diff --git a/agent_assembly/core/assembly.py b/agent_assembly/core/assembly.py
index 0ffaf0b..05be85d 100644
--- a/agent_assembly/core/assembly.py
+++ b/agent_assembly/core/assembly.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import os
+import re
 import sys
 from dataclasses import dataclass, field
 from threading import Lock
@@ -42,12 +43,27 @@
 ENV_CONTROL_PLANE_URL = "AA_CONTROL_PLANE_URL"
 
 _DEFAULT_AGENT_ID = "agent-assembly-default"
+_AGENT_ID_RE = re.compile(r"^[A-Za-z0-9_.-]{1,128}$")
 _VALID_RUNTIME_MODES = {"auto", "ebpf", "proxy", "sdk-only"}
 _VALID_ENFORCEMENT_MODES: frozenset[EnforcementMode] = frozenset({"enforce", "observe", "disabled"})
 _INIT_LOCK = Lock()
 _ACTIVE_CONTEXT: AssemblyContext | None = None
 
 
+def _validate_agent_id(agent_id: str) -> str:
+    """Validate agent_id format before it reaches socket-path interpolation (AAASM-4301).
+
+    Only allows [A-Za-z0-9_.-] up to 128 chars. Reject anything else with a clear ValueError
+    so callers get a fast, actionable error instead of a downstream FileNotFoundError from
+    a malformed socket path.
+    """
+    if not _AGENT_ID_RE.fullmatch(agent_id):
+        raise ValueError(
+            f"invalid agent_id: {agent_id!r}; must match {_AGENT_ID_RE.pattern}"
+        )
+    return agent_id
+
+
 class RuntimePatch(Protocol):
     """Internal monkey-patch mechanism used by framework adapters.
 
@@ -204,7 +220,7 @@ def init_assembly(
         if spawned_by_tool is None:
             spawned_by_tool = _spawn.spawned_by_tool
 
-    resolved_agent_id = agent_id or _DEFAULT_AGENT_ID
+    resolved_agent_id = _validate_agent_id(agent_id or _DEFAULT_AGENT_ID)
 
     global _ACTIVE_CONTEXT
     with _INIT_LOCK:
diff --git a/test/unit/test_assembly.py b/test/unit/test_assembly.py
index b087f5d..ac7e2d9 100644
--- a/test/unit/test_assembly.py
+++ b/test/unit/test_assembly.py
@@ -698,3 +698,95 @@ def test_init_assembly_no_warning_for_loopback_http_with_api_key(
             agent_id="agent-loopback",
         )
     context.shutdown()
+
+
+# AAASM-4301: validate_agent_id enforces a strict character-set + length cap
+# on the agent_id argument before it ever reaches socket-path interpolation.
+
+
+@pytest.mark.parametrize(
+    "agent_id",
+    [
+        "agent-assembly-default",  # the module default
+        "default",
+        "agent-1",
+        "my_agent_2026",
+        "a.b.c",
+        "A",  # single char, upper
+        "a",  # single char, lower
+        "0",  # single char, digit
+        "a" * 128,  # max length
+    ],
+)
+def test_validate_agent_id_accepts_well_formed_values(agent_id: str) -> None:
+    """Well-formed agent_ids match the regex and are returned unchanged."""
+    assert core_assembly._validate_agent_id(agent_id) == agent_id
+
+
+@pytest.mark.parametrize(
+    "agent_id",
+    [
+        "",  # empty string
+        "a" * 129,  # one char over the length cap
+        "../etc/passwd",  # path-traversal attempt
+        "has space",  # whitespace
+        "has/slash",  # path separator
+        "has$special",  # shell metacharacter
+        "unicode-α",  # non-ASCII alpha
+        "trailing\n",  # newline
+    ],
+)
+def test_validate_agent_id_rejects_malformed_values(agent_id: str) -> None:
+    """Malformed agent_ids raise ValueError with the regex pattern in the message."""
+    with pytest.raises(ValueError, match=r"invalid agent_id"):
+        core_assembly._validate_agent_id(agent_id)
+
+
+def test_init_assembly_rejects_malformed_agent_id(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """AAASM-4301 regression: malformed agent_id fails fast at init_assembly entry.
+
+    Without this validation, a path-traversal-like agent_id would be silently
+    interpolated into the runtime socket path, causing a downstream FileNotFoundError
+    when the SDK tries to connect. The validator raises ValueError up-front so the
+    caller gets a clear, actionable error.
+    """
+    monkeypatch.setattr(core_assembly, "_register_adapters", lambda **kwargs: [])
+    monkeypatch.setattr(
+        core_assembly,
+        "_start_network_layer",
+        lambda **kwargs: ("sdk-only", core_assembly._noop_shutdown),
+    )
+
+    with pytest.raises(ValueError, match=r"invalid agent_id"):
+        init_assembly(
+            gateway_url="http://localhost:8080",
+            api_key="test-api-key",
+            agent_id="../attack",
+        )
+
+
+def test_init_assembly_accepts_default_agent_id(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """The module-level _DEFAULT_AGENT_ID must satisfy the validator.
+
+    Guards against a future change that alters _DEFAULT_AGENT_ID to a value the
+    regex rejects, which would break every caller of init_assembly() (no args).
+    """
+    monkeypatch.setattr(core_assembly, "_register_adapters", lambda **kwargs: [])
+    monkeypatch.setattr(
+        core_assembly,
+        "_start_network_layer",
+        lambda **kwargs: ("sdk-only", core_assembly._noop_shutdown),
+    )
+
+    context = init_assembly(
+        gateway_url="http://localhost:8080",
+        api_key="test-api-key",
+    )
+    try:
+        assert context.client.agent_id == core_assembly._DEFAULT_AGENT_ID
+    finally:
+        context.shutdown()