diff --git a/pallas/log.py b/pallas/log.py
index 9db560e..56d747a 100644
--- a/pallas/log.py
+++ b/pallas/log.py
@@ -186,6 +186,8 @@ def setup_logging() -> None:
     """
     formatter = _JSONFormatter()
 
+    # Pallas's own diagnostics level — drives the ``pallas.*`` loggers and
+    # the file sink verbosity.  Unchanged behaviour for operators.
     level_name = (
         os.environ.get("PALLAS_LOG_LEVEL")
         or _level_from_fastagent_config()
@@ -193,6 +195,22 @@ def setup_logging() -> None:
     ).upper()
     level = getattr(logging, level_name, logging.INFO)
 
+    # Root logger level is separate: in interactive ``fast-agent go``
+    # sessions, leaving the root logger at DEBUG lets every third-party
+    # library (``openai``, ``sse_starlette``, ``mcp``, ``anthropic``, …)
+    # spray plain-text DEBUG records straight into the terminal, which
+    # corrupts fast-agent's Rich TUI regardless of whether *our* stderr
+    # handler is attached — the libraries install their own handlers
+    # independently.  Default the root to the higher of (level, INFO) so
+    # we never drag the whole process into DEBUG just because Pallas
+    # wants detailed forwarding traces.  Operators who truly need every
+    # library at DEBUG can opt back in with ``PALLAS_ROOT_LOG_LEVEL=DEBUG``.
+    root_level_name = (
+        os.environ.get("PALLAS_ROOT_LOG_LEVEL")
+        or ("INFO" if level < logging.INFO else level_name)
+    ).upper()
+    root_level = getattr(logging, root_level_name, logging.INFO)
+
     # Build the shared handlers once — they get attached to both the
     # ``pallas`` logger (with ``propagate=False``) *and* the root logger,
     # so third-party libraries' records land in the same file sink.
@@ -226,12 +244,14 @@ def setup_logging() -> None:
         stream_handler.setFormatter(formatter)
         handlers.append(stream_handler)
 
-    # Root logger carries everything from libraries we do NOT own.
-    # We set the level low enough to accept our configured level; each
-    # noisy namespace is dialled back below.  Handlers are attached here,
-    # not on child loggers, so records propagate to a single sink.
+    # Root logger carries everything from libraries we do NOT own.  We
+    # keep it at ``root_level`` (INFO by default) rather than ``level``,
+    # so Pallas's own DEBUG diagnostics don't drag third-party libraries
+    # with them.  Noisy namespaces are dialled back further below.
+    # Handlers are attached here, not on child loggers, so records
+    # propagate to a single sink.
     root = logging.getLogger()
-    root.setLevel(level)
+    root.setLevel(root_level)
     # Avoid duplicate handlers across re-entrant setup_logging() calls
     # (e.g. if the agent subprocess re-imports pallas with an altered
     # config).  We tag ours so a safe idempotent re-attach is possible.
@@ -262,9 +282,33 @@ def setup_logging() -> None:
         # won't see it; at DEBUG it's easy to grep for.
         pallas_logger.info("log file: %s", log_file)
 
-    # Silence noisy HTTP client internals — only surface warnings and above.
-    # Applied *after* root-level configuration so these win.
-    for noisy in ("httpx", "httpcore"):
+    # Silence noisy HTTP/LLM client internals — only surface warnings and
+    # above.  Applied *after* root-level configuration so these win.
+    #
+    # ``openai``, ``anthropic``: their ``_base_client`` emits a DEBUG line
+    # per request ("Sending HTTP Request: POST ...") that floods the TUI.
+    # ``sse_starlette``: logs every SSE chunk at DEBUG, one line per
+    # notifications/progress payload — i.e. every tool-call progress tick.
+    # ``mcp``: the MCP Python SDK debugs raw JSON-RPC traffic.
+    # ``httpx`` / ``httpcore``: request-level debug flood.
+    #
+    # If we're running at DEBUG intentionally for Pallas diagnostics, we
+    # still don't want these libraries spraying DEBUG output; operators
+    # can re-enable a specific one with ``logging.getLogger(...).setLevel``
+    # in their own environment.
+    for noisy in (
+        "httpx",
+        "httpcore",
+        "openai",
+        "openai._base_client",
+        "anthropic",
+        "anthropic._base_client",
+        "sse_starlette",
+        "sse_starlette.sse",
+        "mcp",
+        "mcp.client",
+        "mcp.server",
+    ):
         logging.getLogger(noisy).setLevel(logging.WARNING)
 
     # Suppress successful health probe access log entries.