diff --git a/desktop/src/features/messages/ui/useMentionSendFlow.helpers.test.mjs b/desktop/src/features/messages/ui/useMentionSendFlow.helpers.test.mjs
new file mode 100644
index 00000000000..a783240e83c
--- /dev/null
+++ b/desktop/src/features/messages/ui/useMentionSendFlow.helpers.test.mjs
@@ -0,0 +1,90 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { describeAgentReadinessFailures } from "./useMentionSendFlow.helpers.ts";
+
+test("describeAgentReadinessFailures: no failures blocks nothing and warns about nothing", () => {
+  assert.deepEqual(describeAgentReadinessFailures([]), {
+    blocking: null,
+    warning: null,
+  });
+});
+
+test("describeAgentReadinessFailures: a launch failure for a channel member only warns", () => {
+  // #5099: the agent runs on the user's own server, so this desktop holds no
+  // key for it and the launch can never succeed. It is in the channel and
+  // answers there, so the mention must still go out.
+  const result = describeAgentReadinessFailures([
+    { blocking: false, message: "Backend · Claude: agent has no private key" },
+  ]);
+
+  assert.equal(result.blocking, null);
+  assert.equal(
+    result.warning,
+    "Could not start the mentioned agent; sending the mention anyway: Backend · Claude: agent has no private key",
+  );
+});
+
+test("describeAgentReadinessFailures: the warning never claims the message was sent", () => {
+  // The notice is emitted before Huddle sync, media upload and the send, each
+  // of which can still abort. Past-tense copy here would be a false delivery
+  // confirmation on an error path.
+  for (const count of [1, 2]) {
+    const { warning } = describeAgentReadinessFailures(
+      Array.from({ length: count }, (_, index) => ({
+        blocking: false,
+        message: `Agent ${index}: no private key`,
+      })),
+    );
+
+    assert.match(warning, /sending the mention anyway/);
+    assert.doesNotMatch(warning, /\bsent\b/i);
+    assert.doesNotMatch(warning, /\bdelivered\b/i);
+  }
+});
+
+test("describeAgentReadinessFailures: a blocking failure keeps stopping the send", () => {
+  const result = describeAgentReadinessFailures([
+    { blocking: true, message: "Fizz: Mock agent startup failed." },
+  ]);
+
+  assert.equal(
+    result.blocking,
+    "Could not start agent mention: Fizz: Mock agent startup failed.",
+  );
+  assert.equal(result.warning, null);
+});
+
+test("describeAgentReadinessFailures: a blocking failure wins over a warning", () => {
+  const result = describeAgentReadinessFailures([
+    { blocking: false, message: "Remote: no private key" },
+    { blocking: true, message: "Fizz: startup failed" },
+  ]);
+
+  assert.equal(
+    result.blocking,
+    "Could not start agent mention: Fizz: startup failed",
+  );
+  assert.equal(
+    result.warning,
+    "Could not start the mentioned agent; sending the mention anyway: Remote: no private key",
+  );
+});
+
+test("describeAgentReadinessFailures: several failures of one kind are joined and pluralised", () => {
+  const result = describeAgentReadinessFailures([
+    { blocking: true, message: "A: one" },
+    { blocking: true, message: "B: two" },
+    { blocking: false, message: "C: three" },
+    { blocking: false, message: "D: four" },
+  ]);
+
+  assert.equal(
+    result.blocking,
+    "Could not start agent mentions: A: one; B: two",
+  );
+  assert.equal(
+    result.warning,
+    "Could not start the mentioned agents; sending the mention anyway: C: three; D: four",
+  );
+});
diff --git a/desktop/src/features/messages/ui/useMentionSendFlow.helpers.ts b/desktop/src/features/messages/ui/useMentionSendFlow.helpers.ts
index b2eb3893b7f..7f50694edba 100644
--- a/desktop/src/features/messages/ui/useMentionSendFlow.helpers.ts
+++ b/desktop/src/features/messages/ui/useMentionSendFlow.helpers.ts
@@ -75,3 +75,58 @@ export function isManagedAgentRunning(agent: ManagedAgent) {
 export function isProviderBackedAgent(agent: ManagedAgent) {
   return agent.backend.type === "provider";
 }
+
+/**
+ * A mentioned agent that could not be prepared for the send.
+ *
+ * `blocking` is false only when the agent is already a member of the channel
+ * the message is going to. Launching it on this desktop is then an
+ * optimization: an agent that runs elsewhere — a container on the user's own
+ * server — has no private key on this machine and never will, so its launch
+ * fails every time, yet it is in the channel and still answers. Refusing to
+ * publish loses a valid mention and fixes nothing.
+ *
+ * Everything else blocks, because nothing else would put the agent in the
+ * channel: a failed attach leaves it outside, and a failed launch for an agent
+ * that is only being prepared for a channel this send creates or expands
+ * leaves that channel without the participant it was expanded for.
+ */
+export type AgentReadinessFailure = {
+  blocking: boolean;
+  message: string;
+};
+
+/**
+ * Build the send-blocking message and the send-anyway notice, or null.
+ *
+ * The non-blocking copy is deliberately pre-send tense. It is shown before
+ * Huddle sync, media upload and the send itself, any of which can still fail
+ * and abort, so it must not claim the message was delivered.
+ */
+export function describeAgentReadinessFailures(
+  failures: readonly AgentReadinessFailure[],
+) {
+  const describe = (
+    singular: string,
+    plural: string,
+    selected: readonly AgentReadinessFailure[],
+  ) =>
+    selected.length === 0
+      ? null
+      : `${selected.length === 1 ? singular : plural}: ${selected
+          .map((failure) => failure.message)
+          .join("; ")}`;
+
+  return {
+    blocking: describe(
+      "Could not start agent mention",
+      "Could not start agent mentions",
+      failures.filter((failure) => failure.blocking),
+    ),
+    warning: describe(
+      "Could not start the mentioned agent; sending the mention anyway",
+      "Could not start the mentioned agents; sending the mention anyway",
+      failures.filter((failure) => !failure.blocking),
+    ),
+  };
+}
diff --git a/desktop/src/features/messages/ui/useMentionSendFlow.ts b/desktop/src/features/messages/ui/useMentionSendFlow.ts
index 369974d4122..e33b5fef25e 100644
--- a/desktop/src/features/messages/ui/useMentionSendFlow.ts
+++ b/desktop/src/features/messages/ui/useMentionSendFlow.ts
@@ -38,6 +38,8 @@ import type { AcpRuntime, ChannelType, ManagedAgent } from "@/shared/api/types";
 import { normalizePubkey, truncatePubkey } from "@/shared/lib/pubkey";
 import { buildCustomEmojiTags } from "@/shared/lib/customEmojiTags";
 import {
+  type AgentReadinessFailure,
+  describeAgentReadinessFailures,
   getErrorMessage,
   isManagedAgentRunning,
   isProviderBackedAgent,
@@ -183,7 +185,7 @@ export function useMentionSendFlow({
     ) => {
       if (!capturedChannelId || mentionPubkeys.length === 0) {
         return {
-          errors: [] as string[],
+          errors: [] as AgentReadinessFailure[],
           pubkeys: [] as string[],
         };
       }
@@ -195,13 +197,14 @@ export function useMentionSendFlow({
         ...mentions.memberPubkeys,
         ...preparedParticipantPubkeys.map(normalizePubkey),
       ]);
-      const errors: string[] = [];
+      const errors: AgentReadinessFailure[] = [];
       const pubkeys: string[] = [];
       for (const pubkey of uniqueNormalizedPubkeys(mentionPubkeys)) {
         const agent = managedAgentsByPubkey.get(pubkey);
         if (!agent) {
           continue;
         }
+        const isExistingMember = mentions.memberPubkeys.has(pubkey);
         try {
           if (participantPubkeys.has(pubkey)) {
             if (isProviderBackedAgent(agent)) {
@@ -220,12 +223,13 @@ export function useMentionSendFlow({
           }
           pubkeys.push(pubkey);
         } catch (error) {
-          errors.push(
-            `${agent.name}: ${getErrorMessage(
+          errors.push({
+            blocking: !isExistingMember,
+            message: `${agent.name}: ${getErrorMessage(
               error,
               "Could not prepare agent.",
             )}`,
-          );
+          });
         }
       }
       return {
@@ -452,17 +456,13 @@ export function useMentionSendFlow({
           persistPreflightDraft();
           return;
         }
-        if (agentReadiness.errors.length > 0) {
-          const message =
-            agentReadiness.errors.length === 1
-              ? `Could not start agent mention: ${agentReadiness.errors[0]}`
-              : `Could not start agent mentions: ${agentReadiness.errors.join(
-                  "; ",
-                )}`;
-          setNonMemberPromptError(message);
-          toast.error(message);
+        const readiness = describeAgentReadinessFailures(agentReadiness.errors);
+        if (readiness.blocking) {
+          setNonMemberPromptError(readiness.blocking);
+          toast.error(readiness.blocking);
           return;
         }
+        if (readiness.warning) toast.warning(readiness.warning);
         if (preparedAgentPubkeys.length > 0 && sendChannelId) {
           try {
             await invokeTauri("sync_agents_to_active_huddle", {
diff --git a/desktop/tests/e2e/mentions.spec.ts b/desktop/tests/e2e/mentions.spec.ts
index 06fa8cc7066..69a6007085b 100644
--- a/desktop/tests/e2e/mentions.spec.ts
+++ b/desktop/tests/e2e/mentions.spec.ts
@@ -1553,6 +1553,104 @@ test("mentioning an in-channel provider managed agent deploys it before sending"
   await expect(mentionChip).toBeVisible();
 });
 
+test("mentioning an in-channel agent still sends when its start fails", async ({
+  page,
+}) => {
+  // #5099: the agent runs on the user's own server, so this desktop holds no
+  // key for it and the launch can never succeed. It is a member of the channel
+  // and answers there, so the mention has to be published anyway.
+  const startError = "agent has no private key available";
+  await installMockBridge(page, {
+    managedAgents: [
+      {
+        pubkey: OUT_OF_CHANNEL_PROVIDER_AGENT_PUBKEY,
+        name: "portal",
+        status: "not_deployed",
+        channelNames: ["general"],
+        backend: {
+          type: "provider",
+          id: "portal",
+          config: { region: "test" },
+        },
+      },
+    ],
+    startManagedAgentErrors: [startError],
+  });
+  await page.goto("/");
+  await page.getByTestId("channel-general").click();
+  await expect(page.getByTestId("chat-title")).toHaveText("general");
+
+  const input = page.getByTestId("message-input");
+  await input.fill("Hey @portal");
+
+  const dropdown = autocomplete(page);
+  await expect(dropdown.getByText("portal")).toBeVisible();
+  await input.press("Enter");
+  await page.keyboard.type(" can you help?");
+  await page.getByTestId("send-message").click();
+
+  await expect(
+    page.getByText(startError, { exact: false }).first(),
+  ).toBeVisible();
+  const mentionChip = page
+    .getByTestId("message-row")
+    .last()
+    .locator("[data-mention].agent-mention-highlight", { hasText: "portal" });
+  await expect(mentionChip).toBeVisible();
+});
+
+test("a start failure never reports delivery when the send itself fails", async ({
+  page,
+}) => {
+  // The start notice is emitted before Huddle sync, media upload and the send,
+  // so it has to stay pre-send tense: a later abort must not leave the user
+  // with a message that claims the mention went out.
+  const startError = "agent has no private key available";
+  const sendError = "Mock send failed.";
+  await installMockBridge(page, {
+    managedAgents: [
+      {
+        pubkey: OUT_OF_CHANNEL_PROVIDER_AGENT_PUBKEY,
+        name: "portal",
+        status: "not_deployed",
+        channelNames: ["general"],
+        backend: {
+          type: "provider",
+          id: "portal",
+          config: { region: "test" },
+        },
+      },
+    ],
+    startManagedAgentErrors: [startError],
+    sendMessageErrors: [sendError],
+  });
+  await page.goto("/");
+  await page.getByTestId("channel-general").click();
+  await expect(page.getByTestId("chat-title")).toHaveText("general");
+
+  const input = page.getByTestId("message-input");
+  await input.fill("Hey @portal");
+
+  const dropdown = autocomplete(page);
+  await expect(dropdown.getByText("portal")).toBeVisible();
+  await input.press("Enter");
+  await page.keyboard.type(" can you help?");
+  await page.getByTestId("send-message").click();
+
+  await expect(
+    page.getByText("sending the mention anyway", { exact: false }).first(),
+  ).toBeVisible();
+  // The send aborted: the composer keeps the draft and nothing reached the
+  // timeline, so the notice above must not have claimed delivery.
+  await expect(input).toContainText("portal");
+  await expect(
+    page
+      .getByTestId("message-row")
+      .locator("[data-mention].agent-mention-highlight", { hasText: "portal" }),
+  ).toHaveCount(0);
+  await expect(page.getByText(/message sent/i)).toHaveCount(0);
+});
+
 test("mentioning a non-member managed agent adds and starts it before sending", async ({
   page,
 }) => {