diff --git a/TablePro/Core/AI/AIProvider.swift b/TablePro/Core/AI/AIProvider.swift
index 49051dce..0776d4f3 100644
--- a/TablePro/Core/AI/AIProvider.swift
+++ b/TablePro/Core/AI/AIProvider.swift
@@ -54,4 +54,17 @@ enum AIProviderError: Error, LocalizedError {
             return String(localized: "Streaming failed: \(message)")
         }
     }
+
+    /// Extract human-readable message from provider JSON error responses.
+    /// Supports Anthropic (`{"error":{"message":"..."}}`), OpenAI, and Gemini formats.
+    static func parseErrorMessage(from body: String) -> String? {
+        guard let data = body.data(using: .utf8),
+              let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any],
+              let error = json["error"] as? [String: Any],
+              let message = error["message"] as? String
+        else {
+            return nil
+        }
+        return message
+    }
 }
diff --git a/TablePro/Core/AI/AnthropicProvider.swift b/TablePro/Core/AI/AnthropicProvider.swift
index d34d0f99..37f53ec5 100644
--- a/TablePro/Core/AI/AnthropicProvider.swift
+++ b/TablePro/Core/AI/AnthropicProvider.swift
@@ -18,7 +18,7 @@ final class AnthropicProvider: AIProvider {
 
     init(endpoint: String, apiKey: String) {
         self.endpoint = endpoint.hasSuffix("/") ? String(endpoint.dropLast()) : endpoint
-        self.apiKey = apiKey
+        self.apiKey = apiKey.trimmingCharacters(in: .whitespacesAndNewlines)
         self.session = URLSession(configuration: .ephemeral)
     }
 
@@ -94,32 +94,66 @@ final class AnthropicProvider: AIProvider {
     }
 
     func fetchAvailableModels() async throws -> [String] {
-        // Anthropic doesn't have a models endpoint; return known models
-        [
-            "claude-sonnet-4-5-20250514",
-            "claude-haiku-4-5-20251001",
-            "claude-opus-4-20250514"
-        ]
+        guard let url = URL(string: "\(endpoint)/v1/models") else {
+            throw AIProviderError.invalidEndpoint(endpoint)
+        }
+
+        var request = URLRequest(url: url)
+        request.httpMethod = "GET"
+        request.setValue(apiKey, forHTTPHeaderField: "x-api-key")
+        request.setValue("2023-06-01", forHTTPHeaderField: "anthropic-version")
+
+        let (data, response) = try await session.data(for: request)
+
+        guard let httpResponse = response as? HTTPURLResponse,
+              httpResponse.statusCode == 200,
+              let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any],
+              let models = json["data"] as? [[String: Any]]
+        else {
+            return Self.knownModels
+        }
+
+        let modelIds = models.compactMap { $0["id"] as? String }
+        return modelIds.isEmpty ? Self.knownModels : modelIds
     }
 
+    private static let knownModels = [
+        "claude-sonnet-4-6",
+        "claude-opus-4-6",
+        "claude-haiku-4-5-20251001",
+        "claude-sonnet-4-5-20250929",
+        "claude-opus-4-5-20251101"
+    ]
+
     func testConnection() async throws -> Bool {
         let testMessage = AIChatMessage(role: .user, content: "Hi")
         let request = try buildMessagesRequest(
             messages: [testMessage],
-            model: "claude-sonnet-4-5-20250514",
+            model: "claude-haiku-4-5-20251001",
             systemPrompt: nil,
             maxTokens: 1,
             stream: false
         )
 
-        let (_, response) = try await session.data(for: request)
+        let (data, response) = try await session.data(for: request)
 
         guard let httpResponse = response as? HTTPURLResponse else {
             return false
         }
 
-        // 200 = success, 401 = bad key
-        return httpResponse.statusCode == 200
+        let statusCode = httpResponse.statusCode
+
+        // 200 = full success, 400 = key is valid but request was rejected (e.g. billing)
+        if statusCode == 200 || statusCode == 400 {
+            return true
+        }
+
+        if statusCode == 401 {
+            throw AIProviderError.authenticationFailed("")
+        }
+
+        let body = String(data: data, encoding: .utf8) ?? ""
+        throw mapHTTPError(statusCode: statusCode, body: body)
     }
 
     // MARK: - Private
@@ -209,21 +243,25 @@ final class AnthropicProvider: AIProvider {
         var body = ""
         for try await line in bytes.lines {
             body += line
-            if body.count > 2_000 { break }
+            if (body as NSString).length > 2_000 { break }
         }
         return body
     }
 
     private func mapHTTPError(statusCode: Int, body: String) -> AIProviderError {
+        let message = AIProviderError.parseErrorMessage(from: body) ?? body
+
         switch statusCode {
+        case 400:
+            return .serverError(statusCode, message)
         case 401:
-            return .authenticationFailed(body)
+            return .authenticationFailed(message)
         case 429:
             return .rateLimited
         case 404:
-            return .modelNotFound(body)
+            return .modelNotFound(message)
         default:
-            return .serverError(statusCode, body)
+            return .serverError(statusCode, message)
         }
     }
 }
diff --git a/TablePro/Core/AI/GeminiProvider.swift b/TablePro/Core/AI/GeminiProvider.swift
index 3205157b..136ddfb0 100644
--- a/TablePro/Core/AI/GeminiProvider.swift
+++ b/TablePro/Core/AI/GeminiProvider.swift
@@ -18,7 +18,7 @@ final class GeminiProvider: AIProvider {
 
     init(endpoint: String, apiKey: String) {
         self.endpoint = endpoint.hasSuffix("/") ? String(endpoint.dropLast()) : endpoint
-        self.apiKey = apiKey
+        self.apiKey = apiKey.trimmingCharacters(in: .whitespacesAndNewlines)
         self.session = URLSession(configuration: .ephemeral)
     }
 
@@ -154,13 +154,24 @@ final class GeminiProvider: AIProvider {
         var request = URLRequest(url: url)
         request.httpMethod = "GET"
 
-        let (_, response) = try await session.data(for: request)
+        let (data, response) = try await session.data(for: request)
 
         guard let httpResponse = response as? HTTPURLResponse else {
             return false
         }
 
-        return httpResponse.statusCode == 200
+        let statusCode = httpResponse.statusCode
+
+        if statusCode == 401 || statusCode == 403 {
+            throw AIProviderError.authenticationFailed("")
+        }
+
+        guard statusCode == 200 else {
+            let body = String(data: data, encoding: .utf8) ?? ""
+            throw mapHTTPError(statusCode: statusCode, body: body)
+        }
+
+        return true
     }
 
     // MARK: - Private
@@ -211,21 +222,23 @@ final class GeminiProvider: AIProvider {
         var body = ""
         for try await line in bytes.lines {
             body += line
-            if body.count > 2_000 { break }
+            if (body as NSString).length > 2_000 { break }
         }
         return body
     }
 
     private func mapHTTPError(statusCode: Int, body: String) -> AIProviderError {
+        let message = AIProviderError.parseErrorMessage(from: body) ?? body
+
         switch statusCode {
         case 401, 403:
-            return .authenticationFailed(body)
+            return .authenticationFailed(message)
         case 429:
             return .rateLimited
         case 404:
-            return .modelNotFound(body)
+            return .modelNotFound(message)
         default:
-            return .serverError(statusCode, body)
+            return .serverError(statusCode, message)
         }
     }
 }
diff --git a/TablePro/Core/AI/OpenAICompatibleProvider.swift b/TablePro/Core/AI/OpenAICompatibleProvider.swift
index ca2e3ea5..03d36f8f 100644
--- a/TablePro/Core/AI/OpenAICompatibleProvider.swift
+++ b/TablePro/Core/AI/OpenAICompatibleProvider.swift
@@ -22,7 +22,7 @@ final class OpenAICompatibleProvider: AIProvider {
 
     init(endpoint: String, apiKey: String?, providerType: AIProviderType) {
         self.endpoint = endpoint.hasSuffix("/") ? String(endpoint.dropLast()) : endpoint
-        self.apiKey = apiKey
+        self.apiKey = apiKey?.trimmingCharacters(in: .whitespacesAndNewlines)
         self.providerType = providerType
         self.session = URLSession(configuration: .ephemeral)
     }
@@ -147,7 +147,7 @@ final class OpenAICompatibleProvider: AIProvider {
             let isJSON = contentType.contains("application/json")
 
             if httpResponse.statusCode == 401 {
-                return false
+                throw AIProviderError.authenticationFailed("")
             }
 
             // Non-JSON response means wrong endpoint (e.g., HTML 404 page)
@@ -327,21 +327,23 @@ final class OpenAICompatibleProvider: AIProvider {
         var body = ""
         for try await line in bytes.lines {
             body += line
-            if body.count > 2_000 { break }
+            if (body as NSString).length > 2_000 { break }
         }
         return body
     }
 
     private func mapHTTPError(statusCode: Int, body: String) -> AIProviderError {
+        let message = AIProviderError.parseErrorMessage(from: body) ?? body
+
         switch statusCode {
         case 401:
-            return .authenticationFailed(body)
+            return .authenticationFailed(message)
         case 429:
             return .rateLimited
         case 404:
-            return .modelNotFound(body)
+            return .modelNotFound(message)
         default:
-            return .serverError(statusCode, body)
+            return .serverError(statusCode, message)
         }
     }
 }
diff --git a/TablePro/Views/Settings/AISettingsView.swift b/TablePro/Views/Settings/AISettingsView.swift
index 07932ef8..6a965a9f 100644
--- a/TablePro/Views/Settings/AISettingsView.swift
+++ b/TablePro/Views/Settings/AISettingsView.swift
@@ -472,7 +472,7 @@ private struct AIProviderEditorSheet: View {
                     Text("Test")
                 }
             }
-            .disabled(isTesting)
+            .disabled(isTesting || (draft.type.requiresAPIKey && editingAPIKey.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty))
 
             if case .success = testResult {
                 Text(String(localized: "Connection successful"))
@@ -547,6 +547,11 @@ private struct AIProviderEditorSheet: View {
     // MARK: - Connection Test
 
     func testProvider() {
+        guard !editingAPIKey.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty || !draft.type.requiresAPIKey else {
+            testResult = .failure(String(localized: "API key is required"))
+            return
+        }
+
         let provider = AIProviderFactory.createProvider(for: draft, apiKey: editingAPIKey)
 
         isTesting = true