diff --git a/apps/web/src/app.css b/apps/web/src/app.css
index 301c227c8..9252f4811 100644
--- a/apps/web/src/app.css
+++ b/apps/web/src/app.css
@@ -3722,6 +3722,13 @@ tr.insights-row-clickable:hover {
   font-weight: 600;
   text-decoration: underline;
 }
+.chat-package-rejection-icon {
+  width: 0.9em;
+  height: 0.9em;
+  margin-right: 0.35rem;
+  vertical-align: -0.1em;
+  color: var(--destructive);
+}
 
 /* Sidebar sections: Workbenches above Chats, each with its own label. */
 .shell-panel-section-label {
diff --git a/apps/web/src/chat/deployable-package.test.ts b/apps/web/src/chat/deployable-package.test.ts
index 72dc1b0cf..cc526b71a 100644
--- a/apps/web/src/chat/deployable-package.test.ts
+++ b/apps/web/src/chat/deployable-package.test.ts
@@ -3,6 +3,7 @@ import { describe, expect, test } from "bun:test";
 import {
   deployablePackageFromBody,
   isFiveFieldCron,
+  isPackageRejection,
   resolveMessagePackage,
 } from "./deployable-package";
 
@@ -15,14 +16,15 @@ describe("deployablePackageFromBody", () => {
       `Here's the agent.\n\npackage.json\n\`\`\`\n${PACKAGE_JSON}\n\`\`\`\n\n` +
       `definition.json\n\`\`\`\n${DEFINITION_JSON}\n\`\`\`\n\nPress Deploy.`;
     const result = deployablePackageFromBody(body);
-    expect(result?.pkg.name).toBe("Echo");
+    expect(isPackageRejection(result?.outcome ?? null)).toBe(false);
+    expect(result?.outcome && "name" in result.outcome ? result.outcome.name : null).toBe("Echo");
     expect(result?.strippedBody).toBe("Here's the agent.\n\nPress Deploy.");
   });
 
   test("matches an info-string label instead of a preceding line", () => {
     const body = `\`\`\`json package.json\n${PACKAGE_JSON}\n\`\`\`\n\`\`\`definition.json\n${DEFINITION_JSON}\n\`\`\``;
     const result = deployablePackageFromBody(body);
-    expect(result?.pkg.name).toBe("Echo");
+    expect(result?.outcome && "name" in result.outcome ? result.outcome.name : null).toBe("Echo");
   });
 
   test("reads a comment label with a path prefix on the fence's first line", () => {
@@ -30,7 +32,7 @@ describe("deployablePackageFromBody", () => {
       `Here is the package:\n\n\`\`\`json\n// Scribe/definition.json\n${DEFINITION_JSON}\n\`\`\`\n\n` +
       `\`\`\`json\n# Scribe/package.json\n${PACKAGE_JSON}\n\`\`\`\n\nPress Deploy.`;
     const result = deployablePackageFromBody(body);
-    expect(result?.pkg.name).toBe("Echo");
+    expect(result?.outcome && "name" in result.outcome ? result.outcome.name : null).toBe("Echo");
     expect(result?.strippedBody).toBe("Here is the package:\n\nPress Deploy.");
   });
 
@@ -38,7 +40,8 @@ describe("deployablePackageFromBody", () => {
     const body =
       `\`\`\`json\npackage.json\n${PACKAGE_JSON}\n\`\`\`\n\n` +
       `\`\`\`json\ndefinition.json\n${DEFINITION_JSON}\n\`\`\``;
-    expect(deployablePackageFromBody(body)?.pkg.name).toBe("Echo");
+    const result = deployablePackageFromBody(body);
+    expect(result?.outcome && "name" in result.outcome ? result.outcome.name : null).toBe("Echo");
   });
 
   test("is null when only one of the two files is present", () => {
@@ -46,11 +49,46 @@ describe("deployablePackageFromBody", () => {
     expect(deployablePackageFromBody(body)).toBeNull();
   });
 
-  test("is null when the definition block fails validation", () => {
+  test("rejects with a named field when the definition block fails validation", () => {
     const body =
       `package.json\n\`\`\`\n${PACKAGE_JSON}\n\`\`\`\n` +
-      `definition.json\n\`\`\`\n{"name": ""}\n\`\`\``;
-    expect(deployablePackageFromBody(body)).toBeNull();
+      `definition.json\n\`\`\`\n{"systemPrompt": ""}\n\`\`\``;
+    const result = deployablePackageFromBody(body);
+    expect(isPackageRejection(result?.outcome ?? null)).toBe(true);
+    expect(result?.outcome && "reason" in result.outcome ? result.outcome.reason : null).toBe(
+      "definition.json is missing systemPrompt",
+    );
+  });
+
+  test("rejects with a plain-words reason when definition.json isn't valid JSON", () => {
+    const body =
+      `package.json\n\`\`\`\n${PACKAGE_JSON}\n\`\`\`\n` +
+      `definition.json\n\`\`\`\nnot json\n\`\`\``;
+    const result = deployablePackageFromBody(body);
+    expect(result?.outcome && "reason" in result.outcome ? result.outcome.reason : null).toBe(
+      "definition.json is not valid JSON",
+    );
+  });
+
+  test("takes name from package.json when definition.json omits it", () => {
+    const body =
+      `package.json\n\`\`\`\n${PACKAGE_JSON}\n\`\`\`\n` +
+      `definition.json\n\`\`\`\n{"systemPrompt": "Echo back what you hear."}\n\`\`\``;
+    const result = deployablePackageFromBody(body);
+    expect(result?.outcome && "name" in result.outcome ? result.outcome.name : null).toBe("echo");
+  });
+
+  test("ignores an unknown extra field in definition.json", () => {
+    const body =
+      `package.json\n\`\`\`\n${PACKAGE_JSON}\n\`\`\`\n` +
+      `definition.json\n\`\`\`\n${JSON.stringify({
+        name: "Echo",
+        systemPrompt: "Echo back what you hear.",
+        type: "workflow",
+      })}\n\`\`\``;
+    const result = deployablePackageFromBody(body);
+    expect(isPackageRejection(result?.outcome ?? null)).toBe(false);
+    expect(result?.outcome && "name" in result.outcome ? result.outcome.name : null).toBe("Echo");
   });
 });
 
@@ -71,7 +109,7 @@ describe("resolveMessagePackage", () => {
   test("falls back to the body when there are no attachments", () => {
     const body = `package.json\n\`\`\`\n${PACKAGE_JSON}\n\`\`\`\ndefinition.json\n\`\`\`\n${DEFINITION_JSON}\n\`\`\``;
     const { pkg, renderedBody } = resolveMessagePackage([], body);
-    expect(pkg?.name).toBe("Echo");
+    expect(pkg && "name" in pkg ? pkg.name : null).toBe("Echo");
     expect(renderedBody).toBe("");
   });
 
@@ -88,7 +126,25 @@ describe("resolveMessagePackage", () => {
       ],
       body,
     );
-    expect(pkg?.name).toBe("Attached");
+    expect(pkg && "name" in pkg ? pkg.name : null).toBe("Attached");
     expect(renderedBody).toBe(body);
   });
+
+  test("surfaces a rejection from attachments naming the missing field", () => {
+    const { pkg } = resolveMessagePackage(
+      [
+        { name: "package.json", contentType: "application/json", text: PACKAGE_JSON },
+        {
+          name: "definition.json",
+          contentType: "application/json",
+          text: `{"name": "Attached"}`,
+        },
+      ],
+      "unused body",
+    );
+    expect(isPackageRejection(pkg)).toBe(true);
+    expect(pkg && "reason" in pkg ? pkg.reason : null).toBe(
+      "definition.json is missing systemPrompt",
+    );
+  });
 });
diff --git a/apps/web/src/chat/deployable-package.ts b/apps/web/src/chat/deployable-package.ts
index 8d0f5e8da..99073884c 100644
--- a/apps/web/src/chat/deployable-package.ts
+++ b/apps/web/src/chat/deployable-package.ts
@@ -12,8 +12,12 @@ import { reportError } from "@corbits/error-sink";
 
 import type { MailAttachment } from "./threads-api";
 
-const AgentDefinition = type({
-  name: "string",
+const PackageManifest = type({
+  "name?": "string",
+});
+
+const AgentDefinitionShape = type({
+  "name?": "string",
   systemPrompt: "string",
   "description?": "string",
   "schedule?": "string",
@@ -26,30 +30,97 @@ export function isFiveFieldCron(schedule: string): boolean {
   return schedule.trim().split(/\s+/).length === 5;
 }
 
-export type DeployablePackage = typeof AgentDefinition.infer;
+export interface DeployablePackage {
+  readonly name: string;
+  readonly systemPrompt: string;
+  readonly description?: string;
+  readonly schedule?: string;
+}
 
 export const PACKAGE_MANIFEST_NAME = "package.json";
 export const AGENT_DEFINITION_NAME = "definition.json";
 
-/** The agent package a message's attachments describe, or null when they
- * are just files. Untrusted input: parsed, never cast. */
-export function deployablePackage(
-  attachments: readonly MailAttachment[],
-): DeployablePackage | null {
-  if (!attachments.some((attachment) => attachment.name === PACKAGE_MANIFEST_NAME)) return null;
-  const definition = attachments.find((attachment) => attachment.name === AGENT_DEFINITION_NAME);
-  if (definition === undefined) return null;
-  let body: unknown;
+/**
+ * What a message's attachments (or fenced blocks) resolve to: a parsed
+ * package, a named rejection when the message attempted a package but
+ * got a field wrong, or `null` when the message isn't a package attempt
+ * at all.
+ */
+export type PackageOutcome = DeployablePackage | { readonly reason: string } | null;
+
+export function isPackageRejection(
+  outcome: PackageOutcome,
+): outcome is { readonly reason: string } {
+  return outcome !== null && "reason" in outcome;
+}
+
+/** The plain-words reason a definition.json's arktype error names, e.g.
+ * "definition.json is missing systemPrompt" or "definition.json's
+ * schedule must be a string". Field name comes from the error's path. */
+function definitionRejectionReason(errors: type.errors): string {
+  const first = errors[0];
+  const field = first === undefined ? "value" : String(first.path.at(-1) ?? "value");
+  if (first?.code === "required") return `definition.json is missing ${field}`;
+  return `definition.json's ${field} ${first?.problem ?? "is invalid"}`;
+}
+
+/** Parses the two contract files' text into a package outcome. Returns
+ * `null` only when called on files that were never actually found — the
+ * caller decides that; this always parses given both texts. */
+function parsePackageFiles(
+  manifestText: string,
+  definitionText: string,
+): DeployablePackage | { readonly reason: string } {
+  let manifestBody: unknown;
+  try {
+    manifestBody = JSON.parse(manifestText);
+  } catch (cause) {
+    reportError(cause, { operation: "chat_deployable_package_manifest_parse" });
+    return { reason: "package.json is not valid JSON" };
+  }
+  const manifest = PackageManifest(manifestBody);
+  if (manifest instanceof type.errors) {
+    return { reason: `package.json's ${String(manifest[0]?.path.at(-1) ?? "value")} is invalid` };
+  }
+
+  let definitionBody: unknown;
   try {
-    body = JSON.parse(definition.text);
+    definitionBody = JSON.parse(definitionText);
   } catch (cause) {
-    reportError(cause, { operation: "chat_deployable_package_parse" });
-    return null;
+    reportError(cause, { operation: "chat_deployable_package_definition_parse" });
+    return { reason: "definition.json is not valid JSON" };
+  }
+  const definition = AgentDefinitionShape(definitionBody);
+  if (definition instanceof type.errors) {
+    return { reason: definitionRejectionReason(definition) };
+  }
+
+  const name = definition.name?.trim() || manifest.name?.trim();
+  if (name === undefined || name === "") {
+    return { reason: "definition.json is missing name" };
   }
-  const parsed = AgentDefinition(body);
-  if (parsed instanceof type.errors) return null;
-  if (parsed.name.trim() === "" || parsed.systemPrompt.trim() === "") return null;
-  return parsed;
+  if (definition.systemPrompt.trim() === "") {
+    return { reason: "definition.json is missing systemPrompt" };
+  }
+
+  return {
+    name,
+    systemPrompt: definition.systemPrompt,
+    ...(definition.description !== undefined ? { description: definition.description } : {}),
+    ...(definition.schedule !== undefined ? { schedule: definition.schedule } : {}),
+  };
+}
+
+/** The agent package a message's attachments describe: a parsed package,
+ * a rejection naming what's wrong, or `null` when the attachments aren't
+ * a package attempt at all (no package.json or no definition.json).
+ * Untrusted input: parsed, never cast. */
+export function deployablePackage(attachments: readonly MailAttachment[]): PackageOutcome {
+  const manifest = attachments.find((attachment) => attachment.name === PACKAGE_MANIFEST_NAME);
+  if (manifest === undefined) return null;
+  const definition = attachments.find((attachment) => attachment.name === AGENT_DEFINITION_NAME);
+  if (definition === undefined) return null;
+  return parsePackageFiles(manifest.text, definition.text);
 }
 
 /** Strips wrapping backticks/colon and returns the canonical file name the
@@ -143,25 +214,19 @@ function findNamedFencedBlocks(lines: readonly string[]): Map<string, FencedBloc
 /** The agent package a message body carries as fenced code blocks, plus
  * the body with those blocks removed. Used when attachments are absent —
  * `@intx/tools-mail`'s `mail_send` has no attachments parameter, so this is
- * the mailed-package contract's actual delivery path today. */
-export function deployablePackageFromBody(
-  body: string,
-): { readonly pkg: DeployablePackage; readonly strippedBody: string } | null {
+ * the mailed-package contract's actual delivery path today. `null` when
+ * the body isn't a package attempt (neither file is present). */
+export function deployablePackageFromBody(body: string): {
+  readonly outcome: DeployablePackage | { readonly reason: string };
+  readonly strippedBody: string;
+} | null {
   const lines = body.split("\n");
   const blocks = findNamedFencedBlocks(lines);
   const manifest = blocks.get(PACKAGE_MANIFEST_NAME);
   const definition = blocks.get(AGENT_DEFINITION_NAME);
   if (manifest === undefined || definition === undefined) return null;
-  let parsedBody: unknown;
-  try {
-    parsedBody = JSON.parse(definition.content);
-  } catch (cause) {
-    reportError(cause, { operation: "chat_deployable_package_body_parse" });
-    return null;
-  }
-  const parsed = AgentDefinition(parsedBody);
-  if (parsed instanceof type.errors) return null;
-  if (parsed.name.trim() === "" || parsed.systemPrompt.trim() === "") return null;
+
+  const outcome = parsePackageFiles(manifest.content, definition.content);
 
   const ranges = [manifest, definition].sort((a, b) => a.start - b.start);
   const kept: string[] = [];
@@ -175,20 +240,20 @@ export function deployablePackageFromBody(
     .join("\n")
     .replace(/\n{3,}/g, "\n\n")
     .trim();
-  return { pkg: parsed, strippedBody };
+  return { outcome, strippedBody };
 }
 
-/** The package a message describes and the body to render for it —
+/** The package outcome a message describes and the body to render for it —
  * attachments win when present (the intended path once `mail_send` grows
  * attachments support), otherwise the body's fenced blocks are read and
  * stripped from the rendered text. */
 export function resolveMessagePackage(
   attachments: readonly MailAttachment[],
   body: string,
-): { readonly pkg: DeployablePackage | null; readonly renderedBody: string } {
+): { readonly pkg: PackageOutcome; readonly renderedBody: string } {
   const fromAttachments = deployablePackage(attachments);
   if (fromAttachments !== null) return { pkg: fromAttachments, renderedBody: body };
   const fromBody = deployablePackageFromBody(body);
-  if (fromBody !== null) return { pkg: fromBody.pkg, renderedBody: fromBody.strippedBody };
+  if (fromBody !== null) return { pkg: fromBody.outcome, renderedBody: fromBody.strippedBody };
   return { pkg: null, renderedBody: body };
 }
diff --git a/apps/web/src/chat/message-attachments.tsx b/apps/web/src/chat/message-attachments.tsx
index 29ec6c3e0..0dfcf903a 100644
--- a/apps/web/src/chat/message-attachments.tsx
+++ b/apps/web/src/chat/message-attachments.tsx
@@ -5,10 +5,17 @@
 import { Button } from "@corbits/react-ui";
 import { cronSentence } from "@corbits/workflows/client";
 
+import { WarningCircle } from "@/lib/icons";
+
 import { useDeployAgentMutation } from "../agents-api";
 import { chatPath } from "../chat-path";
 import { Link } from "../navigation";
-import { isFiveFieldCron, type DeployablePackage } from "./deployable-package";
+import {
+  isFiveFieldCron,
+  isPackageRejection,
+  type DeployablePackage,
+  type PackageOutcome,
+} from "./deployable-package";
 import type { MailAttachment } from "./threads-api";
 
 function errorText(cause: unknown): string {
@@ -24,9 +31,10 @@ export function MessageAttachments({
   readonly attachments: readonly MailAttachment[];
   /** Resolved by the caller via `resolveMessagePackage` — attachments and
    * body-carried fenced blocks both land here. */
-  readonly pkg: DeployablePackage | null;
+  readonly pkg: PackageOutcome;
 }) {
   if (pkg === null && attachments.length === 0) return null;
+  if (isPackageRejection(pkg)) return <PackageRejectionCard reason={pkg.reason} />;
   if (pkg === null) {
     return (
       <ul className="chat-attachment-list" aria-label="Attachments">
@@ -42,6 +50,22 @@ export function MessageAttachments({
   return <DeployPackageCard tenantId={tenantId} pkg={pkg} />;
 }
 
+/** Same visual family as the Deploy card, but for a package attempt that
+ * failed to parse — names the reason, offers no button. */
+function PackageRejectionCard({ reason }: { readonly reason: string }) {
+  return (
+    <div className="chat-deploy-card chat-package-rejection-card">
+      <div className="chat-deploy-card-text">
+        <span className="chat-deploy-card-name">
+          <WarningCircle className="chat-package-rejection-icon" aria-hidden="true" />
+          Package couldn&apos;t deploy
+        </span>
+        <span className="chat-deploy-card-error">{reason}</span>
+      </div>
+    </div>
+  );
+}
+
 function DeployPackageCard({
   tenantId,
   pkg,