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@@ -1,4 +1,4 @@
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-# Agent Note: Cache-preserving forked children stay one-shot
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+# Agent Note: Forked children stay one-shot
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Status: implemented
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@@ -12,7 +12,7 @@ The child-scoped `report` return channel is now the largest such addition, and s
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## Decision
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-The cache-preserving compositions bind the fork delegation tool to `backgroundMode: one-shot`: [the base bundle](../../../../packages/bundle/base/cordis.patch.yml), [the ACP example](../../../../examples/acp-agent/cordis.yml), and [the headless example](../../../../examples/headless-agent/cordis.yml). The base bundle leaves `run_in_background` available, because it mounts a task service; the two examples set `enableRunInBackground: false`, because they mount none and a one-shot background start would otherwise fail at call time on a missing `tasks` service. The standard, code, and Cordis CLI presets instead bind fork to `continuable`; their child-scoped `report` additions invalidate the inherited prefix and accept the recomputation cost described here.
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+Every shipped composition inherits the fork delegation tool's `backgroundMode: one-shot` from the [base bundle](../../../../packages/bundle/base/cordis.patch.yml). The base bundle leaves `run_in_background` available because it also mounts the task service needed to settle background work.
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One-shot children — foreground and background alike — are created through `SubagentRuntime.start()`, which never enters the continuable activation-setup registry, so neither `report` nor its prompt section is installed. A forked one-shot child's system prompt and tool schemas therefore equal its parent's, apart from the `persona` and `toolFilter` deltas a deployment opts into per delegation tool.
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@@ -20,9 +20,9 @@ One-shot children — foreground and background alike — are created through `S
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### The restriction is composition, not code
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-`ForkInProcessProvider.prepareContinuable` stays implemented and `ctx.subagents.startContinuable()` accepts `fork`; composition chooses whether the fork tool is one-shot or continuable. `tool-subagent` knows both the provider's `inheritsParentContext` and its own `backgroundMode` at mount, so a load-time rejection of the pair is available and deliberately absent: the pair is not wrong in general. It is costly only while a child-scope delta precedes inherited history, and the package that creates that delta — [`dsh-tool-subagent-report`](../../../../packages/subagent/tool-subagent-report/README.md) — is separately installable and, by its own design, invisible to `tool-subagent`. A deployment that omits the report package can run continuable forked children with the prefix intact. Encoding one roster's consequence as a delegation-tool invariant would make the tool assert something it cannot observe.
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+`ForkInProcessProvider.prepareContinuable` stays implemented and `ctx.subagents.startContinuable()` still accepts `fork`; only the shipped `cordis.yml` rows changed. `tool-subagent` knows both the provider's `inheritsParentContext` and its own `backgroundMode` at mount, so a load-time rejection of the pair was available and is deliberately not added: the pair is not wrong in general. It is wrong only while a child-scope delta precedes inherited history, and the package that creates that delta — [`dsh-tool-subagent-report`](../../../../packages/subagent/tool-subagent-report/README.md) — is separately installable and, by its own design, invisible to `tool-subagent`. A deployment that omits the report package can run continuable forked children with the prefix intact. Encoding one roster's consequence as a delegation-tool invariant would make the tool assert something it cannot observe.
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-The cache-preserving condition is recorded as a `TODO(fork-continuable-prefix-reuse)` marker on `prepareContinuable` and tracked as issue #2124: continuable fork preserves its inherited prefix when the child's system prompt and tool schemas can match the parent's byte for byte.
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+The reintroduction condition is recorded as a `TODO(fork-continuable-prefix-reuse)` marker on `prepareContinuable` itself, the one method the shipped compositions do not call, and tracked as issue #2124: continuable fork reopens when a child's system prompt and tool schemas can match its parent's byte for byte.
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## Alternatives considered
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@@ -30,7 +30,7 @@ The cache-preserving condition is recorded as a `TODO(fork-continuable-prefix-re
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**Stop mounting the fork provider at all.** This was the broader form of the restriction. Rejected because foreground fork *is* the prefix-reusing case and is untouched by the report channel, so a full ban gives up the capability without buying anything the one-shot binding does not already buy — and would leave no shipped composition exercising session seeding.
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-**Use continuable forked children in cache-preserving compositions and accept the loss.** Rejected for the base bundle and ACP/headless examples because the loss is total rather than marginal: reuse breaks ahead of the inherited history, so the child pays full prefill on a transcript it duplicated for the sole purpose of not paying it. The CLI presets make the other tradeoff and retain continuable fork. A deployment that wants a long-lived child with no inherited context already has `spawn`.
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+**Ship continuable forked children and accept the loss.** Rejected because the loss is total rather than marginal: reuse breaks ahead of the inherited history, so the child pays full prefill on a transcript it duplicated for the sole purpose of not paying it. A deployment that wants a long-lived child with no inherited context already has `spawn`.
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**Make `report` visible to every Agent.** A global registration would restore byte-identical prefixes by giving parent and child the same schema and section. Rejected because roots, one-shot children, remote children, and agentless callers would advertise a tool with no derivable recipient, and execution-time rejection would make schema visibility disagree with authority — the scope-local decision the [report tool Agent Note](../feature/2026-07-30-continuable-subagent-report-tool.md) already settled.
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@@ -38,12 +38,12 @@ The cache-preserving condition is recorded as a `TODO(fork-continuable-prefix-re
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## Consequences
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-- The base bundle and ACP/headless examples create only one-shot forked children; their `subagent_fork` returns a result to the caller's turn, and `send_message` addresses only spawned children there. The three CLI presets create continuable forked children.
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-- A one-shot forked child's request prefix stays byte-identical to its parent's unless the deployment configures `persona`, `toolFilter`, or a different LLM route on the fork delegation tool, so the token cost of seeding can buy provider-side reuse. Continuable fork adds `report` before the inherited history and forfeits that reuse.
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-- The fork provider's continuable path has CLI production callers and package-level tests. The same seam accepts one-shot composition, so a bundle or `--patch` overlay can choose either lifecycle without a code change or warning.
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+- No shipped composition creates a continuable forked child; `subagent_fork` returns a result to its caller's turn, and `send_message` addresses only spawned children.
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+- A forked child's request prefix stays byte-identical to its parent's unless the deployment configures `persona` or `toolFilter` on the fork delegation tool, so the token cost of seeding buys provider-side reuse again.
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+- The fork provider's continuable path has no production caller and no assembled-composition coverage. It keeps its package-level tests, and the seam still accepts it, so a bundle or `--patch` overlay can reintroduce it with no code change and no warning.
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- `subagent_fork`'s model-visible schema changes: the continuable background wording is replaced by the one-shot task wording in the base bundle, and disappears entirely from the two examples. The affected keyless snapshot tool-schema sidecars are re-recorded in the same change.
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-- The report obligation reaches spawned children in every continuable composition and forked children in the CLI presets. Its default `next-step` scheduling, authority model, and coverage remain independent of fork composition.
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+- The report obligation's reach narrows to spawned children in shipped deployments. Its default `next-step` scheduling, authority model, and coverage remain independent of fork composition.
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### Accepted risks
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-The one-shot constraint lives in three configuration files and a code comment, not in a gate; the CLI preset rows already choose `backgroundMode: continuable` and incur the prefix loss. Any bundle or profile patch can make either choice without a warning. That is the accepted cost of not encoding one roster's consequence into `tool-subagent`.
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+The constraint lives in three configuration files and a code comment, not in a gate. A future bundle row or profile patch can set `backgroundMode: continuable` on a fork tool and silently reintroduce the prefix loss; nothing fails loud. That is the accepted cost of not encoding one roster's consequence into `tool-subagent`.
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