diff --git a/README.md b/README.md
index d7a4e26..acc436f 100644
--- a/README.md
+++ b/README.md
@@ -88,6 +88,8 @@ await close();
 
 With no principal, every route except the list and counts answers `403`. Another tenant's artifact, or an unknown id, answers `404`. Set a request-body limit on the host for the upload and file-revise routes; they buffer the body before checking `MAX_UPLOAD_BYTES`.
 
+Every write reports `contentSha256`, the hex sha256 stored with the new version: over the UTF-8 text, or over the file's bytes for an upload. `POST /artifacts` and `POST /artifacts/upload` carry it on each artifact, and `POST /artifacts/:id/versions` returns it beside `artifactId` and `version`. It is `null` only for a version written before digests existed.
+
 ### `createWorkflowArtifactRoutes(deps)`
 
 Run-scoped routes for a workflow run, which authenticates with a bearer token and an `x-workflow-run-address` header instead of a session.
@@ -102,6 +104,8 @@ Run-scoped routes for a workflow run, which authenticates with a bearer token an
 | `maxBinaryBytes`  | `number`              | Optional. Byte ceiling for `POST /artifacts/binary`. Defaults to `MAX_UPLOAD_BYTES`.                                                                             |
 | `maxContentChars` | `number`              | Optional. Character ceiling on the resulting content — full text or edited text — for `POST /artifacts` and `PATCH /artifacts/:id/versions`. Defaults to 64,000. |
 
+Every write (`POST /artifacts`, `POST /artifacts/binary`, `POST /artifacts/link-file`, `PATCH /artifacts/:id`) answers `{ data: { id, version, contentSha256 } }`, with the same digest the tenant routes report. For `link-file` the digest covers the stored `preview` text (empty when none is given), not the workspace file at `path`.
+
 ### `runArtifactMigrations(config, { schema })`
 
 Takes the same `DBConfig` and `schema` as Interchange's `runMigrations`. `schema` holds the host's `tenant` and `principal` tables; this package's tables always go in `artifacts`.
@@ -110,7 +114,7 @@ Takes the same `DBConfig` and `schema` as Interchange's `runMigrations`. `schema
 
 `artifacts` is an `@intx/agent` tool pack of three tools. They call the run-scoped routes through the agent's `hub` credential.
 
-- `artifact_write` creates an artifact when no id is given, from content or a workspace file path. With an id it revises, either with the full text or with `edits`: exact passages, each of which must appear once, or nothing is written. `expectedVersion` refuses the write if a newer version exists.
+- `artifact_write` creates an artifact when no id is given, from content or a workspace file path. With an id it revises, either with the full text or with `edits`: exact passages, each of which must appear once, or nothing is written. `expectedVersion` refuses the write if a newer version exists. It returns `{ id, version, contentSha256 }`.
 - `artifact_read` returns the content, a past version with `version`, or a range with `offset` and `limit`.
 - `artifact_search` lists the newest artifacts, filtered by `kind` and by a `query` matched against title and content.
 
diff --git a/e2e/agent-token-mount.test.ts b/e2e/agent-token-mount.test.ts
index d0641e5..d958250 100644
--- a/e2e/agent-token-mount.test.ts
+++ b/e2e/agent-token-mount.test.ts
@@ -4,6 +4,7 @@ import {
   type AgentTokenAuth,
   type ResolvedWorkflowRunScope,
 } from "../src/workflow-mount.js";
+import { sha256Hex } from "../src/artifacts.js";
 import { InlineContentStore } from "../src/content-store.js";
 import { seedArtifact } from "./fixtures.js";
 import { testDb } from "./helpers.js";
@@ -134,9 +135,11 @@ describe("the routes the artifact tools call", () => {
     });
     expect(revised.status).toBe(200);
     expect(
-      (await revised.json()) as { data: { id: string; version: number } },
+      (await revised.json()) as {
+        data: { id: string; version: number; contentSha256: string | null };
+      },
     ).toEqual({
-      data: { id: artifact.id, version: 2 },
+      data: { id: artifact.id, version: 2, contentSha256: sha256Hex("second") },
     });
 
     const read = await app.request(`/artifacts/${artifact.id}/read`, {
diff --git a/e2e/mount.test.ts b/e2e/mount.test.ts
index 2392542..3e69dbd 100644
--- a/e2e/mount.test.ts
+++ b/e2e/mount.test.ts
@@ -185,6 +185,7 @@ describe("POST /artifacts", () => {
     expect(body.artifact).toMatchObject({
       kind: "document",
       source: { origin: "manual" },
+      contentSha256: sha256Hex("body"),
     });
   });
 
@@ -722,6 +723,7 @@ describe("versions", () => {
     expect(await res.json()).toMatchObject({
       version: 2,
       metadata: { kind: "run", supersededByNodeId: null },
+      contentSha256: sha256Hex("v2"),
     });
   });
 
diff --git a/e2e/workflow-mount.test.ts b/e2e/workflow-mount.test.ts
index 17e74d1..cbd250b 100644
--- a/e2e/workflow-mount.test.ts
+++ b/e2e/workflow-mount.test.ts
@@ -5,7 +5,11 @@ import {
   type ResolvedWorkflowRunScope,
 } from "../src/workflow-mount.js";
 import { InlineContentStore } from "../src/content-store.js";
-import { getArtifact, setArtifactArchived } from "../src/artifacts.js";
+import {
+  getArtifact,
+  setArtifactArchived,
+  sha256Hex,
+} from "../src/artifacts.js";
 import { seedArtifact } from "./fixtures.js";
 import { testDb } from "./helpers.js";
 import type { ArtifactDb } from "../src/db.js";
@@ -104,9 +108,10 @@ describe("POST /artifacts", () => {
     );
     expect(res.status).toBe(201);
     const { data } = (await res.json()) as {
-      data: { id: string; version: number };
+      data: { id: string; version: number; contentSha256: string | null };
     };
     expect(data.version).toBe(1);
+    expect(data.contentSha256).toBe(sha256Hex("hello"));
 
     const row = await getArtifact(db, data.id);
     expect(row?.tenantId).toBe("acme");
@@ -409,7 +414,10 @@ describe("POST /artifacts/binary", () => {
       json({ filename: "brief.html", mimeType: "text/html", contentBase64 }),
     );
     expect(res.status).toBe(201);
-    const { data } = (await res.json()) as { data: { id: string } };
+    const { data } = (await res.json()) as {
+      data: { id: string; contentSha256: string | null };
+    };
+    expect(data.contentSha256).toBe(sha256Hex("<h1>rendered</h1>"));
 
     const row = await getArtifact(db, data.id);
     expect(row?.tenantId).toBe("acme");
@@ -520,16 +528,22 @@ describe("creator grant minting", () => {
     ]);
   });
 
-  test("POST /artifacts/link-file mints write+archive creator grants", async () => {
+  test("POST /artifacts/link-file mints write+archive creator grants and digests the preview", async () => {
     const db = await testDb();
     const res = await host(db).request(
       "/artifacts/link-file",
-      json({ title: "Report", kind: "document", path: "out/report.md" }),
+      json({
+        title: "Report",
+        kind: "document",
+        path: "out/report.md",
+        preview: "summary",
+      }),
     );
     expect(res.status).toBe(201);
     const { data } = (await res.json()) as {
-      data: { id: string; version: number };
+      data: { id: string; version: number; contentSha256: string | null };
     };
+    expect(data.contentSha256).toBe(sha256Hex("summary"));
     expect(await grantsFor(db, data.id)).toEqual([
       { action: "archive", origin: "creator" },
       { action: "write", origin: "creator" },
@@ -569,8 +583,11 @@ describe("PATCH /artifacts/:id", () => {
       patchJson({ content: "v2", metadata: { stage: "review" } }),
     );
     expect(res.status).toBe(200);
-    const { data } = (await res.json()) as { data: { version: number } };
+    const { data } = (await res.json()) as {
+      data: { version: number; contentSha256: string | null };
+    };
     expect(data.version).toBe(2);
+    expect(data.contentSha256).toBe(sha256Hex("v2"));
 
     const updated = await getArtifact(db, row.id);
     expect(updated?.metadata).toEqual({ stage: "review" });
diff --git a/src/sidecar-bundle.test.ts b/src/sidecar-bundle.test.ts
index 47bbe18..1db90c9 100644
--- a/src/sidecar-bundle.test.ts
+++ b/src/sidecar-bundle.test.ts
@@ -41,10 +41,15 @@ function env(recorded: Recorded[], respond: () => Response) {
 }
 
 const ok = () =>
-  new Response(JSON.stringify({ data: { id: "art_1", version: 1 } }), {
-    status: 201,
-    headers: { "content-type": "application/json" },
-  });
+  new Response(
+    JSON.stringify({
+      data: { id: "art_1", version: 1, contentSha256: "ab12" },
+    }),
+    {
+      status: 201,
+      headers: { "content-type": "application/json" },
+    },
+  );
 
 const signal = new AbortController().signal;
 
@@ -70,6 +75,11 @@ describe("the artifacts sidecar bundle", () => {
     );
 
     expect(result.isError).toBeUndefined();
+    expect(JSON.parse(String(result.content))).toEqual({
+      id: "art_1",
+      version: 1,
+      contentSha256: "ab12",
+    });
     expect(recorded).toHaveLength(1);
     expect(recorded[0]?.url).toBe("/api/workflow-artifacts/artifacts");
     const headers = recorded[0]?.init?.headers as Record<string, string>;
diff --git a/src/tools.ts b/src/tools.ts
index 660f9b5..73ad166 100644
--- a/src/tools.ts
+++ b/src/tools.ts
@@ -249,7 +249,7 @@ export const ARTIFACT_TOOL_DEFINITIONS: readonly ArtifactToolDefinition[] = [
     name: "artifact_write",
     sideEffect: "write",
     description:
-      "Create or revise an artifact. Every call returns the artifact id and its new version; pass that version as expectedVersion on your next revision. Create: omit artifactId and pass title, kind, and content (or path, for a file you already wrote in your workspace). Revise: pass artifactId with edits to change exact passages, or content to replace the whole text. Prefer edits for small changes. If an edit is refused, nothing was written: read the artifact again and retry.",
+      "Create or revise an artifact. Every call returns the artifact id, its new version and contentSha256, the sha256 of the stored content; pass that version as expectedVersion on your next revision. Create: omit artifactId and pass title, kind, and content (or path, for a file you already wrote in your workspace). Revise: pass artifactId with edits to change exact passages, or content to replace the whole text. Prefer edits for small changes. If an edit is refused, nothing was written: read the artifact again and retry.",
     inputSchema: {
       type: "object",
       properties: {
diff --git a/src/workflow-mount.ts b/src/workflow-mount.ts
index 8de9ebc..f415c38 100644
--- a/src/workflow-mount.ts
+++ b/src/workflow-mount.ts
@@ -129,6 +129,8 @@ export type CreateWorkflowArtifactRoutesDeps = {
 export type CreatedWorkflowArtifact = {
   readonly id: string;
   readonly version: number;
+  /** sha256 (hex) stored with this version; null only on a pre-digest row. */
+  readonly contentSha256: string | null;
 };
 
 // Opaque to this package, same shape `mount.ts` validates: any JSON object,
@@ -358,6 +360,7 @@ export function createWorkflowArtifactRoutes({
     const created: CreatedWorkflowArtifact = {
       id: row.id,
       version: row.version,
+      contentSha256: row.contentSha256,
     };
     return c.json({ data: created }, 201);
   });
@@ -425,6 +428,7 @@ export function createWorkflowArtifactRoutes({
       const created: CreatedWorkflowArtifact = {
         id: row.id,
         version: row.version,
+        contentSha256: row.contentSha256,
       };
       return c.json({ data: created }, 201);
     } catch (err) {
@@ -493,6 +497,7 @@ export function createWorkflowArtifactRoutes({
     const created: CreatedWorkflowArtifact = {
       id: row.id,
       version: row.version,
+      contentSha256: row.contentSha256,
     };
     return c.json({ data: created }, 201);
   });
@@ -567,6 +572,7 @@ export function createWorkflowArtifactRoutes({
     const revised: CreatedWorkflowArtifact = {
       id: written.artifactId,
       version: written.version,
+      contentSha256: written.contentSha256,
     };
     return c.json({ data: revised });
   });