diff --git a/.changeset/fix-server-tool-type-gaps.md b/.changeset/fix-server-tool-type-gaps.md
new file mode 100644
index 0000000..717fd3c
--- /dev/null
+++ b/.changeset/fix-server-tool-type-gaps.md
@@ -0,0 +1,28 @@
+---
+"@openrouter/agent": patch
+---
+
+Fix two type gaps that forced consumers to use `as any` when wiring up
+`callModel` with server tools and chat-format inputs. Both fixes are
+purely additive at the public-type level — `ServerTool` and
+`ServerTool<T>` continue to work exactly as before; no consumer code
+needs to change.
+
+- **Mixed `Array<ClientTool | ServerTool>` now accepts narrow
+  `serverTool()` results without a cast.** Previously `ServerTool<T>`
+  defined `config: Extract<ServerToolConfig, { type: T }>`, which made
+  it invariant over `T` — so `ServerTool<'openrouter:datetime'>` was
+  not assignable to the bare `ServerTool` (= `ServerTool<ServerToolType>`).
+  `ServerTool` is now a conditional generic with a `never` default that
+  collapses to a non-generic structural base (`ServerToolBase`, also
+  newly exported), and narrow variants are represented as an
+  intersection with that base — so any `serverTool(...)` result flows
+  into `Array<ClientTool | ServerTool>` or `Tool[]` directly.
+  `ServerTool<'openrouter:datetime'>` (narrowing `config` at a call
+  site) still compiles as before.
+
+- **`callModel`'s `request.input` now accepts `InputsUnion`** (the SDK's
+  wider message shape returned by `fromChatMessages()`), alongside the
+  existing `Item[]` and plain `string` forms. The docstring on
+  `fromChatMessages()` already claims its output "can be passed directly
+  to `callModel()`"; the types now match.
diff --git a/packages/agent/src/index.ts b/packages/agent/src/index.ts
index 0bd90cf..f0057e0 100644
--- a/packages/agent/src/index.ts
+++ b/packages/agent/src/index.ts
@@ -151,6 +151,7 @@ export type {
   ResponseStreamEvent,
   ResponseStreamEvent as EnhancedResponseStreamEvent,
   ServerTool,
+  ServerToolBase,
   ServerToolConfig,
   ServerToolResultItem,
   ServerToolType,
diff --git a/packages/agent/src/lib/async-params.ts b/packages/agent/src/lib/async-params.ts
index 8856346..8662fe5 100644
--- a/packages/agent/src/lib/async-params.ts
+++ b/packages/agent/src/lib/async-params.ts
@@ -57,7 +57,18 @@ type BaseCallModelInput<
     models.ResponsesRequest[K]
   >;
 } & {
-  input: FieldOrAsyncFunction<Item[]> | string;
+  /**
+   * The input for the model turn. Accepts either:
+   * - A plain `string` prompt.
+   * - An array of typed `Item[]` (the narrow, local item union).
+   * - The SDK's `InputsUnion` shape (a `string` or broader item array)
+   *   — this is what converters like `fromChatMessages()` return, so
+   *   those results assign directly without a cast.
+   *
+   * When a function is provided, it is resolved once per call with the
+   * current turn context before the request is sent.
+   */
+  input: FieldOrAsyncFunction<Item[]> | FieldOrAsyncFunction<models.InputsUnion>;
   tools?: TTools;
   stopWhen?: StopWhen<TTools>;
   /** Typed context data passed to tools via contextSchema. Includes optional `shared` key. */
diff --git a/packages/agent/src/lib/stream-transformers.ts b/packages/agent/src/lib/stream-transformers.ts
index b35b56b..6b6d300 100644
--- a/packages/agent/src/lib/stream-transformers.ts
+++ b/packages/agent/src/lib/stream-transformers.ts
@@ -30,7 +30,7 @@ import {
   isURLCitationAnnotation,
   isWebSearchCallOutputItem,
 } from './stream-type-guards.js';
-import type { ClientTool, ParsedToolCall, ServerTool, Tool } from './tool-types.js';
+import type { ClientTool, ParsedToolCall, ServerToolBase, Tool } from './tool-types.js';
 
 /**
  * Extract text deltas from responses stream events
@@ -261,13 +261,20 @@ type KnownServerToolOutputs = {
  * map via KnownServerToolOutputs; anything else falls back to the
  * provider-side server-tool output union (`OpenRouterServerToolOutput`)
  * so the SDK's forward-compat variants flow through automatically.
+ *
+ * Inference reads `config.type` directly rather than `ServerTool<infer K>`
+ * so it works whether the source is the narrow intersection form or the
+ * wide `ServerToolBase` base — both expose the same `config.type` field.
  */
-type InferServerToolOutput<S> =
-  S extends ServerTool<infer K>
-    ? K extends keyof KnownServerToolOutputs
-      ? KnownServerToolOutputs[K]
-      : OpenRouterServerToolOutput
-    : never;
+type InferServerToolOutput<S> = S extends {
+  readonly config: {
+    readonly type: infer K;
+  };
+}
+  ? K extends keyof KnownServerToolOutputs
+    ? KnownServerToolOutputs[K]
+    : OpenRouterServerToolOutput
+  : never;
 
 /**
  * Union of output item shapes produced by the server tools present in
@@ -275,7 +282,7 @@ type InferServerToolOutput<S> =
  * to every mapped output plus the generic fallback. Unused otherwise.
  */
 type InferServerToolOutputsUnion<TTools extends readonly Tool[]> = InferServerToolOutput<
-  Extract<TTools[number], ServerTool>
+  Extract<TTools[number], ServerToolBase>
 >;
 
 /**
diff --git a/packages/agent/src/lib/tool-types.ts b/packages/agent/src/lib/tool-types.ts
index f4794a8..7a4475a 100644
--- a/packages/agent/src/lib/tool-types.ts
+++ b/packages/agent/src/lib/tool-types.ts
@@ -343,15 +343,13 @@ export type ServerToolConfig = Exclude<
 export type ServerToolType = ServerToolConfig['type'];
 
 /**
- * Structural base type for every server tool. Interface extension (not a
- * distributive conditional) is used so the narrow-T subtype assigns cleanly
- * into the wide-T supertype via nominal inheritance — TypeScript treats
- * `ServerTool<'web_search_2025_08_26'>` as a subtype of `ServerToolBase`
- * without needing to reason about variance through `Extract<..., {type: T}>`.
+ * Structural base type for every server tool. Non-generic so that `Tool`'s
+ * union member doesn't carry a type parameter — this sidesteps the variance
+ * issue where `ServerTool<'openrouter:datetime'>` would fail to assign into
+ * `ServerTool<ServerToolType>` through `Extract<..., {type: T}>`.
  *
- * `Tool` uses `ServerToolBase` as its union member (rather than a generic
- * `ServerTool` parameterized on a union) so specific `ServerTool<T>` values
- * assign into `Tool[]` directly.
+ * `ServerTool` (bare, via its `never` default) collapses to this type, so
+ * `Array<ClientTool | ServerTool>` accepts any narrow variant.
  */
 export interface ServerToolBase {
   readonly _brand: 'server-tool';
@@ -359,29 +357,38 @@ export interface ServerToolBase {
 }
 
 /**
- * A server-executed tool. OpenRouter runs the tool and returns an output
- * item in the response — no execute function lives on the client. When
- * the type parameter `T` is a specific literal, `config` narrows to the
- * SDK shape for that tool. Because this interface `extends ServerToolBase`,
- * any `ServerTool<T>` value is nominally assignable to `ServerToolBase`
- * (and hence to `Tool`) regardless of `T`.
+ * A server-executed tool. Without a type argument, it is the structural base
+ * (equivalent to `ServerToolBase`) — use it as-is in mixed arrays like
+ * `Array<ClientTool | ServerTool>`. With a `type` literal argument, it
+ * narrows `config` to that specific SDK variant — returned by
+ * `serverTool<T>()` and useful when the concrete config shape matters.
+ *
+ * The `[T] extends [never]` default branch keeps bare `ServerTool` equal to
+ * `ServerToolBase`, so specific narrow variants assign into the bare form
+ * via the intersection being a subtype of `ServerToolBase`.
  *
  * @template T The specific server-tool type literal (narrows `config`).
  */
-export interface ServerTool<T extends ServerToolType = ServerToolType> extends ServerToolBase {
-  readonly config: Extract<
-    ServerToolConfig,
-    {
-      type: T;
-    }
-  >;
-}
+export type ServerTool<T extends ServerToolType = never> = [
+  T,
+] extends [
+  never,
+]
+  ? ServerToolBase
+  : ServerToolBase & {
+      readonly config: Extract<
+        ServerToolConfig,
+        {
+          type: T;
+        }
+      >;
+    };
 
 /**
  * Union of every tool kind accepted by `callModel({ tools: [...] })`:
  * client function/generator/manual tools, or OpenRouter server tools.
  * The server branch is the structural base; specific `ServerTool<T>`
- * values flow in via interface extension.
+ * values flow in via the intersection being a subtype of `ServerToolBase`.
  */
 export type Tool = ClientTool | ServerToolBase;
 
diff --git a/packages/agent/tests/unit/consumer-type-ergonomics.test-d.ts b/packages/agent/tests/unit/consumer-type-ergonomics.test-d.ts
new file mode 100644
index 0000000..d6c9e80
--- /dev/null
+++ b/packages/agent/tests/unit/consumer-type-ergonomics.test-d.ts
@@ -0,0 +1,98 @@
+/**
+ * Regression coverage for two consumer-facing type gaps reported against
+ * v0.4.0 that previously required `as any`:
+ *
+ * 1. Mixing `tool()` + `serverTool()` results in a single array typed as
+ *    `Array<ClientTool | ServerTool>` must assign to `callModel`'s `tools`
+ *    parameter without a cast. `serverTool<T>()` returns the narrow
+ *    `ServerTool<T>` (a `ServerToolBase` intersection), which must flow
+ *    into the bare `ServerTool` — bare `ServerTool` collapses to
+ *    `ServerToolBase` via its `never` default.
+ *
+ * 2. `fromChatMessages()` returns the SDK's `InputsUnion`, which must be
+ *    directly assignable to `callModel`'s `request.input` without a cast.
+ *    Previously the input was typed as `Item[] | string`, which is a
+ *    narrower union that `InputsUnion` does not extend.
+ */
+
+import type * as models from '@openrouter/sdk/models';
+import { expectTypeOf } from 'vitest';
+import { z } from 'zod/v4';
+import type { CallModelInput } from '../../src/lib/async-params.js';
+import { fromChatMessages } from '../../src/lib/chat-compat.js';
+import { serverTool, tool } from '../../src/lib/tool.js';
+import type { ClientTool, ServerTool, ServerToolBase, Tool } from '../../src/lib/tool-types.js';
+
+// --- Issue 1: mixed arrays assign without `as any` --------------------------
+
+// Specific narrow factory return types must flow to the bare `ServerTool`
+// form (which collapses to `ServerToolBase`) via intersection subtyping.
+expectTypeOf<ServerTool<'openrouter:datetime'>>().toExtend<ServerTool>();
+expectTypeOf<ServerTool<'openrouter:datetime'>>().toExtend<ServerToolBase>();
+expectTypeOf<ServerTool<'openrouter:datetime'>>().toExtend<Tool>();
+
+// ServerTool (bare, no generic) is the structural base — it should accept
+// any narrow variant assigned to it.
+const _dt: ServerTool = serverTool({
+  type: 'openrouter:datetime',
+});
+const _ws: ServerTool = serverTool({
+  type: 'openrouter:web_search',
+});
+void _dt;
+void _ws;
+
+// Array<ClientTool | ServerTool> accepts a mix without cast.
+const _mixed: Array<ClientTool | ServerTool> = [
+  tool({
+    name: 'save_note',
+    inputSchema: z.object({
+      title: z.string(),
+    }),
+    execute: async () => ({
+      ok: true,
+    }),
+  }),
+  serverTool({
+    type: 'openrouter:datetime',
+  }),
+  serverTool({
+    type: 'openrouter:web_search',
+  }),
+];
+void _mixed;
+
+// Tool[] accepts the same mix.
+const _asTool: Tool[] = [
+  tool({
+    name: 'save_note',
+    inputSchema: z.object({
+      title: z.string(),
+    }),
+    execute: async () => ({
+      ok: true,
+    }),
+  }),
+  serverTool({
+    type: 'openrouter:datetime',
+  }),
+];
+void _asTool;
+
+// --- Issue 2: fromChatMessages() output is assignable to input -------------
+
+// A `CallModelInput`'s `input` field accepts `InputsUnion` directly. We use
+// `Extract` instead of `toExtend` because `input` is a field-or-fn union; we
+// just need the plain data variant to accept `InputsUnion`.
+type _InputField = CallModelInput['input'];
+expectTypeOf<models.InputsUnion>().toExtend<_InputField>();
+
+// And the concrete return of `fromChatMessages()` must be assignable.
+const _converted = fromChatMessages([
+  {
+    role: 'user',
+    content: 'hi',
+  },
+]);
+const _asInput: _InputField = _converted;
+void _asInput;