msd-core/docs/how-to/develop-a-task-content-resolver-capability.md

Develop a task-content resolver capability

Goal: Declare a taskContentResolver in a capability manifest so execute-plan.md resolves a task's <action>/<verify>/<acceptance_criteria>/<read_first>/<done> content from your external issue tracker instead of reading it inline from PLAN.md.

Prerequisites: You already have a capability (capability.json), or you are creating one — see Develop a Capability for MSD 1.5+ first if this is your first one. MSD 1.28 or later (ADR-3646).

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Why this exists

A project that wants an external tracker — beads, Linear, Jira, GitHub Issues — to own task content, not just task status, has no seam for that today: execute-plan.md reads every task's instructions directly out of the PLAN.md task block. taskContentResolver adds one, with a hard-halt guarantee: if your tracker is declared as the source of truth and it fails to resolve, execution stops rather than silently falling back to stale PLAN.md text. That guarantee is the entire point of the feature — a silent fallback would require the tracker and PLAN.md to stay in sync forever, defeating the reason to move content out of PLAN.md in the first place.

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Declare the resolver

Add a taskContentResolver block to your capability's manifest body (role: "feature" only):

{
  "id": "beads",
  "role": "feature",
  "version": "1.0.0",
  "title": "Beads issue tracker",
  "description": "Resolves task content from the bd issue tracker.",
  "tier": "standard",
  "requires": [],
  "runtimeCompat": { "supported": ["*"], "unsupported": [] },
  "skills": [],
  "agents": [],
  "hooks": [],
  "config": {},
  "steps": [],
  "contributions": [],
  "gates": [],

  "taskContentResolver": {
    "trackerPrefix": "beads",
    "invoke": {
      "binary": "bd",
      "args": ["show", "{{id}}", "--json"],
      "timeoutMs": 10000
    }
  }
}

Two fields decide whether the seam works at all:

• trackerPrefix must match the prefix of a task's tracker-id attribute — everything before the first :. Given <task tracker-id="beads:MSD-42">, the prefix is beads and the id passed to your resolver is MSD-42. If a tracker's own ids contain colons, that is fine: only the first colon splits prefix from id, so beads:team:MSD-42 resolves to id team:MSD-42.
• invoke.args must contain the {{id}} placeholder at least once — MSD substitutes it with the task's id before spawning your binary. A declaration whose args never carries the placeholder fails validation at install time, because the id could never reach your resolver.

invoke.timeoutMs is required. An unbounded resolver subprocess is this repo's named Unbounded Subprocesses defect class — declare a bound that matches how long your tracker's lookup actually takes, plus margin.

trackerPrefix must be unique across the merged first-party ∪ overlay capability set; a collision — two installed capabilities both claiming "beads" — is a build-time validation error, not a runtime ambiguity.

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What your resolver must output

execute-plan.md invokes your invoke.binary/invoke.args and expects a single JSON object on stdout when the lookup succeeds:

Field	Type	Required	Maps to
description	string	Yes	The task's <action>
verify	string	No	The task's <verify>
acceptance_criteria	string[]	No	The task's <acceptance_criteria>
read_first	string[]	No	The task's <read_first>
done	string	No	The task's <done>

An absent or empty-string description is treated as "nothing resolved" — execute-plan.md falls back to the task's inline PLAN.md content, the one legitimate pre-migration boundary case (for tasks authored before your tracker migration). This is the only silent fallback path; every other failure is a hard halt.

Exit code and stderr matter. Exit 0 with valid JSON on stdout is the only success path. Anything else — a non-zero exit, a timeout past invoke.timeoutMs, or stdout that fails to parse as JSON — is treated as a resolution failure. Write a clear one-line reason to stderr; it is surfaced verbatim to the person watching execution. Stderr on a successful (exit 0) run is not an error — write informational logs there if your CLI already does; only the exit code and the JSON parse outcome decide success.

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What happens on failure

A resolver that is declared, invoked, and fails — non-zero exit, timeout, or malformed JSON — makes msd_run task resolve-content itself exit non-zero. execute-plan.md treats that as a hard halt: it stops before doing any work on the task, surfaces the tracker-id, the tracker prefix, and your resolver's stderr, and never proceeds to read the task's inline PLAN.md content as a substitute. See ADR-3646 for why this is the load-bearing safety property of the whole feature, and loop-hook-dispatch.md for how this call site differs from the twelve prose-dispatched loop extension points.

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Worked example: a bd/beads-shaped resolver

Suppose bd show MSD-42 --json already returns:

{
  "id": "MSD-42",
  "title": "Add rate-limit check to login",
  "status": "open",
  "description": "Add a rate-limit check to processLogin using the existing RateLimiter.",
  "acceptance": [
    "Login attempts beyond the configured limit return 429",
    "Existing successful-login tests still pass"
  ],
  "notes": "See src/util/rate.ts for the existing limiter."
}

Your resolver is a thin adapter, not bd itself — it maps bd's field names onto the shape execute-plan.md expects and exits non-zero on anything bd itself reports as a failure:

#!/usr/bin/env bash
set -euo pipefail

id="$1"
raw=$(bd show "$id" --json)

node -e '
  const raw = JSON.parse(process.argv[1]);
  const out = {
    description: raw.description || "",
    acceptance_criteria: raw.acceptance || [],
  };
  process.stdout.write(JSON.stringify(out));
' "$raw"

Declare it as the invoke.binary/invoke.args pair (or point invoke.binary at bd directly if its own --json output already matches the expected field names — no adapter needed in that case). Either way, invoke.args must carry {{id}} so MSD can substitute the task's tracker id before spawning it.

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Related

• ADR-3646 — the design decision and rejected alternatives
• Capability manifest → taskContentResolver — the full field table
• loop-hook-dispatch.md — how execute:task differs from the twelve loop extension points
• Develop a Capability for MSD 1.5+ — manifests, registry generation, and federated config