feat(neuron): strip reasoning from chat completions by default

Closes #8.

Reasoning-capable models (Qwen3, DeepSeek-R1, gpt-oss, Mistral
Magistral, …) emit `<think>...</think>` blocks inline in their
content stream. The chat-completions wire format has no slot for
reasoning, so until this change every consumer either parsed the
markers themselves (helexa-acp) or wrote the raw scratchpad
content into their UI (Zed's commit-message generator — visible
as the leaked reasoning block on every generated commit message
against benjy's Qwen3-8B).

## Implementation, model-agnostic by design

The neuron side now does token-level routing without any
hardcoded model knowledge:

1. **At load time** (`detect_reasoning_token_pair` in
   `wire::event`), probe the tokenizer's vocabulary for a known
   reasoning-marker pair: `<think>` / `</think>` (Qwen3,
   DeepSeek-R1, gpt-oss), `[THINK]` / `[/THINK]` (Mistral
   Magistral), and a couple of derivatives. Each marker must
   resolve to a single token id; if both open and close resolve,
   stash on `LoadedModel.reasoning_tokens` (similarly
   `TpLoadedModel`). Non-reasoning models get `None` and pass
   through unchanged.

2. **At inference time**, the three streaming paths
   (`run_inference_streaming` CPU, `stream_inference_via_worker`
   CUDA single-GPU, `chat_completion_tp_stream` CUDA TP) now
   check each sampled token against the pair via the new
   `handle_reasoning_marker` helper before feeding it to the
   detokeniser. Open marker → set `in_reasoning = true`, drop
   the marker. Close marker → unset, drop. Other tokens go
   through `emit_delta(_blocking)` which now picks
   `ReasoningDelta` or `TextDelta` based on state. Markers
   never appear in the streamed output.

3. **In `wire::openai_chat`**, the projector splits into:
   - `project_chat_stream` (unchanged signature; default
     behaviour — drops `ReasoningDelta`)
   - `project_chat_stream_with(rx, …, ChatProjectionConfig)` —
     when `include_thinking: true` and `reasoning_markers:
     Some(_)`, re-wraps reasoning content with the literal
     open/close marker text and emits as content deltas.
     Preserves the on-the-wire shape that helexa-acp's
     `ThinkParser` expects.

4. **HTTP handler** reads `x-include-thinking: true` (case-
   insensitive `1`/`true`/`yes`) from the request headers and
   threads it into the projection config. cortex-gateway already
   forwards arbitrary headers verbatim, so the opt-in works
   end-to-end without gateway changes.

5. **helexa-acp's `openai_chat` provider** sets
   `x-include-thinking: true` on every request so its existing
   `ThinkParser` keeps receiving the marked content stream.
   `ThinkParser` itself is unchanged — needed for endpoints that
   aren't reasoning-aware (OpenRouter, OpenAI directly, etc.).

## Acceptance

- Zed's commit-message generator (vanilla chat-completions
  client, no `x-include-thinking`) gets clean commit messages
  with no `<think>` block.
- helexa-acp sessions continue to render thinking in Zed's
  thought UI via the opt-in path.
- Models without reasoning tokens declared in their tokenizer
  pass through unchanged.
- Implementation contains zero references to "qwen3" or any
  specific model — entirely driven by tokenizer metadata.

## Tests

9 new tests in `wire::event` (token-pair detection across 4
marker conventions, edge cases) and `wire::openai_chat` (default
drop, opt-in re-wrap with multi-chunk reasoning, close-marker on
Finish, fallback when markers absent, off-switch with markers
present). All 213 workspace tests pass; fmt + clippy clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/neuron/src/wire/event.rs

@@ -97,3 +97,134 @@ impl FinishReason {
         }
     }
 }
+
+/// Open/close token IDs for the reasoning marker a loaded model uses
+/// (or `None` for non-reasoning models). The harness reads this once
+/// at load time from the tokenizer's added-tokens table, then the
+/// inference loop checks `next_token` against the pair to flip
+/// between [`InferenceEvent::TextDelta`] and
+/// [`InferenceEvent::ReasoningDelta`].
+///
+/// `open` and `close` text are kept alongside the IDs so wire
+/// projectors that want to re-emit the literal markers (the
+/// opt-in `include_thinking` path on chat completions) don't have
+/// to reach back into the tokenizer for the strings.
+#[derive(Debug, Clone)]
+pub struct ReasoningTokenPair {
+    pub open_id: u32,
+    pub close_id: u32,
+    pub open_text: String,
+    pub close_text: String,
+}
+
+/// Known reasoning-marker conventions. Each is a `(open, close)`
+/// pair of literal token strings. Each modern reasoning model
+/// declares its markers in the tokenizer's `added_tokens` table;
+/// at load time we probe for whichever pair the loaded tokenizer
+/// has and stash both IDs.
+///
+/// Ordering matters only for tie-breaking when a model declares
+/// multiple pairs (shouldn't happen in practice); the first hit
+/// wins.
+const KNOWN_REASONING_MARKERS: &[(&str, &str)] = &[
+    // Qwen3, DeepSeek-R1, gpt-oss, and most other open-weight
+    // reasoning models.
+    ("<think>", "</think>"),
+    // Mistral Magistral.
+    ("[THINK]", "[/THINK]"),
+    // Some older derivatives; harmless to probe.
+    ("<thought>", "</thought>"),
+    ("<reasoning>", "</reasoning>"),
+];
+
+/// Inspect a tokenizer for known reasoning-marker pairs and return
+/// the first match. The tokenizer types this trait is defined over
+/// just need to expose `token_to_id(&str) -> Option<u32>` so this
+/// stays decoupled from the candle crate — the production caller
+/// passes a `tokenizers::Tokenizer`, but tests can fake one.
+///
+/// Returns `None` when no known marker pair is fully declared
+/// (both open AND close token ids must resolve). That's the
+/// pass-through case — non-reasoning models, or reasoning models
+/// whose tokenizer split the markers across multiple tokens (rare
+/// in practice; modern reasoning tokenizers list them as
+/// `added_tokens`).
+pub fn detect_reasoning_token_pair<F>(token_to_id: F) -> Option<ReasoningTokenPair>
+where
+    F: Fn(&str) -> Option<u32>,
+{
+    for (open_text, close_text) in KNOWN_REASONING_MARKERS {
+        let open_id = token_to_id(open_text);
+        let close_id = token_to_id(close_text);
+        if let (Some(open_id), Some(close_id)) = (open_id, close_id) {
+            return Some(ReasoningTokenPair {
+                open_id,
+                close_id,
+                open_text: (*open_text).into(),
+                close_text: (*close_text).into(),
+            });
+        }
+    }
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::collections::HashMap;
+
+    fn lookup<'a>(map: &'a HashMap<&'static str, u32>) -> impl Fn(&str) -> Option<u32> + 'a {
+        |s| map.get(s).copied()
+    }
+
+    #[test]
+    fn detects_qwen3_style_think_markers() {
+        let mut m = HashMap::new();
+        m.insert("<think>", 151648);
+        m.insert("</think>", 151649);
+        let pair = detect_reasoning_token_pair(lookup(&m)).expect("pair detected");
+        assert_eq!(pair.open_id, 151648);
+        assert_eq!(pair.close_id, 151649);
+        assert_eq!(pair.open_text, "<think>");
+        assert_eq!(pair.close_text, "</think>");
+    }
+
+    #[test]
+    fn detects_mistral_magistral_markers() {
+        let mut m = HashMap::new();
+        m.insert("[THINK]", 100);
+        m.insert("[/THINK]", 101);
+        let pair = detect_reasoning_token_pair(lookup(&m)).expect("pair detected");
+        assert_eq!(pair.open_text, "[THINK]");
+    }
+
+    #[test]
+    fn returns_none_when_only_open_marker_present() {
+        // A pathological tokenizer that has `<think>` but not
+        // `</think>` shouldn't half-detect. Pass-through.
+        let mut m = HashMap::new();
+        m.insert("<think>", 1);
+        assert!(detect_reasoning_token_pair(lookup(&m)).is_none());
+    }
+
+    #[test]
+    fn returns_none_for_non_reasoning_tokenizer() {
+        let m: HashMap<&'static str, u32> = HashMap::new();
+        assert!(detect_reasoning_token_pair(lookup(&m)).is_none());
+    }
+
+    #[test]
+    fn first_match_wins_when_multiple_pairs_declared() {
+        // Hypothetical tokenizer with both Qwen-style AND Mistral-style
+        // markers — the `<think>` pair is earlier in the convention
+        // table so it wins.
+        let mut m = HashMap::new();
+        m.insert("<think>", 1);
+        m.insert("</think>", 2);
+        m.insert("[THINK]", 3);
+        m.insert("[/THINK]", 4);
+        let pair = detect_reasoning_token_pair(lookup(&m)).unwrap();
+        assert_eq!(pair.open_id, 1);
+        assert_eq!(pair.close_id, 2);
+    }
+}

crates/neuron/src/wire/mod.rs

@@ -21,4 +21,4 @@ pub mod event;
 pub mod openai_chat;
 pub mod openai_responses;
 
-pub use event::{FinishReason, InferenceEvent};
+pub use event::{FinishReason, InferenceEvent, ReasoningTokenPair, detect_reasoning_token_pair};

crates/neuron/src/wire/openai_chat.rs

@@ -30,13 +30,42 @@ use cortex_core::openai::{ChatCompletionChunk, ChunkChoice};
 use serde_json::json;
 use tokio::sync::mpsc;
 
-use super::event::{FinishReason, InferenceEvent};
+use super::event::{FinishReason, InferenceEvent, ReasoningTokenPair};
 
 /// Output channel buffer size. Mirrors the input side's bound; one
 /// event maps to at most one chunk, so equal capacity keeps the
 /// two ends in sync without surprising memory growth.
 const CHUNK_CHANNEL_CAPACITY: usize = 32;
 
+/// Per-stream config for the chat projector. Used by the
+/// production handler to thread per-request choices (currently:
+/// whether to surface reasoning content) into the projection
+/// without bloating the function signature.
+#[derive(Debug, Clone, Default)]
+pub struct ChatProjectionConfig {
+    /// When `true`, reasoning content is re-wrapped with the
+    /// model's literal open/close markers and emitted as content
+    /// deltas — preserving the on-the-wire shape that
+    /// reasoning-aware clients like helexa-acp's `ThinkParser`
+    /// expect.
+    ///
+    /// When `false` (the default), [`InferenceEvent::ReasoningDelta`]s
+    /// are dropped entirely so consumers that don't know about
+    /// reasoning (Zed's commit-message generator, any vanilla
+    /// OpenAI client) don't have model-internal scratchpad
+    /// material leaking into their UI. The chat-completions wire
+    /// format has no slot for reasoning, so the default chooses
+    /// the safer-for-naïve-clients behaviour.
+    pub include_thinking: bool,
+    /// Open/close marker strings to re-emit when `include_thinking`
+    /// is set. Sourced from the loaded model's
+    /// [`ReasoningTokenPair`]; `None` for non-reasoning models or
+    /// when the caller doesn't have the pair handy (in which case
+    /// `include_thinking` becomes equivalent to dropping reasoning
+    /// because there's nothing to wrap).
+    pub reasoning_markers: Option<ReasoningTokenPair>,
+}
+
 /// Project an [`InferenceEvent`] receiver into a
 /// [`ChatCompletionChunk`] receiver. Spawns one tokio task that
 /// owns the input receiver for the stream's lifetime and exits
@@ -46,15 +75,55 @@ const CHUNK_CHANNEL_CAPACITY: usize = 32;
 /// chunk so the receiver can stay generic (decoupled from
 /// per-request metadata).
 pub fn project_chat_stream(
-    mut rx: mpsc::Receiver<InferenceEvent>,
+    rx: mpsc::Receiver<InferenceEvent>,
     id: String,
     created: u64,
     model_id: String,
+) -> mpsc::Receiver<ChatCompletionChunk> {
+    // Default config: include_thinking off, no marker rewrap.
+    project_chat_stream_with(rx, id, created, model_id, ChatProjectionConfig::default())
+}
+
+/// Same as [`project_chat_stream`] but with a per-stream config
+/// (currently controlling reasoning surfacing). Production
+/// callers that need the opt-in path call this directly; the
+/// shorter wrapper above stays as the no-config convenience.
+pub fn project_chat_stream_with(
+    mut rx: mpsc::Receiver<InferenceEvent>,
+    id: String,
+    created: u64,
+    model_id: String,
+    config: ChatProjectionConfig,
 ) -> mpsc::Receiver<ChatCompletionChunk> {
     let (tx, out_rx) = mpsc::channel::<ChatCompletionChunk>(CHUNK_CHANNEL_CAPACITY);
 
     tokio::spawn(async move {
+        // Track whether the previous event was inside a reasoning
+        // block — used to decide when to emit the literal close
+        // marker on the include_thinking re-wrap path. When this
+        // flips from true → false (a TextDelta or Finish lands
+        // after one or more ReasoningDeltas), we emit the close
+        // marker exactly once.
+        let mut was_in_reasoning = false;
+
         while let Some(event) = rx.recv().await {
+            // Close-marker insertion: if we're leaving a reasoning
+            // chain, emit the literal close marker before the
+            // current event.
+            if was_in_reasoning && !matches!(event, InferenceEvent::ReasoningDelta(_)) {
+                if let Some(marker) = config
+                    .include_thinking
+                    .then_some(())
+                    .and(config.reasoning_markers.as_ref())
+                {
+                    let chunk = content_chunk(&id, created, &model_id, &marker.close_text);
+                    if tx.send(chunk).await.is_err() {
+                        return;
+                    }
+                }
+                was_in_reasoning = false;
+            }
+
             let chunks = match event {
                 InferenceEvent::Start => vec![role_chunk(&id, created, &model_id)],
                 InferenceEvent::TextDelta(text) => {
@@ -66,12 +135,42 @@ pub fn project_chat_stream(
                     }
                     vec![content_chunk(&id, created, &model_id, &text)]
                 }
-                InferenceEvent::ReasoningDelta(_) => {
-                    // Reasoning isn't representable in OpenAI chat
-                    // streaming today. The o-series uses a separate
-                    // `summary` event but it's gated by the
-                    // Responses API; chat-completions just drops it.
-                    continue;
+                InferenceEvent::ReasoningDelta(text) => {
+                    if !config.include_thinking {
+                        // Default path — reasoning has no slot in
+                        // chat completions, so it's dropped. Naïve
+                        // clients (Zed commit-message generator,
+                        // any vanilla OpenAI client) get clean
+                        // output.
+                        continue;
+                    }
+                    let Some(markers) = config.reasoning_markers.as_ref() else {
+                        // Caller asked to include thinking but
+                        // didn't supply markers — best we can do
+                        // is emit the content as visible text.
+                        // Skip the wrap entirely.
+                        if text.is_empty() {
+                            continue;
+                        }
+                        let chunk = content_chunk(&id, created, &model_id, &text);
+                        if tx.send(chunk).await.is_err() {
+                            return;
+                        }
+                        continue;
+                    };
+                    // First chunk of a reasoning block → open
+                    // marker prelude. Subsequent reasoning deltas
+                    // in the same block reuse `was_in_reasoning`
+                    // to skip the prelude.
+                    let mut chunks = Vec::new();
+                    if !was_in_reasoning {
+                        chunks.push(content_chunk(&id, created, &model_id, &markers.open_text));
+                    }
+                    if !text.is_empty() {
+                        chunks.push(content_chunk(&id, created, &model_id, &text));
+                    }
+                    was_in_reasoning = true;
+                    chunks
                 }
                 InferenceEvent::Finish { reason } => {
                     vec![final_chunk(&id, created, &model_id, reason)]
@@ -238,4 +337,165 @@ mod tests {
         assert_eq!(out.len(), 1);
         assert_eq!(out[0].choices[0].delta["content"], "real");
     }
+
+    fn pair() -> ReasoningTokenPair {
+        ReasoningTokenPair {
+            open_id: 0,
+            close_id: 1,
+            open_text: "<think>".into(),
+            close_text: "</think>".into(),
+        }
+    }
+
+    #[tokio::test]
+    async fn include_thinking_rewraps_reasoning_with_literal_markers() {
+        let (tx, rx) = mpsc::channel::<InferenceEvent>(8);
+        let out_rx = project_chat_stream_with(
+            rx,
+            "id".into(),
+            1,
+            "m".into(),
+            ChatProjectionConfig {
+                include_thinking: true,
+                reasoning_markers: Some(pair()),
+            },
+        );
+        tx.send(InferenceEvent::ReasoningDelta("first ".into()))
+            .await
+            .unwrap();
+        tx.send(InferenceEvent::ReasoningDelta("second".into()))
+            .await
+            .unwrap();
+        tx.send(InferenceEvent::TextDelta("answer".into()))
+            .await
+            .unwrap();
+        tx.send(InferenceEvent::Finish {
+            reason: FinishReason::Stop,
+        })
+        .await
+        .unwrap();
+        drop(tx);
+        let out = collect(out_rx).await;
+        // Expected sequence: open marker → reasoning content (2 chunks)
+        // → close marker → visible answer → final chunk.
+        let contents: Vec<&str> = out
+            .iter()
+            .filter_map(|c| c.choices[0].delta["content"].as_str())
+            .collect();
+        assert_eq!(
+            contents,
+            vec!["<think>", "first ", "second", "</think>", "answer"]
+        );
+        assert_eq!(
+            out.last().unwrap().choices[0].finish_reason.as_deref(),
+            Some("stop")
+        );
+    }
+
+    #[tokio::test]
+    async fn include_thinking_closes_marker_at_finish_when_no_trailing_text() {
+        // Edge case: stream ends inside a reasoning block (model
+        // hit max_tokens mid-thought, no visible answer ever).
+        // The Finish event still triggers the close marker so the
+        // stream is balanced.
+        let (tx, rx) = mpsc::channel::<InferenceEvent>(4);
+        let out_rx = project_chat_stream_with(
+            rx,
+            "id".into(),
+            1,
+            "m".into(),
+            ChatProjectionConfig {
+                include_thinking: true,
+                reasoning_markers: Some(pair()),
+            },
+        );
+        tx.send(InferenceEvent::ReasoningDelta("thinking...".into()))
+            .await
+            .unwrap();
+        tx.send(InferenceEvent::Finish {
+            reason: FinishReason::Length,
+        })
+        .await
+        .unwrap();
+        drop(tx);
+        let out = collect(out_rx).await;
+        let contents: Vec<&str> = out
+            .iter()
+            .filter_map(|c| c.choices[0].delta["content"].as_str())
+            .collect();
+        assert_eq!(contents, vec!["<think>", "thinking...", "</think>"]);
+        assert_eq!(
+            out.last().unwrap().choices[0].finish_reason.as_deref(),
+            Some("length")
+        );
+    }
+
+    #[tokio::test]
+    async fn include_thinking_without_markers_emits_content_directly() {
+        // Defensive: if the caller asks for thinking but the
+        // model declared no markers, we still emit the content
+        // rather than dropping it. Better to leak than to lose.
+        let (tx, rx) = mpsc::channel::<InferenceEvent>(4);
+        let out_rx = project_chat_stream_with(
+            rx,
+            "id".into(),
+            1,
+            "m".into(),
+            ChatProjectionConfig {
+                include_thinking: true,
+                reasoning_markers: None,
+            },
+        );
+        tx.send(InferenceEvent::ReasoningDelta("raw".into()))
+            .await
+            .unwrap();
+        tx.send(InferenceEvent::Finish {
+            reason: FinishReason::Stop,
+        })
+        .await
+        .unwrap();
+        drop(tx);
+        let out = collect(out_rx).await;
+        let contents: Vec<&str> = out
+            .iter()
+            .filter_map(|c| c.choices[0].delta["content"].as_str())
+            .collect();
+        assert_eq!(contents, vec!["raw"]);
+    }
+
+    #[tokio::test]
+    async fn include_thinking_off_drops_reasoning_even_with_markers() {
+        // Default behaviour even when markers happen to be
+        // configured. The flag is the gate, not the marker
+        // presence.
+        let (tx, rx) = mpsc::channel::<InferenceEvent>(4);
+        let out_rx = project_chat_stream_with(
+            rx,
+            "id".into(),
+            1,
+            "m".into(),
+            ChatProjectionConfig {
+                include_thinking: false,
+                reasoning_markers: Some(pair()),
+            },
+        );
+        tx.send(InferenceEvent::ReasoningDelta("hidden".into()))
+            .await
+            .unwrap();
+        tx.send(InferenceEvent::TextDelta("visible".into()))
+            .await
+            .unwrap();
+        tx.send(InferenceEvent::Finish {
+            reason: FinishReason::Stop,
+        })
+        .await
+        .unwrap();
+        drop(tx);
+        let out = collect(out_rx).await;
+        let contents: Vec<&str> = out
+            .iter()
+            .filter_map(|c| c.choices[0].delta["content"].as_str())
+            .collect();
+        assert_eq!(contents, vec!["visible"]);
+    }
 }