diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee1979e..14c7478 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ All notable changes to Forgent are documented here. Format based on [Keep a Chan
 
 ## [Unreleased]
 
+### Added
+
+- `src-tauri/src/infrastructure/pty/shell_detector.rs` (T-226) — platform-aware shell detection per ARCHI §11.2. `pub fn detect() -> ShellSpec` returns the program + args that `PtyPool::spawn` will hand to `portable_pty::CommandBuilder`. Unix honours `$SHELL` and falls back to `/bin/bash` when the var is unset or empty; Windows prefers `pwsh` (PowerShell 7) over `powershell.exe` over `%COMSPEC%` / `cmd.exe`, with `-NoLogo` on the PowerShell variants. `ShellSpec` derives `Serialize + Deserialize + TS` (exported to `src/shared/types/ShellSpec.ts`) so it can flow through debug IPC later. The Windows priority chain is extracted into a pure `resolve_windows_shell(pwsh, powershell, comspec)` helper that is unit-tested on every platform — the Linux CI runners exercise the full pwsh > powershell > COMSPEC > cmd.exe fallback ladder without needing real PowerShell binaries. Unix env-var fallback covered by three `temp_env::with_var` tests (SHELL set, unset, empty). `which = "8.0.2"` added as a Windows-only target dependency via `cargo add --target 'cfg(windows)'` so Unix builds stay lean. 283/283 cargo test, `cargo fmt --check`, `cargo clippy --all-targets -- -D warnings`, and `cargo deny check` all green.
+
 ## [0.1.0] - 2026-05-14
 
 First minor release. Bundles every Sprint 2 deliverable (T-219 → T-224) plus the Sprint 1 follow-ups merged since `v0.0.2`. See per-task entries below for details. Highlights:
diff --git a/src-tauri/Cargo.lock b/src-tauri/Cargo.lock
index 802fc46..149c9be 100644
--- a/src-tauri/Cargo.lock
+++ b/src-tauri/Cargo.lock
@@ -348,7 +348,7 @@ dependencies = [
  "rustc-hash 1.1.0",
  "shlex",
  "syn 2.0.117",
- "which",
+ "which 4.4.2",
 ]
 
 [[package]]
@@ -1563,6 +1563,7 @@ dependencies = [
  "tracing-subscriber",
  "ts-rs",
  "uuid",
+ "which 8.0.2",
 ]
 
 [[package]]
@@ -6852,6 +6853,15 @@ dependencies = [
  "rustix 0.38.44",
 ]
 
+[[package]]
+name = "which"
+version = "8.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "81995fafaaaf6ae47a7d0cc83c67caf92aeb7e5331650ae6ff856f7c0c60c459"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "winapi"
 version = "0.3.9"
diff --git a/src-tauri/Cargo.toml b/src-tauri/Cargo.toml
index c05c01e..7ee5523 100644
--- a/src-tauri/Cargo.toml
+++ b/src-tauri/Cargo.toml
@@ -85,3 +85,6 @@ pilot = ["dep:tauri-plugin-pilot"]
 # Auto-generated by `cargo add --optional`; kept as an alias so external
 # tooling that infers feature names from dep names continues to work.
 tauri-plugin-pilot = ["pilot"]
+
+[target."cfg(windows)".dependencies]
+which = "8.0.2"
diff --git a/src-tauri/src/infrastructure/pty/mod.rs b/src-tauri/src/infrastructure/pty/mod.rs
index 9da6085..8b5e628 100644
--- a/src-tauri/src/infrastructure/pty/mod.rs
+++ b/src-tauri/src/infrastructure/pty/mod.rs
@@ -1 +1,3 @@
 //! PTY — `portable-pty` pool, shell detection, stdin injection of Claude CLI commands.
+
+pub mod shell_detector;
diff --git a/src-tauri/src/infrastructure/pty/shell_detector.rs b/src-tauri/src/infrastructure/pty/shell_detector.rs
new file mode 100644
index 0000000..a84898b
--- /dev/null
+++ b/src-tauri/src/infrastructure/pty/shell_detector.rs
@@ -0,0 +1,165 @@
+//! Shell detection — resolves the host shell that `PtyPool::spawn` will hand
+//! to `portable_pty::CommandBuilder`. Unix honours `$SHELL` with `/bin/bash`
+//! fallback; Windows prefers `pwsh` (PowerShell 7) over `powershell.exe` over
+//! `%COMSPEC%` / `cmd.exe`. See ARCHI §11.2.
+
+use serde::{Deserialize, Serialize};
+use ts_rs::TS;
+
+/// Resolved shell program plus the arguments needed to spawn it interactively.
+///
+/// Forgent never invokes `claude` directly — it always spawns the user's
+/// system shell (so `.bashrc`, `.zshrc`, aliases and `PATH` are honoured) and
+/// then injects the Claude CLI command through stdin.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, TS)]
+#[ts(export, export_to = "../../src/shared/types/")]
+pub struct ShellSpec {
+    pub program: String,
+    pub args: Vec<String>,
+}
+
+/// Honour `$SHELL`; fall back to `/bin/bash` when the env var is unset or empty.
+#[cfg(unix)]
+pub fn detect() -> ShellSpec {
+    let program = std::env::var("SHELL")
+        .ok()
+        .filter(|value| !value.is_empty())
+        .unwrap_or_else(|| "/bin/bash".into());
+    ShellSpec {
+        program,
+        args: Vec::new(),
+    }
+}
+
+/// Prefer `pwsh` (PowerShell 7) over `powershell.exe`, falling back to
+/// `%COMSPEC%` and finally `cmd.exe` when neither PowerShell flavour is on PATH.
+#[cfg(windows)]
+pub fn detect() -> ShellSpec {
+    let comspec = std::env::var("COMSPEC").ok();
+    resolve_windows_shell(
+        which::which("pwsh").is_ok(),
+        which::which("powershell").is_ok(),
+        comspec.as_deref(),
+    )
+}
+
+/// Pure resolution of the Windows shell priority chain. Extracted so the
+/// fallback ladder can be exercised on Linux CI runners, which cannot resolve
+/// `pwsh` / `powershell` through `which::which`.
+#[cfg(any(test, windows))]
+fn resolve_windows_shell(
+    pwsh_on_path: bool,
+    powershell_on_path: bool,
+    comspec: Option<&str>,
+) -> ShellSpec {
+    if pwsh_on_path {
+        return ShellSpec {
+            program: "pwsh".into(),
+            args: vec!["-NoLogo".into()],
+        };
+    }
+    if powershell_on_path {
+        return ShellSpec {
+            program: "powershell".into(),
+            args: vec!["-NoLogo".into()],
+        };
+    }
+    let program = comspec
+        .filter(|value| !value.is_empty())
+        .map(String::from)
+        .unwrap_or_else(|| "cmd.exe".into());
+    ShellSpec {
+        program,
+        args: Vec::new(),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    mod windows_resolver {
+        use super::*;
+
+        #[test]
+        fn resolve_prefers_pwsh_when_available() {
+            let spec = resolve_windows_shell(true, true, Some("C:\\Windows\\System32\\cmd.exe"));
+            assert_eq!(spec.program, "pwsh");
+            assert_eq!(spec.args, vec!["-NoLogo".to_string()]);
+        }
+
+        #[test]
+        fn resolve_falls_back_to_powershell_when_pwsh_missing() {
+            let spec = resolve_windows_shell(false, true, Some("C:\\Windows\\System32\\cmd.exe"));
+            assert_eq!(spec.program, "powershell");
+            assert_eq!(spec.args, vec!["-NoLogo".to_string()]);
+        }
+
+        #[test]
+        fn resolve_uses_comspec_when_no_powershell_variants() {
+            let spec = resolve_windows_shell(false, false, Some("D:\\custom\\cmd.exe"));
+            assert_eq!(spec.program, "D:\\custom\\cmd.exe");
+            assert!(spec.args.is_empty());
+        }
+
+        #[test]
+        fn resolve_falls_back_to_cmd_when_comspec_unset() {
+            let spec = resolve_windows_shell(false, false, None);
+            assert_eq!(spec.program, "cmd.exe");
+            assert!(spec.args.is_empty());
+        }
+
+        #[test]
+        fn resolve_falls_back_to_cmd_when_comspec_empty() {
+            let spec = resolve_windows_shell(false, false, Some(""));
+            assert_eq!(spec.program, "cmd.exe");
+            assert!(spec.args.is_empty());
+        }
+    }
+
+    #[cfg(unix)]
+    mod unix {
+        use super::*;
+
+        #[test]
+        fn detect_uses_shell_env_var_when_set() {
+            temp_env::with_var("SHELL", Some("/usr/bin/zsh"), || {
+                let spec = detect();
+                assert_eq!(spec.program, "/usr/bin/zsh");
+                assert!(spec.args.is_empty());
+            });
+        }
+
+        #[test]
+        fn detect_falls_back_to_bin_bash_when_shell_unset() {
+            temp_env::with_var("SHELL", None::<&str>, || {
+                let spec = detect();
+                assert_eq!(spec.program, "/bin/bash");
+                assert!(spec.args.is_empty());
+            });
+        }
+
+        #[test]
+        fn detect_falls_back_to_bin_bash_when_shell_empty() {
+            temp_env::with_var("SHELL", Some(""), || {
+                let spec = detect();
+                assert_eq!(spec.program, "/bin/bash");
+                assert!(spec.args.is_empty());
+            });
+        }
+    }
+
+    #[cfg(windows)]
+    mod windows {
+        use super::*;
+
+        #[test]
+        fn detect_returns_non_empty_program_on_windows_runner() {
+            let spec = detect();
+            assert!(
+                !spec.program.is_empty(),
+                "detect() must always return a usable program"
+            );
+        }
+    }
+}
diff --git a/src/shared/types/ShellSpec.ts b/src/shared/types/ShellSpec.ts
new file mode 100644
index 0000000..44ae534
--- /dev/null
+++ b/src/shared/types/ShellSpec.ts
@@ -0,0 +1,10 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+/**
+ * Resolved shell program plus the arguments needed to spawn it interactively.
+ *
+ * Forgent never invokes `claude` directly — it always spawns the user's
+ * system shell (so `.bashrc`, `.zshrc`, aliases and `PATH` are honoured) and
+ * then injects the Claude CLI command through stdin.
+ */
+export type ShellSpec = { program: string, args: Array<string>, };