diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index dcc2605ab6..be06340d6c 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -377,6 +377,31 @@ export namespace ProviderTransform {
       case "@ai-sdk/deepinfra":
       // https://v5.ai-sdk.dev/providers/ai-sdk-providers/deepinfra
       case "@ai-sdk/openai-compatible":
+        // When using openai-compatible SDK with Claude/Anthropic models,
+        // we must use snake_case (budget_tokens) as the SDK doesn't convert parameter names
+        // and the OpenAI-compatible API spec uses snake_case
+        if (
+          model.providerID === "anthropic" ||
+          model.api.id.includes("anthropic") ||
+          model.api.id.includes("claude") ||
+          model.id.includes("anthropic") ||
+          model.id.includes("claude")
+        ) {
+          return {
+            high: {
+              thinking: {
+                type: "enabled",
+                budget_tokens: 16000,
+              },
+            },
+            max: {
+              thinking: {
+                type: "enabled",
+                budget_tokens: 31999,
+              },
+            },
+          }
+        }
         return Object.fromEntries(WIDELY_SUPPORTED_EFFORTS.map((effort) => [effort, { reasoningEffort: effort }]))
 
       case "@ai-sdk/azure":
@@ -656,9 +681,15 @@ export namespace ProviderTransform {
     const modelCap = modelLimit || globalLimit
     const standardLimit = Math.min(modelCap, globalLimit)
 
-    if (npm === "@ai-sdk/anthropic" || npm === "@ai-sdk/google-vertex/anthropic") {
+    // Handle thinking mode for @ai-sdk/anthropic, @ai-sdk/google-vertex/anthropic (budgetTokens)
+    // and @ai-sdk/openai-compatible with Claude (budget_tokens)
+    if (npm === "@ai-sdk/anthropic" || npm === "@ai-sdk/google-vertex/anthropic" || npm === "@ai-sdk/openai-compatible") {
       const thinking = options?.["thinking"]
-      const budgetTokens = typeof thinking?.["budgetTokens"] === "number" ? thinking["budgetTokens"] : 0
+      // Support both camelCase (for @ai-sdk/anthropic) and snake_case (for openai-compatible)
+      const budgetTokens =
+        typeof thinking?.["budgetTokens"] === "number" ? thinking["budgetTokens"] :
+        typeof thinking?.["budget_tokens"] === "number" ? thinking["budget_tokens"] :
+        0
       const enabled = thinking?.["type"] === "enabled"
       if (enabled && budgetTokens > 0) {
         // Return text tokens so that text + thinking <= model cap, preferring 32k text when possible.
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index b818ab98cf..22d1a70707 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -267,6 +267,56 @@ describe("ProviderTransform.maxOutputTokens", () => {
       expect(result).toBe(OUTPUT_TOKEN_MAX)
     })
   })
+
+  describe("openai-compatible with thinking options (snake_case)", () => {
+    test("returns 32k when budget_tokens + 32k <= modelLimit", () => {
+      const modelLimit = 100000
+      const options = {
+        thinking: {
+          type: "enabled",
+          budget_tokens: 10000,
+        },
+      }
+      const result = ProviderTransform.maxOutputTokens("@ai-sdk/openai-compatible", options, modelLimit, OUTPUT_TOKEN_MAX)
+      expect(result).toBe(OUTPUT_TOKEN_MAX)
+    })
+
+    test("returns modelLimit - budget_tokens when budget_tokens + 32k > modelLimit", () => {
+      const modelLimit = 50000
+      const options = {
+        thinking: {
+          type: "enabled",
+          budget_tokens: 30000,
+        },
+      }
+      const result = ProviderTransform.maxOutputTokens("@ai-sdk/openai-compatible", options, modelLimit, OUTPUT_TOKEN_MAX)
+      expect(result).toBe(20000)
+    })
+
+    test("returns 32k when thinking type is not enabled", () => {
+      const modelLimit = 100000
+      const options = {
+        thinking: {
+          type: "disabled",
+          budget_tokens: 10000,
+        },
+      }
+      const result = ProviderTransform.maxOutputTokens("@ai-sdk/openai-compatible", options, modelLimit, OUTPUT_TOKEN_MAX)
+      expect(result).toBe(OUTPUT_TOKEN_MAX)
+    })
+
+    test("returns 32k when budget_tokens is 0", () => {
+      const modelLimit = 100000
+      const options = {
+        thinking: {
+          type: "enabled",
+          budget_tokens: 0,
+        },
+      }
+      const result = ProviderTransform.maxOutputTokens("@ai-sdk/openai-compatible", options, modelLimit, OUTPUT_TOKEN_MAX)
+      expect(result).toBe(OUTPUT_TOKEN_MAX)
+    })
+  })
 })
 
 describe("ProviderTransform.schema - gemini array items", () => {
@@ -1494,6 +1544,67 @@ describe("ProviderTransform.variants", () => {
       expect(result.low).toEqual({ reasoningEffort: "low" })
       expect(result.high).toEqual({ reasoningEffort: "high" })
     })
+
+    test("Claude via LiteLLM returns thinking with snake_case budget_tokens", () => {
+      const model = createMockModel({
+        id: "anthropic/claude-sonnet-4-5",
+        providerID: "anthropic",
+        api: {
+          id: "claude-sonnet-4-5-20250929",
+          url: "http://localhost:4000",
+          npm: "@ai-sdk/openai-compatible",
+        },
+      })
+      const result = ProviderTransform.variants(model)
+      expect(Object.keys(result)).toEqual(["high", "max"])
+      expect(result.high).toEqual({
+        thinking: {
+          type: "enabled",
+          budget_tokens: 16000,
+        },
+      })
+      expect(result.max).toEqual({
+        thinking: {
+          type: "enabled",
+          budget_tokens: 31999,
+        },
+      })
+    })
+
+    test("Claude model (by model.id) via openai-compatible uses snake_case", () => {
+      const model = createMockModel({
+        id: "litellm/claude-3-opus",
+        providerID: "litellm",
+        api: {
+          id: "claude-3-opus-20240229",
+          url: "http://localhost:4000",
+          npm: "@ai-sdk/openai-compatible",
+        },
+      })
+      const result = ProviderTransform.variants(model)
+      expect(Object.keys(result)).toEqual(["high", "max"])
+      expect(result.high).toEqual({
+        thinking: {
+          type: "enabled",
+          budget_tokens: 16000,
+        },
+      })
+    })
+
+    test("Anthropic model (by model.api.id) via openai-compatible uses snake_case", () => {
+      const model = createMockModel({
+        id: "custom/my-model",
+        providerID: "custom",
+        api: {
+          id: "anthropic.claude-sonnet",
+          url: "http://localhost:4000",
+          npm: "@ai-sdk/openai-compatible",
+        },
+      })
+      const result = ProviderTransform.variants(model)
+      expect(Object.keys(result)).toEqual(["high", "max"])
+      expect(result.high.thinking.budget_tokens).toBe(16000)
+    })
   })
 
   describe("@ai-sdk/azure", () => {