diff --git a/crates/codegen/kigi-config-types/src/lib.rs b/crates/codegen/kigi-config-types/src/lib.rs
index 31b5b90..9d0b948 100644
--- a/crates/codegen/kigi-config-types/src/lib.rs
+++ b/crates/codegen/kigi-config-types/src/lib.rs
@@ -668,6 +668,11 @@ pub struct RemoteSettings {
     /// remote settings verbatim-input flag; env (`KIGI_COMPACTION_VERBATIM_INPUT`) and config override it. `None` = default (true).
     #[serde(default)]
     pub compaction_verbatim_input: Option<bool>,
+    /// Compaction summarizer `tool_choice` (`auto` | `none`) from remote
+    /// settings. Env (`KIGI_COMPACTION_TOOL_CHOICE`) and config override it.
+    /// Some backends reject `tool_choice: "none"` with tools attached.
+    #[serde(default)]
+    pub compaction_tool_choice: Option<String>,
     /// Master switch for jemalloc heap sampling + threshold dumps.
     /// `Some(true)` enables, `Some(false)` kill-switch, `None` = client default off.
     #[serde(default)]
diff --git a/crates/codegen/kigi-sampling-types/src/error.rs b/crates/codegen/kigi-sampling-types/src/error.rs
index 591da18..8c0441b 100644
--- a/crates/codegen/kigi-sampling-types/src/error.rs
+++ b/crates/codegen/kigi-sampling-types/src/error.rs
@@ -352,6 +352,7 @@ pub fn is_context_length_error(message: &str) -> bool {
         || m.contains("maximum prompt length")
         || m.contains("maximum context length")
         || m.contains("context_length_exceeded")
+        || (m.contains("current message") && m.contains("exceeds budget"))
 }
 
 /// Decide whether a [`reqwest::Error`] is worth retrying.
@@ -387,6 +388,7 @@ mod tests {
             "This model's maximum context length is 200000 tokens",
             "invalid_request_error: prompt is too long: 300000 tokens > 200000 maximum",
             "error type: context_length_exceeded",
+            "the current message exceeds budget: 120000 < 250000",
         ] {
             assert!(is_context_length_error(msg), "should match: {msg}");
         }
diff --git a/crates/codegen/kigi-shell/src/agent/config.rs b/crates/codegen/kigi-shell/src/agent/config.rs
index 2b5396c..2311f8f 100644
--- a/crates/codegen/kigi-shell/src/agent/config.rs
+++ b/crates/codegen/kigi-shell/src/agent/config.rs
@@ -2171,6 +2171,19 @@ impl Config {
         )
         .with_segment_detail(self.resolve_compaction_detail())
     }
+    /// Resolve summarizer `tool_choice`: env `KIGI_COMPACTION_TOOL_CHOICE` >
+    /// config > remote settings > default (`auto`).
+    pub(crate) fn resolve_compaction_tool_choice(
+        &self,
+    ) -> crate::util::config::CompactionToolChoice {
+        crate::util::config::resolve_compaction_tool_choice_from(
+            env_string(crate::util::config::ENV_COMPACTION_TOOL_CHOICE).as_deref(),
+            self.features.compaction_tool_choice.as_deref(),
+            self.remote_settings
+                .as_ref()
+                .and_then(|r| r.compaction_tool_choice.as_deref()),
+        )
+    }
     /// Resolve verbatim-input flag: env `KIGI_COMPACTION_VERBATIM_INPUT` > config > remote settings > default `true`.
     pub(crate) fn resolve_compaction_verbatim_input(&self) -> bool {
         BoolFlag::env("KIGI_COMPACTION_VERBATIM_INPUT")
@@ -3696,6 +3709,9 @@ pub struct Features {
     /// Feed the summarizer the verbatim conversation instead of the lossy rewrite; `None` = defer to env/remote settings/default (true).
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub compaction_verbatim_input: Option<bool>,
+    /// Summarizer `tool_choice` (`auto` | `none`); `None` = defer to env/remote settings/default (`auto`).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub compaction_tool_choice: Option<String>,
     /// Snapshot a completed subagent's isolated worktree into a durable git ref
     /// and delete its directory (resume rehydrates from the ref). This is the
     /// per-deployment rollout lever (set in managed_config.toml `[features]`).
diff --git a/crates/codegen/kigi-shell/src/agent/mvp_agent/agent_ops.rs b/crates/codegen/kigi-shell/src/agent/mvp_agent/agent_ops.rs
index 5893122..4057fc7 100644
--- a/crates/codegen/kigi-shell/src/agent/mvp_agent/agent_ops.rs
+++ b/crates/codegen/kigi-shell/src/agent/mvp_agent/agent_ops.rs
@@ -2183,6 +2183,7 @@ impl MvpAgent {
             .cfg
             .borrow()
             .resolve_compaction_verbatim_input();
+        let compaction_tool_choice = self.cfg.borrow().resolve_compaction_tool_choice();
         let two_pass_enabled = self.cfg.borrow().is_two_pass_compaction_enabled();
         let auto_update = self.cfg.borrow().cli.auto_update;
         let client_type = *self.client_type.borrow();
@@ -2568,6 +2569,7 @@ impl MvpAgent {
                     system_prompt_label,
                     compaction_mode,
                     compaction_verbatim_input,
+                    compaction_tool_choice,
                     two_pass_enabled,
                     buffering_settings,
                     origin_client.clone(),
diff --git a/crates/codegen/kigi-shell/src/agent/subagent/handle_request.rs b/crates/codegen/kigi-shell/src/agent/subagent/handle_request.rs
index e82b4db..177b226 100644
--- a/crates/codegen/kigi-shell/src/agent/subagent/handle_request.rs
+++ b/crates/codegen/kigi-shell/src/agent/subagent/handle_request.rs
@@ -1006,6 +1006,7 @@ pub(crate) async fn handle_subagent_request(
             kigi_agent::DEFAULT_SYSTEM_PROMPT_LABEL.to_string(),
             kigi_chat_state::CompactionMode::Summary,
             ctx.resolve_compaction_verbatim_input(),
+            ctx.resolve_compaction_tool_choice(),
             false,
             None,
             None,
diff --git a/crates/codegen/kigi-shell/src/agent/subagent/mod.rs b/crates/codegen/kigi-shell/src/agent/subagent/mod.rs
index 01c5505..1943943 100644
--- a/crates/codegen/kigi-shell/src/agent/subagent/mod.rs
+++ b/crates/codegen/kigi-shell/src/agent/subagent/mod.rs
@@ -397,6 +397,21 @@ impl SubagentSpawnContext {
             .resolve()
             .value
     }
+    /// Subagent summarizer `tool_choice`, mirroring
+    /// `Config::resolve_compaction_tool_choice` (env > config > remote
+    /// settings > default `auto`).
+    pub fn resolve_compaction_tool_choice(&self) -> crate::util::config::CompactionToolChoice {
+        crate::util::config::resolve_compaction_tool_choice_from(
+            crate::agent::config::env_string(crate::util::config::ENV_COMPACTION_TOOL_CHOICE)
+                .as_deref(),
+            self.agent_config
+                .as_ref()
+                .and_then(|c| c.features.compaction_tool_choice.as_deref()),
+            self.remote_settings
+                .as_ref()
+                .and_then(|r| r.compaction_tool_choice.as_deref()),
+        )
+    }
     /// Whether a completed subagent's worktree is snapshotted into a durable ref
     /// and its directory deleted. Resolution mirrors the other subagent gates
     /// (env > config > remote settings > default). Default `false` so it ships dark;
diff --git a/crates/codegen/kigi-shell/src/session/acp_session_impl/spawn.rs b/crates/codegen/kigi-shell/src/session/acp_session_impl/spawn.rs
index 1d3b103..ece7898 100644
--- a/crates/codegen/kigi-shell/src/session/acp_session_impl/spawn.rs
+++ b/crates/codegen/kigi-shell/src/session/acp_session_impl/spawn.rs
@@ -112,6 +112,7 @@ pub(crate) async fn spawn_session_actor(
     system_prompt_label: String,
     compaction_mode: kigi_chat_state::CompactionMode,
     compaction_verbatim_input: bool,
+    compaction_tool_choice: crate::util::config::CompactionToolChoice,
     two_pass_enabled: bool,
     buffering_settings: Option<BufferingSettings>,
     origin_client: Option<crate::http::OriginClientInfo>,
@@ -1032,6 +1033,7 @@ pub(crate) async fn spawn_session_actor(
             previous_model: std::cell::Cell::new(None),
             compaction_mode,
             verbatim_input: compaction_verbatim_input,
+            tool_choice: compaction_tool_choice,
             prefire: crate::session::compaction_config::PrefireState::default(),
             prefix_released: std::sync::atomic::AtomicBool::new(false),
         },
@@ -1535,6 +1537,7 @@ pub(crate) async fn spawn_session_on_thread(
     system_prompt_label: String,
     compaction_mode: kigi_chat_state::CompactionMode,
     compaction_verbatim_input: bool,
+    compaction_tool_choice: crate::util::config::CompactionToolChoice,
     two_pass_enabled: bool,
     buffering_settings: Option<BufferingSettings>,
     origin_client: Option<crate::http::OriginClientInfo>,
@@ -1679,6 +1682,7 @@ pub(crate) async fn spawn_session_on_thread(
                     system_prompt_label,
                     compaction_mode,
                     compaction_verbatim_input,
+                    compaction_tool_choice,
                     two_pass_enabled,
                     buffering_settings,
                     origin_client,
diff --git a/crates/codegen/kigi-shell/src/session/acp_session_tests/cancel_running_task_tests.rs b/crates/codegen/kigi-shell/src/session/acp_session_tests/cancel_running_task_tests.rs
index ce5f952..be6e267 100644
--- a/crates/codegen/kigi-shell/src/session/acp_session_tests/cancel_running_task_tests.rs
+++ b/crates/codegen/kigi-shell/src/session/acp_session_tests/cancel_running_task_tests.rs
@@ -157,6 +157,7 @@ async fn persist_ack_waits_for_disk_flush_before_success() {
                     previous_model: std::cell::Cell::new(None),
                     compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                     verbatim_input: true,
+                    tool_choice: Default::default(),
                     prefire: crate::session::compaction_config::PrefireState::default(),
                     prefix_released: std::sync::atomic::AtomicBool::new(false),
                 },
@@ -611,6 +612,7 @@ async fn first_turn_memory_injection_disabled_does_not_persist_to_chat_history()
                     previous_model: std::cell::Cell::new(None),
                     compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                     verbatim_input: true,
+                    tool_choice: Default::default(),
                     prefire: crate::session::compaction_config::PrefireState::default(),
                     prefix_released: std::sync::atomic::AtomicBool::new(false),
                 },
@@ -871,6 +873,7 @@ async fn cancel_running_task_teardown_clears_running_and_pending_work() {
                     previous_model: std::cell::Cell::new(None),
                     compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                     verbatim_input: true,
+                    tool_choice: Default::default(),
                     prefire: crate::session::compaction_config::PrefireState::default(),
                     prefix_released: std::sync::atomic::AtomicBool::new(false),
                 },
@@ -1865,6 +1868,7 @@ async fn cancel_propagates_to_sampler_handle_so_no_further_emission() {
                     previous_model: std::cell::Cell::new(None),
                     compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                     verbatim_input: true,
+                    tool_choice: Default::default(),
                     prefire: crate::session::compaction_config::PrefireState::default(),
                     prefix_released: std::sync::atomic::AtomicBool::new(false),
                 },
diff --git a/crates/codegen/kigi-shell/src/session/acp_session_tests/idle_resume_tests.rs b/crates/codegen/kigi-shell/src/session/acp_session_tests/idle_resume_tests.rs
index 048867e..6efe802 100644
--- a/crates/codegen/kigi-shell/src/session/acp_session_tests/idle_resume_tests.rs
+++ b/crates/codegen/kigi-shell/src/session/acp_session_tests/idle_resume_tests.rs
@@ -176,6 +176,7 @@ async fn test_e2e_idle_resume_refreshes_model_metadata() {
                     previous_model: std::cell::Cell::new(None),
                     compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                     verbatim_input: true,
+                    tool_choice: Default::default(),
                     prefire: crate::session::compaction_config::PrefireState::default(),
                     prefix_released: std::sync::atomic::AtomicBool::new(false),
                 },
diff --git a/crates/codegen/kigi-shell/src/session/acp_session_tests/inline_auto_compact_flow_tests.rs b/crates/codegen/kigi-shell/src/session/acp_session_tests/inline_auto_compact_flow_tests.rs
index b063a1a..50911b7 100644
--- a/crates/codegen/kigi-shell/src/session/acp_session_tests/inline_auto_compact_flow_tests.rs
+++ b/crates/codegen/kigi-shell/src/session/acp_session_tests/inline_auto_compact_flow_tests.rs
@@ -112,6 +112,7 @@ async fn create_test_actor(
             previous_model: std::cell::Cell::new(None),
             compaction_mode: kigi_chat_state::CompactionMode::Transcript,
             verbatim_input: true,
+            tool_choice: Default::default(),
             prefire: crate::session::compaction_config::PrefireState::default(),
             prefix_released: std::sync::atomic::AtomicBool::new(false),
         },
@@ -539,6 +540,7 @@ async fn create_test_actor_with_memory(
             previous_model: std::cell::Cell::new(None),
             compaction_mode: kigi_chat_state::CompactionMode::Transcript,
             verbatim_input: true,
+            tool_choice: Default::default(),
             prefire: crate::session::compaction_config::PrefireState::default(),
             prefix_released: std::sync::atomic::AtomicBool::new(false),
         },
@@ -1298,6 +1300,7 @@ async fn test_e2e_idle_resume_refreshes_model_metadata() {
                     previous_model: std::cell::Cell::new(None),
                     compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                     verbatim_input: true,
+                    tool_choice: Default::default(),
                     prefire: crate::session::compaction_config::PrefireState::default(),
                     prefix_released: std::sync::atomic::AtomicBool::new(false),
                 },
diff --git a/crates/codegen/kigi-shell/src/session/acp_session_tests/memory_config_tests.rs b/crates/codegen/kigi-shell/src/session/acp_session_tests/memory_config_tests.rs
index 87ef79c..28549e8 100644
--- a/crates/codegen/kigi-shell/src/session/acp_session_tests/memory_config_tests.rs
+++ b/crates/codegen/kigi-shell/src/session/acp_session_tests/memory_config_tests.rs
@@ -163,6 +163,7 @@ async fn create_test_actor_with_memory(
             previous_model: std::cell::Cell::new(None),
             compaction_mode: kigi_chat_state::CompactionMode::Transcript,
             verbatim_input: true,
+            tool_choice: Default::default(),
             prefire: crate::session::compaction_config::PrefireState::default(),
             prefix_released: std::sync::atomic::AtomicBool::new(false),
         },
diff --git a/crates/codegen/kigi-shell/src/session/acp_session_tests/replay_buffer_send_update_tests.rs b/crates/codegen/kigi-shell/src/session/acp_session_tests/replay_buffer_send_update_tests.rs
index 28cd311..7abbeab 100644
--- a/crates/codegen/kigi-shell/src/session/acp_session_tests/replay_buffer_send_update_tests.rs
+++ b/crates/codegen/kigi-shell/src/session/acp_session_tests/replay_buffer_send_update_tests.rs
@@ -116,6 +116,7 @@ pub(super) async fn make_replay_send_update_fixture() -> ReplaySendUpdateFixture
             previous_model: std::cell::Cell::new(None),
             compaction_mode: kigi_chat_state::CompactionMode::Transcript,
             verbatim_input: true,
+            tool_choice: Default::default(),
             prefire: crate::session::compaction_config::PrefireState::default(),
             prefix_released: std::sync::atomic::AtomicBool::new(false),
         },
diff --git a/crates/codegen/kigi-shell/src/session/acp_session_tests/support.rs b/crates/codegen/kigi-shell/src/session/acp_session_tests/support.rs
index 536f5af..5fb8ef7 100644
--- a/crates/codegen/kigi-shell/src/session/acp_session_tests/support.rs
+++ b/crates/codegen/kigi-shell/src/session/acp_session_tests/support.rs
@@ -226,6 +226,7 @@ pub(crate) async fn create_test_actor_ex(
             previous_model: std::cell::Cell::new(None),
             compaction_mode: kigi_chat_state::CompactionMode::Transcript,
             verbatim_input: true,
+            tool_choice: Default::default(),
             prefire: crate::session::compaction_config::PrefireState::default(),
             prefix_released: std::sync::atomic::AtomicBool::new(false),
         },
diff --git a/crates/codegen/kigi-shell/src/session/compaction.rs b/crates/codegen/kigi-shell/src/session/compaction.rs
index fc71ad6..6226160 100644
--- a/crates/codegen/kigi-shell/src/session/compaction.rs
+++ b/crates/codegen/kigi-shell/src/session/compaction.rs
@@ -212,6 +212,7 @@ impl SessionActor {
             &sampling_config,
             self.inference_idle_timeout,
             wall_clock_budget_secs,
+            self.compaction.tool_choice,
         )
         .await
         {
@@ -983,6 +984,7 @@ impl SessionActor {
             sampling_config.clone(),
             self.inference_idle_timeout,
             wall_clock_budget_secs,
+            self.compaction.tool_choice,
         );
         let observer =
             crate::session::helpers::full_replace_compaction::ShellFullReplaceObserver::new(
@@ -2198,6 +2200,7 @@ mod inline_auto_compact_flow_tests {
                 previous_model: std::cell::Cell::new(None),
                 compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                 verbatim_input: true,
+                tool_choice: Default::default(),
                 prefire: crate::session::compaction_config::PrefireState::default(),
                 prefix_released: std::sync::atomic::AtomicBool::new(false),
             },
diff --git a/crates/codegen/kigi-shell/src/session/compaction_config.rs b/crates/codegen/kigi-shell/src/session/compaction_config.rs
index 74773e1..cd2325c 100644
--- a/crates/codegen/kigi-shell/src/session/compaction_config.rs
+++ b/crates/codegen/kigi-shell/src/session/compaction_config.rs
@@ -146,6 +146,9 @@ pub struct CompactionConfig {
     pub compaction_mode: kigi_chat_state::CompactionMode,
     /// When `true`, feed the summarizer the verbatim conversation instead of the lossy rewrite (the retry loop may still fall back).
     pub verbatim_input: bool,
+    /// `tool_choice` for summarizer requests with tools attached (`auto` |
+    /// `none`); some backends reject `"none"`.
+    pub tool_choice: crate::util::config::CompactionToolChoice,
     /// Prefire two-pass state (background NOTE₁ cache + in-flight guard).
     /// `Default` (empty cache, not in-flight).
     pub prefire: PrefireState,
diff --git a/crates/codegen/kigi-shell/src/session/helpers/full_replace_compaction.rs b/crates/codegen/kigi-shell/src/session/helpers/full_replace_compaction.rs
index 2874735..8e78357 100644
--- a/crates/codegen/kigi-shell/src/session/helpers/full_replace_compaction.rs
+++ b/crates/codegen/kigi-shell/src/session/helpers/full_replace_compaction.rs
@@ -70,6 +70,8 @@ pub(crate) struct ShellCompactionSampler {
     /// Wall-clock budget (secs) forwarded to `generate_session_compact` as the
     /// reasoning-runaway backstop; `0` disables it.
     wall_clock_budget_secs: u64,
+    /// Summarizer `tool_choice` forwarded to `generate_session_compact`.
+    tool_choice: crate::util::config::CompactionToolChoice,
     /// Full output of the most recent successful sample (for L5 telemetry).
     last_success: Mutex<Option<CompactOutput>>,
 }
@@ -86,6 +88,7 @@ impl ShellCompactionSampler {
         sampling_config: SamplingConfig,
         idle_timeout: Duration,
         wall_clock_budget_secs: u64,
+        tool_choice: crate::util::config::CompactionToolChoice,
     ) -> Self {
         Self {
             use_short_prompt,
@@ -97,6 +100,7 @@ impl ShellCompactionSampler {
             sampling_config,
             idle_timeout,
             wall_clock_budget_secs,
+            tool_choice,
             last_success: Mutex::new(None),
         }
     }
@@ -135,6 +139,7 @@ impl CompactionSampler for ShellCompactionSampler {
             &self.sampling_config,
             self.idle_timeout,
             self.wall_clock_budget_secs,
+            self.tool_choice,
         )
         .await
         {
diff --git a/crates/codegen/kigi-shell/src/session/helpers/session_compact.rs b/crates/codegen/kigi-shell/src/session/helpers/session_compact.rs
index b57e185..4278034 100644
--- a/crates/codegen/kigi-shell/src/session/helpers/session_compact.rs
+++ b/crates/codegen/kigi-shell/src/session/helpers/session_compact.rs
@@ -339,9 +339,10 @@ where
 /// prefix and force a full prefill on the summarizer call — attaching them
 /// keeps the request prefix byte-identical to the turn requests so the
 /// engine reuses the session's KV cache (the whole point of the verbatim
-/// input path). Tool *use* is forbidden via `tool_choice: none` where the
-/// backend can express it (ChatCompletions, Responses); the Messages wire
-/// enum has no `none`, so that path relies on the prompt instruction alone.
+/// input path). Tool *use* is forbidden by the prompt instruction;
+/// `tool_choice` says what goes on the wire next to the attached tools
+/// (ChatCompletions, Responses — some servers reject `"none"`, hence the
+/// `auto` default). The Messages wire enum has no `none` either way.
 ///
 /// Errors carry a [`CompactFailure`] classification so the caller can
 /// short-circuit retries on deterministic failures (4xx schema violations,
@@ -356,8 +357,17 @@ pub(crate) async fn generate_session_compact(
     sampling_config: &SamplingConfig,
     idle_timeout: std::time::Duration,
     wall_clock_budget_secs: u64,
+    tool_choice: crate::util::config::CompactionToolChoice,
 ) -> Result<CompactOutput, CompactFailure> {
     let num_messages = chat_history.len();
+    let wire_tool_choice = match tool_choice {
+        crate::util::config::CompactionToolChoice::Auto => ToolChoice::auto(),
+        crate::util::config::CompactionToolChoice::None => ToolChoice::none(),
+    };
+    let conversation_tool_choice = match tool_choice {
+        crate::util::config::CompactionToolChoice::Auto => ConversationToolChoice::Auto,
+        crate::util::config::CompactionToolChoice::None => ConversationToolChoice::None,
+    };
     let output = match sampling_config.api_backend {
         ApiBackend::ChatCompletions => {
             let chat_messages: Vec<ChatRequestMessage> =
@@ -373,7 +383,7 @@ pub(crate) async fn generate_session_compact(
                             .map(|t| ToolDefinition::function(t.name, t.description, t.parameters))
                             .collect(),
                     )
-                    .with_tool_choice(ToolChoice::none());
+                    .with_tool_choice(wire_tool_choice);
             }
             let sid = session_id.to_string();
             message.x_kigi_conv_id = Some(sid.clone());
@@ -466,7 +476,10 @@ pub(crate) async fn generate_session_compact(
         ApiBackend::Responses => {
             let request = ConversationRequest {
                 items: chat_history,
-                tool_choice: (!tools.is_empty()).then_some(ConversationToolChoice::None),
+                // Hosted tools count: a hosted-only request with the choice
+                // omitted would leave the hosted tool callable mid-compact.
+                tool_choice: (!tools.is_empty() || !hosted_tools.is_empty())
+                    .then_some(conversation_tool_choice),
                 tools,
                 hosted_tools,
                 model: Some(sampling_config.model.to_owned()),
@@ -1573,6 +1586,7 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_secs(30),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await
         .unwrap_or_else(|_| panic!("compaction must succeed"));
@@ -1661,6 +1675,7 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_secs(30),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await;
         let output = result
@@ -1669,11 +1684,11 @@ mod reasoning_compaction_regression_tests {
         let _ = shutdown_tx.send(());
     }
     /// The compaction request must carry the turn loop's tool definitions
-    /// (prompt-prefix/KV-cache alignment) with `tool_choice: "none"`, and
-    /// must omit both keys when no tools are passed (Chat Completions rejects a bare
-    /// `tool_choice`).
+    /// (prompt-prefix/KV-cache alignment) with the configured `tool_choice`
+    /// (`auto` default; `none` when configured), and must omit both keys when
+    /// no tools are passed (Chat Completions rejects a bare `tool_choice`).
     #[tokio::test]
-    async fn chat_completions_compaction_attaches_tools_with_tool_choice_none() {
+    async fn chat_completions_compaction_attaches_tools_with_configured_tool_choice() {
         use std::sync::{Arc, Mutex};
         let captured: Arc<Mutex<Vec<serde_json::Value>>> = Arc::new(Mutex::new(Vec::new()));
         let cap = captured.clone();
@@ -1726,12 +1741,13 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_secs(30),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await
         .unwrap_or_else(|_| panic!("compaction with tools must succeed"));
         let client = Client::new(config.clone()).unwrap();
         generate_session_compact(
-            chat_history,
+            chat_history.clone(),
             vec![],
             vec![],
             client,
@@ -1739,16 +1755,36 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_secs(30),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await
         .unwrap_or_else(|_| panic!("compaction without tools must succeed"));
+        let tools = vec![ToolSpec {
+            name: "read_file".to_string(),
+            description: Some("Reads a file".to_string()),
+            parameters: json!({ "type" : "object", "properties" : {} }),
+        }];
+        let client = Client::new(config.clone()).unwrap();
+        generate_session_compact(
+            chat_history,
+            tools,
+            vec![],
+            client,
+            acp::SessionId::new("test-session"),
+            &config,
+            std::time::Duration::from_secs(30),
+            0,
+            crate::util::config::CompactionToolChoice::None,
+        )
+        .await
+        .unwrap_or_else(|_| panic!("compaction with tool_choice none must succeed"));
         let bodies = captured.lock().unwrap();
-        assert_eq!(bodies.len(), 2, "mock must have served both requests");
+        assert_eq!(bodies.len(), 3, "mock must have served all three requests");
         let with_tools = &bodies[0];
         assert_eq!(
             with_tools["tool_choice"],
-            json!("none"),
-            "tool use must be disabled at decode time"
+            json!("auto"),
+            "default tool_choice is auto — some backends reject none; the prompt forbids tool use"
         );
         let sent_tools = with_tools["tools"]
             .as_array()
@@ -1764,6 +1800,12 @@ mod reasoning_compaction_regression_tests {
             without_tools.get("tool_choice").is_none(),
             "tool_choice without tools is rejected by OpenAI-compat backends"
         );
+        let none_configured = &bodies[2];
+        assert_eq!(
+            none_configured["tool_choice"],
+            json!("none"),
+            "configured none must reach the wire"
+        );
         let _ = shutdown_tx.send(());
     }
     #[tokio::test]
@@ -1802,6 +1844,7 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_millis(150),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await;
         match result {
@@ -1869,6 +1912,7 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_millis(150),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await;
         match result {
@@ -1940,6 +1984,7 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_millis(150),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await;
         match result {
@@ -2008,6 +2053,7 @@ mod reasoning_compaction_regression_tests {
             &config,
             std::time::Duration::from_millis(150),
             0,
+            crate::util::config::CompactionToolChoice::Auto,
         )
         .await;
         match result {
diff --git a/crates/codegen/kigi-shell/src/util/config/resolve/compaction.rs b/crates/codegen/kigi-shell/src/util/config/resolve/compaction.rs
index b843247..a2a45da 100644
--- a/crates/codegen/kigi-shell/src/util/config/resolve/compaction.rs
+++ b/crates/codegen/kigi-shell/src/util/config/resolve/compaction.rs
@@ -1,6 +1,42 @@
 /// Default auto-compact threshold (% of context window) when no source sets it.
 pub const DEFAULT_AUTO_COMPACT_THRESHOLD_PERCENT: u8 = 85;
 
+/// `tool_choice` sent on compaction summarizer requests when tools are
+/// attached (prefix-cache alignment keeps the tool definitions in the
+/// request). Default `Auto` because some OpenAI-compatible servers reject
+/// `tool_choice: "none"`; the summarization prompt itself forbids tool use.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub enum CompactionToolChoice {
+    #[default]
+    Auto,
+    None,
+}
+
+impl std::str::FromStr for CompactionToolChoice {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.trim().to_ascii_lowercase().as_str() {
+            "auto" => Ok(Self::Auto),
+            "none" => Ok(Self::None),
+            _ => Err(()),
+        }
+    }
+}
+
+pub(crate) const ENV_COMPACTION_TOOL_CHOICE: &str = "KIGI_COMPACTION_TOOL_CHOICE";
+
+pub(crate) fn resolve_compaction_tool_choice_from(
+    env: Option<&str>,
+    config: Option<&str>,
+    remote: Option<&str>,
+) -> CompactionToolChoice {
+    env.and_then(|s| s.parse().ok())
+        .or_else(|| config.and_then(|s| s.parse().ok()))
+        .or_else(|| remote.and_then(|s| s.parse().ok()))
+        .unwrap_or_default()
+}
+
 /// Env-var override for `auto_compact_threshold_percent`. Parsed as `u8`;
 /// out-of-range or unparseable values are ignored.
 pub(crate) const ENV_AUTO_COMPACT_THRESHOLD_PERCENT: &str = "KIGI_AUTO_COMPACT_THRESHOLD_PERCENT";
@@ -127,6 +163,34 @@ pub fn resolve_compaction_wall_clock_budget_secs(gb_global: Option<u64>) -> u64
     resolved
 }
 
+#[cfg(test)]
+mod compaction_tool_choice_tests {
+    use super::{CompactionToolChoice, resolve_compaction_tool_choice_from as resolve};
+
+    #[test]
+    fn precedence_env_config_remote_default() {
+        assert_eq!(resolve(None, None, None), CompactionToolChoice::Auto);
+        assert_eq!(
+            resolve(None, None, Some("none")),
+            CompactionToolChoice::None
+        );
+        assert_eq!(
+            resolve(None, Some("auto"), Some("none")),
+            CompactionToolChoice::Auto
+        );
+        assert_eq!(
+            resolve(Some("none"), Some("auto"), Some("auto")),
+            CompactionToolChoice::None
+        );
+        // Unparseable tiers fall through instead of masking lower tiers.
+        assert_eq!(
+            resolve(Some("bogus"), None, Some("none")),
+            CompactionToolChoice::None
+        );
+        assert_eq!(resolve(Some("NONE "), None, None), CompactionToolChoice::None);
+    }
+}
+
 #[cfg(test)]
 mod compaction_wall_clock_budget_tests {
     use super::resolve_compaction_wall_clock_budget_secs as resolve;