diff --git a/_assembler/tests/determinism.rs b/_assembler/tests/determinism.rs
new file mode 100644
index 0000000..b0c7e0f
--- /dev/null
+++ b/_assembler/tests/determinism.rs
@@ -0,0 +1,96 @@
+//! Determinism + ordering tests for the assembler.
+//!
+//! The assembler module docstring promises:
+//! > Output is deterministic: same manifest + blocks → byte-identical .md
+//!
+//! These tests actually verify that promise. Catches any accidental
+//! `HashMap`-iteration leak, embedded timestamp, or non-stable sort.
+
+mod common;
+
+use common::{assemble_one, seed_tempdir};
+use std::fs;
+
+/// Same input, two runs, byte-identical output.
+#[test]
+fn determinism_same_input_byte_identical() {
+    let (_tmp1, root1) = seed_tempdir();
+    let first = assemble_one(&root1, "code-implementer");
+
+    let (_tmp2, root2) = seed_tempdir();
+    let second = assemble_one(&root2, "code-implementer");
+
+    assert_eq!(
+        first.as_bytes(),
+        second.as_bytes(),
+        "two independent runs produced different bytes"
+    );
+}
+
+/// Same input, ten runs, all byte-identical. Higher chance to catch
+/// hash-map iteration nondeterminism that escapes a 2-run check.
+#[test]
+fn determinism_ten_runs_all_identical() {
+    let mut seen: Option<String> = None;
+    for i in 0..10 {
+        let (_tmp, root) = seed_tempdir();
+        let out = assemble_one(&root, "researcher");
+        match &seen {
+            None => seen = Some(out),
+            Some(prev) => assert_eq!(
+                prev.as_bytes(),
+                out.as_bytes(),
+                "run {i} diverged from run 0"
+            ),
+        }
+    }
+}
+
+/// Block ordering: the order in `manifest.blocks` defines the order
+/// in the output. Reorder the blocks list → output changes, and the
+/// change is localized to the block region (not to frontmatter or
+/// trailing sections).
+#[test]
+fn block_order_controls_output_order() {
+    let (_tmp, root) = seed_tempdir();
+
+    // Baseline: default researcher (baseline, evidence-grading, memory-protocol).
+    let default_out = assemble_one(&root, "researcher");
+
+    // Swap two blocks — write a modified manifest into the same tempdir.
+    let manifest_src = fs::read_to_string(root.join("_manifests/researcher.toml")).unwrap();
+    let swapped = manifest_src.replace(
+        "blocks = [\n    \"baseline\",              # OBLIGATORY\n    \"evidence-grading\",      # OBLIGATORY\n    \"memory-protocol\",       # OBLIGATORY\n]",
+        "blocks = [\n    \"baseline\",\n    \"memory-protocol\",\n    \"evidence-grading\",\n]",
+    );
+    assert_ne!(
+        manifest_src, swapped,
+        "blocks-list replacement did not match — test fixture drifted"
+    );
+    fs::write(root.join("_manifests/researcher.toml"), &swapped).unwrap();
+
+    let swapped_out = assemble_one(&root, "researcher");
+
+    // 1. Output is different.
+    assert_ne!(
+        default_out, swapped_out,
+        "swapping block order did not change output"
+    );
+
+    // 2. Frontmatter unchanged (first `---` through the trailing `---\n\n`
+    //    ends identically — compare the first 500 bytes, which cover
+    //    frontmatter for all our fixtures).
+    let prefix_len = default_out
+        .find("# BASELINE")
+        .expect("BASELINE marker missing in default output");
+    assert_eq!(
+        &default_out[..prefix_len],
+        &swapped_out[..prefix_len],
+        "frontmatter + role drifted when only blocks were reordered"
+    );
+
+    // 3. The "# DOMAIN SCOPE" marker appears in both (tail section unchanged
+    //    by block reordering).
+    assert!(default_out.contains("# DOMAIN SCOPE"));
+    assert!(swapped_out.contains("# DOMAIN SCOPE"));
+}
diff --git a/_assembler/tests/roundtrip.rs b/_assembler/tests/roundtrip.rs
new file mode 100644
index 0000000..972fdd5
--- /dev/null
+++ b/_assembler/tests/roundtrip.rs
@@ -0,0 +1,90 @@
+//! Roundtrip / data-preservation tests.
+//!
+//! The assembler projects the Manifest struct into a Markdown file.
+//! We cannot re-parse a Markdown file back into a Manifest (the
+//! projection is lossy: comments / blank lines / heading formatting),
+//! but we CAN assert that every user-visible string from the manifest
+//! appears verbatim in the generated output — i.e. no field is
+//! silently dropped by a refactor.
+
+mod common;
+
+use common::{assemble_one, seed_tempdir};
+use std::fs;
+
+/// Every `domain_in` bullet, every `forbidden_domain` bullet, every
+/// handoff target + trigger, and the agent name must appear in the
+/// generated output. Covers the code-implementer manifest which has
+/// the richest field population.
+#[test]
+fn every_manifest_string_appears_in_output() {
+    let (_tmp, root) = seed_tempdir();
+    let out = assemble_one(&root, "code-implementer");
+
+    // Parse the same manifest independently with toml crate so we
+    // can iterate its fields without reaching into the private
+    // Manifest struct from main.rs.
+    let toml_text =
+        fs::read_to_string(root.join("_manifests/code-implementer.toml")).unwrap();
+    let parsed: toml::Value = toml::from_str(&toml_text).unwrap();
+
+    let name = parsed["name"].as_str().unwrap();
+    assert!(
+        out.contains(&format!("name: {name}")),
+        "frontmatter missing name"
+    );
+
+    let model = parsed["model"].as_str().unwrap();
+    assert!(
+        out.contains(&format!("model: {model}")),
+        "frontmatter missing model"
+    );
+
+    // Tools are joined with ", ".
+    let tools: Vec<&str> = parsed["tools"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|v| v.as_str().unwrap())
+        .collect();
+    let tools_line = format!("tools: {}", tools.join(", "));
+    assert!(
+        out.contains(&tools_line),
+        "frontmatter tools line missing or wrong order"
+    );
+
+    // domain_in bullets.
+    for item in parsed["domain_in"].as_array().unwrap() {
+        let s = item.as_str().unwrap();
+        assert!(out.contains(s), "domain_in entry missing: {s}");
+    }
+
+    // forbidden_domain bullets.
+    for item in parsed["forbidden_domain"].as_array().unwrap() {
+        let s = item.as_str().unwrap();
+        assert!(out.contains(s), "forbidden_domain entry missing: {s}");
+    }
+
+    // Handoffs: each target AND each trigger appears.
+    for h in parsed["handoff"].as_array().unwrap() {
+        let target = h["target"].as_str().unwrap();
+        let trigger = h["trigger"].as_str().unwrap();
+        assert!(out.contains(target), "handoff target missing: {target}");
+        assert!(out.contains(trigger), "handoff trigger missing: {trigger}");
+    }
+}
+
+/// Double-assembly determinism at the text level: parse + assemble
+/// twice from the very same tempdir (not two separate tempdirs) —
+/// catches any caching or mutable-global drift inside the binary.
+#[test]
+fn double_assembly_same_tempdir_identical() {
+    let (_tmp, root) = seed_tempdir();
+    let first = assemble_one(&root, "patent-compliance");
+    let second = assemble_one(&root, "patent-compliance");
+    assert_eq!(
+        first.as_bytes(),
+        second.as_bytes(),
+        "consecutive runs in same tempdir diverged"
+    );
+}
diff --git a/_assembler/tests/validator_negative.rs b/_assembler/tests/validator_negative.rs
new file mode 100644
index 0000000..4c0445f
--- /dev/null
+++ b/_assembler/tests/validator_negative.rs
@@ -0,0 +1,158 @@
+//! Validator negative-path tests.
+//!
+//! Locks the error contract of validator.rs: each flavour of bad
+//! manifest produces a non-zero exit status AND a stderr message
+//! that names the offending invariant.
+//!
+//! Note: the unsubstituted-`{{placeholder}}` check is being added
+//! in a parallel PR (fix/remaining-findings). That specific test
+//! is deliberately NOT included here; when the check lands, add a
+//! case here and re-run.
+
+mod common;
+
+use common::{run_assemble, seed_tempdir};
+use std::fs;
+use std::path::Path;
+
+/// Write a minimal valid manifest then mutate one field to break it.
+/// Returns the tempdir guard (keeps it alive) and the manifest path.
+fn write_broken(
+    root: &Path,
+    filename: &str,
+    mutate: impl FnOnce(&mut String),
+) -> std::path::PathBuf {
+    let src = fs::read_to_string(root.join("_manifests/researcher.toml")).unwrap();
+    let mut buf = src;
+    mutate(&mut buf);
+    let target = root.join("_manifests").join(filename);
+    fs::write(&target, buf).unwrap();
+    target
+}
+
+fn assert_fails_with(root: &Path, manifest: &Path, needle: &str) {
+    let out = run_assemble(root, &[manifest.to_str().unwrap()]);
+    assert!(
+        !out.status.success(),
+        "expected non-zero exit for broken manifest {}; stdout={:?} stderr={:?}",
+        manifest.display(),
+        String::from_utf8_lossy(&out.stdout),
+        String::from_utf8_lossy(&out.stderr),
+    );
+    let combined = format!(
+        "{}{}",
+        String::from_utf8_lossy(&out.stdout),
+        String::from_utf8_lossy(&out.stderr)
+    );
+    assert!(
+        combined.contains(needle),
+        "stderr did not mention {needle:?}; full output:\n{combined}"
+    );
+}
+
+#[test]
+fn validator_rejects_unknown_block_ref() {
+    let (_tmp, root) = seed_tempdir();
+    // Add an extra block name that doesn't exist on disk.
+    let manifest = write_broken(&root, "broken-unknown-block.toml", |s| {
+        *s = s.replace(
+            "\"memory-protocol\",       # OBLIGATORY\n]",
+            "\"memory-protocol\",\n    \"this-block-does-not-exist\",\n]",
+        );
+    });
+    assert_fails_with(&root, &manifest, "this-block-does-not-exist");
+}
+
+#[test]
+fn validator_rejects_missing_obligatory_block() {
+    let (_tmp, root) = seed_tempdir();
+    // Drop "memory-protocol" from the blocks list.
+    let manifest = write_broken(&root, "broken-missing-obligatory.toml", |s| {
+        *s = s.replace("\"memory-protocol\",       # OBLIGATORY\n", "");
+    });
+    assert_fails_with(&root, &manifest, "memory-protocol");
+}
+
+#[test]
+fn validator_rejects_empty_handoff() {
+    let (_tmp, root) = seed_tempdir();
+    // Strip every `[[handoff]]` table from the manifest.
+    let manifest = write_broken(&root, "broken-no-handoff.toml", |s| {
+        let mut out = String::new();
+        let mut skip = false;
+        for line in s.lines() {
+            if line.trim_start().starts_with("[[handoff]]") {
+                skip = true;
+                continue;
+            }
+            if skip && (line.trim_start().starts_with("[") || line.trim().is_empty()) {
+                // End of the handoff block (next [table] or blank-line gap).
+                if line.trim_start().starts_with("[") && !line.trim_start().starts_with("[[handoff]]") {
+                    skip = false;
+                } else if line.trim().is_empty() {
+                    // Tolerate blank line inside handoff table separator.
+                    continue;
+                }
+            }
+            if !skip {
+                out.push_str(line);
+                out.push('\n');
+            }
+        }
+        *s = out;
+    });
+    assert_fails_with(&root, &manifest, "handoff");
+}
+
+#[test]
+fn validator_rejects_empty_role() {
+    let (_tmp, root) = seed_tempdir();
+    // Replace the role with whitespace only.
+    let manifest = write_broken(&root, "broken-empty-role.toml", |s| {
+        // The researcher manifest uses triple-quoted `role = """..."""`.
+        let start = s.find("role = \"\"\"").expect("role block marker missing");
+        let end_rel = s[start..]
+            .find("\"\"\"\n")
+            .and_then(|_| s[start + 10..].find("\"\"\""))
+            .expect("role closing marker missing");
+        let end = start + 10 + end_rel + 3;
+        let before = &s[..start];
+        let after = &s[end..];
+        *s = format!("{before}role = \"   \"\n{after}");
+    });
+    assert_fails_with(&root, &manifest, "role");
+}
+
+#[test]
+fn validator_rejects_empty_domain_in() {
+    let (_tmp, root) = seed_tempdir();
+    // Replace domain_in array with an empty one.
+    let manifest = write_broken(&root, "broken-empty-domain-in.toml", |s| {
+        let start = s.find("domain_in = [").expect("domain_in marker missing");
+        let end_rel = s[start..].find("]\n").expect("domain_in close marker missing");
+        let end = start + end_rel + 2;
+        let before = &s[..start];
+        let after = &s[end..];
+        *s = format!("{before}domain_in = []\n{after}");
+    });
+    assert_fails_with(&root, &manifest, "domain_in");
+}
+
+#[test]
+fn validate_only_flag_skips_write() {
+    // --validate must NOT write anything under _generated/.
+    let (_tmp, root) = seed_tempdir();
+    let manifest = root.join("_manifests/researcher.toml");
+    let out = run_assemble(&root, &["--validate", manifest.to_str().unwrap()]);
+    assert!(
+        out.status.success(),
+        "--validate on a valid manifest failed: {}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let generated = root.join("_generated/researcher.md");
+    assert!(
+        !generated.exists(),
+        "--validate wrote an output file at {}",
+        generated.display()
+    );
+}