diff --git a/.changeset/sour-parks-trade.md b/.changeset/sour-parks-trade.md
new file mode 100644
index 00000000000..5d58470ce54
--- /dev/null
+++ b/.changeset/sour-parks-trade.md
@@ -0,0 +1,5 @@
+---
+"@effect/ai-openai": patch
+---
+
+Previously, setting `strict: false` on `OpenAiLanguageModel` config caused a 400 "Unknown parameter: 'strict'" response from the OpenAI Responses API, because the flag was spread into the top-level request body instead of being consumed only by the tool and response_format schema builders. The `strict` flag is now stripped from the request body while still controlling `strict` on tool schemas (`prepareTools`) and json_schema response formats (`prepareResponseFormat`).
diff --git a/packages/ai/openai/src/OpenAiLanguageModel.ts b/packages/ai/openai/src/OpenAiLanguageModel.ts
index c4fef761467..287afa629fd 100644
--- a/packages/ai/openai/src/OpenAiLanguageModel.ts
+++ b/packages/ai/openai/src/OpenAiLanguageModel.ts
@@ -287,8 +287,9 @@ export const make = Effect.fnUntraced(function*(options: {
       const include = prepareInclude(providerOptions, config)
       const responseFormat = prepareResponseFormat(providerOptions)
       const verbosity = config.text?.verbosity
+      const { strict: _strict, ...requestConfig } = config
       const request: typeof Generated.CreateResponse.Encoded = {
-        ...config,
+        ...requestConfig,
         input: messages,
         include,
         text: { format: responseFormat, verbosity },