d65890) is green on rust-tests (.), rust-clippy (.), and both kvbm/python bindings jobs. Happy to address anything else you find on a fresh look.

Quick follow-up: pushed a bit more since the comment above.
Merged latest main in — it had moved forward and grown one more exhaustive BackendOutput site (lib/llm/tests/http-service.rs) that needed the new field too.
Also applied the nonblocking suggestions from your review: skip snapshotting jailed_text when migration isn't possible for the request, drop two now-unnecessary token.clone() calls, and reuse scratch buffers across longest_hidden_prefix_suffix calls instead of allocating fresh ones per token.

[jthomson04]

/ok to test 30b62c9

[Efeselimsurekli]

Hello @jthomson04, thanks for approving and running CI.

The failing checks look like runner issues, not the change. sglang-runtime died on an etcd 429 too many requests with a "contact your self hosted runner administrator" message, backend-status-check just fails because of that one, and the frontend / vllm-runtime builds got cancelled at the 60 min timeout. The rust tests, clippy and pre-merge checks are all green.

Could you re-run them when the runners aren't as busy? I can rebase onto main first if that helps.
Thank You.

[jthomson04]

/ok to test 955e989