Sanitize untrusted output in fastmcp list and fastmcp call

[jlowin]

Motivation

• The CLI was printing server-controlled strings (tool names/descriptions, prompt/resource text, and tool output) directly through Rich markup, allowing terminal/Rich markup and ANSI/OSC injection from malicious MCP servers.
• The intent is to prevent terminal spoofing or control-sequence abuse while preserving the human-readable CLI output.

Description

• Add a helper _sanitize_untrusted_text that uses rich.markup.escape and encodes non-printable/control characters as visible \xNN sequences while preserving newlines/tabs and printable UTF-8 content.
• Apply the sanitizer to all CLI text output paths that render server-controlled content in text mode: tool call results and errors, resource text, prompt roles/messages, and fastmcp list output for tools, resources, and prompts.
• Add unit tests verifying that Rich markup and control characters are escaped and that the sanitizer returns expected literal strings; include regression coverage in tests/cli/test_client_commands.py.

Testing

• Ran uv sync successfully to prepare the environment.
• Ran the full test suite with uv run pytest -n auto, which reported unrelated failures/timeouts outside of the CLI change (13 failed, 1 error) in this environment and are not caused by this patch.
• Ran focused CLI tests uv run pytest tests/cli/test_client_commands.py, which passed (78 passed).
• Ran static checks: uv run ruff check passed for the modified files, and uv run prek run --all-files failed to initialize hooks due to an external network checkout error (bootstrapping a third-party hook), unrelated to the code changes.

---

Codex Task

[marvin-context-protocol]

Test Failure Analysis

Summary: The static analysis (prek run --all-files) failed because ruff-format reformatted src/fastmcp/cli/client.py — the file was committed without running the formatter first.

Root Cause: Two console.print(...) calls in src/fastmcp/cli/client.py have lines exceeding the allowed line length. ruff-format auto-reformats them to multi-line style, but those changes weren't committed.

Suggested Solution: Run ruff format src/fastmcp/cli/client.py (or prek run --all-files) locally and commit the result. The diff is minimal — just two console.print(...) calls that need to be split across multiple lines:

# Before (too long):
console.print(f"    {_sanitize_untrusted_text(tool.description)}")
console.print(f"  [cyan]{_sanitize_untrusted_text(str(r.uri))}[/cyan]")

# After (ruff-formatted):
console.print(
    f"    {_sanitize_untrusted_text(tool.description)}"
)
console.print(
    f"  [cyan]{_sanitize_untrusted_text(str(r.uri))}[/cyan]"
)

Detailed Analysis

Failing check: ruff-format — 1 file reformatted

Log excerpt:

ruff format..............................................................Failed
- hook id: ruff-format
- files were modified by this hook

  1 file reformatted, 666 files left unchanged

Full diff applied by ruff-format:

--- a/src/fastmcp/cli/client.py
+++ b/src/fastmcp/cli/client.py
@@ -745,7 +745,9 @@
                     sig = format_tool_signature(tool)
                     console.print(f"  [cyan]{_sanitize_untrusted_text(sig)}[/cyan]")
                     if tool.description:
-                        console.print(f"    {_sanitize_untrusted_text(tool.description)}")
+                        console.print(
+                            f"    {_sanitize_untrusted_text(tool.description)}"
+                        )
                     if input_schema:
@@ -759,7 +761,9 @@
                 if not res:
                     console.print("  [dim]No resources found.[/dim]")
                 for r in res:
-                    console.print(f"  [cyan]{_sanitize_untrusted_text(str(r.uri))}[/cyan]")
+                    console.print(
+                        f"  [cyan]{_sanitize_untrusted_text(str(r.uri))}[/cyan]"
+                    )

Note: The loq (file size limits) check reported 7 violations but explicitly stated loq violations not enforced... yet! — those are warnings and did not cause the failure.

Related Files

• src/fastmcp/cli/client.py — the file that needs formatting; contains the two console.print(...) calls that exceed line length