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Review comments at @src/aiperf/dataset/mmap_cache.py:
- Around line 871-872: Bump MANIFEST_VERSION for the fork-parent message replay
change so cached manifests reflect the updated decoded content, and update the
corresponding version assertion in the mmap cache tests. Preserve the existing
version-history comments and add the new version entry describing this behavior.

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I reviewed this PR at cdb5d1a9b against main (merge-base f10eb50a1). The goal is right: requests sharing a hash prefix need consistent synthetic message roles and boundaries so replay does not lose prefix-cache reuse. That applies to same-level overlapping flat agents (::fa:), not just explicit subagents (::sa:), and does not imply that one agent inherited another's live conversation. I am requesting changes because the implementation can also add content tokens and replace hash-derived content with source-request filler at partial-block boundaries (src/aiperf/dataset/loader/weka_synth_buf.py:358-419); role reconstruction should leave each request's own content intact.

Verified

I ran the exact six-request trace from #1554 through the production loader with the setting off and on. It contains no explicit subagent marker; the overlapping requests form a ::fa:000 flat-agent chain. Its first request has 1,920 content tokens with either setting, with identical token content; the role sequence changes from [user] to [user, assistant, user, user], and its first three messages exactly match the shared source request. All root and later flat-agent content also remains identical. The PR behaves correctly on that example. The failures below are separate partial-block/pullback cases, not a failure of the issue's clean whole-block example.

Blocking: extra reconstructed content, not just extra message delimiters. I reproduced an ordinary explicit subagent through WekaTrace.model_validate and the production WekaTraceLoader.convert_to_conversations, using the PR's deterministic test harness. The decoder renders each synthesized token ID as one whitespace-delimited number, so these counts measure content before any chat-template overhead; no parent segments were fabricated or injected.

Root requests are identical with the setting on and off. The parent's final reconstructed request already contains 1,708 tokens for 26 recorded blocks (1,664 tokens); that parent discrepancy predates this PR. The new child path copies all 1,708 and appends its own ten new blocks (640 tokens): 1,708 + 640 = 2,348, instead of the child's recorded 36 × 64 = 2,304. The regression is propagating that existing parent overhang into a child whose previous initialization honored its budget, not the pre-existing parent discrepancy itself.

Here is a reproducer using this PR's existing test helpers:

uv run --no-sync python - <<'PY'
from aiperf.common.environment import Environment
from tests.unit.dataset.loader.test_weka_child_fork_messages import (
    _blocks, _convert, _messages_at, _req, _subagent, _trace,
)
cut = _blocks(15) + [900]
parent = cut + _blocks(10, 910)
child = parent + _blocks(10, 1000)
trace = _trace('payload-proof', [
    _req(0, _blocks(10), short=20),
    _req(2, _blocks(20), short=10),
    _req(8, cut, api=5),
    _req(14, parent, api=5),
    _subagent(15, [_req(16, child)]),
])
Environment.DATASET.WEKA_TOOL_SHAPED_MESSAGES = False
for enabled in (False, True):
    Environment.DATASET.WEKA_FORK_PARENT_MESSAGES = enabled
    conv = _convert(trace)['payload-proof::sa:a1']
    count = sum(len(m['content'].split()) for m in _messages_at(conv, 0))
    print(f'enabled={enabled}: recorded={len(child) * 64}, content={count}')
PY

enabled=False: recorded=2304, content=2304
enabled=True: recorded=2304, content=2348

The mechanism is explicit: _inherited_segments retains whole parent token buffers at weka_synth_buf.py:406-408; _init_turn_0_from_parent computes tail_n at :366 but only handles positive values. A negative value does not remove or reject the excess. src/aiperf/endpoints/base_endpoint.py:301-314 then appends the raw messages without correcting the content budget, so the additional content survives request assembly.

Blocking: equal length does not guarantee unchanged content. A separate partial-block probe records a 1,537-token child with hashes 0..19 followed by 100..104 (24 complete blocks and a one-token tail). Both settings produce 1,537 content tokens, but the enabled path omits the entire decoded block for hash 9: its 64 tokens are present with the setting off and absent with it on. At the first divergence, token index 576, the enabled path supplies parent filler; 44 inherited filler tokens replace hash-derived content, and the child's sampled tail grows from 1 to 21 tokens. This replaces 64 recorded-block tokens with 64 synthesized filler tokens, rather than merely assigning roles to the same content. The same substitution is reachable through the production loader, with tool shaping disabled.

Why this breaks the product guarantees

• Recorded workload: weka_trace_models.py:32 defines in as the input token count, and weka_synth_buf.py:221-223 promises exact content accounting. Adding 44 content tokens changes the replayed workload before message delimiters are considered.
• Hash identity: weka_synth_buf.py:146-147 and docs/tutorials/weka-trace.md:307 require a hash to decode consistently. Replacing a covered child block with parent filler loses that guarantee even when the total length happens to match.

The distinction is role/boundary inference versus content reconstruction, not parent-versus-child ownership. Flat agents can legitimately share reconstructed prefixes, and SPAWN scheduling does not prohibit the loader from constructing matching messages. The roles are synthetic; their purpose is a consistent rendered prefix, not a claim about actual conversation ancestry.

The description helpfully discloses the overrun, but copying a source request's filler or over-rendered bytes trades recorded token/hash fidelity for prefix matching. Matching synthetic boundaries is desirable; changing the content represented by a request's own hashes or exceeding its recorded input budget is not required to do that. Even a small overrun changes the replayed workload.

Correction

Please separate the shared-prefix role/boundary layout from the content buffers: infer consistent synthetic roles and message starts from the shared hash ancestry, then fill those regions from the current request's own decoded blocks and its own partial/missing-block tail. Preserve both properties: concatenated content token IDs are unchanged by role partitioning, and their count equals the recorded in. Merely rejecting a negative tail_n fixes the overrun but not the equal-length substitution.

Add production-loader regressions for both cases, comparing the complete child token sequence with the setting on and off. The focused Weka suite currently passes (614 passed, 1 skipped, 1 xfailed), but the new oversized-parent test explicitly accepts an overrun, so those passing tests do not establish this invariant. These probes validate reconstructed content accounting; they are not real-server or production-chat-tokenizer measurements.

_{🤖 The agent keeps boundaries without enlarging the brief.}

I reviewed this PR at fe2f3c45a against main (merge-base f10eb50a1). The content-accounting blockers in ajcasagrande’s earlier review are fixed: the child keeps its own content while reusing the parent’s message boundaries. Only two documentation nits remain; nothing blocks merging.

Verified

• Replayed the public #1554 six-request shape with deterministic numeric content and Qwen/Qwen3-0.6B’s chat template (revision c1899de289a04d12100db370d81485cdf75e47ca, transformers 5.15.1): shared prefix increased from 3,842 to 7,693 of 7,695 chat tokens. These are independently measured numbers, not the description’s differently tokenized results.
• 624 Weka tests and 76 cache tests passed; another 3,720 accumulated-turn comparisons preserved content across flag settings. No live inference-server benchmark was run.

Resolved while reviewing

• Earlier content-accounting blockers: the two repros now preserve the full token sequence and exact 2,304 / 1,537 input counts after b5dc4ea1c.
• Tool-result continuation: the exact assistant boundary retains the parent’s call ID; a cut inside it still rolls back.
• Cache invalidation: manifest version is 28, and the flag participates in the cache key.

Deferred to scope

• Parent partial-block hash filling and pre-existing later-turn undercounts remain separate work, as requested.

_{🤖 Shared prefixes stay in the agent’s dossier.}