diff --git a/src/agents/provider-capabilities.test.ts b/src/agents/provider-capabilities.test.ts
index e6dba7c11c4..7c364dc710d 100644
--- a/src/agents/provider-capabilities.test.ts
+++ b/src/agents/provider-capabilities.test.ts
@@ -118,6 +118,26 @@ describe("resolveProviderCapabilities", () => {
     });
   });
 
+  it("preserves built-in fallback capability hints when plugin overrides are partial", () => {
+    resolveProviderCapabilitiesWithPluginMock.mockImplementationOnce(() => ({
+      providerFamily: "anthropic",
+    }));
+
+    expect(resolveProviderCapabilities("anthropic")).toEqual({
+      anthropicToolSchemaMode: "native",
+      anthropicToolChoiceMode: "native",
+      openAiPayloadNormalizationMode: "default",
+      providerFamily: "anthropic",
+      preserveAnthropicThinkingSignatures: true,
+      openAiCompatTurnValidation: true,
+      geminiThoughtSignatureSanitization: false,
+      transcriptToolCallIdMode: "default",
+      transcriptToolCallIdModelHints: [],
+      geminiThoughtSignatureModelHints: [],
+      dropThinkingBlockModelHints: ["claude"],
+    });
+  });
+
   it("normalizes kimi aliases to the same capability set", () => {
     expect(resolveProviderCapabilities("kimi")).toEqual(resolveProviderCapabilities("kimi-code"));
     expect(resolveProviderCapabilities("kimi-code")).toEqual({
diff --git a/src/agents/provider-capabilities.ts b/src/agents/provider-capabilities.ts
index c3424956ecf..8e94dd200aa 100644
--- a/src/agents/provider-capabilities.ts
+++ b/src/agents/provider-capabilities.ts
@@ -119,7 +119,8 @@ export function resolveProviderCapabilities(
   return {
     ...DEFAULT_PROVIDER_CAPABILITIES,
     ...CORE_PROVIDER_CAPABILITIES[normalized],
-    ...(pluginCapabilities ?? PLUGIN_CAPABILITIES_FALLBACKS[normalized]),
+    ...PLUGIN_CAPABILITIES_FALLBACKS[normalized],
+    ...pluginCapabilities,
   };
 }