feat(tools): pr_files_diff with caps

Returns per-file unified diff for a PR, capped at 20KB/file and 200KB
total response. Files exceeding per-file cap report truncated+omitted_lines;
files that would push the response over 200KB go to omitted_files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/tools/pr_files_diff.go

@@ -0,0 +1,171 @@
+package tools
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/allowlist"
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/gitea"
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/registry"
+)
+
+const (
+	maxFileDiffBytes = 20 * 1024
+	maxResponseBytes = 200 * 1024
+)
+
+type PRFilesDiff struct {
+	c *gitea.Client
+	a *allowlist.Allowlist
+}
+
+func NewPRFilesDiff(c *gitea.Client, a *allowlist.Allowlist) *PRFilesDiff {
+	return &PRFilesDiff{c: c, a: a}
+}
+
+func (t *PRFilesDiff) Descriptor() registry.ToolDescriptor {
+	return registry.ToolDescriptor{
+		Name:        "pr_files_diff",
+		Description: "Get a pull request's per-file diff with size caps (20KB/file, 200KB total).",
+		InputSchema: json.RawMessage(`{
+			"type":"object",
+			"properties":{
+				"owner":{"type":"string"},
+				"name":{"type":"string"},
+				"number":{"type":"integer","minimum":1}
+			},
+			"required":["owner","name","number"]
+		}`),
+	}
+}
+
+type prFilesDiffArgs struct {
+	Owner  string `json:"owner"`
+	Name   string `json:"name"`
+	Number int    `json:"number"`
+}
+
+type prFileDiffEntry struct {
+	Path         string `json:"path"`
+	Diff         string `json:"diff"`
+	Truncated    bool   `json:"truncated"`
+	OmittedLines int    `json:"omitted_lines,omitempty"`
+	Additions    int    `json:"additions"`
+	Deletions    int    `json:"deletions"`
+}
+
+func (t *PRFilesDiff) Call(ctx context.Context, raw json.RawMessage) (json.RawMessage, error) {
+	var args prFilesDiffArgs
+	if err := parseArgs(raw, &args); err != nil {
+		return nil, err
+	}
+	if err := t.a.Check(args.Owner); err != nil {
+		return nil, err
+	}
+	if args.Number < 1 {
+		return nil, fmt.Errorf("number must be >= 1: %w", gitea.ErrValidation)
+	}
+
+	files, err := t.c.GetPullRequestFiles(ctx, args.Owner, args.Name, args.Number)
+	if err != nil {
+		return nil, err
+	}
+
+	rawDiff, err := t.c.GetPullRequestDiff(ctx, args.Owner, args.Name, args.Number)
+	if err != nil {
+		return nil, err
+	}
+
+	// Split unified diff by per-file headers ("diff --git a/path b/path")
+	perFile := splitUnifiedDiff(rawDiff)
+
+	out := struct {
+		Files             []prFileDiffEntry `json:"files"`
+		OmittedFiles      []string          `json:"omitted_files,omitempty"`
+		ResponseTruncated bool              `json:"response_truncated"`
+	}{
+		Files: make([]prFileDiffEntry, 0, len(files)),
+	}
+
+	totalBytes := 0
+	for _, f := range files {
+		// look up the diff for this file (best-effort by path match)
+		diffBytes, ok := perFile[f.Filename]
+		if !ok {
+			diffBytes = []byte{}
+		}
+
+		entry := prFileDiffEntry{
+			Path:      f.Filename,
+			Additions: f.Additions,
+			Deletions: f.Deletions,
+		}
+
+		// Per-file cap
+		if len(diffBytes) > maxFileDiffBytes {
+			truncated := diffBytes[:maxFileDiffBytes]
+			omittedLines := bytes.Count(diffBytes[maxFileDiffBytes:], []byte("\n"))
+			entry.Diff = string(truncated)
+			entry.Truncated = true
+			entry.OmittedLines = omittedLines
+		} else {
+			entry.Diff = string(diffBytes)
+		}
+
+		// Response cap — if adding this entry would exceed, push to omitted_files
+		entryEstimate := len(entry.Diff) + 200 // small overhead for path + counts
+		if totalBytes+entryEstimate > maxResponseBytes {
+			out.OmittedFiles = append(out.OmittedFiles, f.Filename)
+			out.ResponseTruncated = true
+			continue
+		}
+		totalBytes += entryEstimate
+		out.Files = append(out.Files, entry)
+	}
+
+	return textOK(out)
+}
+
+// splitUnifiedDiff parses a unified diff and returns a map from filename to that file's
+// portion of the diff. The unified diff format starts each file with a line like
+// "diff --git a/<path> b/<path>".
+func splitUnifiedDiff(d []byte) map[string][]byte {
+	m := map[string][]byte{}
+	scanner := bufio.NewScanner(bytes.NewReader(d))
+	scanner.Buffer(make([]byte, 0, 64*1024), 16*1024*1024) // allow long diffs
+
+	var currentFile string
+	var current bytes.Buffer
+
+	flush := func() {
+		if currentFile != "" {
+			m[currentFile] = []byte(current.String())
+			current.Reset()
+		}
+	}
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		if strings.HasPrefix(line, "diff --git ") {
+			flush()
+			// Parse: "diff --git a/<path> b/<path>"
+			rest := strings.TrimPrefix(line, "diff --git a/")
+			parts := strings.SplitN(rest, " b/", 2)
+			if len(parts) == 2 {
+				currentFile = parts[0]
+			} else {
+				currentFile = ""
+			}
+		}
+		if currentFile != "" {
+			current.WriteString(line)
+			current.WriteByte('\n')
+		}
+	}
+	flush()
+	return m
+}