diff --git a/apps/web/components/github/issue-watch-dialog.tsx b/apps/web/components/github/issue-watch-dialog.tsx
index e14c78e4c4..ec2d148ef2 100644
--- a/apps/web/components/github/issue-watch-dialog.tsx
+++ b/apps/web/components/github/issue-watch-dialog.tsx
@@ -31,6 +31,7 @@ import {
   DEFAULT_ISSUE_WATCH_PROMPT,
 } from "@/components/github/issue-watch-placeholders";
 import { RepoFilterSelector } from "@/components/github/repo-filter-selector";
+import { STEP_DEFAULT, STEP_DEFAULT_LABEL, resolveProfileId } from "@/lib/watcher-profile-default";
 import type {
   RepoFilter,
   IssueWatch,
@@ -300,23 +301,29 @@ function IssueAutomationFields({
       <div className="grid grid-cols-2 gap-4">
         <SelectField
           label="Agent Profile"
-          description="The agent configuration for the task."
-          value={form.agentProfileId}
-          onChange={(v) => setForm((prev) => ({ ...prev, agentProfileId: v }))}
-          placeholder="Select agent profile"
-          items={agentProfiles.map((p) => ({
-            id: p.id,
-            label: p.label,
-            icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
-          }))}
+          description="Optional — falls back to step default."
+          value={form.agentProfileId || STEP_DEFAULT}
+          onChange={(v) => setForm((prev) => ({ ...prev, agentProfileId: resolveProfileId(v) }))}
+          placeholder={STEP_DEFAULT_LABEL}
+          items={[
+            { id: STEP_DEFAULT, label: STEP_DEFAULT_LABEL },
+            ...agentProfiles.map((p) => ({
+              id: p.id,
+              label: p.label,
+              icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
+            })),
+          ]}
         />
         <SelectField
           label="Executor Profile"
-          description="The executor environment for the agent."
-          value={form.executorProfileId}
-          onChange={(v) => setForm((prev) => ({ ...prev, executorProfileId: v }))}
-          placeholder="Select executor profile"
-          items={allExecutorProfiles.map((p) => ({ id: p.id, label: p.name }))}
+          description="Optional — falls back to step default."
+          value={form.executorProfileId || STEP_DEFAULT}
+          onChange={(v) => setForm((prev) => ({ ...prev, executorProfileId: resolveProfileId(v) }))}
+          placeholder={STEP_DEFAULT_LABEL}
+          items={[
+            { id: STEP_DEFAULT, label: STEP_DEFAULT_LABEL },
+            ...allExecutorProfiles.map((p) => ({ id: p.id, label: p.name })),
+          ]}
         />
       </div>
       <div className="space-y-1.5">
diff --git a/apps/web/components/github/review-watch-dialog.tsx b/apps/web/components/github/review-watch-dialog.tsx
index e6b5211e14..b78e3a0791 100644
--- a/apps/web/components/github/review-watch-dialog.tsx
+++ b/apps/web/components/github/review-watch-dialog.tsx
@@ -29,6 +29,7 @@ import {
 import { DEFAULT_REVIEW_WATCH_PROMPT } from "@/components/github/review-watch-placeholders";
 import { ReviewWatchPromptField } from "@/components/github/review-watch-prompt-field";
 import { RepoFilterSelector } from "@/components/github/repo-filter-selector";
+import { STEP_DEFAULT, STEP_DEFAULT_LABEL, resolveProfileId } from "@/lib/watcher-profile-default";
 import type {
   RepoFilter,
   ReviewWatch,
@@ -441,15 +442,18 @@ function ProfileFields({
     <div className="grid grid-cols-2 gap-4">
       <SelectField
         label="Agent Profile"
-        description="The agent configuration used to review the PR."
-        value={form.agentProfileId}
-        onChange={(v) => setForm((prev) => ({ ...prev, agentProfileId: v }))}
-        placeholder="Select agent profile"
-        items={agentProfiles.map((p) => ({
-          id: p.id,
-          label: p.label,
-          icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
-        }))}
+        description="Optional — falls back to step default."
+        value={form.agentProfileId || STEP_DEFAULT}
+        onChange={(v) => setForm((prev) => ({ ...prev, agentProfileId: resolveProfileId(v) }))}
+        placeholder={STEP_DEFAULT_LABEL}
+        items={[
+          { id: STEP_DEFAULT, label: STEP_DEFAULT_LABEL },
+          ...agentProfiles.map((p) => ({
+            id: p.id,
+            label: p.label,
+            icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
+          })),
+        ]}
       />
       <div className="space-y-1.5">
         <div className="flex items-center gap-1.5">
@@ -457,16 +461,19 @@ function ProfileFields({
           <HelpTip text="The repository will be automatically cloned to ~/.kandev/repos/<owner>/<repo> if it is not already present in the workspace." />
         </div>
         <p className="text-xs text-muted-foreground">
-          The executor environment where the agent will run.
+          Optional — falls back to step default. The executor environment where the agent will run.
         </p>
         <Select
-          value={form.executorProfileId || undefined}
-          onValueChange={(v) => setForm((prev) => ({ ...prev, executorProfileId: v }))}
+          value={form.executorProfileId || STEP_DEFAULT}
+          onValueChange={(v) =>
+            setForm((prev) => ({ ...prev, executorProfileId: resolveProfileId(v) }))
+          }
         >
           <SelectTrigger className="cursor-pointer">
-            <SelectValue placeholder="Select executor profile" />
+            <SelectValue placeholder={STEP_DEFAULT_LABEL} />
           </SelectTrigger>
           <SelectContent>
+            <SelectItem value={STEP_DEFAULT}>{STEP_DEFAULT_LABEL}</SelectItem>
             {executorProfiles.map((p) => (
               <SelectItem key={p.id} value={p.id}>
                 {p.name}
diff --git a/apps/web/components/jira/jira-issue-watch-dialog.tsx b/apps/web/components/jira/jira-issue-watch-dialog.tsx
index 22f12097be..ed419dca31 100644
--- a/apps/web/components/jira/jira-issue-watch-dialog.tsx
+++ b/apps/web/components/jira/jira-issue-watch-dialog.tsx
@@ -31,6 +31,7 @@ import {
   JIRA_ISSUE_WATCH_PLACEHOLDERS,
   DEFAULT_JIRA_ISSUE_WATCH_PROMPT,
 } from "@/components/jira/jira-issue-watch-placeholders";
+import { STEP_DEFAULT, STEP_DEFAULT_LABEL, resolveProfileId } from "@/lib/watcher-profile-default";
 import type {
   CreateJiraIssueWatchInput,
   JiraIssueWatch,
@@ -326,22 +327,28 @@ function AutomationFields({
         <SelectField
           label="Agent Profile"
           description="Optional — falls back to step default."
-          value={form.agentProfileId}
-          onChange={(v) => setForm((p) => ({ ...p, agentProfileId: v }))}
-          placeholder="(use step default)"
-          items={agentProfiles.map((p) => ({
-            id: p.id,
-            label: p.label,
-            icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
-          }))}
+          value={form.agentProfileId || STEP_DEFAULT}
+          onChange={(v) => setForm((p) => ({ ...p, agentProfileId: resolveProfileId(v) }))}
+          placeholder={STEP_DEFAULT_LABEL}
+          items={[
+            { id: STEP_DEFAULT, label: STEP_DEFAULT_LABEL },
+            ...agentProfiles.map((p) => ({
+              id: p.id,
+              label: p.label,
+              icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
+            })),
+          ]}
         />
         <SelectField
           label="Executor Profile"
           description="Optional — falls back to step default."
-          value={form.executorProfileId}
-          onChange={(v) => setForm((p) => ({ ...p, executorProfileId: v }))}
-          placeholder="(use step default)"
-          items={allExecutorProfiles.map((p) => ({ id: p.id, label: p.name }))}
+          value={form.executorProfileId || STEP_DEFAULT}
+          onChange={(v) => setForm((p) => ({ ...p, executorProfileId: resolveProfileId(v) }))}
+          placeholder={STEP_DEFAULT_LABEL}
+          items={[
+            { id: STEP_DEFAULT, label: STEP_DEFAULT_LABEL },
+            ...allExecutorProfiles.map((p) => ({ id: p.id, label: p.name })),
+          ]}
         />
       </div>
     </>
diff --git a/apps/web/components/linear/linear-issue-watch-dialog.tsx b/apps/web/components/linear/linear-issue-watch-dialog.tsx
index c52ab75a0f..18258fdd8d 100644
--- a/apps/web/components/linear/linear-issue-watch-dialog.tsx
+++ b/apps/web/components/linear/linear-issue-watch-dialog.tsx
@@ -33,6 +33,7 @@ import {
   useTeamsAndStates,
 } from "./linear-issue-watch-fields";
 import { LINEAR_ISSUE_WATCH_PLACEHOLDERS } from "./linear-issue-watch-placeholders";
+import { STEP_DEFAULT, STEP_DEFAULT_LABEL, resolveProfileId } from "@/lib/watcher-profile-default";
 import {
   ASSIGNED_ANY,
   CREATOR_ANY,
@@ -431,22 +432,28 @@ function AutomationFields({
         <SelectField
           label="Agent Profile"
           description="Optional — falls back to step default."
-          value={form.agentProfileId}
-          onChange={(v) => setForm((p) => ({ ...p, agentProfileId: v }))}
-          placeholder="(use step default)"
-          items={agentProfiles.map((p) => ({
-            id: p.id,
-            label: p.label,
-            icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
-          }))}
+          value={form.agentProfileId || STEP_DEFAULT}
+          onChange={(v) => setForm((p) => ({ ...p, agentProfileId: resolveProfileId(v) }))}
+          placeholder={STEP_DEFAULT_LABEL}
+          items={[
+            { id: STEP_DEFAULT, label: STEP_DEFAULT_LABEL },
+            ...agentProfiles.map((p) => ({
+              id: p.id,
+              label: p.label,
+              icon: p.cli_passthrough ? <CliModeIcon /> : undefined,
+            })),
+          ]}
         />
         <SelectField
           label="Executor Profile"
           description="Optional — falls back to step default."
-          value={form.executorProfileId}
-          onChange={(v) => setForm((p) => ({ ...p, executorProfileId: v }))}
-          placeholder="(use step default)"
-          items={allExecutorProfiles.map((p) => ({ id: p.id, label: p.name }))}
+          value={form.executorProfileId || STEP_DEFAULT}
+          onChange={(v) => setForm((p) => ({ ...p, executorProfileId: resolveProfileId(v) }))}
+          placeholder={STEP_DEFAULT_LABEL}
+          items={[
+            { id: STEP_DEFAULT, label: STEP_DEFAULT_LABEL },
+            ...allExecutorProfiles.map((p) => ({ id: p.id, label: p.name })),
+          ]}
         />
       </div>
     </>
diff --git a/apps/web/e2e/tests/integrations/linear-settings.spec.ts b/apps/web/e2e/tests/integrations/linear-settings.spec.ts
index a08beaaa73..904b677766 100644
--- a/apps/web/e2e/tests/integrations/linear-settings.spec.ts
+++ b/apps/web/e2e/tests/integrations/linear-settings.spec.ts
@@ -1,5 +1,6 @@
 import { test, expect } from "../../fixtures/test-base";
 import { LinearSettingsPage } from "../../pages/linear-settings-page";
+import { assertWatcherAgentProfileResetsToStepDefault } from "./watcher-profile-default-flow";
 
 test.describe("Linear settings", () => {
   test("empty workspace shows form with disabled save/test until secret is filled", async ({
@@ -135,4 +136,8 @@ test.describe("Linear settings", () => {
     // the "<agent> • <profile>" label format.
     await expect(testPage.getByRole("option", { name: new RegExp(passthroughName) })).toBeVisible();
   });
+
+  test("watcher dialog resets the agent profile back to the step default", async ({ testPage }) => {
+    await assertWatcherAgentProfileResetsToStepDefault(testPage);
+  });
 });
diff --git a/apps/web/e2e/tests/integrations/mobile-linear-watcher-profile.spec.ts b/apps/web/e2e/tests/integrations/mobile-linear-watcher-profile.spec.ts
new file mode 100644
index 0000000000..c61acdb386
--- /dev/null
+++ b/apps/web/e2e/tests/integrations/mobile-linear-watcher-profile.spec.ts
@@ -0,0 +1,8 @@
+import { test } from "../../fixtures/test-base";
+import { assertWatcherAgentProfileResetsToStepDefault } from "./watcher-profile-default-flow";
+
+test.describe("Linear watcher dialog (mobile)", () => {
+  test("resets the agent profile back to the step default", async ({ testPage }) => {
+    await assertWatcherAgentProfileResetsToStepDefault(testPage);
+  });
+});
diff --git a/apps/web/e2e/tests/integrations/watcher-profile-default-flow.ts b/apps/web/e2e/tests/integrations/watcher-profile-default-flow.ts
new file mode 100644
index 0000000000..ff2ddab3d0
--- /dev/null
+++ b/apps/web/e2e/tests/integrations/watcher-profile-default-flow.ts
@@ -0,0 +1,46 @@
+import { type Page, expect } from "@playwright/test";
+import { LinearSettingsPage } from "../../pages/linear-settings-page";
+
+// Drives the Linear watcher create dialog: picks a real agent profile, then
+// re-selects "(use step default)" and asserts the field clears back. Shared by
+// the desktop (linear-settings.spec.ts) and mobile (mobile-*.spec.ts) suites so
+// both viewports exercise the same sentinel-reset behavior.
+export async function assertWatcherAgentProfileResetsToStepDefault(testPage: Page) {
+  const settings = new LinearSettingsPage(testPage);
+  await settings.goto();
+
+  await testPage.getByRole("button", { name: /new watcher/i }).click();
+  const dialog = testPage.getByRole("dialog");
+  await expect(dialog).toBeVisible();
+
+  // Scope to the Agent Profile field; the label text is unique within the form.
+  const agentTrigger = dialog
+    .locator("div.space-y-1\\.5")
+    .filter({ hasText: "Agent Profile" })
+    .getByRole("combobox");
+
+  // The select defaults to the sentinel option.
+  await expect(agentTrigger).toContainText("(use step default)");
+
+  // Pick the first real profile — option 0 is the "(use step default)" sentinel.
+  // Radix renders the open list in a portal; scope to its listbox so the option
+  // count can't drift across other (closed) selects in the dialog.
+  await agentTrigger.click();
+  const options = testPage.getByRole("listbox").getByRole("option");
+  // Sentinel + at least one real profile. A bare sentinel means the settings
+  // fixture seeded no agent profiles — fail with a clear count, not a vague
+  // "element not found" timeout on nth(1) below.
+  await expect.poll(() => options.count()).toBeGreaterThan(1);
+  const profileOption = options.nth(1);
+  await expect(profileOption).toBeVisible();
+  const profileLabel = ((await profileOption.textContent()) ?? "").trim();
+  expect(profileLabel.length).toBeGreaterThan(0);
+  await profileOption.click();
+  await expect(agentTrigger).toContainText(profileLabel);
+
+  // Re-select "(use step default)" and confirm the field reverts.
+  await agentTrigger.click();
+  await testPage.getByRole("listbox").getByRole("option", { name: "(use step default)" }).click();
+  await expect(agentTrigger).toContainText("(use step default)");
+  await expect(agentTrigger).not.toContainText(profileLabel);
+}
diff --git a/apps/web/lib/watcher-profile-default.test.ts b/apps/web/lib/watcher-profile-default.test.ts
new file mode 100644
index 0000000000..be289a68e1
--- /dev/null
+++ b/apps/web/lib/watcher-profile-default.test.ts
@@ -0,0 +1,17 @@
+import { describe, it, expect } from "vitest";
+import { STEP_DEFAULT, resolveProfileId } from "./watcher-profile-default";
+
+describe("resolveProfileId", () => {
+  it("collapses the sentinel to an empty string so the payload signals 'use step default'", () => {
+    expect(resolveProfileId(STEP_DEFAULT)).toBe("");
+  });
+
+  it("passes a real profile id through unchanged", () => {
+    expect(resolveProfileId("agent-profile-123")).toBe("agent-profile-123");
+    expect(resolveProfileId("exec-profile-abc")).toBe("exec-profile-abc");
+  });
+
+  it("leaves an already-empty value empty (initial/reset state)", () => {
+    expect(resolveProfileId("")).toBe("");
+  });
+});
diff --git a/apps/web/lib/watcher-profile-default.ts b/apps/web/lib/watcher-profile-default.ts
new file mode 100644
index 0000000000..118f1e102a
--- /dev/null
+++ b/apps/web/lib/watcher-profile-default.ts
@@ -0,0 +1,14 @@
+// Shared sentinel for the "(use step default)" option in the Agent/Executor
+// Profile selects across the four watcher dialogs (Linear, Jira, GitHub issue,
+// GitHub review). The form stores "" to mean "fall back to the workflow step's
+// default profile", and the create/update payloads pass "" through. Radix
+// disallows <SelectItem value="">, so the dropdown item carries this sentinel
+// value and maps back to "" on change.
+export const STEP_DEFAULT = "__step_default__";
+export const STEP_DEFAULT_LABEL = "(use step default)";
+
+// Map a select value back to the stored profile id, collapsing the sentinel to
+// "" so the payload keeps signalling "use step default".
+export function resolveProfileId(value: string): string {
+  return value === STEP_DEFAULT ? "" : value;
+}