diff --git a/README.md b/README.md
index cc1a877..8e36dc0 100644
--- a/README.md
+++ b/README.md
@@ -115,6 +115,9 @@ Configured via YAML file (`--config`). Key settings:
 - `upstream_max_response_size` — max origin response size
 - `downstream_timeout` — client response timeout
 - `signing_key` — HMAC secret for URL signatures
+- `cache_max_bytes` — disk cache size limit in bytes; `0` disables the
+  disk cache entirely; omitted defaults to 75% of the free space on
+  the filesystem containing `<state_dir>/cache/` (minimum 500 MiB)
 
 See `config.example.yml` for all options with defaults.
 
diff --git a/TODO.md b/TODO.md
index 5c291cc..250904e 100644
--- a/TODO.md
+++ b/TODO.md
@@ -12,18 +12,35 @@
 
 pre-1.0. No git tags exist. Recent work extracted the internal/magic,
 internal/allowlist, internal/httpfetcher, and internal/signature
-packages. The gosec findings from the 2026-07-06 survey are resolved:
-the last two open findings (G124, session cookie attributes in
-internal/session) are fixed as of this change, so `make check` is green
-on main.
+packages. The gosec findings from the 2026-07-06 survey are resolved
+and `make check` is green on main. The disk cache is now size-bounded
+with LRU eviction (`cache_max_bytes`), closing the unbounded disk
+growth DoS vector.
 
 # Next Step
 
-P0: implement cache size management and eviction so the disk cannot
-fill up
+P1: implement blocked networks configuration to extend SSRF protection
 
 # Completed Steps
 
+- 2026-08-07 implement cache size management and eviction (closes
+  #51): new `cache_max_bytes` config key validated by the startup
+  framework (explicit values used exactly with no floor, `0` disables
+  the disk cache entirely, omitted defaults to max(75% of free space
+  on the filesystem containing `<state_dir>/cache/`, 500 MiB), logged
+  at startup); processed variants are now tracked in the database
+  (migration 002 adds `variant_content` and an LRU timestamp on
+  `source_content`) so total usage is two SUMs, never a directory scan
+  on the hot path; a background goroutine evicts globally
+  least-recently-used entries (variants and source blobs merged) to
+  the limit, woken by a periodic ticker and by write-pressure
+  notifications from stores; a source blob and ALL of its
+  `source_metadata` references are deleted in one transaction before
+  the file is unlinked, so multi-referenced blobs are never removed
+  while referenced and rows never point at deleted files; a startup
+  reconciliation pass adopts untracked variant files, drops rows for
+  missing files, removes unreachable source blobs, and sweeps stale
+  temp files
 - 2026-08-07 validate configuration on startup, fail fast on bad
   config (closes #52): a config value that is set but unparseable or
   invalid aborts startup naming the key and value (defaults apply only
@@ -79,8 +96,6 @@ fill up
 
 # Future Steps
 
-- P1: implement blocked networks configuration to extend SSRF
-  protection
 - P1: rate limit global concurrent upstream fetches to prevent
   resource exhaustion
 - P1: strip EXIF and other metadata from processed images (privacy)
diff --git a/config.example.yml b/config.example.yml
index 900a0b6..122189e 100644
--- a/config.example.yml
+++ b/config.example.yml
@@ -28,6 +28,13 @@ allow_http: false
 # Maximum concurrent connections per upstream host (default: 20)
 upstream_connections_per_host: 20
 
+# Maximum disk cache size in bytes. Explicit values are used exactly as
+# given; 0 disables the disk cache entirely (every request fetches and
+# processes uncached). When omitted, the default is 75% of the free
+# space on the filesystem containing <state_dir>/cache/ at startup,
+# with a minimum of 500 MiB.
+# cache_max_bytes: 10737418240
+
 # Sentry error reporting (optional)
 sentry_dsn: ""