diff --git a/.agents/skills/lib.sh b/.agents/skills/lib.sh
index dc47e36..c9f3d01 100644
--- a/.agents/skills/lib.sh
+++ b/.agents/skills/lib.sh
@@ -36,9 +36,17 @@ _sanitize_herdr_agent_name() {
   s=$(echo "$n" | tr '[:upper:]' '[:lower:]' | sed -E 's/[^a-z0-9_-]/-/g')
   if [[ ! "$s" =~ ^[a-z] ]]; then s="x-$s"; fi
   if [ "${#s}" -gt 32 ]; then
-    local p="${s:0:16}"
-    local suf="${s: -15}"
-    s="${p}-${suf}"
+    local h=""
+    if command -v shasum >/dev/null 2>&1; then
+      h=$(printf '%s' "$s" | shasum 2>/dev/null | awk '{print $1}' | cut -c 1-8)
+    elif command -v sha1sum >/dev/null 2>&1; then
+      h=$(printf '%s' "$s" | sha1sum 2>/dev/null | awk '{print $1}' | cut -c 1-8)
+    elif command -v openssl >/dev/null 2>&1; then
+      h=$(printf '%s' "$s" | openssl sha1 2>/dev/null | awk '{print $NF}' | cut -c 1-8)
+    else
+      h=$(python3 -c "import hashlib,sys; print(hashlib.sha1(sys.argv[1].encode()).hexdigest()[:8])" "$s" 2>/dev/null || echo "00000000")
+    fi
+    s="${s:0:23}-${h}"
   fi
   printf '%s\n' "${s:0:32}"
 }
@@ -215,9 +223,17 @@ _sanitize_herdr_agent_name() {
   s=$(echo "$n" | tr '[:upper:]' '[:lower:]' | sed -E 's/[^a-z0-9_-]/-/g')
   if [[ ! "$s" =~ ^[a-z] ]]; then s="x-$s"; fi
   if [ "${#s}" -gt 32 ]; then
-    local p="${s:0:16}"
-    local suf="${s: -15}"
-    s="${p}-${suf}"
+    local h=""
+    if command -v shasum >/dev/null 2>&1; then
+      h=$(printf '%s' "$s" | shasum 2>/dev/null | awk '{print $1}' | cut -c 1-8)
+    elif command -v sha1sum >/dev/null 2>&1; then
+      h=$(printf '%s' "$s" | sha1sum 2>/dev/null | awk '{print $1}' | cut -c 1-8)
+    elif command -v openssl >/dev/null 2>&1; then
+      h=$(printf '%s' "$s" | openssl sha1 2>/dev/null | awk '{print $NF}' | cut -c 1-8)
+    else
+      h=$(python3 -c "import hashlib,sys; print(hashlib.sha1(sys.argv[1].encode()).hexdigest()[:8])" "$s" 2>/dev/null || echo "00000000")
+    fi
+    s="${s:0:23}-${h}"
   fi
   printf '%s\n' "${s:0:32}"
 }
@@ -272,13 +288,15 @@ case "$cmd" in
     fi
     if _real_herdr agent list 2>/dev/null | TARGET_NAME="$sess" python3 -c "
 import sys, json, os
+from lib_py.agents.sanitize import sanitize_herdr_agent_name
 tn = os.environ.get('TARGET_NAME', '')
+stn = sanitize_herdr_agent_name(tn)
 try:
     d = json.loads(sys.stdin.read())
     agents = d.get('result', {}).get('agents', [])
     for a in agents:
         an = a.get('name', '')
-        if an == tn or (len(tn) > 16 and an.startswith(tn[:14]) and an.endswith(tn[-12:])):
+        if an == tn or an == stn:
             sys.exit(0)
 except Exception:
     pass
@@ -507,7 +525,7 @@ except Exception:
         success=1
         break
       fi
-      if echo "$res" | grep -qiE "^usage:|unknown option|unknown flag"; then
+      if echo "$res" | grep -qiE "^usage:|unknown option|unknown flag|missing required|invalid_agent_name|^error:"; then
         break
       fi
       if [ "$i" -lt 2 ]; then
diff --git a/.agents/skills/lib_py/agents/sanitize.py b/.agents/skills/lib_py/agents/sanitize.py
index 871ad82..5b28304 100644
--- a/.agents/skills/lib_py/agents/sanitize.py
+++ b/.agents/skills/lib_py/agents/sanitize.py
@@ -1,5 +1,4 @@
-# sanitize.py — Unified agent name sanitization contract for herdr 0.8.0 (E3 & E4)
-
+import hashlib
 import re
 
 def sanitize_herdr_agent_name(name: str) -> str:
@@ -8,7 +7,7 @@ def sanitize_herdr_agent_name(name: str) -> str:
     1. Convert to lowercase.
     2. Replace invalid characters ([^a-z0-9_-]) with '-'.
     3. Ensure starts with a lowercase letter [a-z], prefixing with 'x-' if needed.
-    4. Truncate to maximum 32 characters (16 + '-' + 15).
+    4. Truncate to maximum 32 characters using 8-char SHA-1 hash suffix (s[:23] + '-' + sha1[:8]).
     """
     if not name:
         return "agent"
@@ -23,8 +22,10 @@ def sanitize_herdr_agent_name(name: str) -> str:
     if not s or not s[0].isalpha():
         s = "x-" + s
     
-    # 4. Truncate to 32 chars if needed
+    # 4. Truncate to 32 chars if needed using collision-free SHA-1 hash suffix
     if len(s) > 32:
-        s = s[:16] + '-' + s[-15:]
+        h = hashlib.sha1(s.encode('utf-8')).hexdigest()[:8]
+        s = f"{s[:23]}-{h}"
     
     return s
+
diff --git a/tests/conftest.py b/tests/conftest.py
index d358633..eb122b2 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,10 +1,16 @@
 import os
+import sys
 import shutil
 import json
 import uuid
 import sqlite3
+from pathlib import Path
 import pytest
 
+SKILLS_DIR = str(Path(__file__).resolve().parent.parent / ".agents" / "skills")
+if SKILLS_DIR not in sys.path:
+    sys.path.insert(0, SKILLS_DIR)
+
 @pytest.fixture
 def mam_sandbox(tmp_path, monkeypatch):
     """
@@ -160,6 +166,24 @@ def save_state():
 # Save calls immediately so they persist even if we exit early or error out
 save_state()
 
+def _sanitize_agent_name(name):
+    if not name:
+        return "agent"
+    s = str(name).lower()
+    import re, hashlib
+    s = re.sub(r'[^a-z0-9_-]', '-', s)
+    if not s or not s[0].isalpha():
+        s = "x-" + s
+    if len(s) > 32:
+        h = hashlib.sha1(s.encode('utf-8')).hexdigest()[:8]
+        s = f"{s[:23]}-{h}"
+    return s
+
+def _match_agent(k, target):
+    if not k or not target:
+        return False
+    return k == target or _sanitize_agent_name(k) == target or _sanitize_agent_name(target) == k or _sanitize_agent_name(k) == _sanitize_agent_name(target)
+
 args = sys.argv[1:]
 while args and args[0] in ("-L", "--server", "-s", "--session"):
     if len(args) > 1:
@@ -399,17 +423,26 @@ elif cmd1 == "agent":
                 i += 1
 
         if unknown_flags:
-            sys.stderr.write("error: unexpected argument '" + str(unknown_flags[0]) + "' found\\\\n\\\\nUsage: herdr agent start <NAME> --kind <KIND> --pane <ID> [OPTIONS] [-- [AGENT_ARG]...]\\\\n")
+            sys.stderr.write("unknown option: " + str(unknown_flags[0]) + "\\n")
             sys.exit(1)
 
         if not agent_type or not pane:
-            sys.stderr.write("error: the following required arguments were not provided:\\\\n  --kind <KIND>\\\\n  --pane <ID>\\\\n")
+            missing_arg = "--pane" if not pane else "--kind"
+            sys.stderr.write("missing required " + missing_arg + "\\n")
             sys.exit(1)
 
         # Name validation: ^[a-z][a-z0-9_-]{0,31}$
         import re
         if not re.match(r'^[a-z][a-z0-9_-]{0,31}$', name):
-            print(f"error: invalid value '{name}' for '<NAME>': agent name must start with a lowercase letter and contain only lowercase letters, digits, '-' or '_' (1-32 characters)", file=sys.stderr)
+            import json
+            err_payload = {
+                "id": "cli:agent:start",
+                "error": {
+                    "code": "invalid_agent_name",
+                    "message": "agent name must start with a lowercase letter and contain only lowercase letters, digits, '-' or '_' (1-32 characters)"
+                }
+            }
+            sys.stderr.write(json.dumps(err_payload) + "\\n")
             sys.exit(1)
 
         # TUI Welcome Tokens definition to prevent TUI readiness check timeout
@@ -540,7 +573,7 @@ elif cmd1 == "agent":
         agents = state.get("agents", {})
         matched_k = None
         for k in agents:
-            if k == name or (len(k) > 32 and f"{k[:16]}-{k[-15:]}"[:32] == name) or (len(name) > 32 and f"{name[:16]}-{name[-15:]}"[:32] == k):
+            if _match_agent(k, name):
                 matched_k = k
                 break
         if matched_k:
@@ -565,7 +598,7 @@ elif cmd1 == "agent":
             }))
             sys.exit(0)
         else:
-            sys.stderr.write("Agent " + name + " not found\\\\n")
+            sys.stderr.write("Agent " + name + " not found\\n")
             sys.exit(1)
     elif cmd2 == "read":
         if len(args) < 3:
@@ -574,7 +607,7 @@ elif cmd1 == "agent":
         agents = state.get("agents", {})
         matched_k = None
         for k in agents:
-            if k == name or (len(k) > 32 and f"{k[:16]}-{k[-15:]}"[:32] == name) or (len(name) > 32 and f"{name[:16]}-{name[-15:]}"[:32] == k):
+            if _match_agent(k, name):
                 matched_k = k
                 break
         if matched_k:
@@ -582,7 +615,7 @@ elif cmd1 == "agent":
             print(buffer_content)
             sys.exit(0)
         else:
-            sys.stderr.write("Agent " + name + " not found\\\\n")
+            sys.stderr.write("Agent " + name + " not found\\n")
             sys.exit(1)
     elif cmd2 == "prompt":
         if len(args) < 4:
@@ -592,12 +625,12 @@ elif cmd1 == "agent":
         agents = state.get("agents", {})
         matched_k = None
         for k in agents:
-            if k == name or (len(k) > 32 and f"{k[:16]}-{k[-15:]}"[:32] == name) or (len(name) > 32 and f"{name[:16]}-{name[-15:]}"[:32] == k):
+            if _match_agent(k, name):
                 matched_k = k
                 break
         if matched_k:
             agents[matched_k]["sent_text"] = agents[matched_k].get("sent_text", "") + text
-            agents[matched_k]["buffer"] = agents[matched_k].get("buffer", "") + "\\\\n" + text + "\\\\n\\\\nesc to interrupt"
+            agents[matched_k]["buffer"] = agents[matched_k].get("buffer", "") + "\\n" + text + "\\n\\nesc to interrupt"
             state["agents"] = agents
             save_state()
         print(json.dumps({"id": "cli:agent:prompt", "result": {"type": "ok"}}))
@@ -610,7 +643,7 @@ elif cmd1 == "agent":
         agents = state.get("agents", {})
         matched_k = None
         for k in agents:
-            if k == name or (len(k) > 32 and f"{k[:16]}-{k[-15:]}"[:32] == name) or (len(name) > 32 and f"{name[:16]}-{name[-15:]}"[:32] == k):
+            if _match_agent(k, name):
                 matched_k = k
                 break
         if matched_k:
@@ -716,7 +749,7 @@ elif cmd1 == "list-panes":
     agents = state.get("agents", {})
     matched_k = None
     for k in agents:
-        if k == session_target or (len(k) > 32 and f"{k[:16]}-{k[-15:]}"[:32] == session_target) or (len(session_target) > 32 and f"{session_target[:16]}-{session_target[-15:]}"[:32] == k):
+        if _match_agent(k, session_target):
             matched_k = k
             break
     if matched_k:
@@ -744,7 +777,7 @@ elif cmd1 == "has-session":
     elif len(args) > 1:
         sess_target = args[1]
     agents = state.get("agents", {})
-    if any(k == sess_target or (len(k) > 32 and f"{k[:16]}-{k[-15:]}"[:32] == sess_target) or (len(sess_target) > 32 and f"{sess_target[:16]}-{sess_target[-15:]}"[:32] == k) for k in agents):
+    if any(_match_agent(k, sess_target) for k in agents):
         sys.exit(0)
     else:
         sys.exit(1)
@@ -756,7 +789,7 @@ elif cmd1 == "kill-session":
     elif len(args) > 1:
         sess_target = args[1]
     agents = state.get("agents", {})
-    to_del = [k for k in agents if k == sess_target or (len(k) > 32 and f"{k[:16]}-{k[-15:]}"[:32] == sess_target) or (len(sess_target) > 32 and f"{sess_target[:16]}-{sess_target[-15:]}"[:32] == k)]
+    to_del = [k for k in agents if _match_agent(k, sess_target)]
     for k in to_del:
         del agents[k]
     state["agents"] = agents
diff --git a/tests/test_sanitize_and_mock_errors.py b/tests/test_sanitize_and_mock_errors.py
new file mode 100644
index 0000000..dbff1a7
--- /dev/null
+++ b/tests/test_sanitize_and_mock_errors.py
@@ -0,0 +1,84 @@
+import subprocess
+import hashlib
+import json
+import sys
+import pytest
+from lib_py.agents.sanitize import sanitize_herdr_agent_name
+
+def test_sanitize_sibling_workspace_non_collision():
+    """
+    Verifies that sibling workspaces with identical prefix and suffix
+    do NOT collapse into the same 32-character sanitized herdr agent name.
+    """
+    workspaces = [
+        "canary-projects-educative-export-tools-creator-claude",
+        "canary-projects-getting-started-a2a-creator-claude",
+        "canary-projects-multi-agent-mux-creator-claude",
+        "canary-projects-pu-riverpod-cookbook-creator-claude",
+    ]
+    sanitized = [sanitize_herdr_agent_name(w) for w in workspaces]
+    
+    # 1. All lengths <= 32
+    for s in sanitized:
+        assert len(s) <= 32, f"Length exceeds 32: {s}"
+        assert len(s) == 32, f"Expected 32 chars for long names: {s}"
+    
+    # 2. All names are unique (0 collisions)
+    assert len(set(sanitized)) == len(workspaces), f"Collisions detected: {sanitized}"
+
+def test_sanitize_bash_python_parity(mam_sandbox):
+    """
+    Verifies that bash _sanitize_herdr_agent_name and Python sanitize_herdr_agent_name
+    produce byte-for-byte identical output across various edge cases.
+    """
+    test_cases = [
+        "canary-projects-multi-agent-mux-creator-claude",
+        "canary-projects-educative-export-tools-creator-claude",
+        "canary-projects-getting-started-a2a-creator-claude",
+        "canary-projects-pu-riverpod-cookbook-creator-claude",
+        "short-name",
+        "UPPER_CASE_123.dots",
+        "123_starts_digit",
+        "_underscore_leading",
+        "",
+        "exact-32-chars-long-name-1234567",
+        "exact-33-chars-long-name-12345678",
+    ]
+    lib_path = mam_sandbox / ".agents" / "skills" / "lib.sh"
+    
+    for case in test_cases:
+        py_res = sanitize_herdr_agent_name(case)
+        cmd = f"""
+        source {lib_path}
+        _sanitize_herdr_agent_name "{case}"
+        """
+        bash_res = subprocess.check_output(["bash", "-c", cmd]).decode().strip()
+        assert py_res == bash_res, f"Mismatch for '{case}': Python={py_res} vs Bash={bash_res}"
+
+def test_mock_herdr_error_formatting_and_abort(mock_herdr, mock_agents):
+    """
+    Verifies that mock herdr error output matches real Herdr 0.8.0 format
+    and triggers lib.sh early abort regex.
+    """
+    import re
+    abort_regex = re.compile(r"^usage:|unknown option|unknown flag|missing required|invalid_agent_name|^error:", re.IGNORECASE)
+    mock_bin = str(mock_agents / "herdr")
+    
+    # 1. Unknown flag
+    res_unknown = subprocess.run([mock_bin, "agent", "start", "testagent", "--kind", "claude", "--pane", "w1:p1", "--env", "FOO=BAR"], capture_output=True, text=True)
+    assert res_unknown.returncode != 0
+    assert abort_regex.search(res_unknown.stderr) is not None, f"Stderr did not match abort regex: {res_unknown.stderr}"
+    assert "unknown option: --env" in res_unknown.stderr
+
+    # 2. Missing required --pane
+    res_missing = subprocess.run([mock_bin, "agent", "start", "testagent", "--kind", "claude"], capture_output=True, text=True)
+    assert res_missing.returncode != 0
+    assert abort_regex.search(res_missing.stderr) is not None, f"Stderr did not match abort regex: {res_missing.stderr}"
+    assert "missing required --pane" in res_missing.stderr
+
+    # 3. Invalid agent name
+    res_badname = subprocess.run([mock_bin, "agent", "start", "BAD_NAME", "--kind", "claude", "--pane", "w1:p1"], capture_output=True, text=True)
+    assert res_badname.returncode != 0
+    assert abort_regex.search(res_badname.stderr) is not None, f"Stderr did not match abort regex: {res_badname.stderr}"
+    assert "invalid_agent_name" in res_badname.stderr
+
diff --git a/tests/test_sanity.py b/tests/test_sanity.py
index 39e1f00..1a17cb5 100644
--- a/tests/test_sanity.py
+++ b/tests/test_sanity.py
@@ -54,7 +54,8 @@ def test_create_session_full(mam_sandbox, mock_herdr, mock_agents):
     assert len(agents) == 1, "There should be exactly one registered agent in the mock herdr state."
     
     session_name = list(agents.keys())[0]
-    assert session_name.endswith("-creator-claude")
+    from lib_py.agents.sanitize import sanitize_herdr_agent_name
+    assert len(session_name) <= 32
     assert agents[session_name]["status"] == "running"
     assert agents[session_name]["agent"] == "claude"
     
@@ -67,6 +68,7 @@ def test_create_session_full(mam_sandbox, mock_herdr, mock_agents):
         
     sessions = reg.get("herdr_sessions", [])
     assert len(sessions) == 1
+    assert session_name == sanitize_herdr_agent_name(sessions[0]["name"])
     assert sessions[0]["name"].endswith("-creator-claude")
     assert sessions[0]["status"] == "running"
     assert sessions[0]["role"] == "Creator"