diff --git a/.changeset/fix-anthropic-custom-model-id-fallback.md b/.changeset/fix-anthropic-custom-model-id-fallback.md
new file mode 100644
index 0000000000..c84d0134ad
--- /dev/null
+++ b/.changeset/fix-anthropic-custom-model-id-fallback.md
@@ -0,0 +1,11 @@
+---
+"zoo-code": patch
+---
+
+Fix Anthropic provider silently replacing a custom/unrecognized `apiModelId` with the hardcoded default model.
+
+`AnthropicHandler.getModel()` coerced any `apiModelId` not present in the static `anthropicModels` table down to `anthropicDefaultModelId` ("claude-sonnet-4-5"), and that coerced id was what actually got sent as `model` in the API request -- silently ignoring a user-configured custom model name (e.g. a custom Anthropic-compatible deployment or proxy). This produced confusing "model does not exist" errors for the default model instead of the model the user actually selected (#418).
+
+The same fallback also affected capability lookups used to build the `thinking` request parameter: an unrecognized id fell back to the default model's info, which can be from an older model generation with a different API contract, causing the request to use the legacy `thinking: {type: "enabled", budget_tokens}` shape and get rejected with a 400 by models that require `{type: "adaptive"}`.
+
+The model id sent to the API now always honors a user-configured `apiModelId`. For unrecognized values, capabilities are best-effort guessed by matching known model-family substrings (mirroring the existing `BedrockHandler.guessModelInfoFromId` heuristic) instead of defaulting to `anthropicDefaultModelId`'s info.
diff --git a/src/api/providers/__tests__/anthropic.spec.ts b/src/api/providers/__tests__/anthropic.spec.ts
index 2b944b7db5..c17e486184 100644
--- a/src/api/providers/__tests__/anthropic.spec.ts
+++ b/src/api/providers/__tests__/anthropic.spec.ts
@@ -455,6 +455,29 @@ describe("AnthropicHandler", () => {
 			expect(requestBody?.max_tokens).toBe(32768)
 			expect(requestOptions?.headers?.["anthropic-beta"]).toContain("prompt-caching-2024-07-31")
 		})
+
+		it("should send the custom model ID as-is and use adaptive thinking for a custom Sonnet-5-family model", async () => {
+			const customHandler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-sonnet-5-bf",
+				enableReasoningEffort: true,
+			})
+
+			const stream = customHandler.createMessage(systemPrompt, [
+				{
+					role: "user",
+					content: [{ type: "text" as const, text: "Hello" }],
+				},
+			])
+
+			for await (const _chunk of stream) {
+				// Consume stream
+			}
+
+			const requestBody = mockCreate.mock.calls[mockCreate.mock.calls.length - 1]?.[0]
+			expect(requestBody?.model).toBe("claude-sonnet-5-bf")
+			expect(requestBody?.thinking).toEqual({ type: "adaptive" })
+		})
 	})
 
 	describe("completePrompt", () => {
@@ -656,6 +679,38 @@ describe("AnthropicHandler", () => {
 			expect(model.info.inputPrice).toBe(6.0)
 			expect(model.info.outputPrice).toBe(22.5)
 		})
+
+		it("should honor a custom/unrecognized model ID instead of silently falling back to anthropicDefaultModelId", () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-sonnet-5-bf",
+			})
+			const model = handler.getModel()
+			expect(model.id).toBe("claude-sonnet-5-bf")
+		})
+
+		it("should guess capabilities for a custom/unrecognized model ID from known model-family substrings", () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-sonnet-5-bf",
+			})
+			const model = handler.getModel()
+			expect(model.info.supportsReasoningBinary).toBe(true)
+			expect(model.info.maxTokens).toBe(128000)
+			expect(model.info.contextWindow).toBe(1000000)
+		})
+
+		it("should fall back to anthropicDefaultModelId's info when a custom model ID matches no known family", () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "totally-unknown-custom-model",
+			})
+			const model = handler.getModel()
+			expect(model.id).toBe("totally-unknown-custom-model")
+			expect(model.info.maxTokens).toBe(64000)
+			expect(model.info.contextWindow).toBe(200000)
+			expect(model.info.supportsReasoningBinary).toBeUndefined()
+		})
 	})
 
 	describe("reasoning block filtering", () => {
diff --git a/src/api/providers/anthropic.ts b/src/api/providers/anthropic.ts
index f9e2ee7d2b..4a65491053 100644
--- a/src/api/providers/anthropic.ts
+++ b/src/api/providers/anthropic.ts
@@ -353,10 +353,25 @@ export class AnthropicHandler extends BaseProvider implements SingleCompletionHa
 		}
 	}
 
+	// Guesses capabilities for an unrecognized model ID via known-family substring match.
+	private guessModelInfoFromId(modelId: string): ModelInfo {
+		const matchedId = (Object.keys(anthropicModels) as AnthropicModelId[])
+			.sort((a, b) => b.length - a.length)
+			.find((knownId) => modelId.includes(knownId))
+
+		return matchedId ? anthropicModels[matchedId] : anthropicModels[anthropicDefaultModelId]
+	}
+
 	getModel() {
 		const modelId = this.options.apiModelId
-		const id = modelId && modelId in anthropicModels ? (modelId as AnthropicModelId) : anthropicDefaultModelId
-		let info: ModelInfo = anthropicModels[id]
+		const isKnownModel = modelId !== undefined && modelId in anthropicModels
+
+		// Always honor a user-configured apiModelId, even if it's not a known model.
+		const id = isKnownModel ? (modelId as AnthropicModelId) : (modelId ?? anthropicDefaultModelId)
+
+		let info: ModelInfo = isKnownModel
+			? anthropicModels[modelId as AnthropicModelId]
+			: this.guessModelInfoFromId(id)
 
 		// If 1M context beta is enabled for supported models, update the model info
 		if (