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26. Stage-based dispatch for agent workflow decoupling ​

Date: 2026-04-28

Status ​

Accepted (Option C superseded by ADR 0041 for the event-driven dispatch path)

Context ​

ADR 0008 established that enrolled repos use workflow_dispatch to trigger agent workflows in the .fullsend config repo. Currently, each shim job calls a specific agent workflow by name (triage.yml, code.yml, review.yml). This creates tight coupling: whenever the agent workflow inventory changes — adding a new agent, removing one, or renaming a workflow file — every enrolled repo's shim must be updated and redeployed. (#335).

ADR 0020 established that stages are composed of multiple single-responsibility agents. As stages gain more agents, the cost of shim-to-workflow coupling increases — adding a new agent to a stage should not require touching enrolled repos.

The shim runs in enrolled repos under pull_request_target, where it cannot be modified by PRs (ADR 0009). This is a security property worth preserving — but it also means shim changes require a privileged update to every enrolled repo.

See PR #390 for the implementation.

Options ​

Option A: Direct workflow_dispatch (status quo) ​

The shim in each enrolled repo calls agent workflows by name via workflow_dispatch (gh workflow run triage.yml, gh workflow run code.yml, etc.). Each shim job is bound to a specific workflow file in .fullsend.

  • Simple: one hop, no indirection, easy to trace.
  • Coupled: adding, removing, or renaming an agent workflow requires updating the shim in every enrolled repo.
  • One-to-one: each shim job triggers exactly one workflow. Running multiple agents for the same stage requires adding more shim jobs.

Option B: Dispatcher with repository_dispatch ​

A dispatch-agent.yml workflow in .fullsend receives workflow_dispatch calls from the shim with a stage parameter. It emits a repository_dispatch event (e.g., fullsend-triage) on the config repo. Agent workflows subscribe via on.repository_dispatch.types.

  • Decoupled: the shim knows about stages, not workflows. Agent changes stay in .fullsend.
  • Fan-out: multiple workflows can subscribe to the same event type, running in parallel without coordination logic.
  • Extra hop: one additional workflow execution per event, adding Actions minutes and latency.
  • Direct workflow_dispatch on individual agent workflows still works for testing and debugging.

Option C: Dispatcher with stage-marker scanning ​

A dispatch.yml workflow in .fullsend receives workflow_dispatch calls from the shim with a stage parameter. It scans agent workflow files for # fullsend-stage: markers and triggers each matching workflow via gh workflow run.

  • Decoupled: the shim knows about stages, not workflows. Agent changes stay in .fullsend.
  • Fan-out: multiple workflows can carry the same stage marker, running in parallel without coordination logic.
  • Extra hop: one additional workflow execution per event, adding Actions minutes and latency.
  • Discoverable: stage markers in workflow files make it clear which workflows participate in which stage.
  • Direct workflow_dispatch on individual agent workflows still works for testing and debugging.
  • No dependency on repository_dispatch event semantics — uses the same workflow_dispatch mechanism already established in ADR 0008.

Decision ​

Use Option C. Introduce dispatch.yml as an indirection layer between the shim and agent workflows. The shim calls it with a stage parameter (triage, code, review). The dispatcher scans workflow files in .fullsend for # fullsend-stage: markers matching the requested stage and triggers each match via gh workflow run.

Option B offers atomic fan-out (GitHub triggers all subscribers from a single API call) and a simpler dispatcher (emit one event, no file scanning). However, it introduces a dual-trigger problem: agent workflows would need both on.repository_dispatch.types (for dispatched runs) and on.workflow_dispatch (for manual testing), each with a different input shape — github.event.client_payload vs github.event.inputs. Every agent workflow would need conditional logic to normalize inputs across both paths.

Option C avoids this by keeping workflow_dispatch as the sole trigger model, matching ADR 0008. Agent workflows have one trigger type, one input shape, and remain directly testable from the GitHub Actions UI. The trade-off is a more complex dispatcher (file scanning, sequential dispatch with per-call error handling) and a custom comment-marker convention instead of GitHub's native event subscription.

The shim knows about stages, not workflows. Adding, removing, or replacing agent workflows within a stage requires no shim changes — only changes to the .fullsend config repo.

The dispatcher authenticates gh workflow run calls using an installation token from the orchestrator GitHub App, keeping the PAT (FULLSEND_DISPATCH_TOKEN) confined to the workflow_dispatch boundary between enrolled repos and the config repo.

Consequences ​

  • Agent workflows can be added, removed, or replaced without modifying or redeploying shim workflows in enrolled repos.
  • Multiple workflows can carry the same # fullsend-stage: marker, enabling parallel fan-out within a stage without coordination logic.
  • Agent workflows remain directly testable from the GitHub Actions UI via the "Run workflow" button, using the same workflow_dispatch inputs as dispatched runs.
  • The credential boundary from ADR 0008 is preserved: enrolled repos hold only the dispatch PAT; App PEMs stay in the config repo.
  • An additional workflow execution (the dispatcher) runs on every event, increasing Actions minutes and adding latency to the dispatch path.
  • The # fullsend-stage: marker is a comment-based convention outside GitHub's native event model. It requires contributor documentation and is not validated by GitHub tooling — a missing or malformed marker silently excludes a workflow from dispatch.
  • The dispatcher triggers workflows sequentially via gh workflow run. If a call fails mid-loop, earlier workflows will have already been triggered, producing partial fan-out. Option B's repository_dispatch would have avoided this with atomic event emission.
  • Adding a new agent to a stage is a single-file operation: create a workflow in .fullsend with the appropriate # fullsend-stage: marker. This pattern is repeatable enough to be templated or tooled.
  • Adding a new stage (as opposed to a new agent within an existing stage) still requires changes to the shim template. This decoupling applies to the agent inventory within a stage, not to the stage inventory itself.
  • Orchestration within a stage is limited to parallel fan-out. Sequential execution, conditional chaining, and fan-in between agents within a stage are out of scope — those require the pipeline definition format deferred in ADR 0018.