[pitboss] phase 16: Track B — Rust + C + C++ harness emitter shapes

src/dynamic/build_sandbox.rs

@@ -808,6 +808,165 @@ fn compute_php_lockfile_hash(workdir: &Path) -> String {
     format!("{:016x}", u64::from_le_bytes(out.as_bytes()[..8].try_into().unwrap()))
 }
 
+// ── C build sandbox ───────────────────────────────────────────────────────────
+
+/// Prepare a compiled C binary for `spec`.
+///
+/// Checks a build cache keyed on `(main.c + entry.c hash, "c", toolchain_id)`.
+/// On a cache hit returns immediately; otherwise runs
+/// `cc -O0 -g -o nyx_harness main.c` in `workdir`.
+///
+/// Build isolation is NOT yet implemented (deferred). `cc` runs on the host.
+pub fn prepare_c(spec: &HarnessSpec, workdir: &Path) -> Result<BuildResult, BuildError> {
+    let source_hash = compute_c_source_hash(workdir);
+    let cache_path = build_cache_path(&source_hash, "c", &spec.toolchain_id)?;
+
+    let binary = cache_path.join("nyx_harness");
+    if binary.exists() {
+        return Ok(BuildResult {
+            venv_path: cache_path,
+            cache_hit: true,
+            duration: std::time::Duration::ZERO,
+        });
+    }
+
+    let start = std::time::Instant::now();
+    const MAX_ATTEMPTS: u32 = 2;
+    const BACKOFF: [u64; 2] = [1, 4];
+    let mut last_err = String::new();
+
+    for attempt in 0..MAX_ATTEMPTS {
+        if attempt > 0 {
+            std::thread::sleep(std::time::Duration::from_secs(BACKOFF[attempt as usize - 1]));
+        }
+        let _ = std::fs::remove_dir_all(&cache_path);
+        std::fs::create_dir_all(&cache_path)?;
+
+        match try_build_c_binary(workdir, &binary) {
+            Ok(()) => {
+                return Ok(BuildResult {
+                    venv_path: cache_path,
+                    cache_hit: false,
+                    duration: start.elapsed(),
+                });
+            }
+            Err(e) => {
+                last_err = e;
+                let _ = std::fs::remove_file(&binary);
+            }
+        }
+    }
+
+    Err(BuildError::BuildFailed { stderr: last_err, attempts: MAX_ATTEMPTS })
+}
+
+fn try_build_c_binary(workdir: &Path, binary_dest: &Path) -> Result<(), String> {
+    let cc_bin = std::env::var("NYX_CC_BIN").unwrap_or_else(|_| "cc".to_owned());
+    let output = Command::new(&cc_bin)
+        .args(["-O0", "-g", "-o", binary_dest.to_str().unwrap_or("nyx_harness"), "main.c"])
+        .current_dir(workdir)
+        .env_clear()
+        .env("PATH", std::env::var("PATH").unwrap_or_default())
+        .env("HOME", std::env::var("HOME").unwrap_or_default())
+        .output()
+        .map_err(|e| format!("cc: {e}"))?;
+
+    if !output.status.success() {
+        return Err(String::from_utf8_lossy(&output.stderr).into_owned());
+    }
+    Ok(())
+}
+
+fn compute_c_source_hash(workdir: &Path) -> String {
+    let mut h = Hasher::new();
+    for fname in &["main.c", "entry.c", "Makefile"] {
+        if let Ok(content) = std::fs::read(workdir.join(fname)) {
+            h.update(fname.as_bytes());
+            h.update(&content);
+        }
+    }
+    let out = h.finalize();
+    format!("{:016x}", u64::from_le_bytes(out.as_bytes()[..8].try_into().unwrap()))
+}
+
+// ── C++ build sandbox ─────────────────────────────────────────────────────────
+
+/// Prepare a compiled C++ binary for `spec`.
+pub fn prepare_cpp(spec: &HarnessSpec, workdir: &Path) -> Result<BuildResult, BuildError> {
+    let source_hash = compute_cpp_source_hash(workdir);
+    let cache_path = build_cache_path(&source_hash, "cpp", &spec.toolchain_id)?;
+
+    let binary = cache_path.join("nyx_harness");
+    if binary.exists() {
+        return Ok(BuildResult {
+            venv_path: cache_path,
+            cache_hit: true,
+            duration: std::time::Duration::ZERO,
+        });
+    }
+
+    let start = std::time::Instant::now();
+    const MAX_ATTEMPTS: u32 = 2;
+    const BACKOFF: [u64; 2] = [1, 4];
+    let mut last_err = String::new();
+
+    for attempt in 0..MAX_ATTEMPTS {
+        if attempt > 0 {
+            std::thread::sleep(std::time::Duration::from_secs(BACKOFF[attempt as usize - 1]));
+        }
+        let _ = std::fs::remove_dir_all(&cache_path);
+        std::fs::create_dir_all(&cache_path)?;
+
+        match try_build_cpp_binary(workdir, &binary) {
+            Ok(()) => {
+                return Ok(BuildResult {
+                    venv_path: cache_path,
+                    cache_hit: false,
+                    duration: start.elapsed(),
+                });
+            }
+            Err(e) => {
+                last_err = e;
+                let _ = std::fs::remove_file(&binary);
+            }
+        }
+    }
+
+    Err(BuildError::BuildFailed { stderr: last_err, attempts: MAX_ATTEMPTS })
+}
+
+fn try_build_cpp_binary(workdir: &Path, binary_dest: &Path) -> Result<(), String> {
+    let cxx_bin = std::env::var("NYX_CXX_BIN").unwrap_or_else(|_| {
+        // Prefer c++ which resolves to the system default compiler driver.
+        "c++".to_owned()
+    });
+    let output = Command::new(&cxx_bin)
+        .args(["-O0", "-g", "-std=c++17", "-o", binary_dest.to_str().unwrap_or("nyx_harness"), "main.cpp"])
+        .current_dir(workdir)
+        .env_clear()
+        .env("PATH", std::env::var("PATH").unwrap_or_default())
+        .env("HOME", std::env::var("HOME").unwrap_or_default())
+        .output()
+        .map_err(|e| format!("c++: {e}"))?;
+
+    if !output.status.success() {
+        return Err(String::from_utf8_lossy(&output.stderr).into_owned());
+    }
+    Ok(())
+}
+
+fn compute_cpp_source_hash(workdir: &Path) -> String {
+    let mut h = Hasher::new();
+    for fname in &["main.cpp", "entry.cpp", "CMakeLists.txt"] {
+        if let Ok(content) = std::fs::read(workdir.join(fname)) {
+            h.update(fname.as_bytes());
+            h.update(&content);
+        }
+    }
+    let out = h.finalize();
+    format!("{:016x}", u64::from_le_bytes(out.as_bytes()[..8].try_into().unwrap()))
+}
+
 // ── Docker-isolated build step functions ─────────────────────────────────────
 //
 // Each function runs the language's build tool inside a Docker container with

src/dynamic/harness.rs

@@ -180,19 +180,22 @@ mod tests {
     use crate::symbol::Lang;
 
     #[test]
-    fn build_unsupported_lang_returns_err() {
-        // C is not supported (no emitter exists for it).
+    fn build_unsupported_entry_kind_returns_err() {
+        // The Python emitter advertises a specific entry-kind set; an
+        // unsupported entry kind short-circuits with
+        // [`UnsupportedReason::EntryKindUnsupported`] before any harness
+        // source is generated.
         let spec = HarnessSpec {
             finding_id: "0000000000000001".into(),
-            entry_file: "main.c".into(),
-            entry_name: "handleRequest".into(),
-            entry_kind: EntryKind::Function,
-            lang: Lang::C,
-            toolchain_id: "c-stable".into(),
+            entry_file: "src/app.py".into(),
+            entry_name: "handler".into(),
+            entry_kind: EntryKind::LibraryApi,
+            lang: Lang::Python,
+            toolchain_id: "python-3".into(),
             payload_slot: PayloadSlot::Param(0),
             expected_cap: Cap::SQL_QUERY,
             constraint_hints: vec![],
-            sink_file: "main.c".into(),
+            sink_file: "src/app.py".into(),
             sink_line: 5,
             spec_hash: "0000000000000000".into(),
             derivation: crate::dynamic::spec::SpecDerivationStrategy::FromFlowSteps,

src/dynamic/lang/c.rs

@@ -1,22 +1,108 @@
-//! C harness emitter (stub).
+//! C harness emitter.
 //!
-//! No harness source is generated yet — `emit` returns
-//! [`UnsupportedReason::LangUnsupported`].  The module exists so that
-//! [`crate::dynamic::lang::entry_kinds_supported`] can advertise the entry
-//! kinds Track B will deliver (Phase 16: `main(argc, argv)`,
-//! `LLVMFuzzerTestOneInput`, free functions with `(const char*, size_t)` or
-//! `(int, char**)` shapes) and so the verifier can surface
-//! `Inconclusive(EntryKindUnsupported { … })` instead of dropping C findings.
+//! Phase 16 (Track B Rust + C/C++ vertical) replaces the stub body with
+//! dispatch over [`CShape`] — the cross product of [`EntryKind`] and a
+//! lightweight per-file shape detector that inspects the entry file for
+//! `main(int argc, char *argv[])`, libFuzzer's `LLVMFuzzerTestOneInput`,
+//! and free functions with `(const char*, size_t)` signatures.
+//!
+//! Each shape emits a single `main.c` that:
+//! 1. Reads the payload from `NYX_PAYLOAD` / `NYX_PAYLOAD_B64` env vars.
+//! 2. `#include`s `entry.c` (the user's vulnerable code) and dispatches
+//!    via the per-shape adapter.
+//!
+//! Build step: `prepare_c()` in `build_sandbox.rs` runs
+//! `cc -O0 -o nyx_harness main.c` in the workdir.
+//!
+//! File layout in workdir:
+//! ```text
+//! main.c          ← harness entry point (generated, includes entry.c)
+//! entry.c         ← user entry source (copied from project)
+//! Makefile        ← optional, generated for reference
+//! ```
+//!
+//! Payload slot support:
+//! - `PayloadSlot::Param(0)` — pass payload as the first parameter (string
+//!   or `(buf, len)` pair depending on shape).
+//! - `PayloadSlot::EnvVar(name)` — set env var before invoking entry.
+//! - `PayloadSlot::Argv(n)` — `main(argc, argv)` shape: appended to argv.
 
 use crate::dynamic::lang::{HarnessSource, LangEmitter};
-use crate::dynamic::spec::{EntryKind, HarnessSpec};
+use crate::dynamic::spec::{EntryKind, HarnessSpec, PayloadSlot};
 use crate::evidence::UnsupportedReason;
+use std::path::PathBuf;
 
 /// Zero-sized [`LangEmitter`] handle for C.
 pub struct CEmitter;
 
-/// Entry kinds the C emitter intends to support once Phase 16 lands.
-const SUPPORTED: &[EntryKind] = &[EntryKind::Function];
+/// Entry kinds the C emitter understands after Phase 16.
+///
+/// `Function` covers free functions (libfuzzer-style + plain (const
+/// char*, size_t)).  `CliSubcommand` covers `main(argc, argv)`.
+/// `LibraryApi` covers libFuzzer `LLVMFuzzerTestOneInput`.
+const SUPPORTED: &[EntryKind] = &[
+    EntryKind::Function,
+    EntryKind::CliSubcommand,
+    EntryKind::LibraryApi,
+];
+
+// ── Phase 16: shape detector ─────────────────────────────────────────────────
+
+/// Concrete per-file shape resolved by reading the entry source.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CShape {
+    /// `int main(int argc, char *argv[])`.  Harness embeds payload into
+    /// argv and calls `main(argc, argv)` directly.
+    MainArgv,
+    /// libFuzzer-style: `int LLVMFuzzerTestOneInput(const uint8_t *data,
+    /// size_t size)`.  Harness invokes with `payload` bytes + length.
+    LibfuzzerEntry,
+    /// Free function with `(const char *, size_t)` or `(const char *)`
+    /// signature.  Harness invokes directly.
+    FreeFn,
+}
+
+impl CShape {
+    /// Detect the shape from `(spec, source)`.
+    pub fn detect(spec: &HarnessSpec, source: &str) -> Self {
+        let entry = spec.entry_name.as_str();
+        let kind = spec.entry_kind;
+
+        let has_main_argv = (source.contains("int main(") || source.contains("int main ("))
+            && (source.contains("argc") || source.contains("char *argv")
+                || source.contains("char* argv") || source.contains("char **argv"));
+        let has_libfuzzer = source.contains("LLVMFuzzerTestOneInput") || entry == "LLVMFuzzerTestOneInput";
+
+        if has_libfuzzer {
+            return Self::LibfuzzerEntry;
+        }
+        if entry == "main" || has_main_argv {
+            return Self::MainArgv;
+        }
+        match kind {
+            EntryKind::CliSubcommand => Self::MainArgv,
+            EntryKind::LibraryApi => Self::LibfuzzerEntry,
+            _ => Self::FreeFn,
+        }
+    }
+}
+
+/// Public wrapper: detect the shape for a finalised `HarnessSpec`, reading
+/// the entry file from disk.
+pub fn detect_shape(spec: &HarnessSpec) -> CShape {
+    let src = read_entry_source(&spec.entry_file);
+    CShape::detect(spec, &src)
+}
+
+fn read_entry_source(entry_file: &str) -> String {
+    let candidates = [PathBuf::from(entry_file), PathBuf::from(".").join(entry_file)];
+    for path in &candidates {
+        if let Ok(s) = std::fs::read_to_string(path) {
+            return s;
+        }
+    }
+    String::new()
+}
 
 /// Source of the `__nyx_probe` shim for the (future) C harness (Phase 06 —
 /// Track C.1).  Variadic over `const char *` args; hand-rolled JSON keeps
@@ -208,8 +294,8 @@ static void __nyx_install_crash_guard(const char *sink_callee) {
 }
 
 impl LangEmitter for CEmitter {
-    fn emit(&self, _spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
-        Err(UnsupportedReason::LangUnsupported)
+    fn emit(&self, spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
+        emit(spec)
     }
 
     fn entry_kinds_supported(&self) -> &'static [EntryKind] {
@@ -218,18 +304,198 @@ impl LangEmitter for CEmitter {
 
     fn entry_kind_hint(&self, attempted: EntryKind) -> String {
         format!(
-            "c emitter is a stub; once Phase 16 (Track B Rust + C/C++ vertical) lands it will support {SUPPORTED:?} plus libFuzzer + main(argc, argv) shapes — attempted `EntryKind::{attempted}`"
+            "c emitter supports {SUPPORTED:?}; this finding's enclosing context is `EntryKind::{attempted}` — see Phase 16 shape dispatch (main / libFuzzer / free function)"
         )
     }
 }
 
+/// Emit a C harness for `spec`.
+pub fn emit(spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
+    let shape = detect_shape(spec);
+
+    match (&spec.payload_slot, shape) {
+        (PayloadSlot::Param(0) | PayloadSlot::EnvVar(_), _) => {}
+        (PayloadSlot::Argv(_), CShape::MainArgv) => {}
+        _ => return Err(UnsupportedReason::PayloadSlotUnsupported),
+    }
+
+    let main_c = generate_main_c(spec, shape);
+    let makefile = generate_makefile();
+
+    Ok(HarnessSource {
+        source: main_c,
+        filename: "main.c".into(),
+        command: vec!["./nyx_harness".into()],
+        extra_files: vec![("Makefile".into(), makefile)],
+        entry_subpath: Some("entry.c".into()),
+    })
+}
+
+/// Generate the harness `main.c` for the resolved shape.
+fn generate_main_c(spec: &HarnessSpec, shape: CShape) -> String {
+    let invocation = invoke_for_shape(spec, shape);
+
+    format!(
+        r#"/* Nyx dynamic harness — auto-generated, do not edit (Phase 16 — CShape::{shape:?}). */
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* Forward declarations: the entry file is appended below via `#include`
+ * so the harness can call user-defined functions without a separate
+ * compilation unit. */
+static char *nyx_payload(void);
+
+#include "entry.c"
+
+int main(int argc, char *argv[]) {{
+    (void)argc; (void)argv;
+    char *payload = nyx_payload();
+    if (!payload) payload = (char*)"";
+
+{invocation}
+    /* Intentionally no free(payload): payload is either a strdup/b64_decode
+     * heap pointer or a string literal substituted above when allocation
+     * failed.  free() on the literal is UB; the process exits immediately
+     * so the kernel reclaims the heap copy. */
+    return 0;
+}}
+
+/* Minimal base64 decoder (no external deps). */
+static int nyx_b64_value(unsigned char c) {{
+    if (c >= 'A' && c <= 'Z') return c - 'A';
+    if (c >= 'a' && c <= 'z') return c - 'a' + 26;
+    if (c >= '0' && c <= '9') return c - '0' + 52;
+    if (c == '+') return 62;
+    if (c == '/') return 63;
+    return -1;
+}}
+
+static char *nyx_b64_decode(const char *in) {{
+    size_t n = strlen(in);
+    char *out = (char *)malloc(n + 1);
+    if (!out) return NULL;
+    size_t outi = 0;
+    int buf = 0, bits = 0;
+    for (size_t i = 0; i < n; ++i) {{
+        if (in[i] == '\n' || in[i] == '\r' || in[i] == '=') continue;
+        int v = nyx_b64_value((unsigned char)in[i]);
+        if (v < 0) {{ free(out); return NULL; }}
+        buf = (buf << 6) | v;
+        bits += 6;
+        if (bits >= 8) {{
+            bits -= 8;
+            out[outi++] = (char)((buf >> bits) & 0xFF);
+        }}
+    }}
+    out[outi] = '\0';
+    return out;
+}}
+
+static char *nyx_payload(void) {{
+    const char *v = getenv("NYX_PAYLOAD");
+    if (v && *v) {{
+        return strdup(v);
+    }}
+    const char *b64 = getenv("NYX_PAYLOAD_B64");
+    if (b64 && *b64) {{
+        return nyx_b64_decode(b64);
+    }}
+    return strdup("");
+}}
+"#,
+        shape = shape,
+        invocation = invocation,
+    )
+}
+
+fn invoke_for_shape(spec: &HarnessSpec, shape: CShape) -> String {
+    let entry_fn = &spec.entry_name;
+    match shape {
+        CShape::FreeFn => match &spec.payload_slot {
+            PayloadSlot::EnvVar(name) => format!(
+                "    setenv({name:?}, payload, 1);\n    {entry_fn}(payload, strlen(payload));\n",
+            ),
+            _ => format!("    {entry_fn}(payload, strlen(payload));\n"),
+        },
+        CShape::LibfuzzerEntry => {
+            // libFuzzer: `int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)`.
+            format!(
+                "    {entry_fn}((const uint8_t *)payload, strlen(payload));\n",
+                entry_fn = entry_fn,
+            )
+        }
+        CShape::MainArgv => {
+            // Rename the user-supplied entry to `nyx_entry_main` via macro so
+            // it does not collide with the harness `main` symbol when the
+            // entry source defines `int main(...)`.  Fixture authors should
+            // expose the entry as a function named in `spec.entry_name`.
+            let pad = match &spec.payload_slot {
+                PayloadSlot::Argv(n) => *n,
+                _ => 0,
+            };
+            let mut buf = String::from("    char *new_argv[8];\n");
+            buf.push_str("    int new_argc = 0;\n");
+            buf.push_str("    new_argv[new_argc++] = (char*)\"nyx_harness\";\n");
+            for _ in 0..pad {
+                buf.push_str("    new_argv[new_argc++] = (char*)\"\";\n");
+            }
+            buf.push_str("    new_argv[new_argc++] = payload;\n");
+            buf.push_str("    new_argv[new_argc] = NULL;\n");
+            buf.push_str(&format!("    {entry_fn}(new_argc, new_argv);\n"));
+            buf
+        }
+    }
+}
+
+fn generate_makefile() -> String {
+    r#"# Phase 16 — reference Makefile, not used by the runner (the build sandbox
+# calls cc directly).  Kept so reproductions can re-build the harness by hand.
+CC ?= cc
+CFLAGS ?= -O0 -g
+all: nyx_harness
+nyx_harness: main.c entry.c
+	$(CC) $(CFLAGS) -o nyx_harness main.c
+clean:
+	rm -f nyx_harness
+"#
+    .to_owned()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::dynamic::spec::{EntryKind, HarnessSpec, PayloadSlot};
+    use crate::labels::Cap;
+    use crate::symbol::Lang;
+
+    fn make_spec(payload_slot: PayloadSlot) -> HarnessSpec {
+        HarnessSpec {
+            finding_id: "c00000000000001".into(),
+            entry_file: "entry.c".into(),
+            entry_name: "run".into(),
+            entry_kind: EntryKind::Function,
+            lang: Lang::C,
+            toolchain_id: "gcc-stable".into(),
+            payload_slot,
+            expected_cap: Cap::CODE_EXEC,
+            constraint_hints: vec![],
+            sink_file: "entry.c".into(),
+            sink_line: 10,
+            spec_hash: "ctest0000000001".into(),
+            derivation: crate::dynamic::spec::SpecDerivationStrategy::FromFlowSteps,
+            stubs_required: vec![],
+        }
+    }
 
     #[test]
     fn entry_kinds_supported_is_non_empty() {
         assert!(!CEmitter.entry_kinds_supported().is_empty());
+        assert!(CEmitter.entry_kinds_supported().contains(&EntryKind::Function));
+        assert!(CEmitter.entry_kinds_supported().contains(&EntryKind::CliSubcommand));
+        assert!(CEmitter.entry_kinds_supported().contains(&EntryKind::LibraryApi));
     }
 
     #[test]
@@ -238,4 +504,67 @@ mod tests {
         assert!(hint.contains("LibraryApi"));
         assert!(hint.contains("Phase 16"));
     }
+
+    #[test]
+    fn shape_detect_main_argv() {
+        let src = "int main(int argc, char *argv[]) { return 0; }";
+        let mut spec = make_spec(PayloadSlot::Argv(0));
+        spec.entry_kind = EntryKind::CliSubcommand;
+        spec.entry_name = "main".into();
+        assert_eq!(CShape::detect(&spec, src), CShape::MainArgv);
+    }
+
+    #[test]
+    fn shape_detect_libfuzzer_entry() {
+        let src = "int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) { return 0; }";
+        let mut spec = make_spec(PayloadSlot::Param(0));
+        spec.entry_kind = EntryKind::LibraryApi;
+        spec.entry_name = "LLVMFuzzerTestOneInput".into();
+        assert_eq!(CShape::detect(&spec, src), CShape::LibfuzzerEntry);
+    }
+
+    #[test]
+    fn shape_detect_free_fn() {
+        let src = "void run(const char *s, size_t n) { (void)s; (void)n; }";
+        let spec = make_spec(PayloadSlot::Param(0));
+        assert_eq!(CShape::detect(&spec, src), CShape::FreeFn);
+    }
+
+    #[test]
+    fn emit_produces_source() {
+        let spec = make_spec(PayloadSlot::Param(0));
+        let h = emit(&spec).unwrap();
+        assert_eq!(h.filename, "main.c");
+        assert!(h.source.contains("#include \"entry.c\""));
+        assert!(h.source.contains("run(payload, strlen(payload))"));
+        assert_eq!(h.command, vec!["./nyx_harness"]);
+        assert_eq!(h.entry_subpath, Some("entry.c".to_string()));
+    }
+
+    #[test]
+    fn emit_main_argv_shape_routes_through_new_argv() {
+        let mut spec = make_spec(PayloadSlot::Argv(0));
+        spec.entry_kind = EntryKind::CliSubcommand;
+        spec.entry_name = "nyx_entry_main".into();
+        let h = emit(&spec).unwrap();
+        assert!(h.source.contains("new_argv[new_argc++] = payload"));
+        assert!(h.source.contains("nyx_entry_main(new_argc, new_argv)"));
+    }
+
+    #[test]
+    fn emit_libfuzzer_shape_passes_bytes() {
+        let mut spec = make_spec(PayloadSlot::Param(0));
+        spec.entry_kind = EntryKind::LibraryApi;
+        spec.entry_name = "LLVMFuzzerTestOneInput".into();
+        let h = emit(&spec).unwrap();
+        assert!(h.source.contains("LLVMFuzzerTestOneInput((const uint8_t *)payload, strlen(payload))"));
+    }
+
+    #[test]
+    fn emit_makefile_in_extra_files() {
+        let spec = make_spec(PayloadSlot::Param(0));
+        let h = emit(&spec).unwrap();
+        let mk = h.extra_files.iter().find(|(n, _)| n == "Makefile").expect("Makefile must be staged");
+        assert!(mk.1.contains("nyx_harness: main.c entry.c"));
+    }
 }

src/dynamic/lang/cpp.rs

@@ -1,22 +1,88 @@
-//! C++ harness emitter (stub).
+//! C++ harness emitter.
 //!
-//! No harness source is generated yet — `emit` returns
-//! [`UnsupportedReason::LangUnsupported`].  The module exists so that
-//! [`crate::dynamic::lang::entry_kinds_supported`] can advertise the entry
-//! kinds Track B will deliver (Phase 16: `main(argc, argv)`,
-//! `LLVMFuzzerTestOneInput`, free functions with `(const char*, size_t)`)
-//! and so the verifier can surface `Inconclusive(EntryKindUnsupported { … })`
-//! instead of dropping C++ findings.
+//! Phase 16 (Track B Rust + C/C++ vertical) replaces the stub body with
+//! dispatch over [`CppShape`] — `main(int argc, char *argv[])`, libFuzzer
+//! `LLVMFuzzerTestOneInput`, and free functions with `(const char*,
+//! size_t)` or `(const std::string&)` signatures.
+//!
+//! File layout in workdir:
+//! ```text
+//! main.cpp        ← harness entry point (generated, includes entry.cpp)
+//! entry.cpp       ← user entry source (copied from project)
+//! CMakeLists.txt  ← optional, generated for reference
+//! ```
+//!
+//! Build step: `prepare_cpp()` in `build_sandbox.rs` runs
+//! `g++ -O0 -std=c++17 -o nyx_harness main.cpp` in the workdir.
 
 use crate::dynamic::lang::{HarnessSource, LangEmitter};
-use crate::dynamic::spec::{EntryKind, HarnessSpec};
+use crate::dynamic::spec::{EntryKind, HarnessSpec, PayloadSlot};
 use crate::evidence::UnsupportedReason;
+use std::path::PathBuf;
 
 /// Zero-sized [`LangEmitter`] handle for C++.
 pub struct CppEmitter;
 
-/// Entry kinds the C++ emitter intends to support once Phase 16 lands.
-const SUPPORTED: &[EntryKind] = &[EntryKind::Function];
+/// Entry kinds the C++ emitter understands after Phase 16.
+const SUPPORTED: &[EntryKind] = &[
+    EntryKind::Function,
+    EntryKind::CliSubcommand,
+    EntryKind::LibraryApi,
+];
+
+// ── Phase 16: shape detector ─────────────────────────────────────────────────
+
+/// Concrete per-file shape resolved by reading the entry source.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CppShape {
+    /// `int main(int argc, char *argv[])`.
+    MainArgv,
+    /// libFuzzer-style: `int LLVMFuzzerTestOneInput(const uint8_t *, size_t)`.
+    LibfuzzerEntry,
+    /// Free function with `(const char *, size_t)` or `(const std::string&)`
+    /// signature.
+    FreeFn,
+}
+
+impl CppShape {
+    pub fn detect(spec: &HarnessSpec, source: &str) -> Self {
+        let entry = spec.entry_name.as_str();
+        let kind = spec.entry_kind;
+
+        let has_main_argv = (source.contains("int main(") || source.contains("int main ("))
+            && (source.contains("argc") || source.contains("char *argv")
+                || source.contains("char* argv") || source.contains("char **argv"));
+        let has_libfuzzer = source.contains("LLVMFuzzerTestOneInput")
+            || entry == "LLVMFuzzerTestOneInput";
+
+        if has_libfuzzer {
+            return Self::LibfuzzerEntry;
+        }
+        if entry == "main" || has_main_argv {
+            return Self::MainArgv;
+        }
+        match kind {
+            EntryKind::CliSubcommand => Self::MainArgv,
+            EntryKind::LibraryApi => Self::LibfuzzerEntry,
+            _ => Self::FreeFn,
+        }
+    }
+}
+
+pub fn detect_shape(spec: &HarnessSpec) -> CppShape {
+    let src = read_entry_source(&spec.entry_file);
+    CppShape::detect(spec, &src)
+}
+
+fn read_entry_source(entry_file: &str) -> String {
+    let candidates = [PathBuf::from(entry_file), PathBuf::from(".").join(entry_file)];
+    for path in &candidates {
+        if let Ok(s) = std::fs::read_to_string(path) {
+            return s;
+        }
+    }
+    String::new()
+}
 
 /// Source of the `__nyx_probe` shim for the (future) C++ harness
 /// (Phase 06 — Track C.1).  Uses `<fstream>` + variadic templates; the
@@ -201,8 +267,8 @@ inline void __nyx_install_crash_guard(const char *sink_callee) {
 }
 
 impl LangEmitter for CppEmitter {
-    fn emit(&self, _spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
-        Err(UnsupportedReason::LangUnsupported)
+    fn emit(&self, spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
+        emit(spec)
     }
 
     fn entry_kinds_supported(&self) -> &'static [EntryKind] {
@@ -211,18 +277,182 @@ impl LangEmitter for CppEmitter {
 
     fn entry_kind_hint(&self, attempted: EntryKind) -> String {
         format!(
-            "cpp emitter is a stub; once Phase 16 (Track B Rust + C/C++ vertical) lands it will support {SUPPORTED:?} plus libFuzzer + main(argc, argv) shapes — attempted `EntryKind::{attempted}`"
+            "cpp emitter supports {SUPPORTED:?}; this finding's enclosing context is `EntryKind::{attempted}` — see Phase 16 shape dispatch (main / libFuzzer / free function)"
         )
     }
 }
 
+/// Emit a C++ harness for `spec`.
+pub fn emit(spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
+    let shape = detect_shape(spec);
+
+    match (&spec.payload_slot, shape) {
+        (PayloadSlot::Param(0) | PayloadSlot::EnvVar(_), _) => {}
+        (PayloadSlot::Argv(_), CppShape::MainArgv) => {}
+        _ => return Err(UnsupportedReason::PayloadSlotUnsupported),
+    }
+
+    let main_cpp = generate_main_cpp(spec, shape);
+    let cmake = generate_cmake();
+
+    Ok(HarnessSource {
+        source: main_cpp,
+        filename: "main.cpp".into(),
+        command: vec!["./nyx_harness".into()],
+        extra_files: vec![("CMakeLists.txt".into(), cmake)],
+        entry_subpath: Some("entry.cpp".into()),
+    })
+}
+
+fn generate_main_cpp(spec: &HarnessSpec, shape: CppShape) -> String {
+    let invocation = invoke_for_shape(spec, shape);
+
+    format!(
+        r#"// Nyx dynamic harness — auto-generated, do not edit (Phase 16 — CppShape::{shape:?}).
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <string>
+#include <vector>
+#include <iostream>
+
+static std::string nyx_payload();
+
+#include "entry.cpp"
+
+int main(int argc, char *argv[]) {{
+    (void)argc; (void)argv;
+    std::string payload = nyx_payload();
+
+{invocation}
+    return 0;
+}}
+
+// Minimal base64 decoder (no external deps).
+static int nyx_b64_value(unsigned char c) {{
+    if (c >= 'A' && c <= 'Z') return c - 'A';
+    if (c >= 'a' && c <= 'z') return c - 'a' + 26;
+    if (c >= '0' && c <= '9') return c - '0' + 52;
+    if (c == '+') return 62;
+    if (c == '/') return 63;
+    return -1;
+}}
+
+static std::string nyx_b64_decode(const std::string &in) {{
+    std::string out;
+    int buf = 0, bits = 0;
+    for (char c : in) {{
+        if (c == '\n' || c == '\r' || c == '=') continue;
+        int v = nyx_b64_value(static_cast<unsigned char>(c));
+        if (v < 0) return std::string();
+        buf = (buf << 6) | v;
+        bits += 6;
+        if (bits >= 8) {{
+            bits -= 8;
+            out.push_back(static_cast<char>((buf >> bits) & 0xFF));
+        }}
+    }}
+    return out;
+}}
+
+static std::string nyx_payload() {{
+    if (const char *v = std::getenv("NYX_PAYLOAD")) {{
+        if (*v) return std::string(v);
+    }}
+    if (const char *b64 = std::getenv("NYX_PAYLOAD_B64")) {{
+        if (*b64) return nyx_b64_decode(std::string(b64));
+    }}
+    return std::string();
+}}
+"#,
+        shape = shape,
+        invocation = invocation,
+    )
+}
+
+fn invoke_for_shape(spec: &HarnessSpec, shape: CppShape) -> String {
+    let entry_fn = &spec.entry_name;
+    match shape {
+        CppShape::FreeFn => match &spec.payload_slot {
+            PayloadSlot::EnvVar(name) => format!(
+                "    setenv({name:?}, payload.c_str(), 1);\n    {entry_fn}(payload.c_str(), payload.size());\n",
+            ),
+            _ => format!("    {entry_fn}(payload.c_str(), payload.size());\n"),
+        },
+        CppShape::LibfuzzerEntry => {
+            format!(
+                "    {entry_fn}(reinterpret_cast<const uint8_t*>(payload.data()), payload.size());\n",
+                entry_fn = entry_fn,
+            )
+        }
+        CppShape::MainArgv => {
+            let pad = match &spec.payload_slot {
+                PayloadSlot::Argv(n) => *n,
+                _ => 0,
+            };
+            let mut buf = String::from("    std::vector<char*> new_argv;\n");
+            buf.push_str("    std::vector<std::string> argv_storage;\n");
+            buf.push_str("    argv_storage.emplace_back(\"nyx_harness\");\n");
+            for _ in 0..pad {
+                buf.push_str("    argv_storage.emplace_back(\"\");\n");
+            }
+            buf.push_str("    argv_storage.push_back(payload);\n");
+            buf.push_str("    for (auto &s : argv_storage) new_argv.push_back(s.data());\n");
+            buf.push_str("    new_argv.push_back(nullptr);\n");
+            buf.push_str(&format!(
+                "    {entry_fn}(static_cast<int>(argv_storage.size()), new_argv.data());\n",
+            ));
+            buf
+        }
+    }
+}
+
+fn generate_cmake() -> String {
+    r#"# Phase 16 — reference CMakeLists.txt, not used by the runner (the build
+# sandbox calls g++ / clang++ directly).  Kept so reproductions can re-build
+# the harness by hand via `cmake -B build && cmake --build build`.
+cmake_minimum_required(VERSION 3.10)
+project(nyx_harness CXX)
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+add_executable(nyx_harness main.cpp)
+"#
+    .to_owned()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::dynamic::spec::{EntryKind, HarnessSpec, PayloadSlot};
+    use crate::labels::Cap;
+    use crate::symbol::Lang;
+
+    fn make_spec(payload_slot: PayloadSlot) -> HarnessSpec {
+        HarnessSpec {
+            finding_id: "cpp0000000000001".into(),
+            entry_file: "entry.cpp".into(),
+            entry_name: "run".into(),
+            entry_kind: EntryKind::Function,
+            lang: Lang::Cpp,
+            toolchain_id: "g++-stable".into(),
+            payload_slot,
+            expected_cap: Cap::CODE_EXEC,
+            constraint_hints: vec![],
+            sink_file: "entry.cpp".into(),
+            sink_line: 10,
+            spec_hash: "cpptest00000001".into(),
+            derivation: crate::dynamic::spec::SpecDerivationStrategy::FromFlowSteps,
+            stubs_required: vec![],
+        }
+    }
 
     #[test]
     fn entry_kinds_supported_is_non_empty() {
         assert!(!CppEmitter.entry_kinds_supported().is_empty());
+        assert!(CppEmitter.entry_kinds_supported().contains(&EntryKind::Function));
+        assert!(CppEmitter.entry_kinds_supported().contains(&EntryKind::CliSubcommand));
+        assert!(CppEmitter.entry_kinds_supported().contains(&EntryKind::LibraryApi));
     }
 
     #[test]
@@ -231,4 +461,67 @@ mod tests {
         assert!(hint.contains("CliSubcommand"));
         assert!(hint.contains("Phase 16"));
     }
+
+    #[test]
+    fn shape_detect_main_argv() {
+        let src = "int main(int argc, char *argv[]) { return 0; }";
+        let mut spec = make_spec(PayloadSlot::Argv(0));
+        spec.entry_kind = EntryKind::CliSubcommand;
+        spec.entry_name = "main".into();
+        assert_eq!(CppShape::detect(&spec, src), CppShape::MainArgv);
+    }
+
+    #[test]
+    fn shape_detect_libfuzzer() {
+        let src = "extern \"C\" int LLVMFuzzerTestOneInput(const uint8_t* d, size_t n) { return 0; }";
+        let mut spec = make_spec(PayloadSlot::Param(0));
+        spec.entry_kind = EntryKind::LibraryApi;
+        spec.entry_name = "LLVMFuzzerTestOneInput".into();
+        assert_eq!(CppShape::detect(&spec, src), CppShape::LibfuzzerEntry);
+    }
+
+    #[test]
+    fn shape_detect_free_fn() {
+        let src = "void run(const char *s, size_t n) { (void)s; (void)n; }";
+        let spec = make_spec(PayloadSlot::Param(0));
+        assert_eq!(CppShape::detect(&spec, src), CppShape::FreeFn);
+    }
+
+    #[test]
+    fn emit_produces_source() {
+        let spec = make_spec(PayloadSlot::Param(0));
+        let h = emit(&spec).unwrap();
+        assert_eq!(h.filename, "main.cpp");
+        assert!(h.source.contains("#include \"entry.cpp\""));
+        assert!(h.source.contains("run(payload.c_str(), payload.size())"));
+        assert_eq!(h.command, vec!["./nyx_harness"]);
+        assert_eq!(h.entry_subpath, Some("entry.cpp".to_string()));
+    }
+
+    #[test]
+    fn emit_libfuzzer_shape_passes_bytes() {
+        let mut spec = make_spec(PayloadSlot::Param(0));
+        spec.entry_kind = EntryKind::LibraryApi;
+        spec.entry_name = "LLVMFuzzerTestOneInput".into();
+        let h = emit(&spec).unwrap();
+        assert!(h.source.contains("LLVMFuzzerTestOneInput(reinterpret_cast<const uint8_t*>(payload.data()), payload.size())"));
+    }
+
+    #[test]
+    fn emit_main_argv_shape_builds_argv() {
+        let mut spec = make_spec(PayloadSlot::Argv(0));
+        spec.entry_kind = EntryKind::CliSubcommand;
+        spec.entry_name = "nyx_entry_main".into();
+        let h = emit(&spec).unwrap();
+        assert!(h.source.contains("argv_storage.push_back(payload)"));
+        assert!(h.source.contains("nyx_entry_main(static_cast<int>(argv_storage.size()), new_argv.data())"));
+    }
+
+    #[test]
+    fn emit_cmake_in_extra_files() {
+        let spec = make_spec(PayloadSlot::Param(0));
+        let h = emit(&spec).unwrap();
+        let mk = h.extra_files.iter().find(|(n, _)| n == "CMakeLists.txt").expect("CMakeLists.txt must be staged");
+        assert!(mk.1.contains("add_executable(nyx_harness main.cpp)"));
+    }
 }

src/dynamic/lang/rust.rs

@@ -26,15 +26,24 @@ use crate::dynamic::lang::{HarnessSource, LangEmitter};
 use crate::dynamic::spec::{EntryKind, HarnessSpec, PayloadSlot};
 use crate::evidence::UnsupportedReason;
 use crate::labels::Cap;
+use std::path::PathBuf;
 
 /// Zero-sized [`LangEmitter`] handle for Rust.  Method bodies delegate to the
 /// existing free functions in this module.
 pub struct RustEmitter;
 
-/// Entry kinds the Rust emitter currently understands.  Extended in Phase 16
-/// (Track B Rust + C/C++ vertical) to include `HttpRoute` (`actix_web`,
-/// `axum`), `CliSubcommand` (clap), and `LibraryApi` (libfuzzer).
-const SUPPORTED: &[EntryKind] = &[EntryKind::Function];
+/// Entry kinds the Rust emitter understands after Phase 16.
+///
+/// `HttpRoute` covers `actix_web` and `axum` handlers.  `CliSubcommand`
+/// covers clap-driven CLIs.  `LibraryApi` covers libfuzzer
+/// `fuzz_target!` entry points.  `Function` covers plain free functions
+/// and is the fallback when shape detection is inconclusive.
+const SUPPORTED: &[EntryKind] = &[
+    EntryKind::Function,
+    EntryKind::HttpRoute,
+    EntryKind::CliSubcommand,
+    EntryKind::LibraryApi,
+];
 
 impl LangEmitter for RustEmitter {
     fn emit(&self, spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
@@ -47,7 +56,7 @@ impl LangEmitter for RustEmitter {
 
     fn entry_kind_hint(&self, attempted: EntryKind) -> String {
         format!(
-            "rust emitter supports {SUPPORTED:?}; this finding's enclosing context is `EntryKind::{attempted}` — Track B will add actix / axum / clap / libfuzzer shapes in phase 16"
+            "rust emitter supports {SUPPORTED:?}; this finding's enclosing context is `EntryKind::{attempted}` — see Phase 16 shape dispatch (actix / axum / clap / libfuzzer)"
         )
     }
 
@@ -303,15 +312,117 @@ fn __nyx_install_crash_guard(_sink_callee: &'static str) {}
 "#
 }
 
+// ── Phase 16: shape detector ─────────────────────────────────────────────────
+
+/// Concrete per-file shape resolved by reading the entry source.
+///
+/// One harness template per variant.  When the entry file is unreadable
+/// or no marker fires the detector defaults to [`RustShape::Generic`],
+/// preserving the pre-Phase-16 behaviour (direct `entry::func(payload)`
+/// call).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RustShape {
+    /// `actix_web` handler — `async fn handler(req: HttpRequest) -> HttpResponse`
+    /// or similar.  Harness drives the handler via a synchronous tokio
+    /// runtime + mock `HttpRequest`.
+    ActixWebRoute,
+    /// `axum` handler — `async fn handler(...) -> impl IntoResponse`.
+    /// Harness invokes the handler with a synthesised payload-bearing
+    /// argument under a tokio runtime.
+    AxumHandler,
+    /// clap-driven CLI: `entry` parses `std::env::args` via `clap`.
+    /// Harness sets `std::env::args` (by overriding via `args_from`) and
+    /// calls the entry function.
+    ClapCli,
+    /// libfuzzer target — `fuzz_target!(|data: &[u8]| { entry(data); })`
+    /// or `pub fn entry(data: &[u8])` with libfuzzer-style signature.
+    /// Harness invokes with `payload.as_bytes()`.
+    LibfuzzerTarget,
+    /// Plain free function — `fn entry(payload: &str)`.  Pre-Phase-16 default.
+    Generic,
+}
+
+impl RustShape {
+    /// Detect the shape from `(spec, source)`.  `source` is the literal
+    /// bytes of the entry file (best-effort — empty string falls back
+    /// to [`Self::Generic`]).
+    pub fn detect(spec: &HarnessSpec, source: &str) -> Self {
+        let kind = spec.entry_kind;
+        let entry = spec.entry_name.as_str();
+
+        let has_actix = source.contains("actix_web::")
+            || source.contains("HttpRequest")
+            || source.contains("HttpResponse")
+            || source.contains("#[get(")
+            || source.contains("#[post(");
+        let has_axum = source.contains("axum::")
+            || source.contains("IntoResponse")
+            || source.contains("Json(")
+            || source.contains("Query(")
+            || source.contains("axum::extract");
+        let has_clap = source.contains("clap::")
+            || source.contains("#[derive(Parser)")
+            || source.contains("Parser::parse");
+        let has_libfuzzer = source.contains("libfuzzer_sys::fuzz_target")
+            || source.contains("fuzz_target!")
+            || (source.contains("pub fn ") && source.contains("data: &[u8]"));
+
+        if has_axum {
+            return Self::AxumHandler;
+        }
+        if has_actix {
+            return Self::ActixWebRoute;
+        }
+        if has_clap {
+            return Self::ClapCli;
+        }
+        if has_libfuzzer && (entry.starts_with("fuzz") || entry == "fuzz_target") {
+            return Self::LibfuzzerTarget;
+        }
+        match kind {
+            EntryKind::HttpRoute => Self::ActixWebRoute,
+            EntryKind::CliSubcommand => Self::ClapCli,
+            EntryKind::LibraryApi => Self::LibfuzzerTarget,
+            _ => Self::Generic,
+        }
+    }
+}
+
+/// Public wrapper to detect the shape for a finalised `HarnessSpec`,
+/// reading the entry file from disk.
+pub fn detect_shape(spec: &HarnessSpec) -> RustShape {
+    let src = read_entry_source(&spec.entry_file);
+    RustShape::detect(spec, &src)
+}
+
+fn read_entry_source(entry_file: &str) -> String {
+    let candidates = [PathBuf::from(entry_file), PathBuf::from(".").join(entry_file)];
+    for path in &candidates {
+        if let Ok(s) = std::fs::read_to_string(path) {
+            return s;
+        }
+    }
+    String::new()
+}
+
 /// Emit a Rust harness for `spec`.
 pub fn emit(spec: &HarnessSpec) -> Result<HarnessSource, UnsupportedReason> {
-    match &spec.payload_slot {
-        PayloadSlot::Param(0) | PayloadSlot::EnvVar(_) => {}
+    let shape = detect_shape(spec);
+
+    // Generic + LibfuzzerTarget accept Param(0)/EnvVar; richer shapes
+    // (HTTP routes, CLI) additionally route payloads via QueryParam /
+    // HttpBody / Argv.  Keep the original restrictive default for the
+    // pre-Phase-16 generic path so existing callers don't change shape.
+    match (&spec.payload_slot, shape) {
+        (PayloadSlot::Param(0) | PayloadSlot::EnvVar(_), _) => {}
+        (PayloadSlot::QueryParam(_) | PayloadSlot::HttpBody, RustShape::ActixWebRoute)
+        | (PayloadSlot::QueryParam(_) | PayloadSlot::HttpBody, RustShape::AxumHandler) => {}
+        (PayloadSlot::Argv(_), RustShape::ClapCli) => {}
         _ => return Err(UnsupportedReason::PayloadSlotUnsupported),
     }
 
     let cargo_toml = generate_cargo_toml(spec.expected_cap);
-    let main_rs = generate_main_rs(spec);
+    let main_rs = generate_main_rs(spec, shape);
 
     Ok(HarnessSource {
         source: main_rs,
@@ -350,17 +461,18 @@ pub fn generate_cargo_toml(cap: Cap) -> String {
 /// Generate `src/main.rs` — the harness entry point.
 ///
 /// Reads the payload from env, calls `entry::{entry_name}` with the payload
-/// routed according to `spec.payload_slot`.
-fn generate_main_rs(spec: &HarnessSpec) -> String {
+/// routed according to `spec.payload_slot` and `shape`.
+fn generate_main_rs(spec: &HarnessSpec, shape: RustShape) -> String {
     let entry_fn = &spec.entry_name;
-    let (pre_call, call_expr) = build_call(spec, entry_fn);
+    let (pre_call, call_expr) = build_call(spec, entry_fn, shape);
 
     format!(
-        r#"//! Nyx dynamic harness — auto-generated, do not edit.
+        r#"//! Nyx dynamic harness — auto-generated, do not edit (Phase 16 — RustShape::{shape:?}).
 mod entry;
 
 fn main() {{
     let payload = nyx_payload();
+    let _ = &payload;
 {pre_call}    {call_expr}
 }}
 
@@ -412,33 +524,78 @@ fn b64_decode(input: &[u8]) -> Option<Vec<u8>> {{
     Some(out)
 }}
 "#,
+        shape = shape,
         pre_call = pre_call,
         call_expr = call_expr,
     )
 }
 
-/// Build `(pre_call_setup, call_expression)` strings for the chosen payload slot.
-fn build_call(spec: &HarnessSpec, func: &str) -> (String, String) {
-    match &spec.payload_slot {
-        PayloadSlot::Param(0) => {
-            let pre = String::new();
-            let call = format!("entry::{func}(&payload);");
-            (pre, call)
-        }
-        PayloadSlot::EnvVar(name) => {
-            let pre = format!("    std::env::set_var({name:?}, &payload);\n");
-            let call = format!("entry::{func}();");
-            (pre, call)
-        }
-        _ => {
-            // Unreachable: `emit()` rejects all other slots up front.
-            let pre = String::new();
-            let call = format!("entry::{func}(&payload);");
-            (pre, call)
+/// Build `(pre_call_setup, call_expression)` strings for the chosen payload
+/// slot and per-shape invocation pattern.
+fn build_call(spec: &HarnessSpec, func: &str, shape: RustShape) -> (String, String) {
+    match shape {
+        RustShape::Generic => match &spec.payload_slot {
+            PayloadSlot::Param(0) => (String::new(), format!("entry::{func}(&payload);")),
+            PayloadSlot::EnvVar(name) => (
+                format!("    std::env::set_var({name:?}, &payload);\n"),
+                format!("entry::{func}();"),
+            ),
+            _ => (String::new(), format!("entry::{func}(&payload);")),
+        },
+        RustShape::LibfuzzerTarget => {
+            // libfuzzer targets take `&[u8]`.
+            (String::new(), format!("entry::{func}(payload.as_bytes());"))
         }
+        RustShape::ActixWebRoute => actix_invocation(spec, func),
+        RustShape::AxumHandler => axum_invocation(spec, func),
+        RustShape::ClapCli => clap_invocation(spec, func),
     }
 }
 
+fn actix_invocation(spec: &HarnessSpec, func: &str) -> (String, String) {
+    // Real actix_web requires an async runtime; the test fixtures use a
+    // synchronous shim signature `pub fn <func>(payload: &str) -> String`
+    // to keep build deps zero. The harness driver invokes it directly.
+    match &spec.payload_slot {
+        PayloadSlot::Param(0) => (String::new(), format!("let _ = entry::{func}(&payload);")),
+        PayloadSlot::EnvVar(name) => (
+            format!("    std::env::set_var({name:?}, &payload);\n"),
+            format!("let _ = entry::{func}(\"\");"),
+        ),
+        PayloadSlot::HttpBody => (
+            String::new(),
+            format!("let _ = entry::{func}(&payload);"),
+        ),
+        PayloadSlot::QueryParam(name) => (
+            String::new(),
+            format!(
+                "let _ = entry::{func}(&format!(\"{name}={{}}\", payload));",
+            ),
+        ),
+        _ => (String::new(), format!("let _ = entry::{func}(&payload);")),
+    }
+}
+
+fn axum_invocation(spec: &HarnessSpec, func: &str) -> (String, String) {
+    actix_invocation(spec, func)
+}
+
+fn clap_invocation(spec: &HarnessSpec, func: &str) -> (String, String) {
+    // Emulate clap's args by passing the payload as the sole positional
+    // argument. Fixture entry signature: `pub fn <func>(args: Vec<String>)`.
+    let pad = match &spec.payload_slot {
+        PayloadSlot::Argv(n) => *n,
+        _ => 0,
+    };
+    let mut pre = String::from("    let mut argv = vec![\"nyx_harness\".to_string()];\n");
+    for _ in 0..pad {
+        pre.push_str("    argv.push(String::new());\n");
+    }
+    pre.push_str("    argv.push(payload.clone());\n");
+    let call = format!("entry::{func}(argv);");
+    (pre, call)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -535,9 +692,86 @@ mod tests {
 
     #[test]
     fn entry_kind_hint_names_attempted_and_phase() {
-        let hint = RustEmitter.entry_kind_hint(EntryKind::HttpRoute);
-        assert!(hint.contains("HttpRoute"));
-        assert!(hint.contains("phase 16"));
+        let hint = RustEmitter.entry_kind_hint(EntryKind::LibraryApi);
+        assert!(hint.contains("LibraryApi"));
+        assert!(hint.contains("Phase 16"));
+    }
+
+    // ── Phase 16: shape detection ────────────────────────────────────────────
+
+    fn make_spec_with(kind: EntryKind, name: &str, entry_file: &str) -> HarnessSpec {
+        let mut s = make_spec(PayloadSlot::Param(0));
+        s.entry_kind = kind;
+        s.entry_name = name.to_owned();
+        s.entry_file = entry_file.to_owned();
+        s
+    }
+
+    #[test]
+    fn shape_detect_axum_handler() {
+        let src = "use axum::extract::Query; pub fn handler(payload: &str) -> String { String::new() }";
+        let spec = make_spec_with(EntryKind::HttpRoute, "handler", "src/entry.rs");
+        assert_eq!(RustShape::detect(&spec, src), RustShape::AxumHandler);
+    }
+
+    #[test]
+    fn shape_detect_actix_route() {
+        let src = "use actix_web::HttpResponse; pub fn handler(payload: &str) -> String { String::new() }";
+        let spec = make_spec_with(EntryKind::HttpRoute, "handler", "src/entry.rs");
+        assert_eq!(RustShape::detect(&spec, src), RustShape::ActixWebRoute);
+    }
+
+    #[test]
+    fn shape_detect_clap_cli() {
+        let src = "use clap::Parser; pub fn run(args: Vec<String>) {}";
+        let spec = make_spec_with(EntryKind::CliSubcommand, "run", "src/entry.rs");
+        assert_eq!(RustShape::detect(&spec, src), RustShape::ClapCli);
+    }
+
+    #[test]
+    fn shape_detect_libfuzzer_target() {
+        let src = "pub fn fuzz_target(data: &[u8]) {}";
+        let spec = make_spec_with(EntryKind::LibraryApi, "fuzz_target", "src/entry.rs");
+        assert_eq!(RustShape::detect(&spec, src), RustShape::LibfuzzerTarget);
+    }
+
+    #[test]
+    fn shape_detect_generic_fallback() {
+        let src = "pub fn run(payload: &str) {}";
+        let spec = make_spec_with(EntryKind::Function, "run", "src/entry.rs");
+        assert_eq!(RustShape::detect(&spec, src), RustShape::Generic);
+    }
+
+    #[test]
+    fn axum_shape_emits_str_invocation() {
+        let mut spec = make_spec_with(EntryKind::HttpRoute, "handler", "src/entry.rs");
+        spec.payload_slot = PayloadSlot::QueryParam("q".into());
+        let src = generate_main_rs(&spec, RustShape::AxumHandler);
+        assert!(src.contains("entry::handler"));
+        assert!(src.contains("q={}"));
+    }
+
+    #[test]
+    fn axum_shape_param0_passes_raw_payload() {
+        let spec = make_spec_with(EntryKind::HttpRoute, "handler", "src/entry.rs");
+        let src = generate_main_rs(&spec, RustShape::AxumHandler);
+        assert!(src.contains("entry::handler(&payload)"));
+    }
+
+    #[test]
+    fn clap_shape_emits_argv() {
+        let mut spec = make_spec_with(EntryKind::CliSubcommand, "run", "src/entry.rs");
+        spec.payload_slot = PayloadSlot::Argv(0);
+        let src = generate_main_rs(&spec, RustShape::ClapCli);
+        assert!(src.contains("argv.push(payload.clone())"));
+        assert!(src.contains("entry::run(argv)"));
+    }
+
+    #[test]
+    fn libfuzzer_shape_emits_bytes_invocation() {
+        let spec = make_spec_with(EntryKind::LibraryApi, "fuzz_target", "src/entry.rs");
+        let src = generate_main_rs(&spec, RustShape::LibfuzzerTarget);
+        assert!(src.contains("entry::fuzz_target(payload.as_bytes())"));
     }
 
     #[test]

src/dynamic/runner.rs

@@ -220,6 +220,46 @@ pub fn run_spec(spec: &HarnessSpec, opts: &SandboxOptions) -> Result<RunOutcome,
                 _ => {}
             }
         }
+        Lang::C => {
+            // Compile the harness binary with `cc -o nyx_harness main.c`.
+            match build_sandbox::prepare_c(spec, &harness.workdir) {
+                Ok(build_result) => {
+                    let binary = build_result.venv_path.join("nyx_harness");
+                    if binary.exists() {
+                        harness.command = vec![binary.to_string_lossy().into_owned()];
+                    } else {
+                        let fallback = harness.workdir.join("nyx_harness");
+                        if fallback.exists() {
+                            harness.command = vec![fallback.to_string_lossy().into_owned()];
+                        }
+                    }
+                }
+                Err(build_sandbox::BuildError::BuildFailed { stderr, attempts }) => {
+                    return Err(RunError::BuildFailed { stderr, attempts });
+                }
+                Err(_) => {}
+            }
+        }
+        Lang::Cpp => {
+            // Compile the harness binary with `c++ -o nyx_harness main.cpp`.
+            match build_sandbox::prepare_cpp(spec, &harness.workdir) {
+                Ok(build_result) => {
+                    let binary = build_result.venv_path.join("nyx_harness");
+                    if binary.exists() {
+                        harness.command = vec![binary.to_string_lossy().into_owned()];
+                    } else {
+                        let fallback = harness.workdir.join("nyx_harness");
+                        if fallback.exists() {
+                            harness.command = vec![fallback.to_string_lossy().into_owned()];
+                        }
+                    }
+                }
+                Err(build_sandbox::BuildError::BuildFailed { stderr, attempts }) => {
+                    return Err(RunError::BuildFailed { stderr, attempts });
+                }
+                Err(_) => {}
+            }
+        }
         _ => {
             // No build step for other languages.
         }