diff --git a/mapbox_tilesets/agent_detect.py b/mapbox_tilesets/agent_detect.py
new file mode 100644
index 0000000..7f1b634
--- /dev/null
+++ b/mapbox_tilesets/agent_detect.py
@@ -0,0 +1,67 @@
+"""Detect the AI coding agent (if any) driving this CLI invocation."""
+
+import os
+
+# (agent_id, [env_var, ...]) — table order is precedence order; the first entry with any
+# of its env vars present wins. Presence is the only thing ever tested - a var's value is
+# never read or compared against anything, for any entry. Even a var explicitly set to ""
+# or whitespace counts as present.
+#
+# Ported from mapbox-sdk-js's `lib/helpers/agent-detect.js` (this repo's sibling
+# implementation of the same allowlist - keep the two in sync). Canonical origin:
+# HuggingFace's public `agent-harnesses.ts` registry.
+#
+# "vtcode" and "warp" used to require a specific value (VTCODE == "1", TERM_PROGRAM ==
+# "WarpTerminal") rather than mere presence. Since values are never checked, "warp" was
+# dropped entirely: TERM_PROGRAM is set by most terminal emulators (iTerm2, Apple
+# Terminal, VS Code, Hyper, ...), not just Warp, so an existence check on it would
+# misidentify most terminal sessions as "warp". VTCODE has no such collision risk and
+# stays as a plain presence check.
+#
+# The final entry, "custom-agent", is a catch-all for AI_AGENT/AGENT: these exist so an
+# agent not on this list can still flag its presence, but we only ever check for them,
+# never read their value - an arbitrary, unvalidated string must never be forwarded into
+# telemetry as an "agent id".
+_ALLOWLIST = [
+    ("antigravity", ["ANTIGRAVITY_AGENT"]),
+    ("augment-cli", ["AUGMENT_AGENT"]),
+    ("cline", ["CLINE_ACTIVE"]),
+    ("cowork", ["CLAUDE_CODE_IS_COWORK"]),
+    ("claude-code", ["CLAUDECODE", "CLAUDE_CODE"]),
+    ("codex", ["CODEX_SANDBOX", "CODEX_CI", "CODEX_THREAD_ID"]),
+    ("crush", ["CRUSH"]),
+    ("gemini-cli", ["GEMINI_CLI"]),
+    ("github-copilot", ["COPILOT_MODEL", "COPILOT_ALLOW_ALL", "COPILOT_GITHUB_TOKEN"]),
+    ("goose", ["GOOSE_TERMINAL"]),
+    ("hermes-agent", ["HERMES_SESSION_ID"]),
+    ("kilo-code", ["KILOCODE_FEATURE"]),
+    ("kiro", ["AGENT_CONTEXT_OUT"]),
+    ("openclaw", ["OPENCLAW_SHELL"]),
+    ("opencode", ["OPENCODE_CLIENT"]),
+    ("pi", ["PI_CODING_AGENT"]),
+    ("replit", ["REPL_ID"]),
+    ("trae", ["TRAE_AI_SHELL_ID"]),
+    ("vtcode", ["VTCODE"]),
+    ("zed", ["ZED_TERM"]),
+    ("cursor-cli", ["CURSOR_AGENT"]),
+    ("cursor", ["CURSOR_TRACE_ID"]),
+    ("custom-agent", ["AI_AGENT", "AGENT"]),
+]
+
+
+def detect_agent():
+    """Detect the AI coding agent driving this CLI from environment variables.
+
+    Never reads or logs the full environment - only the matched id is used.
+
+    Returns
+    -------
+    str or None
+        The detected agent id, or None when no agent indicator is present.
+    """
+    for agent_id, env_vars in _ALLOWLIST:
+        for var in env_vars:
+            if var in os.environ:
+                return agent_id
+
+    return None
diff --git a/mapbox_tilesets/utils.py b/mapbox_tilesets/utils.py
index 8f8e2fa..9da8a93 100644
--- a/mapbox_tilesets/utils.py
+++ b/mapbox_tilesets/utils.py
@@ -12,6 +12,8 @@
 import geojson
 import json
 
+from mapbox_tilesets.agent_detect import detect_agent
+
 
 def load_module(modulename):
     """Dynamically imports a module and throws a readable exception if not found"""
@@ -51,7 +53,11 @@ def _get_session(
 ):
     """Get a configured session"""
     s = Session()
-    s.headers.update({"user-agent": "{}/{}".format(application, version)})
+    user_agent = "{}/{}".format(application, version)
+    agent = detect_agent()
+    if agent:
+        user_agent = "{} agent/{}".format(user_agent, agent)
+    s.headers.update({"user-agent": user_agent})
     return s
 
 
diff --git a/tests/test_agent_detect.py b/tests/test_agent_detect.py
new file mode 100644
index 0000000..d2c628d
--- /dev/null
+++ b/tests/test_agent_detect.py
@@ -0,0 +1,93 @@
+import os
+from unittest import mock
+
+from mapbox_tilesets.agent_detect import detect_agent
+
+
+def _detect_with_env(env):
+    # clear=True: these tests run inside various AI-agent shells (e.g. Claude
+    # Code sets CLAUDECODE), so the ambient environment must not leak in.
+    with mock.patch.dict(os.environ, env, clear=True):
+        return detect_agent()
+
+
+def test_no_indicators_returns_none():
+    assert _detect_with_env({}) is None
+
+
+def test_harness_var_wins_over_ai_agent_fallback():
+    # Harness-specific vars take precedence over AI_AGENT/AGENT even when both
+    # are present at once.
+    assert (
+        _detect_with_env({"CLAUDECODE": "1", "AI_AGENT": "something-else"})
+        == "claude-code"
+    )
+
+
+def test_codex_and_claude_code_are_distinct():
+    assert _detect_with_env({"CODEX_THREAD_ID": "abc"}) == "codex"
+    assert _detect_with_env({"CLAUDECODE": "1"}) == "claude-code"
+    assert _detect_with_env({"CLAUDE_CODE": "1"}) == "claude-code"
+
+
+def test_codex_matches_on_any_of_its_vars():
+    assert _detect_with_env({"CODEX_SANDBOX": "1"}) == "codex"
+    assert _detect_with_env({"CODEX_CI": "1"}) == "codex"
+
+
+def test_warp_was_dropped_term_program_is_not_a_safe_existence_only_signal():
+    # TERM_PROGRAM is set by most terminal emulators, not just Warp, so it's
+    # not on the allowlist at all now that presence is the only check.
+    assert _detect_with_env({"TERM_PROGRAM": "WarpTerminal"}) is None
+    assert _detect_with_env({"TERM_PROGRAM": "iTerm.app"}) is None
+
+
+def test_vtcode_matches_on_presence_alone_regardless_of_value():
+    assert _detect_with_env({"VTCODE": "1"}) == "vtcode"
+    assert _detect_with_env({"VTCODE": "0"}) == "vtcode"
+    assert _detect_with_env({"VTCODE": "true"}) == "vtcode"
+
+
+def test_table_order_precedence_among_harness_vars():
+    # antigravity is earlier in the table than cursor - it should win when
+    # both indicators are present.
+    assert (
+        _detect_with_env({"CURSOR_AGENT": "1", "ANTIGRAVITY_AGENT": "1"})
+        == "antigravity"
+    )
+
+
+def test_fallback_ai_agent_used_when_no_harness_match():
+    assert _detect_with_env({"AI_AGENT": "my-cool-tool"}) == "custom-agent"
+
+
+def test_fallback_agent_used_when_no_harness_or_ai_agent_match():
+    assert _detect_with_env({"AGENT": "my-cool-tool"}) == "custom-agent"
+
+
+def test_fallback_ai_agent_takes_precedence_over_agent():
+    # Same result either way - table order still decides precedence, but the
+    # value of AI_AGENT/AGENT is never read, only their presence.
+    assert _detect_with_env({"AI_AGENT": "first", "AGENT": "second"}) == "custom-agent"
+
+
+def test_github_copilot_matches_on_any_of_its_vars():
+    assert _detect_with_env({"COPILOT_MODEL": "gpt"}) == "github-copilot"
+    assert _detect_with_env({"COPILOT_ALLOW_ALL": "1"}) == "github-copilot"
+    assert _detect_with_env({"COPILOT_GITHUB_TOKEN": "abc"}) == "github-copilot"
+
+
+def test_presence_check_only_requires_existence_blank_value_still_counts():
+    # Existence is all that matters - a var set to "" or whitespace still
+    # counts as present, for both a harness var and the AI_AGENT/AGENT
+    # catch-all.
+    assert _detect_with_env({"CLAUDECODE": ""}) == "claude-code"
+    assert _detect_with_env({"CLAUDECODE": "   "}) == "claude-code"
+    assert _detect_with_env({"AI_AGENT": ""}) == "custom-agent"
+    assert _detect_with_env({"AI_AGENT": "   "}) == "custom-agent"
+
+
+def test_fallback_value_is_never_forwarded_even_if_header_unsafe():
+    # The fallback value must never reach the User-Agent header at all - it is
+    # never read, only checked for presence.
+    assert _detect_with_env({"AI_AGENT": "foo\nbar: injected"}) == "custom-agent"
diff --git a/tests/test_utils.py b/tests/test_utils.py
index f3d79c8..05a5f04 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -1,6 +1,7 @@
 import os
 import pytest
 import json
+from unittest import mock
 from mapbox_tilesets.utils import (
     _get_api,
     _get_session,
@@ -22,11 +23,20 @@ def test_get_api():
 
 
 def test_get_session():
-    s = _get_session("my_application", "1.0.0")
+    # Cleared so an ambient agent indicator (e.g. this test running inside
+    # Claude Code, where CLAUDECODE is set) can't leak into the assertion.
+    with mock.patch.dict(os.environ, {}, clear=True):
+        s = _get_session("my_application", "1.0.0")
     assert "user-agent" in s.headers
     assert s.headers["user-agent"] == "my_application/1.0.0"
 
 
+def test_get_session_appends_detected_agent():
+    with mock.patch.dict(os.environ, {"CLAUDECODE": "1"}, clear=True):
+        s = _get_session("my_application", "1.0.0")
+    assert s.headers["user-agent"] == "my_application/1.0.0 agent/claude-code"
+
+
 def test_get_token_parameter():
     token = "token-parameter"
     assert token == _get_token(token)