diff --git a/crates/sc-compose/src/commands/compose_output.rs b/crates/sc-compose/src/commands/compose_output.rs
index 6651e8cf..624e2c4d 100644
--- a/crates/sc-compose/src/commands/compose_output.rs
+++ b/crates/sc-compose/src/commands/compose_output.rs
@@ -126,17 +126,9 @@ fn append_json_record(
         rendered,
     )
     .map_err(CommandError::render_check)?;
-    let value: serde_json::Value = serde_json::from_str(checked.body()).map_err(|error| {
-        CommandError::render_append(
-            anyhow!(error).context(format!(
-                "failed to parse checked JSON from template {}",
-                template_path.display()
-            )),
-            DiagnosticCode::ErrRenderJsonMalformed,
-            Vec::new(),
-        )
-    })?;
-    if !value.is_object() {
+    // The body already passed JSON syntax validation. Inspect only its root
+    // token, avoiding a numeric conversion that would reject valid exponents.
+    if !checked.body().trim_start().starts_with('{') {
         return Err(CommandError::render_append(
             anyhow!("--append requires the rendered output to be a JSON object"),
             DiagnosticCode::ErrRenderAppendNotObject,
diff --git a/crates/sc-compose/tests/cli/fuzz_regressions.rs b/crates/sc-compose/tests/cli/fuzz_regressions.rs
index 3fef7365..2706999a 100644
--- a/crates/sc-compose/tests/cli/fuzz_regressions.rs
+++ b/crates/sc-compose/tests/cli/fuzz_regressions.rs
@@ -426,3 +426,26 @@ fn fuzz_017_nested_raw_values_append_as_one_line_without_changing_lexemes() {
     assert_eq!(envelope["payload"]["bytes_written"], appended.len());
     assert_eq!(envelope["payload"]["appended"], true);
 }
+
+// FUZZ-017 round 3: JSON number grammar is not limited by floating-point range.
+#[test]
+fn fuzz_017_append_preserves_large_exponent_lexemes() {
+    let root = temp_root("fuzz-017-large-exponent");
+    write_file(
+        &root.join("rec.json.j2"),
+        r#"{"n":1e400,"tiny":-1.2300e-4000}"#,
+    );
+    let destination = root.join("log.jsonl");
+    let output = sc_compose()
+        .args(["render", "--file", "rec.json.j2", "--root"])
+        .arg(&root)
+        .arg("--append")
+        .arg(&destination)
+        .output()
+        .unwrap();
+    assert!(output.status.success(), "{output:?}");
+    assert_eq!(
+        std::fs::read_to_string(destination).unwrap(),
+        "{\"n\":1e400,\"tiny\":-1.2300e-4000}\n"
+    );
+}
diff --git a/crates/sc-composer/src/render_check.rs b/crates/sc-composer/src/render_check.rs
index 1dc99c9b..425835bd 100644
--- a/crates/sc-composer/src/render_check.rs
+++ b/crates/sc-composer/src/render_check.rs
@@ -311,20 +311,20 @@ pub fn check_rendered_output_with_meta(
         });
     }
 
-    let Err(error) = serde_json::from_str::<serde_json::Value>(rendered) else {
-        return Ok(CheckedOutput {
-            body: rendered.to_owned(),
-            meta,
-        });
-    };
-
-    let line = error.line();
-    let column = error.column();
-    let byte_offset = byte_offset_at(rendered, line, column, error.classify());
-    // Keep the parser's first syntax failure authoritative. Its recursion
-    // guard reports the opening container at the unsupported depth; only
-    // matching structural evidence changes that failure's classification.
-    if excessive_json_depth(rendered) == Some(byte_offset) {
+    // RawValue validates JSON grammar without converting number lexemes to f64.
+    // Its iterative parser is paired with our explicit nesting bound below.
+    let parse_error = serde_json::from_str::<&serde_json::value::RawValue>(rendered).err();
+    let syntax_offset = parse_error
+        .as_ref()
+        .map(|error| byte_offset_at(rendered, error.line(), error.column(), error.classify()));
+    // Preserve the first failure: an earlier syntax error remains malformed,
+    // while exceeding the supported depth is reported before later failures.
+    if let Some(byte_offset) = excessive_json_depth(rendered)
+        && syntax_offset.is_none_or(|offset| byte_offset <= offset)
+    {
+        let prefix = &rendered[..byte_offset];
+        let line = prefix.bytes().filter(|byte| *byte == b'\n').count() + 1;
+        let column = prefix.rsplit('\n').next().map_or(0, str::len) + 1;
         return Err(OutputCheckError {
             reason: OutputCheckReason::JsonDepthLimit { limit: MAX_JSON_NESTING_DEPTH },
             diagnostics: vec![Diagnostic::new(
@@ -334,6 +334,15 @@ pub fn check_rendered_output_with_meta(
             ).with_path(&meta.template).with_location(line, column)],
         });
     }
+    let Some(error) = parse_error else {
+        return Ok(CheckedOutput {
+            body: rendered.to_owned(),
+            meta,
+        });
+    };
+    let line = error.line();
+    let column = error.column();
+    let byte_offset = byte_offset_at(rendered, line, column, error.classify());
 
     let diagnostic = Diagnostic::new(
         DiagnosticSeverity::Error,
@@ -435,6 +444,28 @@ mod tests {
         }
     }
 
+    #[test]
+    fn json_number_validation_preserves_grammar_beyond_f64_range() {
+        let body = r#"{"n":1e400,"tiny":-1.2300e-4000}"#;
+        assert_eq!(
+            check_rendered_output(OutputFormat::Json, Path::new("numbers.json.j2"), body)
+                .expect("valid number lexemes")
+                .body(),
+            body
+        );
+        let deep = format!("{}1e400{}", "[".repeat(128), "]".repeat(128));
+        let error = check_rendered_output(OutputFormat::Json, Path::new("deep.json.j2"), &deep)
+            .expect_err("depth remains bounded");
+        assert_eq!(
+            error.reason,
+            super::OutputCheckReason::JsonDepthLimit { limit: 127 }
+        );
+        assert_eq!(
+            error.diagnostics[0].code,
+            crate::DiagnosticCode::ErrRenderJsonDepthLimit
+        );
+    }
+
     #[test]
     fn json_depth_boundary_has_a_distinct_typed_failure() {
         for (open, close) in [("{\"a\":", "}"), ("[", "]")] {