diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 7f3b72d0..b72f4718 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 ### Fixed
 
+- Fixed OpenAI Responses custom providers to honor `compat.supportsDeveloperRole: false` for reasoning models ([#5456](https://github.com/earendil-works/pi/issues/5456)).
 - Fixed OpenRouter routing preferences on OpenAI-compatible custom providers to send `compat.openRouterRouting` even when `baseUrl` does not point directly at OpenRouter ([#5347](https://github.com/earendil-works/pi/issues/5347)).
 
 ## [0.78.1] - 2026-06-04
diff --git a/packages/ai/README.md b/packages/ai/README.md
index 6be70212..4190fcb8 100644
--- a/packages/ai/README.md
+++ b/packages/ai/README.md
@@ -926,7 +926,7 @@ const ollamaReasoningModel: Model<'openai-completions'> = {
 
 ### OpenAI Compatibility Settings
 
-The `openai-completions` API is implemented by many providers with minor differences. By default, the library auto-detects compatibility settings based on `baseUrl` for a small set of known OpenAI-compatible providers (Cerebras, xAI, Chutes, DeepSeek, NVIDIA NIM, Together AI, zAi, OpenCode, Cloudflare Workers AI, etc.). For custom proxies or unknown endpoints, you can override these settings via the `compat` field. For `openai-responses` models, the compat field only supports Responses-specific flags.
+The `openai-completions` API is implemented by many providers with minor differences. By default, the library auto-detects compatibility settings based on `baseUrl` for a small set of known OpenAI-compatible providers (Cerebras, xAI, Chutes, DeepSeek, NVIDIA NIM, Together AI, zAi, OpenCode, Cloudflare Workers AI, etc.). For custom proxies or unknown endpoints, you can override these settings via the `compat` field. For `openai-responses` models, the compat field supports Responses-specific flags.
 
 ```typescript
 interface OpenAICompletionsCompat {
@@ -948,7 +948,9 @@ interface OpenAICompletionsCompat {
 }
 
 interface OpenAIResponsesCompat {
-  // Reserved for future use
+  supportsDeveloperRole?: boolean;   // Whether provider supports `developer` role vs `system` (default: true)
+  sendSessionIdHeader?: boolean;     // Whether to send `session_id` from `sessionId` when caching is enabled (default: true)
+  supportsLongCacheRetention?: boolean; // Whether provider supports `prompt_cache_retention: "24h"` (default: true)
 }
 ```
 
diff --git a/packages/ai/src/providers/openai-responses-shared.ts b/packages/ai/src/providers/openai-responses-shared.ts
index 11a977d5..c006c284 100644
--- a/packages/ai/src/providers/openai-responses-shared.ts
+++ b/packages/ai/src/providers/openai-responses-shared.ts
@@ -124,7 +124,8 @@ export function convertResponsesMessages<TApi extends Api>(
 
 	const includeSystemPrompt = options?.includeSystemPrompt ?? true;
 	if (includeSystemPrompt && context.systemPrompt) {
-		const role = model.reasoning ? "developer" : "system";
+		const compat = model.compat as { supportsDeveloperRole?: boolean } | undefined;
+		const role = model.reasoning && compat?.supportsDeveloperRole !== false ? "developer" : "system";
 		messages.push({
 			role,
 			content: sanitizeSurrogates(context.systemPrompt),
diff --git a/packages/ai/src/providers/openai-responses.ts b/packages/ai/src/providers/openai-responses.ts
index 4b35a24c..9afeff29 100644
--- a/packages/ai/src/providers/openai-responses.ts
+++ b/packages/ai/src/providers/openai-responses.ts
@@ -39,6 +39,7 @@ function resolveCacheRetention(cacheRetention?: CacheRetention): CacheRetention
 
 function getCompat(model: Model<"openai-responses">): Required<OpenAIResponsesCompat> {
 	return {
+		supportsDeveloperRole: model.compat?.supportsDeveloperRole ?? true,
 		sendSessionIdHeader: model.compat?.sendSessionIdHeader ?? true,
 		supportsLongCacheRetention: model.compat?.supportsLongCacheRetention ?? true,
 	};
diff --git a/packages/ai/src/types.ts b/packages/ai/src/types.ts
index ed36c7ec..802b8b39 100644
--- a/packages/ai/src/types.ts
+++ b/packages/ai/src/types.ts
@@ -421,6 +421,8 @@ export interface OpenAICompletionsCompat {
 
 /** Compatibility settings for OpenAI Responses APIs. */
 export interface OpenAIResponsesCompat {
+	/** Whether the provider supports the `developer` role (vs `system`). Default: true. */
+	supportsDeveloperRole?: boolean;
 	/** Whether to send the OpenAI `session_id` cache-affinity header from `options.sessionId` when caching is enabled. Default: true. */
 	sendSessionIdHeader?: boolean;
 	/** Whether the provider supports `prompt_cache_retention: "24h"`. Default: true. */
diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 707281c0..c0d45312 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -10,6 +10,7 @@
 
 ### Fixed
 
+- Fixed `models.json` schema support for OpenAI Responses `compat.supportsDeveloperRole` ([#5456](https://github.com/earendil-works/pi/issues/5456)).
 - Fixed tmux setup documentation to require tmux 3.5 for `extended-keys-format csi-u` and document the tmux 3.2-3.4 fallback ([#5432](https://github.com/earendil-works/pi/issues/5432)).
 - Fixed built-in tool expand hints to style closing parentheses consistently ([#5359](https://github.com/earendil-works/pi/issues/5359)).
 
diff --git a/packages/coding-agent/src/core/model-registry.ts b/packages/coding-agent/src/core/model-registry.ts
index 455ab71f..00c81576 100644
--- a/packages/coding-agent/src/core/model-registry.ts
+++ b/packages/coding-agent/src/core/model-registry.ts
@@ -127,6 +127,7 @@ const OpenAICompletionsCompatSchema = Type.Object({
 });
 
 const OpenAIResponsesCompatSchema = Type.Object({
+	supportsDeveloperRole: Type.Optional(Type.Boolean()),
 	sendSessionIdHeader: Type.Optional(Type.Boolean()),
 	supportsLongCacheRetention: Type.Optional(Type.Boolean()),
 });