diff --git a/packages/server/src/server/agent/agent-sdk-types.ts b/packages/server/src/server/agent/agent-sdk-types.ts
index 08eec8714..afeeed9f0 100644
--- a/packages/server/src/server/agent/agent-sdk-types.ts
+++ b/packages/server/src/server/agent/agent-sdk-types.ts
@@ -236,6 +236,11 @@ export type PersistedAgentDescriptor = {
 export type AgentSessionConfig = {
   provider: AgentProvider;
   cwd: string;
+  /**
+   * Provider-agnostic system/developer instruction string.
+   * Mapped by each provider to its native instruction field.
+   */
+  systemPrompt?: string;
   modeId?: string;
   model?: string;
   thinkingOptionId?: string;
diff --git a/packages/server/src/server/agent/providers/claude-agent.ts b/packages/server/src/server/agent/providers/claude-agent.ts
index 6eaf9bc0d..c259712da 100644
--- a/packages/server/src/server/agent/providers/claude-agent.ts
+++ b/packages/server/src/server/agent/providers/claude-agent.ts
@@ -896,6 +896,13 @@ class ClaudeAgentSession implements AgentSession {
       // For "on" we omit maxThinkingTokens (SDK default max).
     }
 
+    const appendedSystemPrompt = [
+      getOrchestratorModeInstructions(),
+      this.config.systemPrompt?.trim(),
+    ]
+      .filter((entry): entry is string => typeof entry === "string" && entry.length > 0)
+      .join("\n\n");
+
     const base: ClaudeOptions = {
       cwd: this.config.cwd,
       includePartialMessages: true,
@@ -904,11 +911,11 @@ class ClaudeAgentSession implements AgentSession {
       canUseTool: this.handlePermissionRequest,
       ...(this.claudePath ? { pathToClaudeCodeExecutable: this.claudePath } : {}),
       // Use Claude Code preset system prompt and load CLAUDE.md files
-      // Append orchestrator mode instructions for agents
+      // Append provider-agnostic system prompt and orchestrator instructions for agents.
       systemPrompt: {
         type: "preset",
         preset: "claude_code",
-        append: [getOrchestratorModeInstructions()].filter(Boolean).join("\n"),
+        append: appendedSystemPrompt,
       },
       settingSources: ["user", "project"],
       stderr: (data: string) => {
diff --git a/packages/server/src/server/agent/providers/codex-app-server-agent.ts b/packages/server/src/server/agent/providers/codex-app-server-agent.ts
index e218c3f9b..ddf44bd5c 100644
--- a/packages/server/src/server/agent/providers/codex-app-server-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-app-server-agent.ts
@@ -1269,7 +1269,13 @@ class CodexAppServerAgentSession implements AgentSession {
     const settings: Record<string, unknown> = {};
     if (match.model) settings.model = match.model;
     if (match.reasoning_effort) settings.reasoning_effort = match.reasoning_effort;
-    if (match.developer_instructions) settings.developer_instructions = match.developer_instructions;
+    const developerInstructions = [
+      match.developer_instructions?.trim(),
+      this.config.systemPrompt?.trim(),
+    ]
+      .filter((entry): entry is string => typeof entry === "string" && entry.length > 0)
+      .join("\n\n");
+    if (developerInstructions) settings.developer_instructions = developerInstructions;
     if (this.config.model) settings.model = this.config.model;
     const thinkingOptionId = this.config.thinkingOptionId;
     if (thinkingOptionId) settings.reasoning_effort = thinkingOptionId;
@@ -1714,6 +1720,9 @@ class CodexAppServerAgentSession implements AgentSession {
       cwd: this.config.cwd ?? null,
       approvalPolicy,
       sandbox,
+      ...(this.config.systemPrompt?.trim()
+        ? { developerInstructions: this.config.systemPrompt.trim() }
+        : {}),
       ...(Object.keys(innerConfig).length > 0 ? { config: innerConfig } : {}),
     })) as CodexThreadStartResponse;
     const threadId = response?.thread?.id;
diff --git a/packages/server/src/server/agent/providers/codex-mcp-agent.ts b/packages/server/src/server/agent/providers/codex-mcp-agent.ts
index dd7fba32b..85be3bd85 100644
--- a/packages/server/src/server/agent/providers/codex-mcp-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-mcp-agent.ts
@@ -2907,7 +2907,10 @@ function buildCodexMcpConfig(
   }
 
 
-  const combinedDeveloperInstructions = [developerInstructions]
+  const combinedDeveloperInstructions = [
+    developerInstructions,
+    config.systemPrompt?.trim(),
+  ]
     .filter(Boolean)
     .join("\n\n");
 
diff --git a/packages/server/src/server/session.ts b/packages/server/src/server/session.ts
index bbb984027..672534059 100644
--- a/packages/server/src/server/session.ts
+++ b/packages/server/src/server/session.ts
@@ -4546,6 +4546,7 @@ export class Session {
     const config: AgentSessionConfig = {
       provider,
       cwd,
+      systemPrompt: VOICE_AGENT_SYSTEM_INSTRUCTION,
       modeId: this.voiceLlmModeId ?? "default",
       ...(model ? { model } : {}),
       mcpServers: {
@@ -4632,8 +4633,6 @@ export class Session {
   private buildVoiceAgentPrompt(userText: string): string {
     const transcription = escapeXmlText(userText.trim());
     return [
-      VOICE_AGENT_SYSTEM_INSTRUCTION,
-      "",
       "<voice_transcription>",
       transcription,
       "</voice_transcription>",