id: "GW20-012"
title: "Fork a new Strand Lane from an existing Strand Lane"
type: "feature"
status: "planned"
milestone: "v20.1.0"
category: "required"
workstream: "api"
issue: "#968"
baseline_commit: "ceb58e656ec5bc85f0ae5991bf2a55599693bbb3"
feedback_sections: [10]
prerequisites: ["GW20-011", "GW20-016"]

Feature

LLM prompt

Complete GW20-012: Fork a new Strand Lane from an existing Strand Lane.
Read this entire task and its current GitHub issue before implementation.
Verify the current mainline and all prerequisite integrations.
Read AGENTS.md and the three TypeScript policy documents.
Use runtime-backed domain values and injected ports.
Keep parsing and codecs at the adapter boundary.
Preserve v20 APIs, stored identities, CRDT semantics, and failure meanings.
Use one coherent issue, PR, and mainline integration. Do not leave a broken intermediate state.
Problem to resolve:
Runtime.fork rejects strand sources. Applications cannot start a child experiment from an existing experiment.
Acceptance checks:
Fork a strand at a validated captured coordinate and reopen the child after process restart.
Later parent-strand and worldline writes do not change the child's starting state.
Child writes do not mutate either parent. Record ancestry and support without cycles.
Retention keeps all referenced parent and overlay support reachable.
Foreign Runtime sources and unsupported ancestry fail before ref publication.
Read these source paths:
src/application/Runtime.ts
src/application/RuntimeStrandLaneBinding.ts
src/application/RuntimeStrandLaneOpening.ts
src/domain/types/StrandDescriptor.ts
Prerequisites: #967, #972
Scope exclusions: General braid collapse, arbitrary cross-repository parents, or a changed v20 descriptor interpretation.
Run all tests and benchmarks in COPY-based Docker without host repository mounts.
Use the project guarded runner and the shared git-locks authority.
Lock host/heavy-work and the exact worker key together for expensive work.
Limit build caches to 20 GiB, test data to 4 GiB, and logs to 128 MiB.
Require 50 GiB free on host and Docker backing storage before heavy work.
Enforce disk accounting, CPU, memory, timeout, and child-process shutdown.
Do not start an unguarded workload. Do not bypass a resource refusal.
Require full touched-code coverage for refactors and an all-green manual SSJS scorecard.
Record source, image, command, fixtures, results, limits, and the regression witness.
Commit only your own files. Do not amend, rebase, force, push, or publish without authorization.
Report incomplete or blocked checks. Do not claim completion from a narrow green test.
Link the issue, coherent PR, and actual mainline integration when those actions are authorized.

1. Background Context

Source: FEEDBACK-git-warp.md, sections 10.
The report SHA-256 is 9b15209d51cd705059a9e6bfebf0f6681ff832924c1462b99ff3cfebde6bacae.
The report uses npm git-warp 20.0.0 and git-cas 6.5.11.
Its runtime results come from macOS host experiments. This planning task did not rerun them.
Source review baseline: ceb58e65.

Tracker: #968.

Read these source and test surfaces before work:

• src/application/Runtime.ts
• src/application/RuntimeStrandLaneBinding.ts
• src/application/RuntimeStrandLaneOpening.ts
• src/domain/types/StrandDescriptor.ts

2. Problem Description

Runtime.fork rejects strand sources. Applications cannot start a child experiment from an existing experiment.

2b. Proposed Solution

Use the common coordinate-based fork operation for a validated strand source. Keep existing worldline fork behavior compatible.

2c. Alternatives considered and rejected

Reject a private-import workaround. It does not provide a supported application contract.
Reject a documentation-only substitute when this task requires runtime behavior.
Keep larger storage and package redesigns under their existing issues.

2d. Acceptance Criteria

• Fork a strand at a validated captured coordinate and reopen the child after process restart.
• Later parent-strand and worldline writes do not change the child's starting state.
• Child writes do not mutate either parent. Record ancestry and support without cycles.
• Retention keeps all referenced parent and overlay support reachable.
• Foreign Runtime sources and unsupported ancestry fail before ref publication.

2e. Test Plan

Golden: Create a worldline, a strand, and a child strand. Write different values to all three and verify isolation after reopen.

Edges: Empty overlay; duplicate name; missing parent support; nested coordinates; GC; cycle rejection.

Known failure modes: preserve typed refusal, atomic publication, and complete evidence. Do not conceal unavailable support.

Fuzz and stress: use fixed fixture sizes and seeds. Apply the roadmap resource guards. Do not start an unbounded campaign.

3. Prerequisites

• Capture and restore historical Strand Lane Coordinates #967 must be integrated. Nested forks require an exact strand coordinate.
• Fork a worldline at a selected historical Coordinate #972 must be integrated. Nested forks use the selected-coordinate fork contract.

4. Scope

In: Runtime.fork rejects strand sources. Applications cannot start a child experiment from an existing experiment.

Out: General braid collapse, arbitrary cross-repository parents, or a changed v20 descriptor interpretation.

Safe intermediate state: one independently mergeable PR passes its relevant checks after its prerequisites.
Existing supported applications remain usable. Historical data remains intact.

5. Why now

This flaw blocks a supported Synapse store workflow or gives its operator incorrect guidance.
Resolve it within v20.1.0 without reducing the existing causal-history commitment.

6. Risks

Source inspection does not establish runtime or performance results.
Preserve historical identities, validation, and evidence. Do not weaken refusal to obtain a green result.
If the fix requires a breaking contract, record the conflict before activation. Do not hide it within a minor release.

7. Definition of Done

All acceptance checks have evidence from the exact candidate.
Relevant lint, typecheck, compatibility, and Docker checks pass.
The manual SSJS scorecard is green. Refactor coverage reaches 100% on touched code.
Record the issue, PR, and mainline integration commit. Record any external release gate.
A required check with missing evidence remains incomplete.

8. Stakeholders

James Ross: git-warp maintainer and acceptance owner.
Synapse store authors: applications need correct writes, reads, history, and operational guidance.
Library and CLI consumers: existing v20 contracts must remain usable.

9. Related Issues

• #792
• #567