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feat(mcp): gate meta-function emission on advertised server capabilities

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Per-server McpServerFeatures (tools fail-open, resources/prompts
fail-closed) now drive which meta-functions are declared, with
gated_meta_function_prefixes as the single gating seam; read/prompt
declarations land together with their handlers. The server-enablement
sentinel keys on the always-emitted search name so resources-only
servers survive role filtering.

Implements plans/mcp-resources-prompts-design.md §4.4/D7 (T5).
This commit is contained in:
Dark-Alex-17
2026-08-25 11:37:41 -06:00
parent ef88b6a2c8
commit 437512fd6d
8 changed files with 433 additions and 87 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/function/mod.rs
+220 -54
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@@ -16,7 +16,9 @@ use crate::config::ensure_parent_exists;
use crate::config::paths;
use crate::mcp::{
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX, MCP_INVOKE_META_FUNCTION_NAME_PREFIX,
MCP_SEARCH_META_FUNCTION_NAME_PREFIX, McpServersConfig, is_mcp_meta_function,
MCP_META_FUNCTION_PREFIXES, MCP_PROMPT_META_FUNCTION_NAME_PREFIX,
MCP_READ_META_FUNCTION_NAME_PREFIX, MCP_SEARCH_META_FUNCTION_NAME_PREFIX, McpServerFeatures,
McpServersConfig, is_mcp_meta_function,
};
use crate::parsers::{bash, python, typescript};
use anyhow::{Context, Result, anyhow, bail};
@@ -409,6 +411,18 @@ impl ToolResult {
}
}
fn gated_meta_function_prefixes(features: &McpServerFeatures) -> Vec<&'static str> {
MCP_META_FUNCTION_PREFIXES
.into_iter()
.filter(|&prefix| match prefix {
MCP_INVOKE_META_FUNCTION_NAME_PREFIX => features.tools,
MCP_READ_META_FUNCTION_NAME_PREFIX => features.resources,
MCP_PROMPT_META_FUNCTION_NAME_PREFIX => features.prompts,
_ => true,
})
.collect()
}
#[derive(Debug, Clone, Default)]
pub struct Functions {
declarations: Vec<FunctionDeclaration>,
@@ -624,7 +638,7 @@ impl Functions {
.retain(|f| !f.name.starts_with(RAG_FUNCTION_PREFIX));
}
pub fn append_mcp_meta_functions(&mut self, mcp_servers: Vec<String>) {
pub fn append_mcp_meta_functions(&mut self, mcp_servers: Vec<McpServerFeatures>) {
let mut invoke_function_properties = IndexMap::new();
invoke_function_properties.insert(
"tool".to_string(),
@@ -682,59 +696,72 @@ impl Functions {
},
);
for server in mcp_servers {
for features in mcp_servers {
let server = &features.name;
let search_function_name = format!("{}_{server}", MCP_SEARCH_META_FUNCTION_NAME_PREFIX);
let describe_function_name =
format!("{}_{server}", MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX);
let invoke_function_name = format!("{}_{server}", MCP_INVOKE_META_FUNCTION_NAME_PREFIX);
let invoke_function_declaration = FunctionDeclaration {
name: invoke_function_name.clone(),
description: formatdoc!(
r#"
for prefix in gated_meta_function_prefixes(&features) {
match prefix {
MCP_INVOKE_META_FUNCTION_NAME_PREFIX => {
self.declarations.push(FunctionDeclaration {
name: invoke_function_name.clone(),
description: formatdoc!(
r#"
Invoke the specified tool on the {server} MCP server. Always call {describe_function_name} first to
find the correct invocation schema for the given tool.
"#
),
parameters: JsonSchema {
type_value: Some("object".to_string()),
properties: Some(invoke_function_properties.clone()),
required: Some(vec!["tool".to_string()]),
..Default::default()
},
agent: false,
};
let search_functions_declaration = FunctionDeclaration {
name: search_function_name.clone(),
description: formatdoc!(
r#"
Find candidate tools by keywords for the {server} MCP server. Returns small suggestions; fetch
schemas with {describe_function_name}.
"#
),
parameters: JsonSchema {
type_value: Some("object".to_string()),
properties: Some(search_function_properties.clone()),
required: Some(vec!["query".to_string()]),
..Default::default()
},
agent: false,
};
let describe_functions_declaration = FunctionDeclaration {
name: describe_function_name.clone(),
description: "Get the full schema or metadata for exactly one MCP catalog item: \
a tool, resource, resource template, or prompt."
.to_string(),
parameters: JsonSchema {
type_value: Some("object".to_string()),
properties: Some(describe_function_properties.clone()),
required: Some(vec!["tool".to_string()]),
..Default::default()
},
agent: false,
};
self.declarations.push(invoke_function_declaration);
self.declarations.push(search_functions_declaration);
self.declarations.push(describe_functions_declaration);
),
parameters: JsonSchema {
type_value: Some("object".to_string()),
properties: Some(invoke_function_properties.clone()),
required: Some(vec!["tool".to_string()]),
..Default::default()
},
agent: false,
});
}
MCP_SEARCH_META_FUNCTION_NAME_PREFIX => {
self.declarations.push(FunctionDeclaration {
name: search_function_name.clone(),
description: formatdoc!(
r#"
Find candidate tools by keywords for the {server} MCP server. Returns small suggestions; fetch
schemas with {describe_function_name}.
"#
),
parameters: JsonSchema {
type_value: Some("object".to_string()),
properties: Some(search_function_properties.clone()),
required: Some(vec!["query".to_string()]),
..Default::default()
},
agent: false,
});
}
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX => {
self.declarations.push(FunctionDeclaration {
name: describe_function_name.clone(),
description: "Get the full schema or metadata for exactly one MCP \
catalog item: a tool, resource, resource template, or \
prompt."
.to_string(),
parameters: JsonSchema {
type_value: Some("object".to_string()),
properties: Some(describe_function_properties.clone()),
required: Some(vec!["tool".to_string()]),
..Default::default()
},
agent: false,
});
}
// The declaration is added alongside its handler.
MCP_READ_META_FUNCTION_NAME_PREFIX => {}
MCP_PROMPT_META_FUNCTION_NAME_PREFIX => {}
_ => debug_assert!(false, "unhandled MCP meta-function prefix: {prefix}"),
}
}
}
}
@@ -1859,6 +1886,19 @@ mod tests {
ToolCall::new(name.to_string(), args, Some("id1".to_string()))
}
fn mcp_features(name: &str, tools: bool, resources: bool, prompts: bool) -> McpServerFeatures {
McpServerFeatures {
name: name.to_string(),
tools,
resources,
prompts,
}
}
fn tools_only(name: &str) -> McpServerFeatures {
mcp_features(name, true, false, false)
}
fn run_async<F: Future>(f: F) -> F::Output {
tokio::runtime::Builder::new_current_thread()
.enable_all()
@@ -2231,7 +2271,7 @@ mod tests {
#[test]
fn functions_append_mcp_meta_creates_three_per_server() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec!["github".to_string()]);
f.append_mcp_meta_functions(vec![tools_only("github")]);
assert_eq!(f.declarations().len(), 3);
assert!(f.contains("mcp_invoke_github"));
assert!(f.contains("mcp_search_github"));
@@ -2241,7 +2281,7 @@ mod tests {
#[test]
fn functions_append_mcp_meta_multiple_servers() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec!["github".into(), "slack".into()]);
f.append_mcp_meta_functions(vec![tools_only("github"), tools_only("slack")]);
assert_eq!(f.declarations().len(), 6);
assert!(f.contains("mcp_invoke_github"));
assert!(f.contains("mcp_invoke_slack"));
@@ -2254,6 +2294,132 @@ mod tests {
assert!(f.is_empty());
}
#[test]
fn functions_append_mcp_meta_resources_only_omits_invoke() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec![mcp_features("res", false, true, false)]);
assert_eq!(f.declarations().len(), 2);
assert!(!f.contains("mcp_invoke_res"));
assert!(f.contains("mcp_search_res"));
assert!(f.contains("mcp_describe_res"));
}
#[test]
fn functions_append_mcp_meta_all_capabilities_emits_three() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec![mcp_features("srv", true, true, true)]);
assert_eq!(f.declarations().len(), 3);
assert!(f.contains("mcp_invoke_srv"));
assert!(f.contains("mcp_search_srv"));
assert!(f.contains("mcp_describe_srv"));
}
#[test]
fn features_from_missing_capabilities_fail_open_for_tools() {
let features = McpServerFeatures::from_capabilities("srv", None);
assert!(features.tools);
assert!(!features.resources);
assert!(!features.prompts);
let mut f = Functions::default();
f.append_mcp_meta_functions(vec![features]);
assert!(f.contains("mcp_invoke_srv"));
}
#[test]
fn gated_prefixes_tools_only() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", true, false, false)),
vec![
MCP_INVOKE_META_FUNCTION_NAME_PREFIX,
MCP_SEARCH_META_FUNCTION_NAME_PREFIX,
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX,
]
);
}
#[test]
fn gated_prefixes_tools_and_resources_include_read() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", true, true, false)),
vec![
MCP_INVOKE_META_FUNCTION_NAME_PREFIX,
MCP_SEARCH_META_FUNCTION_NAME_PREFIX,
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX,
MCP_READ_META_FUNCTION_NAME_PREFIX,
]
);
}
#[test]
fn gated_prefixes_tools_and_prompts_include_prompt() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", true, false, true)),
vec![
MCP_INVOKE_META_FUNCTION_NAME_PREFIX,
MCP_SEARCH_META_FUNCTION_NAME_PREFIX,
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX,
MCP_PROMPT_META_FUNCTION_NAME_PREFIX,
]
);
}
#[test]
fn gated_prefixes_all_capabilities_include_all() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", true, true, true)),
MCP_META_FUNCTION_PREFIXES.to_vec()
);
}
#[test]
fn gated_prefixes_resources_only_omit_invoke() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", false, true, false)),
vec![
MCP_SEARCH_META_FUNCTION_NAME_PREFIX,
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX,
MCP_READ_META_FUNCTION_NAME_PREFIX,
]
);
}
#[test]
fn gated_prefixes_prompts_only_omit_invoke_and_read() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", false, false, true)),
vec![
MCP_SEARCH_META_FUNCTION_NAME_PREFIX,
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX,
MCP_PROMPT_META_FUNCTION_NAME_PREFIX,
]
);
}
#[test]
fn gated_prefixes_resources_and_prompts_omit_invoke() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", false, true, true)),
vec![
MCP_SEARCH_META_FUNCTION_NAME_PREFIX,
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX,
MCP_READ_META_FUNCTION_NAME_PREFIX,
MCP_PROMPT_META_FUNCTION_NAME_PREFIX,
]
);
}
#[test]
fn gated_prefixes_no_capabilities_keep_search_and_describe() {
assert_eq!(
gated_meta_function_prefixes(&mcp_features("srv", false, false, false)),
vec![
MCP_SEARCH_META_FUNCTION_NAME_PREFIX,
MCP_DESCRIBE_META_FUNCTION_NAME_PREFIX,
]
);
}
#[test]
fn functions_find_returns_declaration() {
let mut f = Functions::default();
@@ -2285,7 +2451,7 @@ mod tests {
#[test]
fn functions_mcp_invoke_declaration_has_tool_and_arguments_params() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec!["srv".to_string()]);
f.append_mcp_meta_functions(vec![tools_only("srv")]);
let decl = f.find("mcp_invoke_srv").unwrap();
let props = decl.parameters.properties.as_ref().unwrap();
assert!(props.contains_key("tool"));
@@ -2297,7 +2463,7 @@ mod tests {
#[test]
fn functions_mcp_search_declaration_has_query_and_top_k_params() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec!["srv".to_string()]);
f.append_mcp_meta_functions(vec![tools_only("srv")]);
let decl = f.find("mcp_search_srv").unwrap();
let props = decl.parameters.properties.as_ref().unwrap();
assert!(props.contains_key("query"));
@@ -2307,7 +2473,7 @@ mod tests {
#[test]
fn functions_mcp_describe_declaration_has_tool_param() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec!["srv".to_string()]);
f.append_mcp_meta_functions(vec![tools_only("srv")]);
let decl = f.find("mcp_describe_srv").unwrap();
let props = decl.parameters.properties.as_ref().unwrap();
assert!(props.contains_key("tool"));
@@ -2316,7 +2482,7 @@ mod tests {
#[test]
fn functions_mcp_describe_declaration_has_optional_kind_param() {
let mut f = Functions::default();
f.append_mcp_meta_functions(vec!["srv".to_string()]);
f.append_mcp_meta_functions(vec![tools_only("srv")]);
let decl = f.find("mcp_describe_srv").unwrap();
let props = decl.parameters.properties.as_ref().unwrap();
let kind = props.get("kind").unwrap();