diff --git a/packages/client/src/promise/generated/types.ts b/packages/client/src/promise/generated/types.ts
index 4d51de939f7..ce99934284b 100644
--- a/packages/client/src/promise/generated/types.ts
+++ b/packages/client/src/promise/generated/types.ts
@@ -2130,7 +2130,7 @@ export type ConfigEntry =
                 settings?: ConfigModelSettings
                 headers?: { [x: string]: string }
                 body?: { [x: string]: JsonValue }
-                capabilities?: ModelCapabilities
+                capabilities?: { tools?: boolean; input?: Array<string>; output?: Array<string> }
                 variants?: Array<{
                   id: string
                   settings?: ConfigModelSettings
diff --git a/packages/core/src/config/plugin/provider.ts b/packages/core/src/config/plugin/provider.ts
index 41a7f80d4cd..a3f6265b943 100644
--- a/packages/core/src/config/plugin/provider.ts
+++ b/packages/core/src/config/plugin/provider.ts
@@ -117,13 +117,8 @@ export const Plugin = define({
             if (config.settings !== undefined) model.settings = Provider.mergeOverlay(model.settings, config.settings)
             if (config.headers !== undefined) model.headers = Provider.mergeHeaders(model.headers, config.headers)
             if (config.body !== undefined) model.body = Provider.mergeOverlay(model.body, config.body)
-            if (config.capabilities !== undefined) {
-              model.capabilities = {
-                tools: config.capabilities.tools,
-                input: [...config.capabilities.input],
-                output: [...config.capabilities.output],
-              }
-            }
+            if (config.capabilities !== undefined)
+              model.capabilities = Model.mergeCapabilities(config.capabilities, model.capabilities)
             if (config.variants !== undefined) {
               model.variants ??= []
               for (const variant of config.variants) {
diff --git a/packages/core/src/model.ts b/packages/core/src/model.ts
index 53563d1cc5d..a440578a366 100644
--- a/packages/core/src/model.ts
+++ b/packages/core/src/model.ts
@@ -28,6 +28,16 @@ export type Compatibility = Model.Compatibility
 export const Capabilities = Model.Capabilities
 export type Capabilities = Model.Capabilities
 
+/** Merges partial config capabilities onto a base model's capabilities, defaulting unset fields. */
+export const mergeCapabilities = (config: Partial<Capabilities>, base: Capabilities | undefined) => {
+  const fallback = base ?? Capabilities.default()
+  return {
+    tools: config.tools ?? fallback.tools,
+    input: [...(config.input ?? fallback.input)],
+    output: [...(config.output ?? fallback.output)],
+  }
+}
+
 export const Cost = Model.Cost
 
 export const Ref = Model.Ref
diff --git a/packages/core/src/plugin/provider/opencode.ts b/packages/core/src/plugin/provider/opencode.ts
index 3621683d257..2f65f8bff4f 100644
--- a/packages/core/src/plugin/provider/opencode.ts
+++ b/packages/core/src/plugin/provider/opencode.ts
@@ -9,6 +9,7 @@ import { Credential } from "../../credential.js"
 import { Integration } from "../../integration.js"
 import { IntegrationConnection } from "../../integration/connection.js"
 import { ManagedPolicy } from "../../managed-policy.js"
+import { Model } from "../../model.js"
 import { Provider } from "../../provider.js"
 import { WebSearch } from "../../websearch.js"
 import { ConfigPolicy } from "@opencode/schema/config/policy"
@@ -223,11 +224,7 @@ export const OpencodePlugin = define<HttpClient.HttpClient | Bus.Service | Manag
             model.package = config.package ?? (item.package !== undefined ? undefined : model.package)
             if (item.settings?.baseURL !== undefined && model.settings) delete model.settings.baseURL
             if (config.capabilities !== undefined)
-              model.capabilities = {
-                ...config.capabilities,
-                input: [...config.capabilities.input],
-                output: [...config.capabilities.output],
-              }
+              model.capabilities = Model.mergeCapabilities(config.capabilities, model.capabilities)
             model.settings = Provider.mergeOverlay(
               withoutCredentials(model.settings),
               withoutCredentials(config.settings),
diff --git a/packages/core/test/config/normalization.test.ts b/packages/core/test/config/normalization.test.ts
index 28702520f24..d8ee670b7cb 100644
--- a/packages/core/test/config/normalization.test.ts
+++ b/packages/core/test/config/normalization.test.ts
@@ -203,6 +203,18 @@ describe("ConfigNormalize", () => {
     ])
   })
 
+  test("accepts partial model capabilities without reporting a diagnostic", () => {
+    const result = normalized({
+      providers: {
+        demo: { name: "Demo", models: { demo: { name: "Demo", capabilities: { input: ["text"], output: ["text"] } } } },
+      },
+    })
+    expect(result.diagnostics).toEqual([])
+    expect(result.encoded.providers).toEqual({
+      demo: { name: "Demo", models: { demo: { name: "Demo", capabilities: { input: ["text"], output: ["text"] } } } },
+    })
+  })
+
   test("uses a valid retired provider alias when the canonical legacy entry is malformed", () => {
     const result = normalized({
       provider: {
diff --git a/packages/core/test/config/provider.test.ts b/packages/core/test/config/provider.test.ts
index f660bb14ba6..462fd755d76 100644
--- a/packages/core/test/config/provider.test.ts
+++ b/packages/core/test/config/provider.test.ts
@@ -255,6 +255,70 @@ describe("ConfigProviderPlugin.Plugin", () => {
     }),
   )
 
+  it.effect("merges partial config capabilities onto the base model's existing fields", () =>
+    Effect.gen(function* () {
+      const providers = yield* Provider.Service
+      const models = yield* Model.Service
+      const providerID = Provider.ID.make("custom")
+      const modelID = Model.ID.make("partial")
+      yield* providers.transform((editor) => {
+        editor.models.update(providerID, modelID, (model) => {
+          model.capabilities = { tools: false, input: ["text"], output: ["text"] }
+        })
+      })
+      const entries = [
+        new Document({
+          type: "document",
+          info: decode({
+            providers: {
+              custom: {
+                package: "aisdk:@ai-sdk/openai-compatible",
+                models: { partial: { capabilities: { input: ["text", "image"] } } },
+              },
+            },
+          }),
+        }),
+      ]
+
+      yield* addPlugin(entries)
+
+      expect((yield* models.get(providerID, modelID))?.capabilities).toEqual({
+        tools: false,
+        input: ["text", "image"],
+        output: ["text"],
+      })
+    }),
+  )
+
+  it.effect("resolves a config model with only input/output capabilities using default tools", () =>
+    Effect.gen(function* () {
+      const models = yield* Model.Service
+      const providerID = Provider.ID.make("custom")
+      const modelID = Model.ID.make("demo")
+      const entries = [
+        new Document({
+          type: "document",
+          info: decode({
+            providers: {
+              custom: {
+                package: "aisdk:@ai-sdk/openai-compatible",
+                models: { demo: { name: "Demo", capabilities: { input: ["text"], output: ["text"] } } },
+              },
+            },
+          }),
+        }),
+      ]
+
+      yield* addPlugin(entries)
+
+      expect((yield* models.get(providerID, modelID))?.capabilities).toEqual({
+        tools: true,
+        input: ["text"],
+        output: ["text"],
+      })
+    }),
+  )
+
   for (const scenario of [
     { name: "omitted capabilities", legacy: {}, overrides: {} },
     {
diff --git a/packages/schema/src/config/provider.ts b/packages/schema/src/config/provider.ts
index 1ae813582f5..166a4377b5c 100644
--- a/packages/schema/src/config/provider.ts
+++ b/packages/schema/src/config/provider.ts
@@ -2,7 +2,7 @@ export * as ConfigProvider from "./provider.js"
 
 import { Schema } from "effect"
 import { Money } from "../money.js"
-import { Capabilities, Compatibility, Family, ID, VariantID } from "../model.js"
+import { Compatibility, Family, ID, VariantID } from "../model.js"
 import { Provider } from "../provider.js"
 import { optional } from "../schema.js"
 
@@ -59,6 +59,13 @@ class Cost extends Schema.Class<Cost>("Config.Model.Cost")({
   cache: Cache.pipe(optional),
 }) {}
 
+// Partial: unset fields fall back to the existing/base model's capabilities or Model.Capabilities.default().
+class ModelCapabilities extends Schema.Class<ModelCapabilities>("Config.Model.Capabilities")({
+  tools: Schema.Boolean.pipe(optional),
+  input: Schema.Array(Schema.String).pipe(optional),
+  output: Schema.Array(Schema.String).pipe(optional),
+}) {}
+
 class Limit extends Schema.Class<Limit>("Config.Model.Limit")({
   context: Schema.Int.pipe(optional),
   input: Schema.Int.pipe(optional),
@@ -72,7 +79,7 @@ class Model extends Schema.Class<Model>("Config.Model")({
   compatibility: Compatibility.pipe(optional),
   package: Schema.String.pipe(optional),
   ...ModelOverlays,
-  capabilities: Capabilities.pipe(optional),
+  capabilities: ModelCapabilities.pipe(optional),
   variants: Schema.Struct({
     id: VariantID,
     ...ModelOverlays,