msd-core/docs/adr/0012-command-routing-hub.md

CommandRoutingHub as single dispatch seam for CJS command families

• Status: Superseded by ADR-0174 (2026-05-23); originally Accepted (2026-05-20)
• Date: 2026-05-20

Context

Seven *-command-router.cjs files (phase, phases, roadmap, state, verify, validate, init) each duplicate the same three-part dispatch pattern: (1) check MSD_WORKSTREAM + tryLoadSdk() to decide whether to use the SDK or CJS handler, (2) invoke the selected path, (3) map errors to the error() callback. The duplicated mode-selection logic means a policy change (e.g., adding a new fallback condition) must be applied in eight places. Tests for these routers are mock-heavy — they stub tryLoadSdk, stub getExecuteForCjs, and assert on internal call shapes rather than observable dispatch outcomes. The SDK-vs-CJS fallback decision is smeared across every router, making it impossible to reason about or test the policy in isolation.

Decision

Introduce CommandRoutingHub (msd-core/bin/lib/command-routing-hub.cjs) as the single dispatch seam for all CJS command family routers. The hub contract:

createHub({ mode: 'sdk' | 'cjs', sdkLoader, cjsRegistry, manifest }) -> hub
hub.dispatch({ family, subcommand, args, cwd, raw }) -> Result

Result = { ok: true, data }
       | { ok: false, errorKind, message, details? }

Load-bearing design properties:

• Pure result: the hub never prints to stdout/stderr, never calls process.exit, and never throws. All internal throws are caught and converted to { ok: false, errorKind: 'HandlerFailure' }.
• Mode fixed at construction: mode is set once when createHub is called; it is never re-evaluated per dispatch call. Each adapter (caller) computes mode based on its own env/sdk-load context before constructing the hub.
• No transparent fallback: an SDK-mode hub that encounters an SDK crash or load failure returns { ok: false, errorKind: 'SdkDispatchFailed' } or 'SdkLoadFailed' respectively. It does not silently retry via the CJS registry.
• Closed errorKind enum: the six error kinds (UnknownCommand, InvalidArgs, HandlerRefusal, HandlerFailure, SdkLoadFailed, SdkDispatchFailed) are exported as a frozen ERROR_KINDS object. Callers switch on ERROR_KINDS values, not bare string literals. Adding a new error kind requires amending this ADR.

The router adapter's responsibilities shrink to: determine mode from env, build stubs/registry, construct hub, dispatch, translate the pure Result to output()/error() calls. Each adapter remains a thin CLI-facing translation layer.

phase-command-router.cjs is migrated as the proof-of-concept for this PR. Remaining routers migrate in follow-up issues.

Consequences

• Positive: policy (mode decision, no-throw contract, error taxonomy) is concentrated in one module rather than duplicated across eight. Testing the policy requires only the hub unit tests; adapter tests verify translation correctness (args → dispatch, Result → output/error).
• Positive: future routers can be onboarded by wiring cjsRegistry entries rather than hand-replicating the SDK/CJS conditional block.
• Constraint: adding a new errorKind value requires updating ERROR_KINDS in command-routing-hub.cjs AND amending this ADR. The closed enum is the drift-prevention property; the amendment requirement makes scope of impact explicit.
• Constraint: each adapter must compute mode before hub construction (no lazy re-evaluation). This is intentional — mode ambiguity at dispatch time is a prior source of subtle test flakiness.

Known limitation: SDK-incomplete subcommands

The hub's mode is fixed at construction ('sdk' or 'cjs'). This works cleanly only when every subcommand in a family has an implementation in the active mode. Today some phase subcommands have divergent CJS and SDK implementations. phase.mvp-mode is present in the SDK catalog (command-static-catalog-domain.ts) but its CJS-native implementation (phase.cmdPhaseMvpMode) differs in ROADMAP scan behaviour and error reason codes from the SDK query layer. Routing mvp-mode through the SDK hub would silently change observable CLI behaviour (exit codes, JSON error shape).

The proof-of-concept adapter (phase-command-router.cjs) handles this with an early-return bypass: mvp-mode is intercepted before the dispatch call so it never reaches the hub. This preserves observable behavior but introduces a hub-level abstraction leak — the adapter now carries per-subcommand routing policy that the hub was meant to own.

Future direction (deferred): the hub should consult manifest to detect per-subcommand SDK coverage and route to CJS automatically for subcommands not present in the SDK manifest. That refinement stays inside the global-mode decision — the mode still applies to the family as a whole — and avoids the per-command policy ladder that was explicitly rejected during design. This work is tracked alongside SDK-CJS migration #3524 closure.

References

• Extends ADR-0001 (Dispatch Policy Module) — the hub implements the no-throw + structured-result contract ADR-0001 established for the SDK query layer, applying it to the CJS adapter layer.
• Issue: #3788