diff --git a/ROADMAP.md b/ROADMAP.md
index 6def3e4..ce1fa3b 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -20,7 +20,7 @@ This release focuses on making Cass easier to interrupt, easier to audit, and sa
   - Preserve valid tool-call structure when compacting context; do not leave orphaned tool result messages.
   - Compact or summarize older tool outputs when needed instead of dropping records blindly.
 
-- [ ] **Supersede old file-read outputs.** When the same file is read again in a session, avoid repeatedly sending stale large read outputs to the model.
+- [x] **Supersede old file-read outputs.** When the same file is read again in a session, avoid repeatedly sending stale large read outputs to the model.
   - Keep the historical tool call visible in the conversation record.
   - Replace superseded model-context output with a short note indicating it was omitted because a newer read exists.
   - Be careful with partial reads: a later read of the same file does not always supersede a different line range.
diff --git a/src/agent.rs b/src/agent.rs
index 220e8dc..4b1f2a4 100644
--- a/src/agent.rs
+++ b/src/agent.rs
@@ -7,7 +7,7 @@ use crate::providers::types::ModelMessage;
 use crate::tools::{self, ToolContext, ToolRuntimeEvent};
 use anyhow::Result;
 use serde_json::Value;
-use std::collections::BTreeSet;
+use std::collections::{BTreeMap, BTreeSet};
 use std::path::PathBuf;
 use tokio::sync::mpsc;
 
@@ -247,6 +247,7 @@ fn build_messages(records: &[Record], system: String, config: &Config) -> Vec<Mo
     let mut messages = vec![ModelMessage::System { content: system }];
     messages.extend(records.iter().filter_map(record_to_model_message));
     messages = sanitize_tool_message_structure(messages);
+    supersede_old_read_outputs(&mut messages);
 
     let budget = context_budget_tokens(config);
     if estimate_messages_tokens(&messages) > budget {
@@ -288,6 +289,298 @@ fn record_to_model_message(record: &Record) -> Option<ModelMessage> {
     }
 }
 
+fn supersede_old_read_outputs(messages: &mut [ModelMessage]) {
+    let read_calls = read_tool_calls_by_id(messages);
+    let mut read_outputs = Vec::new();
+    for (message_idx, message) in messages.iter().enumerate() {
+        let ModelMessage::Tool {
+            tool_call_id,
+            name,
+            content,
+        } = message
+        else {
+            continue;
+        };
+        if name != "read" {
+            continue;
+        }
+        let request_specs = read_calls
+            .get(tool_call_id)
+            .map(|call| read_request_specs(&call.arguments))
+            .unwrap_or_default();
+        let sections = parse_read_output_sections(content, &request_specs);
+        if !sections.is_empty() {
+            read_outputs.push(ReadOutputInfo {
+                message_idx,
+                sections,
+            });
+        }
+    }
+
+    for read_idx in 0..read_outputs.len() {
+        let superseded: Vec<bool> = read_outputs[read_idx]
+            .sections
+            .iter()
+            .map(|section| {
+                read_outputs[read_idx + 1..]
+                    .iter()
+                    .flat_map(|later| later.sections.iter())
+                    .any(|later| later.path == section.path && range_covers(later, section))
+            })
+            .collect();
+        if !superseded.iter().any(|v| *v) {
+            continue;
+        }
+
+        let message_idx = read_outputs[read_idx].message_idx;
+        let replacement = superseded_read_content(
+            tool_content(messages, message_idx),
+            &read_outputs[read_idx].sections,
+            &superseded,
+        );
+        if let Some(ModelMessage::Tool { content, .. }) = messages.get_mut(message_idx) {
+            *content = replacement;
+        }
+    }
+}
+
+fn tool_content(messages: &[ModelMessage], idx: usize) -> &str {
+    match &messages[idx] {
+        ModelMessage::Tool { content, .. } => content,
+        _ => "",
+    }
+}
+
+fn read_tool_calls_by_id(messages: &[ModelMessage]) -> BTreeMap<String, StoredToolCall> {
+    let mut calls = BTreeMap::new();
+    for message in messages {
+        let ModelMessage::Assistant { tool_calls, .. } = message else {
+            continue;
+        };
+        for call in tool_calls {
+            if call.name == "read" {
+                calls.insert(call.id.clone(), call.clone());
+            }
+        }
+    }
+    calls
+}
+
+#[derive(Debug, Clone)]
+struct ReadOutputInfo {
+    message_idx: usize,
+    sections: Vec<ReadOutputSection>,
+}
+
+#[derive(Debug, Clone)]
+struct ReadOutputSection {
+    path: String,
+    start_line: usize,
+    end_line: usize,
+    coverage_start: usize,
+    coverage_end: Option<usize>,
+    start_byte: usize,
+    end_byte: usize,
+}
+
+#[derive(Debug, Clone)]
+struct ReadRequestSpec {
+    start_line: usize,
+    end_line: Option<usize>,
+}
+
+fn parse_read_output_sections(
+    content: &str,
+    request_specs: &[ReadRequestSpec],
+) -> Vec<ReadOutputSection> {
+    let mut headers = Vec::new();
+    for (line_start, line) in content_lines_with_offsets(content) {
+        if let Some((path, header_start, header_end)) = parse_read_header(line) {
+            headers.push((line_start, path, header_start, header_end));
+        }
+    }
+
+    let mut sections = Vec::new();
+    for (idx, (start_byte, path, header_start, header_end)) in headers.iter().enumerate() {
+        let end_byte = headers
+            .get(idx + 1)
+            .map(|(next_start, _, _, _)| *next_start)
+            .unwrap_or(content.len());
+        let section_content = &content[*start_byte..end_byte];
+        let (start_line, end_line) =
+            observed_read_range(section_content, *header_start, *header_end);
+        let (coverage_start, coverage_end) = read_coverage(
+            request_specs.get(idx),
+            start_line,
+            end_line,
+            *header_end,
+            section_content.contains("truncated by Cass"),
+        );
+        sections.push(ReadOutputSection {
+            path: path.clone(),
+            start_line,
+            end_line,
+            coverage_start,
+            coverage_end,
+            start_byte: *start_byte,
+            end_byte,
+        });
+    }
+    sections
+}
+
+fn read_request_specs(arguments: &Value) -> Vec<ReadRequestSpec> {
+    if let Some(files) = arguments.get("files").and_then(|files| files.as_array()) {
+        return files
+            .iter()
+            .map(|file| read_request_spec(file.get("lines").and_then(|lines| lines.as_str())))
+            .collect();
+    }
+
+    if arguments.get("path").is_some() {
+        return vec![read_request_spec(
+            arguments.get("lines").and_then(|lines| lines.as_str()),
+        )];
+    }
+
+    Vec::new()
+}
+
+fn read_request_spec(lines: Option<&str>) -> ReadRequestSpec {
+    let Some(lines) = lines.map(str::trim).filter(|lines| !lines.is_empty()) else {
+        return ReadRequestSpec {
+            start_line: 1,
+            end_line: None,
+        };
+    };
+    let Some((start, end)) = lines.split_once('-') else {
+        return ReadRequestSpec {
+            start_line: 1,
+            end_line: Some(0),
+        };
+    };
+    let start_line = if start.is_empty() {
+        1
+    } else {
+        start.parse().unwrap_or(1)
+    };
+    let end_line = if end.is_empty() {
+        None
+    } else {
+        Some(end.parse().unwrap_or(0))
+    };
+    ReadRequestSpec {
+        start_line,
+        end_line,
+    }
+}
+
+fn read_coverage(
+    request: Option<&ReadRequestSpec>,
+    observed_start: usize,
+    observed_end: usize,
+    header_end: usize,
+    truncated: bool,
+) -> (usize, Option<usize>) {
+    let Some(request) = request.filter(|_| !truncated) else {
+        return (observed_start, Some(observed_end));
+    };
+
+    let start = request.start_line.max(1);
+    let end = match request.end_line {
+        None => None,
+        Some(requested_end) if header_end < requested_end => None,
+        Some(requested_end) => Some(requested_end),
+    };
+    (start, end)
+}
+
+fn content_lines_with_offsets(content: &str) -> Vec<(usize, &str)> {
+    let mut out = Vec::new();
+    let mut start = 0;
+    while start < content.len() {
+        let rest = &content[start..];
+        let newline_offset = rest.find('\n');
+        let end = newline_offset
+            .map(|offset| start + offset)
+            .unwrap_or(content.len());
+        out.push((start, &content[start..end]));
+        start = match newline_offset {
+            Some(offset) => start + offset + 1,
+            None => content.len(),
+        };
+    }
+    out
+}
+
+fn parse_read_header(line: &str) -> Option<(String, usize, usize)> {
+    let inner = line.strip_prefix("--- ")?.strip_suffix(" ---")?;
+    let (path, range) = inner.rsplit_once(" lines ")?;
+    let (start, end) = range.split_once('-')?;
+    Some((path.to_string(), start.parse().ok()?, end.parse().ok()?))
+}
+
+fn observed_read_range(
+    section_content: &str,
+    header_start: usize,
+    header_end: usize,
+) -> (usize, usize) {
+    let observed: Vec<usize> = content_lines_with_offsets(section_content)
+        .into_iter()
+        .skip(1)
+        .filter_map(|(_, line)| parse_numbered_read_line(line))
+        .collect();
+    match (observed.first(), observed.last()) {
+        (Some(start), Some(end)) => (*start, *end),
+        _ => (header_start, header_end),
+    }
+}
+
+fn parse_numbered_read_line(line: &str) -> Option<usize> {
+    let (prefix, _) = line.split_once(" | ")?;
+    prefix.trim().parse().ok()
+}
+
+fn range_covers(newer: &ReadOutputSection, older: &ReadOutputSection) -> bool {
+    if older.end_line < older.start_line {
+        return newer.coverage_start <= older.start_line;
+    }
+    if newer.coverage_start > older.start_line {
+        return false;
+    }
+    match newer.coverage_end {
+        Some(end) => end >= older.end_line,
+        None => true,
+    }
+}
+
+fn superseded_read_content(
+    original: &str,
+    sections: &[ReadOutputSection],
+    superseded: &[bool],
+) -> String {
+    let mut out = String::with_capacity(original.len().min(1024));
+    let mut cursor = 0;
+    for (section, is_superseded) in sections.iter().zip(superseded.iter()) {
+        out.push_str(&original[cursor..section.start_byte]);
+        if *is_superseded {
+            out.push_str(&superseded_read_note(section));
+        } else {
+            out.push_str(&original[section.start_byte..section.end_byte]);
+        }
+        cursor = section.end_byte;
+    }
+    out.push_str(&original[cursor..]);
+    out
+}
+
+fn superseded_read_note(section: &ReadOutputSection) -> String {
+    format!(
+        "[Cass omitted this earlier read output for {} lines {}-{} from the model context because a newer read of the same range exists later in the conversation.]\n",
+        section.path, section.start_line, section.end_line
+    )
+}
+
 fn context_budget_tokens(config: &Config) -> usize {
     let context_length = config
         .model_metadata
@@ -566,7 +859,15 @@ mod tests {
         StoredToolCall {
             id: id.to_string(),
             name: "read".to_string(),
-            arguments: json!({"path":"src/main.rs"}),
+            arguments: json!({"files":[{"path":"src/main.rs"}]}),
+        }
+    }
+
+    fn call_with_lines(id: &str, path: &str, lines: &str) -> StoredToolCall {
+        StoredToolCall {
+            id: id.to_string(),
+            name: "read".to_string(),
+            arguments: json!({"files":[{"path":path,"lines":lines}]}),
         }
     }
 
@@ -667,6 +968,248 @@ mod tests {
         );
     }
 
+    #[test]
+    fn repeated_read_of_same_range_supersedes_old_output() {
+        let records = vec![
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call("call_1")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_1".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 1-2 ---\n     1 | old one\n     2 | old two\n"
+                    .into(),
+                ts: now_ts(),
+            },
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call("call_2")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_2".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 1-2 ---\n     1 | new one\n     2 | new two\n"
+                    .into(),
+                ts: now_ts(),
+            },
+        ];
+        let messages = build_messages(&records, "system".into(), &Config::default());
+        assert_valid_tool_structure(&messages);
+
+        let first = messages
+            .iter()
+            .find(|message| matches!(message, ModelMessage::Tool { tool_call_id, .. } if tool_call_id == "call_1"))
+            .unwrap();
+        assert!(matches!(
+            first,
+            ModelMessage::Tool { content, .. }
+                if content.contains("Cass omitted this earlier read output")
+                    && !content.contains("old one")
+        ));
+        let second = messages
+            .iter()
+            .find(|message| matches!(message, ModelMessage::Tool { tool_call_id, .. } if tool_call_id == "call_2"))
+            .unwrap();
+        assert!(matches!(
+            second,
+            ModelMessage::Tool { content, .. } if content.contains("new one")
+        ));
+    }
+
+    #[test]
+    fn later_full_read_supersedes_old_output_even_when_file_shrinks() {
+        let records = vec![
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call("call_1")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_1".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 1-3 ---\n     1 | old one\n     2 | old two\n     3 | deleted\n".into(),
+                ts: now_ts(),
+            },
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call("call_2")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_2".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 1-2 ---\n     1 | new one\n     2 | new two\n".into(),
+                ts: now_ts(),
+            },
+        ];
+        let messages = build_messages(&records, "system".into(), &Config::default());
+
+        let first = messages
+            .iter()
+            .find(|message| matches!(message, ModelMessage::Tool { tool_call_id, .. } if tool_call_id == "call_1"))
+            .unwrap();
+        assert!(matches!(
+            first,
+            ModelMessage::Tool { content, .. }
+                if content.contains("Cass omitted this earlier read output")
+                    && !content.contains("deleted")
+        ));
+    }
+
+    #[test]
+    fn later_partial_read_of_different_range_does_not_supersede_old_output() {
+        let records = vec![
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call_with_lines("call_1", "src/main.rs", "1-2")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_1".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 1-2 ---\n     1 | keep one\n     2 | keep two\n".into(),
+                ts: now_ts(),
+            },
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call_with_lines("call_2", "src/main.rs", "10-12")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_2".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 10-12 ---\n    10 | other\n    11 | lines\n    12 | here\n".into(),
+                ts: now_ts(),
+            },
+        ];
+        let messages = build_messages(&records, "system".into(), &Config::default());
+
+        let first = messages
+            .iter()
+            .find(|message| matches!(message, ModelMessage::Tool { tool_call_id, .. } if tool_call_id == "call_1"))
+            .unwrap();
+        assert!(matches!(
+            first,
+            ModelMessage::Tool { content, .. }
+                if content.contains("keep one")
+                    && !content.contains("Cass omitted this earlier read output")
+        ));
+    }
+
+    #[test]
+    fn later_wider_read_supersedes_covered_partial_output() {
+        let records = vec![
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call_with_lines("call_1", "src/main.rs", "10-12")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_1".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 10-12 ---\n    10 | old\n    11 | old\n    12 | old\n".into(),
+                ts: now_ts(),
+            },
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call_with_lines("call_2", "src/main.rs", "1-20")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_2".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/example.rs lines 1-20 ---\n     1 | new\n    10 | new\n    20 | new\n".into(),
+                ts: now_ts(),
+            },
+        ];
+        let messages = build_messages(&records, "system".into(), &Config::default());
+
+        let first = messages
+            .iter()
+            .find(|message| matches!(message, ModelMessage::Tool { tool_call_id, .. } if tool_call_id == "call_1"))
+            .unwrap();
+        assert!(matches!(
+            first,
+            ModelMessage::Tool { content, .. }
+                if content.contains("Cass omitted this earlier read output")
+                    && !content.contains("old")
+        ));
+    }
+
+    #[test]
+    fn multi_file_read_only_replaces_superseded_sections() {
+        let records = vec![
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call("call_1")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_1".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/a.rs lines 1-1 ---\n     1 | stale\n--- /tmp/b.rs lines 1-1 ---\n     1 | keep\n".into(),
+                ts: now_ts(),
+            },
+            Record::Assistant {
+                content: String::new(),
+                reasoning: String::new(),
+                reasoning_field: None,
+                tool_calls: vec![call("call_2")],
+                ts: now_ts(),
+            },
+            Record::Tool {
+                tool_call_id: "call_2".into(),
+                name: "read".into(),
+                ok: true,
+                content: "--- /tmp/a.rs lines 1-1 ---\n     1 | fresh\n".into(),
+                ts: now_ts(),
+            },
+        ];
+        let messages = build_messages(&records, "system".into(), &Config::default());
+
+        let first = messages
+            .iter()
+            .find(|message| matches!(message, ModelMessage::Tool { tool_call_id, .. } if tool_call_id == "call_1"))
+            .unwrap();
+        assert!(matches!(
+            first,
+            ModelMessage::Tool { content, .. }
+                if content.contains("Cass omitted this earlier read output")
+                    && !content.contains("stale")
+                    && content.contains("keep")
+        ));
+    }
+
     #[test]
     fn invalid_leading_tool_results_are_removed() {
         let messages = sanitize_tool_message_structure(vec![