diff --git a/README.md b/README.md
index 548b9be..985cbaf 100644
--- a/README.md
+++ b/README.md
@@ -88,10 +88,12 @@ ask checks for updates at most once a day. If a new version is available, it let
 
 ## How it works
 
-Under the hood, ask runs Codex, Claude Code, Pi, or OpenCode using the CLI already installed and logged in on your machine.
+Under the hood, ask runs Codex, Claude Code, fx, Pi, or OpenCode using the CLI already installed and logged in on your machine.
 
 For a new session, ask uses your default agent, model, and reasoning. Continuing a session restores those saved choices and the underlying agent session ID. Answer instructions come from the current global setting. Ask starts the selected CLI read-only in your current folder, prints the answer to stdout, and saves the turn automatically.
 
+ask starts fx in `ask` permission mode. Existing fx allow rules still apply, so remove rules that permit changes if you need fx to be strictly read-only.
+
 ## Agents and models
 
 Run `ask --settings` to choose the default agent, model, reasoning, and answer instructions. Use `/settings` inside a session to change that session's model or reasoning, or to edit the global answer instructions. Start a new session to switch agents.
diff --git a/src/harness/fx.rs b/src/harness/fx.rs
new file mode 100644
index 0000000..4c69021
--- /dev/null
+++ b/src/harness/fx.rs
@@ -0,0 +1,299 @@
+use std::ffi::OsString;
+use std::process::Command;
+
+use serde_json::Value;
+
+use super::{Harness, Model, Response, RunOptions};
+use crate::error::{Error, Result};
+
+pub(super) struct Fx {
+    program: OsString,
+}
+
+impl Fx {
+    pub(super) fn new(program: OsString) -> Self {
+        Self { program }
+    }
+
+    fn command(
+        &self,
+        question: &str,
+        session_id: Option<&str>,
+        options: &RunOptions<'_>,
+    ) -> Result<Command> {
+        if options.reasoning.is_some() {
+            return Err(Error::new(
+                "reasoning control is not supported for fx",
+                "choose fx-managed reasoning in 'ask --settings' and try again",
+            ));
+        }
+        let mut command = Command::new(&self.program);
+        command
+            .args(["ask", "--json"])
+            .env("FX_PERMISSION_MODE", "ask");
+        if let Some(session_id) = session_id {
+            command.args(["--resume-id", session_id]);
+        }
+        if let Some(model) = options.model {
+            command.env("FX_MODEL", model);
+        }
+        if let Some(instructions) = options.instructions {
+            command.args(["--system", instructions]);
+        }
+        command.arg(question);
+        Ok(command)
+    }
+}
+
+impl Harness for Fx {
+    fn models(&mut self) -> Result<Vec<Model>> {
+        let output = Command::new(&self.program)
+            .args(["models", "--json"])
+            .output()
+            .map_err(start_error)?;
+        if !output.status.success() {
+            return Err(command_error("could not list fx models", &output.stderr));
+        }
+        let output = String::from_utf8(output.stdout)
+            .map_err(|_| Error::agent("fx", "fx returned a model list that was not valid UTF-8"))?;
+        parse_models(&output)
+    }
+
+    fn ask(
+        &mut self,
+        question: &str,
+        session_id: Option<&str>,
+        options: RunOptions<'_>,
+        _on_delta: &mut dyn FnMut(&str) -> Result<()>,
+    ) -> Result<Response> {
+        let output = self
+            .command(question, session_id, &options)?
+            .output()
+            .map_err(start_error)?;
+        let stdout = String::from_utf8(output.stdout)
+            .map_err(|_| Error::agent("fx", "fx returned output that was not valid UTF-8"))?;
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        let response = parse_response(&stdout).map_err(|error| {
+            let detail = stderr.trim();
+            if detail.is_empty() {
+                error
+            } else {
+                error.detail(detail)
+            }
+        })?;
+
+        if !output.status.success() {
+            return Err(command_error("fx failed", &output.stderr));
+        }
+        Ok(response)
+    }
+}
+
+fn parse_models(output: &str) -> Result<Vec<Model>> {
+    let response: Value = serde_json::from_str(output)
+        .map_err(|error| Error::agent("fx", format!("could not parse fx model list: {error}")))?;
+    let ids = response
+        .get("ids")
+        .and_then(Value::as_array)
+        .ok_or_else(|| Error::agent("fx", "fx returned an invalid model list"))?;
+    let details = response.get("models").and_then(Value::as_array);
+    let models = ids
+        .iter()
+        .map(|value| {
+            let id = value
+                .as_str()
+                .ok_or_else(|| Error::agent("fx", "fx returned an invalid model ID"))?;
+            let (provider, name) = id
+                .split_once('/')
+                .map_or((None, id), |(provider, name)| (Some(provider), name));
+            let source = details
+                .into_iter()
+                .flatten()
+                .find(|model| model.get("id").and_then(Value::as_str) == Some(id))
+                .and_then(|model| model.get("source"))
+                .and_then(Value::as_str);
+            Ok(Model {
+                id: id.to_owned(),
+                name: name.to_owned(),
+                description: source.or(provider).map_or_else(
+                    || "fx model".to_owned(),
+                    |source| format!("{source} provider"),
+                ),
+                is_default: false,
+                reasoning: Vec::new(),
+                default_reasoning: None,
+            })
+        })
+        .collect::<Result<Vec<_>>>()?;
+    if models.is_empty() {
+        Err(Error::agent("fx", "fx did not report any available models"))
+    } else {
+        Ok(models)
+    }
+}
+
+fn parse_response(output: &str) -> Result<Response> {
+    let response: Value = serde_json::from_str(output)
+        .map_err(|error| Error::agent("fx", format!("could not parse fx response: {error}")))?;
+    if response.get("exit_code").and_then(Value::as_i64) != Some(0) {
+        let detail = response
+            .get("error")
+            .and_then(Value::as_str)
+            .unwrap_or("unknown error");
+        return Err(Error::agent(
+            "fx",
+            format!("fx reported an error: {detail}"),
+        ));
+    }
+    Ok(Response {
+        answer: response
+            .get("output")
+            .and_then(Value::as_str)
+            .ok_or_else(|| Error::agent("fx", "fx completed without returning an answer"))?
+            .to_owned(),
+        session_id: response
+            .get("session_id")
+            .and_then(Value::as_str)
+            .filter(|session_id| !session_id.is_empty())
+            .ok_or_else(|| Error::agent("fx", "fx completed without returning a session ID"))?
+            .to_owned(),
+    })
+}
+
+fn start_error(error: std::io::Error) -> Error {
+    if error.kind() == std::io::ErrorKind::NotFound {
+        Error::new(
+            "fx is not installed or not on PATH",
+            "install it, authenticate, then try again",
+        )
+    } else {
+        Error::agent("fx", format!("could not start fx: {error}"))
+    }
+}
+
+fn command_error(message: &str, stderr: &[u8]) -> Error {
+    let detail = String::from_utf8_lossy(stderr);
+    let detail = detail.trim();
+    let message = if detail.is_empty() {
+        message.to_owned()
+    } else {
+        format!("{message}: {detail}")
+    };
+    Error::agent("fx", message)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{Fx, parse_models, parse_response};
+    use crate::harness::RunOptions;
+
+    #[test]
+    fn builds_a_read_only_session_command() {
+        let harness = Fx {
+            program: "fx".into(),
+        };
+        let command = harness
+            .command(
+                "hello",
+                Some("session-1"),
+                &RunOptions {
+                    model: Some("openai/gpt-test"),
+                    reasoning: None,
+                    instructions: Some("Be concise."),
+                },
+            )
+            .unwrap();
+        let args = command
+            .get_args()
+            .map(|arg| arg.to_string_lossy())
+            .collect::<Vec<_>>();
+        let permission_mode = command
+            .get_envs()
+            .find(|(name, _)| *name == "FX_PERMISSION_MODE")
+            .and_then(|(_, value)| value)
+            .unwrap();
+        let model = command
+            .get_envs()
+            .find(|(name, _)| *name == "FX_MODEL")
+            .and_then(|(_, value)| value)
+            .unwrap();
+
+        assert!(args.starts_with(&["ask".into(), "--json".into()]));
+        assert!(
+            args.windows(2)
+                .any(|args| args == ["--resume-id", "session-1"])
+        );
+        assert!(
+            args.windows(2)
+                .any(|args| args == ["--system", "Be concise."])
+        );
+        assert_eq!(permission_mode, "ask");
+        assert_eq!(model, "openai/gpt-test");
+    }
+
+    #[test]
+    fn rejects_reasoning_control() {
+        let harness = Fx {
+            program: "fx".into(),
+        };
+        let error = harness
+            .command(
+                "hello",
+                None,
+                &RunOptions {
+                    model: None,
+                    reasoning: Some("high"),
+                    instructions: None,
+                },
+            )
+            .unwrap_err();
+
+        assert_eq!(error.message(), "reasoning control is not supported for fx");
+    }
+
+    #[test]
+    fn parses_model_ids() {
+        let models = parse_models(
+            r#"{"kind":"models","count":2,"shown_count":2,"more_count":0,"private_models_hidden":false,"ids":["openai/gpt-test","anthropic/claude-test"]}"#,
+        )
+        .unwrap();
+
+        assert_eq!(models[0].id, "openai/gpt-test");
+        assert_eq!(models[0].name, "gpt-test");
+        assert_eq!(models[0].description, "openai provider");
+        assert_eq!(models[1].id, "anthropic/claude-test");
+    }
+
+    #[test]
+    fn parses_subscription_model_ids() {
+        let models = parse_models(
+            r#"{"kind":"models","count":1,"shown_count":1,"more_count":0,"private_models_hidden":false,"ids":["gpt-5.4"],"models":[{"id":"gpt-5.4","source":"Codex"}]}"#,
+        )
+        .unwrap();
+
+        assert_eq!(models[0].id, "gpt-5.4");
+        assert_eq!(models[0].name, "gpt-5.4");
+        assert_eq!(models[0].description, "Codex provider");
+    }
+
+    #[test]
+    fn extracts_answer_and_session() {
+        let response = parse_response(
+            r#"{"output":"hello","exit_code":0,"model":"openai/gpt-test","session_id":"session-1","steps":1,"tool_calls":[]}"#,
+        )
+        .unwrap();
+
+        assert_eq!(response.answer, "hello");
+        assert_eq!(response.session_id, "session-1");
+    }
+
+    #[test]
+    fn surfaces_reported_error() {
+        let error = parse_response(
+            r#"{"output":"","exit_code":1,"model":"","session_id":"","steps":0,"tool_calls":[],"error":"MissingCredentials"}"#,
+        )
+        .unwrap_err();
+
+        assert_eq!(error.message(), "fx reported an error: MissingCredentials");
+    }
+}
diff --git a/src/harness/mod.rs b/src/harness/mod.rs
index c9a3e62..c79e975 100644
--- a/src/harness/mod.rs
+++ b/src/harness/mod.rs
@@ -1,5 +1,6 @@
 mod claude;
 mod codex;
+mod fx;
 mod opencode;
 mod pi;
 
@@ -110,6 +111,20 @@ pub static DEFINITIONS: &[Definition] = &[
         program_env: "ASK_CLAUDE_BIN",
         create: |program| Box::new(claude::Claude::new(program)),
     },
+    Definition {
+        id: "fx",
+        aliases: &[],
+        name: "fx",
+        description: "fx coding agent",
+        default_model: None,
+        default_reasoning: None,
+        reasoning: ReasoningControl::Managed {
+            label: "Managed by fx",
+            explanation: "fx manages reasoning automatically for its selected model.",
+        },
+        program_env: "ASK_FX_BIN",
+        create: |program| Box::new(fx::Fx::new(program)),
+    },
     Definition {
         id: "opencode",
         aliases: &["open-code"],
@@ -203,6 +218,7 @@ mod tests {
         assert_eq!(resolve("codex").unwrap().id, "codex");
         assert_eq!(resolve("claude-code").unwrap().id, "claude");
         assert_eq!(resolve("open-code").unwrap().id, "opencode");
+        assert_eq!(resolve("fx").unwrap().id, "fx");
         assert!(resolve("missing").is_err());
     }
 }