diff --git a/apps/web/test/agents-page.test.tsx b/apps/web/test/agents-page.test.tsx
index a26156df5..f313024f0 100644
--- a/apps/web/test/agents-page.test.tsx
+++ b/apps/web/test/agents-page.test.tsx
@@ -144,6 +144,43 @@ describe("agentModelSettledContent (CL-6848)", () => {
     ).toEqual({ kind: "model", label: "Default" });
   });
 
+  test("an unset model stays Default when the catalog winner changes (CL-6782)", () => {
+    const unset = {
+      status: "ready" as const,
+      data: { name: "triage-bot" },
+    };
+    expect(
+      agentModelSettledContent(unset, [
+        { canonicalName: "claude-sonnet-4", displayName: "Claude Sonnet 4" },
+      ]),
+    ).toEqual({ kind: "model", label: "Default" });
+    expect(
+      agentModelSettledContent(unset, [
+        { canonicalName: "claude-haiku-5", displayName: "Claude Haiku 5" },
+        { canonicalName: "claude-sonnet-4", displayName: "Claude Sonnet 4" },
+      ]),
+    ).toEqual({ kind: "model", label: "Default" });
+  });
+
+  test("a stored model is unchanged when the catalog default becomes a different winner (CL-6782)", () => {
+    const stored = {
+      status: "ready" as const,
+      data: { name: "triage-bot", model: "claude-sonnet-4" },
+    };
+    expect(
+      agentModelSettledContent(stored, [
+        { canonicalName: "claude-sonnet-4", displayName: "Claude Sonnet 4" },
+        { canonicalName: "claude-haiku-5", displayName: "Claude Haiku 5" },
+      ]),
+    ).toEqual({ kind: "model", label: "Claude Sonnet 4" });
+    expect(
+      agentModelSettledContent(stored, [
+        { canonicalName: "claude-haiku-5", displayName: "Claude Haiku 5" },
+        { canonicalName: "claude-sonnet-4", displayName: "Claude Sonnet 4" },
+      ]),
+    ).toEqual({ kind: "model", label: "Claude Sonnet 4" });
+  });
+
   test("a ready model maps to the catalog displayName, not the raw id", () => {
     expect(
       agentModelSettledContent(
diff --git a/packages/settings-ui/src/connections-section.tsx b/packages/settings-ui/src/connections-section.tsx
index efe799633..9dd06bc0f 100644
--- a/packages/settings-ui/src/connections-section.tsx
+++ b/packages/settings-ui/src/connections-section.tsx
@@ -465,6 +465,10 @@ function ModelRoutePanel({
   }
 
   function chooseModel(canonicalName: string) {
+    // Tenant default is an offering-priority write only (CL-6782). Existing
+    // agents keep whatever `capabilities.model` they already store; new
+    // agents may still pick up the default at create. Never PATCH
+    // `/agent-definitions`.
     const target = ordered.find((row) => row.canonicalName === canonicalName);
     applyPatches(
       target === undefined
@@ -485,8 +489,8 @@ function ModelRoutePanel({
         <div>
           <h3 id="model-route-title">Default model & fallbacks</h3>
           <p>
-            Used by Myra and new workbenches unless an agent chooses its own
-            model.
+            Used by Myra and new agents. Changing the default does not rewrite
+            models already stored on existing agents.
           </p>
         </div>
       </div>
diff --git a/packages/settings-ui/src/strings.ts b/packages/settings-ui/src/strings.ts
index a1ecf43a0..2929c8e80 100644
--- a/packages/settings-ui/src/strings.ts
+++ b/packages/settings-ui/src/strings.ts
@@ -193,7 +193,7 @@ export const SETTINGS_STRINGS = {
 
   connectionsSectionTitle: "AI providers",
   connectionsSectionDescription:
-    "Choose the model route Myra and new workbenches use, then connect the providers behind it.",
+    "Choose the model route Myra and new agents use, then connect the providers behind it. Changing the default does not rewrite existing agents.",
   connectionsLoadError: "connections",
   connectionsStatusNotConnected: "Not connected",
   connectionsStatusConnected: "Connected",
diff --git a/packages/settings-ui/test/connections-default-model-pick.test.tsx b/packages/settings-ui/test/connections-default-model-pick.test.tsx
index 7a0cc3cc2..8424e232f 100644
--- a/packages/settings-ui/test/connections-default-model-pick.test.tsx
+++ b/packages/settings-ui/test/connections-default-model-pick.test.tsx
@@ -202,4 +202,78 @@ describe("Global model route", () => {
       container.remove();
     }
   });
+
+  test("choosing a default model PATCHes offerings only, never agent capabilities (CL-6782)", async () => {
+    const calls: { url: string; method: string }[] = [];
+    let patched = false;
+
+    globalThis.fetch = ((url: string, init?: RequestInit) => {
+      calls.push({
+        url,
+        method: init?.method ?? "GET",
+      });
+      if (url === "/api/tenants/ten_1/credentials") {
+        return Promise.resolve(
+          json({ data: [ANTHROPIC_CREDENTIAL], nextCursor: null }),
+        );
+      }
+      if (url === "/api/tenants/ten_1/providers") {
+        return Promise.resolve(
+          json({ data: [ANTHROPIC_PROVIDER], nextCursor: null }),
+        );
+      }
+      if (url === "/api/tenants/ten_1/connections/oauth-configured") {
+        return Promise.resolve(json({}));
+      }
+      if (url === "/api/tenants/ten_1/models") {
+        return Promise.resolve(
+          json(resolvedModels(patched ? 0 : 0, patched ? -1 : 1)),
+        );
+      }
+      if (url === "/api/tenants/ten_1/catalog/offerings") {
+        return Promise.resolve(json({ data: OWN_OFFERINGS, nextCursor: null }));
+      }
+      if (url === "/api/tenants/ten_1/catalog/offerings/offering_haiku") {
+        patched = true;
+        return Promise.resolve(json({ ...OWN_OFFERINGS[1], priority: -1 }));
+      }
+      throw new Error(`unexpected fetch: ${url}`);
+    }) as unknown as typeof fetch;
+
+    const { container, root } = renderSection();
+    try {
+      await settle();
+
+      expect(container.textContent).toContain(
+        "does not rewrite models already stored on existing agents",
+      );
+
+      const defaultModel = container.querySelector(
+        'select[aria-label="Default model"]',
+      ) as HTMLSelectElement | null;
+      act(() => {
+        if (defaultModel !== null) {
+          defaultModel.value = "claude-haiku-5";
+          defaultModel.dispatchEvent(new Event("change", { bubbles: true }));
+        }
+      });
+      await settle();
+
+      const writes = calls.filter(
+        (call) => call.method !== "GET" && call.method !== "HEAD",
+      );
+      expect(writes.length).toBeGreaterThan(0);
+      expect(
+        writes.every((call) =>
+          call.url.startsWith("/api/tenants/ten_1/catalog/offerings/"),
+        ),
+      ).toBe(true);
+      expect(
+        calls.some((call) => call.url.includes("/agent-definitions")),
+      ).toBe(false);
+    } finally {
+      act(() => root.unmount());
+      container.remove();
+    }
+  });
 });