fix(core): keep a failed web_search backend derivation from breaking the tool registry

Reading the generation config to derive the search backend assumed a complete Config surface, so a test double that stubs getModelsConfig() without getGenerationConfig() threw while the registry was being built and took every other tool down with it. Derivation is opportunistic: an unexpected Config shape should cost web search alone, so catch and fall back to the silent no-backend result. Give the subagent-manager double the accessor it was missing.

packages/core/src/subagents/subagent-manager.test.ts

@@ -2307,6 +2307,7 @@ bad`);
         });
         vi.spyOn(mockConfig, 'getModelsConfig').mockReturnValue({
           getResolvedModel: vi.fn().mockReturnValue(undefined),
+          getGenerationConfig: vi.fn().mockReturnValue({}),
         } as unknown as ReturnType<Config['getModelsConfig']>);
       });
 

packages/core/src/tools/web-search.test.ts

@@ -837,6 +837,25 @@ describe('evaluateWebSearchGate auto derivation', () => {
     }
   });
 
+  it('stays silently off instead of throwing when the config surface is incomplete', () => {
+    // The gate runs while the tool registry is being built, for whatever
+    // Config shape the caller has. A missing accessor must cost web search,
+    // not every other tool in the registry.
+    const broken = {
+      getWebSearchSettings: () => undefined,
+      getModel: () => 'some-model',
+      getCurrentAuthType: () => 'openai',
+      getCurrentModelRegistryBaseUrl: () => undefined,
+      getAllConfiguredModels: () => [],
+      getModelsConfig: () => ({}),
+    } as unknown as Config;
+    const gate = evaluateWebSearchGate(broken);
+    expect(gate.ok).toBe(false);
+    if (!gate.ok) {
+      expect(gate.silent).toBe(true);
+    }
+  });
+
   it('prefers an explicitly configured search model over derivation', () => {
     vi.stubEnv(STANDARD.envKey, 'sk-standard');
     const gate = evaluateWebSearchGate(

packages/core/src/tools/web-search.ts

@@ -405,7 +405,22 @@ export function evaluateWebSearchGate(config: Config): WebSearchGateResult {
     // model's provider. An explicit `enabled: true`, or an env-declared
     // backend missing its model, still names the missing piece instead.
     if (settings?.enabled !== true && !settings?.baseUrl) {
-      return resolveAutoBackend(config, settings);
+      try {
+        return resolveAutoBackend(config, settings);
+      } catch (e) {
+        // Derivation is opportunistic and runs while the tool registry is
+        // being built: an unexpected Config shape must cost the user web
+        // search, not every other tool in the registry.
+        gateDebugLogger.debug(
+          `[WebSearch] auto backend derivation threw: ${e instanceof Error ? e.message : String(e)}`,
+        );
+        return {
+          ok: false,
+          silent: true,
+          notice:
+            'WebSearch is not configured and no backend could be derived automatically.',
+        };
+      }
     }
     return {
       ok: false,