diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 695a0dc6..261b335e 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **Custom system prompts missing context**: When using a custom system prompt string, project context files (AGENTS.md), skills, date/time, and working directory were not appended. ([#321](https://github.com/badlogic/pi-mono/issues/321))
+
 ## [0.30.0] - 2025-12-25
 
 ### Breaking Changes
diff --git a/packages/coding-agent/src/core/sdk.ts b/packages/coding-agent/src/core/sdk.ts
index ea1b0a81..5142f5bb 100644
--- a/packages/coding-agent/src/core/sdk.ts
+++ b/packages/coding-agent/src/core/sdk.ts
@@ -550,7 +550,13 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 	if (options.systemPrompt === undefined) {
 		systemPrompt = defaultPrompt;
 	} else if (typeof options.systemPrompt === "string") {
-		systemPrompt = options.systemPrompt;
+		systemPrompt = buildSystemPromptInternal({
+			cwd,
+			agentDir,
+			skills,
+			contextFiles,
+			customPrompt: options.systemPrompt,
+		});
 	} else {
 		systemPrompt = options.systemPrompt(defaultPrompt);
 	}