diff --git a/crates/application/src/agent/lifecycle.rs b/crates/application/src/agent/lifecycle.rs
index 75b85400..48ea6363 100644
--- a/crates/application/src/agent/lifecycle.rs
+++ b/crates/application/src/agent/lifecycle.rs
@@ -11,7 +11,8 @@
 //! [`AgentRuntime`], [`PtyPort`], [`FileSystem`], [`EventBus`]); none knows about
 //! a concrete adapter or Tauri.
 
-use std::collections::HashMap;
+use std::collections::{BTreeSet, HashMap};
+use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
 use domain::ports::{
@@ -24,7 +25,7 @@ use domain::profile::{
     resolve_effort, EffortSelection, McpConfigStrategy, OpenCodeProviderConfig, StructuredAdapter,
     CODEX_CODE_MODE_FEATURES_TABLE, CODEX_CODE_MODE_FEATURES_TOML,
 };
-use domain::sandbox::{compile_sandbox_plan, SandboxContext, SandboxPlan};
+use domain::sandbox::{compile_sandbox_plan, PathAccess, PathGrant, SandboxContext, SandboxPlan};
 use domain::{
     bound_handoff_summary, Agent, AgentId, AgentManifest, AgentOrigin, AgentProfile,
     ContextInjection, ConversationId, ConversationParty, DomainEvent, EffectivePermissions,
@@ -1339,7 +1340,7 @@ fn build_structured_launch_policy(
     profile: &AgentProfile,
     permissions: Option<&EffectivePermissions>,
     network: Option<NetworkPolicy>,
-    project_root: &ProjectPath,
+    codex_git_writable_roots: &[String],
 ) -> Option<StructuredProviderLaunchPolicy> {
     if !matches!(profile.structured_adapter, Some(StructuredAdapter::Codex)) {
         return None;
@@ -1349,8 +1350,8 @@ fn build_structured_launch_policy(
     }
 
     let sandbox_mode = codex_structured_sandbox_mode(permissions).to_owned();
-    let writable_roots = if sandbox_mode == "workspace-write" && !project_root.as_str().is_empty() {
-        vec![project_root.as_str().to_owned()]
+    let writable_roots = if sandbox_mode == "workspace-write" {
+        codex_git_writable_roots.to_vec()
     } else {
         Vec::new()
     };
@@ -1362,6 +1363,77 @@ fn build_structured_launch_policy(
     })
 }
 
+fn codex_git_writable_roots(project_root: &ProjectPath) -> Vec<String> {
+    git_writable_roots_for_project(Path::new(project_root.as_str()))
+}
+
+fn git_writable_roots_for_project(project_root: &Path) -> Vec<String> {
+    let git_marker = project_root.join(".git");
+    let Some(git_dir) = resolve_git_dir(&git_marker) else {
+        return Vec::new();
+    };
+    let Ok(canonical) = git_dir.canonicalize() else {
+        return Vec::new();
+    };
+    if !canonical.is_dir() {
+        return Vec::new();
+    }
+
+    let mut roots = BTreeSet::new();
+    roots.insert(canonical.to_string_lossy().into_owned());
+    roots.into_iter().collect()
+}
+
+fn resolve_git_dir(git_marker: &Path) -> Option<PathBuf> {
+    if git_marker.is_dir() {
+        return Some(git_marker.to_path_buf());
+    }
+    if !git_marker.is_file() {
+        return None;
+    }
+
+    let contents = std::fs::read_to_string(git_marker).ok()?;
+    let gitdir = contents
+        .lines()
+        .find_map(|line| line.trim().strip_prefix("gitdir:").map(str::trim))?;
+    if gitdir.is_empty() {
+        return None;
+    }
+
+    let path = PathBuf::from(gitdir);
+    if path.is_absolute() {
+        Some(path)
+    } else {
+        git_marker.parent().map(|parent| parent.join(path))
+    }
+}
+
+fn enrich_sandbox_plan_with_codex_git_roots(
+    plan: Option<SandboxPlan>,
+    codex_git_writable_roots: &[String],
+) -> Option<SandboxPlan> {
+    let mut plan = plan?;
+    if codex_git_writable_roots.is_empty() {
+        return Some(plan);
+    }
+
+    for root in codex_git_writable_roots {
+        match plan
+            .allowed
+            .iter_mut()
+            .find(|grant| grant.abs_root == *root)
+        {
+            Some(grant) => grant.access.insert(PathAccess::RW),
+            None => plan.allowed.push(PathGrant {
+                abs_root: root.clone(),
+                access: PathAccess::RW,
+            }),
+        }
+    }
+    plan.allowed.sort_by(|a, b| a.abs_root.cmp(&b.abs_root));
+    Some(plan)
+}
+
 fn projection_model(profile: &AgentProfile) -> Option<&str> {
     profile.model.as_deref()
 }
@@ -2157,6 +2229,12 @@ impl LaunchAgent {
         //     remplir `spec.sandbox`. `eff == None` (rien posé) ⇒ `None` : aucun plan,
         //     comportement natif conservé (invariant produit). L'enforcer concret
         //     (Landlock/Noop) est injecté côté PTY au composition root.
+        let codex_git_writable_roots =
+            if matches!(profile.structured_adapter, Some(StructuredAdapter::Codex)) {
+                codex_git_writable_roots(&input.project.root)
+            } else {
+                Vec::new()
+            };
         spec.sandbox = compile_sandbox_plan(
             effective_permissions.as_ref(),
             &SandboxContext {
@@ -2164,11 +2242,13 @@ impl LaunchAgent {
                 run_dir: run_dir.as_str(),
             },
         );
+        spec.sandbox =
+            enrich_sandbox_plan_with_codex_git_roots(spec.sandbox, &codex_git_writable_roots);
         let structured_policy = build_structured_launch_policy(
             &profile,
             effective_permissions.as_ref(),
             network_permission,
-            &input.project.root,
+            &codex_git_writable_roots,
         );
         let resolved_effort = resolve_effort(
             profile.model_reasoning_effort.as_deref(),
diff --git a/crates/application/tests/agent_lifecycle.rs b/crates/application/tests/agent_lifecycle.rs
index fee85f09..c56199c3 100644
--- a/crates/application/tests/agent_lifecycle.rs
+++ b/crates/application/tests/agent_lifecycle.rs
@@ -16,6 +16,7 @@
 //! `LaunchAgent`: `prepare_invocation` → injection (fs write) → `pty.spawn`.
 
 use std::collections::HashMap;
+use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
 
 use async_trait::async_trait;
@@ -615,6 +616,7 @@ struct FakeStructuredFactory {
     starts: Arc<Mutex<Vec<ProfileId>>>,
     efforts: Arc<Mutex<Vec<Option<String>>>>,
     envs: Arc<Mutex<Vec<Vec<(String, String)>>>>,
+    sandboxes: Arc<Mutex<Vec<Option<domain::sandbox::SandboxPlan>>>>,
     policies: Arc<Mutex<Vec<Option<domain::ports::StructuredProviderLaunchPolicy>>>>,
     shutdowns: Arc<Mutex<Vec<SessionId>>>,
     next_session: SessionId,
@@ -627,6 +629,7 @@ impl FakeStructuredFactory {
             starts: Arc::new(Mutex::new(Vec::new())),
             efforts: Arc::new(Mutex::new(Vec::new())),
             envs: Arc::new(Mutex::new(Vec::new())),
+            sandboxes: Arc::new(Mutex::new(Vec::new())),
             policies: Arc::new(Mutex::new(Vec::new())),
             shutdowns: Arc::new(Mutex::new(Vec::new())),
             next_session,
@@ -645,6 +648,10 @@ impl FakeStructuredFactory {
         self.envs.lock().unwrap().clone()
     }
 
+    fn sandboxes(&self) -> Vec<Option<domain::sandbox::SandboxPlan>> {
+        self.sandboxes.lock().unwrap().clone()
+    }
+
     fn policies(&self) -> Vec<Option<domain::ports::StructuredProviderLaunchPolicy>> {
         self.policies.lock().unwrap().clone()
     }
@@ -677,6 +684,7 @@ impl AgentSessionFactory for FakeStructuredFactory {
             .unwrap()
             .push(profile.model_reasoning_effort.clone());
         self.envs.lock().unwrap().push(_env.to_vec());
+        self.sandboxes.lock().unwrap().push(_sandbox.cloned());
         self.policies
             .lock()
             .unwrap()
@@ -738,11 +746,21 @@ fn sid(n: u128) -> SessionId {
     SessionId::from_uuid(Uuid::from_u128(n))
 }
 
+fn fresh_temp_dir(label: &str) -> PathBuf {
+    let dir = std::env::temp_dir().join(format!("idea-agent-lifecycle-{label}-{}", Uuid::new_v4()));
+    std::fs::create_dir_all(&dir).unwrap();
+    dir
+}
+
 fn project() -> Project {
+    project_at("/home/me/proj")
+}
+
+fn project_at(root: &str) -> Project {
     Project::new(
         ProjectId::from_uuid(Uuid::from_u128(1000)),
         "demo",
-        ProjectPath::new("/home/me/proj").unwrap(),
+        ProjectPath::new(root).unwrap(),
         RemoteRef::local(),
         1_700_000_000_000,
     )
@@ -1183,8 +1201,16 @@ fn launch_fixture_with_profile_agent_and_recall(
 }
 
 fn launch_input(agent_id: AgentId) -> LaunchAgentInput {
+    launch_input_with_project(agent_id, project())
+}
+
+fn launch_input_with_project_root(agent_id: AgentId, root: &Path) -> LaunchAgentInput {
+    launch_input_with_project(agent_id, project_at(&root.to_string_lossy()))
+}
+
+fn launch_input_with_project(agent_id: AgentId, project: Project) -> LaunchAgentInput {
     LaunchAgentInput {
-        project: project(),
+        project,
         agent_id,
         rows: 24,
         cols: 80,
@@ -4048,6 +4074,174 @@ async fn codex_structured_policy_carries_exec_safe_sandbox_roots_and_network() {
     );
 }
 
+async fn launch_codex_structured_for_project(
+    project_root: &Path,
+) -> (
+    FakeStructuredFactory,
+    Vec<Option<domain::ports::StructuredProviderLaunchPolicy>>,
+    Vec<Option<domain::sandbox::SandboxPlan>>,
+) {
+    let profile = codex_profile()
+        .with_projector(ProjectorKey::Codex)
+        .with_structured_adapter(StructuredAdapter::Codex);
+    let (mut launch, agent, _fs, pty, _s) = launch_with_projection(
+        profile,
+        Some(ContextInjectionPlan::File {
+            target: "AGENTS.md".to_owned(),
+        }),
+        Some(full_registry()),
+        Some(perm_doc(Posture::Ask)),
+    );
+    let factory = FakeStructuredFactory::new(trace(), sid(890));
+    let structured = Arc::new(StructuredSessions::new());
+    launch = launch
+        .with_structured_routing_mode(StructuredRoutingMode::RequireStructured)
+        .with_structured(Arc::new(factory.clone()), Arc::clone(&structured));
+
+    launch
+        .execute(launch_input_with_project_root(agent.id, project_root))
+        .await
+        .expect("structured launch");
+
+    assert!(
+        pty.spawns().is_empty(),
+        "structured route must not spawn PTY"
+    );
+    (factory.clone(), factory.policies(), factory.sandboxes())
+}
+
+fn assert_codex_git_roots(
+    project_root: &Path,
+    policies: Vec<Option<domain::ports::StructuredProviderLaunchPolicy>>,
+    sandboxes: Vec<Option<domain::sandbox::SandboxPlan>>,
+    expected_roots: Vec<String>,
+) {
+    assert_eq!(policies.len(), 1, "exactly one structured launch policy");
+    let Some(domain::ports::StructuredProviderLaunchPolicy::Codex {
+        sandbox_mode,
+        writable_roots,
+        network_access: _,
+    }) = policies.into_iter().next().unwrap()
+    else {
+        panic!("expected a Codex structured launch policy");
+    };
+    assert_eq!(sandbox_mode, "workspace-write");
+    assert_eq!(
+        writable_roots, expected_roots,
+        "Codex writable_roots must be the resolved Git roots only"
+    );
+    assert!(
+        !writable_roots.contains(&project_root.to_string_lossy().into_owned()),
+        "project root must not be granted as a Codex writable root"
+    );
+
+    assert_eq!(sandboxes.len(), 1, "exactly one structured sandbox plan");
+    let sandbox = sandboxes.into_iter().next().unwrap().expect("sandbox plan");
+    let sandbox_roots = sandbox
+        .allowed
+        .iter()
+        .map(|grant| grant.abs_root.clone())
+        .collect::<Vec<_>>();
+    assert_eq!(
+        sandbox_roots, writable_roots,
+        "OS sandbox Git enrichment must match Codex writable_roots"
+    );
+    assert!(
+        sandbox
+            .allowed
+            .iter()
+            .all(|grant| grant.access == domain::sandbox::PathAccess::RW),
+        "Git roots must be added as write grants only"
+    );
+}
+
+#[tokio::test]
+async fn codex_git_enrichment_adds_standard_git_dir_without_project_root_blanket() {
+    let root = fresh_temp_dir("standard-git");
+    std::fs::create_dir(root.join(".git")).unwrap();
+    let expected_git = root.join(".git").canonicalize().unwrap();
+
+    let (_factory, policies, sandboxes) = launch_codex_structured_for_project(&root).await;
+
+    assert_codex_git_roots(
+        &root,
+        policies,
+        sandboxes,
+        vec![expected_git.to_string_lossy().into_owned()],
+    );
+    let _ = std::fs::remove_dir_all(&root);
+}
+
+#[tokio::test]
+async fn codex_git_enrichment_adds_no_roots_for_non_git_project() {
+    let root = fresh_temp_dir("non-git");
+
+    let (_factory, policies, sandboxes) = launch_codex_structured_for_project(&root).await;
+
+    assert_codex_git_roots(&root, policies, sandboxes, Vec::new());
+    let _ = std::fs::remove_dir_all(&root);
+}
+
+#[tokio::test]
+async fn codex_git_enrichment_resolves_worktree_gitdir_file() {
+    let base = fresh_temp_dir("worktree");
+    let root = base.join("worktree");
+    let git_dir = base.join("common").join("worktrees").join("wt");
+    std::fs::create_dir_all(&root).unwrap();
+    std::fs::create_dir_all(&git_dir).unwrap();
+    std::fs::write(root.join(".git"), "gitdir: ../common/worktrees/wt\n").unwrap();
+    let expected_git = git_dir.canonicalize().unwrap();
+
+    let (_factory, policies, sandboxes) = launch_codex_structured_for_project(&root).await;
+
+    assert_codex_git_roots(
+        &root,
+        policies,
+        sandboxes,
+        vec![expected_git.to_string_lossy().into_owned()],
+    );
+    let _ = std::fs::remove_dir_all(&base);
+}
+
+#[tokio::test]
+async fn codex_git_enrichment_canonicalizes_and_avoids_parent_grants() {
+    let base = fresh_temp_dir("canonical");
+    let real_root = base.join("real");
+    let linked_root = base.join("linked");
+    std::fs::create_dir_all(real_root.join(".git")).unwrap();
+    #[cfg(unix)]
+    std::os::unix::fs::symlink(&real_root, &linked_root).unwrap();
+    #[cfg(not(unix))]
+    std::fs::create_dir_all(&linked_root).unwrap();
+
+    let launch_root = if cfg!(unix) { &linked_root } else { &real_root };
+    let expected_git = real_root.join(".git").canonicalize().unwrap();
+
+    let (_factory, policies, sandboxes) = launch_codex_structured_for_project(launch_root).await;
+
+    assert_codex_git_roots(
+        launch_root,
+        policies.clone(),
+        sandboxes,
+        vec![expected_git.to_string_lossy().into_owned()],
+    );
+    let Some(domain::ports::StructuredProviderLaunchPolicy::Codex { writable_roots, .. }) =
+        policies.into_iter().next().unwrap()
+    else {
+        panic!("expected Codex policy");
+    };
+    assert_eq!(writable_roots.len(), 1, "Git roots must be deduplicated");
+    assert!(
+        !writable_roots.contains(&base.to_string_lossy().into_owned()),
+        "common parent must not be granted"
+    );
+    assert!(
+        !writable_roots.contains(&real_root.to_string_lossy().into_owned()),
+        "repo parent must not be granted"
+    );
+    let _ = std::fs::remove_dir_all(&base);
+}
+
 /// (1d) A non-projectable profile (no projector, no CLAUDE.md, no Codex signal) ⇒
 /// no projection at all, even with a full registry and a posed policy.
 #[tokio::test]
diff --git a/crates/infrastructure/src/session/codex.rs b/crates/infrastructure/src/session/codex.rs
index 38d1e0d4..85cdbe3a 100644
--- a/crates/infrastructure/src/session/codex.rs
+++ b/crates/infrastructure/src/session/codex.rs
@@ -451,18 +451,18 @@ impl CodexExecSession {
     ///
     /// Format RÉEL vérifié 2026-06-10 (codex 0.137.0) :
     /// - Conversation neuve : `codex exec --json --skip-git-repo-check
-    ///   --sandbox workspace-write --add-dir <project-root> <prompt>`.
+    ///   --sandbox workspace-write [--add-dir <explicit-root>...] <prompt>`.
     /// - Reprise (id connu) : `codex exec --json
-    ///   --skip-git-repo-check --sandbox workspace-write --add-dir <project-root> resume
-    ///   <thread_id> <prompt>`.
+    ///   --skip-git-repo-check --sandbox workspace-write [--add-dir <explicit-root>...]
+    ///   resume <thread_id> <prompt>`.
     ///
     /// **Autonomie d'écriture (D3)** : `--sandbox workspace-write` autorise l'agent à
     /// écrire dans son workspace. `codex exec` est déjà non-interactif (aucun prompt
     /// d'approbation possible), donc on ne passe **pas** `--ask-for-approval` : ce flag
     /// appartient à la commande interactive `codex`, pas à la sous-commande `exec` qui
     /// sort sur `error: unexpected argument '--ask-for-approval' found`. Comme l'agent
-    /// tourne depuis son run dir isolé, `--add-dir` expose explicitement le project root
-    /// à la sandbox Codex pour que les écritures Git touchent le vrai workspace.
+    /// tourne depuis son run dir isolé, `--add-dir` expose uniquement les roots
+    /// writable explicitement fournis par la launch policy.
     fn build_spawn_line(&self, prompt: &str) -> SpawnLine {
         let mut args = vec!["exec".to_owned()];
         let conversation_id = self.conversation_id.lock().expect("mutex sain").clone();
diff --git a/crates/infrastructure/src/session/factory.rs b/crates/infrastructure/src/session/factory.rs
index 69e257e3..fec3e547 100644
--- a/crates/infrastructure/src/session/factory.rs
+++ b/crates/infrastructure/src/session/factory.rs
@@ -189,11 +189,7 @@ impl AgentSessionFactory for StructuredSessionFactory {
                         writable_roots.clone(),
                         Some(*network_access),
                     ),
-                    None => (
-                        "workspace-write".to_owned(),
-                        vec![ctx.project_root.clone()],
-                        None,
-                    ),
+                    None => ("workspace-write".to_owned(), Vec::new(), None),
                 };
                 Arc::new(CodexExecSession::new_with_policy_and_overrides(
                     id,
diff --git a/crates/infrastructure/src/session/mod.rs b/crates/infrastructure/src/session/mod.rs
index fd75138f..045b36ee 100644
--- a/crates/infrastructure/src/session/mod.rs
+++ b/crates/infrastructure/src/session/mod.rs
@@ -647,7 +647,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn factory_passes_project_root_to_codex_add_dir() {
+    async fn factory_does_not_pass_project_root_to_codex_add_dir_without_policy() {
         let factory = StructuredSessionFactory::new();
         let (cmd, argv) = make_recording_fake(&[
             r#"{"type":"thread.started","thread_id":"cx-new"}"#,
@@ -679,8 +679,12 @@ mod tests {
         let recorded = std::fs::read_to_string(&argv).expect("argv");
         let args: Vec<&str> = recorded.lines().collect();
         assert!(
-            args.windows(2).any(|w| w == ["--add-dir", "/project/root"]),
-            "factory must relay PreparedContext.project_root to Codex --add-dir, got: {args:?}"
+            !args.windows(2).any(|w| w == ["--add-dir", "/project/root"]),
+            "factory must not invent --add-dir project root without an explicit policy, got: {args:?}"
+        );
+        assert!(
+            !args.contains(&"--add-dir"),
+            "default Codex adapter launch must not carry any implicit --add-dir, got: {args:?}"
         );
         assert!(
             !args.contains(&"--ask-for-approval"),