diff --git a/Cargo.toml b/Cargo.toml
index 85be7191..bf0e783a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -18,7 +18,7 @@ dirsql = { path = "packages/rust" }
 serde = { version = "1", features = ["derive"] }
 # `preserve_order`: a serialized row must keep the projection order the SELECT
 # list asked for, which the default BTreeMap-backed object discards.
-serde_json = { version = "1", features = ["preserve_order"] }
+serde_json = { version = "1", features = ["preserve_order", "float_roundtrip"] }
 regex = "1"
 thiserror = "2"
 tokio = { version = "1", features = ["rt", "sync"] }
diff --git a/packages/python/changelog.d/2026-11-04-exact-json-floats.md b/packages/python/changelog.d/2026-11-04-exact-json-floats.md
new file mode 100644
index 00000000..45059b89
--- /dev/null
+++ b/packages/python/changelog.d/2026-11-04-exact-json-floats.md
@@ -0,0 +1 @@
+**Fixed** JSON numbers from `on-file` hooks and worker functions now parse to the exact double: `123456789.12345679` was stored one ULP off as `123456789.1234568`.
diff --git a/packages/python/e2e-attestations/bot-1586-float-roundtrip-1104.json b/packages/python/e2e-attestations/bot-1586-float-roundtrip-1104.json
new file mode 100644
index 00000000..cd45a869
--- /dev/null
+++ b/packages/python/e2e-attestations/bot-1586-float-roundtrip-1104.json
@@ -0,0 +1,7 @@
+{
+  "command": "uv run python -m pytest tests/e2e/on_file_query_test.py tests/e2e/jsonl_parse_speed_test.py tests/e2e/use_case_transcripts_test.py tests/e2e/query_contract_test.py -x -q",
+  "ran_at": 1793814794,
+  "exit_code": 0,
+  "commit": "4696a76d81c3080df86c416ef418d3d9b2dc0062",
+  "branch": "bot/1586-float-roundtrip-1104"
+}
diff --git a/packages/python/migrations.d/2026-11-04-exact-json-floats.md b/packages/python/migrations.d/2026-11-04-exact-json-floats.md
new file mode 100644
index 00000000..fdf4eabe
--- /dev/null
+++ b/packages/python/migrations.d/2026-11-04-exact-json-floats.md
@@ -0,0 +1,21 @@
+### Exact JSON float parsing
+
+#### Summary
+
+A JSON float an `on-file` hook or worker function prints is stored as the correctly rounded REAL, where some were one ULP off before. No API or configuration changes are required.
+
+#### Required changes
+
+_None._
+
+#### Deprecations removed
+
+_None._
+
+#### Behavior changes without code changes
+
+A REAL column fed from JSON can hold a value one ULP different from before the upgrade: the correct one.
+
+#### Verification
+
+In an empty directory, write `{"v":123456789.12345679}` to `a.ndjson` and run `dirsql query "SELECT v = 123456789.12345679 AS eq FROM './a.ndjson'" --on-file cat --format json`; the output is `[{"eq":1}]`.
diff --git a/packages/rust/changelog.d/2026-11-04-exact-json-floats.md b/packages/rust/changelog.d/2026-11-04-exact-json-floats.md
new file mode 100644
index 00000000..45059b89
--- /dev/null
+++ b/packages/rust/changelog.d/2026-11-04-exact-json-floats.md
@@ -0,0 +1 @@
+**Fixed** JSON numbers from `on-file` hooks and worker functions now parse to the exact double: `123456789.12345679` was stored one ULP off as `123456789.1234568`.
diff --git a/packages/rust/migrations.d/2026-11-04-exact-json-floats.md b/packages/rust/migrations.d/2026-11-04-exact-json-floats.md
new file mode 100644
index 00000000..fdf4eabe
--- /dev/null
+++ b/packages/rust/migrations.d/2026-11-04-exact-json-floats.md
@@ -0,0 +1,21 @@
+### Exact JSON float parsing
+
+#### Summary
+
+A JSON float an `on-file` hook or worker function prints is stored as the correctly rounded REAL, where some were one ULP off before. No API or configuration changes are required.
+
+#### Required changes
+
+_None._
+
+#### Deprecations removed
+
+_None._
+
+#### Behavior changes without code changes
+
+A REAL column fed from JSON can hold a value one ULP different from before the upgrade: the correct one.
+
+#### Verification
+
+In an empty directory, write `{"v":123456789.12345679}` to `a.ndjson` and run `dirsql query "SELECT v = 123456789.12345679 AS eq FROM './a.ndjson'" --on-file cat --format json`; the output is `[{"eq":1}]`.
diff --git a/packages/rust/tests/on_file_float.rs b/packages/rust/tests/on_file_float.rs
new file mode 100644
index 00000000..635e6160
--- /dev/null
+++ b/packages/rust/tests/on_file_float.rs
@@ -0,0 +1,40 @@
+//! A float an `on-file` hook prints is stored as the REAL a correctly
+//! rounded JSON parser reads.
+
+use std::fs;
+
+use dirsql::{DirSQL, Value};
+use tempfile::TempDir;
+
+#[test]
+fn a_real_column_keeps_the_exact_double() {
+    let doubles = [
+        "123456789.12345679",
+        "0.30000000000000004",
+        "1.7976931348623157e308",
+    ];
+    let root = TempDir::new().unwrap();
+    fs::write(
+        root.path().join(".dirsql.toml"),
+        "[[table]]\nname = \"t\"\nddl = \"CREATE TABLE t (v REAL)\"\nglob = \"*.ndjson\"\non-file = \"cat\"\n",
+    )
+    .unwrap();
+    let lines: String = doubles.iter().map(|d| format!("{{\"v\":{d}}}\n")).collect();
+    fs::write(root.path().join("a.ndjson"), lines).unwrap();
+    let db = DirSQL::builder()
+        .root(root.path())
+        .config(root.path().join(".dirsql.toml"))
+        .build()
+        .unwrap();
+    let stored: Vec<Value> = db
+        .query("SELECT v FROM t ORDER BY rowid")
+        .unwrap()
+        .into_iter()
+        .map(|row| row["v"].clone())
+        .collect();
+    let expected: Vec<Value> = doubles
+        .iter()
+        .map(|d| Value::Real(d.parse().unwrap()))
+        .collect();
+    assert_eq!(stored, expected);
+}
diff --git a/packages/rust/tests/on_file_float_e2e.rs b/packages/rust/tests/on_file_float_e2e.rs
new file mode 100644
index 00000000..e0f5081b
--- /dev/null
+++ b/packages/rust/tests/on_file_float_e2e.rs
@@ -0,0 +1,37 @@
+//! End-to-end: a float an `on-file` hook prints reaches SQLite as the same
+//! REAL a correctly rounded JSON parser reads, through the real `dirsql`
+//! binary. Nothing is mocked.
+
+#![cfg(feature = "cli")]
+
+use assert_cmd::prelude::*;
+use tempfile::TempDir;
+
+#[test]
+fn an_on_file_float_round_trips_exactly() {
+    let root = TempDir::new().unwrap();
+    std::fs::write(root.path().join("a.ndjson"), "{\"v\":123456789.12345679}\n").unwrap();
+    let out = std::process::Command::cargo_bin("dirsql")
+        .expect("binary must exist")
+        .args([
+            "query",
+            "SELECT v, v = 123456789.12345679 AS eq FROM './a.ndjson'",
+            "--on-file",
+            "cat",
+            "--format",
+            "json",
+        ])
+        .env("DIRSQL_PROGRESS", "never")
+        .current_dir(root.path())
+        .output()
+        .expect("spawning `dirsql` failed");
+    assert!(
+        out.status.success(),
+        "expected success, stderr: {}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    assert_eq!(
+        String::from_utf8(out.stdout).unwrap(),
+        "[{\"v\":123456789.12345679,\"eq\":1}]\n"
+    );
+}
diff --git a/packages/ts/changelog.d/2026-11-04-exact-json-floats.md b/packages/ts/changelog.d/2026-11-04-exact-json-floats.md
new file mode 100644
index 00000000..45059b89
--- /dev/null
+++ b/packages/ts/changelog.d/2026-11-04-exact-json-floats.md
@@ -0,0 +1 @@
+**Fixed** JSON numbers from `on-file` hooks and worker functions now parse to the exact double: `123456789.12345679` was stored one ULP off as `123456789.1234568`.
diff --git a/packages/ts/e2e-attestations/bot-1586-float-roundtrip-1104.json b/packages/ts/e2e-attestations/bot-1586-float-roundtrip-1104.json
new file mode 100644
index 00000000..dc79d184
--- /dev/null
+++ b/packages/ts/e2e-attestations/bot-1586-float-roundtrip-1104.json
@@ -0,0 +1,7 @@
+{
+  "command": "pnpm exec vitest run --dir tests/e2e query-contract query-params",
+  "ran_at": 1793814888,
+  "exit_code": 0,
+  "commit": "0340207c463df4173a0e59f0993bde832ee2c50e",
+  "branch": "bot/1586-float-roundtrip-1104"
+}
diff --git a/packages/ts/migrations.d/2026-11-04-exact-json-floats.md b/packages/ts/migrations.d/2026-11-04-exact-json-floats.md
new file mode 100644
index 00000000..fdf4eabe
--- /dev/null
+++ b/packages/ts/migrations.d/2026-11-04-exact-json-floats.md
@@ -0,0 +1,21 @@
+### Exact JSON float parsing
+
+#### Summary
+
+A JSON float an `on-file` hook or worker function prints is stored as the correctly rounded REAL, where some were one ULP off before. No API or configuration changes are required.
+
+#### Required changes
+
+_None._
+
+#### Deprecations removed
+
+_None._
+
+#### Behavior changes without code changes
+
+A REAL column fed from JSON can hold a value one ULP different from before the upgrade: the correct one.
+
+#### Verification
+
+In an empty directory, write `{"v":123456789.12345679}` to `a.ndjson` and run `dirsql query "SELECT v = 123456789.12345679 AS eq FROM './a.ndjson'" --on-file cat --format json`; the output is `[{"eq":1}]`.