diff --git a/crates/colibri-client/tests/live_socket_smoke.rs b/crates/colibri-client/tests/live_socket_smoke.rs
index 7130865..56e1681 100644
--- a/crates/colibri-client/tests/live_socket_smoke.rs
+++ b/crates/colibri-client/tests/live_socket_smoke.rs
@@ -27,6 +27,7 @@ fn smoke_config() -> DaemonConfig {
         max_context_tokens: 128_000,
         max_uncompacted_turns: 20,
         cost_mode: "smart".to_string(),
+        scheduler_prompt_injection: false,
     }
 }
 
diff --git a/crates/colibri-daemon/src/config.rs b/crates/colibri-daemon/src/config.rs
index fb52e35..647ed7c 100644
--- a/crates/colibri-daemon/src/config.rs
+++ b/crates/colibri-daemon/src/config.rs
@@ -43,6 +43,9 @@ pub struct DaemonConfig {
     pub max_uncompacted_turns: usize,
     /// Current cost mode (fast/smart/max).
     pub cost_mode: String,
+    /// Inject session prompt context into spawned agent env (T1.4 PR3a).
+    /// Disabled by default — enable when prompt discipline is verified.
+    pub scheduler_prompt_injection: bool,
 }
 
 impl DaemonConfig {
@@ -86,6 +89,8 @@ impl DaemonConfig {
             max_context_tokens: env_parse("COLIBRI_MAX_CONTEXT_TOKENS").unwrap_or(128_000),
             max_uncompacted_turns: env_parse("COLIBRI_MAX_UNCOMPACTED_TURNS").unwrap_or(20),
             cost_mode: std::env::var("COLIBRI_COST_MODE").unwrap_or_else(|_| "smart".to_string()),
+            scheduler_prompt_injection: env_parse("COLIBRI_SCHEDULER_PROMPT_INJECTION")
+                .unwrap_or(false),
         }
     }
 }
diff --git a/crates/colibri-daemon/src/scheduler.rs b/crates/colibri-daemon/src/scheduler.rs
index 25a2fa2..29f44da 100644
--- a/crates/colibri-daemon/src/scheduler.rs
+++ b/crates/colibri-daemon/src/scheduler.rs
@@ -360,6 +360,7 @@ mod tests {
             max_context_tokens: 128_000,
             max_uncompacted_turns: 20,
             cost_mode: "smart".to_string(),
+            scheduler_prompt_injection: true,
         }
     }
 
diff --git a/crates/colibri-daemon/src/socket.rs b/crates/colibri-daemon/src/socket.rs
index a748db6..5b93d07 100644
--- a/crates/colibri-daemon/src/socket.rs
+++ b/crates/colibri-daemon/src/socket.rs
@@ -9,6 +9,8 @@
 
 use std::time::SystemTime;
 
+use std::collections::HashMap;
+
 use colibri_glasspane::DEFAULT_STALL_AFTER;
 use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
 use tokio::net::{UnixListener, UnixStream};
@@ -310,11 +312,41 @@ async fn cmd_spawn_agent(
         args,
         provider,
         model,
-        session_id,
+        session_id: session_id.clone(),
         system_prompt,
         ..Default::default()
     };
 
+    // T1.4 PR3a: inject session prompt context as env var when enabled
+    let mut extra_env = HashMap::new();
+    if state.config.scheduler_prompt_injection {
+        if let Some(ref sid) = session_id {
+            if let Some(session) = state.sessions.get(sid) {
+                let assembly = session.value().build_prompt_assembly().await;
+                let cost_mode =
+                    crate::cost::CostMode::parse(&state.config.cost_mode).unwrap_or_default();
+                let _ = assembly; // trimmed context available for injection
+                let context_json =
+                    serde_json::to_string(&assembly.to_messages()).unwrap_or_default();
+                extra_env.insert("COLIBRI_SESSION_CONTEXT".to_string(), context_json);
+                extra_env.insert(
+                    "COLIBRI_COST_MODE".to_string(),
+                    cost_mode.as_str().to_string(),
+                );
+                info!(
+                    session_id = %sid,
+                    context_bytes = assembly.total_bytes,
+                    cost_mode = cost_mode.as_str(),
+                    "injected session prompt context"
+                );
+            }
+        }
+    }
+
+    // Merge extra env into agent config
+    let mut agent_config = agent_config;
+    agent_config.env.extend(extra_env);
+
     let spawner = Spawner::new(state.config.clone().into());
     match spawner.spawn(agent_config).await {
         Ok(handle) => {
@@ -605,6 +637,7 @@ mod tests {
             max_context_tokens: 128_000,
             max_uncompacted_turns: 20,
             cost_mode: "smart".to_string(),
+            scheduler_prompt_injection: true,
         }
     }