diff --git a/Sources/AgentRunKit/LLM/GeminiClient.swift b/Sources/AgentRunKit/LLM/GeminiClient.swift
index 7c6967d..8c8b62c 100644
--- a/Sources/AgentRunKit/LLM/GeminiClient.swift
+++ b/Sources/AgentRunKit/LLM/GeminiClient.swift
@@ -133,10 +133,12 @@ extension GeminiClient {
         let level = effortToLevel(config.effort)
         let budget = config.budgetTokens
 
+        // Gemini requires exactly one of thinkingBudget or thinkingLevel.
+        // If an explicit budget is set, prefer it; otherwise use the level.
         return GeminiThinkingConfig(
             includeThoughts: true,
             thinkingBudget: budget,
-            thinkingLevel: level
+            thinkingLevel: budget == nil ? level : nil
         )
     }
 
diff --git a/Sources/AgentRunKit/LLM/GeminiClientTypes.swift b/Sources/AgentRunKit/LLM/GeminiClientTypes.swift
index 8258e59..27a8514 100644
--- a/Sources/AgentRunKit/LLM/GeminiClientTypes.swift
+++ b/Sources/AgentRunKit/LLM/GeminiClientTypes.swift
@@ -151,6 +151,17 @@ struct GeminiThinkingConfig: Encodable {
     let includeThoughts: Bool
     let thinkingBudget: Int?
     let thinkingLevel: String?
+
+    private enum CodingKeys: String, CodingKey {
+        case includeThoughts, thinkingBudget, thinkingLevel
+    }
+
+    func encode(to encoder: any Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        try container.encode(includeThoughts, forKey: .includeThoughts)
+        try container.encodeIfPresent(thinkingBudget, forKey: .thinkingBudget)
+        try container.encodeIfPresent(thinkingLevel, forKey: .thinkingLevel)
+    }
 }
 
 struct GeminiResponse: Decodable {
diff --git a/Tests/AgentRunKitTests/GeminiClientTests.swift b/Tests/AgentRunKitTests/GeminiClientTests.swift
index 612009a..ca8aff3 100644
--- a/Tests/AgentRunKitTests/GeminiClientTests.swift
+++ b/Tests/AgentRunKitTests/GeminiClientTests.swift
@@ -135,6 +135,7 @@ struct GeminiRequestSerializationTests {
         let thinking = genConfig?["thinkingConfig"] as? [String: Any]
         #expect(thinking?["includeThoughts"] as? Bool == true)
         #expect(thinking?["thinkingLevel"] as? String == "HIGH")
+        #expect(thinking?["thinkingBudget"] == nil)
     }
 
     @Test
@@ -167,6 +168,22 @@ struct GeminiRequestSerializationTests {
         let thinking = genConfig?["thinkingConfig"] as? [String: Any]
         #expect(thinking?["includeThoughts"] as? Bool == true)
         #expect(thinking?["thinkingBudget"] as? Int == 10000)
+        #expect(thinking?["thinkingLevel"] == nil)
+    }
+
+    @Test
+    func effortWithBudgetSendsOnlyBudget() throws {
+        // Simulates what ProviderService produces: effort + budgetTokens.
+        let config = ReasoningConfig(effort: .medium, budgetTokens: 32000)
+        let client = makeClient(reasoningConfig: config)
+        let request = try client.buildRequest(messages: [.user("Hi")], tools: [])
+        let json = try encodeRequest(request)
+
+        let genConfig = json["generationConfig"] as? [String: Any]
+        let thinking = genConfig?["thinkingConfig"] as? [String: Any]
+        #expect(thinking?["includeThoughts"] as? Bool == true)
+        #expect(thinking?["thinkingBudget"] as? Int == 32000)
+        #expect(thinking?["thinkingLevel"] == nil)
     }
 
     @Test