feat(mcp): add 'describe' tool and update documentation

pkg/mcp/instructions.md

@@ -39,7 +39,7 @@ This is essential because:
 
 **Exceptions:**
 - The `list` tool operates on the cluster, not local files, so it does NOT use a path parameter (it uses namespace instead)
-- The `delete` tool requires exactly one of `path` or `name`; it does NOT support a no-argument CWD mode (the MCP server process has its own working directory unrelated to the Function being managed)
+- The `delete` and `describe` tools each require exactly one of `path` or `name`; they do NOT support a no-argument CWD mode (the MCP server process has its own working directory unrelated to the Function being managed)
 
 ## Deployment Behavior
 
@@ -57,6 +57,7 @@ This is essential because:
 - Before 'deploy' → Read `func://help/deploy`
 - Before 'build' → Read `func://help/build`
 - Before 'list' → Read `func://help/list`
+- Before 'describe' → Read `func://help/describe`
 - Before 'delete' → Read `func://help/delete`
 
 The help text provides authoritative parameter information and usage context.
@@ -131,6 +132,15 @@ A first-time deploy can be detected by checking the func.yaml for a value in the
 - Optional `namespace` parameter to list Functions in specific namespace
 - Returns list of deployed Functions in current/specified namespace
 
+### describe
+
+- **FIRST:** Read `func://help/describe` for authoritative usage information
+- Supports TWO modes (mutually exclusive):
+  1. **Describe by PATH:** Provide 'path' parameter (reads function name from func.yaml at that path)
+  2. **Describe by NAME:** Provide 'name' parameter (describes named function from cluster)
+- Exactly ONE of 'path' or 'name' must be provided, not both
+- Read-only; does not modify local files or cluster resources
+
 ### delete
 
 - **FIRST:** Read `func://help/delete` for authoritative usage information

pkg/mcp/mcp.go

@@ -1,6 +1,7 @@
 package mcp
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"os/exec"
@@ -32,6 +33,14 @@ type Server struct {
 
 type executor interface {
 	Execute(ctx context.Context, subcommand string, args ...string) ([]byte, error)
+	// ExecuteSplit runs the command and returns stdout and stderr captured
+	// into separate buffers. Unlike Execute (which uses CombinedOutput and
+	// therefore offers no guarantee about the relative ordering of stdout
+	// and stderr bytes - they're copied by two independently-scheduled
+	// goroutines), ExecuteSplit gives each stream its own buffer, so callers
+	// that need to parse structured output (e.g. JSON) from stdout can do so
+	// without risk of stderr content (warnings, etc.) corrupting the parse.
+	ExecuteSplit(ctx context.Context, subcommand string, args ...string) (stdout, stderr []byte, err error)
 }
 
 type Option func(*Server)
@@ -108,6 +117,7 @@ func New(options ...Option) *Server {
 	mcp.AddTool(i, buildTool, s.buildHandler)
 	mcp.AddTool(i, deployTool, s.deployHandler)
 	mcp.AddTool(i, listTool, s.listHandler)
+	mcp.AddTool(i, describeTool, s.describeHandler)
 	mcp.AddTool(i, deleteTool, s.deleteHandler)
 	mcp.AddTool(i, configVolumesListTool, s.configVolumesListHandler)
 	mcp.AddTool(i, configVolumesAddTool, s.configVolumesAddHandler)
@@ -138,6 +148,7 @@ func New(options ...Option) *Server {
 	i.AddResource(newHelpResource(s, "Build Help", "help for 'build'", "build"))
 	i.AddResource(newHelpResource(s, "Deploy Help", "help for 'deploy'", "deploy"))
 	i.AddResource(newHelpResource(s, "List Help", "help for 'list'", "list"))
+	i.AddResource(newHelpResource(s, "Describe Help", "help for 'describe'", "describe"))
 	i.AddResource(newHelpResource(s, "Delete Help", "help for delete", "delete"))
 
 	i.AddResource(newHelpResource(s, "Volumes Help", "general help for volumes", "config", "volumes"))
@@ -178,6 +189,17 @@ func (e defaultExecutor) Execute(ctx context.Context, subcommand string, args ..
 	return cmd.CombinedOutput()
 }
 
+func (e defaultExecutor) ExecuteSplit(ctx context.Context, subcommand string, args ...string) (stdout, stderr []byte, err error) {
+	cmdParts := buildArgs(e.s.prefix, subcommand, args)
+	cmd := exec.CommandContext(ctx, cmdParts[0], cmdParts[1:]...)
+	// cmd.Dir not set - inherits process working directory which is the current working directory
+	var outBuf, errBuf bytes.Buffer
+	cmd.Stdout = &outBuf
+	cmd.Stderr = &errBuf
+	err = cmd.Run()
+	return outBuf.Bytes(), errBuf.Bytes(), err
+}
+
 // buildArgs constructs the ordered argument list for execution.
 // An empty subcommand is omitted so that commands like "func --help" are
 // built correctly rather than "func  --help" with a spurious empty argument.

pkg/mcp/mock/executor.go

@@ -9,6 +9,9 @@ import (
 type Executor struct {
 	ExecuteInvoked bool
 	ExecuteFn      func(context.Context, string, ...string) ([]byte, error)
+
+	ExecuteSplitInvoked bool
+	ExecuteSplitFn      func(context.Context, string, ...string) (stdout, stderr []byte, err error)
 }
 
 // NewExecutor creates a new mock executor
@@ -27,3 +30,15 @@ func (m *Executor) Execute(ctx context.Context, subcommand string, args ...strin
 
 	return []byte(""), nil
 }
+
+// ExecuteSplit implements the executor interface, recording invocation
+// details and delegating to ExecuteSplitFn if provided.
+func (m *Executor) ExecuteSplit(ctx context.Context, subcommand string, args ...string) ([]byte, []byte, error) {
+	m.ExecuteSplitInvoked = true
+
+	if m.ExecuteSplitFn != nil {
+		return m.ExecuteSplitFn(ctx, subcommand, args...)
+	}
+
+	return []byte(""), []byte(""), nil
+}

pkg/mcp/tools_describe.go

@@ -0,0 +1,102 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+	fn "knative.dev/func/pkg/functions"
+)
+
+var describeTool = &mcp.Tool{
+	Name:        "describe",
+	Title:       "Describe Function",
+	Description: "Describe a deployed Function: URL, routes, image, namespace, deployer, labels, revision, readiness, and event subscriptions.",
+	Annotations: &mcp.ToolAnnotations{
+		Title:          "Describe Function",
+		ReadOnlyHint:   true,
+		IdempotentHint: true, // Describing the same function multiple times returns consistent results at any point in time.
+	},
+}
+
+func (s *Server) describeHandler(ctx context.Context, r *mcp.CallToolRequest, input DescribeInput) (result *mcp.CallToolResult, output DescribeOutput, err error) {
+	// Validate: exactly one of Path or Name must be provided
+	if (input.Path != nil && input.Name != nil) || (input.Path == nil && input.Name == nil) {
+		err = fmt.Errorf("exactly one of 'path' or 'name' must be provided")
+		return
+	}
+
+	// ExecuteSplit (rather than Execute/CombinedOutput) is required here:
+	// the CLI can write warnings to stderr on an otherwise-successful call
+	// (e.g. permission warnings from the knative describer), and stdout and
+	// stderr copied via CombinedOutput have no guaranteed relative ordering.
+	// Parsing JSON only ever out of a clean, unmixed stdout avoids that
+	// entirely rather than relying on any heuristic about stream ordering.
+	stdout, stderr, err := s.executor.ExecuteSplit(ctx, "describe", input.Args()...)
+	if err != nil {
+		err = fmt.Errorf("%w\nstdout: %s\nstderr: %s", err, string(stdout), string(stderr))
+		return
+	}
+
+	var instance fn.Instance
+	if err = json.Unmarshal(stdout, &instance); err != nil {
+		err = fmt.Errorf("failed to parse describe output: %w\n%s", err, string(stdout))
+		return
+	}
+
+	output = DescribeOutput{
+		Name:          instance.Name,
+		Namespace:     instance.Namespace,
+		URL:           instance.Route,
+		Routes:        instance.Routes,
+		Image:         instance.Image,
+		Ready:         instance.Ready,
+		Deployer:      instance.Deployer,
+		Labels:        instance.Labels,
+		Subscriptions: instance.Subscriptions,
+		Revision:      instance.Revision,
+	}
+	return
+}
+
+// DescribeInput defines the input parameters for the describe tool.
+// Exactly one of Path or Name must be provided.
+type DescribeInput struct {
+	Path      *string `json:"path,omitempty" jsonschema:"Path to the function project directory (mutually exclusive with name)"`
+	Name      *string `json:"name,omitempty" jsonschema:"Name of the function to describe (mutually exclusive with path)"`
+	Namespace *string `json:"namespace,omitempty" jsonschema:"Kubernetes namespace to describe from (default: current or active namespace)"`
+	Verbose   *bool   `json:"verbose,omitempty" jsonschema:"Enable verbose logging output"`
+}
+
+func (i DescribeInput) Args() []string {
+	args := []string{}
+
+	// Either path flag or positional name argument
+	if i.Path != nil {
+		args = append(args, "--path", *i.Path)
+	} else if i.Name != nil {
+		args = append(args, *i.Name)
+	}
+
+	args = appendStringFlag(args, "--namespace", i.Namespace)
+	args = appendBoolFlag(args, "--verbose", i.Verbose)
+
+	// The tool's contract is structured JSON, regardless of caller input.
+	args = append(args, "--output", "json")
+	return args
+}
+
+// DescribeOutput defines the structured output returned by the describe tool.
+type DescribeOutput struct {
+	Name          string            `json:"name" jsonschema:"Function name"`
+	Namespace     string            `json:"namespace,omitempty" jsonschema:"Kubernetes namespace"`
+	URL           string            `json:"url,omitempty" jsonschema:"Primary route URL"`
+	Routes        []string          `json:"routes,omitempty" jsonschema:"All route URLs"`
+	Image         string            `json:"image,omitempty" jsonschema:"Deployed container image"`
+	Ready         string            `json:"ready,omitempty" jsonschema:"Overall readiness (true/false/unknown)"`
+	Deployer      string            `json:"deployer,omitempty" jsonschema:"Deployer backend (knative, k8s, keda)"`
+	Labels        map[string]string `json:"labels,omitempty" jsonschema:"Function labels"`
+	Subscriptions []fn.Subscription `json:"subscriptions,omitempty" jsonschema:"Active event subscriptions"`
+	Revision      string            `json:"revision,omitempty" jsonschema:"Source commit SHA, read from the OCI revision label baked into the built image"`
+}