fix: route deepseek through chat completions

The Anthropic message format drops reasoning blocks that carry no
signature, and DeepSeek never issues one. A reasoning-only assistant
turn — which DeepSeek produces routinely, since it returns
reasoning_content with empty content — therefore lowers to
`content: []`, and the API rejects the whole request with "all messages
must have non-empty content". Once such a turn is in the history every
later request in that session fails, so the session is unrecoverable.

Chat Completions lowers the same turn to `content: ""`, which DeepSeek
accepts, and round-trips reasoning_content as its thinking mode
requires — something the Anthropic route cannot do at all.

Author: razmser

src/models.ts

@@ -41,7 +41,13 @@ type Protocol = "openai" | "anthropic";
 const PROVIDERS = new Map<string, { protocol: Protocol; npm: string }>([
   ["openai", { protocol: "openai", npm: "@ai-sdk/openai" }],
   ["anthropic", { protocol: "anthropic", npm: "@ai-sdk/anthropic" }],
-  ["deepseek", { protocol: "anthropic", npm: "@ai-sdk/anthropic" }],
+  // DeepSeek must use Chat Completions, not the Anthropic protocol: the
+  // Anthropic message format drops reasoning blocks that carry no signature,
+  // and DeepSeek never issues one. A reasoning-only assistant turn then lowers
+  // to `content: []` and the API rejects the whole request with "all messages
+  // must have non-empty content", while `reasoning_content` never survives the
+  // round-trip that DeepSeek's thinking mode requires.
+  ["deepseek", { protocol: "openai", npm: "@ai-sdk/openai-compatible" }],
   ["zai", { protocol: "openai", npm: "@ai-sdk/openai-compatible" }],
   ["moonshotai", { protocol: "openai", npm: "@ai-sdk/openai-compatible" }],
 ]);

test/index.test.ts

@@ -80,6 +80,7 @@ const MODELS_DEV_DATA = {
           output: ["text"],
         },
         cost: { input: 1.5, output: 6, cache_read: 0.15, cache_write: 1.5 },
+        interleaved: { field: "reasoning_content" },
       },
       "deepseek-v4-flash": {
         id: "deepseek-v4-flash",
@@ -99,6 +100,7 @@ const MODELS_DEV_DATA = {
           cache_read: 0.015,
           cache_write: 0.15,
         },
+        interleaved: { field: "reasoning_content" },
       },
     },
   },
@@ -217,8 +219,8 @@ describe("config hook", () => {
       id: "deepseek-v4-pro",
       name: "DeepSeek V4 Pro",
       provider: {
-        api: "https://hub.coreinfra.ai/anthropic/api/v1",
-        npm: "@ai-sdk/anthropic",
+        api: "https://hub.coreinfra.ai/openai/api/v1",
+        npm: "@ai-sdk/openai-compatible",
       },
       attachment: false,
       reasoning: true,
@@ -228,18 +230,15 @@ describe("config hook", () => {
       cost: { input: 1.5, output: 6, cache_read: 0.15, cache_write: 1.5 },
       limit: { context: 1000000, output: 384000 },
       interleaved: { field: "reasoning_content" },
-      headers: {
-        "anthropic-beta":
-          "interleaved-thinking-2025-05-14,fine-grained-tool-streaming-2025-05-14",
-      },
+      headers: {},
     });
 
     expect(models["deepseek-v4-flash"]).toEqual({
       id: "deepseek-v4-flash",
       name: "DeepSeek V4 Flash",
       provider: {
-        api: "https://hub.coreinfra.ai/anthropic/api/v1",
-        npm: "@ai-sdk/anthropic",
+        api: "https://hub.coreinfra.ai/openai/api/v1",
+        npm: "@ai-sdk/openai-compatible",
       },
       attachment: false,
       reasoning: true,
@@ -249,10 +248,7 @@ describe("config hook", () => {
       cost: { input: 0.15, output: 0.6, cache_read: 0.015, cache_write: 0.15 },
       limit: { context: 1000000, output: 384000 },
       interleaved: { field: "reasoning_content" },
-      headers: {
-        "anthropic-beta":
-          "interleaved-thinking-2025-05-14,fine-grained-tool-streaming-2025-05-14",
-      },
+      headers: {},
     });
   });
 

test/models.test.ts

@@ -67,6 +67,7 @@ const MODELS_DEV_FIXTURE = {
           output: ["text"],
         },
         cost: { input: 1.5, output: 6, cache_read: 0.15, cache_write: 1.5 },
+        interleaved: { field: "reasoning_content" },
       },
       "deepseek-v4-flash": {
         id: "deepseek-v4-flash",
@@ -86,6 +87,7 @@ const MODELS_DEV_FIXTURE = {
           cache_read: 0.015,
           cache_write: 0.15,
         },
+        interleaved: { field: "reasoning_content" },
       },
     },
   },
@@ -174,8 +176,8 @@ describe("buildConfigModels", () => {
       id: "deepseek-v4-pro",
       name: "DeepSeek V4 Pro",
       provider: {
-        api: "https://hub.coreinfra.ai/anthropic/api/v1",
-        npm: "@ai-sdk/anthropic",
+        api: "https://hub.coreinfra.ai/openai/api/v1",
+        npm: "@ai-sdk/openai-compatible",
       },
       attachment: false,
       reasoning: true,
@@ -188,19 +190,16 @@ describe("buildConfigModels", () => {
       cost: { input: 1.5, output: 6, cache_read: 0.15, cache_write: 1.5 },
       limit: { context: 1000000, output: 384000 },
       interleaved: { field: "reasoning_content" },
-      headers: {
-        "anthropic-beta":
-          "interleaved-thinking-2025-05-14,fine-grained-tool-streaming-2025-05-14",
-      },
+      headers: {},
     });
 
     const dsFlash = models["deepseek-v4-flash"];
     expect(dsFlash).toEqual({
       id: "deepseek-v4-flash",
       name: "DeepSeek V4 Flash",
       provider: {
-        api: "https://hub.coreinfra.ai/anthropic/api/v1",
-        npm: "@ai-sdk/anthropic",
+        api: "https://hub.coreinfra.ai/openai/api/v1",
+        npm: "@ai-sdk/openai-compatible",
       },
       attachment: false,
       reasoning: true,
@@ -213,10 +212,7 @@ describe("buildConfigModels", () => {
       cost: { input: 0.15, output: 0.6, cache_read: 0.015, cache_write: 0.15 },
       limit: { context: 1000000, output: 384000 },
       interleaved: { field: "reasoning_content" },
-      headers: {
-        "anthropic-beta":
-          "interleaved-thinking-2025-05-14,fine-grained-tool-streaming-2025-05-14",
-      },
+      headers: {},
     });
   });