diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 42b5f6da5..e842a6d8c 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -4,6 +4,8 @@
 
 ### Fixed
 
+- Fixed OpenAI Codex OAuth reasoning effort mapping for `gpt-5.5` by clamping unsupported `minimal` requests to `low`, matching other GPT-5.2+ Codex models.
+
 - Fixed CI type-check failures from test files referencing model IDs no longer returned by upstream `/models` endpoints. `models.generated.ts` is regenerated each CI run, so tests hardcoding stale IDs (`gpt-4o` on github-copilot, `qwen-3-235b-a22b-instruct-2507` on cerebras, `google/gemini-2.0-flash-001` on openrouter) failed `getModel`'s typed lookup. Swapped to currently-served siblings (`gpt-4.1`, `llama3.1-8b`, `google/gemini-2.5-flash`).
 - Fixed Anthropic adaptive-thinking rejection on `claude-opus-4-7` (and the `opus-4-6` family more generally): the request builder now consults a per-(provider, model) capability table and emits the adaptive `thinking.type=adaptive` + `output_config.effort` shape for any model that requires it, instead of the legacy `thinking.type=enabled` + `budget_tokens` shape ([#11](https://github.com/JuliusBrussee/caveman-code/issues/11))
 - Fixed Anthropic 1M context tier not being opted into for `claude-opus-4-5` on the direct Anthropic API and Bedrock: the provider now sends `anthropic-beta: context-1m-2025-08-07` for that family and the model registry overrides its `contextWindow` to 1_000_000 so the modeline / picker / compaction logic see the real ceiling. The opt-in is provider-scoped: the GitHub Copilot Anthropic relay rejects this beta header and exposes 1M-context Claude models as separate model ids instead, so the beta is not sent on Copilot. Discovery of those Copilot-only model ids is out of scope for this PR; a follow-up runtime-discovery layer will surface them ([#12](https://github.com/JuliusBrussee/caveman-code/issues/12))
diff --git a/packages/ai/src/providers/openai-codex-responses.ts b/packages/ai/src/providers/openai-codex-responses.ts
index a544e3d49..1a81910a0 100644
--- a/packages/ai/src/providers/openai-codex-responses.ts
+++ b/packages/ai/src/providers/openai-codex-responses.ts
@@ -335,7 +335,10 @@ function buildRequestBody(
 
 function clampReasoningEffort(modelId: string, effort: string): string {
 	const id = modelId.includes("/") ? modelId.split("/").pop()! : modelId;
-	if ((id.startsWith("gpt-5.2") || id.startsWith("gpt-5.3") || id.startsWith("gpt-5.4")) && effort === "minimal")
+	if (
+		(id.startsWith("gpt-5.2") || id.startsWith("gpt-5.3") || id.startsWith("gpt-5.4") || id.startsWith("gpt-5.5")) &&
+		effort === "minimal"
+	)
 		return "low";
 	if (id === "gpt-5.1" && effort === "xhigh") return "high";
 	if (id === "gpt-5.1-codex-mini") return effort === "high" || effort === "xhigh" ? "high" : "medium";
diff --git a/packages/ai/test/openai-codex-stream.test.ts b/packages/ai/test/openai-codex-stream.test.ts
index d3db8c298..340c19701 100644
--- a/packages/ai/test/openai-codex-stream.test.ts
+++ b/packages/ai/test/openai-codex-stream.test.ts
@@ -404,7 +404,7 @@ describe("openai-codex streaming", () => {
 		await streamResult.result();
 	});
 
-	it.each(["gpt-5.3-codex", "gpt-5.4"])("clamps %s minimal reasoning effort to low", async (modelId) => {
+	it.each(["gpt-5.3-codex", "gpt-5.4", "gpt-5.5"])("clamps %s minimal reasoning effort to low", async (modelId) => {
 		const tempDir = mkdtempSync(join(tmpdir(), "pi-codex-stream-"));
 		process.env.PI_CODING_AGENT_DIR = tempDir;