Report what a reply spent reading its prompt, and pin the clock right

`UsageDelta` gains `prefillMs`, llama-server's own `timings.prompt_ms`, so the
footer under a finished reply is "read 9.5s · 50.3 tok/s · 3:00 PM". Prefill is
the half of a turn that was invisible and is often the larger: measured on the
0.6B here, 1m 4s for the first turn after a model loads against 22ms for the
next, whose prompt the server still had cached.

The clock moves to the end of the line. Everything in front of it is a
provider's own measurement, so a session on another provider has fewer of them
or none, and a reader who has learned where the time is should not have to find
it again because the model changed. The costs grow leftwards into the space
instead, and a test asserts every shape of the line ends with the same thing.

Verified on the emulator against a real llama session: three replies reading
"read 1m 4s · 193 tok/s · 3:54 PM", "read 25ms · 308 tok/s · 3:54 PM" and
"read 22ms · 194 tok/s · 3:54 PM", with the clock in one column.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/src/session/claude/translate.rs

@@ -429,6 +429,7 @@ impl Translator {
                         tokens,
                         context,
                         tokens_per_second: None,
+                        prefill_ms: None,
                     });
                 }
                 // A level snapshot is authoritative at a turn boundary. In
@@ -2554,6 +2555,7 @@ mod tests {
                     tokens: 182,
                     context: None,
                     tokens_per_second: None,
+                    prefill_ms: None,
                 },
                 Event::Status {
                     state: SessionStatus::Idle
@@ -2597,6 +2599,7 @@ mod tests {
                     tokens: 7,
                     context: None,
                     tokens_per_second: None,
+                    prefill_ms: None,
                 },
                 Event::Status {
                     state: SessionStatus::Idle
@@ -2651,6 +2654,7 @@ mod tests {
                 tokens: 173,
                 context: Some(26_131),
                 tokens_per_second: None,
+                prefill_ms: None,
             })
         );
 
@@ -2668,6 +2672,7 @@ mod tests {
                 tokens: 13,
                 context: None,
                 tokens_per_second: None,
+                prefill_ms: None,
             })
         );
     }

server/src/session/codex/translate.rs

@@ -230,6 +230,7 @@ impl Translator {
                         // Cached input is a subset of this figure, not an additional count.
                         context: last.get("inputTokens").and_then(Value::as_u64),
                         tokens_per_second: None,
+                        prefill_ms: None,
                     })
                     .into_iter()
                     .collect()
@@ -247,6 +248,7 @@ impl Translator {
                             tokens: input.unwrap_or(0) + output.unwrap_or(0),
                             context: input,
                             tokens_per_second: None,
+                            prefill_ms: None,
                         });
                     }
                 }
@@ -939,6 +941,7 @@ mod tests {
                 tokens: 18,
                 context: Some(13),
                 tokens_per_second: None,
+                prefill_ms: None,
             }
         );
         assert!(translator.completed());
@@ -1162,6 +1165,7 @@ mod tests {
                 tokens: 42,
                 context: Some(39),
                 tokens_per_second: None,
+                prefill_ms: None,
             }]
         );
         assert_eq!(
@@ -1172,6 +1176,7 @@ mod tests {
                 tokens: 65,
                 context: Some(61),
                 tokens_per_second: None,
+                prefill_ms: None,
             }]
         );
         assert_eq!(

server/src/session/driver.rs

@@ -438,6 +438,17 @@ pub enum Event {
         /// times those were.
         #[serde(default, skip_serializing_if = "Option::is_none")]
         tokens_per_second: Option<f64>,
+        /// How long the provider spent reading the prompt before it began
+        /// answering -- see [`SessionStatus::Reading`], which is this while it
+        /// is happening.
+        ///
+        /// The provider's own measurement or nothing, for the same reason
+        /// `tokens_per_second` above is: the wait this server watched also
+        /// contains the request and whatever else the machine was doing. It is worth reporting because it is the larger half of a
+        /// turn on a long conversation -- 22 seconds against 3 of generation,
+        /// measured on a 14,000-token prompt.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        prefill_ms: Option<u64>,
     },
     /// A compaction that finished, and how much context it recovered.
     ///
@@ -904,6 +915,7 @@ mod tests {
                     tokens: 12,
                     context: Some(30_100),
                     tokens_per_second: None,
+                    prefill_ms: None,
                 }
             ),
             Some(30_100)
@@ -944,6 +956,7 @@ mod tests {
                     tokens: 12,
                     context: None,
                     tokens_per_second: None,
+                    prefill_ms: None,
                 }
             ),
             Some(30_100)

server/src/session/echo.rs

@@ -894,6 +894,8 @@ impl EchoDriver {
                 tokens_per_second: Some(streaming.elapsed().as_secs_f64())
                     .filter(|elapsed| *elapsed > 0.0)
                     .map(|elapsed| words as f64 / elapsed),
+                // Nothing to read: this driver has no prompt to process.
+                prefill_ms: None,
             });
             finish();
         });

server/src/session/llama/mod.rs

@@ -1640,6 +1640,10 @@ fn generate(
     // divided out of the wall time here, which would count the request, the
     // prompt processing and this loop's own scheduling as generation.
     let mut per_second = None;
+    // What it spent reading the prompt, the same way: `prompt_ms` is the
+    // server's own account of prompt processing, and a prompt it had cached is
+    // a small number rather than a missing one.
+    let mut prefill = None;
     // Closes the open block, which is anything the model says that is not more
     // working: the first word of the reply, or a tool call.
     let done_thinking = |thinking: &mut Option<std::time::Instant>| {
@@ -1678,6 +1682,9 @@ fn generate(
         {
             per_second = Some(rate);
         }
+        if let Some(ms) = chunk.pointer("/timings/prompt_ms").and_then(Value::as_f64) {
+            prefill = Some(ms.round() as u64);
+        }
         let Some(delta) = chunk.pointer("/choices/0/delta") else {
             continue;
         };
@@ -1723,6 +1730,7 @@ fn generate(
             tokens,
             context,
             tokens_per_second: per_second,
+            prefill_ms: prefill,
         });
     }
     // A call whose name never arrived is not a call. It happens when a stream
@@ -1967,6 +1975,7 @@ mod tests {
                 tokens: 12,
                 context: Some(12),
                 tokens_per_second: None,
+                prefill_ms: None,
             },
         ]);
         let messages = conversation(&path);

server/src/session/transcript.rs

@@ -997,6 +997,7 @@ mod tests {
                 tokens: 42,
                 context: Some(42),
                 tokens_per_second: None,
+                prefill_ms: None,
             },
             Event::AuthenticationRequired {
                 message: "sign in again".into(),