diff --git a/apps/dispatcher/src/failure-summary.test.ts b/apps/dispatcher/src/failure-summary.test.ts
index 52a8e0d..58e8794 100644
--- a/apps/dispatcher/src/failure-summary.test.ts
+++ b/apps/dispatcher/src/failure-summary.test.ts
@@ -24,6 +24,7 @@ import {
   TRUNCATION_NOTE,
   admissionTimedOutMd,
   appendFailureSummary,
+  failingStepMd,
   failureSummaryMd,
   runSkippedReason,
 } from "./failure-summary";
@@ -273,3 +274,33 @@ describe("appendFailureSummary", () => {
     expect(combined).not.toContain("summary truncated");
   });
 });
+
+describe("failingStepMd", () => {
+  it("names the recorded step and the tag it failed with", () => {
+    const md = failingStepMd(Exit.fail(new ExecFailed({ exitCode: 101, stderrTail: "x" })), {
+      name: "measure",
+      errorTag: "ExecFailed",
+    });
+    expect(md).toBe("**Failing step:** `measure` — `ExecFailed`");
+  });
+
+  it("takes the step a StepFailed names when no step row recorded the failure", () => {
+    const md = failingStepMd(
+      Exit.fail(new StepFailed({ step: "build-report", cause: "exit 2" })),
+      undefined,
+    );
+    expect(md).toBe("**Failing step:** `build-report` — `StepFailed`");
+  });
+
+  it("ignores a step row whose tag differs from the run's failure", () => {
+    const md = failingStepMd(Exit.fail(new SecretsMissing({ keys: ["K"] })), {
+      name: "checkout",
+      errorTag: "ExecFailed",
+    });
+    expect(md).toBe("**Failed outside any step** — `SecretsMissing`");
+  });
+
+  it("is undefined for a success Exit", () => {
+    expect(failingStepMd(Exit.succeed(1), undefined)).toBeUndefined();
+  });
+});
diff --git a/apps/dispatcher/src/failure-summary.ts b/apps/dispatcher/src/failure-summary.ts
index 1b1355f..ec7b510 100644
--- a/apps/dispatcher/src/failure-summary.ts
+++ b/apps/dispatcher/src/failure-summary.ts
@@ -157,6 +157,41 @@ export const failureSummaryMd = (exit: Exit.Exit<unknown, RunError>): string | u
       }),
   });
 
+/** The last `steps` row an execution recorded as `failure`. */
+export type FailedStepRow = { readonly name: string; readonly errorTag: string | null };
+
+/**
+ * Name the step a failed run died in and the tag it failed with — the line a
+ * scheduled run's check-run carries in place of a row only D1 holds. The tag is
+ * the run's typed failure; the step is the one a `StepFailed` names, else the
+ * last failed `steps` row when that row recorded the same tag. A failure no
+ * step recorded reads as outside any step. `undefined` on success and on a
+ * defect or interrupt, which carry no tag.
+ */
+export const failingStepMd = (
+  exit: Exit.Exit<unknown, RunError>,
+  failedStep: FailedStepRow | undefined,
+): string | undefined =>
+  Exit.match(exit, {
+    onSuccess: () => undefined,
+    onFailure: (cause) =>
+      Option.match(Cause.failureOption(cause), {
+        onNone: () => undefined,
+        onSome: (failure) => {
+          // `_tag` read as data for the label, as `describeFailure` in core's step.ts does.
+          const tag = (failure as { _tag?: unknown })._tag;
+          if (typeof tag !== "string") return undefined;
+          const stepName = Match.value(failure).pipe(
+            Match.tag("StepFailed", (e) => e.step),
+            Match.orElse(() => (failedStep?.errorTag === tag ? failedStep.name : undefined)),
+          );
+          return stepName === undefined
+            ? `**Failed outside any step** — \`${tag}\``
+            : `**Failing step:** \`${stepName}\` — \`${tag}\``;
+        },
+      }),
+  });
+
 /**
  * Append a run's failure markdown beneath the generic failure line (blank-line
  * separated, so the markdown renders as its own block), truncating the
diff --git a/apps/dispatcher/src/routes/scheduled.test.ts b/apps/dispatcher/src/routes/scheduled.test.ts
index 39e466c..665f0cc 100644
--- a/apps/dispatcher/src/routes/scheduled.test.ts
+++ b/apps/dispatcher/src/routes/scheduled.test.ts
@@ -64,6 +64,63 @@ describe("handleScheduled", () => {
     warn.mockRestore();
   });
 
+  it("pins a branch-targeting run to the head it resolves and carries the installation for its check-run", async () => {
+    const workflow = makeFakeWorkflow();
+    const env = makeEnv(workflow);
+    const head = "c".repeat(40);
+    const resolveHead = vi.fn(async () => ({ installationId: 4242, sha: head }));
+    const info = vi.spyOn(console, "info").mockImplementation(() => {});
+
+    // `contextful-measures` subscribes to "17 2 * * *" with inputs { repo, ref: refs/heads/main }.
+    await handleScheduled(env, "17 2 * * *", FIRED_AT, resolveHead);
+
+    expect(resolveHead).toHaveBeenCalledWith("fractalboxdev/contextful", "main");
+    const params = workflow.calls[0]!.params as {
+      github: { repo: string; ref: string; sha: string; installation_id?: number };
+      inputs: { sha?: string };
+    };
+    expect(params.github).toEqual({
+      repo: "fractalboxdev/contextful",
+      ref: "refs/heads/main",
+      sha: head,
+      installation_id: 4242,
+    });
+    // The run measures the same commit the check-run lands on.
+    expect(params.inputs.sha).toBe(head);
+    info.mockRestore();
+  });
+
+  it("fires without a check-run when the target repo's head does not resolve", async () => {
+    const workflow = makeFakeWorkflow();
+    const env = makeEnv(workflow);
+    const info = vi.spyOn(console, "info").mockImplementation(() => {});
+
+    await handleScheduled(env, "17 2 * * *", FIRED_AT, async () => undefined);
+
+    const params = workflow.calls[0]!.params as {
+      github: { installation_id?: number };
+      inputs: { sha?: string };
+    };
+    expect(params.github.installation_id).toBeUndefined();
+    expect(params.inputs.sha).toBeUndefined();
+    info.mockRestore();
+  });
+
+  it("does not resolve a head for a run whose inputs name no branch ref", async () => {
+    const workflow = makeFakeWorkflow();
+    const env = makeEnv(workflow);
+    const resolveHead = vi.fn(async () => ({ installationId: 1, sha: "d".repeat(40) }));
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+    await handleScheduled(env, "0 14 * * *", FIRED_AT, resolveHead);
+
+    expect(resolveHead).not.toHaveBeenCalled();
+    expect((workflow.calls[0]!.params as { github: object }).github).not.toHaveProperty(
+      "installation_id",
+    );
+    warn.mockRestore();
+  });
+
   it("collapses duplicate-id create errors as the dedup path (not a hard failure)", async () => {
     const workflow = makeFakeWorkflow();
     // First call records; second call (with same id) throws as a real
diff --git a/apps/dispatcher/src/routes/scheduled.ts b/apps/dispatcher/src/routes/scheduled.ts
index a5ac991..fb99e5b 100644
--- a/apps/dispatcher/src/routes/scheduled.ts
+++ b/apps/dispatcher/src/routes/scheduled.ts
@@ -40,10 +40,48 @@
 // own `executions` D1 row is keyed off the *Workflow instance* id, so a
 // dedup-collapsed second call is a literal no-op end-to-end.
 
+import {
+  getInstallationToken,
+  readBranchHead,
+  resolveRepoInstallationId,
+} from "@fractalboxdev/flare-dispatch-github-app";
 import { toInstanceId } from "../instance-id";
 import { type ScheduleMatch, schedulesByCron } from "../registry";
 import type { Env } from "../env";
 
+/** The installation covering a repo and the head commit of one of its branches. */
+export type ScheduledHead = { readonly installationId: number; readonly sha: string };
+
+/**
+ * Resolve the GitHub App installation for `repo` and the head of `branch`, or
+ * `undefined` when the deploy has no App credentials, the App is not installed
+ * on the repo, or GitHub fails the read — the run then fires without a
+ * check-run, as every scheduled run did before.
+ */
+export type ResolveScheduledHead = (
+  repo: string,
+  branch: string,
+) => Promise<ScheduledHead | undefined>;
+
+/** The live resolver: App JWT → repo installation → installation token → branch ref. */
+export const githubScheduledHead =
+  (env: Env): ResolveScheduledHead =>
+  async (repo, branch) => {
+    const appId = env.GITHUB_APP_ID;
+    const privateKeyPem = env.GITHUB_APP_PRIVATE_KEY;
+    if (appId === undefined || privateKeyPem === undefined) return undefined;
+    try {
+      const installationId = await resolveRepoInstallationId({ appId, privateKeyPem, repo });
+      const token = await getInstallationToken({ appId, privateKeyPem, installationId });
+      const sha = await readBranchHead({ token, repo, branch });
+      return { installationId, sha };
+    } catch (cause) {
+      const message = cause instanceof Error ? cause.message : String(cause);
+      console.warn(`[scheduled] no check-run for ${repo}:${branch} — ${message}`);
+      return undefined;
+    }
+  };
+
 /** Trim a long cron to fit inline in a log message. */
 const fmt = (cron: string): string => (cron.length > 40 ? `${cron.slice(0, 37)}...` : cron);
 
@@ -56,11 +94,13 @@ const fmt = (cron: string): string => (cron.length > 40 ? `${cron.slice(0, 37)}.
  * @param env             binding env (`RUNS_WORKFLOW`).
  * @param cron            `controller.cron` — the expression that fired.
  * @param scheduledTime   `controller.scheduledTime` — epoch ms.
+ * @param resolveHead     installation + branch-head lookup for runs that target a branch.
  */
 export const handleScheduled = async (
   env: Env,
   cron: string,
   scheduledTime: number,
+  resolveHead: ResolveScheduledHead = githubScheduledHead(env),
 ): Promise<void> => {
   const matches = schedulesByCron(cron);
 
@@ -74,7 +114,7 @@ export const handleScheduled = async (
 
   const ctx = { cron, firedAt: scheduledTime };
 
-  await Promise.all(matches.map(async (match) => fireOne(env, match, ctx)));
+  await Promise.all(matches.map(async (match) => fireOne(env, match, ctx, resolveHead)));
 };
 
 /** Fire a single (run, schedule-spec) match, swallowing individual failures. */
@@ -82,6 +122,7 @@ const fireOne = async (
   env: Env,
   match: ScheduleMatch,
   ctx: { cron: string; firedAt: number },
+  resolveHead: ResolveScheduledHead,
 ): Promise<void> => {
   const { run, schedule } = match;
 
@@ -92,17 +133,21 @@ const fireOne = async (
   }
 
   const id = toInstanceId(schedule.idempotencyKey(ctx));
-  const inputs = schedule.inputs(ctx);
+  const declared = schedule.inputs(ctx);
 
   // The Workflow `params` shape `RunWorkflow.run` decodes — same as dispatch.ts.
   // Schedule mode has no GitHub event; the run's own `inputs` carries the
-  // target repo/ref/sha (e.g. `product-demo` records against a deployed URL).
-  // `payload.github` here is just executions-row metadata + the optional
-  // check-run hint — installation_id is omitted, so `Checks` falls through to
-  // the no-op Layer (the run still records its rows, only the PR check-run is
-  // skipped). An operator who wants a check-run on a tracking repo sets
-  // `SCHEDULED_CHECK_RUN_*` env vars — deferred to a follow-up.
-  const target = scheduleGithubTarget(inputs);
+  // target repo/ref/sha. A run whose inputs name a repo and a `refs/heads/`
+  // branch, and no `sha`, is pinned here to that branch's head: the resolved
+  // installation selects the live `Checks` Layer, so `RunWorkflow` opens the
+  // check-run on that commit when the run starts and concludes it with the
+  // run's outcome — the same code push and PR runs take. The head also rides
+  // in `inputs.sha`, so the run executes against the commit the check names.
+  // An unresolved head leaves `installation_id` absent and the inputs as
+  // declared: the run fires, records its rows, and posts no check-run.
+  const pinned = await pinBranchHead(declared, resolveHead);
+  const inputs = pinned?.inputs ?? declared;
+  const target = scheduleGithubTarget(inputs, pinned?.installationId);
   const params = {
     executionId: id,
     run: run.name,
@@ -125,6 +170,28 @@ const fireOne = async (
   }
 };
 
+/**
+ * Resolve the head a branch-targeting run executes against. `undefined` when
+ * the inputs name no repo, no `refs/heads/` ref, or already carry a `sha`, and
+ * when the resolver finds no installation or head.
+ */
+const pinBranchHead = async (
+  inputs: unknown,
+  resolveHead: ResolveScheduledHead,
+): Promise<{ inputs: Record<string, unknown>; installationId: number } | undefined> => {
+  if (typeof inputs !== "object" || inputs === null) return undefined;
+  const obj = inputs as Record<string, unknown>;
+  if (typeof obj.repo !== "string" || typeof obj.ref !== "string" || obj.sha !== undefined) {
+    return undefined;
+  }
+  const branch = obj.ref.startsWith("refs/heads/") ? obj.ref.slice("refs/heads/".length) : "";
+  if (branch === "") return undefined;
+  const head = await resolveHead(obj.repo, branch);
+  return head === undefined
+    ? undefined
+    : { inputs: { ...obj, sha: head.sha }, installationId: head.installationId };
+};
+
 /**
  * Synthesize the `github` block `RunWorkflow` expects. The cron tick carries
  * no GitHub payload, so we lift `repo` / `ref` / `sha` from the run's own
@@ -132,20 +199,20 @@ const fireOne = async (
  * uses these fields only for the executions D1 row and the (no-op) check-run
  * Layer when no `installation_id` is wired.
  *
- * A run that wants a real check-run on its tracking repo should put
- * `installation_id` in its `inputs` and the operator should set
- * `GITHUB_APP_ID` + `GITHUB_APP_PRIVATE_KEY`; that path is wired but no
- * shipped Schedule-mode run uses it yet.
+ * `installationId` comes from `pinBranchHead`; an `installation_id` in the
+ * run's own inputs is the fallback.
  */
 const scheduleGithubTarget = (
   inputs: unknown,
+  installationId?: number,
 ): { repo: string; ref: string; sha: string; installation_id?: number } => {
   const obj = (inputs ?? {}) as Record<string, unknown>;
   const repo = typeof obj.repo === "string" ? obj.repo : "schedule/unknown";
   const ref = typeof obj.ref === "string" ? obj.ref : "refs/heads/main";
   const sha =
     typeof obj.sha === "string" ? obj.sha : typeof obj.ref === "string" ? obj.ref : "main";
-  const installation_id = typeof obj.installation_id === "number" ? obj.installation_id : undefined;
+  const installation_id =
+    installationId ?? (typeof obj.installation_id === "number" ? obj.installation_id : undefined);
 
   // Warn when the run's `schedules[].inputs()` returned a shape without
   // the fields the `executions` row expects — a Schema mismatch between
diff --git a/apps/dispatcher/src/workflow.ts b/apps/dispatcher/src/workflow.ts
index ddf407c..5112666 100644
--- a/apps/dispatcher/src/workflow.ts
+++ b/apps/dispatcher/src/workflow.ts
@@ -28,7 +28,8 @@
 //
 //   * The App credentials (`GITHUB_APP_ID`, `GITHUB_APP_PRIVATE_KEY`) come from
 //     `env`; the installation id rides in the dispatch payload's
-//     `github.installation_id` (PR5's dispatch route fills it in). When any of
+//     `github.installation_id` (PR5's dispatch route fills it in; the
+//     scheduled path resolves it for a run that targets a branch). When any of
 //     the three is absent — local dev without secrets, or a dispatch with no
 //     installation — `makeCFRuntimeLive` selects the *no-op* `Checks` Layer:
 //     the execution runs to completion and records its D1 rows, only the PR
@@ -86,7 +87,13 @@ import { lookupRun } from "./registry";
 import { preAssertedApproval, resolveTargets, runGrant, runsOnFacade } from "./grant-catalog";
 import { selectSandboxNs } from "./sandbox-routing";
 import { queuedSummary, serialQueuedSummary } from "./admission-summary";
-import { appendFailureSummary, failureSummaryMd, runSkippedReason } from "./failure-summary";
+import {
+  appendFailureSummary,
+  type FailedStepRow,
+  failingStepMd,
+  failureSummaryMd,
+  runSkippedReason,
+} from "./failure-summary";
 import { renderResultEmail } from "./notify";
 import { workflowDashboardUrl } from "./dashboard-url";
 import { checkRunNameFor, checkRunTitleFor } from "./check-name";
@@ -1031,9 +1038,28 @@ export class RunWorkflow extends WorkflowEntrypoint<Env> {
       // carry markdown (`AcceptanceFailed.summaryMd` — e.g. `product-demo`'s
       // per-chapter table). Extracted ONCE here and reused by both the
       // check-run failure summary and the notify email below. `undefined` on
-      // success, on a defect/interrupt, and on a summary-less failure — those
-      // render the generic line alone, exactly as before.
-      const failureMd = failureSummaryMd(exit);
+      // success and on a defect/interrupt — those render the generic line alone.
+      //
+      // A failure leads with the step it died in and its error tag, read from
+      // the last `steps` row recorded `failure` — the diagnosis a scheduled run
+      // otherwise leaves only in D1. The read is best-effort: a D1 error drops
+      // the step name, never the verdict.
+      const failedStep = Exit.isFailure(exit)
+        ? yield* Effect.tryPromise(() =>
+            db
+              .prepare(
+                `SELECT name, error_tag AS errorTag FROM steps
+                  WHERE execution_id = ? AND status = 'failure'
+                  ORDER BY completed_at DESC LIMIT 1`,
+              )
+              .bind(payload.executionId)
+              .first<FailedStepRow>(),
+          ).pipe(Effect.orElseSucceed(() => null))
+        : null;
+      const failureMd =
+        [failingStepMd(exit, failedStep ?? undefined), failureSummaryMd(exit)]
+          .filter((part): part is string => part !== undefined && part.trim() !== "")
+          .join("\n\n") || undefined;
 
       // Persist the run output as JSON so `io.priorExecution` can recover it
       // on the next execution in the semantic family. A failed Exit has no
diff --git a/runs/README.md b/runs/README.md
index 135e37e..15169b9 100644
--- a/runs/README.md
+++ b/runs/README.md
@@ -24,6 +24,14 @@ flowchart TB
   class checks ok
 ```
 
+A cron tick carries no commit. A scheduled run whose inputs name a `repo` and a
+`refs/heads/<branch>` ref, and no `sha`, is pinned at the tick to that branch's
+head through the repo's GitHub App installation: the run receives the commit as
+`inputs.sha`, and its `flare-dispatch/<run>` check-run opens on that commit when
+the run starts and concludes with the run's outcome. A failure's summary names
+the failing step and its error tag. A repo without the App installed, or a
+deploy without App credentials, runs the schedule and posts no check-run.
+
 A run is a recipe: it composes primitives, which ride the capability layer, and
 calls capabilities directly where no primitive fits.
 
diff --git a/runs/contextful-measures.test.ts b/runs/contextful-measures.test.ts
index b8e248c..23a1771 100644
--- a/runs/contextful-measures.test.ts
+++ b/runs/contextful-measures.test.ts
@@ -45,13 +45,29 @@ describe("contextful-measures", () => {
         { repo: input.repo, commit: sha, text: report },
       ]);
       expect(handles.artifact.uploads[0]?.name).toBe("measure-report.json");
-      const reportExec = handles.sandbox.execs.find((entry) => entry.command.includes("measure-report"));
+      const reportExec = handles.sandbox.execs.find((entry) =>
+        entry.command.includes("measure-report"),
+      );
       expect(reportExec?.env?.MEASURE_RUN_ID).toBe("contextful_measures_attempt-2");
       expect(reportExec?.env?.MEASURE_RUN_ATTEMPT).toBe("2");
       expect(reportExec?.env?.MEASURE_EXIT).toBe("0");
     }).pipe(Effect.provide(layer));
   });
 
+  it.effect("measures the commit the schedule pinned without re-reading the branch head", () => {
+    const pinned = "b".repeat(40);
+    const { layer, handles } = makeCFRuntimeTest({
+      sandboxProgram: { "measure --tier all": { exitCode: 0 }, "measure-report": { exitCode: 0 } },
+      sandboxFiles: { "/workspace/contextful/measure-report.json": report },
+    });
+    return Effect.gen(function* () {
+      const out = yield* contextfulMeasures.run({ ...input, sha: pinned });
+      expect(out.commit).toBe(pinned);
+      expect(handles.github.branchHeadCalls).toHaveLength(0);
+      expect(handles.sandbox.clones).toEqual([{ repo: input.repo, sha: pinned }]);
+    }).pipe(Effect.provide(layer));
+  });
+
   it.effect("attaches a report even when a measure tier is red", () => {
     const { layer, handles } = makeCFRuntimeTest({
       github: { branchHeads: { "fractalboxdev/contextful:main": sha } },
diff --git a/runs/contextful-measures.ts b/runs/contextful-measures.ts
index c20e5ec..1799bfc 100644
--- a/runs/contextful-measures.ts
+++ b/runs/contextful-measures.ts
@@ -19,6 +19,8 @@ const Input = Schema.Struct({
   repo: Schema.String,
   ref: Schema.String,
   firedAt: Schema.Number,
+  /** The head the scheduled tick pinned; its check-run lands on this commit. */
+  sha: Schema.optional(Schema.String),
   timeoutSec: Schema.optional(Schema.Number),
 });
 const Output = Schema.Struct({
@@ -46,7 +48,11 @@ export const contextfulMeasures = defineRun({
       cron: "17 2 * * *",
       idempotencyKey: ({ firedAt }) =>
         `contextful-measures:${new Date(firedAt).toISOString().slice(0, 10)}`,
-      inputs: ({ firedAt }): typeof Input.Type => ({ repo: REPO, ref: `refs/heads/${BRANCH}`, firedAt }),
+      inputs: ({ firedAt }): typeof Input.Type => ({
+        repo: REPO,
+        ref: `refs/heads/${BRANCH}`,
+        firedAt,
+      }),
     },
   ],
   serialize: (input) => ({
@@ -66,10 +72,10 @@ export const contextfulMeasures = defineRun({
           }),
         );
       }
-      // Resolve HEAD at execution time; a cron tick has no GitHub event SHA.
-      const commit = yield* step("resolve-head", () =>
-        github.branchHead({ repo: REPO, branch: BRANCH }),
-      );
+      // The tick's pinned head, else HEAD at execution time.
+      const commit =
+        input.sha ??
+        (yield* step("resolve-head", () => github.branchHead({ repo: REPO, branch: BRANCH })));
       const executionId = yield* io.executionId;
       const runId = executionId || `contextful-measures:${input.firedAt}`;
       // The ledger runs 65 entries in sequence and compiles its own test
diff --git a/runs/contextful-protocol.test.ts b/runs/contextful-protocol.test.ts
index 478243e..4e3f235 100644
--- a/runs/contextful-protocol.test.ts
+++ b/runs/contextful-protocol.test.ts
@@ -113,6 +113,24 @@ describe("contextful-protocol", () => {
     },
   );
 
+  it.effect("explores the commit the schedule pinned without re-reading the branch head", () => {
+    const pinned = "b".repeat(40);
+    const { layer, handles } = makeCFRuntimeTest({
+      sandboxProgram: {
+        "gate --predecessors --stage toolchain": { exitCode: 0 },
+        randomBytes: { exitCode: 0, stdout: "7\n" },
+        "protocol-differential": { exitCode: 0 },
+        "formal check": { exitCode: 0 },
+      },
+    });
+    return Effect.gen(function* () {
+      const out = yield* contextfulProtocol.run({ ...input, sha: pinned });
+      expect(out.commit).toBe(pinned);
+      expect(handles.github.branchHeadCalls).toHaveLength(0);
+      expect(handles.sandbox.clones).toEqual([{ repo: input.repo, sha: pinned }]);
+    }).pipe(Effect.provide(layer));
+  });
+
   it.effect("rejects a different repository before allocating a sandbox", () => {
     const { layer, handles } = makeCFRuntimeTest({});
     return Effect.gen(function* () {
diff --git a/runs/contextful-protocol.ts b/runs/contextful-protocol.ts
index 26d1980..969ea87 100644
--- a/runs/contextful-protocol.ts
+++ b/runs/contextful-protocol.ts
@@ -20,7 +20,13 @@ const FORMAL_COMMAND = `${PINNED_ENVIRONMENT}cargo run --locked -q -p contextful
 const SEED_COMMAND = `node -e 'console.log(require("node:crypto").randomBytes(8).readBigUInt64LE().toString())'`;
 const REGRESSIONS = "formal/protocol/regressions.jsonl";
 const COMMAND_TIMEOUT_SEC = 1800;
-const Input = Schema.Struct({ repo: Schema.String, ref: Schema.String, firedAt: Schema.Number });
+const Input = Schema.Struct({
+  repo: Schema.String,
+  ref: Schema.String,
+  firedAt: Schema.Number,
+  /** The head the scheduled tick pinned; its check-run lands on this commit. */
+  sha: Schema.optional(Schema.String),
+});
 const Output = Schema.Struct({ commit: Schema.String, seed: Schema.String, logUrl: Schema.String });
 
 /** The gate installs elan; the separate command restores its pinned executable environment. */
@@ -50,9 +56,9 @@ export const contextfulProtocol = defineRun({
           }),
         );
       }
-      const commit = yield* step("resolve-head", () =>
-        github.branchHead({ repo: REPO, branch: "main" }),
-      );
+      const commit =
+        input.sha ??
+        (yield* step("resolve-head", () => github.branchHead({ repo: REPO, branch: "main" })));
       const ws = yield* step("checkout", () => workspace({ repo: REPO, sha: commit }));
       const provisioned = yield* step(
         "toolchain",