fix(ai): support Anthropic eager tool streaming compat

closes #3575

packages/coding-agent/docs/models.md

@@ -11,6 +11,7 @@ Add custom providers and models (Ollama, vLLM, LM Studio, proxies) via `~/.pi/ag
 - [Model Configuration](#model-configuration)
 - [Overriding Built-in Providers](#overriding-built-in-providers)
 - [Per-model Overrides](#per-model-overrides)
+- [Anthropic Messages Compatibility](#anthropic-messages-compatibility)
 - [OpenAI Compatibility](#openai-compatibility)
 
 ## Minimal Example
@@ -195,7 +196,7 @@ If your command is slow, expensive, rate-limited, or should keep using a previou
 | `contextWindow` | No | `128000` | Context window size in tokens |
 | `maxTokens` | No | `16384` | Maximum output tokens |
 | `cost` | No | all zeros | `{"input": 0, "output": 0, "cacheRead": 0, "cacheWrite": 0}` (per million tokens) |
-| `compat` | No | provider `compat` | OpenAI compatibility overrides. Merged with provider-level `compat` when both are set. |
+| `compat` | No | provider `compat` | Provider compatibility overrides. Merged with provider-level `compat` when both are set. |
 
 Current behavior:
 - `/model` and `--list-models` list entries by model `id`.
@@ -269,6 +270,38 @@ Behavior notes:
 - You can combine provider-level `baseUrl`/`headers` with `modelOverrides`.
 - If `models` is also defined for a provider, custom models are merged after built-in overrides. A custom model with the same `id` replaces the overridden built-in model entry.
 
+## Anthropic Messages Compatibility
+
+For providers or proxies using `api: "anthropic-messages"`, use `compat.supportsEagerToolInputStreaming` to control Anthropic fine-grained tool streaming compatibility.
+
+By default pi sends per-tool `eager_input_streaming: true`. If a proxy or Anthropic-compatible backend rejects that field, set `supportsEagerToolInputStreaming` to `false`. Pi will omit `tools[].eager_input_streaming` and send the legacy `fine-grained-tool-streaming-2025-05-14` beta header for tool-enabled requests instead.
+
+```json
+{
+  "providers": {
+    "anthropic-proxy": {
+      "baseUrl": "https://proxy.example.com",
+      "api": "anthropic-messages",
+      "apiKey": "ANTHROPIC_PROXY_KEY",
+      "compat": {
+        "supportsEagerToolInputStreaming": false
+      },
+      "models": [
+        {
+          "id": "claude-opus-4-7",
+          "reasoning": true,
+          "input": ["text", "image"]
+        }
+      ]
+    }
+  }
+}
+```
+
+| Field | Description |
+|-------|-------------|
+| `supportsEagerToolInputStreaming` | Whether the provider accepts per-tool `eager_input_streaming`. Default: `true`. Set to `false` to omit that field and use the legacy fine-grained tool streaming beta header on tool-enabled requests. |
+
 ## OpenAI Compatibility
 
 For providers with partial OpenAI compatibility, use the `compat` field.

packages/coding-agent/src/core/model-registry.ts

@@ -3,6 +3,7 @@
  */
 
 import {
+	type AnthropicMessagesCompat,
 	type Api,
 	type AssistantMessageEventStream,
 	type Context,
@@ -116,7 +117,15 @@ const OpenAIResponsesCompatSchema = Type.Object({
 	// Reserved for future use
 });
 
-const OpenAICompatSchema = Type.Union([OpenAICompletionsCompatSchema, OpenAIResponsesCompatSchema]);
+const AnthropicMessagesCompatSchema = Type.Object({
+	supportsEagerToolInputStreaming: Type.Optional(Type.Boolean()),
+});
+
+const ProviderCompatSchema = Type.Union([
+	OpenAICompletionsCompatSchema,
+	OpenAIResponsesCompatSchema,
+	AnthropicMessagesCompatSchema,
+]);
 
 // Schema for custom model definition
 // Most fields are optional with sensible defaults for local models (Ollama, LM Studio, etc.)
@@ -138,7 +147,7 @@ const ModelDefinitionSchema = Type.Object({
 	contextWindow: Type.Optional(Type.Number()),
 	maxTokens: Type.Optional(Type.Number()),
 	headers: Type.Optional(Type.Record(Type.String(), Type.String())),
-	compat: Type.Optional(OpenAICompatSchema),
+	compat: Type.Optional(ProviderCompatSchema),
 });
 
 // Schema for per-model overrides (all fields optional, merged with built-in model)
@@ -157,7 +166,7 @@ const ModelOverrideSchema = Type.Object({
 	contextWindow: Type.Optional(Type.Number()),
 	maxTokens: Type.Optional(Type.Number()),
 	headers: Type.Optional(Type.Record(Type.String(), Type.String())),
-	compat: Type.Optional(OpenAICompatSchema),
+	compat: Type.Optional(ProviderCompatSchema),
 });
 
 type ModelOverride = Static<typeof ModelOverrideSchema>;
@@ -167,7 +176,7 @@ const ProviderConfigSchema = Type.Object({
 	apiKey: Type.Optional(Type.String({ minLength: 1 })),
 	api: Type.Optional(Type.String({ minLength: 1 })),
 	headers: Type.Optional(Type.Record(Type.String(), Type.String())),
-	compat: Type.Optional(OpenAICompatSchema),
+	compat: Type.Optional(ProviderCompatSchema),
 	authHeader: Type.Optional(Type.Boolean()),
 	models: Type.Optional(Type.Array(ModelDefinitionSchema)),
 	modelOverrides: Type.Optional(Type.Record(Type.String(), ModelOverrideSchema)),
@@ -237,9 +246,9 @@ function mergeCompat(
 ): Model<Api>["compat"] | undefined {
 	if (!overrideCompat) return baseCompat;
 
-	const base = baseCompat as OpenAICompletionsCompat | OpenAIResponsesCompat | undefined;
-	const override = overrideCompat as OpenAICompletionsCompat | OpenAIResponsesCompat;
-	const merged = { ...base, ...override } as OpenAICompletionsCompat | OpenAIResponsesCompat;
+	const base = baseCompat as OpenAICompletionsCompat | OpenAIResponsesCompat | AnthropicMessagesCompat | undefined;
+	const override = overrideCompat as OpenAICompletionsCompat | OpenAIResponsesCompat | AnthropicMessagesCompat;
+	const merged = { ...base, ...override } as OpenAICompletionsCompat | OpenAIResponsesCompat | AnthropicMessagesCompat;
 
 	const baseCompletions = base as OpenAICompletionsCompat | undefined;
 	const overrideCompletions = override as OpenAICompletionsCompat;

packages/coding-agent/test/model-registry.test.ts

@@ -1,7 +1,7 @@
 import { existsSync, mkdirSync, readFileSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import type { Api, Context, Model, OpenAICompletionsCompat } from "@mariozechner/pi-ai";
+import type { AnthropicMessagesCompat, Api, Context, Model, OpenAICompletionsCompat } from "@mariozechner/pi-ai";
 import { getApiProvider } from "@mariozechner/pi-ai";
 import { getOAuthProvider } from "@mariozechner/pi-ai/oauth";
 import { afterEach, beforeEach, describe, expect, test } from "vitest";
@@ -432,6 +432,35 @@ describe("ModelRegistry", () => {
 			expect(compat?.cacheControlFormat).toBe("anthropic");
 		});
 
+		test("compat schema accepts Anthropic eager tool input streaming flag", () => {
+			writeRawModelsJson({
+				demo: {
+					baseUrl: "https://example.com",
+					apiKey: "DEMO_KEY",
+					api: "anthropic-messages",
+					compat: {
+						supportsEagerToolInputStreaming: false,
+					},
+					models: [
+						{
+							id: "demo-model",
+							reasoning: true,
+							input: ["text"],
+							cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+							contextWindow: 1000,
+							maxTokens: 100,
+						},
+					],
+				},
+			});
+
+			const registry = ModelRegistry.create(authStorage, modelsJsonPath);
+			const compat = registry.find("demo", "demo-model")?.compat as AnthropicMessagesCompat | undefined;
+
+			expect(registry.getError()).toBeUndefined();
+			expect(compat?.supportsEagerToolInputStreaming).toBe(false);
+		});
+
 		test("model-level baseUrl overrides provider-level baseUrl for custom models", () => {
 			writeRawModelsJson({
 				"opencode-go": {